De-twinning and Texture Change in an Extruded AM30 Magnesium Alloy during Compression along Normal Direction

Twinning and de-twinning are the salient deformation mechanisms in hexagonal close-packed(hcp)metals. The aim of this study was to examine and quantify the de-twinning process involving a reversible motion of twin boundaries in an extruded AM30 magnesium alloy after re-compression along the normal direction(ND) of pre-compressed samples along the extrusion direction(ED). {1012} extension twins were first introduced at a compressive strain of 3.7% along the ED. The subsequent compressive deformation along the ND induced a gradual shrinkage of twins with increasing cumulative true strain,and the complete de-twinning occurred at a strain of ~7.7%. The twin width decreased linearly with increasing true strain. Texture measurements verified the rotation of c-axes of hcp unit cells towards the anti-compression direction due to {1012} extension twinning after compression along the ED, and a gradual return of c-axes to the initial orientation due to twin shrinking or de-twinning during the following compression along the ND. The {1012} twinning corresponded to the formation of new texture components C{1210}<0001> and D{0110}<0001> and a decrease in the initial texture components A{0001}<2110> and B{0001}<1010>, while the twin shrinking or de-twinning was characterized by a gradual vanishing of components C{1210}<0001> and D{0110}<0001> and an increase in the components A{0001}<2110> and B{0001}<1010>.     

Modified twinning behaviour induced by texture evolution in hot rolled Mg–3Al–1Zn alloy

Abstract

Compressive deformation along the rolling direction (RD) of a hot rolled Mg–3Al–1Zn alloy is applied to investigate the texture evolution and the recompressive yield strength (RYST) along the transverse direction (TD). Preferential orientation of the basal and prismatic planes is generated by the plastic deformation. Precompression along RD results in one plane of {10–10} aligned nearly perpendicular to the normal direction to the rolling plane. As the compressive strain along RD increases, the RYST shows an earlier raised and later decreased trend. The modified twinning mechanism is investigated using X-ray diffraction and electron backscattered diffraction observations. The results reveal that {10–12} twinning in the matrix dominates the recompression along TD, while the formation of {10–12}–{10–12} twins becomes comparatively easier to occur in the previous {10–12} twins for large precompressed samples.     

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.     

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.     

Effects of prior compression on ductility and yielding behaviour in extruded magnesium alloy AZ31

Abstract

In the present paper, the effects of precompression along extrusion direction (ED) on subsequent compression perpendicular to ED were investigated in an extruded magnesium alloy AZ31. The results showed that the yield stress under compression perpendicular to ED increased if there was precompression along ED. The evolution of deformation mechanism was responsible for increase in yield stress because plastic deformation was dominated by both basal slip and {10–12} twinning under compression perpendicular to ED in samples without prestrain, but basal slip was difficult to be activated and {10–12} twinning dominated deformation in samples with precompression. However, because basal slip had no obvious contribution to plastic deformation, the ductility decreased if there was precompression along ED.     

Effects of initial texture on deformation behavior during cold rolling and static recrystallization during subsequent annealing of AZ31 alloy

This study demonstrates that the initial texture of Mg alloy significantly affects the microstructure developed during cold rolling and the recrystallization behavior during subsequent annealing. In a sample with a texture oriented toward the normal direction(ND sample), thick and large-sized shear bands are intensively formed during cold rolling and the deformation is strongly localized along these bands. In contrast,in a sample with a texture oriented toward the transverse direction(TD sample), many {10-12} twins are formed at the early stage of cold rolling, and then, numerous small-sized shear bands are formed in these twins. Results of nanoindentation tests reveal that the cold-rolled ND sample has internal strain energy that is substantially concentrated along the shear bands, whereas the cold-rolled TD sample has a large amount of internal strain energy that is homogeneously distributed throughout the material;this latter behavior is strongly related to the extensive {10-12} twinning and the resultant microstructural variations in the TD sample. During subsequent annealing, recrystallization occurs locally along the shear bands in the ND sample, which leads to the formation of a bimodal grain structure comprising fine recrystallized grains and coarse unrecrystallized grains. In contrast, during subsequent annealing of the TD sample, static recrystallization occurs homogeneously throughout the material, which results in the formation of a uniform grain structure that mostly comprises equiaxed recrystallized grains.     

Influence of different deformation processing on the AZ31 magnesium alloy sheets

AZ31 magnesium alloy sheets were processed by normal rolling (NR), one-pass equal channel angular rolling (1P-ECAR), and cross equal channel angular rolling (C-ECAR) at 400 °C on a die with 105 ° channel angle. The microstructure, texture, and tensile properties of sheets were measured. The results show that ECAR processing can weaken the basal plane texture, thus obviously improve the mechanical properties. The yield ratio σ_s/σ_b decreases and strain hardening exponent n increases along rolling direction (RD) during ECAR, which means that the uniform plastic formability is enhanced. After C-ECAR, the mechanical properties along both the RD and transverse direction (TD) are improved. Different twinning types, fine contraction twinning in the NRed sheets and coarse extension twinning in the ECARed sheets, were observed. The easier activation of twinning and basal 〈a〉 slip leads to the lower yield strength of the ECARed sheets. Dynamic recrystallization (DRX) during the rolling process has great effect on the microstructure of the as-deformed and annealed sheets. The annealed C-ECARed sheets have significant finer and homogenous grains than the annealed NRed sheets, which is attributed to the rarely DRX process during ECAR. The average grain sizes of the annealed C-ECARed samples and NRed samples are 14 and 24 μm, respectively.     

Effect of initial texture on dynamic recrystallization of AZ31 Mg alloy during hot rolling

Wedge-shaped AZ31 plates with two kinds of initial textures were rolled at 573 K to investigate the effect of initial texture on dynamic recrystallization (DRX). The results indicated that the initiation and nucleation of DRX were closely related to the initial texture. The initiation and completion of DRX in the TD-plate were significantly retarded compared with that in the ND-plate. Twin related DRX nucleation was mainly observed in the ND-plate samples; while gain boundary related DRX nucleation was mainly observed in the TD-plate samples. The different DRX behavior between the TD- and ND-plates was attributed to the different deformation mechanism occurring before DRX initiation. For the ND-plate, dislocation glide was considered as the main deformation mechanism accompanied with {1 0 −1 1}-{1 0 −1 2} double twin, which led to the increment of a faster increasing stored energy within the grains. And {1 0 −1 1}-{1 0 −1 2} double twin was mainly found to be DRX nucleation site for the ND-plate. For the TD-plate, {1 0 −1 2} extension twin was the dominant deformation mechanism which resulted in a basal texture with the c-axis nearly parallel to ND. The stored energy caused by dislocation motion was relatively small in the TD-plate before a basal texture was formed, which was considered as the main reason of that DRX was retarded in the TD-plate compared with that in the ND-plate. Based on the difference in deformation mechanism and DRX mechanism caused by the different initial texture, the variation in grain size, micro-texture and misorientation angle distribution in the ND and TD plates were discussed.     

The assisting and stabilizing role played by ω phase during the {112} <111>β twinning in Ti-Mo alloys:A first-principles insight

The ω phase is commonly observed in β-Ti alloys and plays a significant role on various properties of β-Ti alloys.Although many results about the role ofω phase on mechanical properties of β-Ti alloys have been derived from theoretical and experimental studies,the role ofω phase on deformation mechanism hitherto remains elusive and deserves to be further studied.In this work,the role played by ω phase during the {112 } <111>β twinning in Ti-Mo alloys were investigated by first-principles calculations at atomic scale.In the energy favorable interface of(112)β/(10(1)0)ω,we found that partial dislocations slipping on the successive (10(1)0)ω planes ofω phase can lead to the formation of { 112} <111>β twin nucleus.And the twin nucleus grows inwards ω grain interior through atomic shuffle.Thus,a new twinning mechanism of {112 } <111>β assisted by ω phase was proposed.Furthermore,our calculations indicated that the Pearance of ITB (interfacial twin boundary) ω phase can improve the stability of the symmetrical 12 } <111 >β twin boundary (TB),which can well explain the experimental phenomenon that the ITB ω phase always accompanies the formation of {112 } <111>β twin.Finally,a probable microstructure evolution sequence was suggested,namely β matrix → β matrix + athermal ω phase → (112)[11(1)]twin → (112)[11(1)]β twin + ITB ω phase.Our calculations provide new insights on the role played by ω phase during the twinning process of {112} <111>β,which can deepen the understanding on the deformation behaviors of β-Ti alloys.     

Orientation dependencies of mechanical response, microstructure and texture evolution in hot compression of AZ31 magnesium alloy processed by equal channel angular extrusion

The anisotropic mechanical behavior during hot compression of an AZ31 Mg alloy processed by equal channel angular extrusion (ECAE) was evaluated and then discussed in correlation with the concurrent microstructure and texture evolution. The results revealed apparent orientation-dependencies in the mechanical responses, microstructure, and texture development in uniaxial compression along two perpendicular directions. Compression along the transverse direction (TD) led to a higher hardening rate, higher peak stress, and earlier softening than those obtained in compression along the extrusion direction (ED). This can be attributed to the differences in the initial textures prior to compression along the two directions, which led to a more significant contribution of tensile twinning at the early stage of straining and consequently more extensive dynamic recrystallization in loading along TD than along ED. These results suggest that the deformation behavior in compressive loading of the ECAE-processed Mg alloy is highly anisotropic, which needs to be taken into account in their applications.     

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. 相似文献   

On dynamic deformation behavior of WE43 magnesium alloy sheet under shock loading conditions

In the present study, the texture evolution, microstructure and mechanical behavior of WE43 magnesium sheet at high strain rates are investigated. Samples cut along the rolling direction (RD), 45° from the RD, transverse direction (TD) and perpendicular to the RD-TD plane were tested at strain rates of 800, 1200 and 1400 s⁻¹ using Split Hopkinson Pressure Bar. It is observed that after shock loading, the initial weak texture converts to a weak (00.2) basal texture in all samples. Besides, it is found that the strength and ductility increase and twinning fraction decreases with increase in strain rate. Moreover, another effect of increase in strain rate is found to be the higher activation of pyramidal 〈c + a〉 slip systems. In addition, degree of stress and strain anisotropy is low particularly at higher strain rates, which is mainly related to the weak initial texture of the samples. A viscoplastic self-consistent model with a tangent approach is used to analyze the deformation mechanism during shock loading.     

Twinning characteristics in textured AZ31 alloy under impact loading along specified direction

A plate of hot extruded AZ31 alloy with some large grains was selected for impact tests. The occurrence of twinning was investigated. It was found that when impacted along the normal direction (ND) of the plate, {10-11}-{10-12} double twinning is commonly observed in large grains, and {10-12} tension twinning can also be activated although their c-axis is close to the compression direction. In small grains, {10-12} tension twins are favored in which case the extension strain component is parallel to the c-axis. In addition, twins with < 11-20> 75° misorientation to the parent matrix were also observed.     

The development of <100> texture in Fe-Cr-Co-Mo permanentmagnet alloys

The cold-rolled and recrystallization textures of Fe-Cr-Co-Mo permanent magnet alloys are described. The studied composition is Fe-30%Cr-15%Co-3%Mo (in wt.%). The cold-rolled texture can be considered to be {111}<110>, {111}<112>, {100}<110>, and {211}<110>, while the recrystallization texture can be considered to be {111}<100>, {110}<112>, {211}<110>, and {110}<110>. The secondary recrystallization is caused by heat-treating the alloys in the sequence of α, α+γ, α+γ+σ, α phase region. This results in a favorable texture of {110}<110> and <100> direction, aligning along the transverse direction (TD) of the strips. The best magnetic properties obtained in this study were 1.2 T (12.0 kG), iH _c=82.0 kAm^-1 (1025 Oe), and (BH)max= 60.8 kJm^-3 (7.6 MGOe) with TD alloys     

Atomic-resolution studies on reactions between basal dislocations and { 10(1)2 } coherent twin boundaries in a Mg alloy

Reactions between basal 〈a60〉 dislocations and {1012} coherent twin boundaries(CTBs) in a Mg alloy were studied with atomic resolution. Individual dislocation-CTB reactions produced steps with residual dislocations and multiple t winnin g dislocations(TDs) gliding away, consequently resulting in TB migration. Reactions between {1012} CTBs and low-angle grain boundaries composed of basal 〈a60〉 dislocations created either basal-prismatic/prismatic-basal interfaces or asymmetric tilt grain boundaries, depending on whether TDs gliding away or not. Not only the emission of TDs by dislocation-TB reactions may drive TB migration, but also the resultant steps or facets along TBs can act as TD sources to facilitate TB migration. Our results indicate that roughness or severe loss of local coherency induced by dislocation-TB reactions does not intrinsically impede TB migration in Mg alloys. Dislocation-TB reactions may provide another feasible strategy to improve the ductility of Mg alloys, in addition to other techniques like grain refinement and texture modification.     

AZ31镁合金的变形织构和协调变形机理

进行变形速率可控的单向拉伸试验,研究了变形织构与滑移和孪生等协调变形机理对AZ31镁合金综合性能的影响。结果表明:在沿挤压方向拉伸过程中,变形织构使{0002}晶面Schmid因子较低,基面滑移难以开动,屈服强度高。在沿45°拉伸过程中,变形织构使柱面取向晶粒处于发生{0002}滑移的最佳位置,基面取向晶粒的棱柱面滑移也处于最佳位置,屈服强度低而延伸率高。沿横向拉伸的力学性能主要受孪晶影响,由于大量孪晶诱发裂纹,延伸率最低。试样在45°和横向拉伸时产生的大量拉伸孪晶,是出现{0002}双峰织构的诱因。     

Deformation twinning in zircaloy 2

Abstract

Fully recrystallised zircaloy 2 samples were subjected to different degrees of uniaxial compression. Grains of high Taylor factors showed {1012}〈1011〉 deformation twins, noticeable up to 13–16% compression. Twinning strongly affected the crystallographic texture and also brought in clear differences in stored energy and residual stress between the suspected parent and product grains/orientations of twinning. At later stages of deformation, where presence of twinning was insignificant, aforementioned heterogeneity was further supplemented by heterogeneity in microstructure – clear presence of fragmenting and non-fragmenting grains. Direct observations on twin fraction, twin deviation and twin continuity had shown an apparent peak in twinning by ～7·5% compression, an observation explainable through a simple model of twin decay by in grain misorientation development.     

Deformation behaviour of ultrafine grained Mg–7Gd–5Y–1.2Nd–0.5Zr alloy under high strain rates

The mechanical behaviour of Mg–7Gd–5Y–1.2Nd–0.5Zr (wt. %) alloy with ultrafine grains was measured by split Hopkinson pressure bar method under the strain rates of 1000, 1500, and 2000 s^?1 at room temperature. Dynamic tests were carried out along extrusion direction (ED), transverse direction (TD), and normal direction (ND). The results demonstrated that the flow stress increased with the increase of strain rate, showing a positive strain rate strengthening effect. There was no obvious anisotropy in dynamic compression along ED, TD, and ND, which was caused by rare earth elements and multi-pass deformation. This led to the adoption of plastic deformation mode dominated by non-basal slip and participated by tension twinning.     

Detwining in Mg alloy with a high density of twin boundaries

To investigate the role of preexisting twin boundaries in magnesium alloys during the deformation process, a large number of {10-12} tensile twins were introduced by a radial compression at room temperature before hot compressive tests with both low and high strain rates. Unlike the stable twins in Cu-based alloys with low stacking fault energies, {10-12} twins in Mg alloy are extremely unstable or easy to detwin through {10-12}-{10-12} re-twinning. As a result, non-lenticular residual twins and twin traces with misorientation of 5°–7° were present, as confirmed by electron backscatter diffraction. The extreme instability of the twins during compression indicates that both twin and detwinning require extremely low resolved shear stresses under our experimental conditions.     

Analyses of deformation twinning in the extruded magnesium alloy AZ31 after compressive and cyclic loading

The influence of different loading conditions on the microstructural development of extruded magnesium alloy AZ31 was investigated by optical microscopy and electron backscattered diffraction. Extruded magnesium profiles exhibit a significant asymmetry in the mechanical properties, due to the low activation energy of the extension twinning system \( \left\{ {10\overline{1} 2} \right\}\langle {10\overline{1} 1}\rangle,\) when compressing along the extrusion direction. For the analyses of this twinning system, compression tests with different applied strains 0.4 ≤ ε ≤ 11% were performed for two extrusion products exhibiting different microstructures. The main deformation mechanisms during cyclic loading are the formation of extension twins during compression and the detwinning during subsequent tensile loading. The strain-controlled fatigue tests were carried out with applied strain amplitudes 0.3 ≤ ε_A ≤ 5%. The tests were stopped at characteristic numbers of cycles N in the tensile or compression maximum of the hysteresis loop. The microstructural investigations deliver information about the type of twinning and the size, shape, local distribution, and volume fraction of twins as a function of the plastic deformation. These results will be discussed with regard to the microstructure of the initial state material and to the applied load.