Compressive Creep behavior of a hot-rolled Mg-Y binary alloy and Mg-Y-Zn ternary alloys was investigated at 650 K. Creep strength of Mg-Y alloys was significantly improved by the addition of zinc. In Mg-Y-Zn alloys, many "planar faults" were formed on (0 0 0 1) matrix planes at high temperatures even with small addition of 0.02 mol% zinc. The stacking fault energy decreased by the simultaneous addition of yttrium (Y) and zinc and many a-dislocations on basal planes were extended. The separation width of the extended dislocations is stabilized by the segregation of yttrium and zinc and the separation width significantly increases under the interaction of partial dislocations on different basal planes. The excellent creep strength in Mg-Y-Zn was explained by the decrease of the mobility of these widely extended dislocations.
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