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。     

晶粒位向对AZ31挤压板材Hall-Petch关系的影响

本文研究了AZ31镁合金挤压板材热处理后不同晶粒尺寸下各角度的拉伸性能与变形行为,通过分析拉伸曲线和变形后的位错类型来确定不同角度下的拉伸变形机制。结果发现:沿板材法向0°,22.5°,45°,67.5°,90°拉伸后的屈服强度与晶粒尺寸符合Hall-Petch(H-P)关系,但H-P参数值各不相同,0°具有最高的σ_0和最低的K_y,其变形机制由拉伸孪晶为主导与棱柱面a滑移共同作用;45°拉伸时具有最低的σ_0和最高的K_y,变形机制为基面a滑移主导;90°下的H-P参数值与0°相接近,c位错及a+c位错开始出现,棱柱面滑移与拉伸孪晶共同主导变形机制,孪晶可以协调变形改变晶粒取向和促进非基面滑移的产生。     

柱状多晶Mg-Gd-Y合金形变组织演变及形变硬化机制

利用定向凝固技术制备了生长取向集中于■、基面〈a〉滑移取向因子(Schmidfactor, SF)大于0.4的柱状多晶Mg-6.38Gd-0.45Y合金,并研究了实验合金室温拉伸形变行为。结果表明,形变初期,软取向柱状晶内首先启动■拉伸孪生协调应变。形变过程中■,拉伸孪晶界快速、大范围扩展,吞噬基体并使基体取向逐渐转为■,于是启动■压缩孪生和■双孪生协调应变。压缩孪晶和双孪晶易形成压缩孪晶带群,并贯穿整个晶粒,滑移或扩展的位错及拉伸孪晶界与压缩孪晶带群交织在一起,产生形变硬化,提高合金强度的同时也形成应力高度集中分布区域,成为微裂纹形成之地。     

一种镍基单晶高温合金压缩变形的各向异性

将一种镍基单晶高温合金在室温沿不同晶体取向压缩变形,研究了在试样表面形成的滑移线和微观组织的变化。结果表明：这种合金〈111〉取向试样的屈服强度最高,〈001〉取向的强度最低,〈110〉取向的强度居中。〈001〉和〈110〉取向的试样在压缩变形过程中主要启动八面体滑移系,其中〈110〉取向明显观察到双滑移系的开动;而〈...     

TWIP钢拉伸变形过程中微观组织的原位观察

采用扫描电镜对TWIP钢的拉伸过程进行了原位观察,研究结果表明:在TWIP钢的拉伸过程中,具有孪晶界的晶粒内部首先发生变形,并产生一定程度的加工硬化;随后其余部分晶粒发生转变,形成对变形有利的取向,从而变形得到扩展,最终得到非常大的无颈缩延伸.拉伸过程中,微裂纹首先在夹杂物和晶界处萌生,并在应力集中的作用下发生扩展连接,最终完成断裂过程.TWIP钢在变形过程中产生了大量的应变条痕,其大部分为滑移线,小部分为形变孪晶.由于孪晶诱导塑性效应,孪晶晶粒发生扭折并形成了台阶状结构,从而导致了试样表面的"褶皱"现象.     

镁合金塑性变形中孪生的研究

介绍了镁合金变形过程中孪生的晶体学、位错机理以及几何位向学;探讨了孪晶的形核、长大与演变机制;分析了孪生过程对塑性变形的作用;论述了影响孪生的几种基本因素,包括晶粒取向、变形温度、变形速度、晶粒尺寸、预变形.研究结果表明,镁合金塑性变形过程中孪生变形的作用在于,通过孪生过程改变晶粒取向或通过孪晶间或孪晶与滑移之间的相互作用,诱发新的滑移和孪生;孪晶也可抑制裂纹的产生和扩展,从而提高变形镁合金的室温塑性.     

高应变速率下等径角挤压高纯粗晶铝中的形变孪晶与退火孪晶

在室温下对铸态高纯粗晶铝进行一道次高应变率动态等径角挤压(D-ECAP)变形,利用电子背散射衍射技术(EBSD)研究挤压过程中所形成的孪晶。结果表明:利用D-ECAP能够在粗晶铝中同时制备出形变孪晶和退火孪晶,但两者在形态、Kernel平均取向差(KAM)以及与相邻晶粒的取向差三个方面存在较大差异。D-ECAP高应变率和大剪切变形使高层错能铝中形成了百微米级的形变孪晶,形变孪晶的形态为透镜状,后续变形使得孪晶界偏离∑3 60°〈111〉取向关系且KAM值主要集中于0.6°~1.8°。高应变率剪切变形下形成的大量层错和复杂的位错组态以及高形变储存能在变形温升的作用下促进了退火孪晶的形成。退火孪晶的形态较不规则,但孪晶界的取向关系更接近于∑3 60°〈111〉且KAM值主要集中于0.2°~0.5°。     

电工钢中柱状晶热压缩时取向的变化及对析出的影响

利用电子背散射衍射(EBSD)技术和场发射电子扫描显微镜(FESEM)研究了电工钢中柱状晶不同初始取向50%热压缩变形后的演变规律及其对粒子析出行为的影响。结果表明:〈100〉晶粒形变后仍为〈100〉取向,只发生绕ND∥〈100〉方向的转动,具有遗传性,粒子析出数量少。柱状晶长轴与压缩轴的偏转度增加,取向的稳定性变差,超过40°,可形成〈111〉取向,期间发生复杂的滑移系交互作用,产生较高的缺陷密度,促进粒子析出。MnS粒子主要在晶内析出,变形温度高、形变量低,粒子均无法充分析出。回复时,形变晶粒内高的储能和缺陷密度是促进粒子进一步析出的必要条件。     

工业纯钛的低温拉伸行为研究

研究了工业纯钛TA2在-196℃到25℃时的拉伸性能及塑性变形机制。结果表明,随着温度的降低,TA2屈服强度和抗拉强度逐渐升高,断后伸长率先升高后降低,在-140℃达到最大值,断面收缩率逐渐升高,在-80℃达到79%后保持稳定。25℃、-80℃、-120℃、-196℃拉伸试样中均观察到孪晶、高密度位错、变形组织。25℃时,拉伸试样中孪晶极少。-80℃、-120℃、-196℃时孪晶数量大幅度增加,且不同温度拉伸样品中孪晶形态差异很大。25℃时,TA2拉伸过程中塑形变形为滑移主导,随着温度降低,挛生在拉伸塑性变形过程中发挥的作用逐渐增大。     

直接时效热连轧GH4169合金的力学性能与变形特征

通过对热连轧GH4169合金进行直接时效处理、组织形貌观察和力学性能测试,研究了直接时效热连轧合金的力学性能和变形特征.结果表明,直接时效热连轧GH4169合金组织由细小晶粒组成,具有明显的形变孪晶特征,且有细小#$和#%相在合金中弥散析出,可提高合金强度.在实验温度范围内,合金的抗拉和屈服强度随着温度的升高而逐渐下降;在拉伸过程中,直接时效热连轧GH4169合金的变形特征是孪晶变形和位错的双取向滑移;在拉伸后期,大量微孔沿晶界或晶内形成并聚合形成裂纹,致使合金发生韧性断裂.     

Numerical formability assessment in single crystals of magnesium

In this paper a rate-sensitive elastic–viscoplastic crystal plasticity constitutive model (CPCM) together with the Marciniak–Kuczynski (M–K) approach have been used to assess the formability of a magnesium single crystal sheet by simulating the forming limit diagrams (FLDs). Sheet necking is initiated from an initial imperfection in terms of a narrow band. A homogeneous deformation field is assumed inside and outside the band, and conditions of compatibility and equilibrium are enforced across the band interfaces. Thus, the CPCM only needs to be applied to two regions, one inside and one outside the band. The FLDs have been simulated under two conditions: (a) the plastic deformation mechanisms are basal, pyramidal 〈c + a〉, and prismatic slip systems, and (b) the plastic deformation mechanisms are basal, pyramidal 〈c + a〉, and prismatic slip systems, as well as extension and contraction twinning systems. The FLDs have been generated for two grain orientations. In the first orientation pyramidal 〈c + a〉 and extension twinning systems, and in the second orientation basal and pyramidal 〈c + a〉 slip systems, as well as contraction twinning systems have favourable orientation for activation. The effects of shear strains outside the necking band, strain rate sensitivity, and c/a ratio on the simulated FLDs in the two grain orientations have been individually explored.     

GH4169合金楔横轧加工过程中动态再结晶及织构演变

为研究GH4169合金楔横轧加工过程中动态再结晶及织构演变规律,采用金相显微镜(OM)和电子背散射衍射(EBSD)对30%,50%两种断面收缩率下GH4169合金楔横轧件表层与心部的微观组织、晶体取向及织构进行分析。结果表明:GH4169合金楔横轧加工过程中,随着动态再结晶的发生,晶体取向逐渐变得随机化分布;轧制表层大角度晶界数量较轧件心部多,轧件表层织构强度变化不大,心部织构强度明显增强;经过楔横轧变形后织构发生转动,原始态织构类型为{001}〈110〉,{111}〈110〉,{111}〈011〉,轧制后主要织构类型为{001}〈010〉,{112}〈110〉,{110}〈111〉,{110}〈112〉;GH4169合金楔横轧件动态再结晶及织构演变规律是由楔横轧特殊变形特点决定的。     

Texture effect on microyielding of wrought magnesium alloy AZ31

Uniaxial tension tests and elastoplastic self-consistent (EPSC) simulations were conducted to study the effect of texture on the microyielding of highly textured AZ31 plate. The onset of microyielding is always associated with the activation of basal 〈a〉 slip irrespective of starting texture, while the value of the microyield stress does depend on the texture. The macroscopic yield stress is largely affected by the critical resolved shear stress of the deformation mode with the highest Schmid factor. An inverse relation is found between the microyield strength normalized by the macroscopic yield strength and the average Schmid factor of prism 〈a〉 slip, which is useful for a rough estimate of the microyield strength.     

Study of Deformation Mechanisms in Titanium by Interrupted Rolling and Channel Die Compression Tests

The mechanisms of small plastic deformation of titanium (T40) during cold rolling and channel die compression by means of "interrupted in situ" EBSD orientation measurements were studied. These interrupted EBSD orientation measurements allow to determine the rotation flow field which leads to the development of the crystallographic texture during the plastic deformation. Results show that during rolling, tension twins and compression twins occur and various glide systems are activated, the number of grains being larger with twins than with slip traces. In channel die compression, only tension twins are observed in some grains, whereas slip traces can be spotted in almost all observed grains. The different stress conditions and different strain rates existing under the two modes of deformation lead to the activation of different deformation mechanisms.     

AZ31B镁合金压-压循环载荷下变形行为及变形机制演化EI北大核心CSCD

为探讨AZ31B挤压态镁合金棒材沿径向取样的循环变形特征,开展了0.75%,1.0%,2.0%和4.0%应变幅下应变控制的非对称压-压循环变形实验。结果表明:在小应变幅(0.75%,1.0%)下,循环变形的滞回曲线表现出较好的对称性;在大应变幅(2.0%,4.0%)下,滞回曲线对称性差,且在滞回曲线上出现拐点;随着循环周次增加,塑性应变幅呈现下降趋势,材料均表现出循环硬化行为,在小应变幅下循环拉伸阶段对材料硬化率远大于压缩阶段的硬化率,而在大应变幅下这种区别并不明显。分析表明,沿径向取向的〈1120〉丝织构镁合金,小应变幅下位错滑移在整个寿命周期内作用更大;大应变幅下,随着塑性变形的增加,循环过程中变形机制发生演化,较低临界剪切应力(critical resolved shear stress,CRSS)的基面位错和拉伸孪生不能完全满足变形要求,较高CRSS滑移系启动及残余孪晶使得滞回曲线出现拐点;循环变形过程中不完全的孪生-去孪生过程使基体中存在大量残余孪晶,影响了循环变形过程的硬化率,同时降低了疲劳寿命。     

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.     

Significant effect of as-cast microstructure on texture evolution and magnetic properties of strip cast non-oriented silicon steel

In this study, two types of as-cast microstructure produced by strip casting were cold rolled and annealed to investigate the effect of initial microstructure on the textural evolution and magnetic properties of non-oriented silicon steel. The results indicated that the cold-rolled sheets of coarse-grained strip with pronounced {100} components exhibited stronger λ fiber (〈100〉//ND) and weaker γ fiber (〈111〉//ND) texture as composed to the fine-grained strip with strong Goss ({110}〈001〉) texture. After annealing, the former was dominated by η fiber (〈001〉//RD) texture with a peak at {110}〈001〉 orientation, while the latter consisted of strong {111}〈112〉 and relatively weak {110}〈001〉 texture. In addition, a number of precipitates of size ～30–150?nm restricted the grain growth during annealing, resulting in recrystallization of grain size of ～46?μm in the coarse-grained specimen and ～41?μm in the fine-grained specimen. Ultimately, higher magnetic induction (～1.72?T) and lower core loss (～4.04?W/kg) were obtained in the final annealed sheets of coarse-grained strip with strong {100} texture.     

Influence of texture on twin boundary cracks in fatigued austenitic stainless steel

It is well-known that crack initiation in fatigued austenitic steel (316L) specimens is dominated at lower deformation amplitudes by twin boundaries (TBs). For medium plastic strain amplitudes, it is shown here that the propagation of short cracks starting at TBs can be explained when both the surface tractions caused by elastic anisotropy as well as the related slip processes are considered. This conclusion has been obtained from grain orientation measurements along damaged TBs using the electron backscatter diffraction technique in the scanning electron microscope. The frequency of the damaged TBs strongly depends on the meso-texture given by the distribution of 60° 〈111〉 rotation axes of the twins in the pole figure. The texture was determined by automatic orientation mapping. Specimens, which were machined transverse to the rolling direction of the plate, show more damaged TBs than those machined parallel. Consequently, the risk of TB cracks can be reduced by favorable alignment of the specimens with respect to the rolling direction.     

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   

金属钛拉伸的分子动力学研究

本文利用分子动力学的研究方法,采用了钛的嵌入式原子势,建立了沿[0001]晶向和[0110]方向拉伸模型,结果表明:两种方向的拉伸均包含弹性变形阶段、屈服阶段、颈缩阶段、断裂阶段.沿[0001]方向拉伸时,滑移系少,取向偏离软取向.变形时屈服强度为3.55GPa,屈服应变为0.063,断裂时的应变达到0.55.沿[01...