Crystal nucleation and growth in Pd-Ni alloys: a molecular simulation study

Using molecular simulation methods, we determine the microscopic mechanisms underlying crystal nucleation and growth (up to crystallites of a 2.5 nm radius) in various Pd-Ni alloys under ambient pressure. For all compositions, the critical nucleus is predominantly of the face-centered cubic structure. The concentration in Pd is higher in the nucleus than in the liquid alloy. However, our results show that the structure of the critical nucleus strongly depends on the composition. Increasing the mole fraction in Ni in the alloy leads to a decrease in the face-centered cubic character of the critical nucleus. Simulations of the growth of critical nuclei into large post-critical nuclei show that these trends also govern crystal growth of Pd-Ni. Throughout growth, the concentration in Pd in the nuclei always remains higher than in the liquid alloy. We also observe two different growth mechanisms. When the Pd mole fraction is greater than 0.5, we obtain large face-centered cubic post-critical nuclei with a few stacking faults. On the contrary, when Ni is predominant, the post-critical nuclei rapidly lose their structural identity during crystal growth, leading to highly polycrystalline nuclei.