Effects of discontinuous precipitate (DP) and continuous precipitate (CP) on the tensile elongation and fracture mechanism of aging-treated AZ91D Mg alloy were investigated via quantitative microscopy and fractography. Specimens with various distributions of DP and CP were prepared under a wide range of aging conditions. As quantitative microstructural factors of DP and CP, area fraction of DPs and CP/α-Mg interfaces per unit volume were measured by optical microscopy (OM) and scanning electron microscopy (SEM). To explain the tensile fracture mechanism, a deformation twinning behavior was analyzed using OM and electron back-scattered diffraction, and area fraction of stepped cleavage facet was quantitatively measured on a large number of fractographs. An increase in the area fraction of DPs caused a slight decrease in the tensile elongation and this reduction was attributed to cracking at the DP along grain boundary. For the specimens with high CP/α-Mg interfaces per unit volume, a large number of fine {10−12} tension twins were produced within a few percent of tensile elongation as well as premature fracture occurred. In this investigation, the premature fracture mechanism of the aging-treated AZ91D alloy with a large amount of fine CP distribution was explained in relation with the increased probability of {10–12} tension twin boundary cracking, although activation of {10–12} tension twinning can be a way to facilitate deformation of some Mg alloys in previous researches. Validity of the suggested premature fracture mechanism was supported by the quantitative measurement of area fraction of the stepped cleavage facet on the fractography as indirect evidence of the twin boundary cracking.

通过定量显微镜和断口扫描技术研究了不连续沉淀（DP）和连续沉淀（CP）对时效处理的AZ91D Mg合金的拉伸伸长和断裂机理的影响。在广泛的老化条件下制备了具有不同分布的DP和CP的样品。作为DP和CP的定量微结构因子，通过光学显微镜（OM）和扫描电子显微镜（SEM）测量每单位体积的DP和CP /α-Mg界面的面积分数。为了解释拉伸断裂的机理，使用OM和电子反向散射衍射分析了变形孪生行为，并在大量的断口扫描仪上定量测量了阶梯状分裂面的面积分数。DP的面积分数的增加引起拉伸伸长率的轻微降低，并且这种减少归因于DP沿晶界的裂纹。对于每单位体积具有高CP /α-Mg界面的标本，在百分之几的拉伸伸长率范围内会产生大量的精细{10-12}拉伸孪晶，并且会发生过早断裂。在这项研究中，尽管{10-12}被激活，但随着{10-12}张力孪晶边界裂纹的可能性增加，解释了具有大量细CP分布的时效处理的AZ91D合金的过早断裂机理。以前的研究认为，张力孪晶可能是促进某些镁合金变形的一种方法。