Tension twin-induced premature fracture mechanism of aging-treated AZ91D magnesium alloy

Highlights: • Effects of DP and CP on the elongation of the aging-treated AZ91D alloy was quantitatively evaluated. • Precipitation of CPs drastically reduced the tensile elongation of the AZ91D alloy. • Deformation twinning behavior of the AZ91D alloy was analyzed using OM and EBSD. • Quantitative fractography analysis was performed using SEM. • Premature fracture of the aging-treated AZ91D was attributed to the {10–12} tension twin boundary cracking. 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.