High temperature effect on short crack behaviour in hypoeutectic Al-Si alloys and its modelling

Hypoeutectic Al-Si alloys used for components facing the combustion chamber of automotive engines necessitate excellent high-temperature mechanical properties. In this study, the effect of high temperature on short crack behaviour of hypoeutectic Al-Si alloys is investigated experimentally. The results show that the short crack growth rate is affected by thermal softening and precipitate coarsening. At 200 °C, the slight thermal softening and the positive effect of short-term precipitate coarsening causes the crack growth rates remain unchanged compared to room temperature (RT). At 250 °C, the crack growth rate remains essentially constant during the early stage, while with increasing testing time, the crack growth rate increases rapidly due to the precipitate coarsening. At 300 °C, the crack growth rate is significantly enhanced due to thermal softening and precipitate coarsening. Due to thermal softening, the elastic modulus and yield stress (σ_ys) decrease with increasing temperature. Due to time and temperature dependent precipitate coarsening, the σ_ys drops sharply with the testing time at 250 °C and 300 °C. At 200 °C, the σ_ys increases first and then decreases with the increase of testing time. The transgranular mode dominates the short crack growth at both RT and high temperatures. The grain boundaries have an apparent suppression effect on crack growth. Finally, the relationships between σ_ys and temperature are established for crack tip opening displacement range (ΔCTOD) calculation. The short crack growth rates can be well characterized and predicted by ΔCTOD.