Critique of mechanisms of formation of deformation,annealing and growth twins: Face-centered cubic metals and alloys

The formation of deformation, annealing and growth twins in face-centered cubic materials is discussed. Slip precedes deformation twinning, and twins form from the interaction between primary and secondary slip dislocations having co-planar, but different, Burgers vectors. The influence of several metallurgical variables on twinning can only be rationalized in terms of the model. Annealing twins form due to growth accidents on differently inclined {1 1 1} facets present on a migrating grain boundary. Growth twins also form by growth accidents on the {1 1 1} planes.     

Deformation twins in nanocrystalline body-centered cubic Mo as predicted by molecular dynamics simulations

This work studies deformation twins in nanocrystalline body-centered cubic Mo, including the nucleation and growth mechanisms as well as their effects on ductility, through molecular dynamics simulations. The deformation processes of nanocrystalline Mo are simulated using a columnar grain model with three different orientations. The deformation mechanisms identified, including dislocation slip, grain-boundary-mediated plasticity, deformation twins and martensitic transformation, are in agreement with previous studies. In 〈1 1 0〉 columnar grains, the deformation is dominated by twinning, which nucleates primarily from the grain boundaries by successive emission of twinning partials and thickens by jog nucleation in the grain interiors. Upon arrest by a grain boundary, the twin may either produce continuous plastic strain across the grain boundary by activating compatible twinning/slip systems or result in intergranular failure in the absence of compatible twinning/slip systems in the neighboring grain. Multiple twinning systems can be activated in the same grain, and the competition between them favors those capable of producing continuous deformation across the grain boundary.     

Mechanism of twinning-induced grain boundary engineering in low stacking-fault energy materials

The current status of “grain boundary engineering” is overviewed, i.e. the deliberate manipulation of grain boundary crystallography in polycrystals in order to produce a material containing grain boundaries which have superior properties compared to average boundaries. A particular focus of attention is annealing twinning in low stacking-fault energy (SFE) materials as a means to achieve this aim, since the role of such twinning in improving the grain boundary network has not hitherto been satisfactorily explained. The twinning is discussed from the viewpoint of strain retention, imposition of crystallographic constraints at grain junctions, “relative specialness” rather than “absolute specialness” and proposal of a new “Σ3 regeneration model” to explain how twins can enhance the Σ3 boundary fraction in the network.     

The Formation of Twins in Al-10Zn-3Mg-1.8Cu Alloy by Cryomilling

Aluminum alloys with high stacking fault energies have difficulty forming deformation twins at room temperature and at a low strain rate. In this study, several deformation twins were found in Al-10.0Zn-3.0Mg-1.8Cu alloy powders after being cryomilled for a certain period of time. Annealing twins were also found in the same powders after they had been cryomilled first and then placed at an ambient temperature for a longer period of time, up to 2 years. It is suggested that the Venables model on deformation twins is plausible to explain the twin propensity formed in these powders. The study of the formation mechanism of both deformation and annealing twins concludes that twinning deformation can reduce the grain boundary energy of the alloy powders when the powders are cryomilled first and then placed at the ambient temperature for a longer period of time, up to 2 years.     

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。     

Yield point elongation due to twinning in a magnesium alloy

Fine-grained magnesium alloy AZ31 displays a yield elongation when deformed such that yielding occurs by twinning. That is, following yielding there is a plateau in the stress-strain curve. The present paper presents a microstructural analysis of the twins in deformed samples. A major aim is to explain the yield elongation and in particular why it decreases and eventually disappears with increasing grain size. It is shown that during the Lüders yield elongation twins initiate twinning events in neighbouring grains and in this manner twinning spreads its way progressively over the sample. This occurs at a twinning frequency of approximately one twin per grain. A criterion for the presence of a Lüders strain is developed based on twin transfer across boundaries. It is shown that higher Lüders strains and stresses are expected for finer grain sizes. The key to understanding the effect is that it arises from the condition for Lüders band propagation whereby the twins on the Lüders band front must stimulate, on average, one twin each within the fresh material ahead of the front, at a constant value of applied stress. An important part of the derivation followed here is that at the higher stresses seen in fine-grained samples, the twin aspect ratio is larger and consequently the strain at the grain level corresponding to a single twinning event is higher in finer-grained samples.     

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.     

Deformation twins and twinning at ambient temperature in cryomilled A1-Zn-Mg-Cu alloy powders

The nanocrystalline Al-Zn-Mg-Cu alloy powders were synthesized using cryomilling. Deformation twins and twinning at ambient temperature were observed in Al-Zn-Mg-Cu powders. Experimental results indicate that high strain rate at low temperature during cryomilling induces the formation of deformation twins by the climbing of the restricted dislocations in nano-scale grains, and the cryomilled powders at high energies and nonequilibrium state reduce the grain boundary energies by twinning.     

制备工艺对GH4169合金组织结构与蠕变行为的影响

通过对等温锻造和热连轧工艺制备的GH4169合金进行蠕变性能测试和组织形貌观察,研究制备工艺对GH4169合金组织结构及蠕变行为的影响.结果表明:在热连轧期间,合金发生孪晶变形和位错滑移;与等温锻造相比,热连轧合金中的高密度位错具有形变强化的作用,可提高合金的蠕变抗力.在蠕变期间,等温锻造合金仅发生孪晶变形,而热连轧合金的变形机制是孪晶和位错滑移,其中,合金在热连轧期间形成的高密度位错可诱发蠕变位错发生单取向或多取向滑移,可减缓应力集中,抑制或延缓裂纹在晶界处萌生是使该合金具有较长蠕变寿命的主要原因.蠕变后期,裂纹在与应力轴垂直的晶界处萌生,并沿晶界扩展、发生解理断裂是2种工艺制备合金的蠕变断裂机制.     

The role of mechanical twinning on microcrack nucleation and crack propagation in a near-γ TiAl alloy

The role of mechanical twinning on microcrack nucleation and crack propagation in a near-γ TiAl alloy was investigated in 4-point bend specimens using selected area channeling patterns and electron channeling contrast imaging to obtain the true crystal orientation image crystal defects. Two types of bend specimens were used, a conventional specimen that was deformed to a surface strain of about 1.4%, but not fractured, and a bi-layer notched specimen with an aluminum backing that prevented catastrophic fracture so that crack arrest features could be examined. Most grains showed activity of 1–4 twinning systems, and their activity correlated closely with the Schmid factor. Grain boundary microcracks were most frequently found where twins interacted with the grain boundary, and crystallographic analysis indicated that the twinning shear caused these cracks to open. In the crack growth specimen, extensive twinning and dislocation activity occurred in the grain with the arrested crack tip, but with continued straining, these same twins caused grain boundary microcracks to form and link-up to form full intergranular cracks, resulting in renucleation of the primary failure crack.     

Deformation twinning in bulk nanocrystalline metals: Experimental observations

Deformation twins have been observed in nanocrystalline (nc) fcc metals with medium-to-high stacking fault energies such as aluminum, copper, and nickel. These metals in their coarse-grained states rarely deform by twining at room temperature and low strain rates. Several twinning mechanisms have been reported that are unique to nc metals. This paper reviews experimental evidences on deformation twinning and partial dislocation emissions from grain boundaries, twinning mechanisms, and twins with zero-macro-strain. Factors that affect the twinning propensity and recent analytical models on the critical grain sizes for twinning are also discussed. The current issues on deformation twinning in nanocrystalline metals are listed.     

On the stress state dependence of the twinning rate and work hardening in twinning-induced plasticity steels

The influence of the stress state on the twinning rate and work hardening is studied in the case of an Fe–Mn–C TWIP steel strained in uniaxial tension, simple shear and rolling. The resulting stress–strain responses exhibit marked differences. The twinning rate, number of activated twinning systems in each grain, twin thickness and transmission of twins across grain boundaries are dependent on the imposed stress state during straining. Relationships between twin features and macroscopic work hardening rate are established.     

Introducing Grain Boundary Influenced Stochastic Effects into Constitutive Models

Twinning is an important deformation mechanism in hexagonal close-packed (hcp) metals such as Mg, Zr, Ti, and Be. Twinning in hcp materials is a multiscale process that depends on microstructural and mechanical response details at the mesoscale, microscale, and atomic scales. Twinning can generally be understood as a two-step process, a nucleation step followed by propagation. The nucleation of twins is governed by both material details such as the defect configurations at potential nucleation sites within grain boundaries, as well as the highly local mechanical field near grain boundaries. These two factors, the material and mechanical, must align for a successful nucleation event. In this article, we present a stochastic constitutive law for nucleation of twins and describe its implementation into a homogenized crystal plasticity simulation. Twin nucleation relies on the dissociation of grain boundary defects under stress into the required twinning partials. This dissociation is considered to follow a Poisson process where the parameters of the Poisson distribution are related to the properties of the grain boundaries. The rate of the process is a direct function of the local stress concentration at the grain boundary. These stress concentrations are randomly sampled from a distribution calibrated to full-field crystal plasticity simulations.     

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.     

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     

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.     

Effects of tensile twinning on the stretch formability of Mg

This study examined the influences of twinning during the Erichsen test. {10-12} tensile twinning plays the critical role in Mg alloys. Alloy elements and grain size are important factors that determine the formation of {10-12} tensile twins at room temperature. Mg-6.0Zn(Z6) and Mg-6.0Zn-0.3Ca(ZX60) alloys were fabricated and their grain size was varied under different annealing conditions. Tensile twinning is promoted by the addition of Ca, as assessed from measurements of the microstructure and the viscoplastic self-consistent calculations. The coarse-grain Ca-containing alloy showed the largest amount of tensile twinning. However, the stretch formability increased with grain size up to a certain point and then decreased. It can be inferred that microstructures with large grains activated the tensile twin, which became origins of cracks. The results of the small Erichsen test showed that tensile twinning contributes to high stretch formability releasing the stress concentration in the grain boundaries but the interaction between twin and slip causes cracks as the grain size increases.     

Revealing the strain-hardening behavior of twinning-induced plasticity steels: Theory,simulations, experiments

We present a multiscale dislocation density-based constitutive model for the strain-hardening behavior in twinning-induced plasticity (TWIP) steels. The approach is a physics-based strain rate- and temperature-sensitive model which reflects microstructural investigations of twins and dislocation structures in TWIP steels. One distinct advantage of the approach is that the model parameters, some of which are derived by ab initio predictions, are physics-based and known within an order of magnitude. This allows more complex microstructural information to be included in the model without losing the ability to identify reasonable initial values and bounds for all parameters. Dislocation cells, grain size and twin volume fraction evolution are included. Particular attention is placed on the mechanism by which new deformation twins are nucleated, and a new formulation for the critical twinning stress is presented. Various temperatures were included in the parameter optimization process. Dissipative heating is also considered. The use of physically justified parameters enables the identification of a universal parameter set for the example of an Fe–22Mn–0.6C TWIP steel.     

Internal strain and texture development during twinning: Comparing neutron diffraction measurements with crystal plasticity finite-element approaches

Different approaches to the modeling of twinning are examined within the framework of a crystal plasticity finite-element code. The model predictions are compared with in situ neutron diffraction experiments previously carried out on zirconium and magnesium alloys. The experiments are used to evaluate different model assumptions regarding the stress state inside newly formed twins at inception, as well as different assumptions concerning the interaction between twin and parent grain during subsequent twin growth. In particular the relaxation in some grain orientations that is experimentally observed, and is associated with twin-induced stress relief, can be captured by the model under appropriate assumptions.