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Nanoscale serration characteristics of additively manufactured superalloys
Abstract
Structural elements made of nickel-based superalloys usually operate at high temperatures. Many surface failure mechanisms such as creep, fatigue, fretting fatigue, corrosion, and stress corrosion cracking start from the surface. Thus, studying the surface mechanical properties and deformation behavior of materials is vital in order to draw a correlation between the surface properties and failures. Herein, a nanoindentation technique was implemented to characterize surface properties of a widely used additively manufactured (AM) superalloy, i.e., Inconel 625. The specimens were tested in a range of temperatures in a vacuum chamber. Serrated plastic flow characterized by pop-in events during nanoindentation, known as Portevin-Le Chatelier (PLC) effect, was observed in all three tested elevated temperatures. This phenomenon depicts itself as bursts of plasticity in the loading section of the load-displacement curves. These bursts were studied comprehensively to explore incipient plasticity. Hertzian contact mechanics were implemented to extract the maximum shear stress beneath the indenter from the pop-in loads. The results show that the initial pop-in load increases as the temperature increases. The average initial pop-in load increases by almost four times from 300 °C to 650 °C. This was attributed to the formation of strengthening precipitates. The average size of the serrations and the magnitude of dislocation nucleation increase from 300 °C to 500 °C and decrease from 500 °C to 650 °C. The serrations are attributed to the dislocation generation and movement as well as their interaction with the solute atoms and precipitates.
In this research, a room temperature multicycle nanoindentation technique was implemented to evaluate the effects of the laser peening (LP) process on the surface mechanical behavior of additively manufactured (AM) Inconel 625. Repetitive deformation was introduced by loading-unloading during an instrumented nanoindentation test on the as-built (No LP), 1-layer, and 4-layer laser peened (1LP and 4LP) conditions. It was observed that laser-peened specimens had a significantly higher resistance to penetration of the indenter and lower permanent deformation. This is attributed to the pre-existing dislocation density induced by LP in the material which affects the dislocation interactions during the cyclic indentation. Moreover, high levels of compressive stresses, which are greater in the 4LP specimen than the 1LP specimen, lead to more effective improvement of surface fatigue properties. The transition of the material response from elastic-plastic to almost purely elastic in 4LP specimens was initiated much earlier than it did in the No LP, and 1LP specimens. In addition to the surface fatigue properties, hardness and elastic modulus were also evaluated and compared.
Additively manufactured (AM) nickel-based superalloys, such as Inconel 625 (IN 625), are promising candidates for complex systems with high operating temperatures. In this work, an analysis was conducted to extract material properties such as hardness, elastic modulus, and creep properties of both AM IN 625 and traditionally manufactured (TM) IN 625 after annealing through an instrumented nanoindentation technique from room temperature up to 700 °C. The results for both specimens were compared and indicated that creep deformation decreases with the temperature increase up to 500 °C where no creep was observed for the AM IN 625 specimens at 500 °C.
We developed a combined finite element and CALPHAD based model of the Laser Powder Bed Fusion (LPBF) process for AA7075 alloy that considers the effect of feedstock composition and print parameters. A single-pass of a laser on a layer of AA7075 alloy powder has been considered. Sensitivity of temperature evolution and melt pool geometry to variation in the stoichiometry of the feedstock powder and laser source characteristics have been studied. Our results indicate that deviation (up to 10%) of the feedstock composition from the AA7075 raises the maximum temperature and increases melt pool size. Excess Cu content shows the largest melt pool width and depth among all the cases. The peak temperature is higher than the standard feedstock composition in all cases, except when the Cu concentration is reduced. Increasing the scan power also results in a higher peak temperature and a larger melt pool size. Furthermore, the temperature's rise time increases by lowering the scan speed.
Thermal tensile and creep properties provide important information for selecting the suitable service conditions for metal materials. In this study, the effects of temperature on the tensile and creep properties, microstructure evolution, and deformation mechanisms of thermal-rolled Inconel 625 sheets were systematically investigated at 650 and 750 °C. The results showed that type B + C serrations dominated the tensile deformation at 650 °C. As the tensile temperature increased to 750 °C, type C serrations occurred in the early stages of the deformation and gradually transformed to type B serrations with the increase of the strain level. The microscopic observations through transmission electron microscopy (TEM) indicated that type B and C serrations were related to the carbon atom–moving dislocation interactions and nucleation–growth process of twin deformation, respectively. Moreover, the creep tests at 650 °C exhibited a better creep resistance than that at 750 °C. The results from the electron backscatter diffraction and TEM indicated combined grain boundary sliding and dislocation slipping as the main creep mechanisms at 650 °C. Moreover, the creep stress can accelerate the precipitation of the γ″ phases at 650 °C, which plays an important role in improving creep resistance, thereby improving the efficiency in strengthening Ni-based alloys. In addition, δ phase precipitation occurred during creep loading at 750 °C. The δ phases/matrix interfaces can act as nucleation sites to initiate dynamic recrystallization, which greatly affects the reduction of the work hardening rate and consequently, decreasing the creep resistance at 750 °C.
The aim of this work was to investigate the influence of laser shock peening on the topography, microstructure, surface roughness and the mechanical properties of the Inconel 625 nickel alloy. Examination of the topography and microstructure of the nickel alloy after laser treatment was carried out by means of scanning electron microscopy as well as atomic force microscopy. The roughness of the surface was measured by WYKO NT9300 equipment. Nanohardness test was carried out using a nanoindenter NHT 50-183 of CSM Instruments equipped with a Berkovich diamond indenter. Additionally, transmission electron microscopy was used to examine the microstructural changes on the surface layer after laser treatment. The investigations showed that the laser process produced an ablation and melting of the surface layer and, hence, increased the surface roughness of the Inconel 625. On the other hand, the presence of the slip bands on the surface and on the cross section of the treated material, a high density of dislocations and a higher hardness of the treated region indicated that the laser shock processing caused severe plastic deformation of the surface layer. Additionally, due to the high plastic deformation, cracking of the carbide precipitates was observed.
Creep is a serious concern reducing the efficiency and service life of components in various structural applications. Multi-principal element alloys are attractive as a new generation of structural materials due to their desirable elevated temperature mechanical properties. Here, time-dependent plastic deformation behavior of two multi-principal element alloys, CoCrNi and CoCrFeMnNi, was investigated using nano-indentation technique over the temperature range of 298 K to 573 K under static and dynamic loads with applied load up to 1000 mN. The stress exponent was determined to be in the range of 15 to 135 indicating dislocation creep as the dominant mechanism. The activation volume was ~25b3 for both CoCrNi and CoCrFeMnNi alloys, which is in the range indicating dislocation glide. The stress exponent increased with increasing indentation depth due to higher density and entanglement of dislocations, and decreased with increasing temperature owing to thermally activated dislocations. The results for the two multi-principal element alloys were compared with pure Ni. CoCrNi showed the smallest creep displacement and the highest activation energy among the three systems studied indicating its superior creep resistance.
Additive manufacturing, often known as three-dimensional (3D) printing, is a process in which a part is built layer-by-layer and is a promising approach for creating components close to their final (net) shape. This process is challenging the dominance of conventional manufacturing processes for products with high complexity and low material waste1. Titanium alloys made by additive manufacturing have been used in applications in various industries. However, the intrinsic high cooling rates and high thermal gradient of the fusion-based metal additive manufacturing process often leads to a very fine microstructure and a tendency towards almost exclusively columnar grains, particularly in titanium-based alloys1. (Columnar grains in additively manufactured titanium components can result in anisotropic mechanical properties and are therefore undesirable2.) Attempts to optimize the processing parameters of additive manufacturing have shown that it is difficult to alter the conditions to promote equiaxed growth of titanium grains3. In contrast with other common engineering alloys such as aluminium, there is no commercial grain refiner for titanium that is able to effectively refine the microstructure. To address this challenge, here we report on the development of titanium–copper alloys that have a high constitutional supercooling capacity as a result of partitioning of the alloying element during solidification, which can override the negative effect of a high thermal gradient in the laser-melted region during additive manufacturing. Without any special process control or additional treatment, our as-printed titanium–copper alloy specimens have a fully equiaxed fine-grained microstructure. They also display promising mechanical properties, such as high yield strength and uniform elongation, compared to conventional alloys under similar processing conditions, owing to the formation of an ultrafine eutectoid microstructure that appears as a result of exploiting the high cooling rates and multiple thermal cycles of the manufacturing process. We anticipate that this approach will be applicable to other eutectoid-forming alloy systems, and that it will have applications in the aerospace and biomedical industries. Titanium–copper alloys with fully equiaxed grains and a fine microstructure are realized via an additive manufacturing process that exploits high cooling rates and multiple thermal cycles.
Dislocation structures, chemical segregation, γ′, γ″, δ precipitates, and Laves phase were quantified within the microstructures of Inconel 718 (IN718) produced by laser powder bed fusion additive manufacturing (AM) and subjected to standard, direct aging, and modified multi-step heat treatments. Additionally, heat-treated samples still attached to the build plates vs. those removed were also documented for a standard heat treatment. The effects of the different resulting microstructures on room temperature strengths and elongations to failure are revealed. Knowledge derived from these process-structure-property relationships was used to engineer a super-solvus solution anneal at 1020 °C for 15 min, followed by aging at 720 °C for 24 h heat treatment for AM-IN718 that eliminates Laves and δ phases, preserves AM-specific dislocation cells that are shown to be stabilized by MC carbide particles, and precipitates dense γ′ and γ″ nanoparticle populations. This “optimized for AM-IN718 heat treatment” results in superior properties relative to wrought/additively manufactured, then industry-standard heat treated IN718: relative increases of 7/10 % in yield strength, 2/7 % in ultimate strength, and 23/57 % in elongation to failure are realized, respectively, regardless of as-printed vs. machined surface finishes.
Nanoindentation has been used for a long time to investigate the mechanical properties of materials. A well-known displacement burst, the so-called “pop-in,” is usually observed during nanoindentation tests. In particular for the first pop-in has been well studied because it should be directly related to the fundamental mechanical strength of the local volume beneath the indenter. The first pop-in load measured for a defect-free local volume is corresponding to the onset load of homogeneous dislocation nucleation, while measured for a local volume with immobile defects is that of heterogeneous dislocation nucleation. The first pop-in is a thermally activated event under loading; thus, the pop-in load exhibits both temperature and loading-rate dependencies. Although theoretical studies have successfully explained the temperature and loading-rate dependencies, to the best of our knowledge, an atomistic prediction for specific materials which takes into account the stress state beneath the indenter has not yet been reported. In this study, we propose an atomistically informed multiscale (two-scale) modeling to predict the temperature and loading-rate dependencies of the first pop-in load for the homogeneous dislocation nucleation considering an atomistically estimated stress state beneath the indenter and stress-dependent activation energies of dislocation nucleation. Body-centered-cubic Fe and Ta are chosen as target metals, although the method is generally applicable to any ductile material. In addition to the homogeneous dislocation nucleation, we also developed a heterogeneous nucleation theory to explain the difference between the experimentally and atomistically obtained homogeneous nucleation pop-in loads.
Additive manufacturing is an emerging technology that challenges traditional manufacturing methods. However, the corrosion behaviour of additively manufactured parts must be considered if additive techniques are to find widespread application. In this paper, we review relationships between the unique microstructures and the corresponding corrosion behaviour of several metallic alloys fabricated by selective laser melting, one of the most popular powder-bed additive technologies for metals and alloys. Common issues related to corrosion in selective laser melted parts, such as pores, molten pool boundaries, surface roughness and anisotropy, are discussed. Widely printed alloys, including Ti-based, Al-based and Fe-based alloys, are selected to illustrate these relationships, and the corrosion properties of alloys produced by selective laser melting are summarised and compared to their conventionally processed counterparts.
Self-healing materials with the ability to partially or completely restore their mechanical properties by healing the damage inflicted on them have great potential for applications where there is no or only limited access available to conduct a repair. Here, we demonstrate a bio-inspired new design for self-healing materials, where unit cells embedded in the structure are filled with a UV-curable resin and act as reservoirs for the self-healing agent. This design makes the repeated healing of mechanical damage possible. When a crack propagates and reaches one of these embedded reservoirs, the healing agent is released into the crack plane through the capillary action, and after polymerization through UV light exposure, bonds the crack faces. The structures here were fabricated using a stereolithography technique by a layer-by-layer deposition of the material. “Resin trapping” as a unique integration technique is developed for the first time to expand the capability of additive manufacturing technique for creating components with broader functionalities. The self-healing materials were manufactured in one step without any needs for any sequential stages, i.e. filling the reservoir with the healing agent, in contrast with the previously reported self-healing materials. Multiscale mechanical tests such as nanoindentation and three-point bending confirm the efficiency of our method.
Inconel 625 is a nickel-based alloy that is mainly used in high-temperature applications. Inconel 625 exhibits an unstable plastic flow at elevated temperatures characterized by serrated yielding, well-known as the Portevin-Le Chatelier effect. The evaluation of the mechanical properties of Inconel 625 at high temperatures is the aim of this work. The tensile tests were executed in temperatures ranging from room temperature to 1000 °C with strain rates of 2 × 10−4 to 2 × 10−3 s−1. The creep tests were executed in the temperature range of 600–700 °C and in the stress range of 500–600 MPa in a constant load mode. The optical and scanning electron microscopes were used for surface fracture observation. In the curves obtained at 200–700 °C the serrated stress-strain behavior was observed, which was related to the dynamic strain aging effect. The yield strength and the elongation values show anomalous behavior as a function of the test temperature. An intergranular cracking was observed for a specimen tensile tested at 500 °C that can be attributed to the decohesion of the carbides along the grain boundaries. The fracture surface of the specimen tensile tested at 700 °C showed the predominance of transgranular cracking with tear dimples with a parabolic shape.
Additive manufacturing (AM) nickel-based superalloys have been demonstrated to equate or exceed mechanical properties of cast and wrought counterparts but their tribological potentials have not been fully realized. This study investigates fretting wear behaviors of Inconel 625 against the 42 CrMo4 stainless steel under flat-on-flat contacts. Inconel 625 is prepared by additive manufacturing (AM) using the electron beam selective melting. Results show that it has a high hardness (335 HV), superior tensile strength (952 MPa) and yield strength (793 MPa). Tribological tests indicate that the AM-Inconel 625 can suppress wear of the surface within a depth of only ~2.4 μm at a contact load of 106 N after 2 × 104 cycles. The excellent wear resistance is attributed to the improved strength and the formation of continuous tribo-layers containing a mixture of Fe2O3, Fe3O4, Cr2O3 and Mn2O3.
The effect of hydrogen and strain rate on nanoindentation creep of austenitic stainless steel was investigated by nanoindentation loading and creep tests. The loading segment mainly reflects dislocation nucleation, but creep mainly indicates dislocation movement. The strain rate significantly influences dislocation nucleation, resulting in increased creep displacement with strain rate. The interaction between hydrogen and dislocation is responsible for the increased creep displacement in 310S steel. Hydrogen greatly enhances the displacement burst after small strain rate due to increased dislocation nucleation rate. Hydrogen slightly enhances creep of 304 steel as hydrogen diffuses through strain-induced α′ martensite to accumulate under the indenter before creep and enhances dislocation movement.
Dynamic strain aging (DSA), observed macroscopically as serrated plastic flow, has long been seen in nickel-base superalloys when plastically deformed at elevated temperatures. Here we report the absence of DSA in Inconel 625 made by additive manufacturing (AM) at temperatures and strain rates where DSA is present in its conventionally processed counterpart. This absence is attributed to the unique AM microstructure of finely dispersed secondary phases (carbides, N-rich phases, and Laves phase) and textured grains. Based on experimental observations, we propose a dislocation-arrest model to elucidate the criterion for DSA to occur or to be absent as a competition between dislocation pipe diffusion and carbide–carbon reactions. With in situ neutron diffraction studies of lattice strain evolution, our findings provide a new perspective for mesoscale understanding of dislocation–solute interactions and their impact on work-hardening behaviors in high-temperature alloys, and have important implications for tailoring thermomechanical properties by microstructure control via AM.
316L stainless steel samples have been prepared by selective laser melting (SLM) using a pulsed laser mode and different laser powers and scanning patterns. The as-fabricated samples were found to be dominated by clusters of nano-sized γ needles or cells. TEM imaging shows that these needles contain a high population of dislocations while TEM-EDX analysis reveals high chemical homogeneity throughout the as-fabricated samples as evidenced by the fact that there is even no micro-/nano-segregation at interfaces between neighbouring γ needles. The good chemical homogeneity is attributed to the extremely high cooling rate after SLM (>106 °C/s) and the formation of Si- and Mn-oxides that distribute randomly in the current samples. The laser-processed samples show both superior strength and ductility as compared with conventionally manufactured counterparts. TEM examination on the deformed specimens reveals a significantly high density of dislocations and a great number of twinning within nano-needles, suggesting that the plastic deformation has been governed by both gliding of dislocations and twinning deformation, which is believed to be responsible for the simultaneous acquisition of superior strength and ductility. Finally, laser power shows a much more dominant role than laser scanning pattern in porosity and grain size development for the SLM-processed 316L stainless steel samples.
Structural elements made of nickel-based superalloys are often plastically formed at elevated temperature (e.g. by stamping) so detailed characterization of their deformation behaviour is crucial. Some alloys exhibit unstable plastic flow at elevated temperature characterized by serrated yielding, known as Portevin-Le Chatelier (PLC) effect. This phenomenon is usually attributed to dynamic strain ageing (DSA). The PLC effect is observed in many nickel-based superalloys within a certain range of deformation temperature and strain rate. The aim of presented research was to investigate deformation behaviour of 625 nickel-based superalloy in tensile tests in the temperature range of 200-1100ºC and for strain rates of 5·10-4 and 5·10 2 s 1. The effect of temperature and strain rate on the mechanical properties of the 625 alloy and character of serrations on the stress-strain curves was analysed. Strain rate sensitivity parameter dependence on deformation temperature was determined.
Metal-based additive manufacturing, or three-dimensional (3D) printing, is a potentially disruptive technology across multiple industries, including the aerospace, biomedical and automotive industries. Building up metal components layer by layer increases design freedom and manufacturing flexibility, thereby enabling complex geometries, increased product customization and shorter time to market, while eliminating traditional economy-of-scale constraints. However, currently only a few alloys, the most relevant being AlSi10Mg, TiAl6V4, CoCr and Inconel 718, can be reliably printed; the vast majority of the more than 5,500 alloys in use today cannot be additively manufactured because the melting and solidification dynamics during the printing process lead to intolerable microstructures with large columnar grains and periodic cracks. Here we demonstrate that these issues can be resolved by introducing nanoparticles of nucleants that control solidification during additive manufacturing. We selected the nucleants on the basis of crystallographic information and assembled them onto 7075 and 6061 series aluminium alloy powders. After functionalization with the nucleants, we found that these high-strength aluminium alloys, which were previously incompatible with additive manufacturing, could be processed successfully using selective laser melting. Crack-free, equiaxed (that is, with grains roughly equal in length, width and height), fine-grained microstructures were achieved, resulting in material strengths comparable to that of wrought material. Our approach to metal-based additive manufacturing is applicable to a wide range of alloys and can be implemented using a range of additive machines. It thus provides a foundation for broad industrial applicability, including where electron-beam melting or directed-energy-deposition techniques are used instead of selective laser melting, and will enable additive manufacturing of other alloy systems, such as non-weldable nickel superalloys and intermetallics. Furthermore, this technology could be used in conventional processing such as in joining, casting and injection moulding, in which solidification cracking and hot tearing are also common issues. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Nanoindentation became a versatile tool for testing local mechanical properties beyond hardness and modulus. By adapting standard nanoindentation test methods, simple protocols capable of probing thermally activated deformation processes can be accomplished. Abrupt strain-rate changes within one indentation allow determining the strain-rate dependency of hardness at various indentation depths. For probing lower strain-rates and excluding thermal drift influences, long-term creep experiments can be performed by using the dynamic contact stiffness for determining the true contact area. From both procedures hardness and strain-rate, and consequently strain-rate sensitivity and activation volume can be reliably deducted within one indentation, permitting information on the locally acting thermally activated deformation mechanism. This review will first discuss various testing protocols including possible challenges and improvements. Second, it will focus on different examples showing the direct influence of crystal structure and/or microstructure on the underlying deformation behavior in pure and highly alloyed material systems.
Compression behavior of the Al0.5CoCrCuFeNi high-entropy alloy (HEA) was studied at different temperatures from 673 K to 873 K at a low strain rate of 5 × 10−5/s to investigate the temperature effect on the mechanical properties and serration behavior. The face-centered-cubic (fcc) structure is confirmed at the lower temperature of 673 K and 773 K, and a structure of mixed fcc and body-centered cubic (bcc) is identified at a higher temperature of 873 K after compression tests using high-energy synchrotron x-ray diffraction. By comparing the stress–strain curves at different temperatures, two opposite directions of serrations types were found, named upward serrations appearing at 673 K and 773 K and downward serrations at 873 K, which may be due to dynamic strain aging.
Haynes 282 is a newly introduced Ni-based superallony, developed to provide a combination of high-temperature mechanical properties, thermal stability and processability. The present contribution investigates the effect of dynamic strain aging (DSA) on the deformation behaviour of Haynes 282 during monotonic and cyclic loading. It is shown that DSA (presumably related to carbon diffusion based on rough estimates of the activation energy) completely dominates the development of the stress during cycling at intermediate temperatures, leading to extensive cyclic hardening and serrated yielding. However, no clear effects on the fatigue life or the resulting dislocation structure could be observed. The tensile properties were not severely affected, in spite of the presence of extensive serrated yielding, although a reduction in ductility was observed in the DSA temperature regime. During monotonic loading at lower strain rates indications of an additional DSA mechanism due to substitutional elements were observed.
Today, laser additive manufacturing (LAM) is used in more and more industrial applications. Due
to a new freedom in design it offers a high potential for weight saving in lightweight applications,
e.g., in the aerospace industry. However, most design engineers are used to design parts for
conventional manufacturing methods, such as milling and casting, and often only have limited
experience in designing products for additive manufacturing. The absence of comprehensive
design guidelines is therefore limiting the further usage and distribution of LAM. In this paper,
experimental investigations on the influence of part position and orientation on the dimension
accuracy and surface quality are presented. Typical basic shapes used in lightweight design have
been identified and built in LAM. Thin walls, bars, and bore holes with varying diameters were
built in different orientations to determine the process limits. From the results of the experiments,
comprehensive design guidelines for lightweight structures were derived in a catalog according to
DIN 2222 and are presented in detail. For each structure a favorable and an unfavorable example is
shown, the underlying process restrictions are mentioned and further recommendations are given.
The present study deals with the development and characterization of a Mo-Ti-Si alloy by pressure-assisted sintering for high-temperature applications. The three-phase alloy was found to be consisted of (Mo,Ti)3Si-type silicide and discontinuous Mo-rich and Ti-rich α-(Mo, Ti)ss phases. The alloy was characterized for mechanical properties, creep, and wear behavior. Elastic modulus and hardness of all the constituent phases were determined using nanoindentation technique. Creep properties of the alloy determined from the compression creep tests in vacuum were found to be better than the single-phase Mo5Si3 and Ti5Si3 at the tested temperature of 1000 °C. The coefficient of friction values were found to be 0.59, 0.55 and 0.4 at the testing loads of 10, 15 and 20 N, respectively, at 10 Hz frequency in reciprocal testing mode at room temperature, while 0.42 at 500 °C with a load of 15 N. The dominant wear mechanism was proposed based on the wear scar analysis.
This work demonstrates the feasibility of fabricating bulk nanostructured high modulus steels in-situ by additive manufacturing. This ideal match of novel processes and alloy concepts opens up new pathways for lightweight design by producing light, stiff, strong and ductile components with minimal geometric restraints. On the example of an Fe – Ti – B alloy, a conventional processing sequence of melting and casting pre-alloys, gas-atomisation and laser powder bed fusion (selective laser melting) led to finely dispersed metastable particle and matrix phases. A simple annealing step transformed them into the desired equilibrium constituents of ductile ferrite (matrix) and light and stiff TiB2 (particles), with only minimal changes in particle size (about 20–150 nm in diameter) and distribution (mainly on the matrix grain boundaries). This nano-scaled composite structure promises an extremely attractive property profile, i.e. an increased stiffness/ratio at elevated strength and without deteriorated ductility. However, the not yet optimized parameters of the laser fusion process led to the formation of few pores and cracks, which prevented the complete assessment of the property profile of the manufactured samples. Material and processing strategies for the further development of this promising lightweight design approach – including the suitability of other powder metallurgy processing routes – are outlined and discussed.
Functionally graded materials (FGMs) have attracted much research interest in the industry due to their graded material properties, which result from gradually distributed compositions or structures. In recent years, metallic FGMs have been widely studied, and additive manufacturing (AM) has become an important approach to build metallic FGMs. This paper aims to provide an overview of the research progress in metallic FGMs fabricated by laser metal deposition (LMD), an AM process that is widely used in metallic materials. Firstly, the unique material properties and advantages of FGMs are introduced. Then, typical recent findings in transition path design, fabrication, and characterization for different types of metallic FGMs via LMD are summarized and discussed. Finally, challenges in fabricating metallic FGMs via LMD are discussed, and other related aspects in the area of FGMs such as model representation and numerical simulation are proposed for further investigation.
Laser powder bed fusion (LPBF) is a proven additive manufacturing (AM) technology for producing metallic components with complex shapes using layer-by-layer manufacture principle. However, the fabrication of crackfree high-performance Ni-based superalloys such as Hastelloy X (HX) using LPBF technology remains a challenge because of these materials’ susceptibility to hot cracking. This paper addresses the above problem by proposing a novel method of introducing 1 wt.% titanium carbide (TiC) nanoparticles. The findings reveal that the addition of TiC nanoparticles results in the elimination of microcracks in the LPBF-fabricated enhanced HX samples; i.e. the 0.65% microcracks that were formed in the as-fabricated original HX were eliminated in the as-fabricated enhanced HX, despite the 0.14% residual pores formed. It also contributes to a 21.8% increase in low-angle grain boundaries (LAGBs) and a 98 MPa increase in yield strength. The study revealed that segregated carbides were unable to trigger hot cracking without sufficient thermal residual stresses; the significantly increased subgrains and low-angle grain boundaries played a key role in the hot cracking elimination. These findings offer a new perspective on the elimination of hot cracking of nickel-based superalloys and other industrially relevant crack susceptible alloys. The findings also have significant implications for the design of new alloys, particularly for high-temperature industrial applications.
Inconel 718 is one of the most commonly employed alloys for metal additive manufacturing (MAM) and has a wide range of applications in aircraft, gas turbines, turbocharger rotors, and a variety of other corrosive and structural applications involving temperatures of up to ∼700 °C. Numerous studies have investigated different aspects of the mechanical behaviour of additively manufactured (AM) Inconel 718. This study analyses the observations from more than 170 publications to provide an unbiased engineering overview for the mechanical response of AM Inconel 718 (and its variations and spread among different reports). First, a brief review of the microstructural features of AM Inconel 718 is presented. This is followed by a comprehensive summary of tensile strength, hardness, fatigue strength, and high-temperature creep behaviour of AM Inconel 718 for different types of MAM techniques and for different process and post-process conditions.
Despite the potential and promising future of additive manufacturing (AM), the quality and performance of the products from metal additive manufacturing has been susceptible to skepticism. Herein, a nanoindentation method was implemented to compare the hardness, Young’s modulus, and fracture toughness in AM and hot-rolled Inconel 718. Results indicated that Young’s modulus and hardness of AM IN718 were nearly 25% and 50% smaller, respectively, compared to hot-rolled specimens. Indentation fracture toughness measurements showed that the traditionally fabricated specimens had a fracture toughness close to the reported range, but that of the AM samples was almost 25% smaller.
Laser peening (LP) is a post‐processing method used to optimize the service lives of critical components for various applications by increasing the material's resistance to surface‐related failures, such as fatigue, corrosion‐fatigue, wear, and stress corrosion cracking. Herein, the effects of LP on the nanomechanical properties of additively manufactured Inconel 718 are reported. Additively manufactured cubic specimens peened without a protective overlay were evaluated under vigorous microscopy and nanomechanical studies. Depth sensing through hardness and modulus mapping was conducted to evaluate the plastic deformation and hardness enhancement introduced by the process. X‐ray diffraction was used to measure the residual stresses to correlate hardness and residual stresses. The results showed that three layers of LP induces significant compressive residual stresses in the surface and subsurface (up to the depth of ≈2 mm) of the material and a moderate increase in hardness (≈20% to 30%) with high thermal stability of compressive stresses (retaining 60% of initial stress) after even 50 hours of annealing at 593 °C. Slight changes in the elastic modulus were recorded in LP specimens. This article is protected by copyright. All rights reserved.
Medium-Mn steel is the newly developed steel acting as a promising candidate of the 3rd-generation advanced high strength steels. In the present study, the effect of hydrogen on the mechanical behavior and martensite transformation process in a duplex medium-Mn steel is investigated by in-situ nanoindentation test. The mechanical response of individual phase: ferrite, and retained austenite by introducing hydrogen is studied. With the presence of hydrogen, the reduction of activation energy for dislocation nucleation was verified by using a stress-biased, statistical thermal activation model. The stacking fault energy (SFE) was reduced, which was revealed by electron channeling contrast (ECC) technique. Magnetic force microscopy (MFM) revealed a suppression of retained austenite to α′-martensite transformation with the presence of hydrogen, which is related to hydrogen induced SFE reduction and enhanced slip planarity.
In this work, temperature dependent mechanical properties of conventional and additive manufactured Inconel 718 (IN718) after solution and aging (SA) heat treatment were measured at room temperature, 350 °C, and 650 °C. Nanoindentation technique was used to obtain material properties including hardness, reduced modulus, creep rate, creep stress exponent, and thermal activation volume. The indentation size effect (ISE) was analyzed at each temperature. The reduced modulus was measured to decrease by 26% for the conventional samples and by 22% for AM samples as temperature increases from room temperature to 650 °C. The nanoindentation hardness decreases by 19% for the conventional samples and 16% for the AM samples. During the creep tests at 650 °C, the creep stress exponent was found in the range of 8–18, indicating that the
deformation at the examined nano/micro scale is controlled by the dislocation climb mechanism.
The use of additive manufacturing for prototyping and manufacturing purposes is becoming a crucial topic in both industry and research. Multi Jet Fusion printing technology possesses advantages in fabricating engineering-quality parts, but the dependency of their mechanical characteristics on process parameters is not well understood. In this work, multiscale mechanical analyses were conducted with the goal of investigating the effect of build orientation on Multi Jet Fusion manufactured polyamide 12 samples. Tensile tests of samples printed in two orientations showed that build orientation resulted in different strain and stress behaviors, with vertical samples having 25% smaller fracture strains, but slightly higher (5.8%) fracture stresses than horizontal samples. Nanoindentation showed vertical samples with reduced elastic modulus (Er) values double that of horizontal, and with hardness values nearly 30% larger. Nanoscratch tests showed that vertical samples had an almost 50% increase in lateral forces. Experimental and numerical creep analyses demonstrated higher creep resistance of vertical samples as compared to the horizontal specimens. Additionally, macroscale friction measurements showed that the friction coefficient of horizontal samples was independent of the direction of sliding while coefficient of friction for vertical samples showed a 60% drop when sliding directions were varied. These results point to a dependence on print orientation in the resulting mechanical and tribological properties.
The uniaxial tensile experiments are performed on a nickel-based superalloy with different initial microstructures at intermediate temperatures (473–973) K and low strain rates (0.0001–0.001) s⁻¹. Effects of initial microstructures on plastic deformation mechanisms, serrated flow features, and fracture characteristics are analyzed. It is clearly demonstrated that the plastic deformation mechanism of serrated flow is irrelevant to the initial microstructures, and the dislocation across slip in the matrix is responsible for the serrated flow. However, the effects of initial microstructures on fracture mechanisms are obvious, i.e., the main fracture mechanism of the ST (solution treated) and HS (solution plus γ′/γ″ phases aging precipitation) superalloys are the nucleation and growth of micro-voids, which accelerates the break of carbides and interfacial failure of carbides/matrix. While for the SAT (solution plus δ phases aging precipitation) superalloy, δ phases and carbides play a significant role in the nucleation of micro-voids. Furthermore, the increase of the deformation temperature reduces the elongation to fracture of the ST and HS superalloys. However, the better tensile ductility is obtained with raising the deformation temperatures for the SAT superalloy.
In the present work, we develop a series of binary Cu-Zr thin film metallic glasses (TFMGs) with tunable nanoscale structures by using direct-current (DC) magnetron sputtering technique. Depending on the chemical composition and the sputtering parameters, Cu-Zr TFMGs show variable nanoscale structures from nanocolumn to nanogranula. As compared to the metallic glassy ribbons with identical composition, the Cu-Zr TFMGs with nanoscale structures exhibit novel and unique thermal and mechanical properties. From nanoindentation study, it is found that the TFMGs with nanoscale structures display obvious reduced hardness (about 48–52%) and increased elastic modulus (about 7–23.8%) as compared to the metallic glassy ribbons. Moreover, the serrated flow behavior, as commonly observed from the load-displacement response in the nanoindentation of metallic glasses (MGs) is absence for nanostructured TFMGs, indicating the profound effects of nanoscale structural features on the plastic yielding and flow in TFMGs. This study provides a detail insight into a strategy of designing nanostructured TFMGs and obtaining tunable properties which is of great interest in the potential application of micro/nanoscale devices.
Nanoindentation tests were performed to investigate the nano-scale plastic deformation in the Al0.5CoCrCuFeNi high entropy alloys at room temperature (RT) and 200 °C, respectively. Serrated plastic flow, manifested as discrete bursts of plasticity on the load-displacement curves, was observed for both temperatures during the loading period, and its behavior and dependence on the temperature was analyzed from both the experimental and theoretical perspectives. The application of a mean-field theory indicated that the displacement bursts exhibited a temperature-dependent power-law distribution, and the universal exponents, κ and λ, were computed to be 1.5 and 0.04, respectively. With the use of the computed universal exponents, a critical annealing temperature for the slip-avalanche size distribution was estimated to be 1120 °C. Creep occurred during the nanoindentation holding period and exhibited very large stress exponent, implying that the dislocation glide-climb is the dominant mechanism. The creep simulations with a two-layer viscoplastic model further revealed that the deformation at a higher temperature (e.g., 200 °C) featured a greater and faster-growing plastic zone underneath the indenter, implying more pronounced dislocation activities.
Compression experiments of the Al0.5CoCrCuFeNi high-entropy alloy (HEA) under displacement control were conducted at different temperatures ranging from 673 K to 873 K with a strain rate of 5 × 10⁻⁵/s to study its serration behavior. Samples after compression tests were investigated, using the synchrontron-diffraction technique and transmission-electron microscopy. By comparing the stress-strain curves at different temperatures, two opposite directions of serrations were observed, named the upward serration appearing at 573 K and 673 K and the downward serration at 773 K and 873 K. The different directions of serrations were discussed in terms of not only the relationships among the stress vs. strain, stress vs. time, and strain vs. time, but also the interactions among dislocations, atoms, and nanoparticles. Finally, the temperature effect on the flow serration is discussed by referring to a theoretical framework for the initiation of the serrated flow. Beyond a critical high temperature, the initiation of the serrated flow becomes swiftly difficult, and ultimately the plastic flow in the full deformation range turns smooth. The theoretical prediction of the normal behavior is essentially in qualitative agreement with the experimental observation in the present work, i.e., the critical strain to initiate the serration decreases with increasing the temperature.
Superalloy Inconel 625 has been widely used in selective laser melting (SLM). Since SLM-induced microstructure with columnar grains, strong texture, porosity, and undesired properties, heat treatment is often used to tune the microstructure and mechanical properties. However, the microstructure evolution of IN 625 from SLM to heat treatment is poorly understood. In this study, IN 625 samples were SLMed and then heat treated at elevated temperatures. Microstructure evolution characteristics of the processed IN 625 alloy have been characterized using optical metallography, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), X-ray diffraction, and micro indentation. Fine dendrite microstructures with strong texture parallel to layer build-up direction was observed in the as-SLM samples due to rapid cooling and epitaxial growth. High dislocation density and high microhardness were found in the γ matrix which also contains high Z-contrast precipitates. After annealing at high temperatures, random grain growth accompanied by dislocation annihilation and twinning occurs. The decrease in lattice parameter and the prevalence of large grain boundary misorientation in the γ matrix suggests that SLM-induced residual stress be significantly reduced. In addition, the uncertainty of mechanical properties due to the process variations was quantified.
In situ electrochemical nanoindentation has been used to study the effect of hydrogen on the nanomechanical response of Alloy 718. Observations show that hardness increase as a result of hydrogen charging. Also, the hydrogen charging gives a reduced pop-in load and pop-in width. This is related to a reduction in the energy needed for dislocation nucleation and the mobility of the dislocations in the presence of hydrogen. Two grains with different orientations has been tested here. The pop-in load and width obtained in the (101) orientation was more affected by the presence of hydrogen than those achieved in the (111) oriented grain.
Nanoindentation, in combination with scanning probe microscopy, has been used to measure the hardness and Young's modulus in the hydride and matrix of a high burn-up neutron-irradiated Zircaloy-2 cladding material in the temperature range 25–300 °C. The matrix hardness was found to decrease only slightly with increasing temperature while the hydride hardness was essentially constant within the temperature range. Young's modulus decreased with increasing temperature for both the hydride and the matrix of the high burn-up fuel cladding material. The hydride Young's modulus and hardness were higher than those of the matrix in the temperature range.
Ti and Ti-TiB composite materials were produced by selective laser melting (SLM). Ti showed an α΄ microstructure, whereas the Ti-TiB composite revealed a distribution of needle-like TiB particles across an α-Ti matrix. Hardness (H) and reduced elastic modulus (Er) were investigated by nanoindentation using loads of 2, 5 and 10 mN. The results showed higher H and Er values for the Ti-TiB than Ti due to the hardening and stiffening effects of the TiB reinforcements. On increasing the nanoindentation load, H and Er were decreased. Comparison of the nanoindentation results with those derived from conventional hardness and compression tests indicated that 5 mN is the most suitable nanoindentation load to assess the elastic modulus and hardness properties. The wear resistance of the samples was related to their corresponding H/Er and H³/Er² ratios obtained by nanoindentation. These investigations showed that there is a high degree of consistency between the characterization using nanoindentation and the wear evaluation from conventional wear tests.
This study presents the investigation on the serrated flow behavior of a Pd-based bulk metallic glass under nanoindentation. Firstly, a method of curve-fitting and statistical analysis was proposed that found to successfully extract the serration events from the deformation of bulk metallic glasses during nanoindentation. Then the serrated flow at various loading rates were systematically analyzed by using the proposed method. The results showed that the lower loading rates promote the serrated flow, while the individual serration was more likely to be triggered at higher loading rates. Combined with the intrinsic micromechanism of bulk metallic glasses, we conjectured the serrated flow features may be attributed to the evolution of liquid-like region in bulk metallic glasses.
Serrated flow is a result of unstable plastic flow, which occurs during tensile and compression tests on some dilute alloys. This phenomenon is referred as the Portevin Le-Chatelier effect (PLC effect). The aim of this research was to investigate and analyze this phenomenon in Inconel 625 solution strengthened superalloy. The tested material was subjected to tensile tests carried out within the temperature range 200-700 °C, with three different strain rates: 0.002 1/s, 0.01/s, and 0.05 1/s and additional compression tests with high deformation speeds of 0.1, 1, and 10 1/s. The tensile strain curves were analyzed in terms of intensity and the observed patterns of serrations Using a modified stress drop method proposed by the authors, the activation energy was calculated with the assumption that the stress drops’ distribution is a direct representation of an average solute atom’s interaction with dislocations. Subsequently, two models, the standard vacancy diffusion Bilby-Cottrell model and the realistic cross-core diffusion mechanism proposed by Zhang and Curtin, were compared. The results obtained show that the second one agrees with the experimental data. Additional microstructure analysis was performed to identify microstructure elements that may be responsible for the PLC effect. Based on the results, the relationship between the intensity of the phenomenon and the conditions of the tests were determined.
This book covers virtually all technical aspects related to the selection, processing, use, and analysis of superalloys. The text of this new second edition has been completely revised and expanded with many new figures and tables added. In developing this new edition, the focus has been on providing comprehensive and practical coverage of superalloys technology. Some highlights include the most complete and up-to-date presentation available on alloy melting. Coverage of alloy selection provides many tips and guidelines that the reader can use in identifying an appropriate alloy for a specific application. The relation of properties and microstructure is covered in more detail than in previous books.
Fundamental deformation mechanisms of FCC materials under indentation have been probed at the grain scale. Experimental tests have been conducted on large-grained annealed and cold-worked polycrystalline nickel samples with a Berkovich indenter. Indentation axes have been chosen to be close to the three main crystallographic directions [001], [101] and [111]. Pile-ups and slip traces have been revealed around the residual imprints by analysing topographic measurements obtained by atomic force microscopy. It is shown that the indenter orientation in each indentation plane drives pile-ups and slip traces which in turn contain precious information about the crystallographic orientation and the hardening state of the studied grain. Imprint topographies after pile-up formation therefore carry information that one can exploit to assess some intrinsic material properties at the grain scale. A 3D finite element modelling of the nanoindentation test at the grain scale has been developed, making use of crystal plasticity constitutive laws. Six different virtual materials having the same macroscopic behaviour have been built. The simulation results show a good agreement with experimental tests and also a great pile-up sensitivity to interaction matrix components. These results pave the way to the interaction matrix identification using an inverse finite element method.
In recent years, numerous partitioned-domain methods have been developed to describe dislocation behavior at length scales that are usually inaccessible to most classical atomistic methods. These methods retain full atomistic detail in regions of interest while using a continuum description to reduce the computational burden elsewhere. In most of these methods, however, lattice defects in the continuum are either implemented via constitutive relations, lattice elasticity with dislocation field interactions, or are not permitted at all. In such approaches, the transit of dislocations across the atomistic/continuum interface appeals to approximate heuristics intended to minimize the effects of the interface due to the change from atomistic to continuum degrees of freedom. The concurrent atomistic-continuum (CAC) method, originally developed for addressing dynamic dislocation behavior by Xiong et al. (2011), permits dislocations to propagate in a continuum domain that employs a piecewise continuous finite element description with interelement displacement discontinuities. The method avoids ghost forces at interface between atomistically resolved and coarse-grained domains. CAC has subsequently been used to investigate complex dislocation behavior in face-centered cubic (FCC) metals (Xiong et al., 2012b, a, c, 2015). In this paper, we propose a quasistatic 3-D method to carry out sequential energy-minimized simulations at 0 K. This facilitates study of structure evolution along minimum energy pathways, avoiding over-driven conditions of high rate molecular dynamics. Parallelization steps in code implementation are described. Applications are presented for the quasistatic CAC method in FCC metal plasticity. Comparisons are made with a fully-resolved atomistic method for generalized stacking fault energy, core structure and stress field of a single 60∘ mixed type dislocation, surface indentation, and 60∘ mixed type dislocation migration through the interface between atomistic and coarse-grained domains. It is shown that 3-D CAC simulations are useful in substantially reducing the number of degrees of freedom while preserving key characteristics of dislocation structure, stacking faults, and plasticity, including the net Burgers vector and long range fields of interacting dislocations.
Selective laser melting (SLM), an additive manufacturing process, is capable of manufacturing metallic parts with complex shapes directly from computer-aided design (CAD) models. SLM parts are created on a layer-by-layer manner, making it more flexible than traditional material processing techniques. In this paper, Inconel 625 alloy, a widely used material in the aerospace industry, were chosen as the build material. Scanning electron microscopy (SEM), electron back scattering diffraction (EBSD) and X-ray diffraction (XRD) analysis techniques were employed to analyze its microstructure. It was observed that the molten pool was composed of elongated columnar crystal. Due to the rapid cooling speed, the primary dendrite arm space was approximately 0.5 μm and the hardness of SLM state was very high (343 HV). The inverse pole figure (IPF) indicated that the growing orientation of the most grains was <001> due to the epitaxial growth and heat conduction. The XRD results revealed that the austenite structure with large lattice distortion was fully formed. No carbides or precipitated phases were found. After heat treatment the grains grew into two microstructures with distinct morphological characters, namely, rectangular grains and limited in the molten pool, and equiaxed grains along the molten boundaries. Upon experiencing the heat treatment, MC carbides with triangular shapes gradually precipitated. The results also identified that a large number of zigzag grain boundaries were formed. In this study, the grain formation and microstructure, and the laws of the molten pool evolution were also analyzed and discussed.
Volume 7 covers the basic principles and techniques of powder metallurgy (PM) as it applies to specific metal/alloy families. It addresses powder manufacturing and characterization along with compaction, sintering, and full density processing. It also provides information on metal injection molding and conventional press and sinter powder metallurgy as well as materials and processes in current use. The volume opens with an introductory review of the history of powder metallurgy and relevant material standards. For information on the print version of Volume 7, ISBN: 978-1-62708-089-3, follow this link.
The microstructure of Nd60Al10Ni10Cu20−xFex (x=0, 5, 7, 10, 15, 20) alloys can change from homogeneous phase to a composite structure consisting of amorphous phase plus clusters or nanocrystals by adjusting the Fe content. The effect of microstructure on the plastic deformation behavior in this alloy system is studied by using nanoindentation. The alloys with homogeneous amorphous structure exhibit pronounced flow serrations during the loading process of nanoindentation. The addition of Fe changes the plastic deformation behavior remarkablely. No flow serration is observed in the alloys with high Fe content for the indentation depth of 500 nm. The mechanism for the change of plastic serrated flow behavior is discussed.