Strain-path effects on the evolution of microstructure and texture during the severe-plastic deformation of aluminum

Microstructure and texture evolution during the severe-plastic deformation (SPD) of unalloyed aluminum were investigated to establish the effect of processing route and purity level on grain refinement and subgrain formation. Two lots of aluminum with different purity levels (99.998 pct Al and 99 pct Al) were subjected to large plastic strains at room temperaturevia four different deformation processes: equal-channel angular extrusion (ECAE), sheet rolling, conventional conical-die extrusion, and uniaxial compression. Following deformation, microstructures and textures were determined using orientation-imaging microscopy. In commercial-purity aluminum, the various deformation routes yielded an ultrafine microstructure with a ∼1.5-μm grain size, deduced to have been formedvia a dynamic-recovery mechanism. For high-purity aluminum, on the other hand, the minimum grain size produced after the various routes was ∼20 μm; the high fraction of high-angle grain boundaries (HAGBs) and the absence of subgrains/deformation bands in the final microstructure suggested the occurrence of discontinuous static recrystallization following the large plastic deformation at room temperature. The microstructure differences were underscored by the mechanical properties following four ECAE passes. The yield strength of commercial-purity aluminum quadrupled, whereas the high-purity aluminum showed only a minor increase relative to the annealed condition.     

Microstructural evolution and superplasticity of Al-5.8Mg-0.23Mn alloys processed by reciprocating extrusion

This study developed the reciprocating extrusion method to refine the inclusions and grain structure of Al-5.8Mg-0.23Mn alloys to enhance their strength and superplasticity without prior homogenization treatment. Alloy cast billets were extruded with an extrusion ratio of 10:1 at 450 °C for one, five, or ten passes. The grain size was reduced to 4.6 μm, and the coarse inclusions refined to 2 μm, after ten passes. A subgrain structure was formed in the interior of the fine grains, indicating that dynamic recrystallization occurred during extrusion. In this study, dynamic recrystallization in the billet was repeatedly induced by a number of extrusion passes until a limiting grain size was obtained. Thereafter, dynamic recrystallization was no longer activated because grain boundary sliding, instead of dislocation gliding, accommodated the deformation strain required for extrusion. The alloys extruded in ten-passes extrusion were found to be stronger and more ductile than commercial Al-Mg alloys and showed improved superplastic behavior at 500 °C not only from low to high strain rate but also with a small flow stress of less than 30 MPa. These advantages demonstrate that reciprocating extrusion can produce Al-Mg alloys with improved mechanical properties making them good candidates for high-strain-rate superplastic forming.     

A model for microstructure evolution in adiabatic shear bands

A mechanical subgrain rotation model is proposed to account for the recrystallized grains which have been observed to form in adiabatic shear bands in a number of materials. The model is based on a “bicrystal” approach using crystal plasticity theory to predict the evolution of subgrain misorientations. These mechanically induced rotations are shown to occur at the high strain rate associated with adiabatic shear band formation. Recrystallized grain formation is proposed to occur by the formation and mechanical rotation of subgrains during deformation, coupled with boundary refinement via diffusion during shear band cooling. This model is referred to as progressive subgrain misorientation recrystallization and appears to account for shear band microstructures in a variety of metals.     

Microstructural Evolution and Mechanical Response of Equal-Channel Angular Extrusion-Processed Al-40Zn-2Cu Alloy

Microstructural evolution, tensile properties, and impact toughness of an aluminum-zinc-copper (Al-40Zn-2Cu) alloy subjected to repetitive equal-channel angular extrusion (ECAE) up to four passes following either route A or route B_C were investigated. The experimental results reveal that the ECAE eliminated as-cast dendritic microstructure along with casting defects such as microporosities almost completely. The ECAE-processed samples consisted of mostly elongated microconstituents via route A and equiaxed microconstituents via route B_C. The high stresses imposed in ECAE lead to the fragmentation of the copper-rich θ phase into smaller particles with significant fragmentation occurring in the first pass and additional breaking in the subsequent passes in both routes. The ECAE processing simultaneously increased both the strength and ductility of the alloy as compared to the as-cast state, regardless of the processing route and number of passes. The deformation behavior of as-cast Al-40Zn-2Cu alloy has changed from brittle to ductile mode after ECAE due to the microstructural refinement, deformation-induced homogenization, and reduction of porosities. The limited impact toughness of as-cast alloy was significantly improved by multipass ECAE, especially in route A.     

Adiabatic shear bands on the titanium side in the titanium/mild steel explosive cladding interface: Experiments, numerical simulation, and microstructure evolution

The microstructure and microtexture in adiabatic shear bands (ASBs) on the titanium side in the titanium/mild steel explosive cladding interface are investigated by means of optical microscopy, scanning electron microscopy/electron backscattered diffraction (SEM/EBSD), and transmission electron microscopy (TEM). Highly elongated subgrains and fine equiaxed grains with low dislocation density are observed in the ASBs. Microtextures (25 deg, 75 deg, 0 deg), (70 deg, 45 deg, 0 deg), and (0 deg, 15 deg, 30 deg) formed within the ASBs suggest the occurrence of the recrystallization. The grain boundaries within ASBs are geometrically necessary boundaries (GNBs) with high angles. Finite element computations are performed to obtain the effective strain and temperature distributions within the ASBs under the measured boundary conditions. The rotation dynamic recrystallization (RDR) mechanism is employed to describe the kinetics of the nanograins’ formation and the recrystallized process within ASBs. During the deformation time (about 5 to 10 μs), the following processes take place: dislocations accumulate to form elongated cell structures, cell structures break up to form subgrains, and subgrains rotate and finally form recrystallized grains. The small grains within ASBs are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

铝合金等通道挤压晶粒细化机制

本文讨论铝合金在等通道挤压过程中的晶粒细化机制。发生的晶粒细化主要通过三种机制完成：1)取向分裂诱发形变带;2)应变集中产生的宏观或微观剪切带;3)高角度晶界随应变增加。形变条件和路径、模具几何及材料参数决定形变组织的演化。亚结构和显微剪切带的取向与模具剪切面一致但在原则上与材料的晶体位错滑移系统无关。形变带的晶体取向倾向接近在路径A下稳定织构的取向。在高应变,由于显微组织的压缩和拉长造成的晶界面积增加成为主要晶粒细化机制。变形至一定应变后,形变进入稳态,晶粒细化不再发生。     

Deformation Structures of Pure Titanium during Shear Deformation

The deformation microstructure of commercial pure (CP) titanium formed in the theoretical shear zone of an equal channel angular pressing (ECAP) die during 3 or 4 passes is investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The typical feature of the microstructure is that ultrafine grains coexist with coarse elongated grains with a high density of low-angle grain boundaries (LAGBs). Dislocation tangle zones (DTZs), dislocation cells (DCs), and subgrains are generated during shear deformation. The primary twin type has been found to be { 10[`1]2 }. \left\{ {10\bar{1}2} \right\}. Grain refinement appears to progress by continuous dynamic recrystallization (CDRX), in which dislocation movement to LAGBs leads to their evolution into high-angle grain boundaries (HAGBs).     

Recrystallization Phenomena During Friction Stir Processing of Hypereutectic Aluminum-Silicon Alloy

Microstructural evolution and related dynamic recrystallization phenomena were investigated in overlapping multipass friction stir processing (FSP) of hypereutectic Al-30 pct Si alloy. FSP resulted in the elimination of porosities along with the refinement of primary silicon particles and alpha aluminum grains. These alpha aluminum grains predominantly exhibit high angle boundaries with various degrees of recovered substructure and dislocation densities. The substructure and grain formation during FSP take place primarily by annihilation and reorganization of dislocations in the grain interior and at low angle grain boundary. During multipass overlap FSP, small second phase particles were observed to form, which are accountable for pinning the grain boundaries and thus restricting their growth. During the multipass overlap FSP, the microstructure undergoes continuous dynamic recrystallization by formation of the subgrain boundary and subgrain growth to the grain structure comprising of mostly high angle grain boundaries.     

Microstructure evolution and mechanical behavior of bulk copper obtained by consolidation of micro- and nanopowders using equal-channel angular extrusion

The consolidation of copper micro- and nanoparticles (325 mesh, 130 nm, and 100 nm) was performed using room-temperature equal-channel angular extrusion (ECAE). The effects of extrusion route, number of passes, and extrusion rate on consolidation performance were evaluated. The evolution of the microstructure and the mechanical behavior of the consolidates were investigated and related to the processing route. Possible deformation mechanisms are proposed and compared to those in ECAE-processed bulk Cu. A combined high ultimate tensile stress (470 MPa) and ductility (∼20 pct tensile fracture strain) with near-elasto-plastic behavior was observed in consolidated 325-mesh Cu powder. On the other hand, early plastic instability took place, leading to a continuous softening in flow stress of bulk ECAE-processed copper. Increases in both strength and ductility were evident with an increasing number of passes in the bulk samples, which appears to be inconsistent with grain-boundary-moderated deformation mechanisms for a microstructure with an average grain size of 300 to 500 nm. Instead, this increase is attributed to microstructural refinement and to dynamic recovery and bimodal grain-size distribution. Near-perfect elastoplasticity in consolidated 325-mesh Cu powder is explained by a combined effect of strain hardening accommodated by large grains in the bimodal structure and softening caused by recovery mechanisms. Compressive strengths as high as 760 MPa were achieved in consolidated 130-nm copper powder. Although premature failure occurred during tensile loading in 130-nm consolidated powder, the fracture strength was still about 730 MPa. The present study shows that ECAE consolidation of nanoparticles opens a new possibility for the study of mechanical behavior of bulk nanocrystalline (NC) materials, as well as offering a new class of bulk materials for practical engineering applications.     

Modeling recrystallization kinetics,grain sizes,and textures during multipass hot rolling

A physically based model for the evolution of recrystallization microstructures and textures during hot rolling of aluminum is presented. The approach taken differs from similar models developed for steels. The present model is based on recent experimental investigations directed toward identifying the nature of the nucleation sites for recrystallized grains of different crystallographic orientations. Particle stimulated nucleation (PSN) and nucleation from cube bands and grain boundary regions have been incorporated in the model. The multipass aspect complicates the modeling due to partial recrystallization between the rolling passes. Two different approaches have been suggested to handle this. The model has been applied to predictions of recrystallization kinetics, recrystallized grain sizes, and recrystallization textures during multipass hot rolling of aluminum. The predictions are reasonable compared to experimental results.     

Quantitative Microstructural Characterization of Thick Aluminum Plates Heavily Deformed Using Equal Channel Angular Extrusion

A detailed quantitative analysis of the microstructure has been performed in three orthogonal planes of 15-mm-thick aluminum plates heavily deformed via two equal channel angular extrusion (ECAE) routes. One route was a conventional route A with no rotation between passes. Another route involved sequential 90?deg rotations about the normal direction (ND) between passes. The microstructure in the center of these plates, and especially the extent of microstructural heterogeneity, has been characterized quantitatively and compared with that in bar samples extruded via either route A or route Bc with 90?deg rotations about the longitudinal axis. Statistically robust data were obtained in this work using gallium enhanced microscopy and EBSD mapping of large sample areas. For the plate processed using route A, the fraction of high-angle boundaries was found to strongly depend on the inspection plane, being smallest in the plane perpendicular to the ND (plane Z), where the largest subgrain size and most profound microstructural heterogeneities were also revealed. In comparison, the plate extruded with 90?deg rotations about the ND was less heterogeneous and contained smaller subgrains in plane Z. Comparing the plate and bar samples, the most refined and least heterogeneous microstructure was observed in the route Bc bar sample. The differences in the microstructure are reflected in the hardness data; the hardness is lowest after ECAE via route A and greatest in the bar sample processed using route Bc.     

Evolution of Grain-Boundary Microstructure and Texture in Interstitial-Free Steel Processed by Equal-Channel Angular Extrusion

The equal-channel angular extrusion (ECAE) of Ti-bearing interstitial-free (IF) steel was performed following two different routes, up to four passes, at a temperature of 300 °C. The ECAE led to a grain refinement to submicron size. After the second pass, the grain size attained saturation thereafter. The microstructural analysis indicated the presence of coincident-site lattice (CSL) boundaries in significant fraction, in addition to a high volume fraction of high-angle random boundaries and some low-angle boundaries after the deformation. Among the special boundaries, Σ3 and Σ13 were the most prominent ones and their fraction depended on the processing route followed. A deviation in the misorientation angle distribution from the Mackenzie distribution was noticed. The crystallographic texture after the first pass resembled that of simple shear, with the {112}, {110}, and {123} aligned to the macroscopic shear plane.     

Development of adiabatic shear bands in annealed 316L stainless steel: Part II. TEM studies of the evolution of microstructure during deformation localization

The evolution of adiabatic shear localization in an annealed AISI 316L stainless steel has been investigated and was reported in Part I of this paper (Met. Trans. A, 2006, Vol. 37A, pp. 2435–446). In the present research (Part II), a comprehensive transmission electron microscopy (TEM) examination was conducted on the microstructural evolution of shear localization in this material at different loading stages. The TEM results indicate that elongated subgrain laths and an avalanche of dislocation cells are the major characteristics in an initiated band. Development of the substructures within shear bands is controlled by dynamic recovery and continuous dynamic recrystallization. The core of shear bands was found to consist of fine equiaxed subgrains. Well-developed shear bands are filled with a mixture of equiaxed, rectangular, and elongated subgrains. The equiaxed subgrains, with a typical size less than 100 nm, are postulated to result from either the breakdown and splitting of subgrain laths or the reconstruction of subcells.     

Dynamic restoration mechanisms in Al-5.8 At. Pct Mg deformed to large strains in the solute drag regime

An Al-5.8 at. pct Mg (5.2 wt pct Mg) alloy was deformed in torsion within the solute drag regime to various strains, up to the failure strain of 10.8. Optical microscopy (OM) and transmission electron microscopy (TEM) were used to analyze the evolution of the microstructure and to determine the dynamic restoration mechanism. Transmission electron microscopy revealed that subgrain formation is sluggish but that subgrains eventually (ε ≈ 1) fill the grains. The “steady-state” subgrain size (λ ≈ 6 μm) and misorientation angle (θ ≈ 1.6 deg) are reached by ε ≈ 2. These observations confirm that subgrains eventually form during deformation in the solute drag regime, though they do not appear to significantly influence the strength. At low strains, nearly all of the boundaries form by dislocation reaction and are low angle (θ < 10 deg). At a strain of 10.8, however, the boundary misorientation histogram is bimodal, with nearly 25 pct of the boundaries having high angles due to their ancestry in the original grain boundaries. This is consistent with OM observations of the elongation and thinning of the original grains as they spiral around the torsion axis. No evidence was found fordiscontinuous dynamic recrystallization, a repeating process in which strain-free grains nucleate, grow, deform, and give rise to new nuclei. It is concluded that dynamic recovery in the solute drag regime gives rise togeometric dynamic recrystallization in a manner very similar to that already established for pure aluminum, suggesting that geometric dynamic recrystallization may occur generally in materials with a high stacking-fault energy (SFE) deformed to large strains.     

TEM studies on recovery and recrystallisation in Equal Channel Angular Extrusion processed Al-3%Mg alloy

Equal Channel Angular Extrusion (ECAE) is a promising severe plastic deformation (SPD) process which can produce polycrystalline materials with ultra-fine grains (UFG) of sub micrometer range or nanometer range. Large plastic shear deformation induced by the high applied pressure in ECAE material processing is the prime reason behind the grain refinement. The focus of the present work is to study the evolution of dislocation microstructure during dynamic recovery (due to intense strain deformation) and static recovery (due to static annealing after deformation) in commercial Al-3%Mg alloy processed by ECAE. It is observed that local concentrations of shear strain can take place and high angle boundary (HAGB) segments are formed initially at random locations. When thermal energy is provided, during static annealing, the boundary segments get further defined and extended. This leads to the formation of very fine size grains with high mis-orientations which subsequently develop into an ultra-fine grain distribution in the material. Also, it appears dynamic recrystallisation (DRX) occurring during the deformation itself is a general phenomenon leading to refinement of grains. Transmission Electron Microscopy (TEM) is the characterizing tool used in the present study. The influence of precipitates/second phase particles on the deformation characteristics and on the increased degree of grain fragmentation is also detailed.     

Cu-P-Cr-Ni-Mo耐候钢双相区多道次轧制模拟的组织演变

 为了实现Cu-P-Cr-Ni-Mo耐候钢的铁素体晶粒细化从而充分提高其强塑性,通过热模拟压缩试验,利用金相、SEM、EBSD等微观组织分析方法研究了其在双相区的多道次压缩变形过程中的组织演变。结果表明,试验钢在变形过程中,第二相（马氏体、贝氏体）呈条带状分布于铁素体基体上,随着道次增加,铁素体晶粒逐步细化,第5道次变形后得到1.8 μm左右的超细晶铁素体。前期铁素体晶粒细化的主要机制是形变强化铁素体相变,即多道次的累积大变形使组织内畸变能增大,铁素体形核点增多,促进铁素体快速析出,形成细小铁素体晶粒;后面几道次变形中,随着应变量继续增大,在铁素体晶粒内形成大量亚晶界,且亚晶界逐步累积扭转成大角度晶界,分割原来的粗大晶粒,发生铁素体连续动态再结晶细化。     

Precipitation and grain refinement in a 2195 Al friction stir weld

The microstructure across a friction stir weld in aluminum alloy 2195 was analyzed to reveal the precipitation processes, grain evolution mechanisms, and crystallographic texture within that weld. The complex microhardness variations across the weld are explained by the observed precipitation sequence, in which the original precipitates coarsen and dissolve during welding, and are then replaced by different precipitates, which form during cooling. The grain development from the thermomechanically affected zone (TMAZ) into the weld nugget reveals that subgrains form within the TMAZ grains and develop increasing boundary misorientations through continuous dynamic recrystallization by subgrain rotation to eventually form the refined grains observed within the weld nugget. Within the weld nugget, a {112}〈110〉 texture is observed, corresponding to a high strain/high temperature shear strain component.     

Numerical Simulation of Austenite Recrystallization in CSP Hot Rolled C-Mn Steel Strip

An integrated mathematical model is developed to predict the microstructure evolution of C-Mn steel during multipass hot rolling on the CSP production line,and the thermal evolution,the temperature distribution,the deformation,and the austenite recrystallization are simulated.The characteristics of austenite recrystallization of hot rolled C-Mn steel in the CSP process are also discussed.The simulation of the microstructure evolution of C-Mn steel ZJ510L during CSP multipass hot rolling indicates that dynamic recrystallization and metadynamic recrystallization may easily occur in the first few passes,where nonuniform recrystallization and inhomogeneous grain size microstructure may readily occur;during the last few passes,static recrystallization may occur dominantly,and the microstructure will become more homogeneous and partial recrystallization may occur at relatively low temperature.