Effect of I(Mg3YZn6)-, W(Mg3Y2Zn3)- and LPSO(Mg12ZnY)-phases on tensile work-hardening and fracture behaviors of rolled Mg–Y–Zn alloys

Abstract

The effects of secondary phases (W, Mg3Y2Zn3), long-period stacking-ordered (LPSO, Mg12ZnY) and icosahedral (I, Mg3YZn6) phases on the tensile properties and work-hardening behavior of Mg–Y–Zn alloys (Mg–1.32Zn–1.79Y, ZW12; Mg–4.4Zn–2.2Y, ZW42; and Mg–5.2Zn–1.1Y, ZW51) prepared after application of differential speed rolling on their cast microstructures were studied at room temperature. The volume fractions of LPSO, W and I-phase in ZW12, ZW42 and ZW51 were 4.7, 6.3 and 4.8%, respectively. The rolled alloys exhibited a similar level of yield strength but different work hardening characteristics and tensile fracture behavior. The rolled ZW51 with I-phase particles exhibited the highest work-hardening rate and the largest tensile elongation (with ductile fracture). The ZW42 with W-phase particles exhibited a work-hardening rate slightly lower than that of the rolled ZW51, despite having the larger amount of secondary phase, and premature fracture occurred far before neck formation in ZW42, while fracture occured after neck formation in ZW51. The fractured surface showed that the coherent interface between the I-phase and Mg matrix phase is significantly more resistant to cracking than is the incoherent interface between the W-phase and Mg matrix phase. The rolled ZW12 with LPSO-phase, forming a coherent interface with the Mg matrix phase, exhibited a considerably lower work-hardening rate than the rolled ZW51, though they had the similar amounts of secondary phase. Thus, the uniform strain of the rolled ZW12 was quite small compared with that of the rolled ZW51, though fracture occurred after necking in both alloys. The same work-hardening and tensile elongation behaviors were retained after annealing treatment on the rolled alloys.