Effect of strain rate on the defect susceptibility of tensile properties to porosity variation

The contribution of the strain rate to the dependence of tensile properties on the microporosity variation in A356 casting alloy was investigated in terms of the defect susceptibility of tensile properties to the microporosity variation with variations of the strain rate as well as the relative contribution of microporosity and strain rate to overall tensile properties. The test samples were prepared using a low-pressure die-casting process and subsequent T4 treatment (12-h at 540 °C), and the tensile test was carried out at room temperature for strain rates varying in the 1.4×10−4∼1.4×10−1 s−1 range. The overall dependence of tensile properties on the strain rate is not described precisely owing to remarkable deviations in the data that mainly arose from the variation of the fractographic porosity; these deviations can be clearly attributed to the variability in the defect susceptibility coefficient of the tensile properties to the microporosity variation. The defect susceptibility coefficient of ultimate tensile strength (UTS) to microporosity variation increases with the strain rate, whereas the defect susceptibility coefficient of elongation decreases. Although the UTS for the sample with a low microporosity level increases with increasing strain rates, the UTS above a certain porosity level is affected adversely and decreases with the increasing strain rates. However, the nominal level of tensile elongation on the variation of the strain rate clearly decreases with the increase of the microporosity. The fractographic porosity practically decreases with the increase of the strain rate, and the overall fracture path between the micro-voids depends practically upon a certain transition of the fracture mode of Si particles accompanying the variation of the strain rate. Additionally, the damage evolution of eutectic Si particles is transited from a mixed mode of cracking and debonding failure to a failure mode which the debonding failure dominates as the strain rate increases.