Microstructural evolution and hot tensile behavior of Mg-3Zn-0.5Zr alloy subjected to multi-pass friction stir processing

The microstructures and hot tensile behaviors of ZK30 alloys subjected to single- and multi-pass friction stir processing (FSP) were systematically investigated. Following single-pass FSP (S-FSP), coarse grains underwent refinement to 1-2 μm, with a distinct basal texture emerging in the stir zone (SZ). Additionally, second-phase particles were fragmented, dispersed, and partially dissolved. Multi-pass FSP (M-FSP) further enhanced the homogeneity of the microstructure, reduced texture intensity differences, and decreased the fraction of second-phase particles by 50%. Both S-FSP and M-FSP SZs demonstrated superplasticity at strain rates below 1×10 -3 s -1 and at temperatures of 250-350 °C. The S-FSP SZ exhibited an elongation of 390% at 250 °C and 1×10 -4 s -1 , while the M-FSP SZ achieved an elongation of 406% at 350 °C and 1×10 -3 s -1 . The superplastic deformation of SZ was co-dominated by grain boundary sliding (GBS) and the solute-drag mechanism in S-FSP and mainly by GBS in M-FSP.