FGH4096粉末高温合金的再结晶形核机制

利用OM和TEM对FGH4096粉末高温合金的再结晶组织进行了系统的观察和分析,证实有3种再结晶形核机制存在,即原始颗粒边界形核、应变诱导蝶状γ’相形核和孪晶叠加形核.通过研究微观偏析,弯曲褶皱边界的形成和孪晶叠加效应,原子扩散和位错运动建立了形核模型.     

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.     

Atomistic simulation of tension deformation behavior in magnesium single crystal

The deformation behavior in magnesium single crystal under c-axis tension is investigated in a temperature range between 250 K and 570 K by molecular dynamics simulations. At a low temperature, twinning and shear bands are found to be the main deformation mechanisms. In particular, the {1012} tension twins with the reorientation angle of about 90° are observed in the simulations. The mechanisms of {1012} twinning are illustrated by the simulated motion of atoms. Moreover, grain nucleation and growth are found to be accompanied with the {1012} twinning. At temperatures above 450 K, the twin frequency decreases with increasing temperature. The {1012} extension twin almost disappears at the temperature of 570 K. The non-basal slip plays an important role on the tensile deformation in magnesium single crystal at high temperatures.     

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.     

Exploring the Formation Mechanism of Deformation Twins in CrMnFeCoNi High Entropy Alloy

The deformation twins initiated in CrMnFeCoNi high entropy alloy at cryogenic temperature are experimentally studied. Under the external loading, a three-dimensional shear stress concentration originating from dislocation tangling at both the grain boundaries and twin boundaries could be formed, which promotes emission of partial dislocations from the planar defects and is thus considered to be the key factor for twin formation. A sympathetic nucleation mechanism is proposed to describe the nucleation behaviors of twins.     

Nanoscale growth twins in sputtered metal films

This article reviews recent studies on the mechanical properties of sputtered copper and 330 stainless-steel films with {111} nanoscale growth twins preferentially oriented perpendicular to growth direction. The mechanisms of formation of growth twins during sputtering, unusually high strengths, and excellent thermal stability of nanotwinned structures are highlighted.     

Deformation-twinning-related features of primary recrystallization of (110)[001] single crystals of the Fe-3% Si-0.5% Cu alloy

In the regions of the deformed cube-on-edge single crystal of the Fe-3% Si-0.5% Cu alloy with an enhanced density of deformation twins, the nucleation of new grains during primary recrystallization (PR) occurs via several mechanisms. The majority of nuclei of PR are formed on twins as on substrates. The newly formed grains of PR are characterized by the presence of a cube-on-edge texture and are in special misorientations of the Σ3 type with respect to the twin orientation and of the Σ9 (Σ27) type with respect to the octahedral matrix. The mechanism of the formation of nucleation centers for PR on the twins as on substrates can be a splitting of the nonequilibrium special boundary Σ3. A significant role of special misorientations in the formation of the Goss texture upon PR has been shown experimentally.     

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.     

Relating Initial Texture to Deformation Behavior During Cold Rolling and Static Recrystallization Upon Subsequent Annealing of an Extruded WE43 Alloy

Deformation behaviors during cold rolling and static recrystallization behaviors upon subsequent annealing of an extruded WE43 alloys with different initial textures were investigated in this study. Three types of differently textured WE43 initial alloys were labeled as samples Ⅰ, Ⅱ and Ⅲ. The results showed that multiple twinning modes and basal slip dominated the deformation of samples during cold rolling. Cold-rolled sample Ⅰ activated the larger number of double twins with high strain energy...     

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。     

Deformation twinning in TiAl: Effects of defect clustering

Possible roles of point defect clustering in the formation of deformation twins in γ-TiAl are critically assessed by reviewing the available models of dislocation-assisted twin nucleation and experimental data on deformation twinning in Ti-56 at.% Al single crystals and two-phase Ti-47 at.% Al alloys. According to the pole mechanism for twinning in the Ll₀ structure, a reasonable combination of the stress concentration (n≈r27) and the vacancy supersaturation (c/c₀≈13) is needed to overcome the critical stages of twin formation. The so-called radiation-induced ductility reported in Ti-47 at.% Al alloys is attributed to the effective formation of twin embryos in the presence of interstitial-type Frank loops and the subsequent nucleation and growth of twins during plastic deformation.     

Investigation of deformation twinning in a fine-grained and coarse-grained ZM20 Mg alloy: Combined in situ neutron diffraction and acoustic emission

Neutron diffraction and acoustic emission were used in a single in situ experiment in order to study the deformation twinning of two ZM20 Mg alloys with significantly different grain sizes at room temperature. The combination of these two techniques facilitates the distinction between twin nucleation and twin growth. It is shown that yielding and immediate post-yielding plasticity in compression along the extrusion direction is governed primarily by twin nucleation, whereas plasticity at higher strains is presumably governed by twin growth and dislocation slip. It is further shown that, in the fine-grained alloy, collaborative twin nucleation in many grains dominates yielding, whereas twin nucleation in the coarse-grained alloy is progressive and occurs over a larger strain range. In addition, it is shown that, despite twin nucleation stresses increasing with decreasing grain size, roughly the same overall volume fraction of twins is formed in both fine and coarse parent grains. This confirms the difficulty of the alternative deformation modes and suggests a negligible suppressive effect of grain size on twinning in the case of the strongly textured fine-grained alloy. The current results also show that twins in the coarse-grained alloy are born less relaxed with respect to surrounding polycrystalline aggregate than those in the fine-grained alloy. This is believed to lead to lower reversal stresses acting on twin grains in the coarse-grained alloy upon unloading and thus to less untwinning and thus to a smaller pseudoelastic-like hysteresis.     

Twin formation during the atomic deposition of copper

Vapor deposited copper films with 〈111〉 growth texture usually contain twin plates stacked normal to the growth direction. However, twins are rarely seen when growth occurs in other principle crystallographic directions. Atomistic modeling indicated that during 〈111〉 growth, adatoms occupied either parent or twin surface lattice sites with almost equal probability, resulting in a high nucleation density of twin domains. During growth on either {110} or {100} surfaces, adatoms were only able to occupy parent lattice sites, and no twin nucleation occurred. A phase field method was used to model the twin domain evolution during 〈111〉 growth as a function of deposition rate and temperature. Simulation results indicated that twin domains evolved by rapid lateral expansion with very little vertical thickening. The lateral expansion was found to be fast compared with the deposition rate, and as a result twin domains usually grew to occupy the entire width of a growth column, in good agreement with experimental observations. The model indicated that the twin structures formed during the 〈111〉 growth of copper are not directly controllable by either the processing temperature or the deposition rate.     

Deformation twinning mechanisms from bimetal interfaces as revealed by in situ straining in the TEM

During in situ transmission electron microscopy indentation of layered Cu/Nb composites fabricated by accumulative roll bonding, we observed the nucleation and growth of deformation twinning in Cu. Combining dislocation theory and interface defect structure analysis, we propose that the processes of nucleation and thickening of deformation twins proceed as follows: (1) nucleation and emission of a nanometer thick twin nucleus due to the dissociation of interfacial misfit dislocations or the interaction between a run in Nb slip dislocation and the interface or both; (2) emission of a second twin nucleus by the same mechanism, but at some distance along the interface from the first; (3) in the region in-between them, twin thickening occurs by relatively rapid emission of oppositely signed twinning dislocations in order to reduce the backstresses due to the twins in processes 1 and 2; and finally (4) the continuation of this “alternating emission” growth mechanism such that eventually the entire domain in-between the two initially separated fine twin nuclei transforms into a thick twin.     

Simulations of stress-induced twinning and de-twinning: A phase field model

Twinning in certain metals or under certain conditions is a major plastic deformation mode. Here we present a phase field model to describe twin formation and evolution in a polycrystalline fcc metal under loading and unloading. The model assumes that twin nucleation, growth and de-twinning is a process of partial dislocation nucleation and slip on successive habit planes. Stacking fault energies, energy pathways (γ surfaces), critical shear stresses for the formation of stacking faults and dislocation core energies are used to construct the thermodynamic model. The simulation results demonstrate that the model is able to predict the nucleation of twins and partial dislocations, as well as the morphology of the twin nuclei, and to reasonably describe twin growth and interaction. The twin microstructures at grain boundaries are in agreement with experimental observation. It was found that de-twinning occurs during unloading in the simulations, however, a strong dependence of twin structure evolution on loading history was observed.     

化学气相沉积金刚石微球的生长机制研究

采用微波等离子体化学气相沉积法,在过饱和碳离子浓度条件下,在单晶硅衬底上制备了球形结构的多晶金刚石微球,通过控制沉积气压与温度的变化,研究了金刚石由石墨生长区向纳米晶的球形结构、再到具有良好结晶性的金刚石生长区的过渡过程。结果表明:沉积气压与温度的升高导致微球的粒径增大,微球由sp3、sp2键共存相转变为较纯的金刚石相;在一定的碳离子过饱和度和气压、温度范围内,微球的形成主要受二次形核过程的控制。气压和温度升高后,微球呈<110>取向生长,微球的形成主要受（111）面高密度孪晶和层错缺陷的控制,揭示了化学气相沉积金刚石不同生长区内二次形核机制与孪晶层错机制诱导的金刚石微球的生长过程。      

Analysis of deformation twins and the partially dehydrogenated microstructure in nanocrystalline magnesium hydride (MgH2) powder

Cryo-stage transmission electron microscopy (TEM), supported by Density Functional Theory (DFT), is employed to explore the microstructure of magnesium hydride (MgH₂) powders. Mechanical milling results in deformation twinning of the hydride. The crystallography of the twins is established. DFT analysis shows that the twin unit cell is just as thermodynamically stable as the undeformed α-MgH₂ matrix. It is hypothesized that the twins contribute significantly to the observed milling-induced kinetic enhancement by acting as high diffusivity paths for hydrogen. Energy-filtered TEM analysis on partially desorbed MgH₂ demonstrates that nucleation and growth of metallic magnesium occurs non-uniformly. Larger powder particles are a composite of isolated magnesium grains heterogeneously nucleated on the remaining hydride. Smaller particles are either fully transformed to magnesium or remain entirely a hydride. There is little evidence for any “core–shell” structure. It is also shown that in situ hydrogen desorption in the TEM is not representative of the elevated-temperature ex situ sequence.     

强基面织构AZ31合金压缩过程中的孪晶对行为

挤压态AZ31合金在室温下沿挤压方向进行压缩变形,合金中产生大量的拉伸孪晶。综合分析了孪晶对的斯密特因子(SF)和应变兼容因子(m_f),其中孪晶对包括相连的孪晶对和非相连的孪晶对。结果表明:相连的孪晶对优先在取向差约为25°的相邻晶粒的晶界上形核。大约88%的相连孪晶对具有很高的斯密特因子,大约76%的相连孪晶对具有很高的应变兼容因子。低斯密特因子的孪晶对的发生能够通过高应变兼容因子进行解释。大约23%的非相连孪晶对的应变兼容因子接近于0。     

On the twinning occurrence in bulk semiconductor crystal growth

Based on known theories of twinning in semiconductor crystal growth, a new model is proposed to study the occurrence of twins during the solidification of photovoltaic multicrystalline silicon ingots. It is expected that twins will appear on facets existing at the grain boundary–solid–liquid triple line. Necessary conditions for the existence of facets are derived and it is shown that twinning remains a function of the probability of nucleation of twinned nuclei. It is demonstrated that this probability is in qualitative agreement with the experimental observation for cases where the grain orientation is such that an angle of 132° occurs between a facet and a grain boundary. However, full validation of the model requires accurate values of interfacial energies at the melting point, which are currently lacking.     

Nucleation mechanisms of dynamic recrystallization for G3 alloy during hot compression

Dynamic recrystallization(DRX) mechanisms of a nickel-based corrosion-resistant alloy, G3, were investigated by hot compression tests with temperatures from 1050 to 1200 ℃ and strain rates from 0.1 to 5.0 s~(-1).Deformation microstructure was observed at the strain from 0.05 to 0.75 by electron backscatter diffraction(EBSD) and transmission electron microscope(TEM).Work hardening rate curves were calculated to analyze the effect of deformation parameters on the nucleation process.Results indicate that strain-induced grain boundary migration is the principal mechanism of DRX. Large annealing twins promote nucleation by accumulating dislocations and fragmenting into cell blocks. Continuous dynamic recrystallization is also detected to be an effective supplement mechanism, especially at low temperature and high strain rate.