Evolution of Twinning During Cyclic Loading of Magnesium Alloy ZM21 Examined by Quasi-in-Situ EBSD

Abstract

Most complex engineering components experience varied loading in use. We conducted successive experiments of cyclic tensile deformation followed by electron-backscattered diffraction imaging of the same area on each of several ZM21 magnesium alloy sample to investigate the microstructure and texture evolution during cyclic loading. In this way we correlated the evolving deformed microstructure of cyclically loaded magnesium to the initial grain orientations. To investigate the effects of the strain path, the behaviour of samples with and without pre-compression was compared. Twins can propagate from grain to grain and twin chains will form in the material without any pre-deformation. Twin chains are a result of twin transmission at grain boundaries and twin transmission frequency decreases with increasing grain boundary angles. In the absence of pre-compression, the fraction of extension twins increases from 0.003 to 0.019 after two cycles of tensile loading to a strain of 0.03, after which it changes only slightly as strain is increased. Twin chains in the pre-compressed specimen are activated without any obvious dependence on the matrix orientation. The fraction of extension twinning is 0.2 for the pre-compressed specimen. After a cycle of tensile loading to a total plastic strain of 0.015, 90% of the extension twins undergo detwinning. The detwinning starts just after reverse loading. Secondary twins forming within primary twin interfaces start to nucleate in the microstructure of the pre-compressed material at an early stage (at plastic strain values of 0.056). This is different from the nucleation of secondary twins in the material without pre-compression that occurs at a strain of 0.104. The early formation of secondary twinning is due to the presence of residual primary twins in the microstructure of the pre-compressed material as a result of detwinning of the initial twins.