[Show abstract][Hide abstract] ABSTRACT: A polycrystalline Ni (99.882% purity) bar sample was subjected to surface mechanical grinding treatment (SMGT) at ambient temperature. Gradient microstructures along depth from the treated surface were generated owing to a graded variation of strain and strain rates, including dislocation structures, submicron-sized structures, and nanostructures, respectively. In the subsurface layer of 10–80 μm deep, 2-dimensional laminated structures with low angle boundaries and strong deformation textures were formed of which the average thickness is ∼20 nm, one order of magnitude smaller than that of the ultrafine structures in Ni induced by conventional severe plastic deformation. The extraordinary grain refinement was ascribed to the high strain rates and high strain gradients that enhance accumulation of geometric necessary dislocations with a suppressed recovery dynamics. Deformation of the nano-laminated structures is governed by dislocation slip, and supplemented by deformation twining at the nanoscale, eventually leading to fragmentation into nano-sized equiaxed grains.
[Show abstract][Hide abstract] ABSTRACT: The structural heterogeneity of a polycrystalline Ni subjected to dynamic plastic deformation to a strain of 2.3 was characterized, and its influence on the structural coarsening behaviour during post annealing was investigated. Structural heterogeneity on the large scale manifests itself by formation of two types of layers: low misoriented regions (LMRs) and highly misoriented regions (HMRs). On the small scale, the heterogeneity was characterized by different distributions of boundaries and textures in each layer. LMRs contain only low angle boundaries and one dominating crystallographic orientation. In contrast HMRs contain both low and high angle boundaries (>15°) and the texture is mixed with close to the compression axis. During annealing, LMRs coarsen uniformly and recrystallization nucleation is difficult to form. In HMRs, the structural evolution is heterogeneous and recrystallization nuclei are readily formed. The importance of structural heterogeneity during structural design for high performance nanostructure was highlighted.
IOP Conference Series Materials Science and Engineering 08/2015; 89(1). DOI:10.1088/1757-899X/89/1/012056
[Show abstract][Hide abstract] ABSTRACT: By means of surface mechanical rolling treatment (SMRT), a gradient nanostructured (GNS) surface layer was formed on AISI 316L stainless steel. The mean grain size is ∼30 nm in the topmost surface layer and increases with depth. Tension–compression fatigue measurements were performed on the SMRT sample under the stress-controlled mode. In comparison with the coarse-grained sample, the fatigue strength of the SMRT sample is significantly enhanced in both the low- and high- cycling fatigue regimes. Meanwhile, the fatigue ratio is evidently elevated with an increasing tensile strength in the SMRT sample. The initiation and growth of cracks, the cyclic deformation behaviors, as well as effects of strength and residual stresses, have been investigated to clarify the fatigue mechanism of the SMRT sample. The results emphasized the GNS surface layer enhances the fatigue property by suppressing the initiation of cracks and accommodating a remarkable cyclic plastic strain amplitude.
[Show abstract][Hide abstract] ABSTRACT: A bulk nanostructured CuCrZr alloy consisting of nanotwins and nanograins was prepared by dynamic plastic deformation at liquid nitrogen temperature. A tensile strength of 700 MPa and an electrical conductivity of 78.5% International Annealed Copper Standard are obtained in the nanostructured CuCrZr alloys processed by means of this one-step deformation without aging treatment. The reason for the increased strength without the sacrifice of its high electrical conductivity was discussed.
Scripta Materialia 04/2015; 99. DOI:10.1016/j.scriptamat.2014.11.032 · 3.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The performance of 2-Mercapto-5-benzimidazolesulfonic acid (MBIS) in a Watts bath in crack electro-healing was investigated. Potentiostatic and galvanostatic voltammetry, current efficiency measurements were performed for electrochemical investigation. The morphology of the healed crack indicates that the healing crystals grow in a controllable manner with the addition of MBIS under forced convection with the healing crystals fill preferentially in the crack tip to the crack sidewalls. No obvious defects were observed along the interface between the substrate and the healing crystals, which is attributed the much higher current efficiency of MBIS. Annual-ring like defects are observed but with a much longer width compared with PEI. The formation mechanism of annual ring is discussed. Possible approach to get defect-free crack healing is also prospected.
Journal of The Electrochemical Society 02/2015; 162(6):D222-D228. DOI:10.1149/2.1021506jes · 3.27 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: By means of cold spray, a Zn-Al coating was successfully deposited on an interstitial-free (IF) steel sheet. The formation of interfacial compounds between the coating and the IF steel was studied during diffusion annealing at 400 °C. And its correlations with the stripping behaviors of the coating were investigated by using a three-point bending method. The results showed that Fe-Zn and Fe-Al-Zn compounds begin to form at the coating/substrate interface after an annealing duration of 60 min, and the stripping resistance increases slightly before that duration and then decreases significantly by further increasing annealing duration. The enhanced stripping resistance at the earlier stage might be due to the modifications of microstructure and deformation compatibility of the sprayed coating, while the decreased stripping resistance at the later stage is related to the high stress-concentration at the interface of the formed brittle Fe-Al-Zn phase and the Zn-Al coating.
[Show abstract][Hide abstract] ABSTRACT: Nanolaminated (NL) structure has been produced in an interstitial-free steel by means of surface mechanical grinding treatment. The NL structure is characterized by an average lamella thickness of similar to 20 nm and also exhibits a strong deformation texture. Various dislocation substructures and individual dislocations exist inside these lamellae from submicron size to a few nanometers. Due to this extraordinary grain refinement, the NL structure exhibits a record hardness of 5.3 +/- 0.6 GPa.
Scripta Materialia 01/2015; 95(1). DOI:10.1016/j.scriptamat.2014.10.003 · 3.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Dry sliding tribological properties and worn subsurface microstructures were investigated in Cu–Al alloys with Al concentrations of 0–2.2 wt%. It was found that the wear volume of Cu–Al alloys decreases with an increasing Al content below 0.5 wt%, and increases at higher Al contents above 0.5 wt%. For each sample, a worn subsurface layer is generated consisting of dynamic recrystallization (DRX) structures beneath the top nanostructured mixing layer (NML). DRX grain sizes decrease monotonically with an increasing Al content. As the DRX grain sizes are larger than 0.7 μm, the wear-off process is dominated by cracking and peel-off of the NML. For the samples with finer DRX grains, cracking takes place within the DRX layer, of which peeling-off becomes a dominant mechanism. Such a wear mechanism transition is in agreement with the quantitative correspondence between wear volume and DRX grain size, indicating that the minimum wear volume (i.e., the maximum wear resistance) exists at an average DRX grain size of about 0.7 μm.
[Show abstract][Hide abstract] ABSTRACT: A novel type of duplex microstructure is generated in a single-phase austenitic steel (AISI 316L; X2CrNiMo19-12), consisting of plastically compliant recrystallized austenitic grains as the matrix containing coarse non-recrystallized grains with a nanotwinned austenitic (nt-c) structure as strengthening inclusions. This novel type of single-phase yet duplex microstructured steel exhibits an excellent combination of strength and ductility. We study the plastic co-deformation mechanisms between the nanotwinned and the recrystallized grains under tension using electron back-scatter diffraction (EBSD) and transmission electron microscopy (TEM). At tensile strains below 5%, the nt-c grains nearly deform homogeneously in conjunction with the surrounding statically recrystallized (SRX) grains without generating notable strain localization near their interfaces. The aniso-tropic plastic deformation of the nt-c grains with predominant shear parallel to the twin boundaries results in a higher dislocation density in the neigh-boring SRX grains. As the strain exceeds 12%, localized deformation occurs within the nt-c grains in the form of shear banding. A strain gradient is developed in the surrounding SRX grains as a function of distance from the nt-c/SRX interface. Deformation twinning is observed in the SRX grains near the nt-c grains, while away from nt-c grains dislocation slip dominates the deformation. The strengthening effect of the strong and ductile nt-c grains may offer a novel approach to strengthen austenitic steels and related alloys by generating a nanotwinned/recrystallized duplex microstructure.
[Show abstract][Hide abstract] ABSTRACT: By means of surface mechanical attrition treatment (SMAT), a gradient nanostructured surface layer was fabricated on a ferritic-martensitic (F-M) steel plate. Its aluminizing behaviors were investigated during a packed aluminization process and a subsequent diffusion annealing treatment at lower temperatures. In comparison with the initial sample, a much thicker Al5Fe2 layer was formed on the SMAT sample after the packed aluminization, with a growth constant of ~ 3 times higher at 600 °C. The transformation kinetics from Al5Fe2 phase into AlFe phase and α-(Fe,Al) solid solution are also enhanced in the subsequent annealing treatment at ~ 700 °C. The enhanced aluminizing kinetics originates from the increased atomic diffusivities by numerous grain boundaries and a higher concentration of vacancies in the nanostructured surface layer. With the determined growth kinetics, a duplex aluminizing process was demonstrated for achieving a gradient surface layer with lower-Al containing aluminides below the tempering temperature of F-M steel.
[Show abstract][Hide abstract] ABSTRACT: Dynamic plastic deformation followed by recovery annealing of an austenitic stainless steel results in the formation of a hierarchical microstructure consisting of nanotwinned austenitic grains (>55 vol.%) mixed with nanograins and dislocation structures. The sample exhibits a yield strength of 1055 MPa and a uniform elongation of ∼5.2% with a considerable work hardening. Such a remarkable tensile ductility originates from the intrinsic plasticity of the nanotwinned austenitic grains in which dislocation density is reduced after the recovery annealing.
Scripta Materialia 08/2014; s 84–85:31–34. DOI:10.1016/j.scriptamat.2014.04.008 · 3.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Lamellar nanostructures were induced in a plain martensitic low-carbon steel by using dynamic plastic deformation at room temperature. The nanostructured steel was hardened after annealing at 673 K for 20 min, with a tensile strength increased from 1.2 GPa to 1.6 GPa. Both the remained nanostructures and annealing-induced precipitates in nano-scale play key roles in the hardening.
Journal of Materials Science and Technology -Shenyang- 08/2014; 30(8). DOI:10.1016/j.jmst.2014.03.008 · 1.61 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: With surface mechanical grinding treatment, a gradient nanograined (GNG) surface layer is produced on a bulk coarse-grained (CG) pure Cu, where the grain size increases gradually from 20 nm (topmost surface) to micrometer scale. Microhardness measurements of the GNG/CG sample after tension revealed that tension induces softening for grains smaller than 165 nm and induces hardening above this size. This critical size agrees with the strain-induced saturation grain size of Cu subjected to severe plastic deformation.
Scripta Materialia 04/2014; 77:17–20. DOI:10.1016/j.scriptamat.2014.01.006 · 3.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Heavy plastic deformation may refine grains of metals and make them very strong. But the strain-induced refinement saturates at large strains, forming three-dimensional ultrafine-grained (3D UFG) structures with random orientations. Further refinement of this microstructure is limited because of the enhanced mobility of grain boundaries. Very-high-rate shear deformation with high strain gradients was applied in the top surface layer of bulk nickel, where a 2D nanometer-scale laminated structure was induced. The strongly textured nanolaminated structure (average lamellar thickness of 20 nanometers) with low-angle boundaries among the lamellae is ultrahard and ultrastable: It exhibits a hardness of 6.4 gigapascal--which is higher than any reported hardness of the UFG nickel--and a coarsening temperature of 40 kelvin above that in UFG nickel.
[Show abstract][Hide abstract] ABSTRACT: A gradient nanograined (GNG) surface layer was formed on a martensitic stainless steel bar sample by means of the surface mechanical grinding treatment (SMGT). The average grain size is similar to 25 nm on the topmost surface layer and increases gradually with increasing depth. The torsion fatigue strength is elevated by 38% with the GNG surface layer compared with the original material. An additional 8% increment in fatigue strength is achieved after a post-annealing treatment of the SMGT sample. By analysing the microstructure, hardness, surface roughness and residual stress distribution in the SMGT samples, we believe that the enhanced fatigue resistances originate from the GNG structure with a hard surface layer and a high structural homogeneity.
[Show abstract][Hide abstract] ABSTRACT: Cyclic deformation was studied in Cu samples with a gradient nanograined (GNG) surface layer. Compared with the coarse-grained sample, the Cu samples with a GNG surface layer exhibit a greatly enhanced fatigue limit under stress-controlled cyclic deformation. The cyclic deformation induced an abnormal grain coarsening that initiated from the subsurface layer and grew along 45 degrees to the stress axis toward the top surface layer, where the fatigue cracks were formed. (c) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
Scripta Materialia 05/2013; 68(10):801–804. DOI:10.1016/j.scriptamat.2013.01.031 · 3.22 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Bulk nanostructured Cu sample with nano-scale twin bundles embedded in nano-sized grains was synthesized by using dynamic plastic deformation (DPD) technique. Dry sliding tribological properties of the DPD Cu and the coarse grained (CG) Cu samples were investigated under liquid nitrogen temperature (LNT) in comparison with room temperature (RT) conditions. Experimental results show that the wear volume under LNT was much larger than that under RT for both sets of Cu samples. The DPD Cu sample exhibits almost same wear volume compared with the CG Cu when sliding at LNT, which is quite different from the enhanced wear resistance for the DPD Cu sample sliding at RT. A similar steady worn subsurface structure was formed for two Cu samples, which was constituted by heavily deformed nanostructured mixing layer (NML) and ultra-fine grained dynamic recrystallization (DRX) layer. Comparing with the worn subsurface structure at RT, lower sliding temperature could effectively suppress grain growth within the DRX layer, resulting in a cracked NML and extremely fine grains in the DRX layer. When sliding at LNT, quick propagation of cracks from NML into DRX layer and flaking rate of NML are determining the wear process which results in a high wear rate.
[Show abstract][Hide abstract] ABSTRACT: To minimize pores or voids that left after electro-healing by plain solution without additives, controllable electro-healing was investigated in pure Ni plates with a through-thickness crack in a Watts bath by adding polyethyleneimine (PEI) and saccharin. Controllable electro-healing means that the growth velocity of healing crystals from crack tips is much faster than that from sidewalls of the crack, which reduces the number of pores owing to depletion of healing solution. Rotating disk electrode (RDE) was introduced to evaluate the electrochemical behavior in crack tips and crack center. Voltammograms and galvanostatic measurements indicated that PEI exhibits a convection-dependent inhibition effect on Ni deposition. The addition of saccharin will much enhance the inhibition behavior of PEI, controllable electro-healing was achieved with the presence of both PEI and saccharin.
Journal of The Electrochemical Society 03/2013; 160(6):D289-D293. DOI:10.1149/2.140306jes · 3.27 Impact Factor