Abnormal grain growth mechanism and inhibiting strategy by a tailored two-step solution treatment of ZK60 Mg alloy

Mg profiles extruded through porthole die inherently contain longitudinal welds, which can readily induce the abnormal grain growth (AGG) in solution treatment at elevated temperature. Here, a strategy of two-step (primary and secondary) solution is tailored to inhibit AGG. The results indicate that AGG during primary solution can only be suppressed when the temperature drops below 300 °C, while these samples experienced AGG during secondary solution. It is interesting that the width of abnormal grain after secondary solution significantly decreases from 1,473 to 71 µm with increasing the holding time of primary solution from 3 to 84 h. This inhibiting effect results in notable enhancements in mechanical properties, where the elongation, initial fracture points, and maximum bending force are improved by 88.58 %, 32.63 %, and 128.50 %, respectively. The dislocation density and types of the precipitated phases after primary solution are the main factors for inhibiting AGG. First, the dislocation density decreases with increasing the time of primary solution, reducing the stored energy in grain interior. Second, as the primary solution time is extended, MgZn₂ phases greatly become coarsening, accompanied by a transition in their relationship with α-Mg from coherent to semi-coherent. Moreover, the quantities of Mg₄Zn₇ (non-coherent with α-Mg) and Mg₂Zn₃ (semi-coherent with α-Mg) undergo a substantial increase, contributing to a high interfacial energy that effectively inhibits the grain boundary migration during secondary solution.