Microscopic phase-field simulation for precipitation process of Ni<sub>60</sub>Al<sub>20</sub>V<sub>20</sub> medium entropy alloy
Authors: Yang Yi-Bo , Zhao Yu-Hong , Tian Xiao-Lin , Hou Hua ,   School of Materials Science and Engineering, North University of China, Taiyuan 030051, China
Published: December 2020

Medium entropy alloys have attracted much attention because of their excellent physical and chemical properties. Nano-scaled <i>L</i>1<sub>2</sub> 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 Ni<sub>60</sub>Al<sub>20</sub>V<sub>20</sub> 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 <i>γ'</i> (<i>L</i>1<sub>2</sub>-Ni<sub>3</sub>Al) and <i>θ</i> (<i>DO</i><sub>22</sub>-Ni<sub>3</sub>V) 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 Ni<sub>60</sub>Al<sub>20</sub>V<sub>20</sub> medium entropy alloys, which are of <inline-formula><tex-math id="M1">\begin{document}$ {\gamma }' $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200154_M1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200154_M1.png"/></alternatives></inline-formula> phase with <i>L</i>1<sub>2</sub> structure and of <inline-formula><tex-math id="M2">\begin{document}$ \theta $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200154_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20200154_M2.png"/></alternatives></inline-formula> phase with <i>DO</i><sub>22</sub> structure.The two ordered phases constitute a pseudo binary system. The <i>L</i>1<sub>0</sub> phase precipitates at the same time as <i>DO</i><sub>22</sub>, and the <i>L</i>1<sub>0</sub> phase gradually transforms into the <i>L</i>1<sub>2</sub>-<i>γ′ </i>phase, while the traditional Ni<sub>75</sub>Al<sub>7.5</sub>V<sub>17.5</sub> alloy first precipitates <i>L</i>1<sub>0</sub> phase, and then the <i>DO</i><sub>22</sub> phase precipitates at the boundary of anti-phase domain of <i>L</i>1<sub>2</sub> phase. In the transition from L1<sub>0</sub> to <i>L</i>1<sub>2</sub>, α position of fcc lattice is occupied by Ni atom, and the <i>β</i> position is occupied by Al atom and V atom. The congruent ordering of atoms results in the formation of <i>θ</i> single-phase ordered domain of <i>DO</i><sub>22</sub> structure, followed by spinodal decomposition; the non-classical nucleation of <i>L</i>1<sub>0</sub> structure gradually transforms into <i>L</i>1<sub>2</sub>-<i>γ′</i> 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 Ni<sub>60</sub>Al<sub>20</sub>V<sub>20</sub> medium entropy alloy lengthens with temperature increasing. This study can be applied to the design of Ni-Al-V medium entropy alloy.