Abstract
The influence of ultimate tensile strength, 0.2% proof stress and elongation on the thermo-mechanical fatigue life has been studied for Cu added AC4C aluminum cast alloys. Hardness changes have been also investigated during isothermal aging at 180°C together with the detailed TEM observation of precipitate microstructures. The thermo-mechanical fatigue life decreased with increasing 0.2% proof stress. In the Cu added AC4C alloys, high density of needle shaped precipitates of the Q′ phase were observed in the over-aged condition. Furthermore, it was found that Cu retarded both the coarsening of the precipitates and resultantly softening of the matrix. This is because that the misfit of the Q′ phase with the matrix is smaller than that of the β′ phase founded in AC4C aluminum alloys. The retarded softening of the matrix causes the decrease in the thermo-mechanical fatigue life due to the preferential crack propagation through the fractured eutectic Si particles during thermo-mechanical fatigue loading. The optimized over-aging conditions, however, resulted in the decreased 0.2% proof stress and are extremely effective to improve the thermo-mechanical fatigue life of the Cu added AC4C aluminum alloy.
