中文摘要：

通过SEM原位观测方法试验研究了铸造镁铝系列合金的微观破坏机理，得到了α-Mg颗粒大小、形状、β（Mg17，Al12）相分布对破坏机理的贡献与微结构的改性对其强度韧度的影响。结果表明：微裂纹主要萌生于晶界并沿界面扩展，其中由于铸造镁铝合金的“准脆性”，疲劳裂纹稳态扩展范围在微米量级，疲劳裂纹扩展平面微观尺度下与外力方向不完全垂直，裂纹扩展机理表现为ModeⅠ＋ModeⅡ的耦合结果。因此，线性破坏力学中的应力强度因子AK不能有效地评价铸造镁铝合金的疲劳扩展。

英文摘要：

The micro-fracture mechanism of cast magnesium alloys was investigated based on in-situ observation with scanning electron microscope （SEM）. The effects of the α-Mg grain size and shape as well as the phases （Mg17 Al12 ） distribution on the fatigue crack initiation and propagation, and effect of the addition of Ca to magnesium alloy on the mechanical properties were examined. The fatigue cracks mainly initiated and propagated at the α-Mg boundaries. These cracks stably propagated in a scale of micrometers and did not absolutely propagate perpendicularly to the maximal principal stress direction. The fatigue crack propagating mechanism was dependant on the coupling of mode I and mode Ⅱ. Therefore, the stress intensity factor ΔK was not suitable for estimating the fatigue crack growth rate in microscope.