Dense Dislocation Walls Research Articles

Microstructural evolution and strain hardening behavior during plastic deformation of Fe–12Mn–8Al–0.8C steel

Microstructural evolution and deformation mechanism of a Fe–12Mn–8Al–0.8C steel were investigated. The steel exhibited a good combination of strength over 900MPa and ductility about 46%. The microstructure of the Fe–12Mn–8Al–0.8C steel in solid solution state was a mixture of the (α+γ+κ) phases. The κ phase carbides ((Fe,Mn)3AlCx) were formed by a spinodal decomposition during quenching. The deformation behavior of the austenite in the early deformation stage was fully determined by the size of the dislocation substructure, namely, dislocations alignment and Taylor lattices. With strain increasing, a gradual transition from Taylor lattices to highly dense dislocation walls and microbands was observed in austenite. Ferrite exhibited randomly distributed dislocations at a low strain and high dense dislocation tangles at a strain of 20%. TEM observations revealed ordered κ-carbide precipitates were sheared by slip bands at a strain of 30%. The present steel presented a multiple-stage strain hardening behavior which was associated with the changes of such dislocation substructures. The steel also exhibited a high initial strain hardening rate and a high specific strength about 130.5MPacm3/g.

Modeling and simulation on ultrafine-graining based on multiscale crystal plasticity considering dislocation patterning

Ultrafine-grained metals whose grain size is less than one micron have attracted interest as high strength materials. However, a mechanism of ultrafine-graining based on evolution of dislocation structures has not been clarified. In this study, we derive reaction–diffusion equations for dislocation patterning of dislocation cell structures and subgrains. In order to express the generation of dislocation pattern responding to deformation progress, information of slip rate and stress and effect of interactions between slip systems on formation of cell structures are introduced into the reaction rate coefficients of reaction–diffusion equations. Moreover, we propose a multiscale crystal plasticity model based on dislocation patterning. Then we carry out pseudo-three-dimensional FE-FD hybrid simulation for severe compression of FCC polycrystal using the present model. Some processes of ultrafine-graining, i.e., generation of dislocation cell structures, subgrains, dense dislocation walls and lamella subdivisions with high angle boundaries are numerically reproduced, and we investigate the effect of dislocation behavior on the processes of ultrafine-graining.

Strain-induced refinement and thermal stability of a nanocrystalline steel produced by surface mechanical attritiontreatment

A surface layer with a depth-dependent nanocrystalline microstructure was formed on reduced activation ferrite-martensitic (RAFM) steel by means of surface mechanical attrition treatment (SMAT). Equiaxed nano-sized grains in the surface layer were characterized by XRD, SEM and TEM, and the result indicated that the grain size decreased gradually with increasing deformation strain and strain rate. Compared with the size and number of carbides in the matrix, smaller and fewer carbides were detected in the SMAT layer, which indicated the refining and dissolving process of carbide during the drastic deformation in the SMAT process. Sub-grain boundaries (sub-GB) and high dense dislocation walls (DDWs) in ferrite were found in deformation layer, and dislocation accumulation, rearrangement and annihilation during the drastic deformation were discussed in detail. Both TEM images and XRD results demonstrate that the nanocrystalline layer has excellent thermal stability after annealing at 823K.

Banding in single crystals during plastic deformation

A rigid-plastic rate-independent crystal plasticity model capable of capturing banding in single crystals subjected to homogeneous macroscopic deformation is proposed. This model treats the single crystal as a ‘stack of domains’. Individual domains deform homogeneously while maintaining velocity and traction continuity with their neighbors. All the domains collectively accommodate the imposed deformation. The model predicts lattice orientation evolution, slip distribution, strain localization and band orientation in copper single crystals with imposed plane strain deformation. In quantitative agreement with experimental observations reported in the literature, macroscopic shear banding and regular deformation banding are predicted in initially copper and rotated cube oriented single crystals, respectively, while banding is not predicted in initially Goss oriented single crystals. The model does not, however, predict the experimentally observed orientation of smaller scale dislocation boundaries such as dense dislocation walls.

The evolution of cold-rolled deformation microstructure of {0 0 1}〈1 1 0〉 grains in Ta–7.5 wt%W alloy foils

The deformation structure of {0 0 1}〈1 1 0〉 grains in cold-rolled Ta–7.5 wt%W alloy foils has been investigated by transmission electron microscopy (TEM) and electron backscattering diffraction (EBSD). The results show that there are two kinds of {0 0 1}〈1 1 0〉 grains formed during the cold-rolled deformation process. Type A grains are surrounded by α-fiber, while type B grains are surrounded by γ-fiber. Dislocation analysis reveals a continuous evolution of dislocation substructures from dislocation arrays, to high dense dislocation walls, and to microbands in type A grains, while in type B grains from dislocation mesh to dislocation cells as a function of plastic strain. By both the analysis of EBSD and TEM, a new deformation mode in type A grains is developed.

Effect of eutectic Si on surface nanocrystallization of Al–Si alloys by surface mechanical attrition treatment

Pure Al (AA196) and Al–Si (A356) alloy were subjected to surface nanocrystallization through surface mechanical attrition treatment (SMAT). Strain induced microstructure evolution, including grain refinement of Al matrix and Si particles, was examined using transmission electron microscopy. Nanocrystallization of Al matrix in both pure Al and A356 alloy occurs through formation of dislocation cells separated by dense dislocation walls and dislocation tangle within the original coarse grains or subdivided subgrains. During SMAT, the brittle Si phase is preferentially refined through direct breakage. The Al matrix refinement process is greatly facilitated by the dispersed small Si particles, because such particles can not only act as obstacles for dislocation movement, but also promote dislocation generation and multiplication. Comparing to pure Al, thicker nanocrystalline zone can be obtained with much smaller stabilized nano-metered grains in the Al–Si alloy. Due to the low diffusion rate of Si in Al matrix at low temperature, there is no obvious re-dissolve of Si particles into Al during SMAT process.

Grain refinement process of pure iron target under hypervelocity impact

The structure of ultrafine grain is formed at the crater bottom of pure iron target under hypervelocity impact. The microstructures of different layers at the crater bottom were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The cross-section observation was performed to reveal the grain refinement process driven by plastic deformation. Firstly, low energy dislocation structures (LEDS) such as dense dislocation walls (DDWs) and dislocation tangles (DTs) refine the original grains and form intersecting lamellar structures. With increasing strain, DDWs and DTs transform into subboundaries with small misorientations to separate lamellar structure to cells. Subboundaries are converted to high misorientation grain boundaries, so ultrafine grains are formed. The formation of ultrafine grains was discussed in the dynamic recrystallization process due to the large strain and strain rate induced by spherical shock wave.

The hot-deformability and quantitative description of the microstructure of hot-deformed Fe–Ni superalloy

The paper presents the results of research concerning the influence of hot plastic forming parameters on the deformability and structure of a Fe-Ni austenitic alloy. The research was performed on a torsion plastometer in the range of temperatures of 900-1150 °C, at a strain rate 0.1 and 1.0 s-1. Plastic properties of the alloy were characterized by the worked out flow curves and the temperature relationships of flow stress and strain limit. The structural inspections were performed on microsections taken from plastometric samples after so called "freezing". The stereological parameters as the recrystallized grain size, inhomogenity and grain shape have been determined. Functional relations between the Zener-Hollomon parameter and the peak stress and the mean grain size have been developed and the activation energy of the hot plastic deformation has been estimated. The examination of substructure on TEM allowed the calculation of structural parameters: the average subgrain area and the mean dislocation density. A detailed investigation has shown that the substructure is inhomogeneous, consists of dense dislocation walls, subgrains and recrystallized regions.

Tensile deformation micromechanisms for bulk metallic glass matrix composites: From work-hardening to softening

A Ti-based bulk metallic glass matrix composite (BMGMC) with a homogeneous distribution of dendrites and the composition of Ti 46Zr 20V 12Cu 5Be 17 is characterized by a high tensile strength of ∼1640 MPa and a large tensile strain of ∼15.5% at room temperature. The present BMGMC exhibits the largest tensile ductility and highest fracture absorption energy under the stress–strain curve of all dendrite-reinforced BMGMCs developed to date. Tensile deformation micromechanisms are explored through experimental visualization and theoretical analyses. After tension, fragmentation of the dendrites, rather than crystallization within the glass matrix and/or atom debonding near the interface of dual-phase composites, is responsible for the high tensile ductility. The subdivisions within the interior of dendrites are separated by shear bands and dense dislocation walls, and local separation of dendrites under modes I and II prevails. The multiplication of dislocations, severe lattice distortions, and even local amorphization dominate within the dendrites. Good structural coherency of the interface is demonstrated, despite being subjected to significant plastic deformation. Theoretical analyses reveal that the constitutive relations elastic–elastic, elastic–plastic, and plastic–plastic of dual-phase BMGMC generally correspond to the (1) elastic, (2) work-hardening, and (3) softening deformation stages, respectively. The capacity for work-hardening is highly dependent on the large plastic deformation of the dendrites and the high yield strength of the glass matrix. The present study provides a fundamental basis for designing work-hardening dual-phase BMGMCs exhibiting remarkably homogeneous deformation.

Microstructural evolution in NiTi alloy subjected to surface mechanical attrition treatment and mechanism

Both nanocrystalline and amorphous phases are observed from the near surface of nickel titanium shape memory alloy (NiTi SMA) with the B2 austenite phase after surface mechanical attrition treatment (SMAT). The microstructure and phase changes are systematically studied by cross-sectional and plane-view transmission electron microscopy. The strain induces grain refinement and it is accompanied by increased strain in the surface layer triggering the onset of highly dense dislocations and dislocation tangles (DTs), formation of the martensite plate via stress-induced martensite (SIM) transformation (B2 to B19′), and dislocation lines (DLs) as well as dense dislocation walls (DDWs) inside the martensite plate leading to the subdivision of the martensite plate. In addition, reverse martensite transformation (B19′ to B2) and amorphization take place concurrently in the surface region, and successive subdivision and amorphization finally result in the formation of well separated nanocrystalline and amorphous phases in the near surface. The average grain size of the nanocrystallites is about 20 nm. Owing to the almost complete reverse martensite transformation as well as thermal stability, the strain-induced nanocrystalline structure has the B2 austenite phase in the surface layer and no transformation occurs.

Microstructures in polycrystalline pure copper induced by high-current pulsed electron beamdeformation structures

In order to investigate the superfast deformation mechanism of metal, the high-current pulsed electron beam (HCPEB) technique is employed to irradiate the polycrystalline pure copper. The microstructure of the irradiated sublayer is investigated by using transmission electron microscopy. It is suggested that the stress with very high value and strain rate is introduced within the sublayer after HCPEB irradiation. The dislocation cell and the tangle dislocation formed by cross slip are the dominant defects after one-pulse HCPEB irradiation, whereas, dense dislocation walls and twins are the central microstructures after five- and ten-pulse irradiation. The diffusion and the climb of the atomic plane can cause the formation of the steps at the grain boundary and (or) the twin boundary. Based on the structure characteristics of the irradiated surface, the possible deformation mechanism induced by HCPEB irradiation is discussed.

A Multiscale Crystal Plasticity Simulation on Ultrafine-Graining Based on Dislocation Patterning

Ultrafine-grained metals whose grain size is less than one micron have attracted interest as high strength materials. However, a mechanism of ultrafine-graining based on evolution of dislocation structures has not been clarified. In this study, we derive reaction-diffusion equations for dislocation patterning of dislocation cell structures and subgrains. In order to express the generation of dislocation pattern responding to deformation progress, information of slip rate and stress and effect of interactions between slip systems on formation of cell structures are introduced into the reaction rate coefficients of reaction-diffusion equations. Moreover, we propose a multiscale crystal plasticity model based on dislocation patterning. Then we carry out two-dimensional FE-FD simulation for severe compression of FCC polycrystal using the present model. Some processes of ultrafine-graining, i.e., generation of dislocation cell structures, subgrains, dense dislocation walls and lamella subdivisions with high angle boundary is numerically reproduced, and we investigate the effect of dislocation behaviors on the processes of ultrafine-graining.

The Effect of Initial Microstructure on Surface Nanocrystallization of Quenched and Tempered Steel

Nanocrystalline surface layer was fabricated on a quenched and tempered Cr-Si alloy steel by using Surface mechanical treatment. The microstructure features of various sections in the surface layer were characterized by using transmission electron microscopy (TEM). By analyzing the microstructural characteristic at different depths in the treated surface, the effect of the initial microstructures on grain refinement process of quenched and tempered steel was investigated. Experimental evidence showed the initial subgrains with small angle boundary and lower dislocation density were firstly developed into Lamellar-type dislocation cells (DCs) with dense dislocation walls (DDWs). Some initial subboundaries were moved to DDWs by dislocation activities. The width of lamellar-type DCs was 2-3 times of that of initial lathy subgrains. The size of the DCs and subgrains formed in the interim of refinement process was not uniformity. On the top surface the cementite granules were decomposed or fragmented to hyperfine particles, and the size of the grains tended to uniformity. Experimental analysis indicated the configuration of microstructure was affect by the initial microstructure in the initial stage and the interim of the grain refinement process. Surface nanocrystallization of Cr-Si steel can be attributed to dislocation activities.

Dislocation boundaries and disclinations formed within the cube-oriented grains during tensile deformation of aluminium

The evolution of dislocation boundaries, comprising a distinct equiaxed cell block structure formed within the cube-oriented grains during tensile straining of an aluminium alloy AA5005, has been investigated in detail using transmission electron microscopy. The observed dislocation substructure displayed a hierarchical character resulting from a gradual development of larger-angle planar dense dislocation walls (DDWs), bounding roughly equiaxed cell blocks, on a background of slightly misoriented tangled cell boundaries. These walls displayed a complex, non-crystallographic character largely governed by the macroscopically imposed deformation geometry. A high degree of freedom in dislocation rearrangement, allowed by the present experimental conditions, resulted in a “random” arrangement of DDWs, in which misorientation vectors were largely scattered and misorientations across consecutive walls displayed a tendency to become more cumulative with increasing strain. The present observations appear to support the dislocation/disclination concept, describing the evolution of DDWs as nucleation, growth and immobilization of partial disclination dipoles.

Grain refinement of LY2 aluminum alloy induced by ultra-high plastic strain during multiple laser shock processing impacts

The plastic deformation behavior and the effects of the impact time on the LY2 aluminum (Al) alloy during multiple laser shock processing (LSP) impacts were investigated. The residual stress in the near-surface region was determined by X-ray diffraction. In addition, the micro-structural features of the hardening layer were characterized by scanning electron microscopy, optical microscopy and transmission electron microscopy. It was found that the micro-structure was obviously refined due to the ultra-high plastic strain induced by multiple LSP impacts. The minimum grain size in the top surface after multiple LSP impacts was about 100–200 nm. The grain refinement process after multiple LSP impacts can be described as follows: (i) the formation and development of dislocation lines in original grains; (ii) dislocation tangles (DTs) and the formation of dense dislocation walls (DDWs); (iii) transformation of DTs and DDWs into subgrain boundaries; and (iv) evolution of the continuous dynamic recrystallization in subgrain boundaries to refined grain boundaries.

The vacancy defect clusters in polycrystalline pure nickle induced by high-current pulsed electron beam

High-current pulsed electron beam (HCPEB) technique is employed to irradiate the samples of polycrystalline pure nickel. The microstructures of the irradiated surface layers are investigated by using transmission electron microscopy (TEM). After HCPEB irradiation, very high value of residual stress is induced in the irradiated surface layer, which leads to the formation of dense dislocation walls and twins. Furthermore, a larger number of vacancy defect clusters including dislocation loops, stacking fault tetrahedra (SFT) and voids are also formed. Among three vacancy defect clusters, the number of SFT is much more than that of two other vacancy defect clusters. The lower dislocation density near the regions with dense SFT is observed and voids are likely to be present in these regions. It suggests that the stress with very high value and strain rate induced by rapid heating and cooling due to HCPEB irradiation could cause the shifting of the whole atomic plane synchronously.

Effect of Dislocation Mechanisms during Extrusion of Nanostructured Aluminum Powder Alloy

Nanostructured Al-3.0Fe-0.42Cu-0.37Mn powder alloy was deformed by extrusion over a temperature range of 375 °C to 425 °C, a ram speed range of 1 to 30 mm/s, and an extrusion rate of 10:1. Flow stresses and strain rates were calculated from the experimental ram pressures and speeds. The stress–strain-rate–temperature relationship in the extrusion of the nanostructured alloy was found to be similar to that in hot-worked conventional materials. The extrusion, torsion, compression, and creep data of nanostructured and conventional materials, extending over ten orders of magnitude of strain rate and over two orders of magnitude of stress, were correlated by a hyperbolic-sine constitutive equation, because the power and exponential laws lose linearity at high and low stresses, respectively. The hyperbolic-sine equation is widely used to correlate the hot-working behavior of conventional materials. It was concluded that the hot working of nanostructured powders is a thermally activated process in which the rate-controlling mechanism is the climb of edge dislocations. Microstructural changes in the consolidated alloys as a function of the extrusion conditions were investigated. An analysis was made of the dislocation behavior in very small grains of nanostructured metal by transmission electron microscopy (TEM) and we identified a dislocation structure and the different ways it appears in 40- to 100-nm Al-alloy grains. We also discuss the thermally activated propagation of dislocations and their interactions with shear bands/grain boundaries (SBs/GBs), and dislocation loops. Microstructural features including low-angle GBs, high-angle GBs, and equilibrium GBs and subgrain boundaries were observed. Dislocation structures under a deformation condition were studied to investigate the microstructural evolutions, which revealed some unique microstructural features such as dislocation tangle zones (DTZs) and dense-dislocation walls (DDWs), and the recovery process is discussed herein.

Surface nanostructure formation mechanism of 45 steel induced by supersonic fine particles pombarding

By means of supersonic fine particles bombarding (SFPB), a nanostructured surface layer up to 15 μm was fabricated on a 45 steel plate with ferrite and pearlite phases. To reveal the grain refinement mechanism of SFPB-treated 45 steel, microstructure features of various sections in the treated surface were systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Grain size increases with an increase of depth from the treated surface. Plastic deformation and grain refinement processes are accompanied by an increase in strain. Plastic deformation in the proeutectoid ferrite phases has precedence over the pearlite phases. Grain refinement in the ferrite phases involves: the onset of dislocation lines (Dls), dislocation tangles (DTs) and dense dislocation walls (DDWs) in the original grains; the formation of fine lamellar and roughly equiaxed cells separated by DDWs; by dislocation annihilation and rearrangement, the transformation of DDWS into subboundaries and boundaries and the formation of submicron grains or subgrains; the successive subdivision of grains to finer and finer scale, resulting in the formation of highly misoriented nano-grains. By contrast, eutectoid cementite phase accommodated strain in a sequence as follows: onset of elongated, bended and shear deformation under deformation stress of ferrites, short and thin cementites with a width of about 20-50 nm and discontinuous length were formed. Shorter and thinner cementites were developed into ultra-fine pieces under the action of high density dislocation and strains. At the top surface, some cementites were decomposed under severe plastic deformation. Experimental evidences and analysis indicate that surface nanocrystallization of 45 steel results from dislocation activities, high strains and high strain rate are necessary for the formation of nanocrystallites.

Multiscale Crystal Plasticity Simulation with Pseudo-Three-Dimensional Model on Ultrafine-Graining Based on Evolution of Dislocation Structures

In this study, we develop a multiscale crystal plasticity model that represents evolution of dislocation structure on formation process of ultrafine-grained metal based both on dislocation patterning and geometrically necessary dislocation accumulation. A computation on the processes of ultrafine-graining, i.e., generation of dislocation cell and subgrain patterns, evolution of dense dislocation walls, its transition to micro-bands and lamellar dislocation structure and formation of subdivision surrounded by high angle boundaries, is performed by use of the present model. Dislocation patterning depending on activity of slip systems is reproduced introducing slip rate of each slip system into reaction-diffusion equations governing self-organization of dislocation structure and increasing immobilizing rate of dislocation with activation of the secondary slip system. In addition, we investigate the effect of active slip systems to the processes of fine-graining by using the pseudo-three-dimensional model with twelve slip systems of FCC metal.

The Investigation of Surface Nanocrystallization of Structural Steel Induced by Supersonic Fine Particles Bombarding

The nanostructured surface layers were fabricated on a hardened and tempered chrome-silicon alloy steel and a normalized medium carbon steel by mean of Supersonic Fine Particles Bombarding (SFPB). The microstructure features in the treated surface layer were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations. Experimental results show the nanostructured surface layer is fabricated on both samples after SFPB treatment. The microstructure of the top surface is characterized by uniformly distributed nano-scale grains with equiaxed shape and random crystallographic orientations. The mean size of equiaxed nanocrystallites on the top surface layer is approximately 15-20nm for the SFPB treated medium carbon steel and Chrome-silicon alloy steel. During severe deformation the grain refinement in ferrite and cementite phases is observed, the cementite phases are exposed to breaking and dissolution due to mechanical alloying resulting in the formation of a supersaturated solid solution of carbon in α-Fe matrix. In the ferrite phase, the grains are refined by the process of dislocation actives and forming cell structures separated by dense dislocation walls (DDWs), as well as evolution of dislocation to subboundaries and grain boundaries.