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。     

准静态压缩纯钛的微观结构和织构演变

对纯钛圆柱样品进行准静态压缩试验,研究纯钛在准静态条件下的微观结构和织构演变及其变形机制。结果发现:准静态压缩纯钛中均形成了{2110}、{1211}和{2211}3种类型形变孪晶,微观结构演变经历了形变孪晶细化晶粒、孪晶达到饱和和晶粒破碎细化阶段。织构演变也经历了初始双峰织构转变为圆环状分布、圆环状分布转变为基面织构和基面织构增强3个阶段。微观结构和织构演变分析表明,准静态压缩纯钛的变形机制是:低应变阶段(ε≤0.3)以形变孪生为主,孪生饱和后(ε0.3)转变为以位错滑移为主,其孪生饱和临界应变为0.3。     

高应变速率对纯钛塑性变形的影响

利用动态塑性变形(DPD)和准静态压缩变形(QSC)技术对纯钛圆柱样品进行对比压缩试验,研究了不同应变速率下纯钛形变孪晶和微结构演变。结果发现:2种变形方式的变形机制相似,低应变时以形变孪生为主,孪生饱和后转变为位错滑移主导;高应变速率促进了形变孪晶的产生,激发{4211}压缩孪晶的形成,同时使变形机制转变临界应变提前至0.2;纯钛在高应变速率和高应变(ε≥0.6)下出现绝热剪切带(ASB)。     

高纯钛压缩孪生行为研究

利用背散射电子衍射(EBSD)技术对高纯钛形变组织中同一个晶粒内部出现的{1122}和{1124}压缩孪生进行了研究。结果表明:{1124}孪晶总是伴随{1122}孪晶在同一晶粒中产生,在变形组织中没有发现单独存在的{1124}孪晶;这种极少出现的{1124}压缩孪晶主要由{1122}孪晶与传统晶界或{1122}孪生变体之间交互作用改变了局部应力状态而诱发的。在同一个晶粒中,由{1122}孪晶诱发的{1124}孪晶更倾向于同其中一个{1122}孪生变体具有相同的转轴。此外,根据晶体对称性及相同晶粒中{1122}和{1124}孪晶之间的取向关系,{1122}和{1124}孪晶之间可发生孪晶反应并形成4种不同类型的孪晶反应界面。     

氧含量对纯钛动态变形行为的影响

通过准原位EBSD和SEM观察研究了纯钛、Ti-0.2% O和Ti-0.4% O（质量分数）多晶体在高应变速率下的变形行为。结果表明：在5%应变的动态压缩变形下,纯钛中的孪生行为非常活跃,多数晶粒内的孪生系被激活,且半数晶粒中出现多种孪生变体;而滑移迹线分析表明,仅有50%的晶粒内开动了滑移系。随着氧含量的增加,孪生晶粒比例及孪晶面积占比均呈下降趋势,同时发生多滑移与交滑移。XRD分析表明,溶质氧原子导致晶格畸变,提高了α-Ti的c/a,有利于位错滑移。活跃的位错滑移行为抑制了孪晶的形成,并且氧原子钉扎位错也会阻碍孪晶界的扩展,导致孪生行为不再活跃。此外,氧含量每增加0.2%（质量分数）,纯钛的动态屈服强度就能增加约390 MPa。这种固溶强化现象主要源于晶格畸变,也受到被钉扎位错及多滑移和交滑移产生的割阶影响。     

挤压镁合金AZ31的压缩过程中的组织及织构演化

对常规挤压态镁合金AZ31压缩过程的组织及织构演化进行了扫描电镜-电子背散射衍射(SEM-EBSD)原位观察。结果表明材料的初始组织为等轴晶,晶粒的平均尺寸为76微米,晶粒内部未发现形变孪晶。材料的初始织构类型为典型的{11-20}丝织构,即大多数晶粒的<11-20>晶向平行于棒材的挤压方向(ED)。在压缩过程中,多数晶粒内部开始出现拉伸孪晶,随着压缩应变的增加,孪晶片层不断增厚,导致晶内的孪晶合并成大的孪晶并占据晶粒内部的大部分区域进而使孪晶的体积分数不断增加。随着压缩压缩应变的增加初始丝织构不断减弱并有新的基面织构形成。实验表明压缩过程中的{10-12}<10-11>孪生而非滑移是引起压缩过程中织构演化的主要原因。     

动态塑性变形下AZ31镁合金的孪生特征

沿热轧退火态AZ31镁合金轧制方向进行室温动态塑性变形,采用场发射扫描电子显微镜研究{10ī2}孪生的变体类型及结构特征。结果表明:在{10ī2}孪生机制主导塑性变形的初级阶段(ε8%),AZ31镁合金中大部分晶粒内部仅产生一种{10ī2}孪生变体或变体对,所以孪晶结构呈彼此平行状,将晶粒分割成片层组织;这种片层组织的厚度随着应变量的增加而减小,从5.55μm减小到2.49μm;少数晶粒形核产生不同的{10ī2}孪生变体,且这种晶粒的数量随着塑性变形量的增大而增加;当ε8%,孪生体积分数接近饱和,位错滑移成为镁合金主要的变形机制。     

镁单晶沿不同取向压缩的变形机制分子动力学研究

基于分子动力学理论,建立沿C轴以及10 10(垂直C轴)方向进行单轴压缩的模型,结合两种模型的应力-应变曲线,分析镁单晶沿不同取向压缩的微观变形机制。结果表明,沿C轴压缩时模型的压缩弹性模量较大,说明该取向难变形。且该模型先发生基面不全位错滑移(柏氏矢量b→1=1/3 10 10)以及锥面位错滑移(柏氏矢量b→2=1/6 02 23),其次在位错畸变区形核产生{10 11}孪晶。此外,在晶体内部观察到两种不同类型的{10 11}孪晶变体。沿垂直C轴方向压缩过程中,首先会形成大量的紊乱点,为位错以及孪晶的产生提供形核点。进一步加载时,会出现{10 12}孪生过程,且{10 12}孪晶迅速吞噬基体,模型变为沿C轴方向压缩变形,最后在位错堆积的畸变区形核生成{10 11}二次孪晶。     

AZ31镁合金在以孪生主导的塑性变形中力学行为的模拟（英文）

从实验和黏塑性自洽(VPSC)模拟两个方面定量分析具有织构的AZ31镁合金中孪晶数量及其与应力-应变曲线的关系。沿着两个不同的方向进行压缩以启动{1012}拉伸或者{1011}压缩孪生。{1012}拉伸孪晶在塑性变形的初始阶段形核并且长大到完全吞并母体。当沿着法向压缩时,{1011}孪生和{1011}-{1012}二次孪生在应变量为0.05时开始启动,并且这些孪晶的数量一直增加直到材料断裂,即应变量大于0.15。当沿着横向压缩应变量为0.06时,也会启动{1011}孪生和{1011}-{1012}二次孪生,然后在已经完全发生{1012}孪生的晶粒中大量启动。应用适当的参数,VPSC模型可以准确地判断拉伸孪晶、压缩孪晶和二次孪晶的启动和流动应力以及变形织构。从模拟中可以看出,孪生和滑移具有相同的硬化参数。     

高强Ti-26合金室温压缩性能和显微组织演变研究

通过单道次压缩实验研究了压缩速率0.001~0.1 s~(-1)和压缩量20%~60%条件下Ti-26合金室温压缩变形行为,分析了Ti-26合金室温压缩力学性能,显微组织演变和变形机制。研究结果表明:相同压缩量条件,压缩速率为0.1 s~(-1)时,真应力-应变曲线存在明显不连续屈服现象,且屈服强度对压缩速率变化更敏感。随着压缩速率、压缩量增加,大变形区晶粒由等轴状变为细长纤维状,且两者增加越大,晶粒伸长程度越显著。室温小变形量条件,合金微观组织存在明显滑移和少量孪晶,位错滑移和孪生协调进行并促进合金室温变形;大变形量条件,滑移占据更多单个β晶粒并向周围β晶粒扩展,大量滑移将少量孪晶完全吞噬,位错滑移成为合金室温变形的唯一机制。     

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 mechanisms of pure Ti during equal channel angular pressing

The microstructural development of commercially pure titanium was investigated to elucidate the mechanisms of grain refinement and strain accommodation during equal channel angular pressing. The samples were processed at 623 K via route C, in which the sample was rotated 180° around its longitudinal axis between the passes. TEM micrographs of the sample undergoing the first pass revealed that the strain imposed by the pressing is accommodated mainly by {1011} deformation twinning. During the second pass, the deformation mechanism was changed to dislocation slip. TEM analysis indicated that the slip system consisted of alternating twin bands containing dislocations ofa slip on a prismatic plane and ofa+c slip on a pyramidal plane. Microstructural evolution in commercially pure titanium subjected to equal channel angular pressing was discussed based on the preferred orientation formed during the first pass and resolved shear stress for the slip systems.     

Exploring Plastic Deformation Mechanism of Multilayered Cu/Ti Composites by Using Molecular Dynamics ModelingEI北大核心CSCD

Multilayered metallic composites have attracted great interest because of their excellent characteristics. In recent years, the mechanical behavior of Cu/Ti composites is described in terms of macroscopic or mesoscopic scales, but the micromechanism regarding dislocation slip, twinning and shear banding at heterogeneous interfaces remains unclear. In this work, the molecular dynamics method is used to study the uniaxial tensile and plane strain compression deformation of the Cu/Ti multilayered composites with characteristic initial crystal orientations. The simulation results show that under the tensile load, dislocations are preferentially nucleated at the heterogeneous interface between Cu and Ti, and then slip along {111} plane within the Cu layers. The corresponding mechanism is confined layer slip. With the multiplication of dislocations, dislocations interact with each other, and intrinsic stacking faults and deformation twins are formed in Cu layers. However, no dislocation slip or twinning is activated within the Ti layers at this stage of deformation. As the load increases, the stress concentration at the Cu/Ti interface leads to the fracture of the composites. For the composites under plane strain compression, the stress concentration at the Cu/Ti interface triggers the formation of shear bands in the Ti layer, and there are only very limited dislocations within the shear bands and their adjacent area. With the increase of applied strain, the common action of various deformation mechanisms causes the grains to rotate, and the disorder degree of complex atoms increases. In addition, the micro-plastic deformation mechanism and mechanical properties of Cu/Ti complex with different initial orientations and strain rates are significantly different. The results reveal the microscopic deformation mechanism of the laminated composites containing hcp metals.     

Deformation Mechanisms of Alpha-Uranium Single Crystals

The operative deformation elements in α-uranium single crystals under compression at room temperature have been determined as a function of the compression directions. The deformation mechanisms noted may be arranged with respect to their frequency of occurrence and ease of operation in the following order: 1—(010)-[100] slip, 2—{130} twinning, 3—{~172} twinning, and 4—under special conditions of stress application, kinking, cross-slip, {~176} twinning, and {011} slip. The composition planes of the {172} and {176} systems were found to be irrational. Cross-slip was shown to be associated with the major (010) slip system, coupled with localized interaction of slip on the (001) planes. The mechanism of kinking was found to be similar to that observed in other metals in that it occurred chiefly when the compression direction was nearly parallel to the principal slip direction [100] and was associated with a lattice rotation about an axis contained in the slip plane and normal to the slip direction: the [001] in the uranium lattice. The resolved critical shear stress for slip on the (010)-[100] system was found to be 0.34 kg per mm². In a single test it was shown that under compression in suitable directions twinning on the {130} also occurs at 600°C     

Mg-3Y合金大应变量轧制过程中的孪生辅助动态再结晶及组织演变

采用预挤压加单道次大应变量热轧制的方法制备了Mg-3Y（质量分数,%）合金板材。并研究了大应变量轧制过程中不同孪晶类型对合金动态再结晶（DRX）及组织演变的影响。结果表明,在挤压比为8:1的预挤压过程中,合金内部发生了几乎完全的动态再结晶。而在接下来的大应变量热轧制过程中,孪生变形尤其是■压缩孪晶及■双孪晶在协调合金的塑性应变中发挥了重要作用。此外,大量动态再结晶在压缩孪晶及双孪晶内部发生,并扩展到非孪晶区域,有效缓解了轧制过程中的内应力集中。上述2个过程对提高合金在大应变量轧制中的成形性均起到了促进作用。     

Microstructural Evolution and Biodegradation Response of Mg-2Zn-0.SNd Alloy During Tensile and Compressive Deformation

The effect of deformation behavior on the in vitro corrosion rate of Mg-2Zn-0.5Nd alloy was investigated experimentally after uniaxial tensile and compressive stress.The microstructure and texture were characterized using electron backscattered diffraction and X-ray diffraction,while potentiodynamic polarization and immersion tests were used to investigate the cor-rosion response after deformation.The result reveals that applied compressive stress has more dominant effect on the corro-sion rate of Mg-2Zn-0.5Nd alloy as compared to tensile stress.Both tensile and compressive strains introduce dislocation slip and deformation twins in the alloy,thereby accelerating the corrosion rate due to the increased stress corrosion related to dislocation slips and deformation twins.The { 10(1)2} tension twinning and prismatic slip were the major contributors to tensile deformation while basal slip,and { 10(1)2} tension twin were obtainable during compressive deformation.The twinning activity after deformation increases with the plastic strain and this correlates with the degradation rate.     

压下量对WE43镁合金变形机理和孪生行为的影响（英文）

研究最后一道次压下量对铸态WE43镁合金在480℃直接热轧过程中位错滑移和孪生行为的影响。结果表明,在480℃轧制时柱面滑移始终是主要的滑移模式。另外,不同压下量下激活的孪生类型不同。较小的压下量(2%)下激活的主要是{■}拉伸孪生,而在较大的压下量(12%和20%)下激活的主要是{■}压缩孪生和■二次孪生。Schmid因子计算表明,{■}孪晶变体选择遵循Schmid定律,而{■}孪晶变体选择不遵循Schmid定律。即使单道次压下量达到20%也几乎没有发现动态再结晶晶粒,这可能是因为没有达到动态再结晶所需要的变形量。     

AZ31镁合金挤压板材的中温变形机理及软化机制(英文)

研究AZ31镁合金挤压板材在473～523K的温度范围内。应变速率0.001～1.0s-1压缩时的流变应力行为,计算板材沿挤压方向压缩时的激活能,并结合光学显微镜和透射电子显微镜探讨合金软化机制和变形机理之间的联系。结果表明,在中温下沿挤压方向压缩时,AZ31挤压态镁合金的变形激活能为174.18kJ/mol。这说明,由热激活位错交滑移所控制的动态再结晶是合金中温变形的主要软化机制。位错滑移是中温变形的主要变形机理,而孪生的作用则不大。其主要的动态再结晶机制为持续动态再结晶,并伴随少量的孪生动态再结晶。     

低温轧制纯锆的形变孪晶及织构演变

通过在77K温度下进行不同变形量的低温轧制实验,研究了具有强烈单轴织构的工业纯锆板材在低温轧制变形条件下的孪生行为及变形机理。采用扫描电镜（SEM）和电子背散射衍衬（EBSD）分析和表征了变形材料的微观组织和织构。结果表明,在沿C轴加载的低应变条件下{1022}〈1123〉压缩孪生是主要的变形机制,同时在{1022}〈1123〉压缩孪晶中产生了二次孪晶（{10}-2}〈10T1〉拉伸孪晶）以协调变形。施密特因子计算及孪晶分布的EBSD模拟结果表明,在低温变形条件下的孪生模式的选择是由施密特因子的数值大小决定的。探讨并解释了轧制过程中随着应变量增加由孪生所导致的织构演变。     

LOW CYCLE FATIGUE BEHAVIORS AND MICROSTRUCTURES OF Ti–2Al–2.5Zr WITH FINE GRAIN AT RT AND 77 K

对细晶Ti--2Al--2.5Zr合金进行了室温/低温(77 K)疲劳实验及微观组织观察. 结果表明: 室温低应变幅Δε_t/2(=0.5%, 1.0%)下,合金表现为循环软化; 室温高应变幅(1.5%, 2.0%)下, 则表现为循环应力饱和; 77 K时, 不同应变幅下均表现为循环硬化, 且随应变幅升高, 循环硬化程度增强. 疲劳寿命测试结果表明: 低温疲劳寿命始终高于室温. 断口SEM观察表明, 室温和低温下, 疲劳裂纹扩展区均有明显的疲劳条纹,疲劳裂纹以穿晶方式扩展, 室温下伴随有大量二次裂纹, 低温下的二次裂纹数量明显减少. TEM观察表明: 低温下孪生是合金主要的变形方式, 包括{1011}和{1121}型孪晶. 疲劳变形位错组态为: 室温较低应变幅(0.5%, 1.0%)下, 形成位错线和局部位错缠结; 室温下应变幅提高到1.5%和2.0%时,\{1010}柱面和{1121}锥面滑移同时开动, 位错组态演化为亚晶和明显的位错胞. 77 K下, 应变幅2.0%时形成沿 柱面平行分布的位错带; 77 K下应变幅升高到4.5%时, 多滑移形成相互垂直的位错线. 低温诱发形变孪晶是Ti--2Al--2.5Zr低温疲劳寿命升高的原因.