Formation mechanism of cluster-arranged layers in Mg-Y-Zn alloy: A density functional theory study

A sparsely introduced basal intrinsic 2-type stacking fault (I₂-SF) with a dense segregation of clusters (cluster-arranged layer; CAL) in α-Mg exerts a sufficient strengthening effect with a reduced content of additive elements. Moreover, the dynamic nucleation and growth of CALs during deformation largely improves the creep resistance. This paper analyzes the cosegregation behaviors of yttrium (Y) and zinc (Zn) atoms at an I₂-SF in bulk and at basal edge dislocations using density functional theory calculations. We also study the modification of the generalized stacking-fault energy (GSFE) curves associated with the cosegregation. The segregation energies of Y and Zn atoms in the I₂-SF are relatively small during the initial segregation of a cluster, but increases stepwise as the cluster grows. After introducing Y and Zn atoms in the I₂-SF in an energetically stable order, we obtain an L1₂-type cluster resembling that reported in the literature. Small structural changes driven by vacancy diffusion produce an exact L1₂-type cluster. Meanwhile, the core of the Shockley partial dislocation generates sufficient segregation energy for cluster nucleation. Migration of the Shockley partial dislocation and expansion of the I₂-SF part are observed at a specific cluster size. The migration is triggered by a large modification of the GSFE curve and destabilization of the hexagonal close-packed stacking (hcp) by the segregated atoms. At this point, the cluster has reached sufficient size and continues to follow the growth in the I₂-SF part. According to our findings, the CAL at elevated temperature is formed through repeated synchronized behavior of cluster nucleation at the Shockley partial dislocation, dislocation migration triggered by the destabilized hcp stacking, and following of cluster growth in the I₂-SF part of the dislocation.