外来夹杂物颗粒对枝晶生长形态影响的相场法研究

在KKS相场模型基础上提出了一种耦合取向场的二元合金相场模型.通过该模型分析了外来夹杂物颗粒与晶粒之间的取向错配对于枝晶生长的影响，模拟结果表明外来夹杂物颗粒的大小、位置、数量以及与晶粒的取向差等因素均对枝晶生长形态有较大影响，外来夹杂物颗粒会造成枝晶臂的偏转与分叉，从而最终形成各种形貌的非规则枝晶. 关键词： 相场法 枝晶 外来夹杂物颗粒 晶体取向     

单晶Ni52Mn24Ga24中马氏体变体择优取向的物理表征

关键词：     

晶体取向对定向凝固平界面失稳行为的影响

采用丁二腈-丙酮透明模型合金研究了不同晶体取向的晶粒在定向凝固条件下的平界面失稳过程.实验选择了三个界面失稳后具有不同生长形态的典型晶粒作为研究对象, 分别为择优生长枝晶、倾斜枝晶和海藻晶.结果表明可发展为择优生长枝晶的晶粒的平界面失稳孕育时间和初始扰动波长最小,海藻晶次之, 倾斜枝晶最大,这与以往的解析结果和相场模拟结果一致. 同时,实验观察发现可发展为择优生长枝晶和倾斜枝晶的晶粒的界面非稳态演化过程与海藻晶显著不同,这表明平界面失稳的非稳态演化过程与晶体取向相关.     

Influence of dispersoids on microstructure evolution and work hardening of aluminium alloys during tension and cold rolling

The influence of dispersoids on work hardening of aluminium during tension and cold rolling has been studied by comparing Al–Mn alloys containing similar amounts of solutes but various dispersoid densities. The microstructure evolution with deformation strain was examined in transmission and scanning electron microscopy. It is found that a high density of fine dispersoids strengthens the materials significantly, but their strengthening effect diminishes as the strain increases. From a series of Bauschinger tests, it is found that the internal stress, due to particles, increases rapidly at the initial stage of deformation, but saturates at strains larger than 5%. It is concluded that the internal stress makes a small contribution to the work hardening and contributes to less than 10% of the total flow stress during monotonic loading at strains larger than 5%. The work-hardening behaviour has been correlated to the corresponding microstructure, and the strengthening mechanisms are discussed.     

Modelling work hardening of aluminium alloys containing dispersoids

The influence of dispersoids on tensile deformation behaviour has been studied by comparison of aluminium alloys containing different dispersoid densities. It was found that a fine dispersion of non-shearable particles led to an increased work hardening at the initial plastic deformation, but the effect was opposite at higher strains. The reason has been attributed to the generation of geometrically necessary dislocations (GNDs). A new model has been proposed for the evolution of GNDs based on a balance of storage and dynamic recovery of GNDs. The model predicts a rapid saturation of GNDs and a reduced work hardening at small strains, consistent with the experimental results.     

Numerical study of the process of plastic deformation localization by an example of high-speed compression of a hollow single crystal cylinder

The effect of the crystallographic orientation of a single crystal hollow cylinder on features of creation and evolution of plastic deformation in it under conditions of high-speed axisymmetric load is studied. An advantage of the proposed loading scheme is the simultaneous implementation of all loading variants within the chosen crystallographic base plane of the cylinder and reaching different degrees of deformation over the cross section of the sample. Using the molecular-dynamic modeling, the difference in deformation properties of the loaded sample has been shown depending on the chosen crystallographic orientation of the base plane. Results of the investigation can be used to understand the main mechanisms of the plastic deformation of crystalline bodies.     

An Infinitesimal Deformation Approach to the Crystallographic Theory of the Martensite Phase Transformation: Application to Au-Cd,Fe-Ni AND In-Tl Alloys

Using the infinitesimal deformation approach, a crystallographic analysis of the austenite-martensite transformation from the cubic to orthorhombic phase - which predicts crystallographic parameters such as habit plane, orientation relationship between austenite and martensite, rotation matrix and total shape deformation matrix - is derived from a knowledge of the crystal structures of the initial and final phase only. The numerical values coming from orientation relationships obtained for Au-47.5 Cd Fe-Ni and In--Tl alloys are compared with predictions of the phenomenological crystallographic theory, infinitesimal deformation approach and experimental data.     

Hall effect of hot electrons in n-PbTe

Measurements of the Hall-coefficient were performed on n-PbTe at 80K in different crystallographic directions as a function of electric field up to 1.1kV/cm and magnetic fields up to 0.6T. The observed behaviour gives evidence for the presence of strong equivalent intervalley transfer. As the crystallographic orientation had no significant influence on the threshold and the appearance of an instability, equivalent intervalley scattering was excluded as a possible mechanism for this instability.     

Elastic-plastic deformation and fracture of shock-compressed single-crystal and polycrystalline copper near melting

The Hugoniot elastic limit, the yield strength, and the spall strength of polycrystalline M1 copper and single-crystal (110) and (111) copper are determined during shock compression up to 8 GPa in the temperature range 20–1080°C from an analysis of the free-surface velocity profiles recorded with VISAR laser velocimeter. The measurements show that all copper samples exhibit strong athermal hardening (increase in the Hugoniot elastic limit) near the melting temperature. Copper single crystals have a very low elastic limit in the temperature range up to 600°C, this limit increases sharply as the temperature increases to 1000°C, and it depends on the crystallographic orientation of a single crystal. The temperature dependence of the spall strength has a threshold character for all copper samples. Copper single crystals demonstrate higher resistance to spall fracture; however, near the melting temperature, the difference between the spall strengths of the copper single crystals and M1 copper becomes insignificant, 50% of the initial level.     

Atomistic simulation analysis of the effects of void interaction on void growth and coalescence in a metallic system

Material deformation caused by the interaction between defects is a significant factor of material fracture failure. The present study employs molecular dynamics simulations of single-void and double-void crystalline Ni atomic systems to investigate inter-void interactions. Furthermore, simulations showing the evolution of dislocations for three different crystallographic orientations are conducted to study the void growth and coalescence. The simulations also consider the effect of the radius of the secondary void on dislocation evolution. The results show that double-void systems are more prone to yield than single-void systems. Further microstructural analysis indicates that the interaction between voids is realized by dislocation reactions. The simulation results of the dislocation evolution of the three orientations reveal that a relationship exists between the evolution of the dislocation density and the stress-strain curve. At the initial stage of dislocation, the dislocation grows slowly, and consists of Shockley partial dislocation. The dislocation growth rate then increases significantly in the sharply declining stage of the stress-strain curve, where most of dislocations are Shockley partial dislocation. Analysis of the dislocation length during the overall simulation indicates that the dislocation length of the [110] orientation is the longest, followed by that of the [111] orientation and the [100] orientation, which has the shortest dislocation length.     

Grain-boundary hardening in polycrystalline copper-based solid solutions

A study is made of the contribution of grain-boundary hardening to the overall hardening in a polycrystalline material on the basis of Ashby's model. Yield curves are used for copper-based solid solutions in polycrystalline and singlecrystal forms. It is shown that the contribution from statistically accumulated dislocations to the yield stress in a polycrystalline specimen reflects the behavior of the corresponding single crystal. The contribution from grain boundaries to the yield stress can be described in terms of the additional dislocation density due to the joint grain deformation in the aggregate up to high strains. At low strains, the main role in hardening of a polycrystalline material is played by the grain boundaries. This extends up to larger strains as the strain temperature is reduced and the alloy-element concentration increases.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 47–52, January, 1984.     

Room temperature precipitation in quenched Al–Cu–Mg alloys: a model for the reaction kinetics and yield strength development

The microstructural evolution during low temperature ageing of two commercial purity alloys (Al–1.2Cu–1.2Mg–0.2Mn and Al–1.9Cu–1.6Mg–0.2Mn?at.%) was investigated. The initial stage of hardening in these alloys is very rapid, with the alloys nearly doubling in hardness during 20?h ageing at room temperature. The microstructural evolution during this stage of hardening was investigated using differential scanning calorimetry (DSC), isothermal calorimetry and three–dimensional atom probe analysis (3DAP). It is found that, during the hardening, a substantial exothermic heat evolution occurs and that the only microstructural change involves the formation of Cu–Mg co–clusters. The kinetics of cluster formation is analysed and the magnitude of the hardening is discussed on the basis of a model incorporating solid solution hardening and modulus hardening originating from the difference in modulus between Al and clusters.     

Crystallographic fringe orientation diffraction efficiency in bismuth silicon oxide

We report the first experimental measurement of fringe-angle applied electric field scaling of space charge growth and of crystallographic orientation effects in the initial development of the diffraction efficiency of thick holograms produced by the photorefractive effect in a bismuth silicon oxide (BSO) crystal. Diffraction efficiencies of holograms made by interfering two plane waves on the [¯110] face are measured as a function of the angle between the fringe pattern and the applied electric field. As the crystal is rotated relative to the interference fringes, the applied field may be scaled to yield identical space charge growth. Polarization-dependent diffraction measurements agree with the theory of a birefringent grating when optical activity is included as a separate, serial effect. Both the rotation-scaled applied electric fields and the crystallographic variations in the birefringent diffraction grating are consistent with charge transport processes in which the initial space charge fields are perpendicular to the interference fringes over growth times extending nearly into the steady state regime.     

The memory effect in Eu<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> single crystals exposed to high hydrostatic pressure

The influence of high-pressure hydrostatic compression and subsequent annealing on the structural properties of β’-phase single crystals of europium molybd ate was studied by IR spectroscopy and X-ray diffraction. It was demonstrated that after compression, the IR spectra exhibit drastic changes and do not depend on the initial crystallographic orientation of a single crystal. The high-pressure compression of single crystals followed by their annealing was found to result in the formation of β’-Eu₂(MoO₄)₃ single crystals with the same crystallographic orientation as that of the initial samples.     

Crystallographic contribution to the formation of the columnar grain structure in cobalt films deposited at oblique incidence

The geometric and crystallographic structures for cobalt films deposited at 75° and 45° by sputtering were investigated on the basis of optical and magnetic measurements and X-ray analysis. The substrate temperature was 332 K and the film thickness ranged from 20 to 900 nm. The alignments of columnar grains below and above 50 nm are perpendicular and parallel to the incidence plane, respectively, and the packing density of columnar grains constituting the parallel alignment decreases with increasing thickness at both incidence angles. At the high incidence angle of 75° the parallel alignment is more well-defined and its packing density is lower. The conclusions from these results are as follows. (1) Above 50 nm the crystal habit induces the two-degree orientation of the HCP phase through the geometric selection. Higher incidence angle enhances the geometric selection. (2) The crystal size necessary to exhibit the crystal habit is independent of the incidence angle.     

A crystal plasticity model of a formation of a deformation band structure

The formation of deformation bands with the typically alternating sign of the misorientation across their boundaries is interpreted as spontaneous deformation instability caused by anisotropy of hardening. To analyse the nature of the fragmentation, a model of a rigid-plastic crystal domain deformed by symmetric double slip in a plane-strain compression is considered. The basic reason for the deformation band existence is that a local decrease in number of active slip systems in the bands is energetically less costly than a homogeneous deformation by multislip. However, such model of the bands predicts their extreme orientation and their width tends to zero. This trend is modified by hardening caused by a build up of the band boundaries and by a dislocation bowing (Orowan) stress. The model provides an explanation of observed orientation of the bands, their width and the significant change in the structural morphology seen as the band reorientation occurs at large strains. The predictions are in a favourable agreement with the available observations.     

Thickness-dependent orientation evolution in nickel thin films grown on yttria-stabilized zirconia single crystals

Microstructure evolution along with crystallographic orientation change as a function of film thickness was investigated in Ni thin films grown on (100) yttria-stabilized zirconia (YSZ) single crystal substrates. Texture development with two different orientation relationships, OR1: Ni {111}//YSZ {100} and OR2: Ni {100}//YSZ {100}, cube on cube orientation were identified by X-ray diffraction and transmission electron microscopy depending on the film thickness. The observed orientation transition reveals the existence of a critical thickness (～320?nm) favoring two different orientations in sputtered Ni films on YSZ (100) substrate.     

Shape Memory Effect and Superelasticity in Ti–Ni and Fe–Ni–Co–Ti Single Crystals

The dependence of shape memory effect and superelasticity on the crystal orientation, size, and particle volume fraction is studied for Ti–Ni and Fe–Ni–Co–Ti single crystals. The shape memory effect and superelasticity are shown to decrease with increase in the particle volume fraction and depend on the number of crystallographic variants of particles that can vary due to aging under loading. The influence of disperse particles undergoing no martensite transformations on the martensitic crystal growth and their fine structure is examined.     

Thermal etching of sodium chloride III. Etching of non-cubic surface

Previous work on the thermal etching of (100) surfaces of NaCl single crystals is extended to the other surface orientations by using polycrystalline samples. The results obtained with (110), (111), (520) and (144) surfaces are used to corroborate the correctness of the mechanisms proposed in a previous paper to explain the thermal etching in air of (100) surfaces; it is also shown that these mechanisms are only dependent on crystal structure and not on the surface orientation.When such surfaces were treated with wet silk, concentrated HC1, and Moran's reagent, a very different behaviour was observed for each orientation. The reactivity is very sensitive to crystallographic orientation, being very low in the case of the (144) surface.     

Deformation behaviour of ultrafine and nanosize-grained Mg alloy synthesized via mechanical alloying

Bulk Mg–5Al alloys were consolidated from powders that had been mechanically alloyed over different milling durations. The microstructural evolution, and physical and mechanical properties of the alloys were investigated. Mechanical measurements revealed a change in deformation behaviour after certain milling durations. At short milling duration, high yield strength was obtained through dislocation strengthening mechanisms predominantly by grain refinement and to a lesser extent by solid solution strengthening and particle dispersion strengthening. However, at longer milling durations, low yield strength was observed and the strengthening mechanisms at work in short milling durations appeared to be no longer effective. Enhanced ductility with no work hardening behaviour was observed in samples with a mean grain size of 45?nm. It appeared that the significantly large increase in the grain boundary regions played an important role in the room temperature deformation of the alloys. The possibility of a diminishing effect of the dislocation strengthening mechanisms and the onset of grain boundary deformation modes for the softening phenomenon and the absence of work hardening at some nanoscale grain sizes are discussed.