In situ fracture observation and fracture toughness analysis of squeeze cast AZ51–xSn magnesium alloys

Effects of Sn addition on microstructural modification and microfracture mechanisms of squeeze cast AZ51–xSn magnesium alloys were investigated by in situ fracture and fractographic observations. Microstructural analyses indicated that Mg2Sn particles as well as Mg17Al12 particles were precipitated mainly along solidification cell boundaries, and the volume fraction of Mg2Sn particles increased with increasing Sn addition, thereby leading to the increase in strength. According to observation of microfracture processes of the AZ51–7Sn alloy, the fracture proceeded mainly along cell boundaries as the crack initiated at the notch tip propagated mainly along intercellular Mg17Al12 and Mg2Sn particles. In the AZ51 and AZ51–3Sn alloys, on the other hand, the deformation and fracture proceeded into cells rather than into cell boundaries as twins were actively developed inside cells, although some microcracks were initiated at Mg17Al12 or Mg2Sn particles. This resulted in the improved fracture properties, which was also confirmed by the R-curve analysis. These findings suggested that the addition of 3–5 wt.% Sn was effective in improving both tensile and fracture properties, which was explained by mechanisms such as development of twins in the Mg matrix, blocking of crack growth, and crack blunting.