Effects of Solution Heat Treatment on Microstructure and Mechanical Properties of the Mg-4.5Zn-4.5Sn-2Al-0.6Sr Alloy

Abstract

Microstructure and mechanical properties of the Mg-4.5Zn-4.5Sn-2Al-0.6Sr alloy are investigated both in the as-cast condition and after the different three-step solution heat treatments (a solution heat treatment of 310 °C × 4 h + 340 °C × 28 h followed by a high-temperature solution treatment) to explore the optimal solution treatment cycle. The as-cast alloy contains a microstructure consisting of the α-Mg matrix, Mg2Sn, Mg51Zn20, Mg32(Al, Zn)49, and MgSnSr phases. After the solution heat treatment, all the Mg51Zn20, the Mg32(Al, Zn)49 phases, and most of the Mg2Sn phase are dissolved into the matrix, only the MgSnSr phase and a minority of the Mg2Sn phase are remained in the granular form or the fine dot-like. The volume fraction of the residual second phases decreases from 5.61 to 1.84% with the increasing solution time from 0 to 4 h at 420 °C and it decreases from 2.9 to 0.4% with the increasing solution temperature from 420 to 480 °C for 2 h. The alloy that experiences the solution treatment of 310 °C × 4 h + 340 °C × 28 h + 460 °C × 2 h exhibits the highest strength and the best plasticity among all the solution-treated alloys. Therefore, the optimal solution treatment is 310 °C × 4 h + 340 °C × 28 h + 460 °C × 2 h. The residual second phases in the alloy that experiences the optimal solution treatment are confirmed to be the Mg2Sn phase and the MgSnSr phase which are related to their relatively high thermal stability. The ultimate tensile strength and the elongation to rupture of the as-solutionized alloy are 238 MPa and 12%, respectively, about 25 MPa and 2.4% higher than the counterparts of the as-cast alloy.