CREEP OF A Mg-Zn-Y ALLOY AT ELEVATED TEMPERATURES

Mg alloys containing long-period stacking ordered (LPSO) phases have superior strength at elevated temperatures, which makes them potential creep resistant materials. We have studied the creep behavior of a Mg97Zn1Y2 (at.%) alloy. No creep strain was detected for the Mg97Zn1Y2 alloy at 150 degrees C under a tensile stress of 70MPa in 100h. The creep strains were measured to be about 0.01% and 0.28% at 200 degrees C and 250 degrees C, respectively, under a tensile stress of 70MPa in 100h, showing excellent creep resistance. Fracture occurred with a creep strain of 7.55% after 42h of tensile creep under a tensile stress of 70MPa at 300 degrees C. The microstructures were characterized by transmission electron microscopy and scanning-transmission electron microscopy techniques, in order to understand the creep deformation behavior and microstructural origin of the excellent creep resistance of this alloy. LPSO phase was found playing an important role in the alloy's creep resistance. Generation and motion of basal "a" dislocations led to bending of LPSO phase. No voids were formed at LPSO/Mg interfaces. Suzuki segregation occurred widely along stacking faults resulting from dissociation of either "a" or "a + c" dislocations in the Mg matrix, which hindered dislocation motion and thus played an important role in strengthening the Mg grains that are softer than the LPSO phases. This segregation should have considerable contribution to the alloys excellent creep resistance at elevated temperatures. Growth of stacking faults on basal planes in Mg can effectively restrict motion of non-basal dislocations. The excellent creep resistance of the Mg97Zn1Y2 (at.%) alloy at elevated temperatures is intimately associated with the LPSO strengthening phase, wide stacking faults with Zn and Y segregation formed during solidification, and dynamic Suzuki segregation along nanometer-sized stacking faults produced by dislocation dissociation.