**4. Conclusions**

In situ tension in SEM combined with EBSD and DIC techniques was applied to investigate fracture behavior in a Mg-Gd-Y alloy. Intergranular cracking was found at the triangle grain boundary area with serious inhomogeneous strain distribution conditions. The difference in strain amplitude between linked grain boundaries along the tension direction led to severe deformation incompatibility at the triangle point, which makes it a weak point for crack initiation. As the tension strain increases, the crack extends from the weak point along the grain boundary and causes intergranular cracking.

**Author Contributions:** Formal analysis, J.S.; Methodology, J.S., J.L., Q.C. and L.L.; Project administration, J.S.; Writing—original draft, J.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the National Natural Science Foundation of China (Grant No. 52001188) and the Natural Science Foundation of Shandong Province (ZR2020QE161).

**Conflicts of Interest:** The authors declare no conflict of interest.
