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。     

Effect of initial texture on fatigue properties of extruded ZK60 magnesium alloy

The effect of initial texture on cyclic deformation behavior of extruded ZK60 magnesium (Mg) alloy was experimentally investigated under strain‐controlled loading with the strain amplitudes at 4%, 1%, and 0.35%. The testing specimens were taken from extrusion direction (ED), transverse direction (TD), and a material precompressed to 9.4% along the ED (ED_?9.4%). At a high strain amplitude of 4%, the cyclic deformation modes of ED and ED_?9.4% specimens are similar, and they experience twinning exhaustion → slip and detwinning exhaustion → slip during each loading cycle. At a medium strain amplitude of 1%, twinning‐detwinning is involved in the cyclic deformation, but different deformation mechanisms were observed in the 3 different specimens. Partial twinning‐complete detwinning mode dominates the cyclic deformation in the ED specimen, while partial detwinning‐retwinning mode occurs in the ED_?9.4% specimen. For the TD specimen, both basal slip and tension twinning occur during cyclic deformation. At a low strain amplitude of 0.35%, dislocation slips dominate the deformation for the ED specimen with a few observable tension twins. For the ED_?9.4% specimen, initially twined texture increases the ductility of the material and enhances fatigue life as compared with the other 2 specimens.     

Comparative study on local and global mechanical properties of 2024 T351, 2024 T6 and 5251 O friction stir welds

The effect of the microstructure heterogeneity on the global and local tensile properties of friction stir welded joints in 5251 (O temper) and AA2024 (T351 and T6 tempers) aluminium alloys has been investigated. Micro-tensile tests parallel to the welding direction have been carried out in the regions representative of the main microstructural zones. The digital image correlation technique (DIC) has been used during transverse tensile tests for mapping the strain distribution and to determine the local stress–strain curves. A 3-D finite elements model has been developed to predict the weld behaviour from the tensile curves of the individual regions of the weld.The tensile properties of the 5251 O weld are relatively homogeneous leading to high ductility and fracture in the base material. In contrast, the tensile properties of the various regions of the 2024 T351 and 2024 T6 welds are very heterogeneous and essentially controlled by the state of precipitation. The thermo-mechanically affected zone is the weakest region where the strain localises during a transverse tensile test. The 2024 T6 base material is stronger than the 2024 T351 alloy, leading to a more pronounced strain localisation during transverse tensile tests and a lower overall ductility. Local tensile data obtained by strain mapping are in good agreement with the curves obtained by micro-tensile tests, and these results can be safely used in a finite elements model to predict the behaviour of the overall weld assembly.     

Strain rate and orientation-dependent strain hardening of Mg–9Li–Al using crystal plasticity

A crystal plasticity finite element model was proposed considering slip and twinning interactions. The grain morphology and crystallographic orientations were introduced into the model to describe the microstructure of duplex polycrystalline Mg–9Li–Al. The activation of the slip systems and the strain localisation with respect to initial grain orientations were investigated. In addition, the effects of phase distributions and volume fractions on the macroscopic responses and on strain hardening rates were analysed. The results show that the strain hardening is rate-dependent but the texture is less sensitive to strain rate. The distribution of a phase and its volume fraction play primary roles in governing the mechanical response.     

The Outstanding Contribution of Basal Slip in Substructure Development during Friction Stir Processing of Magnesium Alloys

This research reveals the critical role of basal slip in the substructure development during friction stir processing of a magnesium alloy. In this respect, the intragranular lattice rotation axes are considered to identify the activity of different slip systems. The applied shear strain during the procedure is stored in the matrix through slip-induced rotations at the grain level. The rotations around distinct Taylor axes produce “slip domains” separated by necessary boundaries from the parent grains, significantly contributing in grain refinement. The basal slip is easily activated in grains holding different stored energy; however, the nonbasal slip has a higher dependency on the amount of local applied strain. Determining the contribution of different slip systems in strain accommodation reveals that the basal slip imposes the highest fraction of low-angle boundaries into the microstructure leading to the development of the ultimate grain boundary structure.     

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.     

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.     

Large load friction stir welding of Mg–6Al–0.4Mn–2Ca magnesium alloy

Friction stir welded (FSW) magnesium alloys usually exhibit a lower yield strength and elongation compared with base materials. In this study, large load FSW associated with an extremely low welding speed and rotation rate were applied to a non-combustive Mg–6Al–0.4Mn–2Ca magnesium alloy to modify the microstructure and texture in the weld zone and improve the mechanical properties of the joint. The twin structure in the stir zone provided adequate barriers for dislocation motion for strengthening and created more local sites for nucleating and accommodating dislocations, thereby elevating ductility and strain hardening in the transverse tensile test. The results showed that the yield strength and elongation of the joint were enhanced to 98% and 126% of the base material levels, respectively.     

FATIGUE DAMAGE IN AUSTENITIC-FERRITIC DUPLEX STAINLESS STEELS

Fatigue damage in two austenitic-ferritic duplex stainless steels, with the structure of a natural composite and different levels of nitrogen content, was studied in low-cycle fatigue. Both steels show initial cyclic hardening followed by softening and a long stabilisation period. The cyclic stress-strain curve increases with the nitrogen content while Manson-Coffin curves of both steels intersect at medium fatigue lives. The study of the surface relief reveals intensive slip markings both in ferrite and in austenite. Their density is influenced by the nitrogen content. Both the intensity and density of the persistent slip band (PSB) markings are higher in the ferrite. Crack initiation was found to appear predominantly in PSBs in the ferritic grains at the low strain amplitudes, and in the ferritic and austenitic grains at the highest strain amplitudes. The level of the cyclic stress-strain response and the fatigue lives are discussed in terms of the cyclic strain localisation and of the effect of texture and nitrogen content on the strength and fatigue damage. The increased strength of the austenitic phase, due to high nitrogen alloying, results in cyclic slip localisation in the ferrite, and the decrease of fatigue life, compared with the steel with the lower nitrogen content.     

The effects of texture in titanium alloys for engineering components under fatigue

The mechanical response of textured Ti 6/4 plate material is assessed through an evaluation of monotonic properties under tension and torsion loading and fatigue testing of plain section and notched specimen geometries. Significant variations in modulus, yield strength, ultimate tensile strength and ductility are demonstrated for testpieces taken from the plate materials parallel to either the transverse or longitudinal rolling direction. Cyclic performance is also shown to be sensitive to orientation with different cyclic stress–strain curves defined in the two orientations. The relationship between the principal stress axis and the dominant basal plane texture is shown to control fatigue crack initiation lives and the ultimate mode of fracture. Whilst loading parallel to the transverse direction offers the strongest monotonic and cyclic stress–strain response, fatigue tests performed on specimens orientated parallel to the longitudinal rolling direction provide the optimum cyclic life. These effects are discussed with reference to the inherent, anisotropic mechanical response of α+β titanium alloys, which results from the hexagonal crystallographic form of the α phase and the availability of preferential slip systems. It is argued that the anisotropic response could be utilised to an engineering advantage by matching critical stressing directions to the specific properties offered by the texture.     

Characterization of weld seam properties of extruded magnesium hollow profiles

Extrusions of hollow profiles with weld seams were conducted using the magnesium alloy ME21 applying various extrusion ratios. Subsequent analysis of the profiles’ microstructure was performed comparing weld free with weld seam containing material using (polarized) light optical microscopy (LOM). Additionally, the local texture and microstructure in the weld-free material as well as in the weld seam region has been examined with a scanning electron microscope coupled with electron backscatter diffraction technique (SEM-EBSD). The weld-free material and the weld seam are characterized by recrystallized microstructures, whereas few residual cast grains were identified. The local texture distinctively changes from the weld-free material to the weld seam. The texture of the weld-free material is comparable with the typical ME21 sheet texture. In the weld seam area, a pole density is found, which is distributed towards the transverse direction (TD) combined with a split and broadening of the pole density in the extrusion direction (ED). This texture influences the mechanical anisotropy due to the dependence of the activation of basal 〈a〉-slip and \( \{ 10\bar{1}2\} \;\langle 10\bar{1}1\rangle \)-extension twinning on the loading direction in favorably oriented grains.     

Microstructural and textural evolution of pure titanium during differential speed rolling and subsequent annealing

The microstructural and textural evolution of pure titanium during differential speed rolling (DSR) at 500 °C and subsequent annealing were investigated using electron backscattered diffraction analysis. Twinning only occurs in the initial stage of DSR, and further deformation is dominated by dislocation slip. The as-rolled microstructure is characterized by large deformed grains, which form a major component of the material, and some dynamically recrystallized (DRXed) grains mainly existing in shear band regions. A dramatic change in the rolling texture, from the transverse direction (TD) split texture with basal poles largely tilted at ±40° to the single-peak basal texture, occurs during DSR. This change in texture is accelerated during the late stage of DSR and may be attributed to the gradual lattice rotation of deformed grains caused by enhanced basal slip activity. The basal texture changes back to the TD-split texture with basal poles tilted at ±25° after annealing due to consumption of deformed grains with basal orientations from DRXed grains with the TD-split texture. Subsequent extensive grain growth changes the orientation of the a-axis from parallel to the TD to parallel to the rolling direction, which is the result of the preferential grain growth in that direction.     

Experimental and numerical investigations of bonding interface behavior in stationary shoulder friction stir lap welding

Stationary shoulder friction stir lap welding (SSFSLW) was employed to weld 2024 aluminum alloy. A coupled Eulerian-Lagrangian (CEL) model was developed to investigate the lap interface behavior during SSFSLW. Numerical results of material movement and equivalent plastic strain were in good agreement with the experimental work. With increasing welding speed, the distances from the hook tip to the top surface of the upper workpiece on the retreating side (RS) and the advancing side (AS) increase, while the distance between two wave-shaped alclads decreases. A symmetric interface bending is observed on the AS and the RS during plunging, while the interface bending on the AS is bigger than that on the RS during welding. The peak temperature of the interface on the AS is higher than that on the RS. The equivalent plastic strain gradually increases as the distance to the weld center decreases, and its peak value is obtained near the bottom of the weld.     

一种新型固有应变法的焊接变形仿真

以T形焊接接头为算例,结合热弹塑性有限元法和映射应变分量法研究角焊缝角变形的产生机制,认为固有剪切应变才是产生角变形的主要根源,而不是传统思维认为的横向固有正应变.随后,以此观点为基础,对比分析了两种不同的加载方位对横向约束度、横向固有应变及残余角变形的影响.沿焊缝表面斜向加载更接近于实际测量值和热弹塑性模拟值,表明了此种加载方位的合理性,为更合理的利用固有应变法预测焊接残余变形提供了有益的参考.     

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.     

Deformation behaviour of Al–Cu–Li alloy containing T1 precipitates

In the present work, a systematic investigation of crystallographic texture evolution and strain hardening behaviour was undertaken to comprehend the deformation behaviour in the presence of T₁ (Al₂CuLi) precipitates. Characteristic texture components symbolising multiple slip condition such as Copper and S were observed upon rolling which is in contrast with other Al alloys containing shearable precipitates. Strain hardening ability was also observed to be remarkably high in the presence of T₁ precipitates. The texture and strain hardening results are compared with another age hardenable Al alloy (Al–Mg–Si alloy) containing shearable precipitates to clearly bring out the difference in the nature of T₁ precipitates.     

钢-竹组合工字形梁界面滑移及变形分析

基于初等梁变形理论和界面滑移与应变差关系,建立了钢-竹组合工字形梁界面相对滑移微分方程,推导出3种常见荷载作用下的组合梁钢-竹界面滑移和应变差解析解,在此基础上依据虚功原理提出了界面滑移引起的跨中附加挠度理论计算公式,从而形成了钢-竹组合工字形梁考虑滑移效应的跨中挠度计算方法,进一步通过6根梁的模型试验,探讨了界面应变差、界面滑移分布以及组合梁变形,并与试验结果进行了比较。理论分析和试验研究结果表明:钢-竹界面应变差的理论计算值与试验结果吻合较好,受压区和受拉区应变差分布基本相同;组合梁的纯弯区段,界面滑移曲线斜率增大,而弯剪区段则逐渐减小,界面滑移在梁端达到最大值,且在整个弯剪区段保持较大水平,因此在该区域布置连接件,可有效提高组合梁的整体工作性能;考虑界面滑移效应后的理论分析结果与未考虑滑移效应的换算截面法相比,更接近组合梁真实的变形,平均误差由11.5%减小为1.64%,随着变形的增大,基于界面滑移效应的钢-竹组合梁变形分析方法的优越性将更为突出。     

Hot workability analysis with processing map and texture characteristics of as-cast TX32 magnesium alloy

Hot deformation behavior of as-cast TX32 (Mg–3Sn–2Ca) alloy has been studied in uniaxial compression in the temperature and strain rate ranges of 300–500 °C and 0.0003–10 s^?1 with a view to characterize the evolution of microstructure and texture. On the basis of the temperature and strain rate dependence of flow stress, a processing map has been developed and the crystallographic orientation information on the deformed specimens has been obtained from electron back scatter diffraction micro-texture analysis. The processing map revealed two domains of dynamic recrystallization in the temperature and strain rate ranges of (1) 300–350 °C and 0.0003–0.001 s^?1 and (2) 390–500 °C and 0.005–0.6 s^?1. Specimens deformed at peak in Domain 1 exhibited maximum intensity of basal poles located at about 35–45° to the compression axis while those deformed at peak in Domain 2 showed near-random texture. Schmid factor analysis of different slip systems operating in the two domains suggests that basal + prismatic slip causes the basal texture in Domain 1 while second-order pyramidal slip randomizes the texture in Domain 2.     

Texture development in two-pass ECAE-processed beryllium

Texture development and substructure evolution are described for pure beryllium given two-pass equal channel angular extrusion (ECAE) processing following routes A and C. These routes impose different strain paths between the first and second passes—the former cross-shearing and the latter reversal. Polycrystal calculations that are in good agreement with the texture measurements suggest that basal slip and, secondly, prismatic slip are operative in both passes of both routes. Multi scale polycrystal modeling is shown to effectively predict texture evolution to strains of two caused by both ECAE processing routes. Shear-like deformation textures observed in the second pass of route C are explained by differences in deformation characteristics between the first and second passes.     

Crystal plasticity modeling of damage accumulation in dissimilar Mg alloy bi-crystals under high-cycle fatigue

Damage accumulation in Mg AZ31–AZ80 alloy bi-crystals under fatigue loading at room temperature is studied using a modified version of the crystal plasticity finite element model of Abdolvand and Daymond. The model accounts for strain accommodation by both slip and tensile twinning, and is first shown to reasonably describe monotonic single crystal Mg experimental data from the literature. The high cycle fatigue behavior was then investigated in misoriented dissimilar alloy bi-crystals through stress-controlled simulations up to 1000 cycles. Nine different orientation combinations were simulated and the fatigue damage evolution, defined as the cumulative shear strain amplitude, were compared and analyzed. The bi-crystal geometry was used to simulate possible microstructure combinations occurring, for instance within an idealized friction stir weld. Findings suggest that when either of the alloy bi-crystal grains is oriented for basal slip, poor fatigue performance can occur by twinning or slip localization depending upon the neighboring orientation.