Orientations of nuclei during static recrystallization in a cold-rolled Mg-Zn-Gd alloy

The static recrystallization process of a cold-rolled Mg-Zn-Gd alloy was tracked by a quasi-in-situ electron backscatter diffraction method to investigate the orientations of nuclei.The results show that orientation distribution of nuclei is associated with nucleation mechanism.The continuous static recrystallization nuclei display similar orientations to the parent grains with TD orientation.Differently,discontinuous static recrystallization nuclei formed within the parent grains(TD-45~0 orientation) show random orientations and a variety of misorientation angles but preferred axes <5273> or <5270>.Interestingly,a special oriented nucleation is found.Discontinuous static recrystallization nuclei originated from boundaries of the parent grain(TD-70° orientation) show concentrated TD orientations in another side due to the preferred misorientation relationship 70°<1120>(∑18 b).It is speculated that these two special misorientation relationships are related to the dislocation type.     

EBSD Investigation on Oriented Nucleation in IF Steels

The mechanism responsible for the formation of recrystallization texture in cold-rolled Ti bearing interstitial free （IF） steel sheets was investigated using electron back-scatter diffraction （EBSD）. In addition, the origin of nuclei with specific orientations was studied. The formation of recrystallization texture was explained by oriented nucleation. Most nuclei have a high misorientation angle of 25-55° with the surrounding deformed matrices, but no specific orientation of misorientation axis between the nucleus and the surrounding deformed matrix is observed. The stored energy of deformed grains is in the decreasing order of the {111}〈112〉,{111}〈110〉, {112}〈110〉 and {001}〈110〉 orientations. New {111}〈110〉 grains are nucleated within deformed {111}〈112〉 grains and new {111}〈112〉 grains originate in the deformed {111}〈110〉 grains.     

镁合金AZ31高温形变机制的织构分析

利用X射线衍射和背散射电子衍射方法测定了镁合金AZ3l高温动态再结晶和超塑形变时的宏观和微观织构,分析了晶粒内部的形变机制.结果表明,在动态再结晶和超塑形变过程中,晶粒内部的滑移机制仍起重要作用,表现为再结晶晶粒出现择优取向以及一些晶粒可充分均匀形变成长条状.宏观织构的测定表明,具有不同初始织构的两类样品高温动态再结晶时,新晶粒有不同的取向择优过程,形成相似的织构;长条形变晶粒内部开动的滑移系也有一定的差异.分析了不同温度下相同的织构对应的不同塑变机理取向成像分析表明,基面织构取向的晶粒间总伴随着较高比例的小角晶界和30°(0001)的取向关系,这是六方结构的六次对称性限制了动态再结晶时(亚)晶粒间取向差的有效增大的缘故.     

Evolution of microstructure of an iron aluminide during severe plastic deformation by heavy rolling

Severe plastic deformation is generally achieved using novel techniques such as Equal Channel Angular Pressing (ECAP) or High Pressure Torsion (HPT), but may also be achieved by more conventional methods such as very heavy rolling. Microstructure evolution is examined in an iron aluminide intermetallic rolled to strains up to 3.3 using Transmission Electron Microscopy (TEM) and orientation determinations by Kikuchi line analysis. After the highest strains the microstructure is still characterized as a recovered submicron-scale dislocation structure, with generally low angles across the various boundaries, and a high density of dislocations inside these boundaries. The structures observed show a dependence on orientation of the underlying parent grain, with [001] orientations showing poorer rearrangement to cellular structures than grains with [113–111] orientations.     

Orientation dependent mechanical properties of commercially pure (cp) titanium

The present investigation is an attempt at correlating the crystallographic orientation and mechanical properties of hexagonal commercially pure titanium (cp-titanium). Annealed cp-titanium sheets are subjected to tensile deformation along the rolling direction, along 45° to the rolling direction and along 90° to the rolling direction respectively. Crystallographic textures and mechanical properties of these cp-titanium samples are investigated in the present study. The hardness of different grains/orientations is estimated through nanoindentation, grain average misorientation, orientation estimated elastic stiffness and Taylor factor measurements. It is observed that the hardness of the grains close to basal orientation is higher compared to non-basal orientations. It is further observed that the estimated bulk mechanical properties of cp-titanium have a direct relationship with the volume fraction of basal grains/orientations.     

Microstructure characterization based on the type of deformed grains in cold-rolled,Cu-added,bake-hardenable steel

The electron backscatter diffraction technique has been used to characterize the microstructure of deformed grains in cold-rolled, Cu-added, bake-hardenable steel. A new scheme based on the kind and number of average orientations, as determined from a unique grain map of the deformed grains, was developed in order to classify deformed grains by type. The α-fiber components, γ-fiber components and random orientations, those which could not be assigned to either γ-fiber or α-fiber components, were used to define the average orientation of unique grains within individual deformed grains. The microstructures of deformed grains in as-rolled specimens were analyzed based on the Taylor factor, stored energy, and misorientation. The relative levels and distributions of the Taylor factor, the stored energy and the misorientation were examined in terms of the types of deformed grains.     

The effect of SiC nanoparticles on deformation texture of ARB-processed steel-based nanocomposite

In this study, the influence of SiC nanoparticles on deformation texture of steel-based nanocomposite fabricated by accumulative roll bonding process was investigated. It was found that there was a texture transition from the rolling texture to the shear texture for both pure interstitial free steel and steel-based nanocomposite. However, the texture transition occurred in different cycles for the pure steel (the third cycle) and steel-based nanocomposite (the first cycle). It was realized that the fraction of low misorientation angle grain boundaries was decreased and the fraction of high misorientation angle grain boundaries was increased by the number of cycles. Also, recrystallization occurred in the pure steel and steel-based nanocomposite samples after the third and first cycles, respectively. In addition, the occurrence of recrystallization in steel-based nanocomposite was sooner than that of pure steel. At the early stage of dynamic recrystallization in processed steels, the {011}< 100 >-oriented grains were evolved and the fraction of grains with α-fiber and γ-fiber orientations was slightly decreased. The formation of the rolling texture in the steel-based nanocomposite samples was different from the typical rolling texture for the pure steel samples, due to the presence of the SiC nanoparticles in the nanocomposite. The weak rolling texture was attributed to the high stored energy of deformation, which was, in turn, due to low deformation temperature.     

Q235碳素钢应变强化相变中铁素体的取向特征

利用背散射电子衍射取向成像技术分析了在热模拟单向压缩条件下Q235碳素钢应变强化相变中铁素体晶粒的取向（差）变化特点。结果表明，奥氏体的状态影响应变诱导出的铁素体的取向，奥氏体的动态再结晶使应变诱导出的铁素体的取向随机分布，在铁素体的内部基本上没有小角晶界，随着形变温度的降低和应变量的增大，铁素体取向的择优性变强，铁素体内部的小角晶界增加，这是细小铁素体动态再结晶的表现，相变，形变以及铁素体的动态再结晶都影响＜111＞方向线织构的形成。     

The Formation of Twins in Recrystallized Binary Al-1.3%Mn

Local texture analysis in sheets of cold-rolled and recrystallization-annealed binary Al-1.3%Mn revealed a number of twin grain boundaries, including one twin chain of third generation. The formation mechanism of these twins was traced by means of electron backscattering diffraction analysis of the twinned grains and the surrounding matrix at the early stages of recrystallization. The results obtained suggest that the twins do not form by conventional recrystallization twinning, but the twin boundaries are rather the result of an impingement between newly formed recrystallized grains and subgrains of the as-deformed matrix, which were already in twin relation. Consequently, the formation of twins does not lead to the generation of new orientations and, hence, does not affect the recrystallization texture. Besides the twins, other coincidence site–lattice orientation relations were frequently observed, which also could be tracked to the as-deformed state.     

Coupled simulations of texture evolution during deformation and recrystallization of fcc and bcc metals

Thermo-mechanical processing to produce optimum grain structure and texture is essential for the successful utilization of commercial aluminum alloys and steels as sheet products. Several modeling techniques have been developed in the past with a reasonably good predictive capability for bulk deformation textures. However, prediction of texture evolution during recrystallization remains very challenging because of uncertainties involved in predicting the mechanisms that lead to nuclei formation and crystallographic orientations of the nuclei, and the uncertainties involved in predicting the grain boundary properties that determine the growth kinetics of the nuclei. We present some of our recent work in modeling the recrystallization textures following cold deformation in polycrystalline bcc metals and hot-deformation in fcc metals.     

Treatment of misorientation data to determine the fraction of recrystallized grains in a partially recrystallized metal

A method has been developed to differentiate recrystallized from unrecrystallized grains in partially annealed samples based on the point-to-point misorientation within the grains. Recrystallized grains exhibit a low average point-to-point misorientation whereas the contrary is the case for deformed grains. The analysis for recrystallized fraction was calibrated based on this distinguishing feature, and it was found that the average point-to-point misorientation within a grain in a fully recrystallized sample was less than 0.7°. Based on this calibration, partially recrystallized samples were analysed for their degree of recrystallization, and the technique was validated using microhardness measurements. The analysis of the misorientation data was in excellent agreement with the hardness data. There are three factors which distinguish the current method in comparison to the earlier work: a fixed limit is maintained on the minimum number of pixels which may constitute a grain; pattern quality is not considered; and the recrystallization criterion is calibrated to the experimental data. Nevertheless, in the early stages of annealing, where recovery is likely to have the most influence on the drop in hardness, the analysis of electron backscattered diffraction (EBSD) data underpredicts the recrystallized fraction. The analysis was also conducted on a cold-rolled, nonannealed sample, and from this, the error of the technique is estimated to be a maximum of 0.06 recrystallized volume fraction.     

3%Si电工钢铸坯中柱状晶的形变、再结晶行为及织构演变规律

利用电子背散射衍射(EBSD)技术和X射线衍射仪(XRD)研究3%(质量分数)Si电工钢铸坯中柱状晶的形变、再结晶行为及织构演变规律。结果表明:柱状晶长轴分别沿轧向、横向和法向放置,具有不同的初始织构。热轧后,表层形成的3种剪切取向中高斯取向较容易形成。中心区,RD样品中的α和γ线轧制取向,TD样品中的强γ线取向,ND样品中的强{100}取向以及各样品中的立方取向,均表现出明显的初始取向依赖性。冷轧后,RD,TD样品中的强{111}〈112〉取向来自热轧板中的高斯取向,ND样品中的强旋转立方取向遗传自初始{100}取向。受初始取向偏差及大晶粒尺寸影响,ND样品中的旋转立方取向晶粒内取向梯度较大。退火后,样品中心大尺寸的{100}取向晶粒是柱状晶初始取向遗传性的表现。     

Microstructure and microtextural studies of friction stir welded aluminium alloy 5052

The present investigation presents a composite picture of the microstructural developments in a friction stir welded (FSW) AA5052. Optimized, defect free and chemically homogeneous, FS weld was generalized in four regions – base material (BM), nugget, advancing side (AS) and retreating side (RS), using standard nomenclatures. Each region had its signature of microstructural features. AS had clear indications of shear and of grain fragmentation. The nugget region, on the other hand, had nearly equiaxed grains, with strong in-grain misorientation and presence of grain-interior dislocation structure ruling out contributions from static recrystallization. Equiaxed grains of the nugget region had typical onion ring structure – each ring did approximately correspond to one dominant family of orientation. Microstructural developments, as obtained from relative grain refinement, in-grain misorientation development, relative banding, etc., were most significant in nugget followed by AS and then by RS. Heterogeneous plastic deformation and thermal activation through localized heating/friction were the apparent causes. Most of the friction stir welded specimen fractured away from the nugget and showed ductile mode of failure.     

Promotion of texture weakening in magnesium by alloying and thermomechanical processing: (I) alloying

The recrystallization and texture evolution of four Mg–Zn–Ce sheets with a warm-rolled microstructure obtained through two stages that can be characterised as rough rolling and finish rolling were investigated at different stages of post-rolling annealing. On annealing, the same regions of the microstructure, located by hardness indentations, were examined and tracked by electron backscatter diffraction (EBSD). Furthermore, intragranular misorientation axes (IGMA) analysis was used to investigate the associated deformation mechanisms in the as-deformed material. By combining these two methods, the development of the recrystallization microstructure was investigated and important aspects, such as preferential nucleation sites, correlation between activated deformation mechanism and initial orientation of the recrystallized grains, were studied. The results showed that the Mg–1Zn–1Ce alloy, which had the highest Ce/Zn ratio, showed the weakest as-rolled texture and the most homogenous distribution of shear banding/twinning. The IGMA analysis also showed that in Mg–1Zn–1Ce, other types of dislocations rather than basal 〈a〉 were activated; in particular, prismatic 〈a〉 type was activated during deformation. Therefore, the weakening of recrystallization texture during rolling resulting from the addition of RE elements was linked with a change in dynamic recrystallization (DRX) behaviour. Since the Mg–1Zn–1Ce alloy corresponds to the highest level of Ce in solid solution, the observed texture weakening was possibly due to decreasing grain boundary mobility as a result of solute partitioning of RE elements to dislocations and grain boundaries.     

Grain boundary migration: misorientation dependence

The ability of grain boundaries (GB) to move has been found to be strongly dependent on crystallography, i.e. misorientation of the adjacent grains and orientation (inclination) of the GB in a crystal. Boundary mobility is rate-controlling in recrystallization and grain growth and thus, affects microstructure evolution and texture formation. This paper deals with recent advances in our understanding of misorientation and inclination dependence of grain boundary migration.     

Microstructure and texture evolution in low carbon steel deformed by differential speed rolling (DSR) method

The study examined the microstructural and textural evolution of low carbon steel samples fabricated using a differential speed rolling (DSR) process with respect to the number of operations. For this purpose, the samples were deformed by up to 4-pass of DSR at room temperature with a roll speed ratio of 1:4 for the lower and upper rolls, respectively. The DSR technique applied to low carbon steel samples resulted in a microstructure composed of ultrafine ferrite grains, approximately 0.4 µm in size, after 4-pass with a high-angle grain boundary fraction of ~65 %. The microstructural features of the ferrite phase indicated the occurrence of continuous dynamic recrystallization, beginning with the formation of a necklace-like structure of ultrafine equiaxed grains around the elongated grains, which were formed in the early stages of deformation, and ending with ultrafine recrystallized grains surrounded by boundaries with high angles of misorientations. In the pearlite phase, the microstructural changes associated with DSR deformation were presented by the occurrence of bending, kinking, and breaking of the cementite lamellar plates. In addition, the evolution of texture after DSR processing was affected by shear deformation and rolling deformation, leading to the formation of a texture composed of fractions of components with shear texture orientations such as {110} 〈001〉 (Goss) and orientations close to {112} 〈111〉, in addition to rolling texture components consisting mainly of α-fiber and γ-fiber.     

Texture evolution during hot-rolling and recrystallization in B2-type FeAl, NiAl and CoTi intermetallic compounds

The texture evolution during the hot-rolling and the recrystallization of B2-type Fe–48Al, Ni–50Al and Co–50Ti (expressed by at.%) intermetallic compounds were investigated. By hot-rolling at 973 K, Fe–48Al showed a microstructure with coarse grains elongated along rolling direction, while Ni–50Al and Co–50Ti showed a deformed microstructure featured by the heavily distorted (elongated) grains and/or the deformation bands. The hot-rolling texture of Fe–48Al was composed of {111}<uvw>, while those of Ni–50Al and Co–50Ti were composed of {111}<110> and {111}<112>, respectively. After annealing, the recrystallized grains were preferentially nucleated at the grain boundaries for Fe–48Al, and in the heavily distorted regions or the deformation bands for Ni–50Al and Co–50Ti. The orientations of the recrystallized grains were similar with those of the deformed matrix, especially for Ni–50Al and Co–50Ti. The recrystallization textures were generally more dispersive than the hot-rolling texture. Based on these results, the texture evolution during the hot rolling and the recrystallization of the B2-type intermetallic compounds were discussed.     

Modeling the crystallographic changes in processing of Al alloys

The evolution of deformation and recrystallization (RX) textures in 6016 Al alloy is analyzed in the current study by means of experimental measurements and numerical simulations. The deformation texture is modeled with various Taylor-type homogenization models whereas the development of RX texture is analyzed by evaluation of energy stored during the plastic deformation in grains of various orientations employing crystal plasticity (CP) calculations. It is shown that the main features of texture which evolve during discontinuous RX could be reproduced by taking into consideration both a microgrowth selection criterion and orientation selection based on crystallographically resolved stored energy of deformation. The influence of the strain heterogeneities on the development of RX texture is analyzed on the basis of CP and results derived from finite element calculations.     

取向电工钢中Goss晶粒生长的取向环境

将以MnS为主要抑制剂的普通取向电工钢作为实验材料,检测并分析脱碳样品的宏观织构及脱碳和随后加热至925℃时样品的微观织构,统计分析了Goss与周围晶粒的取向差分布。根据取向差原理计算了脱碳样品主要织构组分内各取向晶粒的取向差环境。结果表明,取向电工钢脱碳退火后Goss晶粒与周围晶粒的取向差分布呈现大角度特征,主要取向差分布范围为30°-45°,而非Goss晶粒与周围晶粒的取向差分布则呈现更多小角度特征。二次再结晶后,Goss晶粒与周围晶粒的取向差分布仍然以大角度特征为主。