中文摘要：

用原位SEM观测研究了粉末冶金BeAl材料应力控制的疲劳机制.断裂时,低应力疲劳比高应力的平均累积塑性应变小得多.前者的累积塑性应变变化速率随着循环次数增大明显减小,后者则接近线性变化规律,表明高应力水平下的疲劳损伤累积更接近线性假设.低应力疲劳可从表面观测到一条疲劳主裂纹,由微观尺度下的累积塑性应变控制;高应力水平疲劳加载初期很快有宏观尺度的累积塑性应变,使Be与Al结合相界面同时产生较多裂纹.裂纹主要沿Al和Be结合强度较弱的表面路径扩展,随后沿纵深方向扩展,在材料内部裂纹扩展方向会变为与加载方向平行的Be基滑移面方向.当裂纹达到临界尺寸,局部塑性应变控制变为主应力方向控制,促使裂纹向前方扩展,宏观断口呈现出有层次的撕裂型.

英文摘要：

Mechanisms of fatigue fracture were investigated with the stress-controlled tests by in-situ scanning electronics microscopy （SEM） for the BeAl alloy of Powder Metallurgy （PM）. The accumulated plastic strain under low-stress fatigue was much lower than that under high-stress fatigue. It＇s also shown that the accumulated plastic strain rate obviously decreased with increasing cycle numbers for the low-stress fatigue, while increased linearly for the high-stress fatigue. This makes clear that the assumption of the linearly accumulated fatigue damage is more reasonable for the high-stress fatigue. One fatigue main microcrack on the surface could be observed on the sample controlled by the accumulated plastic strain at low-stress fatigue. Macroscopic plastic strain will produce many cracks in the Be-Al phase interface at high-stress fatigue. The cracks first propagate in the direction dictated by the weak interface of Be and Al, then grow towards in depth. The cracks may propagate along Be basal slip plane parallel to the loading direction in the material When the crack size reaches a critical value, the cracks dictated by localized plasticity will propagate ahead, controlled by the principal tensile axis. All these macroscopic surface fractures show the hiberarchy and tearing characteristics.