单晶合金激光熔凝过程中晶向对单晶完整性的影响

运用几何模型对单晶合金激光熔凝过程中激光扫描方向与[100]方向夹角ξ变化时熔池内的枝晶生长方向和速度进行了计算,研究了ξ增大时不同晶向区域的分布变化规律.发现随着ξ的增大, 0]区域增大,[010]区域减小,且熔池两边不同部分速度差别增强.根据速度变化规律,构造出了熔池不同部位的过冷区域变化图,说明了可能出现新晶粒的趋势变化,并与实验结果进行了比较,揭示了在晶向不同的交界区域产生新晶粒的内在机理. 关键词： 单晶合金 激光熔凝 晶向 组成过冷     

Microstructure and residual stress of laser rapid formed Inconel 718 nickel-base superalloy

The microstructure and residual stress of laser rapid formed (LRFed) nickel-base superalloy Inconel 718 was investigated. The as-deposited microstructure of an LRFed Inconel 718 alloy is composed of columnar dendrites growing epitaxially along the deposition direction, and the columnar dendrites transformed to unevenly distributed equiaxed grains after annealing treatment at high temperature. Residual stress evaluation in microstructure scale by Vickers micro-indentation method indicates that the residual thermal stress is unevenly distributed in the LRFed sample, and it has a significant effect on the recrystallization during solution annealing treatment. The residual stress is introduced by rapid heating and cooling during laser rapid forming. There is an alternative distribution between high residual stress regions and low residual stress regions, within a single deposited layer, resulting in a similar distribution of recrystallized grain size.     

Shock compression of [001] single crystal silicon

Silicon is ubiquitous in our advanced technological society, yet our current understanding of change to its mechanical response at extreme pressures and strain-rates is far from complete. This is due to its brittleness, making recovery experiments difficult. High-power, short-duration, laser-driven, shock compression and recovery experiments on [001] silicon (using impedance-matched momentum traps) unveiled remarkable structural changes observed by transmission electron microscopy. As laser energy increases, corresponding to an increase in peak shock pressure, the following plastic responses are are observed: surface cleavage along {111} planes, dislocations and stacking faults; bands of amorphized material initially forming on crystallographic orientations consistent with dislocation slip; and coarse regions of amorphized material. Molecular dynamics simulations approach equivalent length and time scales to laser experiments and reveal the evolution of shock-induced partial dislocations and their crucial role in the preliminary stages of amorphization. Application of coupled hydrostatic and shear stresses produce amorphization below the hydrostatically determined critical melting pressure under dynamic shock compression.     

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.     

Pyramid-like spikes in a single crystal superalloy produced by picosecond laser irradiation

Pyramid-like spikes in a single crystal superalloy were investigated upon irradiation with picosecond (ps) laser pulses (200 ps, 800 nm, 1 kHz) under different laser fluences and pulse numbers. Both sides and grooves of pyramid-like spikes were covered with ripples, which had a period of ～760 nm. The pyramid-like spike separation increased obviously with increasing laser fluence. Microstructural investigations indicate that the pyramid-like spikes were initiated with subsequent pulses from a smooth surface with corrugations and ripples. The coexistence of capillary waves for spikes and capillary waves for ripples in the melted material can be used to explain the formation of the pyramid-like spikes.     

Synthesis of [100] Wurtzite InN Nanowires and [011] Zinc-Blende InN Nanorods

One-dimensional wurtzite InN nanowires and zincblende InN nanorods are prepared by chemical vapour deposition （CVD） method on natural cleavage plane （110） of GaAs. The growth direction of InN nanowires is [100], with wurtzite structure. The stable crystal structure of InN is wurtzite （w-InN）, zincblende structure （z-InN） is only reported for 2D InN crystals before. However, in this work, the zincblende InN nanorods [011] are synthesized and characterized. The SEM and TEM images show that every nanorod shapes a conical tip, which can be explained by the anisotropy of growth process and the theory of Ehrlich Schwoebel barrier.     

Microstructure evolution and mechanical property of pulsed laser welded Ni-based superalloy

For evaluating the microstructure evolution and mechanical property of Ni-based Hastelloy C-276 weld joint by the pulsed laser welding, the influence of pulsed laser welding on the microstructure and mechanical property of the weld joint is investigated by the analysis of the microstructure morphology, microhardness, phase structure and tensile property. The results indicate that, in the fusion zone three sections are divided on the basis of the patterns of grain structures. In the weld joint, the element segregation is found, but the trend of brittle phase׳s formation is weakened. The weld microhardness presents just a little higher than that of base metal, and there is no obvious the softened heat affected zone. Meanwhile in the weld joint, the phase structure is still the face-center cubic with the tiny shift of peak positions and widened Full Width at Half-Maximum. The yield strength of weld joint is the same as that of base metal, and the tensile strength is nearly 90% of that of base metal. The decreased tensile strength is mainly attributed to the dislocation piling-up.     

Electrical and magnetic properties of single crystal [NEt4]2[CuII(mnt)2]

We report electrical and magnetic studies of [NEt₄]₂[Cu^II(mnt)₂]. This crystal is composed of chains of theplanar [Cu^II(mnt)₂]^?2 anions (space group $\text{P}\text{1}$ and z = 1) which exhibit only weak magnetic interactions. The material behaves as a semiconductor; from 300–400°K the conductivity increases by six orders of magnitude and the resistivity values above 300°K are comparable to those of some of the better known wide band-gap inorganic semiconductors. In contrast with the behavior of other linear chain systems, at room temperature the conductivity along the chain (σ_∥) is less than that perpendicular to the chain (σ_⊥). As the temperature is increase, the anisotropy ratio, σ_∥/σ_⊥, becomes greater than unity and increases to 1.6 × 10² at 400°K.     

Characterization of preferential orientation of martensitic variants in a single crystal of NiMnGa

We report the detailed observation of martensitic variants in NiMnGa single crystals. The variants that are twinned with each other in different ways can be clearly identified in our single crystals by optical observation. We also investigated the preferential orientation of the martensitic variants in NiMnGa single crystals. We observed the motion of the variant boundary in response to application of a magnetic field. This observation can be used to explain phenomenologically the magnetic-field-induced strain. In the single crystal with composition Ni₅₂Mn₂₄Ga₂₄, martensite with seven modulated layers (7M) shows preferentially oriented variants. A completely recoverable two-way shape-memory behavior was also observed by measuring the free sample in three different directions during a complete temperature cycle. It was found that the largest strains in the [001] and [010] directions occur in different temperature ranges.     

Grain refinement and texture evolution during high precision machining of a Ni-based superalloy

Abstract

The grain refinement and texture evolution in the surface gradient microstructure of a Ni-based superalloy induced by high speed machining was studied in this research. The direct evidence of grain refinement induced by dislocation–twin interaction was revealed and the detailed grain refinement process was summarised as deformation twinning, dislocation-twin reaction, localied thinning of nanotwin lamellae and final fracture. The underlying dislocation–twin interaction mechanism was elucidated from the crystallographic perspective. Using electron backscatter diffraction and precession electron diffraction techniques, a multiscale texture analysis covering undeformed coarse grain region, ultrafine grain region and nanograin region was carried out. The texture evolution with decreasing depth to the machined surface was identified as cube in the bulk interior and a mixture of rotated cube {0?0?1}<1?1?0>, cube {1?0?0}<0?0?1>, copper {1?1?2}<1?1?1 > and Goss {1?1?0}<0?0?1> textures in the topmost 1.3-μm-thick nanograin layer. The intrinsic thermomechanical effects of high precision machining are responsible for crystallographic texture transformation.     

Understanding reversed temperature dependence of diffusional solidification time in single crystal superalloy brazement

A fully implicit two-dimensional moving-mesh finite element simulation model, without symmetry assumption, recently developed by the present authors was used to study the cause of anomalous reversed temperature dependence of diffusional solidification completion time, t _f, during brazing of single crystal nickel-based superalloys. Contrary to the general concept of competition between solute diffusivity and solubility, as predicted by analytical models, numerical calculations coupled with experimental observation showed that the anomalous behavior is caused by departure from parabolic liquid–solid interface migration with time. Application of the new numerical model coupled with the understanding provided by the study resulted in a viable approach for minimizing the time required to produce reliable brazed joints free of deleterious eutectic and stray-grains in single crystal superalloys.     

Deformation band evolution in [110] Al single crystals strained in tension

Several types of deformation bands form during uniaxial extension of Al single crystals for which the tensile axis is initially parallel to [110]. The objectives of the present work are to analyse crystal orientation evolution in the deformation bands and adjoining regions, and to integrate the experimental observations with a crystal mechanics model. The most prominent deformation bands contain secondary slip traces and exhibit crystal rotations consistent with unpredicted slip on a secondary slip system. These special bands of secondary slip (SBSS) become more closely aligned with the tensile axis as extension increases. The evolution of SBSS inclination with extension indicates that SBSS form initially as kink bands and that SBSS boundaries are immobile. SBSS grow during straining by expansion of the volume of material in which secondary slip operates. Deformed matrix (DM) bands are zones between SBSS; primary slip predominates in DM bands. Small intra-DM bands result from spatial variation of the shear amplitudes for the two primary slip systems. The evolution of intra-DM band inclination with extension indicates that intra-DM bands form initially as kink bands and that the band boundaries are mobile, at least to some extent.     

蠕变镍基单晶高温合金微观结构与界面特征的X射线小角散射研究

利用SAXS技术对蠕变过程中不同尺度范围的微观结构变化分析表明X射线小角散射(SAXS)与中子小角散射(SANS)测量的二维散射图具有明显的差异,由散射强度曲线的变化说明了蠕变过程中二次析出γ'相形貌和不同区域尺寸特征的改变情况.分析结果表明二次析出γ'相存在两类特征尺寸,在蠕变过程中沿[100]或[010]方向的变化趋势类似,均是在第一和第二阶段有所减小,在第三阶段又有所增大,相较而言,特征尺寸较大的γ'相变化也较为显著.二次析出γ'相在蠕变第二阶段元素扩散最严重,相表面最粗糙,在第三阶段两相界面又进一 关键词： 单晶高温合金 二次析出γ'相 X射线小角散射 微观结构