Microstructure evolution and mechanical properties of Mg-Nd-Y alloy in different friction stir processing conditions

Microstructure evolution and its effect on mechanical properties of Mg-Nd-Y alloy during normal friction stir processing (NFSP) and submerged friction stir processing (SFSP) were investigated. It was found that microstructure of the as-cast Mg-Nd-Y alloy was mainly composed of coarse α-Mg dendrites and net-shaped Mg12Nd phases. After NFSP and SFSP, α-Mg dendrites were greatly refined with the average grain size of 2.7 μm and 1.9 μm, respectively. The finer grain size obtained by SFSP was attributed to its rapid cooling rate. Moreover, the Mg12Nd networks were changed into small particles. The microstructure of SFSP specimen was more homogeneous than that of the NSFP specimen, in which coarse grain band could be found. Tensile tests indicated that the as-cast alloy exhibited the lowest ultimate tensile strength (UTS) of 178 ± 3.5 MPa, the lowest yield strength (YS) of 172 ± 3 MPa and the lowest elongation of 3.4 ± 0.5% because of the coarse grains and large eutectic phases. Due to microstructure refinement, the UTS, YS and elongation of NFSP and SFSP specimens were increased to 291 ± 4.6 MPa, 270 ± 5 MPa, 7.5 ± 0.5% and 297 ± 6.9 MPa, 281 ± 3.9 MPa, 20.3 ± 1.8%, respectively.