含共格畸变的立方合金粗化机制的原子层面计算机模拟研究

基于微观相场动力学模型，编制了包含共格畸变能的二元立方合金沉淀过程微观组织演化的计算机模拟程序，开展了不同共格畸变能作用下，溶质浓度为20at％的二元镍基合金的粗化机制的计算机模拟。研究发现：共格畸变能为零时，粗化遵循LSW机制，仅仅由颗粒的尺寸大小决定：随着共格畸变能的增大，粗化过程遵循混合机制，由沉淀颗粒的大小和位向（颗粒间的相对位置）共同决定；当共格畸变增大到一定程度时，粗化过程纯粹由颗粒间的位向决定，处于弹性“软”方向上的颗粒优先长大，而处于弹性“软”方向外的颗粒将消失掉。     

MICROSCOPIC PHASE-FIELD SIMULATION COUPLED WITH ELASTIC STRAIN ENERGY FOR γ′(Ni3Al) PRICIPITATION IN Ni-Al ALLOY

基于微观相场动力学模型和微观弹性能理论,对Ni-Al合金中沉淀相Ni3Al(γ′)形貌演化、早期沉淀机制和后期粗化过程进行了原子层面的计算机模拟.结果表明:沉淀过程中γ′相形貌由早期随机分布的圆形或不规则状逐渐向方形转变,其排列的取向性也越来越明显,最后形成周边圆滑的长方块状颗粒,沿[10]和[01]弹性"软"方向规则排列.弹性应变能作用下的粗化过程遵循优先选择的原则,位于弹性"软"方向上的颗粒不断长大和粗化,位于弹性"软"方向外的颗粒逐渐消失,沉淀后期在基体中形成高度择优取向的微观组织.低浓度Ni-Al合金中γ′相的早期沉淀机制为非经典形核长大机制,其演化序列为:过饱和固溶体→非化学计量比有序相→化学计量比平衡γ′相→长大.     

Ni75AlxV25-x合金沉淀相粗化行为的微观相场模拟

基于共格应变的微观弹性理论和微观相场动力学模型,模拟了Ni75AlxV25-x合金沉淀过程的组织演化和粗化行为．结果表明：两种沉淀相颗粒在粗化阶段均表现出明显的取向性．θ相（Ni3V）沿[100]方向的生长趋势明显超过[001]方向,γ’相（Ni3Al）沿（001）方向排列．二相析出顺序与Al含量有关,先析出相的长大和粗化两个阶段比较分明,后析出相的长大和粗化过程同时进行．两种有序相发生粗化以相邻颗粒碰撞并融合为较大颗粒的方式进行．不同浓度的合金两相析出先后顺序及体积分数不同,但最终形貌基本一致,两相均呈长方块状分布,并且具有（001）θ‖{100}γ＇的取向关系．     

镍基合金粗化过程中共格沉淀相的形貌演变

基于微观相场模型,研究了镍基合金粗化过程中沉淀颗粒在弹性应变能作用下的微观组织变化。结果表明,镍基合金在粗化过程中,弹性应变能的大小通过改变颗粒中心沿[10]方向和[11]方向的溶质浓度分布,影响共格沉淀相的形貌变化。     

Ni-Cr-Al合金中D022相和L12相沉淀过程微观相场模拟

基于微观相场动力学模型和微观弹性理论,对Ni75Cr19Al6.0合金的沉淀过程进行了研究.结果表明,合金沉淀初期,DD22相以失稳分解机制从无序基体中析出,L12相以非经典形核长大机制析出,两相均呈现不规则形状且随机分布.共格沉淀相和母相之间的点阵错配度引起的弹性场对沉淀过程产生明显影响,D022相和L12相在沉淀后期表现出强烈的各向异性特征,其形貌均转变为长方块状,沿弹性"软"方向([100]和[001])规则分布,沉淀后期在基体中形成高度择优取向的微观组织.粗化过程中,D022相和L12相的平均半径的立方与时效时间整体上并不满足线性关系,弹性约束系统中的长大规律发生变化.     

界面错配应力对铝合金粗化机制影响的微观相场模拟

采用微观相场法研究铝基合金中界面错配应力对沉淀相粗化过程的影响.结果表明:界面错配应力为零时,粗化机制符合经典LSW机制;错配应力较大时,粗化机制是位向控制粗化机制,沉淀相颗粒沿弹性"软"方向规则分布;中等界面错配应力时,粗化机制为兼具位向控制和LSW机制的混合机制;界面错配应力促进沉淀相延弹性软方向生长;沉淀相的粗化过程和长大过程交叠进行.     

共格沉淀析出过程的微观结构演化相场法模拟

采用微观相场模型,对共格沉淀析出过程微观结构的演化进行计算机模拟,并探讨不同作用程度的弹性应变能对沉淀相形貌的影响规律。研究发现:当共格弹性应变能为零时,沉淀相为球状弥散分布的颗粒或"海绵状"结构,呈各向同性特征;随应变能作用的增强,沉淀相分布的取向性越来越明显,趋于沿弹性"软"方向分布,形成类方格阵、纵横层状结构等,呈各向异性特征。模拟结果和相关实验结果一致。     

相场法研究弹性畸变能对Ni-Al合金有序化过程的影响

采用微观弹性相场动力学模型,研究由共格错配产生的弹性畸变能对Ni-Al合金沉淀过程中γ′(Ni3Al)相有序化过程的影响。模拟结果表明,对于不同的沉淀机制,如非经典形核、非经典形核与失稳分解的混合机制及失稳有序化,弹性畸变能均阻碍Al原子在β格点的占位,从而阻碍有序化过程的进行。同一时间步长下,考虑弹性畸变能的合金,其γ′相内浓度和长程序参数曲线的高度和宽度均小于忽略弹性畸变能的合金,曲线分布更加弥散。弹性畸变能改变了γ′相的空间排列及微观形貌,最终形成沿弹性"软方向"择优排列的共格微观组织。     

温度对Ni-Al-Fe合金早期沉淀过程的影响

采用三元微观相场动力学模型，模拟研究了973，1173，1253K时效温度下，Ni75Al15Fe10合金中γ’相的早期沉淀过程。通过对该合金原子显微形貌，长程序参数和浓度及Fe原子在不同格点占位分数随时间演化的模拟计算分析得出：随着温度的升高，Ni75Al15Fe10合金的沉淀机制由等成分有序化兼失稳分解机制向非经典形核长大机制过渡；γ’相沉淀析出的孕育期变长；Fe原子在B格点的占位越来越困难，占位几率降低；合金沉淀析出的γ’相为单一Ll2结构的Ni3（AlFe）复合相。     

相场方法研究Ni-Al-V合金的竞争沉淀过程（英文）

采用微观相场动力学模型,在原子层面上研究Ni75AlxV25-x合金中γ′相和θ相的沉淀过程及微观结构演化。模拟实验结果表明,Al浓度对沉淀序列具有重要的影响。低Al浓度合金中,先析出θ(Ni3V)有序相,随后析出γ′(Ni3Al)有序相;中Al浓度合金中,θ有序相及γ′有序相几乎同时析出;高Al浓度合金中,γ′有序相先析出,θ有序相随后析出。沉淀过程中,θ有序相和γ′有序相长大和粗化过程中存在竞争关系,无论先析出哪种相,θ有序相在粗化的竞争过程中占有优势。因此,沉淀最后形成择优取向的显微组织。     

Simulation of the coarsening behavior of intermetallic precipitates in Ni75AlxV25-x alloys

The coarsening behavior of L12 and D022 in Ni75AlxV25-x （x,at.%） alloys including coherent strain was investigated using the microscopic phase-field model. The simulation results indicate that the shape transition and spatial correlation of L12 and D022 are caused by the morphological-dependent anisotropic elastic interactions in the system. The coarsening process of the particles is by means of neighbor particles impingement and aggregation into larger ones. For the strain-induced interactions between the precipitates,the LSW theory is altered for the coarsening behavior of L12 and D022. In addition,the simulation reveals that the growth and coarsening of D022 present two obvious stages at lower Al concentration regions and proceed simultaneously at high Al concentration regions. The growth and coarsening processes of L12 at the same regions is reverse to those of D022.     

Retarded coarsening phenomenon of γ′ particles in Ni-based alloy

An anomalous coarsening behavior has been observed after aging an alloy Ni-14.5 at .%Al-5.9 at. % Mo at 1453 K. It was found that the initial cubic γ′ (ordered Ll₂ Ni₃Al phase) particles began to split into smaller ones after short aging (t < 3 min). After continued aging, the particles agglomerated into large groups of many particles, and the distribution of the particles became non-uniform. The kinetics of coarsening did not obey the typical linear dependence between the cube of the particle size and aging time. Instead, a retardation of the coarsening process was found. Convergent beam electron diffraction (CBED) was applied to measure the distribution of the lattice strains around particles. The result indicated that there was an inhomogeneous distribution of lattice strain in the matrix around the coherent particles.     

冷轧医用b钛合金显微组织和力学性能演变规律研究

基于微观相场动力学模型,对应变诱发镍基合金粗化过程共格沉淀相的形貌不稳定性及其发展规律进行了研究.结果表明,应变能作用下,粗化过程中沉淀颗粒界面曲率发生变化,通过沿[10]方向尺寸减小与[11]方向尺寸增大,转变为方形.低应变能作用下,沉淀颗粒形貌由圆形转变为周边圆滑,边长向外凸的方形颗粒;中等应变能作用下,沉淀颗粒形貌由圆形变为边长向内凹的方形;高应变能作用下,圆形沉淀颗粒先转变为方形,再从方形颗粒中心“分裂”为2个长条状颗粒,进一步转变为双透镜形状.     

Coarsening dynamics of self-accommodating coherent patterns

The coarsening kinetics of self-accommodating coherent domain structures is investigated using computer simulations based on a continuum phase-field model. The domain structures are produced from coherent hexagonal to orthorhombic phase transformations. It is found that the long-range elastic interactions arising from the lattice accommodation among different orientation domains of the orthorhombic phase dominate the domain morphologies and the kinetics of domain coarsening. It is shown that the long-range elastic interactions result in several new features for the domain coarsening as compared to normal grain growth. For example, the domain growth rate is reduced significantly and the growth exponent becomes a function of the relative contribution of the elastic energy reduction to the total driving force. In general, the elastic interaction is in favor of fine domains. Although triple junctions are dominant in the microstructure, a significant amount of quadrojunctions exist throughout the domain coarsening process. The average number of sides of the domain is also reduced.     

Microscopic phase-field study on aging behavior of Ni_(75)Al_(17)Zn_8 alloy

The precipitating kinetics of Ni75Al17Zn8 alloy was studied at both 873K and 973K by microscopic phase-field model.The calculation results show that the order-disorder transformation experiences the matrix→lowly-ordered L10 phase→L12 phase at 973 K.And the nucleation of L12 particles belongs to the spinodal decomposition mechanism.As temperature increases,orderings of Al and Zn atoms are resisted,but coarsening of L12 particles is promoted.The value of coarsening kinetic exponents approaches to 1/2.In addition,the discussions about Ni-Al anti-site defect and Zn substitutions for Ni site and Al site exhibit that the higher the temperature,the more distinctive the Ni-Al anti-site defect,but the less the Zn substitution.     

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.     

Microscopic phase-field simulation coupled with elastic strain energy for precipitation process of Ni-Cr-Al alloys with low Al content

The precipitation process of Ni-Cr-Al alloy with low Al content was studied at atomic scale based on the microscopicphase-field kinetic model coupled with elastic strain energy.The aim is to investigate the effect of elastic strain energy onprecipitation mechanism and morphological evolution of the alloy.The simulation results show that in the early stage of precipitation,D022 phase and L12 phase present irregular shape,and they randomly distribute in the matrix.With the progress of aging,L12 phaseand D022 phase change into the quadrate shape and their orientations become more obvious.In the later stage,L12 phase and D022phase present quadrate shape with round corner and align along the[100]and[010]directions,and highly preferential selectedmicrostructure is formed.The mechanism of early precipitation of L12 phase in Ni-17%Cr-7.5%Al(mole fraction)alloy is the mixedstyle of non-classical nucleation growth and spinodal decomposition and the D022 phase is the spinodal decomposition.Themechanisms of early precipitation of L12 phase and D022 phase in Ni-12.5%Cr-7.5%Al alloy are both the non-classical nucleationand growth.The coarsening process follows the rule of preferential selected coarsening.     

Microstructure evolution of immiscible alloys during rapid cooling through miscibility gap

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Mechanism of precipitation in a Cu-2.5 pct fe alloy

The precipitation process in a Cu-2.5 pct Fe alloy was studied experimentally by means of X-ray diffraction and hardness measurements and light and electron microscopic observations. It was concluded that γ-Fe and α-Fe precipitate simultaneously, γ-Fe dominating in the early stages. During later stages, the particles of γ-Fe dissolve, providing the copper lattice with iron atoms which diffuse to the more stable α-Fe particles. The α-Fe particles continue to grow until all evidence of the γ-Fe is gone.