A1-Li合金回归机制的计算机模拟

基于离散格点形式的微扩散方程(Langevin方程)，对A1-Li合金在回归过程中δ-A13Li相溶解进行了原子层面计算机模拟，分析了析出相在回归过程中原子图像和序参数的演化，进而探讨了A1-Li合金的回归机制．首次探明A1-Li合金的回归过程有序相演化序列为化学计量比δ相→非化学计量比有序相→无序基体，并论证了亚稳区合金的回归过程更趋向于失稳区合金时效过程的逆过程；失稳区合金的回归过程更趋向于亚稳区合金时效过程的逆过程。     

Al-Zr中间合金热处理过程的微观相场模拟

利用微观相场方法对3种不同成分的Al-Zr合金的时效过程进行计算机模拟.结果显示:Al-3at%Zr、Al-6at%Zr和Al-15at%Zr3种合金时效过程中形成的Ll<,2>结构的Al<,3>Zr有序相的沉淀机制分别为失稳分解机制、失稳分解加非经典形核混合机制、非经典形核机制:有序相形成经历的过程为过饱和固溶体→G.P.区→非化学计量比有序相一化学计量比有序相,开始阶段形成的G.P.区中,一些尺寸和浓度较小的区域随着演化的进行逐渐溶解消失,一些尺寸和浓度较大的则进一步长大有序化并最终形成化学计量比有序相.     

Al-Li合金回归再时效的微观相场模拟

基于离散格点形式的微观相场动力学扩散方程,以Al-11.7Li(原子分数,%)合金为对象,模拟了Al-Li合金时效,经历了3种不同程度的回归再时效过程.这3种回归为:完全回归(δ'相全部溶解)、不完全回归(δ'相未全部溶解,少量存在)以及部分回归(δ'相部分溶解).结果表明:对于该浓度合金,完全回归再时效获得的组织形貌与直接时效的相当;而不完全回归再时效得到的颗粒密度大于前两者,颗粒尺寸更加细化,分布更均匀,弥散度也高,不完全回归再时效能够有效地改善Al-Li合金的微观组织;部分回归再时效获得的δ'相粗大且数量少.     

Al-Li合金回归过程的数值模拟

基于离散格点形式的Langevin方程,以Al-12%Li合金为对象,模拟合金完全回归的过程。结果表明,Al-Li合金完全回归过程经过Li的固溶,δ’相的回归,δ’相的无序化;Al-Li合金完全回归,其温度需要达到一定条件才可以发生,否则即使延长回归时间,有序相也只发生部分回归;温度越高,Al-Li合金完全回归时间越短,Li含量越低,发生完全回归的温度越低。     

δ''相在外场下早期沉淀机制的计算机模拟

基于离散格点形式的微扩散方程(Langevin方程)和非平衡自由能函数,编制了引入原子间相互作用能变化的Al3Li(δ')相沉淀原子层面计算机模拟程序.该程序包容亚稳区到失稳区的全部温度、成分范围和孕育期至粗化的全过程,可以处理与时间相关的过程问题.开展了不同原子间相互作用势下原子图像、序参数的计算机模拟,进而探讨了最近邻原子相互作用能(W1)对有序相沉淀的影响机制.发现过渡区合金形核前出现短暂的等成分有序化阶段.探明随W1的增大,有序相沉淀的孕育期缩短,形核率增加,合金有序化速度和原子簇聚速度加快,在所研究的时间范围内达到的长程序参数和成分偏离序参数最大值增大.随W1的增大,有序相的析出呈现出失稳分解的特征.     

外场下失稳区合金沉淀机制的计算机研究

基于离散格点形式的微扩散方程(Langevin方程)和非平衡自由能函数，编制了引入原子间相互作用能变化的Al3Li(δ′)相沉淀原子层面计算机模拟程序。该程序包容亚稳区到失稳区的全部温度、成分范围，孕育期至粗化的全过程，可以处理与时间相关的过程问题。开展了不同原子间相互作用势下原子图像、序参数的计算机模拟，进而探讨了最近邻原子相互作用能(W1)对有序相沉淀的影响机制。探明过渡区合金先发生等成分有序化，后进行失稳分解；随W1的增大，有序相沉淀的孕育期缩短，单相有序畴数量增多，面积减小，合金达到最大有序化的时间早，所能达到的最大有序化程度大。W1值较大，会促进合金有序化和原子簇聚，阻碍无序相的形成。     

低铝含量Ni75AlxV25-x合金早期沉淀过程的原子尺度计算机模拟

采用微观相场模型, 基于离散格点形式的微观扩散方程和非平衡自由能函数, 编制了三元Ni75AlxV25-x合金的原子层面计算机模拟程序.模拟发现: 低铝含量Ni75AlxV25-x合金的θ相先于γ′相析出, 其沉淀机制为等成分有序化 失稳分解; γ′相在θ相的相界处非经典形核, 二者均先形成非化学计量比有序相, 之后向化学计量比有序相转变.     

δ′相在外场下早期沉淀机制的计算机模拟

基于离散格点形式的微扩散方程(Langevin方程)和非平衡自由能函数,编制了引入原子间相互作用能变化的Al3Li(δ′)相沉淀原子层面计算机模拟程序。该程序包容亚稳区到失稳区的全部温度、成分范围和孕育期至粗化的全过程,可以处理与时间相关的过程问题。开展了不同原子间相互作用势下原子图像、序参数的计算机模拟,进而探讨了最近邻原子相互作用能(W1)对有序相沉淀的影响机制。发现过渡区合金形核前出现短暂的等成分有序化阶段。探明随W1的增大,有序相沉淀的孕育期缩短,形核率增加,合金有序化速度和原子簇聚速度加快,在所研究的时间范围内达到的长程序参数和成分偏离序参数最大值增大。随W1的增大,有序相的析出呈现出失稳分解的特征。     

Al-Li合金分级时效的微观相场模拟

基于离散格点形式的微观相场动力学扩散方程，以Al-10at％Li合金为对象，对3种不同的时效工艺进行计算模拟。结果表明，对于该合金，采用137℃预时效＋190℃终时效工艺比另2种时效工艺获得的组织更均匀，颗粒更细化、弥散度更高。Al-10at％Li合金的单级时效和分级时效的沉淀相δ’（Al3Li）由失稳分解＋形核长大的混合方式形成。分级时效可以有效地提高有序相的体积分数和颗粒密度。     

三元体系γ''相和θ相沉淀早期计算机研究

基于三元合金沉淀动力学相场方程的微观离散格点形式建立模型,首次对Ni75Al11V14合金的γ′和θ2个有序相析出的早期过程进行了模拟,并计算了γ′有序相的成分序参数和长程序参数.结果表明:Ni75Al11V14合金中γ′相的沉淀机制为非经典形核长大兼有失稳分解特征,先形成非化学计量比有序相,然后才形成化学计量比有序相;非化学计量比θ有序相在γ′相界处形成,距离γ′相界越远,其有序程度越低,θ有序畴存在2种类型:水平分布和垂直分布,和它所依附的γ′相界面取向有关.     

Evolution of precipitates of Al-Cu alloy during equal-channel angular pressing at room temperature

The evolution of precipitates and hardness changes in Al-Cu alloys during equal-channel angular pressing（ECAP） at room temperature were investigated by hardness measurement, X-ray diffraction analysis and transmission electron microscopy. The results show that with the increase of the total equivalent strain during ECAP from 0 to 8.4, the hardness of specimens with metastable θ″ phase increases first and decreases in later period. The hardness increases successively in specimens containing metastable θ′ phase and equilibrium θ phase. It is believed that the evolutions of hardness are related to the mechanism of re-dissolution of precipitates. A critical nuclei size concept is provided to express the mechanism of such re-dissolution of three precipitates in Al-Cu alloys.     

Magnetic-induced tricritical point in alloys and the low-temperature Fe−Ni and Fe−Ni−Cr phase diagrams

Phase equilibria at temperatures above and below the tricritical point of binary alloys are discussed in terms of thermodynamic principles. The existence of a second spinodal curve for the disordered phase within the magnetic-induced miscibility gap is rationalized. The relative interplay of the chemical and magnetic contributions to the stabilities of various types of phase equilibria in binary alloys is also discussed. The effect of magnetic contribution to the Fe?Ni and Fe-Ni-Cr phase stabilities at low temperatures is reviewed. The calculated phase diagrams of the Fe?Ni and Fe-Ni-Cr systems considering the magnetic and nonmagnetic terms are compared with experimental results. These results were obtained either from meteorites or from synthesized alloys subjected to irradiation. The meteorites, which have cooled slowly in space, may have achieved metastable/stable equilibrium conditions. The many defects introduced by irradiation for the synthesized alloys may have helped to achieve metastable/stable equilibrium conditions.     

The use of diffusion multiples to examine the compositional dependence of phase stability and hardness of the Co-Cr-Fe-Mn-Ni high entropy alloy system

High entropy alloys (HEAs) or Multi-principal element alloys (MEAs) are a relatively new class of alloys. These alloys are defined as having at least five major alloying elements in atomic percent from 5% to 35%. There are hundreds of thousands of equiatomic compositions possible and only a fraction have been explored. This project examines diffusion multiples as a method to accelerate alloy development in these systems. Co-Cr-Fe-Mn-Ni quinary diffusion multiples were successfully created. Using these multiples, a quinary region of disordered of FCC was formed and examined using EDS and nanoindentation methods. From these techniques, maps of common HEA parameters (Ω, δ, ΔS_mix, ΔH_mix and Δχ) proposed in literature could be calculated and directly compared to observed phase stability. Similarly, hardness was examined as function of compositional complexity and atomic mismatch in the quinary disordered region in order to directly test the severe lattice distortion hypothesis. It was found that proposed HEA parameters were ineffective at single phase stability limits in the Co-Cr-Fe-Mn-Ni system. It was also observed that hardness did not correlate well to the maximum compositional complexity or to the maximum in atomic mismatch. This indicates the severe lattice distortion hypothesis is not the primary contributor to strengthening in the Co-Cr-Fe-Mn-Ni HEA system.     

Effects of Cd and Sn on double-peak age-hardening behaviors of Al-Si-Cu-Mg cast alloys

The effects of trace elements Cd and Sn on precipitation process of Al-Si-Cu-Mg cast alloys were investigated in the present research.It is shown that the addition of Cd and Sn not only increases remarkably the aging peak hardness and reduces the time to reach aging peak,but also eliminates the double-aging-peak phenomenon which appears in Al-Si-Cu-Mg alloys.In Al-Si-Cu-Mg alloys the first aging peak corresponds to GP zones(especially GPⅡ) ,and the second one is caused by metastable phases.The obvious time interval of transition from GPⅡ to metastable phases associates with the double-aging-peak phenomenon.The results of DSC and TEM show that Cd/Sn elements suppress the formation of GPⅠzone,stimulate the formation of θ",θ' and θ phases,and then shorten remarkably the temperature intervals of each exothermic peak.Because the transition interval between GPⅡzone and metastable phases is shortened by Cd/Sn in Al-Si-Cu-Mg cast alloys,θ' phase coexists with θ" phase in matrix of ageing peak condition,which causes effective hardening on the alloys,and at the same time,eliminates the double-aging-peak phenomenon.     

Continuum Separation of Nanoscale Phase in Thermal Aging Fe-Cr Alloys: Phase-Field Simulation and Experiment

JOM - The separation of Cr-enriched α′ phase in Fe-(25–45)&nbsp;at.% Cr alloys is studied from the metastable region to the unstable region. The three-dimensional phase-field...     

Self-organized layered growth phenomena of diffusion couples with spinodal decomposition in binary alloys

In order to investigate the formation mechanisms of the layered growth phenomena in diffusion couples with spinodal decomposition, a phase field model combined with elastic strain field was employed. Microstructure evolutions of diffusion couple with spinodal decomposition in binary alloys were numerically simulated by considering concentration fluctuation and elastic anisotropy. The simulation results indicate that the number of the periodical layers decreases with the increase of initial concentration fluctuation, even with large elastic anisotropy. The growth of layered microstructures can be attributed to the directional diffusion enhanced by initially discontinuous chemical potential at the interface.     

共格沉淀析出过程的模拟Ⅰ--微观结构演化

采用相场模型对共格沉淀析出过程微观结构演化进行模拟研究.模拟结果表明,应力场的存在将会对相变过程中析出相形态产生显著的影响,通过界面能与弹性应变能的相互竞争析出相在不同阶段呈现不同的形态如模量结构、网格结构、三明治多畴结构、沉淀宏观点阵结构以及板条状等;此外,点阵错配度中等(2%～4%)时,粒子的粗化过程将出现应力诱导反向粗化现象,这种粗化现象取决于粒子的点阵错配度与体积分数.     

Microstructure and Mechanical Properties of Mg–RE–TM Cast Alloys Containing Long Period Stacking Ordered Phases: A Review

Casting magnesium alloys hold the greatest share of magnesium application products due to their short processing period, low cost and near net shape forming. Compared with conventional commercial magnesium alloys or other Mg–RE-based alloys, the novel Mg–RE–TM cast alloys with long period stacking ordered(LPSO) phases usually possess a higher strength and are promising candidates for aluminum alloy applications. Up to now, two ways: alloying design and casting process control(including subsequent heat treatments), have been predominantly employed to further improve the mechanical properties of these alloys. Alloying with other elements or ceramic particles could alter the solidifi cation pattern of alloys, change the morphology of LPSO phases and refi ne the microstructures. Diff erent casting techniques(conventional casting, rapidly solidifi cation, directional solidifi cation, etc.) introduce various microstructure characteristics, such as dendritic structure, nanocrystalline, metastable phase, anisotropy. Further heat treatments could activate the transformation of various LPSO structures and precipitation of diverse precipitates. All these evolutions exert great impacts on the mechanical properties of the LPSO-containing alloys. However, the underlying mechanisms still remain a subject of debate. Therefore, this review mainly provides the state of the art of the casting magnesium alloys research and the accompanying challenges and summarizes some topics that merit future investigation for developing high-performance Mg–RE–TM cast alloys.     

FeMnCoCr系亚稳高熵合金的研究进展

高熵合金是一种原子排列有序,化学无序的新型多主元合金.通过改变合金元素的种类和浓度,能够调控合金系统层错能及显微组织的相稳定性,进而诱发形变孪晶、马氏体相变等塑性变形机制,最终使合金获得突出的综合力学性能.这种高熵合金的设计理念称为"亚稳工程".亚稳高熵合金的显微组织、相结构及变形机制与合金体系的层错能密切相关.在Fe...     

Ni75Al15Mn10合金γ′相沉淀过程以及原子占位的微观相场模拟

基于微观相场模型和微观弹性理论,对Ni75Al15Mn10合金γ′相沉淀过程以及原子占位进行了原子层面的计算机模拟。结果表明:合金在1273K进行时效,沉淀早期先析出L10结构,之后随着有序度的增加,逐渐转变为L12结构;原子的有序化早于成分簇聚,γ′相的沉淀机制为等成分有序化+失稳分解的混合机制;γ′有序相的体积分数比γ无序相小,且γ′和γ相的体积分数比值约为60%;Al原子主要占据β格点(γ′相顶角位置),αⅡ和αⅠ格点主要由Mn原子占据,且在αⅡ格点占位几率高于αⅠ格点,Mn原子主要占据Ni位,形成的γ′相为单一的(Ni,Mn)3Al相。