**1. Introduction**

The elongation of wrought magnesium at ambient temperature can achieve 20% or even more, which is larger than that of traditional aluminum alloys [1–4]. However, the formability of wrought magnesium still cannot be compared with aluminum alloys at ambient temperature, which impedes the development of wrought magnesium.

It is common to think that damage nucleation during the deformation process occurs at locations with large strain concentrations, where substantial heterogeneous deformation occurs. If large local strains are effective in accommodating the required geometry changes, they may prevent damage nucleation, whereas it is conceivable that damage may nucleate where insufficient strain or shape accommodation occurs. Such variability in shape accommodation is connected to crystal orientations and crystallographic deformation mechanisms. In previous studies, cracking was thought to be related to {10–12} tension twinning [5], {10–11}–{10–12} double twinning [6,7], persistent slip bands [8] or grain boundaries [9]. J. Koike et al. [7] thought {10–11}–{10–12} double twinning would lead to the formation of large surface steps, cracks and final failure. An Luo et al. [10] thought that fatigue cracks mainly exist as transgranular cracks, and the crack propagation path is related to the orientation of target grains and the loading direction. Small cracks usually initiate on basal planes in grains with a larger Schmid factor.

This paper focused on the microstructure evolution and fracture behavior in the Mg-Gd-Y alloy, which is critical to its ductility. In situ tension tests were used in combination with electron backscattered diffraction (EBSD) and digital image correlation (DIC) to correlate the activation of deformation modes and fracture behavior. Strain accommodation in the grain boundary area was carefully analyzed, and the effects of grain orientation and grain boundary compatibility were discussed.
