Abstract:The creep behavior of the 35% (volume fraction) Mg₂B₂O_5w reinforced AZ31 magnesium matrix composites was investigated under thermal cycling conditions with varied external applied stresses of 7, 14, 21 and 42 MPa, respectively. The results show that the creep behavior of Mg₂B₂O_5w/AZ31 under the thermal cycling with external applied stress is primary creep stage at first, followed by steady creep stage and finally unsteady and quick creep. The composite sample subjected to thermal cycling and applied stress of 42 MPa occurs an unsteady and quick creep after a shortly steady creep. The drastic fluctuation of strain rates has less effect on the strain increment trend when the composite sample subjected to thermal cycling and applied stress of 21 MPa. The tilt angles of hysteresis loops decrease with increasing the applied stresses. The lower the applied stress is, the higher the strain rate sensitivity exponent m is. Superplasticity behavior also occurs for the composite isothermal creep experiment at 400℃. However, the strain rate of the composite isothermal creep is much lower than that of the composites creeps when the composite samples are subjected to thermal cycling and applied stresses. The average thermal residual stress in the matrix Mg alloy approaches null at elevated temperature of about 300 ℃. The residual strains mainly appear at upper temperature during the heating of cycling.

Key words: Mg based composites; Mg₂B₂O₅ whisker; thermal cycling; superplasticity; strain rate