Effects of V/Ⅲ ratio on a-plane GaN epilayers with an InGaN interlayer

The effects of V/Ⅲgrowth flux ratio on a-plane GaN films grown on r-plane sapphire substrates with an InGaN interlayer are investigated.The surface morphology,crystalline quality,strain states,and density of basal stacking faults were found to depend heavily upon the V/Ⅲratio.With decreasing V/Ⅲratio,the surface morphology and crystal quality first improved and then deteriorated,and the density of the basal-plane stacking faults also first decreased and then increased.The optimal V/Ⅲratio growth condition for the best surface morphology and crystalline quality and the smallest basalplane stacking fault density of a-GaN films are found.We also found that the formation of basal-plane stacking faults is an effective way to release strain.     

A systematic investigation of stacking faults in magnesium via first-principles calculation

The stacking faults of crystal magnesium have been studied systematically by means of first-principles calculation within the generalized gradient approximation (GGA). The generalized stacking fault (GSF) energy surfaces for four kinds of basal stacking faults as well as other non-basal stacking faults in the prismatic and pyramidal planes have been gained using a supercell approach with the supercell tilling technique. The most likely slip directions for the formation of these stacking faults in the corresponding slip plane were determined, and the generalized stacking fault energy curves along the most likely slip directions were derived, then the stable and unstable stacking energies were obtained and discussed. The present results are helpful for further investigation of dislocations and the correlative mechanical properties.     

基于准连续介质方法模拟纳米多晶体Ni中裂纹的扩展

通过准连续介质方法模拟了纳米多晶体Ni中裂纹的扩展过程.模拟结果显示:裂纹尖端的应力场可以导致晶界分解、层错和变形孪晶的形成等塑性形变,在距离裂纹尖端越远的位置,变形孪晶越少,在裂纹尖端附近相同距离处,层错要远多于变形孪晶.这反映了局部应力的变化以及广义平面层错能对变形孪晶的影响.计算了裂纹尖端附近区域原子级局部静水应力的分布.计算结果表明:裂纹前端晶界处容易产生细微空洞,这些空洞附近为张应力集中区,并可能促使裂纹沿着晶界扩展.模拟结果定性地反映了纳米多晶体Ni中的裂纹扩展过程,并与相关实验结果符合得很好     

Assessment of the thermodynamic dimension of the stacking fault energy

Especially with respect to high Mn and other austenitic TRansformation and/or TWinning Induced Plasticity (TRIP/TWIP) steels, it is a current trend to model the stacking fault energy of a stacking fault that is formed by plastic deformation with an equilibrium thermodynamic formalism as proposed by Olson and Cohen in 1976. In the present paper, this formalism is critically discussed and its ambiguity is stressed. Suggestions are made, how the stacking fault energy and its relation to the formation of hexagonal ?-martensite might be treated appropriately. It is further emphasized that a thermodynamic treatment of deformation-induced stacking fault phenomena always faces some ambiguity. However, an alternative thermodynamic approach to stacking faults, twinning and the formation of ?-martensite in austenitic steels might rationalize the specific stacking fault arrangements encountered during deformation of TRIP/TWIP alloys.     

金属铝中层错能的缀加球面波法第一性原理计算

将层错周期地置入超晶胞中,当超晶胞有足够大的设置尺度时,相邻超晶胞中的层错彼此间互作用很小,从而以如此方式计算单一层错能量成为可能。用第一性原理途径(缀加球面波方法)计算了金属铝中的内禀和外延层错能,其理论值分别是Γ_ISF=154erg/cm²和Γ_ESF=138erg/cm²,与实验值符合较好。 关键词：     

Shrinking stacking fault through glide of the Shockley partial dislocation in hard-sphere crystal under gravity

Disappearance of a stacking fault in the hard-sphere crystal under gravity, such as reported by Zhuet al. [Nature 387, 883 (1997)], has successfully been demonstrated by Monte Carlo simulations. We previously found that a less ordered (or defective) crystal formed above a bottom ordered crystal under stepwise controlled gravity [Moriet al. J. Chem. Phys. 124, 174507 (2006)]. A defect in the upper defective region has been identified with a stacking fault for the (001) growth. We have looked at the shrinking of a stacking fault mediated by the motion of the Shockley partial dislocation; the Shockley partial dislocation terminating the lower end of the stacking fault glides. In addition, the presence of crystal strain, which cooperates with gravity to reduce stacking faults, has been observed.     

氧化锌纳米带中的面缺陷

面缺陷是纳米带中非常普遍和非常重要的一类缺陷。在有些情况下，面缺陷对于高表面能指数面的出现起着决定性的作用。同时。它们可以诱导纳米带沿着特殊的方向生长。面缺陷可以是孪晶或双晶，层错和由杂质原子聚集在特定原子面所形成的间隙原子层。在本文中。利用透射电子显微术，我们将介绍氧化锌纳米带中被发现的几种面缺陷。我们确认了两种孪晶／双晶结构，它们的孪晶面分别是（01^-13）和（^-2112）面。基面层错有I1和I2两种。在大尺寸的纳米带中，I1基面层错可以折叠到（2^110）面形成棱面层错。当少量的In离子掺入氧化锌纳米带后，我们发现伴随着杂质In在基面的聚集，形成了两种倒反畴界。     

Lattice imperfection studies in polycrystalline materials by x-ray diffraction line-profile analysis

This review concerns our recent investigations with a series of binary fcc Ag- and Cu-base alloys (viz Ag-Ga, Ag-Ge, Ag-Al and Cu-Ga, Cu-Ge) from detailed analyses of x-ray diffraction line profiles, the importance of which has been briefly summarized. The theoretical formulations of the Warren-Averbach’s method of Fourier analysis of peak-shapes along with the methods of peak-shift and peak-asymmetry have been outlined. A preview on the significant studies carried out earlier with Ag- and Cu-base (fcc) binary alloys has been made in short. A detailed analyses on the recorded profiles in the present considerations revealed, in general, quantitative estimates of several microstructural parameters characterising the deformed state of the materials namely, propensity of stacking faults (intrinsic, extrinsic and twin faults), rms microstrains, coherent domain sizes, long-range residual stresses, lattice parameter changes, dislocation density and stacking fault energy. The results indicate a general trend of increase in the concentrations of stacking faults, primarily, of intrinsic character, with increase in solute concentrations; which are solely responsible for the observed peak-shifts as well as domain size broadening. Small asymmetry in the profiles is due to the presence of extrinsic stacking faults, relatively less in magnitude compared to the intrinsic ones while the deformation twin faults are almost absent—an observation with significance. The dislocation density, quite appreciable in magnitude, has been evaluated from the anisotropic values of the coherent domain sizes and rms microstrains. The stacking fault energies of pure Ag and Cu, an important parameter have also been estimated and compared with those obtained from electron microscopy. Annealing experiments with a Ag-5·8% Al alloy, aluminium being a precipitating solute, do not reveal any detectable evidence of solute segregation at the stacking faults. The occurrence of stacking faults in the alloy systems has been correlated with a number of physical factors involving solvent-solute types.     

Detecting electronic states at stacking faults in magnetic thin films by tunneling spectroscopy

Co islands grown on Cu(111) with a stacking fault at the interface present a conductance in the empty electronic states larger than the Co islands that follow the stacking sequence of the Cu substrate. Electrons can be more easily injected into these faulted interfaces, providing a way to enhance transmission in future spintronic devices. The electronic states associated with the stacking fault are visualized by tunneling spectroscopy, and its origin is identified by band structure calculations.     

单轴应变驱动铁bcc—hcp相转变的微观模拟

用分子动力学方法模拟了沿〈001〉晶向应变加载和卸载情况下单晶铁中体心立方(bcc)与六方密排(hcp)结构的相互转变,分析了相变的可逆性和微结构演化特征.微观应力的变化显示样品具有超弹性性质,而温度变化表明在相变和逆相变过程中均出现放热现象.相变起始于爆发式均匀形核,晶核由块状颗粒迅速生长为沿{011}晶面的片状分层结构; 而卸载逆相变则从形核开始就呈现片状形态,且相界面晶面指数与加载相变完全一致,表现出形态记忆效应.在两hcp晶核生长的交界面易形成面心立方(fcc)堆垛层错. fcc通过在hcp晶粒内     

TEM study on relationship between stacking faults and non-basal dislocations in Mg

Recent interest in the study of stacking faults and non-basal slip in Mg alloys is partly based on the argument that these phenomena positively influence mechanical behaviour. Inspection of the published literature, however, reveals that there is a lack of fundamental information on the mechanisms that govern the formation of stacking faults, especially I1-type stacking faults (I1 faults). Moreover, controversial and sometimes contradictory mechanisms have been proposed concerning the interactions between stacking faults and dislocations. Therefore, we describe a fundamental transmission electron microscope investigation on Mg 2.5 at. % Y (Mg–2.5Y) processed via hot isostatic pressing (HIP) and extrusion at 623 K. In the as-HIPed Mg–2.5Y, many 〈c〉 and 〈a〉 dislocations, together with some 〈c + a〉 dislocations were documented, but no stacking faults were observed. In contrast, in the as-extruded Mg–2.5Y, a relatively high density of stacking faults and some non-basal dislocations were documented. Specifically, there were three different cases for the configurations of observed stacking faults. Case (I): pure I2 faults; Case (II): mixture of I1 faults and non-basal dislocations having 〈c〉 component, together with basal 〈a〉 dislocations; Case (III): mixture of predominant I2 faults and rare I1 faults, together with jog-like dislocation configuration. By comparing the differences in extended defect configurations, we propose three distinct stacking fault formation mechanisms for each case in the context of slip activity and point defect generation during extrusion. Furthermore, we discuss the role of stacking faults on deformation mechanisms in the context of dynamic interactions between stacking faults and non-basal slip.     

β—SiC外延层中晶体缺陷的观察

用腐蚀法研究了β-SiC外延层中的晶体缺陷。腐蚀剂为熔融氢氧化钾。三角形尖底蚀坑对应于位错。在β-SiC中的全位错为立方晶系的73°位错和60°位错。不同堆垛方式的β-siC生长层相遇时将形成{111}交界层错,其腐蚀图象为平行于<110>方向的直线。60°位错可分解为两个1/6<112>SchockLey不全位错,并夹着一片{111}层错构成扩展位错。三个1/6<110>压杆位错与三片{111}层错可构成层错锥体。正、反堆垛的β-SiC可形成尖晶石律双晶,双晶面为(111)。腐蚀法和X射线劳厄法证实了这种双晶的存在。 关键词：     

Deformation induced microtwins and stacking faults in aluminum single crystal

Microtwins and stacking faults in plastically deformed aluminum single crystal were successfully observed by high-resolution transmission electron microscope. The occurrence of these microtwins and stacking faults is directly related to the specially designed crystallographic orientation, because they were not observed in pure aluminum single crystal or polycrystal before. Based on the new finding above, we propose a universal dislocation-based model to judge the preference or not for the nucleation of deformation twins and stacking faults in various face-centered-cubic metals in terms of the critical stress for dislocation glide or twinning by considering the intrinsic factors, such as stacking fault energy, crystallographic orientation, and grain size. The new finding of deformation induced microtwins and stacking faults in aluminum single crystal and the proposed model should be of interest to a broad community.     

硅蹼中位错和层错的X射线貌相研究

利用X射线投影貌相术观察和分析了硅蹼中的位错和层错。在生长态硅蹼中,除观察到柏氏矢量为1/2<110>的刃型、螺型与60°全位错以及柏氏矢量为1/6<112>的Shockley刃型半位错外,还观察到平行于硅蹼表面的大面积层错和蹼中的60°,30°Shockley半位错。位错在热处理过程中运动并发生位错反应形成近六角形的位错网络。热处理改变生长态硅蹼中层错的组态和衬度,并由于杂质聚集破坏了Shockley半位错的消象法则。还观察到层错象中的位错。对所观察的结果都分别作了分析和简要的讨论。 关键词：     

Dislocation and stacking fault core fields in fcc metals

Atomistic models were used to determine the properties of dislocation core fields and stacking fault fields in Al and Cu using embedded atom method (EAM) potentials. Long-range, linear elastic displacement fields due to nonlinear behaviour within dislocation cores, the core field, for relevant combinations of Shockley partial dislocations for edge, screw, and mixed (60° and 30°) geometries were obtained. Displacement fields of stacking faults were obtained separately and used to partition the core field of dissociated dislocations into core fields of partial dislocations and a stacking fault expansion field. Core field stresses were derived from which the total force, including the Volterra field plus core field, between dislocations for several dislocation configurations was determined. The Volterra field dominates when the distance between dislocations exceeds about 50b but forces due to core fields are important for smaller separation distances and were found to affect the equilibrium angle of edge dislocation dipoles and to contribute to the force between otherwise non-interacting edge and screw dislocations. Interactions among the components of a dissociated dislocation modify the equilibrium separation for Shockley partials suggesting that methods that determine stacking fault energies using measurements of separation distances should include core fields.     

Investigation of V-shaped extended defects in a 4H–SiC epitaxial film

Abstract

We investigated two types of V-shaped extended defects on the basal plane in epitaxial 4H-SiC by synchrotron X-ray topography, photoluminescence imaging/spectroscopy and transmission electron microscopy (TEM). One is the (2, 5) stacking fault (in Zhdanov notation) bounded by two partial dislocations with the Burgers vector b ± 1/4[0?0?0?1]; the other is the (2, 3, 3, 5) stacking fault bounded by partial dislocations with b = ±1/4[0?0?0?1]. The core of the partial dislocations associated with the (2, 3, 3, 5) fault has an out-of-plane component (Frank component) and three in-plane components (Shockley components); the three Shockley components are cancelled out in total. The electronic structures of the (2, 5) and (2, 3, 3, 5) stacking faults were further examined by photoluminescence spectroscopy and first-principles calculations. It is suggested that the (2, 5) and (2, 3, 3, 5) stacking faults both have an interband state at a similar energy level, although they differ structurally.     

Microstructures of a single-crystal nickel-base superalloy after thermo-mechanical fatigue

The microstructures of a single-crystal nickel-base superalloy were observed by transmission electron microscopy (TEM) before and after thermo-mechanical fatigue. Both dislocation configurations and γ′ precipitate morphologies under in-phase (IP) testing were found to be clearly different from those under out-of-phase (OP) testing. Under IP testing, dense hexagonal dislocation networks were found on the horizontal interfaces, and irregular dislocation networks on the vertical interfaces. With decreasing mechanical strain amplitude, rafting of γ′ precipitates was more pronounced. Under OP testing, no dislocation networks were found on the γ/γ′ interfaces; also, γ′ precipitates were sheared by superlattice stacking faults, and were not clearly rafted. The characteristic dislocation networks and partials of the stacking fault are analysed and the rafting mechanism of the γ′ precipitates is discussed.     

晶体生长的层错机制及其生长动力学

面心立方晶体中层错矢量为1/6<112>和1/3<111>的堆垛层错在生长面{111}上的露头处产生了亚台阶(sub-step)。基于亚台阶的原子图象的分析,本文讨论了层错生长机制及其生长动力学,证明了亚台阶处的成核势垒总是小于二维成核势垒,解释了近年来所观察到的堆垛层错作为生长台阶源的实验事实。 关键词：     

Dislocation formation and twinning from the crack tip in Ni3Al: molecular dynamics simulations

The mechanism of low-temperature deformation in a fracture process of L12 Ni3Al is studied by molecular dynamic simulations.Owing to the unstable stacking energy,the [01ˉ1] superdislocation is dissociated into partial dislocations separated by a stacking fault.The simulation results show that when the crack speed is larger than a critical speed,the Shockley partial dislocations will break forth from both the crack tip and the vicinity of the crack tip;subsequently the super intrinsic stacking faults are formed in adjacent {111} planes,meanwhile the super extrinsic stacking faults and twinning also occur.Our simulation results suggest that at low temperatures the ductile fracture in L12 Ni3Al is accompanied by twinning,which is produced by super-intrinsic stacking faults formed in adjacent {111} planes.     

Stacking fault energy of cryogenic austenitic steels

Stacking fault energy and stacking fault nucleation energy are defined in terms of the physical nature of stacking faults and stacking fault energy, and the measuring basis for stacking fault energy. Large quantities of experimental results are processed with the aid of a computer and an expression for calculating stacking fault energy has been obtained as γ³⁰⁰_SF(mJ·m^-2)=γ⁰_SF+1.59Ni-1.34Mn+0.06Mn²-1.75Cr+0.01Cr²+15.21Mo-5.59Si-60.69(C+1.2N)^1/2 + 26.27(C+1.2N)(Cr+Mn+Mo)^1/2+0.61[Ni·(Cr+Mn)]^1/2.