SCN-Cam共晶合金的定向凝固研究

采用实时观测装置和定向凝固系统研究了SCN-Cam(Succinonitrile-wt%Camphor,wt%为质量分数)模型合金的凝固过程.实验结果表明,SCN-23.6%Cam共晶合金在常规条件下形成规则的共晶组织,共晶间距随界面推移速度的增大而减小;加入超声振动时,共晶合金生长出初生相;SCN-21%Cam亚共晶...     

Dependency of deformation twinning on grain orientation in an FCC and a HCP metal

Twinning plays important roles in HCP metals and those FCC metals with low stacking fault energy. The structural difference of two types of metals makes quite different contributions of twinning to plasticity. The variety of grain orientation in polycrystalline metals causes the inhomogeneous occurrence of twinning and further distinct transformation kinetics of twinning as strain increases and texture develops. This changes finally the work hardening behavior and mechanical properties. This paper reveals the dependency of twinning on grain orientation in an FCC TWIP (twinning induced plasticity) steel with high Mn content and in a magnesium alloy using electron-backscatter-diffraction (EBSD) technique, and analyzes the characteristics of twinning in the two types of metals by Schmid factor calculation. In addition, the relation of twinning and shear banding, as well as their influence on properties are discussed.     

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.     

Grain size effect on deformation twinning and detwinning

This article systematically overviews the grain size effect on deformation twinning and detwinning in face-centered cubic (fcc) metals. With decreasing grain size, coarse-grained fcc metals become more difficult to deform by twinning, whereas nanocrystalline (nc) fcc metals first become easier to deform by twinning and then become more difficult, exhibiting an optimum grain size for twinning. The transition in twinning behavior from coarse-grained to nc fcc metals is caused by the change in deformation mechanisms. An analytical model based on observed deformation physics in nc metals, i.e., grain boundary emission of dislocations, provides an explanation of the observed optimum grain size for twinning in nc fcc metals. The detwinning process is caused by the interaction between dislocations and twin boundaries. Under a certain deformation condition, there exists a grain size range where the twinning process dominates over the detwinning process to produce the highest density of twins.     

亚稳β型钛合金中的{332}〈113〉变形孪晶

{332}113变形孪晶是亚稳β型钛合金变形过程中的一种独特变形机制。该类型孪晶具有特殊性质并且对亚稳β型钛合金力学性能具有显著影响。本文总结了{332}113变形孪晶的研究状况和特性,重点介绍了{332}113变形孪晶形成的几种代表性模型。通过分别对这些模型的假设条件以及需要进一步解释和验证的科学问题进行分析,旨在为理解和揭示{332}113变形孪晶的变形机制提供有用的参考信息。     

Investigation of the Mechanical Twinning of Antimony Single Crystals by Nanoindentation

The twinning of antimony single crystals in nanoindentation has been investigated. The role of twinning in the formation of the impression of the Berkovich pyramid on the surface (111) of antimony single crystals has been established. A new method of investigation of the mechanical twinning of single crystals has been developed. For the first time, the duration of the process of twinning has been determined experimentally by a direct technique.     

Parametric study of stress state development during twinning using 3D finite element modeling

A deformation twinning model which simulates the characteristic twin shear and corresponding grain reorientation has been developed using a 3D finite element method. This model has been used to study how twinning affects the stress state in both the parent grain and twin, and the stress states that are energetically favorable for twinning. The component of shear stress on the twin plane and direction is primarily responsible not only for whether twinning can occur, but also the energetically favorable twin volume fraction. A map predicting twin volume fraction as a function of parent grain deviatoric stress has been developed.     

Insight from in situ microscopy into which precipitate morphology can enable high strength in magnesium alloys

Magnesium alloys, while boasting light weight, suffer from a major drawback in their relatively low strength. Identifying the microstructural features that are most effective in strengthening is therefore a pressing challenge. Deformation twinning often mediates plastic yielding in magnesium alloys. Unfortunately, due to the complexity involved in the twinning mechanism and twin-precipitate interactions, the optimal precipitate morphology that can best impede twinning has yet to be singled out. Based on the understanding of twinning mechanism in magnesium alloys, here we propose that the lamellar precipitates or the network of plate-shaped precipitates are most effective in suppressing deformation twinning. This has been verified through quantitative in situ tests inside a transmission electron microscope on a series of magnesium alloys containing precipitates with different morphology. The insight gained is expected to have general implications for strengthening strategies and alloy design.     

Twinning modes of Er<Subscript>2</Subscript>Si<Subscript>2</Subscript>O<Subscript>7</Subscript>

Theory of deformation twinning by Bilby and Crocker is applied to calculate the twinning elements for all possible twinning modes in monoclinic Er₂Si₂O₇ TYPE C and TYPE D. The magnitude of shear strain was also calculated for each twinning mode. The criteria of small shear strain and minimum shuffling is applied to predict the operative twinning mode for monoclinic Er₂Si₂O₇ TYPE C and TYPE D. These predications of the theory are compared with the available experimental information and may act as guideline for future experimental work.     

Deformation twinning in nanocrystalline materials

Nanocrystalline (nc) materials can be defined as solids with grain sizes in the range of 1-100 nm. Contrary to coarse-grained metals, which become more difficult to twin with decreasing grain size, nanocrystalline face-centered-cubic (fcc) metals become easier to twin with decreasing grain size, reaching a maximum twinning probability, and then become more difficult to twin when the grain size decreases further, i.e. exhibiting an inverse grain-size effect on twinning. Molecular dynamics simulations and experimental observations have revealed that the mechanisms of deformation twinning in nanocrystalline metals are different from those in their coarse-grained counterparts. Consequently, there are several types of deformation twins that are observed in nanocrystalline materials, but not in coarse-grained metals. It has also been reported that deformation twinning can be utilized to enhance the strength and ductility of nanocrystalline materials. This paper reviews all aspects of deformation twinning in nanocrystalline metals, including deformation twins observed by molecular dynamics simulations and experiments, twinning mechanisms, factors affecting the twinning, analytical models on the nucleation and growth of deformation twins, interactions between twins and dislocations, and the effects of twins on mechanical and other properties. It is the authors’ intention for this review paper to serve not only as a valuable reference for researchers in the field of nanocrystalline metals and alloys, but also as a textbook for the education of graduate students.     

Twinning modes of Nd2Si2O7

Theory of deformation twinning by Bilby and Crocker [B.A. Bilby, A.G. Crocker, Proc. R. Soc. 288 (1965) 241] is applied to calculate the twinning elements for all possible low index twinning modes in tetragonal, orthorhombic and monoclinic Nd₂Si₂O₇. The magnitude of shear strain was also calculated for each twinning mode. The criteria of small shear strain and minimum shuffling is applied to predict the operative twinning modes for tetragonal, orthorhombic and monoclinic Nd₂Si₂O₇. These predictions of the theory are compared with the available experimental information for analogue oxides and may act as guideline for future experimental work.     

Calculation of the interaction energy of the twinning dislocations of a wedge twin based on the mesoscopic dislocation model

The energy of interaction of the twinning dislocations in a wedge crystal twin was calculated based on the mesoscopic dislocation model. It has been established that the interaction energy of the twinning dislocations increases moderately with increase in the length of the twin. It is shown that the dependence of the interaction energy of the twinning dislocations on the width of the twin is near-parabolic.     

The polycrystalline plasticity due to slip and twinning during magnesium alloy forming

In order to simulate the magnesium alloy-forming processes accurately, it is necessary to consider the plastic anisotropy. In this paper, a new rate-independent constitutive model for polycrystalline plastic deformation by slip and twinning has been formulated, and then introduced into a FEM program. Metal flow is assumed to occur by crystallographic slip on given slip and twinning systems within each crystal. Each integration point represents a single crystal. Then uniaxial compression and cup drawing of Mg alloy are studied by using a rate-independent polycrystalline plasticity finite element analysis. In this paper, the ear distributions of the polycrystal are predicted for different typical initial orientation cases. The values of the twinning factors associated with slip system deformation are deduced. It is found that the twinning factors vary with the value of the stress. The basal slip and twinning system plays the dominant role in the deformation of magnesium alloy, which might be the most important contribution to strain hardening.     

Correlation between ductile-to-brittle transition behaviour and twinning in ferritic stainless steel

In this work, the 18Cr–2Mo ferritic stainless steel was treated with and without warm rolling at 573 K with 66.5% reduction after conventional hot rolling process. It was shown that ductile-to-brittle transition behaviour could be closely related to deformation twinning and parameters affecting critical temperature for twinning would also inevitably affect ductile-to-brittle transition temperature. This correlation between ductile-to-brittle transition behaviour and twinning was in good agreement with the Cottrell–Petch model. A lowered transition temperature and an improved toughness after introducing warm rolling process and corresponding annealing process could be mainly explained in terms of refining the recrystallised grains and reducing the volume fraction of grains having orientations favourable for twinning, which decrease the critical temperature at which twins form.     

EBSD analysis of {10–12} twinning activity in Mg–3Al–1Zn alloy during compression

The {10–12} twinning activity of Mg–3Al–1Zn magnesium alloy during uniaxial compression at room temperature has been investigated by electron backscatter diffraction. The results indicated that the twinning activity was closely related with two angles: one was the angle between the c-axis and the compression direction and the other was the angle between the a-axis and the titling direction in the basal plane for a given relation between the c-axis and the compression direction. These two parameters can be used to explain which twinning variant will operate under the given strain path. For the grains containing a single {10–12} twinning variant, the {10–12} twinning variant occurred in a wide range of Schmid factor values (0 < Schmid factor < 0.5) and the Schmid factor rank of 1 or 2 was the most commonly observed. By contrast, for the grains containing two {10–12} twinning variants, the {10–12} twinning activity exhibited a stronger orientation dependence and the combinations of Schmid factor ranks 1–3 and 1–2 were the most commonly observed.     

Twins,disorders and other demons

Dramatic improvements in both the hardware and software used to perform X-ray crystallographic studies on single crystals have led to the publication of thousands of new structures each year. These improvements have also made it possible to successfully resolve a larger percentage of structural problems such as disorder and complicated twinning. Examples are included of twins that were first thought to be disorders structures with both twinning and disorder, and four-fold twinning.     

室温压缩AZ91镁合金显微组织及β-Mg17Al12相析出动力学

对常温压缩粗镁直接熔炼AZ91镁合金时效处理后的组织及β-Mg17Al12相析出动力学进行研究。结果表明:AZ91镁合金在常温压缩过程中出现大量的孪晶,为β-Mg17Al12相的析出提供了大量的形核基底;时效时β-Mg17Al12相优先在晶界、孪晶界析出,尤其易在孪晶与晶界、孪晶交接处析出并长大,且孪晶内析出的β-Mg17Al12相与α-Mg基体保持一定的位向关系;时效时间越长,析出的β-Mg17Al12相越多,温度越高,析出定量β-Mg17Al12相所需时间越短;结合实验数据,由JMAK方程计算得到AZ91镁合金析出β-Mg17Al12相激活能为23.8～37.9kJ/mol。     

Orientation dependence of deformation twinning in Cu single crystals

Cu single crystals were subjected to dynamic compression plastic deformation to investigate orientationdependent twinning.The experimental results showed that twinning is closely related to the ratio of the maximum Schmid factor for twinning partial(m_T)to the maximum Schmid factor for perfect dislocation(m_S),i.e.,m_T/m_S,rather than m_T.The twin volume fraction V_Tincreases with the m_T/m_Svalue and the most favorable orientation for twinning has the maximum m_T/m_Svalue(1.15).The relationships of m_T/m_Swith both effective stacking fault energy γ_(eff)and threshold stress for twinning τ_Twere established for understanding orientation-dependent twinning.Further insights into the orientation-dependent twinning and guidance for developing bulk high density nanotwinned materials are provided.     

X-ray Diffractions of Deformation Structure in Polycrystalline Fe-32Mn-5Si Alloy

The change of microstructure with strain was investigated in a Fe-32Mn-5Si austenitic alloy at room temperature by X-ray diffraction profile analysis. The experimental results show that the Fe-32Mn-5Si alloy is deformed by the strain-induced γ→ε transformation and the twinning except dislocation slip at room temperature. The amount of strain-induced ε-martensite, the stacking fault probability and the twinning probability all exhibit parabolic relationship with increasing strain. The stacking fault probability is higher than the twinning probability.     

Deformation twinning during impact – numerical calculations using a constitutive theory based on multiple natural configurations

When materials such as Armco iron, titanium etc., are subject to impact it can be observed that two basic inelastic processes take place – slip and deformation twinning. Of these processes, inelasticity associated with the slip mechanism has received considerable attention. For example, Zerilli and Armstrong (1988) modeled the Taylor impact test for a variety of materials using traditional plasticity theories. They found that there was a significant discrepancy between the theoretical and experimental results for some materials. They attributed this to the fact that they had neglected deformation twinning in their models. Subsequent metallurgical studies have indicated that twinning had indeed taken place in these materials. In this study, we focus on the inelastic processes solely due to deformation twinning (i.e., neglecting slip). We model these processes using the approach of Rajagopal and Srinivasa (1995, 1997) and Srinivasa et al., (1997), the results of which are briefly summarized in section 2.1. In order to better understand the twinning process, we study the Taylor impact test for a 2-D slab under the assumption that only deformation twinning takes place and solve the governing dynamical equations by using the finite element method. The results show that the twinned zone is concentrated near the point of impact and indeed it contributes significantly to the overall permanent shape change due to the impact.