Ni60Al20V20中熵合金沉淀过程微扩散相场法模拟

Microscopic phase-field simulation for precipitation process of Ni60Al20V20 medium entropy alloy

纳米级L12结构的γ′有序相形态、析出过程和原子排布等对镍基中熵合金强化具有重要作用.本文采用微扩散相场动力学模型探究Ni60Al20V20中熵合金沉淀过程微观机理,以原子占据晶格位置的几率为场变量描述微结构变化,结合反演算法,通过分析γ′相和θ相原子图像演化,序参数变化,体积分数变化等,探讨了γ′(Ni3Al)和θ(DO22)有序相的沉淀机制.研究结果表明:Ni60Al20V20中熵合金无序相有序化动力学过程中,L10相和DO22相同时析出,L10相逐渐转变为L12-γ′相,而传统Ni75Al7.5V17.5合金沉淀序列是先析出L10相,L10转变为L12相后,DO22相在L12相的反相畴界处析出.L10向L12转变时,面心立方晶格a位被Ni原子占据,b位被Al原子和V共同占据.原子等成分有序化形成DO22结构q单相有序畴,随后失稳分解;L10结构非经典形核,逐渐转化为L12-γ′相并失稳分解.Ni-Al第一近邻原子间相互作用势随温度线性升高,随长程序参数增加逐渐增加;Ni60Al20V20中熵合金孕育期随温度升高而时间变长.本文研究适用于Ni-Al-V中熵合金设计.

Medium entropy alloys have attracted much attention because of their excellent physical and chemical properties. Nano-scaled L12 structure ordered phase plays an important role in strengthening the mechanical properties of medium entropy alloys, and its local atomic arrangement plays a decisive role in yield strength of medium entropy alloys. In this paper, the microscopic mechanism of the precipitation process of Ni60Al20V20 medium entropy alloy is studied by using the micro diffusion phase field dynamics model, in which the probability of atoms to occupy the lattice position is taken as a field variable to describe the configuration of atoms and the morphology of precipitates. In this model, the shape and concentration of precipitate phase, the position and appearance of new phase cannot be set in advance. Combined with the inversion algorithm, the precipitation mechanism of ordered phases of γ′(L12-Ni3 Al) and q(DO22-Ni3V) is discussed by analyzing the evolution of atomic images, the change of order parameters and volume fraction. The result shows that two kinds of ordered phases are precipitated in the kinetical process of disordered phase ordering into Ni60Al20V20 medium entropy alloys, which are of γ′ phase with L12 structure and of θ phase with DO22 structure.The two ordered phases constitute a pseudo binary system. The L10 phase precipitates at the same time as DO22, and the L10 phase gradually transforms into the L12-g′ phase, while the traditional Ni75Al7.5V17.5 alloy first precipitates L10 phase, and then the DO22 phase precipitates at the boundary of anti-phase domain of L12 phase. In the transition from L10 to L12, a position of fcc lattice is occupied by Ni atom, and the b position is occupied by Al atom and V atom. The congruent ordering of atoms results in the formation of q single-phase ordered domain of DO22 structure, followed by spinodal decomposition;the non-classical nucleation of L10 structure gradually transforms into L12-g′ phase and spinodal decomposition. The interaction potential between the first-nearest-neighbor atoms of Ni-Al increases linearly with temperature, and increases gradually with the increase of long range order parameters. The incubation period of Ni60Al20V20 medium entropy alloy lengthens with temperature increasing. This study can be applied to the design of Ni-Al-V medium entropy alloy.