Analysis of the microstructure evolution during tensile testing at room temperature of high-manganese austenitic steel

The microstructure and texture evolution of low-stacking fault energy high-manganese austenitic steel during tensile testing at room temperature was studied by means of interrupted tests. Untested material shows fully recrystallized austenitic grains and an almost random texture. During deformation, two deformation mechanisms, mechanical twinning and dislocation gliding, compete with each other. The governing mechanism of a single grain will depend on its crystallographic orientation relative to tensile direction. As a result of the strong interaction between grain orientation and twinning activity at low tensile strain, both grains with and without deformation twins can be observed. However, at high strain all the grains exhibited twin bands. Tensile tested samples were characterized by the presence of a fiber texture with the <111> and <100> directions parallel to the tensile direction.     

Plastic accommodation at homophase interfaces between nanotwinned and recrystallized grains in an austenitic duplex-microstructured steel

The plastic co-deformation behavior at the homophase interfaces between the hard nanotwinned grain inclusions and the soft recrystallized matrix grains in a duplex-microstructured AISI 316L austenitic stainless steel is examined through the analysis of long-range orientation gradients within the matrix grains by electron backscatter diffraction and transmission electron microcopy. Our analysis reveals that the mechanical accommodation of homophase interfaces until a macroscopic strain of 22% is realized within a small area of soft grains (about four grains) adjacent to the homophase interface. The activation of deformation twinning in the first two grain layers results in the occurrence of a ‘hump’ in the orientation gradient profile. We ascribe this effect to the role of deformation twinning on the generation of geometrically necessary dislocations. The smooth profile of the orientation gradient amplitude within the first 10 grain layers indicates a gradual plastic accommodation of the homophase interfaces upon straining. As a consequence, damage nucleation at such interfaces is impeded, resulting in an enhanced ductility of the single phase duplex-microstructured steel.     

Dynamic deformation behavior and microstructural evolution during high-speed rolling of Mg alloy having non-basal texture

The dynamic deformation behaviors and resultant microstructural variations during high-speed rolling(HSR) of a Mg alloy with a non-basal texture are investigated. To this end, AZ31 alloy samples in which the basal poles of most grains are predominantly aligned parallel to the transverse direction(TD) are subjected to hot rolling with different reductions at a rolling speed of 470 m/min. The initial grains with a TD texture are favorable for {10–12} twinning under compression along the normal direction(ND); as a result, {10–12} twins are extensively formed in the material during HSR, and this consequently results in a drastic evolution of texture from the TD texture to the ND texture and a reduction in the grain size. After the initial grains are completely twinned by the {10–12} twinning mechanism, {10–11} contraction twins and {10–11}-{10–12} double twins are formed in the {10–12} twinned grains by further deformation.Since the contraction twins and double twins have crystallographic orientations that are favorable for basal slip during HSR, dislocations easily accumulate in these twins and fine recrystallized grains nucleate in the twins to reduce the increased internal strain energy. Until a rolling reduction of 20%, {10–12}twinning is the main mechanism governing the microstructural change during HSR, and subsequently,the microstructural evolution is dominated by the formation of contraction twins and double twins and the dynamic recrystallization in these twins. With an increase in the rolling reduction, the average grain size and internal strain energy of the high-speed-rolled(HSRed) samples decrease and the basal texture evolves from the TD texture to the ND texture more effectively. As a result, the 80% HSRed sample, which is subjected to a large strain at a high strain rate in a single rolling pass, exhibits a fully recrystallized microstructure consisting of equiaxed fine grains and has an ND basal texture without a TD texture component.     

Dynamic recrystallization behavior and microstructural evolution of Mg alloy AZ31 through high-speed rolling

High-speed rolling (HSR) is known to improve the workability of Mg alloys significantly, which makes it possible to impose a large reduction in a single pass without fracture. In the present study, dynamic recrystallization (DRX) behavior and microstructural and textural variations of Mg alloy AZ31 during a HSR process were investigated by conducting rolling with different imposed reductions in the range of 20%–80% at a high rolling speed of 470 m/min and 400 °C. High-strain-rate deformation during HSR suppresses dislocation slips but promotes twinning, which results in the formation of numerous twins of several types, i.e., {10–12} extension twins, {10–11} and {10–13} contraction twins, and {10–11}–{10–12} double twins. After twinning, high strain energy is accumulated in twin bands because their crystallographic orientations are favorable for basal slips, leading to subsequent DRX at the twin bands. Accordingly, twinning activation and twinning-induced DRX behavior play crucial roles in accommodating plastic deformation during HSR and in varying microstructure and texture of the high-speed-rolled (HSRed) sheets. Area fraction of fine DRXed grains formed at the twin bands increases with increasing rolling reduction, which is attributed to the combined effects of increased strain, strain rate, and deformation temperature and a decreased critical strain for DRX. Size, internal strain, and texture intensity of the DRXed grains are smaller than those of unDRXed grains. Therefore, as rolling reduction increases, average grain size, stored internal energy, microstructural inhomogeneity, and basal texture intensity of the HSRed sheets gradually decrease owing to an increase in the area fraction of the DRXed grains.     

Microstructure evolution of Mg-Zn-Zr magnesium alloy against soft steel core projectile

The study aimed to shed light on the post deformation and damage behavior of an extruded Mg-Zn-Zr alloy under a ballistic impact.The results revealed that the initial microstructure consisted of both{0001} basal and {10(1)0} prismatic fiber texture.After impact,adiabatic shear bands,pronounce dif-ferent twinning in big grains,,,and types of dislocations,and grain refinement through twinning induce recrystallization accommodated the strain,and absorbed ～65.7 ％ of the energy during impact carried by a soft steel projectile.Interestingly,the deformation behavior at the top broad sides of the crater was entirely different.The weak basal texture was changed to a strong prismatic texture,which was further proved by typical sigmoidal compressive stress-strain curves.A revised model for the development of the ultra-fine grains adjacent to the crater has been proposed.The microhardness and yield strength was ～33 ％ and ～40 ％ higher and chiefly ascribed to strain hardening in ultra-fine grained near the surface of the perforation path.The exit of the perforation path was severely damaged and forms onion-shaped concentric rings which were comprised of melted zones,dimples,and cracks.Based on the all interesting findings,this study can be a clue for the development of the lightweight Mg alloy for military and aerospace applications.     

Texture and microstructure evolution in ultrafine-grained AZ31 processed by EX-ECAP

Extruded AZ31 alloy was processed by equal channel angular pressing (ECAP) up to 12 passes at 180 °C following route B_c, i.e. rotating the sample 90° between individual passes. Microstructure evolution was investigated using EBSD and TEM, as a function of strain imposed by ECAP. The first ECAP pass resulted in the formation of a new texture component which relates to the bimodal grain structure observed in this specimen. The grains larger than 10 μm show the orientation changes corresponding to the ECAP shear, which is characterised by the rotation of the basal poles by approximately 40° from the initial orientation. The fine grains with the average size of 1 μm maintain the initial orientation. The character of the bimodal grain structure and the distinct texture components between large and small grains remained unchanged up to 4 ECAP passes. Further ECAP pressing to 8 and 12 passes leads to a grain refinement through the whole sample volume and the orientation changes of all grains corresponding to the ECAP shear.     

New crystal plasticity constitutive model for large strain deformation in single crystals of magnesium

A new rate-dependent elastic–viscoplastic crystal plasticity constitutive model (CPCM) to simulate the large strain deformation in magnesium alloys is presented. The observed intragranular plastic deformation mechanisms of primary extension, primary contraction, and secondary extension (double) twinning are accounted for. The basal and non-basal slip systems in the parent grain, primary and double twins were also incorporated in the model. The crystallographic planes and directions of various slip and twinning systems are calculated. The slip-induced shear in the parent grain, as well as primary and secondary twinned regions are simulated. The twinning-induced shear from the primary and secondary twinned regions are also computed. In the model the texture evolution in the parent, as well as primary and secondary twinned regions are tracked. Separate resistance evolution functions for all the slip and twinning systems were considered. The interactions between various slip and twinning systems are accounted for in a comprehensive manner. Using the proposed CPCM, the plastic deformation in a magnesium single crystal in simple shear strain path is simulated. The contributions of various plastic deformation mechanisms to the macroscopic plastic deformation of the magnesium single crystal in this strain path are presented. The importance of identifying the active plastic deformation in a given strain path on a magnesium single crystal for a reliable model prediction was shown with an example.     

低温等通道转角挤压中定向凝固纯铜的组织及性能演变

采用光学显微镜(OM)和XRD技术对干冰冷却后的定向凝固纯铜(99.99%)经等通道转角挤压(ECAP)时的微观组织演变规律进行研究,并测试了ECAP后定向凝固纯铜的硬度及导电性能。结果表明,定向凝固纯铜在低温下经A和C路径变形后易于形成取向一致的纤维组织,并且保持(111)面的择优取向特征,而经Bc路径变形后,柱状晶破碎,形成均匀的等轴晶,且各晶面逐渐趋于随机取向;经过1道次变形后,各路径硬度大幅增加,约为原来的1.8倍,在随后的挤压中,硬度增加缓慢,经4道次ECAP后,Bc路径的硬度有所下降;在低应变下,晶粒取向的一致性使得导电率增加;随着应变的增加,晶格畸变使得电子发生散射,使导电率略有降低。     

Crystallographic texture evolution and martensite transformation in the strain hardening process of a ferrite-based low density steel

The strain-induced martensite transformation is of great importance in the strain hardening process of ferrite based low-density steel.Based on the microstructure analysis,the texture evolution and martensite transformation behavior in the strain hardening process were studied.The results show that martensite transformation accompanied by TWIP effect and high density dislocations maintains the con-tinuous hardening stage.As the strain increases,the texture of retained austenite evolves towards the F orientation{111}〈112〉,which is not conducive to martensite transformation.After the strain of 5％,the number of austenite grains with high Schmid factor orientations is gradually increased,and then signif-icantly reduced when the strain is over 10％due to the occurrence of martensitic transformation,which results in a high martensitic transformation rate.However,the unfavorable orientation and the reduced grain size of austenite slow down the martensite transformation at the final hardening stage.Moreover,because of the coordination deformation of austenite grains,strain preferentially spreads between adja-cent austenite grains.Consequently,the martensite transformation rate in strain hardening process is dependent on the orientation and grain size evolution of austenite,leading to a differential contribution to each strain hardening stage.     

A crystal plasticity finite element analysis of cross-grain deformation heterogeneity in equal channel angular extrusion and its implications for texture evolution

A crystal plasticity finite element (CPFE) method was applied to evaluate cross-grain deformation heterogeneity and its implication on texture evolution during equal channel angular extrusion (ECAE) of pure copper. The simulations were conducted for one to four passes of ECAE via route C, assuming simple shear in each pass at the macroscopic level. Analyses of the stress and strain distributions reveal considerable deformation heterogeneities across individual grains in the polycrystal. The grain interactions are found to be remarkable after even-numbered passes and they partly contribute to the retained shear textures. The CPFE model captures very well the experimental textures after odd-numbered passes; however, it is not able to model the measured textures subsequent to even-numbered passes, and the results are only slightly improved as compared to a visco-plasticity self-consistent polycrystal model. These results suggest that dedicated considerations of deformation heterogeneities at both the macro- and meso-levels are necessary in modeling texture evolution during severe plastic deformation.     

Nano grained AZ31 alloy achieved by equal channel angular rolling process

Equal channel angular rolling (ECAR) is a severe plastic deformation process which is carried out on large, thin sheets. The grain size could be significantly decreased by this process. The main purpose of this study is to investigate the possibility of grain refinement of AZ31 magnesium alloy sheet by this process to nanometer. The effect of the number of ECAR passes on texture evolution of AZ31 magnesium alloy was investigated. ECAR temperature was controlled to maximize the grain refinement efficiency along with preventing cracking. The initial microstructure of as-received AZ31 sheet showed an average grain size of about 21 μm. The amount of grain refinement increased with increasing the pass number. After 10 passes of the process, significant grain refinement occurred and the field emission scanning electron microscopic (FESEM) micrographs showed that the size of grains were decreased significantly to about 14-70 nm. These grains were formed at the grain boundaries and inside some of the previous larger micrometer grains. Observation of optical microstructures and X-ray diffraction patterns (XRD) showed the formation of twins after ECAR process. Micro-hardness of material was studied at room temperature. There was a continuous enhancement of hardness by increasing the pass number of ECAR process. At the 8th pass, hardness values increased by 53%. At final passes hardness reduced slightly, which was attributed to saturation of strain in high number of passes.     

高纯铝多向锻造大塑性变形过程的数值模拟及实验研究

采用多向锻造的方法研究室温下锻造道次对高纯铝组织的影响,并用三维DEFORM软件对实验过程进行模拟。结果表明:经3次多向锻造后,高纯铝试样横截面上形成1个X形的细晶区及4个粗晶区,随锻造道次增至9,细晶区的面积不断扩大,粗晶区的面积不断缩小,但细晶区与粗晶区的晶粒尺寸差异并未消除。当高纯铝试样心部的等效应变量达到2.5时,心部再结晶晶粒尺寸达到70μm,继续增加心部的等效应变至6.0,心部的晶粒不再随等效应变量的增加而细化,达到晶粒细化的极限。而当试样边部难变形区和自由变形区的等效应变量增至4.0时,其再结晶晶粒仍随等效应变量的增加而细化,未达到晶粒细化极限。这表明局部等效应变量及局部变形方式均是影响高纯铝晶粒细化的重要因素。     

Effect of equal channel angular pressing on grain refinement and texture evolution in a biomedical alloy Ti13Nb13Zr

Evolution of texture and concomitant grain refinement during Equal Channel Angular Pressing (ECAP) of Ti13Nb13Zr alloy has been presented. Sub-micron sized equiaxed grains with narrow grain size distribution could be achieved after eight pass at 873 K. A characteristic ECAP texture evolved in α phase till four passes while the evolution of characteristic ECAP texture in the β phase could be observed only beyond the fourth pass. On increasing the deformation up to eight passes, the texture in α phase weakens while the β phase shows an ideal ECAP texture. A weaker texture, low dislocation density and high crystallite size values in α phase suggest the occurrence of dynamic recrystallization. The absence of texture evolution in β phase till four passes can be attributed to local lattice rotations. The characteristic ECAP texture in the eight pass deformed sample is attributed to delayed dynamic recrystallization in the β phase.     

EBSD Characterization of the Laser Remelted Surface Layers in a Commercially Pure Mg

Laser surface melting has been applied on a commercially pure Mg. The microstructure and texture modifications encountered in the surface layers were carefully investigated by using electron backscattered diffraction （EBSD） technique. Due to the melting followed by rapid solidification and cooling, a layer having graded microstructures and texture formed. At the bottom of the melted layer, the solidified Mg grains have an elongated shape with a 〈0001 〉 basal fibre texture nearly parallel to sample normal direction, while equiaxed grains were observed in the top melted layer having a much weaker basal fibre texture. Solidification twinning and deformation twinning were found in the vicinity of the melt/substrate interface where the Mg grains grow larger due to the heating. In addition, no epitaxial type grain growth was observed at the melt/substrate interface.     

Equal channel angular pressing of a TWIP steel: microstructure and mechanical response

A Fe–20.1Mn–1.23Si–1.72Al–0.5C TWIP steel with ultrafine grain structure was successfully processed through equal channel angular pressing (ECAP) at warm temperature up to four passes following the B _C route. The microstructure evolution was characterized by electron backscattered diffraction to obtain the grain maps, which revealed an obvious reduction in grain size, as well as a decrease in the twin fraction, with increasing number of ECAP passes. The texture evolution during ECAP was analyzed by orientation distribution function. The results show that the annealed material presents brass (B) as dominant component. After ECAP, the one pass sample presents A ₁* and A ₂* as the strongest components, while the two passes and four passes samples change gradually toward \( B/\bar{B} \) components. TEM analysis shows that all samples present twins. The twin thickness is reduced with increasing the number of ECAP passes. Nano-twins, as a result of secondary twinning, are also observed in the one and two passes samples. In the four passes sample, the microstructure is extensively refined by the joint action of ultrafine subgrains, grains and twins. The mechanical behavior was studied by tensile samples, and it was found that the yield strength and the ultimate tensile strength are significantly enhanced at increasing number of ECAP passes. Although the ductility and strain hardening capability are reduced with ECAP process, the present TWIP steel shows significant uniform deformation periods with positive work hardening rates.     

Microstructure and texture evolution during accumulative roll bonding of aluminium alloys AA2219/AA5086 composite laminates

Accumulative roll bonding of two aluminium alloys, AA2219 and AA5086 was carried out up to 8 passes. During the course of ARB, the deformation inhomogeneity between the two alloy layers results in interfacial instability after the 4th pass, necking of the AA5086 layers after the 6th pass and fracture along the necked regions after the 7th and 8th pass. The EBSD analysis shows deformation bands along the interfaces after 8 passes of ARB. The ARB-processed materials predominantly show characteristic deformation texture components. The weak texture after the 2nd pass results from the combination of a weakly-textured starting AA2219 layer and a strongly-textured starting AA5086 layer. A strong deformation texture forms due to the high imposed strain after a higher number of ARB passes. Subgrain formation and related shear banding induces copper/S components in the case of the small elongated grains, while planar slip leads to the formation of brass component in the large elongated grains.     

Novel severe plastic deformation technique—accumulated extrusion (AccumEx)

Accumulated extrusion, a novel severe plastic deformation technique based on conventional extrusion process, is proposed and has been validated on commercial pure aluminium sheets. Four sheets were extruded together at 75% reduction, and this product was recut into four pieces and reextruded up to eight passes to a strain of 13.2. The tensile strength increased up to 200?MPa after six passes. The elongation to failure was 21% after one pass and 6% after six passes. Ultrafine grains with average grain size of 600?nm were observed after eight passes. The refinement process was monitored along all three directions. Texture evolution played an influential role on the misorientation profile and high angle grain boundary fraction.     

Evolution of microstructure and texture in an ultrafine-grained Al6082 alloy during severe plastic deformation

The evolution of microstructure and texture in Al6082 precipitation-hardened alloy during equal-channel angular pressing (ECAP) was studied. It was found that although the dislocation density and the subgrain size saturated after 1 pass, the size of grains bounded by high angle boundaries reached its minimum value only after 4 passes. Furthermore, the grain orientation distribution changes between 4 and 8 passes, indicating the development of grain boundary structure even after the saturation of the parameters of the microstructure. As a result of this evolution, the initial texture of the commercial alloy was diminished after 8 passes and the grain orientation distribution became to be close to random case.     

Multi-pass large strain rolling of as-cast AZ31 magnesium alloy

As-cast AZ31 magnesium alloy subjected to multi-pass large strain rolling was investigated. A successive rolling process up to three passes was carried out at 370°C with a pass reduction of 30%. Deformation microstructure characteristics prove that the dynamic recrystallisation (DRX) mode changed with the increase of rolling passes. In the first pass, DRX related to twinning played a dominant role. But in the third pass, DRX grains mainly appeared around the pre-existing grain boundaries. The ultimate strength and elongation of rolled sheets after three passes rolling are enhanced by 37 and 39%, respectively, compared to the as-cast alloy. Meanwhile, the tensile fracture mode was ductile fracture which was different from the ductile–brittle fracture of as-cast.     

Dependence of deformation mechanisms on grain orientations and their changes calculated based on Sachs model in magnesium alloy AZ31

During deformation, the orientation of a grain influences not only the deformation mechanisms (slip or twinning) and the specific selection of activated slip or twinning systems for that grain, but also the kinetics of different types of transformation. Schmid factor analysis was applied to determine the orientation dependency of deformation mechanisms in magnesium alloys AZ31 in this work. The orientation changes after the operation of the specific deformation mechanisms were also calculated based on Sachs model. It was found that different deformation mechanisms proceeded differently according to theoretical predictions. Basal slip occurred when basal planes of grains were tilted toward ND around TD. Prismatic slip dominated when basal planes were approximately perpendicular to TD. Calculation results also indicated that the operating of pyramidal 〈a〉 slip can not be neglected. {10 2} twinning was favorable when basal planes were approximately normal to RD and {10 1} twinning was analyzed to be related to the grains with basal orientations. The operating of 〈a + c〉 slip could greatly suppress the activating of twinning by our Schmid factor analysis. Basal orientations with TD and RD scattering can favor basal slip and tension twinning, respectively, after the operation of compression twinning based on the Schmid factor calculations.