加载热循环对Mg_2B_2O_(5w)/AZ31B复合材料塑性的影响

Effect of applied stress combined with thermal cycling on plasticity of Mg_2B_2O_(5w)/AZ31B composites

对35%(体积分数)的Mg2B2O5w/AZ31复合材料分别在外加应力为7、14、21和42 MPa时进行加载热循环实验,研究其蠕变行为。结果表明:加载热循环分为初始蠕变阶段、稳定蠕变阶段和失稳变形阶段;外加载荷为42 MPa时,热循环由稳定蠕变阶段迅速进入失稳变形阶段;当外加载荷为21 MPa时,应变速率的急剧变化对累积残余应变影响较小;热循环滞后环倾角随外加载荷的增大而减小;Mg2B2O5w/AZ31复合材料的外加载荷越低,应变速率敏感指数越高,而在400℃的高温蠕变虽也呈现超塑性行为,但应变速率比加载热循环大幅下降。在加载热循环过程中,300℃时基体中的平均内应力接近于零,热循环残余应变的产生主要发生在热循环的高温阶段。

The creep behavior of the 35% （volume fraction） Mg2B2Osw 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 Mg2B2Osw/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~C. 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.