Microstructural cyclic deformation mechanisms and their relation to small fatigue crack initiation and early propagation behavior were experimentally investigated in a rare earth-containing magnesium alloy (Mg-Gd-Y-Zr).The results indicate that basal slip is the dominant deformation mechanism,especially in coarse grains,and eventually leads to fatigue crack initiation.Early crack propagation behavior was strongly influenced by local microstructural heterogeneities in grain size and orientation.Three kinds of microstructures—favorably-oriented coarse grains,fine grain clusters and unfavorably-oriented coarse grains—are discussed in terms of their deformation mechanisms and resistance to crack propagation.These microstructural effects caused highly variable crack propagation rates within the first ~200 μm of cracks.Furthermore,by means of high resolution transmission electron microscope (HRTEM),solute segregation and dislocations around the crack initiation site were investigated in fatigued Mg-10Gd-Y alloy.Co-segregation of Gd and Y atoms along basal plane was detected in the shape of lamella structure,and this was ascribe to the attraction for alloying atoms by motion of basal dislocation during cyclic loading.The Gd/Y segregation bands inhibited the glide of subsequent dislocation and initiated dislocation motion within the segregated bands,and hence they may enhance the fatigue strength through the retard of fatigue crack initiation along the basal plane.