Effect of stacking faults in nanograins on the tensile properties of Mg-Y-Nd-Gd-Zr alloys subjected to ultrasonic surface rolling processing

In this study, an Mg-Y-Nd-Gd-Zr alloy was subjected to ultrasonic surface rolling processing (USRP), and a deformation layer approximately 651 mu m deep was observed. It shows a double-gradient microstructure as the grain size increased and the twin density decreased. The grain size at the top surface layer was refined to 147.08 nm, and several stacking faults (SFs) were observed in the nanograins. Two main types of SFs were observed, including intrinsic I-1 SFs bounded by 1/6<20(2)over bar3> Frank partial dislocations and intrinsic I-2 SFs bounded by 1/3<10(1)over bar0> Shockley partial dislocations. The SFs induced a strong hindrance to dislocation motion, improving the yield strength of the alloy. Also, the dislocation caused by SFs increased the stress concentration of the nanograins and promoted premature fracture failure at the top surface of the alloy, reducing its plasticity after USRP. In addition, the tensile fracture of the alloy changed from plastic fracture mechanism dominated by micropore aggregation before USRP to brittle fracture mechanism dominated by cleavage planes and steps after USRP.