Atomistic mechanism underlying nucleation in Al-Cu alloys with different compositions and cooling rates

Nucleation, significantly influenced by local structures in metallic melts, is crucial for controlling the microstructure of solidified materials. However, the connection between local atomic structures in melts and crystal nucleation is still an open issue. In this work, a systematic investigation on local structures and crystal nucleation in Al-Cu alloys with different Cu contents have been carried out utilizing molecular dynamics simulations. The results show that the local atomic structures and nucleation pathways of Al-Cu melts strongly depend on the composition and the cooling rate. With the increase of Cu content as well as the cooling rate, the population of icosahedral configurations around Al and Cu simultaneously grows, leading to a poorer diffusivity and stronger dynamic heterogeneity of Al and Cu atoms. Based on isoconfigurational ensemble simulations, the casual link between icosahedral short-range order and DH was established. Moreover, we revealed that the kinetic pathways of nucleation are distinct with the promotion of Cu content and cooling rate, indicated by the absence or appearance of intermediate BCC phase during the nucleation stage. We attribute this feature to the variation of the icosahedral structures in Al-Cu melts, namely, the increase of icosahedral short-range order leads to the transition of nucleation pathway and the formation of intermediate BCC phase. Our findings not only shed light on the homogeneous nucleation mechanism of binary alloys, but also provide a guidance for controlling polymorph selection.