Early stages of decomposition of alloys (spinodal or nucleation)

Progress in theory and experiment on the early stage of decomposition of alloys is reviewed. In the first half of this paper, theoretical treatments on decomposition of alloys by spinodal and nucleation mechanism and also continuum theories and the discrete lattice theories are comparatively reviewed. In the latter half of this paper, experimental results on Al-Zn, Ni-Al, Cu-Co and Al-Ag alloys are discussed.     

Interactions between grain boundary and compositional domain boundary during spinodal decomposition in nanocrystalline alloys

Due to the large grain boundary (GB) volume fraction in nanocrystalline materials, interactions between GB and compositional domain boundary (CDB) play an important role in determining the nanoscale-modulated domain structures during spinodal decomposition. In the present paper, the phase field crystal model is employed to investigate the interactions between GB and CDB. Simulation results show that CDB coarsening can drive the GB migration and bring the impingement of particles with different orientations; the large volume fraction of GB can increase the dislocation volume fraction in CDBs but does not change its proportion in the whole defects number; the crossover point of the coarsening dynamic comes from the block effect of GB with large volume fraction.     

Isotropic averaging for cell-dynamical-system simulation of spinodal decomposition

Formulae have been developed for the isotropic averagings in two and three dimensions. Averagings are employed in the cell-dynamical-system simulation of spinodal decomposition for inter-cell coupling. The averagings used in earlier works on spinodal decomposition have been discussed.     

A soluble kinetic model for spinodal decomposition

We compare the two-dimensional voter model with approximate theories for spinodal decomposition. The cluster size distribution and the short-time dynamics of the voter model are studied by means of a Monte Carlo simulation. The time-dependent structure factor and the long-time scaling of the voter dynamics are known analytically.This paper is dedicated to Nico van Kampen on the occasion of his 67th birthday.     

Fracture complexity of pressure vessel steels

Abstract

Significant volume of literatures are already available in the published domain reporting the mechanical and fracture behaviour of different pressure vessel steels under various testing conditions and other potential circumstances. There have been limited researches available in the open domain to correlate the tensile properties of these steels with their corresponding fracture features at various testing temperatures, which are primarily aimed at in the current investigation. A comprehensive literature review has been performed to realise this fact critically. There has been high probability that fracture features are the signature of the entire deformation history which was operated in the material. In order to understand this hypothesis, many tensile experiments are carried out at a constant strain rate by systematic variation in temperature of a reactor pressure vessel steel. The initial inclusion content and their distribution pattern are kept unaltered for all the specimens before tests, and temperatures are varied methodically to vary the nucleation sites of micro-voids (i.e. carbides, phase interfaces, etc.) which result in change of ductile fracture features. Conventional metallographic technique has been employed to characterise the microstructures at various temperatures. Fractographic characterisation of all broken tensile specimens is done to measure the two-dimensional fracture features (i.e. dimple geometry, extent of tearing ridge pattern and dimple number density) under secondary mode of imaging in scanning electron microscope. Quantitative fractography and image processing have been extensively employed to measure the two-dimensional fractographic features. An excellent correlation has been drawn between the ductile fractographic features, microstructures and corresponding tensile properties of the material as a function of test temperature. This study brings to the fore that from the systematic fractographic features, it is possible to determine reasonably the mechanical and fracture properties of a material, when the microstructure is known.     

调幅分解及形核对相分离作用机理研究

以SCN-30wt%H₂O, SCN-50 wt%H₂O和SCN-80 wt%H₂O三组透明体系, 在恒温场下实现了形核和调幅分解两种过程; 在此基础上, 施加温度梯度, 研究了第二相液滴的迁移运动规律. 结果表明, 相分离在临界成分体系以调幅分解方式进行, 在另外两种体系中以形核长大方式进行; 调幅分解与形核过程相比, 反应进行得更快, 液滴长大到同一尺寸所需时间仅为形核所需时间的1/3—1/2. 且临界成分体系有更大的不混溶间隙, 所以第二相液滴具有更多迁移时间, 揭示了偏晶体系相分离过程中在临界成分处易获得壳-核组织的内在机理. 在单向温度场中, 测量了不同半径的液滴迁移速率, 并且与理论Marangoni迁移速率值作比较, 发现液滴迁移速率和Marangoni理论迁移速率符合较好. 说明了在较好地抑制自然对流条件下Marangoni迁移对于相分离过程起主要作用.     

Phase-field study of spinodal decomposition under effect of grain boundary

Grain boundary directed spinodal decomposition has a substantial effect on the microstructure evolution and properties of polycrystalline alloys. The morphological selection mechanism of spinodal decomposition at grain boundaries is a major challenge to reveal, and remains elusive so far. In this work, the effect of grain boundaries on spinodal decomposition is investigated by using the phase-field model. The simulation results indicate that the spinodal morphology at the grain boundary is anisotropic bicontinuous microstructures different from the isotropic continuous microstructures of spinodal decomposition in the bulk phase. Moreover, at grain boundaries with higher energy, the decomposed phases are alternating α/β layers that are parallel to the grain boundary. On the contrary, alternating α/β layers are perpendicular to the grain boundary.     

The nonlinear Cahn-Hilliard equation: Transition from spinodal decomposition to nucleation behavior

The behavior of the nonlinear Cahn-Hilliard equation for asymmetric systems,c _t =²(±c+Bc ²+c ³-² c) within the unstable subspinodal region is explored. Energy considerations and amplitude equation methods are employed. Evidence is given for a transition from periodically structuredspinodal behavior to nucleation behavior somewhere within the traditional spinodal. A mechanism for describing a time-dependent lengthening of the dominant wavelength is explored.     

多晶凝固及后续调幅分解过程的晶体相场法模拟

采用晶体相场模型,模拟了二元合金多晶凝固及后续调幅分解全过程.结果表明,晶体相场模型可完整再现包括形核、生长、粗化、晶界形成等多晶生长过程以及圆满完成从凝固到调幅分解的多相变过程. 关键词： 晶体相场模型 多晶凝固 调幅分解 组织演化     

The role of dimensionality in the kinetic Ising model of spinodal decomposition: Evidence from zero-temperature quenches

We study a three-dimensional Ising lattice gas model with spin-exchange dynamics quenched from infinite to zero temperature. We consider a wide range of values of the binary composition (i.e., magnetization) and annealed vacancy concentration. We find that, as in two dimensions, the system freezes in a configuration very far from equilibrium, and that the interface energy per bond in the frozen state, which is very large, in all cases takes very nearly the same values as in two dimensions. We discuss the implications of these results regarding the irrelevance of dimensionality in this problem.     

The Manning factor for direct exchange and ring diffusion mechanisms

Abstract

In this paper, we consider lattice-based diffusion kinetics for the direct exchange and ring mechanisms as possible proxy diffusion mechanisms for diffusion in liquid alloys. For these mechanisms, we assessed the Manning factor that arises from the Darken–Manning relation relating the interdiffusion coefficient and tracer diffusion coefficients and which can be obtained experimentally. The maximum values of the Manning factor for these two mechanisms occur when the exchange only takes place between the atoms of different type but not between the atoms of the same type. These values have strong composition dependence and reach a value of 2 (ignoring tracer correlation factors) for the direct exchange mechanism at equal compositions of the two components in binary alloys. But for the three atom ring mechanism, these values as a function of composition have a much more complicated form that sits below the direct exchange mechanism for compositions between 10 and 90%. When all exchanges (allowed by a mechanism) occur with approximately the same probability, then the Manning factor is about unity for all compositions.     

Phase separation and domain structure of iron-based ordering alloys

The process of ordering with phase separation and domain structure of Fe-Al and Fe-Ga alloys in the regions of mixed phases A2 + B2 and A2 + DO₃ have been investigated by transmission electron microscopy. Observation by dark field micrographs with superlattice reflections revealed a complicated process of phase separation strongly depending on alloy composition and temperature, which was in good agreement with thermodynamics based on Landau's phenomenological theory.     

Shear band formation and fracture behavior of nanocrystalline (Co,Fe)-based alloys

The crystal structure, crystallization, fracture behavior and mechanical properties of (Co_{1 ? x} Fe _x )₈₉Zr₇B₄ (x = 0–0.7) nanocrystalline ribbons were investigated. The crystallization peaks of the amorphous ribbons tend to shift to higher temperatures with increasing Fe content. After annealing at 475°C for 3600 s, the main crystallization product is hcp-(Co,Fe) for the Co-rich composition (x = 0), bcc-(Co,Fe) for high Fe contents (x ≥ 0.3) and a mix of bcc, and fcc for intermediate compositions (0.025 ≤ x ≤ 0.15). The relative strain at fracture decreases dramatically (ε_f < 0.01) for x ≥ 0.15, whereas for lower Fe content it has a maximum (ε_f > 0.037) at x = 0.025 and 0.050 resulting in excellent resistance against fracture. The brittle ribbons (x ≥ 0.15) showed smooth fracture surface with dimples less than 230 nm in diameter, small localized or absent shear bands and large Vickers hardness (>1200 kg mm^?2). On the other hand, the Co-rich ribbons with greater ductility (x = 0.025, 0.05) exhibit a vein pattern filled with voids (features ～2–11 µm), extensive shear banding and lower Vickers hardness (<1050 kg mm^?2).     

A simple model to predict long-range atomic ordering temperatures in Cu-based shape memory alloys

The order–disorder and order–order phase transition temperatures in the austenitic phase of Cu-based shape memory alloys were used to obtain a set of first- and second-neighbour pair interchange energies. To this end, a mean field model was postulated. Then, the applicability to different alloys of this simple model was analysed. It was found that a good agreement with the experimental phase diagram is obtained for Cu–Zn–Al, Cu–Al–Ni and Cu–Al–Be alloys using composition-independent parameters. It was also found that for Cu–Al–Mn alloys, composition-dependent pair interchange energies need to be employed.     

Fe40—45Cr30—35Co20—25Mo0—4Zr0—2合金微观结构对力学性能的影响

研究了高矫顽力型FeCrCo合金力学性能和微观结构间的关系.对抗弯强度和断裂韧性的测试 结果表明，高矫顽力型FeCrCo合金的力学性能远低于普通FeCrCo合金.透射电镜照片显示高 矫顽力合金的两相成分差较大；x射线衍射结果反映高矫顽力合金衍射峰强度较弱而半高宽 较大，说明高矫顽力合金中调幅分解进行彻底.合金中α₁，α₂两 相成分差的扩 大使晶格错配度增加，强度提高，韧性降低.这就是高矫顽力合金力学性能变差的原因. 关键词： 力学性能 调幅分解 抗弯强度 断裂韧性     

Spinodal Decomposition for Multicomponent Cahn–Hilliard Systems

We consider the initial-stage phase separation process in multicomponent Cahn–Hilliard systems through spinodal decomposition. Relying on recent work of Maier-Paape and Wanner, we establish the existence of certain dominating subspaces determining the behavior of most solutions originating near a spatially homogeneous state. It turns out that, depending on the initial concentrations of the alloy components, several distinct phenomena can be observed. For ternary alloys we observe the following two phenomena: If the initial concentrations of the three components are almost equal, the dominating subspace consists of two copies of the finite-dimensional dominating subspace from the binary alloy case. For all other initial concentrations, only one copy of the binary dominating subspace determines the behavior. Thus, in the latter case we observe a strong mutual coupling of the concentrations in the alloy during the initial separation process.     

Mode I and Mode II interlaminar fracture toughness of CFRP/magnesium alloys hybrid laminates

The fiber metal laminates (FML), consisting of carbon fiber reinforced polymer prepregs and magnesium alloys sheets, were introduced, and the Mode I (peel) and Mode II (shear) interlaminar fracture toughness of the FMLs were investigated. The results show that the Mode I interlaminar toughness (0.23 kJ/m²) of the FMLs is much lower than the Mode II interlaminar toughness (5.81 kJ/m²), due to the fact that the effects of mechanical interlock to hinder crack propagates is smaller under Mode I loading conditions than under Mode II. The FMLs mainly show adhesive failure and interfacial failure under Mode I loading conditions, while for Mode II loading, it exhibits a degree of epoxy cohesive failure except the adhesive failure and interfacial failure.     

可膨胀过热水系统多气核演化过程与临界过热度分析

使用分子动力学方法,研究过热水系统均质沸腾核化过程.采用Langevin 动力学方法控制体积可变系统的温度与压力,更好地模拟了沸腾实际物理过程.得到了液相系统体积连续膨胀、分子间距逐渐增大,最终稳定在汽相的现象学规律.当过热温度较高时,亚稳态液相系统可能在局部形成不同大小的近球形区域：气核,这些气核是不稳定的,处于不断演化之中.通过分析分子所受引力与斥力的共同作用,得到了气核形成与消亡以及多个气核融合的机理.比较了模拟结果与经典沸腾理论的差异,提出了气核生长是比气泡生长更为微观过程的认识.通过研究不同过热 关键词： 过热水系统 分子动力学 气核 临界过热度