等通道挤压变形奥氏体不锈钢中孪晶细化机理

采用SEM和TEM分析了等通道挤压奥氏体不锈钢中孪晶的细化过程.结果表明,一道次变形后,原退火孪晶受剪切断裂,并在一些区域形成小的形变孪晶;随着挤压道次增加,孪晶通过孪生和滑移的方式进一步变形,滑移由晶界开始并向晶粒内部扩展,最后将大的孪晶破碎,在孪晶层状结构内部通过孪生方式形成二次孪晶,在随后的变形过程中,逐渐形成微米级孪晶组织.八道次挤压后形成纳米级的晶粒和细小的微孪晶组织.     

镍基合金表面五重孪晶形成的分子动力学模拟

五重孪晶结构能够改善合金的表面性能,而关于合金五重孪晶化表面的研究较少报道。基于分子动力学模拟和纳米压痕方法,采用嵌入原子势函数(EAM)和等温等压系综(NPT),使用半径为14 nm的圆柱压头以40 m/s的压痕速度沿着[112]晶向对单晶镍基合金持续压痕,采用共领域分析法对合金在应力诱导作用下的变形行为进行了分析。结果表明,非共格孪晶界形成于四个不同{111}滑移面交叉中心附近。交叉中心处白色高能原子发射不全位错,堆垛层错产生。随着不全位错持续发射,孪晶得以形核、生长,孪晶界相继形成,最终五重孪晶形成于合金表面。合金表面中五重孪晶的形成并非源于晶界连续不断发射不全位错,而是与压痕过程中合金表面能量增加以及非共格孪晶界息息相关。     

原子尺度下纯锌中{1012}孪晶界的结构表征（英文）

通过高分辨透射电子显微镜研究原子尺度下轧制变形后的纯锌中{1012}孪晶界的结构和迁移机制,发现在{1012}孪晶界上存在基面/柱面锯齿,并且在同一个{1012}孪晶中存在两种结构不同的孪晶界:由{1012}共格孪晶界和微小基面/柱面锯齿组成的孪晶界,以及由连续基面/柱面锯齿组成的不包含{1012}共格孪晶界的孪晶界。基面/柱面锯齿的形成与密排六方金属的轴比无关,因为基面/柱面锯齿的形成在能量上和几何上都是有利的。基于位错理论,孪晶界的迁移是由孪生位错在共格孪晶界上的滑移以及界面位错在基面/柱面锯齿上的攀移来实现的。     

孪晶片层厚度对纳米孪晶Cu疲劳性能的影响

通过恒应力幅控制拉--拉疲劳实验, 比较了脉冲电解沉积制备的不同孪晶片层厚度纯Cu样品的疲劳寿命和疲劳耐久极限. 结果表明: 在应力疲劳下, 样品的疲劳寿命与疲劳耐久极限均随孪晶片层厚度的减小而提高. 疲劳样品的宏观表面变形形貌(SEM观察)和微观结构(TEM观察)表明:
当平均孪晶片层厚度为85 nm时, 材料的塑性形变由位错滑移和剪切带共同承担, 进而疲劳裂纹沿剪切带萌生; 而当平均孪晶片层厚度为32 nm时, 材料的塑性形变由位错--孪晶界交互作用主导, 从而导致疲劳裂纹沿孪晶界形成.     

纳米孪晶金属塑性变形机制

本文综述了纳米孪晶金属材料的塑性变形机制.通过分析纳米孪晶二维结构变形时可启动的滑移位错类型,揭示纳米孪晶金属塑性变形的3种位错机制,即位错塞积并穿过孪晶界机制,Shockley不全位错诱导孪晶界迁移机制以及贯穿位错在孪晶片层内受限滑移机制.通过改变加载方向与孪晶界面的相对取向可实现这3类位错机制的可控转变.     

纳米孪晶Cu中局部剪切应变诱导的退孪生行为

对择优取向纳米孪晶结构Cu样品进行室温轧制变形.微观结构研究发现,当变形压下量为15%时,样品中出现了与轧制方向呈30o~45°方向(最大剪切应力方向)分布的退孪生带.退孪生带中孪晶片层明显粗化,孪晶界上出现大量Shockley位错.塑性变形过程中较小应变时,纳米孪晶Cu中局部退孪生机制是协调局部剪切应变的主要机制.     

Mg-9Gd-4Y-0.6Zr合金的孪晶超塑性

对Mg-9Gd-4Y-0.6Zr合金Ext-T5态进行高温拉伸时发现,合金在400 ℃时产生孪晶超塑性,在晶界上有大量的再结晶和晶内有大量的孪晶共存的奇异现象;孪晶的生成调整了晶粒取向并释放了变形过程中产生的局部应力集中,导致组织中不产生裂纹和空洞;合金的变形机制是一种新的超塑性变形机制,即在晶界上以再结晶方式进行晶界滑移和在晶内以孪生方式进行协调的超塑性变形机制.     

Fe-C-Mn-Si钢中奥氏体共格孪晶界对贝氏体铁素体变体选择的影响

将C含量(质量分数)分别为0.05%和0.4%的Fe-C-Mn-Si钢进行等温处理得到贝氏体组织,采用EBSD技术对奥氏体共格孪晶界上形成的贝氏体铁素体变体进行分析.结果表明,2种钢中的贝氏体铁素体与母相奥氏体均成近似K-S取向关系.奥氏体孪晶界两侧形成取向相同的变体对.此变体对形成后,孪晶界基本不再显现.晶体学分析表明,共格孪晶界两侧可能出现的变体对最多不超过3组,且这3组变体对的惯习面均与孪晶界平行,因此,贝氏体铁素体变体都将沿孪晶界生长.含C量为0.05%的Fe-C-Mn-Si钢中奥氏体孪晶界上只观察到一组贝氏体铁素体变体对的形成,这是因为C含量较低,贝氏体铁素体生长速度较快,消除了其它变体对的形核机会,先形核的变体对一旦形核就迅速覆盖整个孪晶面.而在含C量为0.4%的Fe-C-Mn-Si钢中,由于C含量较高,贝氏体铁素体生长速度较慢,3组变体对均有机会形核,因此,在孪晶界上可以观察到这3组变体对同时出现.     

Fe-C-Mn-Si钢中奥氏体共格孪晶界对贝氏体铁素体变体选择的影响

将C含量(质量分数)分别为0.05%和0.4%的Fe-C-Mn-Si钢进行等温处理得到贝氏体组织,采用EBSD技术对奥氏体共格孪晶界上形成的贝氏体铁素体变体进行分析.结果表明,2种钢中的贝氏体铁素体与母相奥氏体均成近似K-S取向关系.奥氏体孪晶界两侧形成取向相同的变体对.此变体对形成后,孪晶界基本不再显现.晶体学分析表明,共格孪晶界两侧可能出现的变体对最多不超过3组,且这3组变体对的惯习面均与孪晶界平行,因此,贝氏体铁素体变体都将沿孪晶界生长.含C量为0.05%的Fe-C-Mn-Si钢中奥氏体孪晶界上只观察到一组贝氏体铁素体变体对的形成,这是因为C含量较低,贝氏体铁素体生长速度较快,消除了其它变体对的形核机会,先形核的变体对一旦形核就迅速覆盖整个孪晶面.而在含C量为0.4%的Fe C Mn-Si钢中,由于C含量较高,贝氏体铁素体生长速度较慢,3组变体对均有机会形核,因此,在孪晶界上可以观察到这3组变体对同时出现.     

利用分子动力学研究梯度纳米孪晶Cu的微观变形机理

提出了一种新的纳米结构材料即梯度纳米孪晶界结构,并利用分子动力学方法计算了梯度纳米孪晶Cu的单轴拉伸和压痕的变形过程,分析了纳米孪晶界分布对位错机制的影响.结果表明,梯度纳米孪晶界主导的塑性变形可分为2类,不全位错主导了较厚的孪晶片层的塑性变形,较细孪晶片层的塑性变形由全位错主导.此外,提高孪晶界密度可以有效改善材料的强度和硬度.     

Double Extension Twin and Its Related Compound Twin Structures in MgEI北大核心CSCD

This paper summarizes recent research progresses on {10 (1) over bar2}-{10 (1) over bar2} double extension twin and its related compound twin structures in Mg. Tension-compression asymmetry of Mg with strong texture can be greatly alleviated through sequential multi-directional deformations, which consist of several sequential bi-axial deformations. There exist 36 possible double extension twin variants, which can be classified into four misorientation groups according to their misorientations with respect to the grain matrix. One of the groups appears with a much higher frequency than the others, which cannot be perfectly explained by Schmid factor (SF) rule. Primary and secondary extension twins form intergranular and intragranular compound twin structures without any one-for-all mechanism. SF rule and m' factor, which evaluates how much twinning shear can pass through an interface, partly or even totally fail to explain the formation of the compound twin structures, presenting challenge to make clear mechanism of twin formation under complex loading conditions. It is suggested that modelling on the formation of intragranular compound twin structure and experimental characterization of interfacial structures of primary twin- twin boundary and secondary twin boundary should be paid much attention in the future.     

Twinning-Associated Boundaries in Hexagonal Close-Packed Metals

In this article, we highlighted twinning-associated boundaries that play crucial roles in nucleation, growth, and interactions of deformation twins. According to microscopic characterizations and atomistic simulations in Mg, three types of boundaries are reviewed, including (I) prismatic-basal boundaries associated with twin nucleation via pure-shuffle mechanism, (II) serrated coherent twin boundaries associated with twin growth and shrinkage via glide and climb of twinning dislocations, and (III) tilt prismatic–prismatic and basal–basal boundaries associated with co-zone twin–twin interactions. More importantly, these boundaries affect twinning and detwinning processes that may correspond to twinning-induced hardening and seem universally associated with twins in hexagonal close-packed metals.     

基于原子尺度模拟研究HCP镁中的一次及二次孪生模式

通过分子动力学模拟（MD）,研究在HCP镁中的一个对称倾斜晶界与基面滑移的位错相互作用而激发的变形孪晶,也就是孪晶形核与长大的过程（或者是孪晶界迁移,TBM）。{1^-1^-21}孪晶在该过程中是最易被激发的孪生模式。一旦这样的孪晶形成了,它们就会不断长大。该种孪晶界迁移是由单纯的原子位置局域调整造成的。在模拟过程中同时也产生了二次孪晶{1^-1^-22}。该二次孪晶模型的孪晶形核与长大需要克服的能垒与{1^-1^-21}孪晶不同。同时,二次孪晶的孪晶界迁移过程是通过孪晶界上的锥形滑移而激发的。     

Macroscopic dislocation model of a wedge twin with allowance for the shape of its boundaries

Stress fields for wedge twins with various shapes of boundaries are calculated on the basis of a macroscopic dislocation model. It is shown that the deviation of a wedge twin from a symmetric shape leads also to the loss of symmetry in the stress-field distribution near the twin. Cases of a uniform and an equilibrium distribution of twinning dislocations at twin boundaries are considered.     

纳米面心立方金属的变形机制及影响其力学性能的因素

综述了纳米面心立方金属的变形机制随晶粒尺寸的减小而发生的变化,即变形机制由晶界处发射不全位错、形成孪晶转变为晶界滑移、晶粒转动.当变形机制为晶界处发射不全位错、形成孪晶时,存在最佳孪晶形成晶粒尺寸范围,此时的孪晶形核应力最小.另一方面,随着晶粒尺寸的减小,在变形机制发生转变的临界晶粒尺寸附近存在韧-脆断裂方式的转变.提高孪晶密度、在纳米晶材料中加入微米晶相形成双峰晶粒材料可以提高纳米晶材料的塑性,得到更好的综合机械性能.     

Structure of twin boundaries in Mn-based shape memory alloy: A HRTEM study and the strain energy driving force

The paramagnetic–antiferromagnetic transition and the cubic-tetragonal martensitic transformation in Mn-based alloys occur at the same temperature. The shape memory effect depends on the morphology of twins and the motion of twin boundaries. High-resolution electron microscopy was carried out across the twin boundary. Abnormal lattice distortion was observed and determined to be a twinning dislocation. A model for the defect was established based on the elasticity theory of Eshelby and Khachaturyan. The twinning was found effectively to reduce the strain energy and the introduction of twinning dislocations reduces it further. The strain energy related to these defects was only 2/7 relative to the value in their absence, providing a thermodynamic mechanism for the formation of these twinning dislocations.     

DEFORMATION AND WORK HARDENING OF HADFIELD MANGANESE STEEL

The work hardening of Hadfield manganese steel arises mainly form two causes;1)a largeamount of twinning makes perfect dislocations or Shockley dislocations to be blocked at thecoherent twin boundaries.2)the deformation leads to strong unsymmetrical distortion due tothe occurrence of a large number gf Mn-C pairs or Mn-vacancy carbon-carbon cluster.Ithas been demonstrated by TEM observation and internal friction tests.As deformation pro-ceeds,the slip constantly accomodates twinning shear giving rise to stress relaxation.Thus,the Hadfield steel exhibits both rapid work hardening and good ductility.     

The coalescence of twins in bismuth single crystals

The process of the coalescence of twin boundaries in bismuth single crystals has been studied. It has been established that the twin coalescence can occur without the formation of an interface between them or with the formation of a double twin boundary, depending on the direction of twin growth and orientation of twinning planes.     

On the mechanism of twin formation in Fe–Mn–C TWIP steels

Although it is well known that Fe–Mn–C TWIP steels exhibit high work-hardening rates, the elementary twinning mechanisms controlling the plastic deformation of these steels have still not been characterized. The aim of the present study is to analyse the extended defects related to the twinning occurrence using transmission electron microscopy. Based on these observations, the very early stage of twin nucleation can be attributed to the pole mechanism with deviation proposed by Cohen and Weertman or to the model of Miura, Takamura and Narita, while the twin growth is controlled by the pole mechanism proposed by Venables. High densities of sessile Frank dislocations are observed within the twins at the early stage of deformation, which can affect the growth and the stability of the twins, but also the strength of these twins and their interactions with the gliding dislocations present in the matrix. This experimental evidence is discussed and compared to recent results in order to relate the defects analysis to the macroscopic behaviour of this category of material.