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.     

四近邻对势对Ni75Al5V20合金沉淀机制影响的 微观相场模拟

利用微观相场动力学模型,研究了4近邻对势对Ni75Al5V20合金沉淀机制的影响.结果表明,不改变4近邻对势,DO22相和Ll2相沉淀机制均为失稳分解与非经典形核混合型机制.随Ni-Al4近邻对势V4Ni-Al向负值方向绝对值增大,Ll2相沉淀机制转变为失稳分解.随Ni-V4近邻对势V4Ni-V增大,DO22相沉淀机制转变为失稳分解,Ll2相转变为非经典形核机制,Ll2相中的Al浓度平衡值增大.随Al-V 4近邻对势V4Al-V向负值方向绝对值增大,对沉淀机制的影响趋势与V4Al-V增大类似;随V4Al-V向正值方向增大,与V4Al-V减小的影响类似.     

Ni75Al5.5V19.5合金沉淀行为的微观相场模拟

利用微观相场方法模拟了Ni75Al5.5V19.5合金两种有序相D022和L12随温度变化时的不同沉淀过程.发现过冷度在相变驱动力中处于主导地位.高温下D022相首先析出,之后L1 2相在D022相和无序相的界面以及D022相形成的界面处析出,L1 2相长大时同时向无序基体和D022相推移;低温下L1 2相先析出,D022相在L1 2相的界面处形核长大,同时L1 2相通过失稳分解由非化学计量相向平衡相转变,其尺寸减小.该合金最终两相体积分数同时达到平衡并基本相等.随温度降低,L1 2相由高温时的非经典形核与失稳分解的混合机制向低温时的失稳分解转变;D022相的沉淀机制由非经典形核与失稳分解的混合机制向低温时的非经典形核转变.     

Ni-Cr-Al合金有序化早期原子尺度的模拟

基于三元体系的离散格点的微观相场模型，模拟研究温度1123K时三元合金Ni—Cr-Al有序化早期过程，通过研究沉淀过程原子图像和序参数曲线的演化，发现低饱和度合金Ni-15at．％Cr-10．5at．％Al沉淀机制为非经典形核长大，沉淀过程包括D022有序相的形核长大，Ll2有序相的形核和D022有序相向Ll2有序相的转化3个阶段。由于Cr原子替代Al原子位置，在Ll2有序相内两种原子占位几率接近，形成Ni3（CrxAl1-x）结构的Ll2有序相，并且与D022有序相共存。高饱和度合金沉淀机制为等成分有序化和失稳分解混合机制，并且有序化和簇聚过程比低饱和度合金提前。     

低Al含量Ni-Cr-Al合金有序化及相边界演化的数值模拟

基于三元微观相场模型,模拟研究了873K时低Al含量三元合金Ni-Cr-Al有序化及相边界演化过程。研究发现,合金Ni-11Cr-7Al(原了百分数,下同)沉淀机制为非经典形核长大,沉淀过程中L12有序相伴随着DO22有序相的形核长大,同时,部分DO22有序相向L12有序相转化。由于Cr原子替代Al原子位置,在L12有序相内两种原子占位几率接近,形成Ni3(CrxAl1-x)结构的L12有序相,并且与DO22有序相共存。     

温度对Ni-Cr-Al合金中Cr原子替代规律的影响

利用微观相场动力学模型,模拟研究温度对Ni-12at%Cr-14.5at%Al合金沉淀过程中Cr原子的替代规律。在873～1173K范围内,随着温度的提高,合金沉淀机制由等成分有序化+失稳分解转化为非经典形核机制,沉淀孕育期逐渐延长;L12相体积分数增加,而D022相体积分数减少,在1173K时,D022相消失,最终形成单一的L12相。     

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.     

微观相场法研究沉淀早期两相竞争对筏化的影响

基于微观相场动力学模型,研究沉淀早期不同成分Ni-Al-V合金时效时有序相的生长和组织生长方向的变化。结果表明:沉淀早期有序相间的竞争生长会很大程度地影响合金的形貌及生长方向。在一定的温度、应力场作用下,Al含量较低时,DO22有序相先析出,DO22相颗粒数量较多、尺寸较小,有序相的筏化不明显,合金整体上无显著的筏化现象;Al含量稍高时,L12相先析出,DO22相颗粒数量较少、尺寸较大,组织在[010]方向上发生一定程度的择优生长;Al含量较高时,L12相先析出,DO22相颗粒数量继续减少、尺寸继续增大,组织在[100]方向上显示出较强的筏化趋势;较低温度时效的合金L12有序相析出提前,组织的筏化趋势较弱。     

Transition from Metastablity to Instability in Dynamics for the Precipitation of d￠(Al3Li)

A computer simulation study on dynamics for the precipitation of d￠(Al3Li) ordered particles from a disordered matrix (a) in binary Al-Li alloys is performed using the microscopic Langevin equation. A unique precipitation mechanism is found near the ordering spinodal line. Different from the classical nucleation mechanism in the me-tastable region and the congruent ordering followed by spinodal decomposition in the instable region, a nonstoichi-ometric single ordered phase with composition fluctuations is formed by non-classical nucleation, and this ordered phase decomposes spinodally. It can be concluded that the precipitation dynamics of d￠ phase from metastability to instability is gradual, and no sharp transition occurs near the mean-field spinodal line as the mean-field theory pre-dicts.     

Ni75Al7.5V17.5相变过程中L12相合金元素占位几率演化的微观相场模拟（英文）

基于微观相场模型,研究Ni75Al7.5V17.5合金在相变过程中L12相内合金元素演化与DO22相生长之间的关系。研究表明,在L12相向DO22相转变的过程中,DO22相的长大可以分为两阶段。在早期,L12相内合金元素的成分基本不变,DO22相的长大主要受L12相间有序畴界的减少所控制。在后期,DO22相长大所需的V一部分来自L12相内部,一部分来自L12相体积的减少,其中,相内部为DO22相长大所提供的V则主要来自Al格点位置,由于L12相体积减少而富余的Al向L12相内部扩散迁移并主要占据Al格点位置,富余的Ni则同时向L12相内部和DO22相内部扩散,主要占据L12相的Ni位置。DO22相长大的后期主要受L12相内反位缺陷和第三组元的演化所控制。     

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.     

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。     

包含共格畸变的立方合金沉淀机制的计算机模拟

利用包含共格畸变能的微观扩散方程对立方合金的沉淀过程进行计算机模拟。结果表明：共格畸变阻碍了过渡区合金沉淀过程的进行，随共格畸变能的增大，沉淀相分布的取向性越来越明显，由原来离散分布的等轴粒子状向椭片状转变。靠近高浓度合金的过渡区合金，随共格畸变能的增大，其沉淀机制由非经典形核长大和大稳分解的混合特征逐渐向非经典形核长大型的沉淀特征转变。     

相场法预测低铝合金Ni75AlxV25-x的早期沉淀过程

采用微观相场模型,通过分析原子图像、有序相内序参数分布及有序相体积分数随时间的变化,在原子尺度对低Al浓度Ni75AlxV25-x合金的早期沉淀动力学进行了计算机预测．研究发现该合金沉淀过程中,析出D022结构的θ(Ni3V)相和L12结构的γ‘(Ni3Al)相,二者构成伪二元系;θ相先于γ‘析出,θ相的沉淀机制为等成分有序化 失稳分解,γ‘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;随Al浓度增加,θ相析出变慢,所占比例减少,而γ’相反之,析出越来越早且所占比例增加。     

Al-Zn对称成分Spinodal分解合金的组织、结构与相成分

利用组织分析方法和X射线衍射分析, 研究了Al-Zn对称成分Spinodal分解合金的组织、结构与成分. 结果表明, Al-Zn对称成分合金经固溶处理后可获得单相组织, 但该组织在室温下将发生Spinodal分解和Zn的析出. 在钢模铸造的快速冷却条件下, 该合金的相组成已经接近于平衡态, 但存在较多的缺陷和较大的成分不均匀性. 利用实测晶格常数的方法可以近似确定Spinodal分解和不连续析出等高分散度两相组织中富Al固溶体的相成分.     

微观相场法研究镍基合金相变时的成分演化及界面定向迁移机制

基于耦合外应力场下的微观相场模型,研究了Ni_(75)Al_(7.5)V_(17.5)三元合金相变过程中相分解、生长过程中合金成分演化及应力作用下相间界面的定向迁移规律。研究表明:早期DO_(22)相长大所需的V来自于在L1_2相间有序畴界处,而后期则来自于其内部,且早期DO_(22)相长大的速度高于后期的长大速度;时效过程有5种异相间界面形成,而A类界面的迁移为相迁移的主要类型;相变初期A、B类界面较少,合金形貌变化不大,随着相变的进行,A类界面数量增加,相的生长及分解过程加剧,导致后期A类界面减少、B类界面增多,合金体系达到平衡状态;在压应力作用下,A类界面沿应力方向迁移,促进了DO_(22)相在此方向上的生长,导致合金"筏状"组织的形成。     

时效温度对Ni-Al-V合金筏化影响的微观相场模拟

基于微观相场动力学模型,以Ni<,75>Al<,25-x>V<,x>合金为对象,研究时效温度变化对沉淀相竞争生长机制及筏状组织形成的影响.结果表明:相的生长和粗化存在各向异性和竞争机制;当具有生长各相异性的DO<,22>相数量较少、尺寸较大时,易形成单一方向排布的组织,从而促使具有生长各向同性的L12相也沿着特定的方向生长,最终形成具有高度择向生长的"筏状"组织,且随着时效温度的降低,沉淀相提前析出;若L12相先析出,时效温度越低,DO<,22>相越多,组织筏化越不明显;若DO<,22>先析出,时效温度越低,DO<,22>相越少,组织筏状越显著.     

Nucleation of Ni–Fe alloy near the spinodal

In molecular dynamics simulations, “non-classical” nucleation around the spinodal in Fe–Ni alloys is observed by controlling the composition. With increasing Fe concentration, metastable body-centered cubic (bcc) clusters are formed during pre-crystallization under the influence of the spinodal, and then grow into face-centered cubic (fcc)-ordered critical nuclei. When the composition reaches 75 at.% Fe, this transformation is suppressed and the bcc, rather than fcc, symmetry dominates the structure of critical nuclei, a typical nucleation behavior near the spinodal. Further increase in the Fe concentration depresses nucleation below the spinodal temperature. As a consequence, two transient bcc phases characterized by high and low densities appear upon reaching the critical size.     

Temporal evolution of non-equilibrium γ' precipitates in a rapidly quenched nickel base superalloy

The temporal evolution of non-equilibrium γ' precipitates in a rapidly quenched and isothermally annealed commercial nickel base superalloy has been investigated by coupling transmission electron microscopy and atom probe tomography. When subjected to rapid quenching from above the γ' solvus temperature, the supersaturated single phase γ matrix appears to undergo compositional phase separation possibly via spinodal decomposition to form solute-rich and solute-depleted regions. The regions that have a depletion in Cr and Co undergo an ordering process resulting in the γ' domains which exhibit a far–from equilibrium composition. Upon isothermal annealing, the γ/γ' interface sharpens and the compositions of both γ and γ' phases approach equilibrium. The influence of a non-classical mechanism of γ' precipitation on the size distribution of precipitates as well as the precipitate and matrix compositions, and its subsequent evolution during isothermal annealing has been discussed.     

Non-classical homogeneous precipitation mediated by compositional fluctuations in titanium alloys

This paper presents experimental evidence of homogeneous precipitation of the α-phase within the β matrix of a titanium alloy, and then accounts for this phase transformation by a new, non-classical mechanism involving compositional fluctuations, based on the pseudo-spinodal concept [1]. This mechanism involves local compositional fluctuations of small amplitude which, when of a certain magnitude, can favor thermodynamically certain regions of the β matrix to transform congruently to the α-phase but with compositions far from equilibrium. Subsequently, as measured experimentally using the tomographical atom probe, continuous diffusional partitioning between the parent β- and product α-phases during isothermal annealing drives their compositions towards equilibrium. For a given alloy composition, the decomposition mechanism is strongly temperature dependent, which would be expected for homogeneous precipitation via the compositional fluctuation-mediated mechanism but not necessarily for one based on classical nucleation theory. The applicability of this mechanism to phase transformations in general is noted.