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Microstructure characteristics and a deformation mechanism of fine-grained tungsten heavy alloy under high strain rate compression
Abstract
The dynamic behavior of fine-grained 93W–4.9Ni–2.1Fe–0.03Y (wt.%) alloy with an average grain size of ∼6 μm was investigated under the strain rates of 1.2 × 103, 1.5 × 103, 1.8 × 103 and 1.9 × 103 s−1. Microstructures in the tested specimens were characterized by optical microscopy and transmission electron microscopy. The results show that the fine-grained alloy exhibit low rate-sensitivity and strain hardening. The slope of the elastic deformation stage corresponding to the stress–strain curve of 1.9 × 103 s−1 is much smaller than those of the other curves due to slight premature instability. In addition, adiabatic shear bands (ASBs) were formed in the specimen with the strain rate of 1.9 × 103 s−1. Numerous dislocations and some twins in the W and γ-(Ni, Fe) could be observed in the ASB, indicating that no sharp temperature increase occurred in the specimen, and the adiabatic shear failure is probably independent of the significant thermal softening effect during high strain rate compression. The significant thermal softening independence of the adiabatic shear failure in the alloy is mainly attributed to the contributions stemming from the grain refinement and second-phase particles of yttrium.
... [1][2][3]. The mechanical properties of WHAs are often tailored to withstand severe conditions by controlling their W content (30-99 wt%) or W particle size [4][5][6][7]. To date, liquid phase sintering (LPS) is the most common and practical method for preparing WHAs [8]. ...
... Furthermore, the utilization of a fine raw W powder and the incipient duration of the liquid in the LMD process contributed to the resulting fine W particles. Although fine-grain WHAs exhibit improved mechanical properties, adiabatic shear propensities, and especially penetration performances, there is no practical industrial process at present that can achieve such materials [6,[29][30][31]. Compared with the W particle sizes of over 30 μm employed in conventional LPS WNiFe fabrication [10], and considering the capability of LMD to prepare large components owing to its higher build rates and larger build volumes [16], LMD has significant potential for the preparation of fine-grain WHAs for the fabrication of structural parts. ...
... As shown in Fig. 5, LMD 50WNiFe and 75WNiFe exhibited excellent mechanical strengths under quasi-static tension, and a higher W volume fraction resulted in a better strength. Fig. 5 shows both the tensile curves and a comparison of the strengths of the prepared LMD samples and WHAs prepared by LPS [6,10,[33][34][35][36][37][38]. Usually, LPS WNiFe alloys with higher W contents have higher strengths. ...
Preparing bulk WNiFe alloys with high strengths and low W content (<80 wt%) is challenging. Here, we present a strategy, powder-fed laser melting deposition (LMD), to solve this problem. The LMD-prepared WNiFe (LMD WNiFe) samples show high mechanical strengths and low W contents of 50 and 75 wt%. The W particles in the LMD WNiFe samples uniformly distribute in the matrix (γ), exhibiting fine particle sizes of approximately one-fifth of those of conventional 93WNiFe alloys prepared by liquid phase sintering (LPS). The ultimate tensile strengths of the 50 and 75 wt% samples are 1120 and 1316 MPa, respectively, which are 24.44 and 59.33% higher than those of the LPS-prepared 93WNiFe alloy (LPS 93WNiFe). These results suggest a useful strategy for preparing low-W-content, high-strength WNiFe alloys with fine, uniformly distributed W particles. This finding offers new potential applications of WNiFe alloys in additive manufacturing.
... But in some cases where temperature rise is not sufficient enough for melting to happen, the hardening effect overtakes the thermal softening. Dynamic recrystallization becomes the dominant mechanism to control the formation and development of ASBs [10][11][12][13][14][15]. Rittel et al. [16,17] suggested that the majority of plastic deformation work exists in the form of dynamic stored energy. ...
... This is different from the dynamic stress-strain curves of ductile materials which exhibit distinguished hardening stage and unstable deformation stage [18,23]. The similar mechanical response was found in the finegrained tungsten heavy alloy under high strain rate compression [14] where the flow stress curve was nearly flat in the plastic deformation region. This is termed as the process of plasticity instability. ...
... The formation of equiaxed grains and subgrains through breaking down and splitting was initiated by dynamic recovery and then dominated by dynamic recrystallization. On the contrary, Rittel et al. [16] had a different view that the temperature rise prior to shear localization has a very small effect on the onset of ASB formation, which is supported by Clos et al. [43] and Gong et al. [14] who reported a minor temperature rise prior to localization. Formation of ASBs at the temperature of À 140°C in tungsten under dynamic compression further demonstrates that the initial temperature has minor influence on ASBs formation [44], although the low temperature can suppress dynamic recrystallization to a certain degree. ...
The microstructure evolution of adiabatic shear bands (ASBs) in AISI 52100 produced under high strain rate impact loading was investigated by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). A full view of the shear band formation and development was presented. The results reveal that no strain hardening effect occurred in the AISI 52100 under the dynamic loading. Two distinct regions of the ASBs and the transition region were observed. Driven by the high shear deformation and assisted by a thermal softening effect, the formation of ASBs started in the core region under the dominant mechanism of dynamic recrystallization. It is followed by the formation of neighbouring region under the co-action of both dynamic recovery and dynamic recrystallization. Then the transition region formed which was controlled by dynamic recovery. The core region of ASBs was consisted of subgrains and fine equiaxed grains. The grains were refined either through partitioning and breaking down the elongated subgrains by entangled dislocations clusters or through splitting and rotating the elongated subgrains into small grains under a synergistic effect of shearing and twisting. The amount and size of cementites were greatly reduced in ASBs due to cementite dissolution induced by large plastic deformation and dynamic recrystallization.
... Several works on enhancing the adiabatic shear failure of coarse-grained tungsten heavy alloys have been carried out over the last decade. Gong et al. [87] studied the dynamic behavior of a fine-grained alloy of 93W-4.9Ni-2.1Fe-0.03Y in the strain range of 1200 to 1900 s −1 . The shear band occurred in the alloy tested at the strain rate of 1900 s −1 and is represented in Figure 13. ...
... The adiabatic shear band is visible in the specimen dynamically tested at a strain rate of 1.9 × 10 3 s −1[87]. Reproduced with permission from Gong et al., Materials Science and Engineering: A; published by Elsevier, 2010. ...
Tungsten heavy alloys are two-phase metal matrix composites that include W–Ni–Fe and W–Ni–Cu. The significant feature of these alloys is their ability to acquire both strength and ductility. In order to improve the mechanical properties of the basic alloy and to limit or avoid the need for post-processing techniques, other elements are doped with the alloy and performance studies are carried out. This work focuses on the developments through the years in improving the performance of the classical tungsten heavy alloy of W–Ni–Fe through doping of other elements. The influence of the percentage addition of rare earth elements of yttrium, lanthanum, and their oxides and refractory metals such as rhenium, tantalum, and molybdenum on the mechanical properties of the heavy alloy is critically analyzed. Based on the microstructural and property evaluation, the effects of adding the elements at various proportions are discussed. The addition of molybdenum and rhenium to the heavy alloy gives good strength and ductility. The oxides of yttrium, when added in a small quantity, help to reduce the tungsten’s grain size and obtain good tensile and compressive strengths at high temperatures.
... Tungsten heavy alloys (WHAs) are a typical class of two-phase composites, mainly consisting of spherical tungsten particles interspersed in a matrix phase with lower melting elements (Ni, Fe, Cu, Cr, Mo and Co) [1][2][3]. These alloys provide a unique combination of properties, such as high density, excellent mechanical properties and good corrosion resistance, making them increasingly attractive in many practical applications, such as radiation shielding, mass balance for aerospace, vibrating masses for cell phones and kinetic energy penetrators [4][5][6][7][8]. Due to the extremely high melting point of tungsten (about 3420°C), the consolidation of WHAs is generally performed through liquid-phase sintering above the eutectic temperature. ...
... In general, the ultimate tensile strength of the sintered WHAs is normally limited in the range from 800 to 1000 MPa, mainly depending on the mass ratio of tungsten to matrix phase or preparation conditions. It is well known that good mechanical properties are vital for structural materials in any practical application, particularly in kinetic energy penetrators whose attractive penetration performance strongly relies on the excellent mechanical properties [7,8]. Therefore, to improve the overall mechanical properties of the sintered WHAs in the future is a constant focus in this field. ...
... Comparison in hardness, density, microstructure and mechanical properties between two different alloys of tungsten; W-Ni-Cu and W-Ni-Fe has also been made [5,6]. Moreover, the microstructures, physical and mechanical properties of WHAs sintered at various temperatures were investigated [7][8][9]. Also, the microstructures and mechanical properties of oxides dispersion WHAs were studied and were shown as a potential improvement [10,11]. ...
The long rods of tungsten heavy alloy (WHA) which contain W–5Ni–1.5Fe (wt.%) were fabricated by using warm powder extrusion. The elemental powders were first mixed, and then the mixture was prepared for warm back extrusion by adding a proper binder at 5–10 wt.% concentrations. The extrusion temperature was finely adjusted to facilitate the powder extrusion and eliminate cracks based on an extensive pre-study. Thermal debinding was also utilized prior to vacuum sintering. The results showed that the extruded rods with high length-to-diameter ratios could be obtained at the proposed binder content and extrusion temperature. The metallurgical and the mechanical characteristics of the extruded rods were evaluated and discussed with respect to the preparation route.
... (1), the flow stress of WHA at high strain rate increase with the decrease of W grain size and volume fraction of the matrix phase. Furthermore, the deformation of WHA starts with deformation of the matrix phase during high strain rate compression [19]. For the 95WHA, most of Al 2 O 3 particles are dispersed within the matrix phase (as shown in Fig. 1), leading to an enhanced deformation resistance of the matrix. ...
... Some of the factors identified to influence the occurrence of ASB in metallic alloys include strain rate sensitivity [19], microstructure [7,13], specimen geometry and dimension [19,20], local defects [19], notches [21], stacking fault energy [22], deformation temperature and grain size [23]. The presence of oxide dispersions or second phase particles has also been reported to influence the capability of tungsten heavy alloys (WHA) to resist ASB formation under dynamic shock loading [24,25]. ...
Cylindrical specimens of AA 2017 alloy in T451, T651 and O temper conditions were subjected to quasi-static compressive loading at a strain rate of 3.2×10-3 s-1 and dynamic impact loading at strain rates between 3000s-1 and 8000s-1. The effects of strain rates and temper condition on damage evolution were investigated. Deformation under quasi-static loading was dominated by strain hardening while high strain rates deformation was characterized by two-peak stress-strain curves, indicating simultaneous occurrence of hardening and softening controlling deformation. The degree of softening after the first peak is highest in the annealed sample while the age-hardened specimens showed higher degree of softening beyond the second peak. Both deformed and transformed shear bands were observed in the impacted age-hardened specimens, whereas only deformed bands developed in the annealed specimens. The critical strains for the onset of the thermomechanical instability leading to the formation of adiabatic shear bands are about the same for both T451 and T651 specimens, but much higher for the annealed (O) specimens. The age-hardened specimens fractured in a manner consistent with ductile fracture mode that is characterized by the sequential nucleation, growth and coalescence of micro-voids inside transformed shear bands.
... These properties can be improved or deteriorated with many factors such as alloying, composition, heat treatment, adding reinforcement, methods, production parameters, and environment. When considering WHAs, the most effective factors determining the properties are individual constituents, volume fraction of reinforcement and matrix phase, distributions of solid particles, particle size and shape, interface bonding, and microstructural factors [127][128][129][130][131][132][133][134]. ...
Tungsten heavy alloys (WHAs) belong to a group of two-phase composites, based on W-Ni-Cu and W-Ni-Fe alloys. Due to their combinations of high density, strength, and ductility, WHAs are used as radiation shields, vibration dampers, kinetic energy penetrators and heavy-duty electrical contacts. This paper presents recent progresses in processing, microstructure, and mechanical properties of WHAs. Various processing techniques for the fabrication of WHAs such as conventional powder metallurgy (PM), advent of powder injection molding (PIM), high-energy ball milling (MA), microwave sintering (MW), and spark-plasma sintering (SPS) are reviewed for alloys. This review reveals that key factors affecting the performance of WHAs are the microstructural factors such as tungsten and matrix composition, chemistry, shape, size and distributions of tungsten particles in matrix, and interface-bonding strength between the tungsten particle and matrix in addition to processing factors. SPS approach has a better performance than those of others, followed by extrusion process. Moreover, deformation behaviors of WHA penetrator and depleted uranium (DU) Ti alloy impacting at normal incidence both rigid and thick mild steel target are studied and modelled as elastic thermoviscoplastic. Height of the mushroomed region is smaller for í µí»¼ = 0.3 and it forms sooner in each penetrator as compared to that for í µí»¼ = 0.2.
... These properties can be improved or deteriorated with many factors such as alloying, composition, heat treatment, adding reinforcement, methods, production parameters, and environment. When considering WHAs, the most effective factors determining the properties are individual constituents, volume fraction of reinforcement and matrix phase, distributions of solid particles, particle size and shape, interface bonding, and microstructural factors [127][128][129][130][131][132][133][134]. ...
Tungsten heavy alloys (WHAs) belong to a group of two-phase composites, based on W-Ni-Cu and W-Ni-Fe alloys. Due to their combinations of high density, strength, and ductility, WHAs are used as radiation shields, vibration dampers, kinetic energy penetrators and heavy-duty electrical contacts. This paper presents recent progresses in processing, microstructure, and mechanical properties of WHAs. Various processing techniques for the fabrication of WHAs such as conventional powder metallurgy (PM), advent of powder injection molding (PIM), high-energy ball milling (MA), microwave sintering (MW), and spark-plasma sintering (SPS) are reviewed for alloys. This review reveals that key factors affecting the performance of WHAs are the microstructural factors such as tungsten and matrix composition, chemistry, shape, size and distributions of tungsten particles in matrix, and interface-bonding strength between the tungsten particle and matrix in addition to processing factors. SPS approach has a better performance than those of others, followed by extrusion process. Moreover, deformation behaviors of WHA penetrator and depleted uranium (DU) Ti alloy impacting at normal incidence both rigid and thick mild steel target are studied and modelled as elastic thermoviscoplastic. Height of the mushroomed region is smaller for and it forms sooner in each penetrator as compared to that for .
... Such an adiabatic phenomenon escalates the temperature of the material locally rather than uniformly. Therefore, regarding the facilitative effect of the thermal softening phenomenon on the plastic deformation mechanism, the plastic strain of these fine adiabatic local sites possessing higher temperature increases relative to other locations, and the homogeneous strain is disturbed in the form of thermomechanical instability [10][11][12][13][14]. Also, increasing the average strain and flow stress values enhance this thermomechanical instability within the material flow [10,15]. ...
The present study aims at detecting the critical criteria and corresponding critical impact energy for initiation of strain localization during explosive cladding of the Inconel 625 superalloy as a cladding material and low-carbon steel as a substrate. The results do not reveal adiabatic shear bands, which are the main signs of strain localization, within the superalloy in all studied impact energies up to 205 kJ. At impact energies greater than 78–114 kJ, strain localization is observed in low-carbon steel, and microcracks develop within the adiabatic shear bands. The Johnson-Cook model is used to explains the results obtained and to study the thermomechanical behavior of materials.
... Such an adiabatic phenomenon escalates the temperature of the material locally rather than uniformly. Therefore, regarding the facilitative effect of thermal softening phenomenon on plastic deformation mechanism, the plastic strain of these fine adiabatic local sites possessing higher temperature rises relative to other locations, and homogeneous strain will be disturbed in the form of thermomechanical instability [12][13][14][15][16]. Also, increasing the average strain and flow stress values will heighten this instability within the material flow [12,17]. ...
Though explosive cladding is a viable potential solid state method for cladding different materials together, complicated behavior of materials under ballistic impacts raises the probability of interfacial shear failure. This study aims to relate the failure of explosive cladding of Inconel 625 and plain carbon steel to utilized impact energy, and consequently finding appropriate cladding parameters to prevent interfacial shear failure. The shear strength representing the adhesion strength is used as a failure criterion. According to the obtained results, by increasing the impact energy to an optimum value, the adhesion strength starts to increase. However, after an optimum value, any further increment of impact energy drops the shear strength significantly, which makes the cladding process fail. The outcomes reveal the decisive role of plastic strain localization caused by high impact energies in this failure, where local development of microcracks through adiabatic shear bands in the steel raises the chance of failure. Consequently, an attempt is made to find the optimum cladding parameters to prevent strain localization and failure of cladding.
... Nickel, iron and copper serve as a binder matrix, which holds the brittle tungsten grains together and makes the alloys ductile and easy to machine. These alloys provide a unique combination of properties, such as high density, excellent mechanical properties and good corrosion resistance, making them increasingly attractive in many practical applications, such as radiation shielding, mass balance for aerospace, vibrating masses for cell phones and kinetic energy penetrators [3][4][5][6][7][8]. Tungsten content in conventional heavy alloys varies from 90 to 98 weight percent and is the reason for their high density (between 16.5 and 18.75 g/cc). ...
A novel reduction technique has been developed to synthesize nano-sized tungsten heavy alloys powders and compared with the same powders processed by mechanical alloying technique. In the first method, nano-sized tungsten heavy alloys powders have been obtained by reduction of precursors obtained by spray drying of several appropriate aqueous solutions , which were made from salts containing tungsten, cobalt, and nickel. By adjusting the stoichiometry of the component of the solutions, it is possible to obtain the desired chemical composition of the tungsten heavy alloys powders. In the second method, highly pure elemental powders of tungsten heavy alloys have been mechanically alloyed in a tumbler ball mill for different milling time. The investigated tungsten heavy alloy powders with the composition (95%W-3.5%Ni-1.5%Fe), (93%W-4.5%Ni-1.0%Fe-1.5%Co), and (90%W-6%Ni-4%Cu) have been prepared using both methods. The prepared powders have been compacted at 70 bar (200 MPa) and sintered in vacuum furnace at 1400˚C. Vacuum sintering was carried out to achieve full densification of the produced tungsten heavy alloys. The investigated materials were going to be evaluated the physical and mechanical properties of the sintered parts such as density; electrical conductivity, hardness, and transverse rupture strength. The results reveal that, the grain size of alloys fabricated by chemical reduction technique (53.1-63.8 nm) is smaller than that fabricated by mechanical alloying technique (56.4-71.4 nm).
Tungsten heavy alloys (WHAs) are widely used in the preparation of kinetic penetrators. However, the penetrating performance of alloys, measured by the “self-sharpening effect”, is often limited by the low sensitivity of the adiabatic shear band (ASB). To address this issue, the 90W–7Ni–3Fe rods with deformation degrees of 20% and 40% were prepared by rotary swaging (RS) at high strain rates (4000 s⁻¹ and 6000 s⁻¹, respectively). The deformation and ASB behavior of RS-treated alloys were compared to those of undeformed samples. According to the results, RS pretreatment significantly increased the dynamic penetration strength and ASB sensitivity of the alloys. Besides, the characterization of the recrystallized microstructure within ASB and the calculation of the adiabatic temperature rise indicated that microstructure softening could be the primary cause for the initiation of ASB, which was consistent with the rotational dynamic recrystallization (RDR) mechanism. Moreover, the higher strain energy of the 40% RS deformation sample with high densities of dislocations and twins promoted the subgrain rotation and RDR initiation, providing a recrystallization prerequisite for the propagation and proliferation of ASB. The recrystallized twins formed after the completion of the RDR in the 40% RS deformation sample in order to adapt to the reloading impeded the movement of dislocations, which became a stress concentration site promoting microdamage more likely to nucleate and expand, making the ASB more prone to fracture. Therefore, this study is of great significance for the development of self-sharpening WHA kinetic penetrators.
The present work investigates the mechanical behavior and microstructural evolution mechanisms under quasi-static and dynamic loading in a dual-phase 90W–7Ni–3Fe alloy. Interrupted compressed tests are performed at strain rates of 0.01 and 6000 s⁻¹. The results show positive strain-rate sensitivity on the yielding strength is available for 90W–7Ni–3Fe alloy, and the strain hardening effect weakens with increasing the strain rates due to the adiabatic environment upon dynamic loadings. Under quasi-static loading, the dislocation structure in the W particles and the deformation twinning in the γ-(Ni, Fe) phase continued to increase during deformation, resulting in the alloy exhibiting significant strain hardening over a long period of time. While thermal softening effect dominates during plastic flow upon dynamic loading. With increasing deformation, the temperature increase can accelerate dislocation realignment and annihilation in both two phase, resulting in the formation of a dislocation substructure. In addition, the orientation relationship between the W/γ-(Ni, Fe) interface is defined as Kurdjumov-Sachs, which facilitates the dislocation slip transfer from the γ-(Ni, Fe) phase to the W particles and completes the plastic co-deformation of the 90W–7Ni–3Fe alloy. These results not only advance the understanding of the deformation mechanisms in tungsten heavy alloy under different application environment but also offer the design of tungsten alloys should pay more attention to the dual-phase interface structure.
In this work, Ni/TiC composites were synthesized by the laser cladding technique (LCT). A scanning electron microscope (SEM), X-ray diffractometer (XRD), microhardness meter, electrochemical workstation, and friction and wear tester examined the microstructure, surface morphology, phase structure, microhardness, wear, and corrosion resistances of the Ni/TiC composites. These results indicated the Ni/40TiC composite contained finer equiaxed crystals than the Ni and Ni/20TiC composites. In addition, numerous TiC particles in the Ni/40TiC composite impeded growth of the nickel crystals, which resulted in the fine microstructure of the Ni/40TiC composite. The Ni, Ni/20TiC, and Ni/40TiC composites exhibited face-centered cubic (f c c) lattices. The average microhardness values of the Ni/20TiC and Ni/40TiC composites were approximately 748 HV and 851 HV, respectively. The Ni/40TiC composite had the lowest friction coefficient (0.43) among all three coatings, and only some shallow scratches appeared on the surface of the Ni/40TiC composite. The corrosion potential (E) of Ni/40TiC exceeded the Ni/20TiC composite, and both were larger than the Ni composite, which indicated the Ni/40TiC composite had outstanding corrosion resistance and the Ni composite had poor corrosion resistance. The corrosion current densities (i) of Ni, Ni/20TiC, and Ni/40TiC composites were 5.912, 4.405, and 3.248 μA/cm², respectively.
Semi-circular bend specimens are used to measure the brittle materials' fracture toughness, such as brittle metals, rocks, and ceramic materials. The quasi-static and dynamic model I fracture toughness of tungsten heavy alloy, a typical brittle metal material, are measured using the semi-circular bend specimens under quasi-static compression and dynamic impact loadings. For quasi-static cases, the specimens are loaded on a universal testing machine with a constant loading speed. While for dynamic cases, the specimens are executed on a modified split Hopkinson pressure bar system with the impact pressures from 0.2 to 0.4 MPa. The results show that the dynamic fracture toughness of tungsten alloy increases with the increase of the loading rate. Further, a rate-dependent bilinear traction-separation law is introduced to explain the dynamic fracture behaviors of tungsten alloy. The parameters of the rate-dependent law are obtained from the experiments and the finite element simulations. Finally, the dynamic simi-circular bend simulations using the cohesive element with rate-dependent cohesive law are established to simulate the crack propagation, and the simulation results are in good agreement with the test data.
In this work, the effect of NiB on densification, phase, microstructure, distortion and mechanical properties of conventionally sintered ternary 90W-6Ni-4Co heavy alloy has been investigated without altering (Ni+NiB)/Co ratio. No intermetallic phase was detected in the heavy alloys upon addition of NiB. Instead, the partial and complete exchange of Ni with NiB in 90W-6Ni-4Co has elicited relatively better densification (>99% theoretical), higher bulk hardness (HV3 ~ 389–396 VHN), higher hardness of matrix (~352 HV0.025) when the alloys were sintered at 1500 ℃. In addition to W grains and matrix, the completely NiB-substituted heavy alloy (90W-6NiB-4Co) sintered at 1500 ℃ exhibits the occurrence of W-rich third phase which contains ~75 wt.% of W. Although the samples prevented slumping at 1475 ℃, elephant foot formation was found inevitable in the base composition (90W-6Ni-4Co) when sintering was carried out at 1500 ℃. However, the decreased distortion parameter (from 24.8 to 8.5) with optimal NiB addition affirms the enhanced shape retention capability of 90W-3Ni-3NiB-4Co and 90W-6NiB-4Co heavy alloys. Room temperature compression test of WHAs sintered at 1475 ℃ was performed at a fixed strain rate of 0.1 s⁻¹. No crack formation was observed in NiB containing samples up to 40% deformation. Moreover, the systematic addition of 3–6 wt.% NiB resulted in lower coefficient of thermal expansion of NiB-containing heavy alloys in the temperature range of 400–600 ℃.
Liquid phase sintering (LPS) is a proven technique for preparing large-size tungsten heavy alloys (WHAs). However, for densification, this processing requires that the matrix of WHAs keeps melting for a long time, which simultaneously causes W grain coarsening that degenerates the performance. This work develops a novel ultrashort-time LPS method to form bulk high-performance fine-grain WHAs based on the principle of laser additive manufacturing (LAM). During LAM, the high-entropy alloy matrix (Al0.5Cr0.9FeNi2.5V0.2) and W powders were fed simultaneously but only the matrix was melted by laser and most W particles remained solid, and the melted matrix rapidly solidified with laser moving away, producing an ultrashort-time LPS processing in the melt pool, i.e., laser ultrashort-time liquid phase sintering (LULPS). The extreme short dwell time in liquid (~1/10,000 of conventional LPS) can effectively suppress W grain growth, obtaining a small size of 1/3 of the size in LPS WHAs. Meanwhile, strong convection in the melt pool of LULPS enables a nearly full densification in such a short sintering time. Compared with LPS WHAs, the LULPS fine-grain WHAs present a 42% higher yield strength, as well as an enhanced susceptibility to adiabatic shear banding (ASB) that is important for strong armor-piercing capability, indicating that LULPS can be a promising pathway for forming high-performance WHAs that surpass those prepared by conventional LPS.
The susceptibility to adiabatic shear banding (ASB) of tungsten heavy alloys (WHAs) is a key factor to determine their penetration performance for kinetic energy penetrators (KEPs). This work reports a novel WHA with strong susceptibility to ASB that uses a precipitation-hardening high-entropy alloy (HEA) as the binding phase of W particles. The results find that nanoscale L12 precipitates and high dislocation density in the matrix contribute a high dynamic yield strength of ∼2300 MPa. Also, a narrow ASB with a width of ∼12 μm forms in the novel WHA, which is only 6% of the width of ASB in traditional WHAs like W-NiFe, exhibiting a great susceptibility to ASB. The redissolution of precipitates into the matrix because of temperature rise during shearing could sharply soften the shearing location, which is suggested to be a very important factor of destabilizing mechanism to promote the development of ASB.
In this study, bulk metallic glass composite (BMGC) reinforced with a novel spiral structure of tungsten wires was prepared. The composite exhibits a high ultimate strength of 2830 MPa and a large plasticity of 26% under quasi-static compression. The dynamic compressive mechanical properties of this BMGC were also systematically investigated by split Hopkinson pressure bar technique. It is found that the yield stress of the composite under uniaxial compression increases firstly and then decreases with increasing the strain rate. To describe the strain-rate dependence of the yielding behavior of the composite, a constitutive model considering adiabatic shear localization was established. Furthermore, the deformation and fracture behaviors of the composite under quasi-static and dynamic compression were discussed in detail. Different from the tungsten fiber reinforced bulk metallic glass composites (BMGCs), the spiral tungsten wire reinforced BMGC fractured in a shear mode, which is distinctly favorable to the self-sharpening capability. These findings could deepen the understanding of deformation behaviors and failure modes of BMGCs and promote their applications.
In this study, the effect of the strain rates on the plastic deformation behavior and fracture mechanism of 90W-7Ni-3Fe heavy alloy prepared by liquid-phase sintering was investigated. Stress–strain tests were carried out in tensile and compression under quasi-static condition with the strain rate ranging from 10⁻³ to 1 s⁻¹. Dynamic test was examined using a compression split Hopkinson bar at strain rates ranging from 1000 to 1700 s⁻¹ in order to evaluate the effects of high strain rate on dynamic plastic deformation behavior. Quasi-static tests results show that the strength and hardness of this alloy increase with increasing strain rate. Results indicate that the as-sintered 90W-7Ni-3Fe heavy alloy has obvious strain-hardening characteristic under tensile or compression deformation under quasi-static condition. At this time, strain hardening is the main factor. Nevertheless, strain-hardening and strain-softening behaviors of the specimens occur simultaneously under dynamic compression condition. The effect of compressive stress on the work hardening is more obvious. A strong relevance of work-hardening effect on strain rate is also found. And the fracture mode of quasi-static and dynamic compression samples is also different.
The aim of present study is to investigate the strain rate sensitivity of a Ni-based superalloy under the tensile loading. The yield strength of the alloy increases with the increasing of strain rate, especially when the strain rate is higher than 10¹ s⁻¹. But with the increasing of strain rate, the fracture elongation decreases at first, then increases rapidly and shows a minimum value at the strain rate of 10² s⁻¹. The manner in which the dislocations pass through the strengthening phases may be different during the plastic deformation at different strain rates and it is the main reason of the appearance of inflection point in the change of the strength with the strain rate. The obvious increase in plasticity of the alloy at high strain rate is considered as the changing of plastic deformation mechanism due to the appearance of twining.
Rod penetrators with 95W–3.75Ni–1.25Fe fine-grained tungsten heavy alloy (fine-grained 95W) and conventional tungsten heavy alloy rod penetrators with the same chemical composition (conventional 95W) were subjected to ballistic impact to compare their penetration performance. “Self-sharpening” behavior and an average 10.5% increase in penetration depth compared to conventional 95W penetrators. An acute head remained on the fine-grained 95W rod with SEM results revealing many micro-cracks and small debris on surface layer of the rod head. The stress-strain curves collected in the Split Hopkinson Pressure Bar (SHPB) experiment showed that critical failure strain values of the fine-grained 95W were 0.12 and 0.39 at strain rate of 2×10³ s⁻¹ and 3.9×10³ s⁻¹, respectively, approximately 40% and 10% lower than those of the conventional 95W. The dynamic strength values of fine-grained 95W were 2100 MPa and 2520 MPa, respectively, which were 500 MPa and 520 MPa higher than those of the conventional 95W. The relationship among microstructure, mechanical property and “self-sharpening” behavior of fine-grained 95W is discussed in this work.
In this study, split Hopkinson pressure bar was used to evaluate the dynamic deformation behavior of the 93W-5.6Ni-1.4Fe heavy alloy (93WHA) prepared by spark plasma sintering (SPS) and conventional liquid-phase sintering (CLS). The influence of the microstructural characteristics (such as W grain size, W-W contiguity and volume fraction of the matrix) on the dynamic deformation behavior was investigated. In contrast to the conventional liquid-phase sintered 93WHA, the spark plasma sintered 93WHAs exhibit high yield strength and flow stress during high strain rate compression, due to the decreased mean matrix thickness (the mean matrix thickness is related to the W grain size, W-W contiguity and volume fraction of the matrix). The decreased matrix mean thickness and increased number of grain boundaries in the spark plasma sintered 93WHAs result in an increase of aspect ratio of W grains in the core of the deformed specimen and a decreased width of shear band along the direction of maximum shear stress.
The microstructures and mechanical properties of hypoeutectoid U- Nb alloy laser welded joint were investigated by optical microscopy (OM), X- ray diffractometer (XRD), transmission electron microscopy (TEM), split hopkinson pressure bar (SHPB) and other analysis apparatus. The results show that the microstructure of hypoeutectoid U-Nb alloy base metal is α-U+γ-U lamellar pearlite under isothermal heat treatment, while the laser welding seam is composed of α' lath martensite for pre-heated or α' twin martensite for no pre-heated with orthogonal crystal structure. The quasi-static tensile strength of welded joint (about 400 MPa) is much less than base metal and microstructures of weld, for the main reason of incomplete penetration weld and low fracture toughness. Between dynamic impact loading for base and welded joint, the strain rate of welded joint is lower than base metal, and the yield strength of welded joint is higher. Also, the compressive stress-strain curves indicated that the flow stresses for welded joint increased with the increase of strain rate and the obvious effect of strain rate hardening has been observed. At strain rate of 2000 s-1, selected plastic deformation taking place in welded joint is due to the tremendous difference mechanic properties between weld seam and base metal, and the adiabatic shear band(ASB) only appears in the rest of welded joint.
The aim of present study is to investigate the strain rate sensitivity of a Ni-based superalloy under the tensile loading. The yield strength of the alloy increases with the increasing of strain rate, especially when the strain rate is higher than 101 s-1. But with the increasing of strain rate, the fracture elongation decreases at first, then increases rapidly and shows a minimum value at the strain rate of 102s-1. The manner in which the dislocations pass through the strengthening phases may be different during the plastic deformation at different strain rates and it is the main reason of the appearance of inflection point in the change of the strength with the strain rate. The obvious increase in plasticity of the alloy at high strain rate is considered as the changing of plastic deformation mechanism due to the appearance of twining.
The tensile deformation behavior of a nickel-base superalloy under the dynamic loads was investigated the by the optical microscope, scanning electron microscope and transmission electron microscope. The results shown that the yield strength of the alloy increases with the increasing of strain rate, eapacily at the strain rate higher than 101 s-1. While with the increasing of strain rate, the fracture elongation decreases at first, then increases rapidly and shows a minimum value at the strain rate of 102 s-1. The manner in which the dislocations go through the strengthening phases may be different during the plastic deformation at different strain rates and it is the main reason of the appearance of inflection point in the change of the strength with the strain rate. The obvious increasing of plasticity of the alloy at high strain rate is depended on the deformation twining during the plastic deformation.
In traditional aeroengine manufacturing industry, the variation and mechanism of the mechanical property of superalloy used for rotating parts under the actual dynamic load is not given full considerations during its structure design. The mechanical property and deformation behavior of the alloys under the dynamic load have significant difference compared with that under the static load, and therefore the study on the deformation behavior of the alloy under the dynamic load is important for the safety of rotating parts used under the severe service conditions. The effect of microstructural changes of long-term aging GH4169 alloy on the mechanical properties through tensile testing at strain rates ranging from 10 1 to 10 3 s 1 was examined in this paper. The tensile deformation behavior of the alloy and the mechanisms were also discussed. The results showed that the strength of the alloy depends strongly on the aging time, the fracture elongation decreases with the increasing aging time and remains unchanged when aged for 500 h when tensile tested at the strain rates ranging from 10 1 to 10 3 s -1. And when the strain rate is high up to 10 3 s -1, the elongation depends strongly on the aging time and the degradation of ductility by the long-term aging happens ahead of time, but the aging time has no obvious effect on the strength of the alloy. Through tensile testing at the strain rate of 10 3 s -1, it is too late to release the blocked dislocation motion in the way of dislocation decomposition or climb in the alloy. And there is no peaking size effect of the strengthening phase in the alloy with the aging time ranging from 0 to 1000 h and there is no obvious effect of the aging time on the strength of the alloy. The ability of accommodation of plastic deformation by grain boundaries reduces under the dynamic loads due to the existence of precipitate free zones around 6 phase at the grain boundary in the alloy by long-term aging, and thus the ductility of the alloy by aging for a shorter time decreases rapidly when tensile tested at the strain rate of 10 3 s -1.
High speed stamping process is used to high strength and high electrical conductivity phosphor bronze with extremely high strain rates more than 103s-1. This study on the dynamic tensile behaviour and deformational mechanism is to optimise the high speed stamping processes and improve geometrical precision in finished products. Thus, the tensile properties and deformation behaviour of C5191 phosphor bronze under quasi-static tensile condition at a strain rate of 0.001s-1 by electronic universal testing machine, and dynamic tensile condition at strain rate of 500, 1000 and 1500s-1 by split Hopkinson tensile bar (SHTB) apparatus were studied. The effects of strain rate and the deformation mechanism were investigated by means of SEM and TEM. The results showed that the yield strength and tensile strength of C5191 phosphor bronze under high strain rates deformation increased by 32.77% and 11.07% respectively compared with quasi-static condition, the strain hardening index increases from 0.075 to 0.251, and the strength of the material strain rates sensitivity index change from 0.005 to 0.022, which presented a clear sensitive to strain rates. Therefore, it is claimed that the dominant deformation mechanism was changed by the dislocation motion under different strain rates, and the ability of plastic deformation of C5191 phosphor bronze increased due to the number of movable dislocations increased significantly, started multi-line slip, and the soft effect of adiabatic temperature rise at the strain rate ranging from 500 to 1500s-1.
Tensile properties and deformation behavior of DP780 steel were studied using servo-hydraulic high-speed material testing machine, SEM and TEM. The effects of strain rate and the mechanism were investigated. The results showed that the strength and ductility of DP780 steel remained almost unchanged as the strain rate increased at strain rates lower than 10(0) s(-1). When the strain rate was over 10(1) s(-1), the strength and the strain-hardening coefficient increased remarkably. Ductility of DP780 steel increased significantly at the strain rates ranging from 3 x 10(1) to 5 x 10(2) s(-1). The deformation resistance increased with increasing the strain rate due to the stronger short range resistance induced by the acceleration of dislocation motion in the ferrite matrix. Increasing strain rate up to 3 x 10(1) s(-1) resulted in a considerable increase of the amount of mobile dislocation, which was the main reason for the increasing uniform elongation and fracture elongation of DP780 steel at the strain rate ranging from 3 x 10(1) to 5 x 10(2) s(-1). Interface of ferrite-martensite in DP780 steel was the main location for pile-up of dislocation, crack initiation and propagation. The ability of inhomogeneous plastic deformation of DP780 steel increased due to the decreasing plastic strain energy difference between the ferrite matrix and ferrite-martensite interface and the consequent delaying initiation and propagation of microvoids at ferrite-martensite interface induced by the increasing work hardening degree of ferrite matrix with the increasing strain rate.
Fine grain 93W-4.9Ni-2.1Fe-0.03Y% alloy was strengthened by rapid hot extrusion. The dynamic mechanical properties and failure behavior were investigated for the rapid hot extruded fine grain tungsten heavy alloy at high strain rates. The results show that the flow stresses of the as-extruded fine-grained tungsten alloy at relatively lower strain rates are equivalent, of the order of 2000 MPa. Tungsten particles on the fracture surfaces are severely deformed and fracture into fine pieces which adhere to the soften matrix phase. This phenomenon is enhanced with the strain rate increasing. Observation of the sectioned surfaces reveals that the tungsten particles are heavily shear deformed along the fracture surface, but no plastic deformation is observed when it gets into the interior, indicating the localized shearing marks. The experimental results have proved that the failure of the extruded fine-grained tungsten heavy alloy is adiabatic shearing failure.
The effects of tungsten contents (80-88wt.%) and different Ni:Cu ratio (7:3-3:7 by weight) on microstructure and dynamic mechanical properties of W-Ni-Cu alloys were investigated. Results show that at the same sintering conditions, with tungsten content increasing from 80 wt % to 88 wt %, tungsten grains increase slightly. Spherical and more uniform tungsten grains distribute in matrix phase in 85W alloys with Ni:Cu ratio of 1:1 than that of 3:7 and 7:3. The results also show that the dynamic yield strength of W-Ni-Cu alloys goes up with tungsten content increasing, but keep similar deformation capability. With strain rate increasing in the range of 2600-4200s-1, 85W displays strain rate softening effect. Adiabatic shear bands are formed at strain rate over 3200s-1.
Methodology for the monitoring of an acoustic emission (AE) signal was employed to investigate the deformation and/or fracture processes together with damage mechanisms for RHA steel and WHA in high-strain-rate compressive loadings. Cylindrical test specimens were loaded with an incident bar and the AE activity was monitored in real time during the entire impact by using a resonant type AE sensor connected to the specimen with a waveguide. Post-failure observations were conducted to correlate the particular features in the acoustic emission signal that corresponded to the specific types of damage mechanisms. AE characteristics from the specimens were firstly examined to distinguish the different AE signals from various possible damage mechanisms. AE signals were analyzed in terms of AE amplitude and normalized cumulative AE energy, and were classified into three different signal types based on the waveform and the peak frequency which was obtained by a short time Fourier transform (STFT). Furthermore the behavior of RA (ratio of rise time to amplitude) value was used for characterization of the damage mechanisms which could be confirmed by the SEM observation after the test. As a result, the behavior of the three classified different AE signal types according to the waveform and the peak frequency successfully represented the deformation/fracture processes of the armor materials exhibiting various and multiple damage mechanisms. It was elucidated that each classified signal type was associated with the specific damage mechanism within the specimen, and the AE characteristics were summarized in association with the individual damage mechanism of the armor materials.
The dynamic mechanical properties of 93W–4.9Ni–2.1Fe alloys in the form of extruded rods sintered by microwave heating were investigated under dynamic compression using a split Hopkinson Pressure Bar. The microstructure and microhardness values of the sintered specimens after dynamic compression were analyzed and tested. The results show that the deformation amount and microhardness of specimens increase with increasing strain rate. When the strain rate is 3000 S− 1, the deformation amount is increased to the maximum value of 59.8%, and the microhardness values of the tungsten grains and the matrix phase are also promoted to the maximum values of 7.66 and 6.92 Gpa, respectively. The formation of cracks during compressive deformation initiates before the appearance of the adiabatic shear bands. As the strain rate increases, cracks initiating at the edge of specimens gradually propagate to the bulk alloy, and the adiabatic shear band is observed at about 45° to the loading direction under the strain rate of 3000 S− 1. These findings suggest that tungsten-based alloys extruded rods sintered by microwave heating would be an ideal material with excellent self-sharpening and penetration performance for penetrators.
High strain rate–dependent deformation behaviors are important in design and optimization of armor structural materials. Herein, the static tensile and the dynamic compressive behaviors of the practical materials including Al5083, rolled homogeneous armor steel and tungsten heavy alloy were investigated by means of a universal testing machine and a split Hopkinson pressure bar apparatus, respectively. The test was performed at high strain rates (1200–3100 s−1) to obtain a detailed understanding of the responses of the materials. A finite element analysis was then carried out using an elastic–plastic failure material model considering a user-defined parameter determined from the split Hopkinson pressure bar tests. Both flow and peak stresses of the materials were different corresponding to the mechanical properties and strain rates. The dynamic yield stresses are generally larger than static yield stresses, particularly in Al5083. As shown in the results, the experimentally obtained true stress–strain behaviors give a good agreement with those from finite element analyses. In addition, observations of the impacted zone in the specimen showed that a few cracks propagated along the specimen’s original periphery. In order to determine the level of deformation, strain rate sensitivity, m, at strain of 0.1 was also calculated for all materials; rolled homogeneous armor steel and tungsten heavy alloy had lower strain rate sensitivity than Al5083. The proposed procedure shows proper agreements between numerical predictions and experimental results such as stress–strain relations, peak stresses and deformation. The methodology coupled with the experimental data reflecting the dynamic compressive properties provides a more accurate prediction of the strain rate–dependent behavior of armor structural materials.
Like all cladding methods, interfacial features dominate the ultimate mechanical and metallurgical behavior of explosive cladded samples. Therefore, knowing the effect of explosive welding parameters on interfacial characteristics can be helpful in adjusting the welding parameters to obtain final desired properties. The present study aims to relate the load ratio and standoff distance as two main explosive welding parameters to interfacial features of explosive cladded Inconel 625/plain carbon steel bimetal plate such as interfacial structure, interfacial local melting phenomenon, localization of plastic strain, hardness variation across the interface and adhesion strength. As the results indicate, low impact energies are accompanied by linear interfacial structure, and increasing the impact energy initiates the waves at the interface. However, excessive impact energies lead to spoiling the wavy structure locally. Moreover, by raising the impact energy through increasing the load ratio and standoff distance, the locally melted zones appear in the Inconel side in vicinity of the interface. Higher impact energies promote the continuity of these interfacial cast layers. Though chemical elements of two materials are mixed together in these regions, no sign of intermetallic compounds formation is observed. According to the obtained results, raising the impact energy localizes the plastic deformation of steel side, resulting in the formation of adiabatic strain bands (ASB). The microhardness profiles reveal the hardening effect of collision in the vicinity of the interface with the exception of samples where the localization of strain in steel side hinders the hardening effect. Furthermore, the results of the shear test show that adhesion strength possesses an optimum value versus the increment of impact energy. By raising the load ratio and the standoff distance, the adhesion strength improves, but applying excessive collision energies drops this value significantly.
One-pass rapid hot extrusion of fine-grained 93W–4.9Ni–2.1Fe–0.03Y (wt.%) alloy with an average grain size of ∼10μm was performed at 1150°C with an extrusion speed of ∼100mm/s and an extrusion ratio of ∼3.33:1. Microstructure and mechanical properties of the as-extruded alloy were investigated. The results show that the tungsten particles of the as-extruded alloy are severely elongated along the extrusion direction and the aspect ratios of these elongated particles are 5–8. Three crystallographic textures {001}〈110〉, {111}〈110〉 and {110}〈110〉 arose after rapid hot extrusion and the total volume fraction of these texture components was approximately 30%. Many lath-shaped subgrains with a small misorientation and low density dislocations could be observed in tungsten phase and γ-(Ni, Fe) phase respectively. These microstructure characteristics indicate that slight dynamic recovery-recrystallization process occurred during rapid hot extrusion. In contrast to as-sintered alloy, the as-extruded alloy possessed much higher ultimate tensile strength and hardness (HRC) but a relatively lower ductility (1570MPa vs. 995MPa; HRC48 vs. HRC29 and 6.5% vs. 24%). In addition, the fracture morphology shows that the predominant failure mode for the as-extruded alloy is cleavage failure of the tungsten particles, while the ductile rupture of the γ-(Ni, Fe) phase that can be frequently observed in the as-sintered alloy nearly disappeared after rapid hot extrusion.
The objective of this study is to investigate the dynamic deformation and fracture behavior of an oxide-dispersed (OD) tungsten
heavy alloy fabricated by mechanical alloying (MA). The tungsten alloy was processed by adding 0.1 wt pct Y2O3 powders during MA, in order to form fine oxides at triple junctions of tungsten particles or at tungsten/matrix interfaces.
Dynamic torsion tests were conducted for this alloy, and the test data were compared with those of a conventional liquid-phase
sintered (LPS) specimen. A refinement in tungsten particle size could be obtained after MA and multistep heat treatment without
an increase in the interfacial area fraction between tungsten particles. The dynamic test results indicated that interfacial
debonding between tungsten particles occurred over broad deformed areas in this alloy, suggesting the possibility of adiabatic
shear-band formation. Also, oxide dispersion was effective in promoting interfacial debonding, since the fine oxides acted
as initiation sites for interfacial debonding. These findings suggest that the idea of forming fine oxides would be useful
for improving self-sharpening and penetration performance in tungsten heavy alloys.
Single crystals and polycrystals of aluminium containing non-deformable second-phase particles of silicon, have been deformed, and the resultant structures investigated by microscopy and by X-ray and microtexture techniques. The particle size is found to influence the scale of the deformation bands formed, and there is evidence that particles may affect the nucleation of these bands. The deformed materials were recrystallized, and the effect of particle stimulated nucleation on the weakening of the rolling texture is discussed with reference to a computer simulation. In contrast, the recrystallization texture of particle-containing single crystals deformed on only two slip systems is sharp, and it is shown that the texture components are consistent with plasticity theory.
Dynamic testing of statically predeformed specimens of magnesium (AM50) shows that the failure strain increases with the level of prestrain. Interrupted and other dynamic tests show that the temperature rise prior to localization has only a minor influence on adiabatic shear band (ASB) formation. For all the tests, the dynamic deformation energy until ASB formation is found to be relatively constant, indicating that ASB is dependent almost solely on dynamic deformation processes, with quasistatic and thermal effects prior to localization being very marginal. We suggest the concept of a constant dynamic mechanical energy (toughness) as a quantitative criterion for ASB formation--this concept being related physically to the dynamic stored energy of cold work. All in all, the tests indicate that ASB failure is more dependent on energy considerations that on strain criteria, as has been considered until now.
Tungsten-based heavy metal alloys containing 90–97% W are two-phase composites combining high density, high strength and relatively high ductility. W content and manufacturing parameters have a strong influence on the deformation and fracture behavior of the alloys. Specimens with different content of W were prepared. The microstructure was studied after successive stages of plastic deformation, allowing the delineation of the weak regions of importance as fracture starting points. It was confirmed that the material plasticity increases substantially with decreasing tungsten content and that the deformation capability is determined by the condition of the W–W and W–matrix zone interfaces which is different at the compression and tension modes of deformation. A correlation between microstructure and mechanical properties was found and an explanation for the improved plasticity with lower tungsten content is given.
On the basis of observations made by electron microscopy of the
distribution of dislocations around undeformable particles embedded in a
matrix of plastic crystalline metal, it is possible to see a clear
transition from 'laminar plastic flow' to 'rotational flow' at a
critical strain which depends mostly upon the particle size, larger
particles displaying smaller critical strains. These patterns are
produced by plastic relaxation of the internal stress caused by
deformation. Although there seems to be no secure way of predicting the
patterns on the basis of continuum mechanics, nor on an atomistic basis
of dislocation mechanics, it is possible by a combination of dislocation
theory and continuum patterns of flow to devise simple models which go
some way to explaining the transition and to predicting the distribution
of misorientation in the rotational structures. The results have
important consequences for understanding recrystallization phenomena, as
well as overall work-hardening behaviour. They suggest that indentation
plasticity might behave similarly.
The tungsten heavy alloys (WHAs) with fibrous grains were obtained by hot-hydrostatic extrusion at 950°C with plastic deformation ratio of 75%. Dynamic mechanical behaviors under uniaxial dynamic compression were systematically investigated with the angles between the loading direction and the extruding direction being 0°, 45° and 90°. The testing results show obvious difference in dynamic behaviors and susceptibility to adiabatic shear band (ASB) for different specimens. In the 0° specimens, no localized flow is observed. The 45° specimens exhibit slight localized shearing. In the 90° specimens, localized ASB was firstly observed at an angle of 45° with respect to the fibrous orientation followed by cracking, which is greatly desirable for kinetic energy penetration applications. Microstructure analyses reveal that the high susceptibility to ASB of the 90° specimens result from adiabatic temperature rising during dynamic loading and reduced strain hardening caused by the special micro-strained condition and fiber distribution.
Dynamic and quasi-static torsional deformation behavior of a 93W–4.9Ni–2.1Fe tungsten heavy alloy (WHA) was investigated in order to evaluate the possibility of forming adiabatic shear bands. Dynamic torsional tests were conducted using a torsional Kolsky bar for the four WHA specimens fabricated through each processing condition, e.g. sintering, heat-treatment, swaging, and aging, and then the test data were compared with those of the quasi-static tests. The dynamic torsional test results indicated that the shear stress increases, while the shear strain decreases, with the maximum shear stress increasing in the order of the as-sintered, the as-heat-treated, the as-swaged, and the as-aged specimens. The as-swaged and the as-aged specimens showed a higher possibility of the adiabatic shear band formation because of the abrupt drop of shear stress between the maximum shear stress and the final fracture points and because of the radical reduction of shear strain at the maximum shear stress point. The observation of the deformed areas of the dynamically fractured torsional specimens revealed that the shear deformation was homogeneously distributed in a wide area in the as-sintered and the as-heat-treated specimens, whereas it was concentrated on the central area of the gage section in the as-swaged and the as-aged specimens. This torsional behavior correlated well with the shear stress–shear strain curves, suggesting that the torsional Kolsky bar technique is a good tool for helping evaluate the possibility of the adiabatic shear band formation.
Single crystals of (001)[110] orientation of aluminum silicon alloys containing second-phase particles have been deformed in channel die compression. The crystals deform to produce two complimentary stable orientations close to the ideal copper components (112)[1[bar 1]1] and ([bar 1][bar 1]2)[111]. This behavior is similar to that of single crystals of pure aluminum of the same orientation, although the scale of the microstructure is coarser in the present case. In the vicinity of the particles, deformation zones containing highly misoriented material of the orientation of the complementary copper component are found. It is shown that this behavior is consistent with a model in which the presence of the particle results in the suppression of slip on certain slip systems in regions adjacent to the particle.
The dynamic behaviors of tungsten heavy alloy (WHA) processed by hot-hydrostatic extrusion (HE) and hot torsion (HT) were investigated. The HE + HT WHAs exhibit significant improved susceptibility to forming adiabatic shear bands (ASBs) and a remarkably high flow stress of 2400 MPa under uniaxial dynamic compression. With increasing plastic strain of HT, an obviously increasing tendency of localized shearing was observed. Elongated subgrains with an average width of 200-300 nm bonded together and formed parallel lamellar bands within the ASBs of WHA. Crown copyright (c) 2008 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.
This paper studied the densification behavior of nanocrystalline composite powders of 93W–4.9Ni–2.1Fe (wt.%) and 93W–4.9Ni–2.1Fe–0.03Y synthesized by sol-spray drying and hydrogen reduction process. The X-ray diffraction (XRD) analysis showed that γ-(Ni, Fe) phase was formed in the final obtained powders. Powders morphology characterized by scanning electron microscope (SEM) showed that the 93W–4.9Ni–2.1Fe nanocrystalline composite powders exhibited larger agglomeration and grain size compared with the 93W–4.9Ni–2.1Fe–0.03Y nanocrystalline composite powders. Both kinds of green compacts can obtain full density if sintered at 1410°C for 1h. When sintering temperature was above 1410°C, the sintering density for both compacts decreased rapidly. In addition, the sintering density, densification rate and grain coarsening rate of 93W–4.9Ni–2.1Fe compacts were higher than those of 93W–4.9Ni–2.1Fe–0.03Y. The effect of trace yttrium on the densification behavior of nanocrystalline composite powders was also discussed.
The microstructure and mechanical properties of mechanically alloyed oxide dispersion strengthened (ODS) tungsten heavy alloys were investigated. Three different mechanical alloying processes, such as one-step mechanical alloying process, two-step mechanical alloying process, and mechanical alloying and mixing process, were performed in order to control the microstructure and mechanical properties of ODS tungsten heavy alloys. The partially stabilized zirconia (PSZ) dispersoids tend to be dispersed in tungsten grains rather than the binder matrix when the powders were prepared by two-step mechanical alloying or mechanical alloying and mixing process. The yield strength of ODS tungsten heavy alloy increased with decreasing the binder mean thickness, but was not dependent on location of oxide dispersoids. The elongation and high temperature strength of ODS tungsten heavy alloys increased with increasing the content of PSZ dispersoids.
Using UCSD's recovery Hopkinson technique [Nemat-Nasser, S., Isaacs, J.B., Starrett, J.E., 1991. Hopkinson techniques for dynamic recovery experiments. Proc. R. Soc. London, A135, 371], enhanced for high-temperature compression experiments [Nemat-Nasser, S., Isaacs, J., 1997. Direct measurement of isothermal flow stress of metal at elevated temperatures and high strain rates. Acta. Met., 45, 907], two techniques are developed to create adiabatic shearbands in tungsten heavy alloy (WHA) samples at high strain rates (3000 to 5000/s), and the results are compared with those obtained at low strain rates (10−3/s). In the first technique, a cylindrical sample is subjected to a single compression pulse and is recovered without being subjected to any additional loading. The change in the surface temperature of the sample is measured during its high-strain-rate deformation, using an infrared technique. In the second scheme, a small circular cylindrical sample is constrained at both ends by thin confining rings and then subjected to axial compression in the recovery Hopkinson bars at various initial temperatures. Barreling of the sample takes place under somewhat controlled conditions and hence, this experiment is referred to as the `controlled barreling' test. The confining rings promote shearband formation. Since the samples have been subjected to only a single compression pulse, the experiments allow correlating the resulting microstructural changes with the corresponding temperature and loading histories. SEM observations revealed that intergranular fracture occurs within the shearbands in the Fe–Ni matrix. Some transgranular fracture was also observed. The high-strain-rate controlled barreling test performed at the initial room temperature, invariably leads to adiabatic shearbanding. However, at suitably high initial temperatures (e.g., 500–700°C), the shearbanding gives way to a diffused yet highly localized deformation. In addition, the strain, strain-rate, and temperature dependency of the flow stress of this material is quantified, based on high-strain-rate isothermal and adiabatic, and quasi-static flow stress measurements. These and related features are discussed in this paper.
In this paper, Y2O3 dispersed 93W–4.9Ni–2.1Fe alloy with fine grains (FG) of about 3–5μm has been fabricated by liquid-phase sintering of their respective nanocomposite powders. The dynamic properties of FG-WHA have been investigated under uniaxial dynamic compression using Split Hopkinson Pressure Bar. The results show that the FG-WHA exhibits an almost ideal elastic–plastic behavior with no work hardening and susceptibility of the adiabatic shear localization at lower strain rates than the traditional WHA. Two obvious adiabatic shearbands have been observed at the strain rate of 1900s−1. The adiabatic shearbands origins in the crack tip region in the surface area of the specimen, propagating deeply into the specimen along orientations at 45° to the impact direction. The bands have much narrower width of about 10–25μm than that for traditional WHA (the width of bands is about 200μm according to Kim et al. (1998a,b)). Within the center of the bands, the tungsten grains are severely elongated to be fibrous and orient along the propagation directions of the adiabatic shear bands, exhibiting plastic flow localized instability.
Many two-phase alloys work-harden much faster than do pure single crystals. This is because the two phases are not equally easy to deform. One component (often dispersed as small particles) deforms less than the other, or not at all, so that gradients of deformation form with a wavelength equal to the spacing between the phases or particles. Such alloys are ‘plastically non-homogeneous’, because gradients of plastic deformation are imposed by the microstructure. Dislocations are stored in them to accommodate the deformation gradients, and so allow compatible deformation of the two phases. We call these ‘geometrically-necessary’ dislocations to distinguish them from the ‘statistically-stored’ dislocations which accumulate in pure crystals during straining and are responsible for the normal 3-stage hardening. Polycrystals of pure metals are also plastically non-homogeneous.The density and arrangement of the geometrically-necessary dislocations can be calculated fairly exactly and checked by electron microscopy and x-ray techniques. The rate at which they accumulate with strain is conveniently described by the ‘geometric slip distance’, a characteristic of the microstructure. Their arrangement is quite different from that of the statistically-stored dislocations, which may make them particularly susceptible to recovery effects, even at low temperatures.Geometrically-necessary dislocations control the work hardening of the specimen when their density exceeds that of the statistically-stored ones. They contribute to hardening in two ways: by acting as individual obstacles to slip, and (collectively) by creating a long-range back-stress, with wave-length equal to the particle spacing.With the exception of single-phase single crystals, almost all metals and alloys are plastically non-homogeneous to some extent. The model provides an explanation for the way in which the stress-strain curve is influenced by a dispersion of particles, and by grain size.
The microstructures of adiabatic shear bands formed by high-speed impact in a tungsten heavy alloy penetrator were investigated in the present study. The penetrator was highly deformed at high strain rate by high-speed impact, and microstructural observation of the remained penetrator and the debris was conducted after the impact test. Heavily elongated tungsten particles and reaction products such as tungsten oxides were observed on the surface region of the debris presumably due to the local temperature rise occurring upon high-speed impact. A few adiabatic shear bands were observed in the regions near the surface cracks of the remaining penetrator, and their width was wide in comparison to the shear band formed in armor plates. The crack had general trends to propagate along the shear band, but sometimes changed its propagation path along the interfaces between adhering tungsten particles. It was suggested from the microstructural observation of the shear bands that in order to improve the penetration performance of the tungsten heavy alloy, the minimization of the tungsten–tungsten particle interfaces and the optimization of the fabricating process were required.
The microstructures and mechanical properties of mechanically alloyed oxide dispersion strengthened tungsten heavy alloys were investigated. Elemental powders of tungsten (W), nickel (Ni), iron (Fe) and partially stabilized zirconia (PSZ) were mechanically alloyed by a planetary mill. Mechanically alloyed powders were consolidated by liquid phase sintering at temperature ranged 1465–1485 °C for 1 h in hydrogen atmosphere to obtain full densification. The W grain size decreases with increasing PSZ content. The oxide dispersoids were dispersed both within W grains and W/W and W/matrix interfaces. The yield strength of oxide dispersion strengthened tungsten heavy alloy was not dependent on PSZ content but dependent on microstructural factor, while the elongation decreases with increasing PSZ content. However, the high temperature yield strength increases with increasing content of PSZ dispersoid and the PSZ dispersoids act as the strengthening agent at high temperature deformation.
Flaw-free bulk Cu specimens with a high density of nano-scale mechanical twins were produced by means of dynamic plastic deformation at liquid nitrogen temperature. The nano-twinned Cu exhibits a tensile yield strength of 600 MPa and an elonga-tion-to-failure of 11%. The significant strengthening originates from the effective blockage of glide dislocations by numerous twin boundaries and dislocations.
A comparison of shear instability in U-Tivs W alloy during high strain rate deformation is presented. Experimental quasi-static and dynamic deformation data are used
to formulate the constitutive description based on the mechanical threshold stress model (MTS). The MTS model is used to predict
the deformation behavior of U-Ti and W-alloy beyond the conventionally achievable experimental capabilities. We suggest that
uranium alloys are more prone to catastrophic-localized deformation (adiabatic shearing) due to the existence of a soft high-temperature
phase, which is reached by uranium alloys undergoing large strains at high strain rate of deformation.
An experiment has been designed to check a previously proposed equivalence of the effects of changes in strain rate and in temperature upon the stress‐strain relation in metals. It is found that this equivalence is valid for the typical steels investigated. The behavior of these steels at very high rates of deformation may, therefore, be obtained by tests at moderate rates of deformation performed at low temperatures. The results of such tests are described. Aside from changing the isothermal stress‐strain relation, an increase of strain rate tends to change the conditions from isothermal to adiabatic. It is found that at low temperatures, the adiabatic stress‐strain relation in the plastic range is radically different from the isothermal, having an initial negative rather than a positive slope. This initial negative slope renders unstable homogeneous plastic deformation.
A series of experiments is described in which the local temperature and local strain are measured during the formation of an adiabatic shear band in a low alloy structural steel (HY-100). The specimen employed consists of a short thin-walled tube and the required rapid deformation rates are imposed by loading the specimen in a torsional Kolsky bar (split-Hopkinson bar). The local temperature is determined by measuring the infrared radiation emanating at twelve neighboring points on the specimen's surface, including the shear band area. Indium-antimonide elements are employed for this purpose to give the temperature history during deformation. In addition, high speed photographs are made of a grid pattern deposited on the specimen's surface, thus providing a measure of the strain distribution at various stages during shear band formation. By testing a number of specimens, it is possible to form a picture of the developing strain localization process, of the temperature history within the forming shear band, and of the consequent loss in the load carrying capacity of the steel. It appears that plastic deformation follows a three stage process which begins with a homogeneous strain state, followed by a generally inhomogeneous strain distribution, and finally by a narrowing of the localization into a fine shear band. It is estimated that the shear band propagates at a speed of about 510 m/s in the material tested. Results also include data on the stress-strain behavior of HY-100 steel over the temperature range —190°C to 250°C and at quasi-static as well as dynamic strain rates.
We have investigated the dynamic mechanical behaviors of representative body-centered cubic (b.c.c.) refractory metals with ultrafine grained (UFG, grain size (d) smaller than 500 nm, but larger than 100 nm) and nanocrystalline (NC, d smaller than 100 nm) microstructures. The UFG/NC microstructures were produced by either bottom-up or top-down fabrication. The dynamic compressive behavior was evaluated by means of uni-axial high strain rate Kolsky bar tests. For some of the metals, due to the special technique used for their fabrication, the specimen dimensions were very small. Therefore, a miniaturized desk-top Kolsky bar system was used to measure the dynamic behavior. Our experimental results indicate that the majority of the UFG/NC b.c.c. metals are prone to localized plastic flow under uni-axial dynamic compression, with UFG-Ta being an exception. This tendency is a consequence of the combined effect of reduced strain hardening capacity and strain rate sensitivity (SRS), increased yield strength, and enhanced adiabatic heating. The experimental observations are discussed in connection with mechanics-based models.
The concepts of twinning shears and twinning modes are introduced. The early attempts to predict these features are presented. This is followed by a detailed discussion of the formal theories of Bilby and Crocker and Bevis and Crocker for predicting these elements. Their formalisms are applied to predict twinning modes in single lattice structures, superlattices, hexagonal close packed structure and other double lattice structures. Wherever possible the predicted modes are compared with those observed.The description of fully coherent, rational twin interfaces is presented, and the concepts of elementary, zonal, complementary and partial twinning dislocations are discussed. It is suggested that the irrational K1 twin interfaces may be faceted on the microscopic scale, and these facets may be coherent.Homogeneous and heterogeneous nucleation of twins are discussed. The growth of twins by the nucleation of twinning dislocations on planes parallel and contiguous to the coherent twin boundary is considered. Various dislocation models proposed for the formation of twins in b.c.c., f.c.c., diamond cubic, zinc-blende and h.c.p. structures are critically reviewed. In some cases the supporting experimental evidence is presented. Additionally, the effects of deformation temperature, imposed strain-rate, alloying and doping, prestrain, precipitates and second phase disperions on deformation twinning are discussed.Mechanistic details regarding the accommodation processes occurring at twins terminating within a crystal, slip-twin, twin-slip and twin-twin intersections are reviewed and are compared with the experimental results. The role of twins in the nucleation of fracture in materials is also considered.
A quasistatic stability analysis is performed for the problem of a strain-softening material undergoing dynamic one-dimensional simple shear. This analysis is basically algebraic in nature and deals strictly with the underlying linearized problem. A simple algebraic criterion for the onset of instability is developed. An application of the criterion to two steels is then made and compared with experimental data, and qualitative agreement is observed. The analysis is then refined in a special case to include the time-dependence of the underlying parallel flow in the stability method. For a simple constitutive assumption of a thermoviscous materials), it is found that the refined analysis can yield a somewhat different stability criterion than the quasistatic analysis. The importance of this for the problem under consideration is also discussed.
The high strain rate behavior of two different tungsten-based composites (WBCs) has been investigated using a combination of compression Kolsky bar and high strain rate pressure—shear plate impact experiments. The two materials investigated are (a) a W/(WNiFe) “heavy alloy” containing 91 wt.% W with a WNiFe matrix/binder phase and (b) a W/Hf composite containing 72 wt.% W and Hf matrix/binder phase. For each materials, stress-strain curves are obtained in compression and shear over the strain rate range of 103 to 105 s−1. The flow stresses observed in the W/Hf composite during these dynamic deformations are comparable to those observed in the W/(WNiFe) material, in spite of the lower tungsten content of the W/Hf composite. Further, the W/Hf composite displays approximately the same degree of strain-rate hardening as the W/(WNiFe) composite. The observed behaviorof the W/(WNiFe) composite is compared with the predictions of several phenomenological models for high-rate deformations, including the Johnson-Cook and Zerilli-Armstrong models.
We have systematically investigated the quasi-static and dynamic mechanical behavior (especially dynamic failure) of ultra-fine grained (UFG) tungsten (W) under uniaxial compression. The starting material is of commercial purity and large grain size. We utilized severe plastic deformation to achieve the ultrafine microstructure characterized by grains and subgrains with sizes of ∼500 nm, as identified by transmission electron microscopy. Results of quasi-static compression show that the UFG W behaves in an elastic–nearly perfect plastic manner (i.e., vanishing strain hardening), with its flow stress approaching 2 GPa, close to twice that of conventional coarse grain W. Post-mortem examinations of the quasi-statically loaded samples show no evidence of cracking, in sharp contrast to the behavior of conventional W (where axial cracking is usually observed). Under uniaxial dynamic compression (strain rate ∼103 s−1), the true stress–true strain curves of the UFG W exhibit significant flow softening, and the peak stress is ∼3 GPa. Furthermore, the strain rate sensitivity of the UFG W is reduced to half the value of the conventional W. Both in situ high-speed photography and post-mortem examinations reveal shear localization and as a consequence, cracking of the UFG W under dynamic uniaxial compression. These observations are consistent with recent observations on other body-centered cubic metals with nanocrystalline or ultrafine microstructures. The experimental results are discussed using existing models for adiabatic shear localization in metals.
Dynamic recrystallization (DRX) of tungsten was investigated through the microstructural examination of a recovered shaped charge slug, using both optical and transmission electron microscopy. The microstructure was refined, indicating the DRX process occurred during deformation. No twins or elongated subgrains were observed due to the extremely high strain rate deformation process. This proves that the DRX process was dominated by dislocation movements. Compared with previous reports of copper and tantalum shaped charges, the tungsten liner deforms more like copper than tantalum.
Dynamic deformation and failure behavior of a tungsten heavy alloy (93W) under complex stress condition are studied using a split Hopkinson pressure bar (SHPB) apparatus. Cylindrical, step-cylindrical and truncated-conic specimens are used to generate different stress condition in an attempt to induce strain localization in the alloy. The microstructure of the specimens after tests is examined by optical microscopy and scanning electronic microscopy (SEM). It is found that in all the specimens, except the cylindrical ones, intense strain localization in the form of shear bands is initiated at stress concentration sites. In order to analyze the stress condition of different specimen geometry, finite element simulations are also presented. The Johnson-Cook model is employed to simulate the thermo-viscoplastic response of the material. It is found that dynamic deformation and failure modes are strongly dependent on the geometry of the specimens. The stress condition controlled by specimen geometry has significant influence on the tendency for shear band formation. The adiabatic shear band has general trends to initiate and propagate along the direction of maximum shear stress. It is suggested that further studies on the control of the stress condition to promote shear band formation be conducted in order to improve the penetration performance of the tungsten heavy alloy.
Liquid sintered tungsten (W) heavy alloy with a 92.5W–5.25Ni–2.25Fe composition was examined using a compression split-Hopkinson bar to realize the effects of strain rate and temperature on dynamic impact deformation behaviour. Both stress and strain were measured for specimens tested at temperatures ranging from 25 to 1100°C and strain rates ranging from 8×102 to 4×103 s−1. The relationship between flow stress, strain rate and temperature was determined and the results have been successfully modeled by a proposed constitutive equation incorporating the effect of strain, strain rate and temperature. Our results show that flow stress increases with increasing strain rate. Alternatively, high temperature reduces flow stress significantly and improves the degree of thermal softening. During impact, initial cracking occurs preferentially either at tungsten–tungsten grain boundaries or at the tungsten–matrix interface, and failure is dominated principally by a mixture fracture model. Metallographic examinations show a dramatic increase in microcrack density and deformation of tungsten grains as strain rate and temperature are increased. Additionally, changes in microhardness are also found to correlate with changes in strain rate and temperature.
The microstructures and mechanical properties of mechanically alloyed oxide-dispersed tungsten heavy alloys were investigated. Elemental powders of tungsten, nickel, and iron of a composition corresponding to 93W–5.6Ni–1.4Fe (wt.%) were mechanically alloyed with additional Y2O3 powders in a tumbler ball mill for 72 h. Mechanically alloyed powders were solid-state sintered at 1300 °C for 1 h followed by secondary liquid-phase sintering at 1470 °C for a sintering time ranging from 4 to 90 min. Liquid-phase sintering of oxide-dispersed tungsten heavy alloys at 1485 °C for 1 h was also carried out. Oxide-dispersed tungsten heavy alloys containing 0.1–5 wt.% Y2O3 can be obtained and oxide dispersoids effectively inhibit the coarsening of tungsten particles during sintering. A high temperature compression test of tungsten heavy alloys at 800 °C showed that the strength of mechanically alloyed oxide-dispersed tungsten heavy alloys increased with the oxide content. In split-Hopkinson bar tests at a strain rate of 104 s−1, the oxide-dispersed tungsten heavy alloy showed a tendency for premature failure during localized shear deformation due to debonding at the oxide/matrix interfaces.
A micromechanics study is made of the rate-dependent thermal softening behavior of a tungsten matrix composite containing glassy particles. Under adiabatic compression of the composite, the elastic glassy particles thermally soften at relatively high strains, enhancing the thermal softening of the tungsten-based composite, thus reducing the strain rate sensitivity and fostering shear localization. To guide the microstructural design of the particle-modified tungsten-based composite in penetration applications, systematic predictions are made for the stress-strain behavior of the composite under overall adiabatic compression with different temperature-dependent behaviors, sizes, volume fractions of the particle and different applied strain rates. The temperature-dependent behavior of the particles is characterized by a set of exponential functions using two non-dimensional parameters and a reference temperature. The plastic behavior of tungsten is taken to be power-law strain and strain rate hardening. It is found that the radius r of the particles has very little influence on the composite behavior if r ⩽ 10 μm. It is also found that both the onset and the rate of thermal softening of the composite depend critically on the applied strain rate. Owing to thermal softening of the glassy particles, the strain-rate sensitivity of the composite is reduced.
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