OCC技术制备的单晶铜线材冷拔组织TEM分析

采用TEM系统分析了OCC技术制备的单晶铜线材在冷拔过程中形成的位错胞、几何必须界面以及孪晶,发现当真应变小于0.94时,冷拔单晶铜线材的微观组织为位错胞。当真应变大于0.94时,其微观组织主要包括位错胞和几何必须界面两类组织。其中位错胞沿冷拔方向拉长,横界面上为等轴状。随变形量的增加,位错胞尺寸减小,当真应变大于1.39时,位错胞尺寸不再随塑性变形量的增加而减小,其直径保持在0.28 μm左右。当变形量比较小时,两类几何必须位错界面属于晶体学界面,当变形量比较大时,几何必须位错界面与冷拔方向平行。同时,分析还发现冷拔单晶铜线材中存在少量由凝固过程形成并遗传到形变组织以及在冷拔过程直接形成的两类孪晶。     

孪生诱发软化与强化效应的Cu晶体塑性行为模拟

基于晶体塑性理论,考虑孪生软化效应建立了描述孪晶形核、增殖和长大的位错密度基晶体塑性有限元模型。应用该模型揭示了不同晶体取向Cu单晶拉伸变形过程中位错滑移、孪生激活及其交互作用下的宏观塑性行为演化规律,进一步分析了Cu多晶拉伸变形过程中晶粒间交互作用对孪生软化、应变硬化等宏观塑性行为的影响。结果表明：孪生具有明显的取向效应,在孪生主导塑性条件下,Cu单晶塑性变形过程中孪晶增殖导致应力-应变曲线存在明显的应力突降现象,其塑性变形分为滑移、孪生及位错与孪晶交互作用3个阶段;此外,随着饱和孪晶体积分数增加,Cu单晶塑性变形过程中第3阶段的应变硬化率也随之提升。进一步模拟Cu多晶拉伸变形的塑性行为可知,在晶粒间交互作用下孪晶形核、增殖和长大过程中不会出现应力突降现象,与Cu单晶相比整个塑性变形过程具有更高的应变硬化率;Cu多晶塑性变形过程中位错密度在晶界处出现集中现象,孪晶也容易在晶界处形成。     

孪生诱发软化与强化效应的Cu晶体塑性行为模拟

基于晶体塑性理论,考虑孪生软化效应建立了描述孪晶形核、增殖和长大的位错密度基晶体塑性有限元模型。应用该模型揭示了不同晶体取向Cu单晶拉伸变形过程中位错滑移、孪生激活及其交互作用下的宏观塑性行为演化规律,进一步分析了Cu多晶拉伸变形过程中晶粒间交互作用对孪生软化、应变硬化等宏观塑性行为的影响。结果表明：孪生具有明显的取向效应,在孪生主导塑性条件下,Cu单晶塑性变形过程中孪晶增殖导致应力-应变曲线存在明显的应力突降现象,其塑性变形分为滑移、孪生及位错与孪晶交互作用3个阶段;此外,随着饱和孪晶体积分数增加,Cu单晶塑性变形过程中第3阶段的应变硬化率也随之提升。进一步模拟Cu多晶拉伸变形的塑性行为可知,在晶粒间交互作用下孪晶形核、增殖和长大过程中不会出现应力突降现象,与Cu单晶相比整个塑性变形过程具有更高的应变硬化率;Cu多晶塑性变形过程中位错密度在晶界处出现集中现象,孪晶也容易在晶界处形成。     

应变速率对含空洞的镁孪晶界面塑性变形机制的影响

利用嵌入原子势的分子动力学模拟,研究了应变速率对含空洞的镁孪晶界面塑性变形机制的影响。结果表明,塑性变形的主要形式包括不全位错、滑移带和堆垛层错;应变速率不会改变试样的杨氏模量,应变速率愈大屈服应力愈大;随着应变速率增大,位错和滑移带的数量增加,堆垛层错的数目先增加后减小,位错运动自由行程的平均长度减小;随着变形进行,位错源不断产生新位错,导致位错密度提高;高应变速率时,晶界处容易形成应力集中,并会有微裂纹产生。     

塑性变形中晶粒变形与细化机制的影响因素

讨论了层错能、应变速率和变形温度等因素在塑性变形制备超细晶/纳米晶材料的变形过程中,对变形机制与晶粒细化机制的影响.研究表明,随着层错能的降低,晶粒的变形机制会由位错滑移向机械孪生转变,有利于晶粒的细化.应变速率的增加与变形温度的降低有利于抑制位错动态回复、增加流变应力,促使晶粒进一步细化.     

层错能对面心立方金属形变机制与力学性能的影响

层错能在面心立方(fcc)金属塑性变形和损伤过程中具有重要作用，本文主要总结了以下研究结果：(1)随层错能降低，fcc金属滑移方式逐渐从易于交滑移的波状滑移方式转变为平面滑移方式，直至发生变形孪生；(2)为了理解不同位错密度fcc金属中层错能的变化趋势，采用有效层错能的概念，随位错密度增加，有效层错能也随之升高；(3)层错能降低不是决定fcc金属形变孪生发生的唯一因素，通过第一原理计算模拟滑移和孪生之间的竞争关系，建立了fcc金属形变孪生临界判据；(4)通过对高层错能、中等层错能以及低层错能fcc金属疲劳位错组态的实验观察和分析，总结了fcc金属中形成规则驻留滑移带的判定条件；(5)随Al含量增加，Cu-Al合金层错能降低导致平面滑移程度增加，其拉伸强度和均匀延伸率呈现同步提高趋势；(6)采用指数应变硬化模型可精确描述Cu-Al合金拉伸加工硬化过程，进而预测了不同合金成分和微观组织状态Cu-Al合金屈服强度-抗拉强度-均匀延伸率之间的定量关系；(7)随Al含量增加，Cu-Al合金疲劳强度升高；在相同应变幅下，随Al含量增加，其低周疲劳寿命也升高。表明合金成分明显影响fcc金属形变损伤机...     

基于三维离散位错动力学的fcc结构单晶压缩应变率效应研究

基于位错理论建立了Ni单晶微柱压缩变形过程的三维离散位错动力学模型,该模型考虑了晶体塑性变形过程中位错所受的外载荷、位错间相互作用力、位错线张力及自由表面镜像力的影响。应用该模型研究了Ni单晶微柱压缩变形过程中流动应力和变形机制的应变率效应,同时,结合理论分析研究了应变率对流动应力中有效应力、位错源激活应力和位错间弹性相互作用力的影响。结果表明:当应变率较低时,Ni单晶微柱压缩变形中位错源激活应力主导流动应力,位错源激活数量较少,初始位错密度对流动应力影响很小,呈现单滑移变形;随着应变率增加,晶体变形过程中的流动应力随之增加,流动应力中位错源激活应力所占比例逐渐减小,有效应力逐渐主导流动应力,同时激活多个滑移系内的位错源来协调塑性变形;应变率越高,各激活滑移系内的塑性应变贡献相差越小,单晶微柱变形逐渐由单滑移向多滑移机制转变;在高应变率条件下,晶体初始位错密度越高塑性变形过程中流动应力越小。     

动态压缩条件下高纯钛微观组织特征研究

利用光学显微镜(OM)、背散射电子衍射(EBSD)技术及透射电子显微镜(TEM)对高纯钛低-中应变动态压缩变形的微观组织特征进行了研究。结果表明：随着应变量(ε)的增加,晶粒内部通过孪晶与孪晶,孪晶与位错以及位错与位错之间的交互作用逐步细化原始晶粒;变形初期,形变孪生以{11-22}孪晶为主,当ε达到0.2后,{10-12}孪晶转变为主要形变孪生类型,孪生改变了原始晶粒的取向,进一步促进晶粒内部的位错滑移。高纯钛动态压缩变形经历了由位错滑移到形变孪生,再到位错滑移主导的过程,但位错滑移和孪生始终共同作用协调动态压缩变形。     

动态压缩条件下高纯钛微观组织特征

利用光学显微镜(OM)、背散射电子衍射(EBSD)技术及透射电子显微镜(TEM)对高纯钛低-中应变动态压缩变形的微观组织特征进行了研究。结果表明:随着应变量(ε)的增加,晶粒内部通过孪晶与孪晶,孪晶与位错以及位错与位错之间的交互作用逐步细化原始晶粒;变形初期,形变孪生以{1122}孪晶为主,当ε达到0.2后,{1012}孪晶转变为主要形变孪生类型,孪生改变了原始晶粒的取向,进一步促进晶粒内部的位错滑移。高纯钛动态压缩变形经历了由位错滑移到形变孪生,再到位错滑移主导的过程,但位错滑移和孪生始终共同作用协调动态压缩变形。     

氮对冷变形高氮奥氏体不锈钢微观结构的作用

利用透射电镜(TEM)对氮含量为0.66%和1.2%的高氮奥氏体不锈钢的冷变形微观结构进行了对比研究.结果表明,氮含量的增加提高了材料的层错能,强烈促进位错的平面滑移.冷变形过程中不断形成机械孪晶,机械孪晶随变形量的增加而减小,在60%变形量时只有几百纳米长.运用扩展位错和短程有序理论对氮的作用机理的分析表明,氮合金化引起的短程有序是促进位错平面滑移、提高材料加工硬化能力的主要因素,低层错能引起的位错扩展也有重要作用.     

中低应变量冷轧AA1050铝合金中晶粒取向与形变位错界面的演变

采用ECC和EBSD技术研究了AA1050铝合金冷轧至中低应变量下位错界面结构的演变. 结果表明, 冷轧至中低应变量下形成典 型的位错胞块结构,其微观组织结构强烈依赖于晶粒的取向. 变形后主要形成三种典型的组织结构: Type A------两组交叉的几何必须位错界面 (GNBs) 结构,Type B------一组GNBs结构, Type C------近似等轴的普通胞状结构. 绝大多数Copper, Brass和Goss取向的晶粒具有Type A结构, S取向的晶粒具有Type B结构, 而Cube取向的晶粒主要具有Type C结构. 变形过程中的GNBs不仅具有与RD成±(30°---40°) 的宏观取向, 而且大多数GNBs位错界面近似平行于由最大Schmid因子所确定的{111}滑移面的迹线.     

Temperature dependence of microstructure and texture in cold drawn aluminum wire

The effect of strain and drawing temperature on the evolution of microstructure and fiber textures of aluminum wires drawn at room temperature and cryogenic temperature was investigated by TEM and EBSD observations. The results show that low angle boundaries frequency increases and high angle boundaries frequency decreases with strain increasing when the strain is low. At high strain, most of grain and dislocation boundaries are parallel to the drawn direction and low angle boundaries frequency decreases and high angle boundaries frequency increases with strain increasing. The decrease of deformation temperature leads to microstructure finer and low angle boundaries frequency increasing. Texture analysis indicates that volume fraction of complex texture component decreases with strain increasing and a mixture of 〈111〉 and 〈100〉 fiber texture forms at high strain. 〈111〉 is stable at low strains but 〈100〉 becomes stable at high strain. The decrease of temperature can enhance the stability of 〈111〉 orientation at high strain.     

Study on Deformation Structure and Texture of Pure Zirconium with Large Grain Size Rolled at Liquid Nitrogen Temperature

主要研究大晶粒退火态纯锆在液氮温度下均匀轧制时的形变组织特征及孪生机制。利用光学显微镜、扫描电镜、电子背散射衍衬（EBSD）、X射线衍射等对不同变形量样品的变形组织和织构进行了研究,重点结合软件对EBSD结果进行组织重构和机理分析。结果表明,液氮温度轧制时大晶粒纯锆中产生的孪晶类型为C1{112}<11>、T1{102}<10>和T2{111}<11>孪晶,其中C1{112}<11>孪晶最容易产生且为主要孪生类型。变形开始时,3种孪晶的数量迅速增多,而小角度晶界含量较少;变形量增大到30%时,小角度晶界含量占优势。变形初始阶段孪生优先于滑移进行, 且孪生变形是最主要的变形方式,当变形量为30%时,孪晶协调的位错滑移成为主要的变形方式。变形过程中织构类型未发生变化,保持基面双峰织构（偏离ND方向±30°左右）,但强度随着变形量的增大呈减小趋势     

Deformation twinning in nanocrystalline Al by molecular-dynamics simulation

We use a recently developed, massively parallel molecular-dynamics code for the simulation of polycrystal plasticity to elucidate the intricate interplay between dislocation and GB processes during room-temperature plastic deformation of model nanocrystalline-Al microstructures. Our simulations reveal that under relatively high stresses (of 2.5 GPa) and large plastic strains (of ˜12%), extensive deformation twinning takes place, in addition to deformation by the conventional dislocation-slip mechanism. Both heterogeneous and homogeneous nucleation of deformation twins is observed. The heterogeneous mechanism involves the successive emission of Shockley partials from the grain boundaries onto neighboring slip planes. By contrast, the homogeneous process takes place in the grain interiors, by a nucleation mechanism involving the dynamical overlap of the stacking faults of intrinsically and/or extrinsically dissociated dislocations. Our simulations also reveal the mechanism for the formation of a new grain, via an intricate interplay between deformation twinning and dislocation nucleation from the grain boundaries during the deformation. The propensity for deformation twinning observed in our simulations is surprising, given that the process has never been observed in coarse-grained Al and that the well-known pole mechanism cannot operated for such a small grain size. It therefore appears that the basic models for deformation twinning should be extended with particular emphasis on the role of grain-boundary sources in nanocrystalline materials.     

Stress-driven migration of simple low-angle mixed grain boundaries

We investigated the stress-induced migration of a class of simple low-angle mixed grain boundaries (LAMGBs) using a combination of discrete dislocation dynamics simulations and analytical arguments. The migration of LAMGBs under an externally applied stress can occur by dislocation glide, and was observed to be coupled to the motion parallel to the boundary plane, i.e. tangential motion. Both the migration and tangential velocities of the boundary are directly proportional to applied stress but independent of boundary misorientation. Depending on the dislocation structure of the boundary, either the migration or tangential velocity of the boundary can switch direction at sufficiently high dislocation climb mobility due to the dynamics of dislocation segments that can climb out of their respective slip planes. Finally, we show that the mobility of the LAMGBs studied in this work depends on the constituent dislocation structure and dislocation climb mobility, and is inversely proportional to misorientation.     

Interior dislocation behavior in superplastic deformation of 8090 Al-Li alloy

1lNTRODUCTl0NInteriordislocationslipisoneoftheimpor-tantaccomodationmechanisminsuperplasticde-f....ti..[1~10J,inwhichthegrainsarerefinedbydynamicrecrystallization.Butthesuperplas-ticdeformationprocessleadstodifferentgrainsizesatvariousstagesandcausesachangeingrainboundaryslidingquantity,italsomakestlieaccommodatingmechanismsatvariousstagesshowdifferentfunctions,especiallydislocationslipmechanism.Therefore,itissignificantf0rthisinvestigationtoreplenishthecurrenttheoryofsuperplasticdeformati…     

On the influence of the grain boundary misorientation on the plastic deformation of aluminum bicrystals

Aluminum bicrystals with symmetric 〈1 1 2〉 tilt boundaries and misorientations of 8.7° (small angle), 15.4° (transition), and 31.5° (large angle) were deformed in a channel die experiment in order to study the influence of misorientation on the deformation at grain boundaries. Samples were characterized by strain measurements and microtexture mappings. The experiments were compared to crystal plasticity finite element simulations. We studied strain heterogeneity at the macroscopic and at the microscopic level. Even macroscopically homogeneous areas showed microscopic heterogeneity in the form of bands of different sets of glide systems. We observed clear effects of the grain boundary misorientation on the deformation kinematics close to the boundaries. The 8.7° grain boundary did not show any orientation change which was interpreted in terms of free dislocation penetration. In contrast, the 15.4° and 31.5° bicrystals showed orientation changes which were attributed to dislocation pile-ups.     

Dense dislocation walls and microbands aligned with slip planes-theoretical considerations

Some of the dislocation boundaries in cold deformed f.c.c. metals at low and intermediate strains lie on crystallographic slip planes and others have a macroscopic direction with respect to the sample axes (i.e. they are non-crystallographic). A model for the occurrence of the former type of dislocation boundaries is proposed. The model combines slip pattern analysis and dislocation theory. It is assumed (i) that the dislocations in the boundaries are generated by slip, (ii) that the deformation temperature is low enough to exclude dislocation climb and (iii) that the driving force for formation of boundaries is minimisation of the energy stored in the boundaries. Formation of crystallographic boundaries is predicted if two active slip systems in the same slip plane account for a large fraction of the total slip. For single crystals the agreement between predicted and experimentally observed crystallographic and non-crystallographic boundaries is excellent. For different grain orientation in poly crystalline aluminium specimens, the agreement between prediction and experiment is satisfactory in view of the complexity of polycrystal studies compared to studies of single crystals.     

Twinning during hot compression of ZK30 ＋ 0.3Yb magnesium alloy

The twinning process of ZK30＋0.3Yb magnesium alloy was studied. The results show that twinning occurs at the initial stage of deformation, and decreases during further deformation. The original grain is fragmented after small straining. It is investigated that the twinning boundary activates the occurrence of the non-basal slip system due to the stress concentration at the vicinity of twin boundary introduced by the dislocation pile-ups at the vicinity of twinning boundary. The rearrangement of dislocation after dislocation climb introduces new grain boundary. Simultaneously, twinning occurs to form “polygonization” due to the stress concentration relaxation, and the “polygonization” will transform into low angle boundary to refine the original grain under the shear stress with further straining.     

晶界对近片层TiAl高温动态力学行为的数值模拟

基于晶体塑性理论,给出了同时考虑位错滑移、形变孪晶和晶界变形的近片层组织TiAl本构模型;在此基础上,建立基于Voronoi算法的近层片TiAl三维多晶有限元模型,并在晶粒交界处引入壳单元来描述晶界;利用上述有限元模型,对不同温度(室温、500和700℃)和不同拉伸应变率(10-3、320、800和1 350 s-1)下近层片TiAl的塑性力学行为进行数值模拟。结果显示:模拟得到的应力塑性应变曲线与试验结果吻合较好,能够反映近层片TiAl在不同温度和应变率下的材料响应;由于晶界的存在,晶粒内的应力分布会发生明显改变,晶界附近产生一定的应力集中。此外,晶界对孪晶存在一定的阻碍作用,使得晶界附近实体单元的孪晶体积分数要略低于多晶整体的平均孪晶体积分数。