2005 Volume 46 Issue 1 Pages 111-117
Out-of-phase type thermo-mechanical fatigue tests have been performed for Al-Si cast alloys with the temperature range of 323–523 K and the applied mechanical strain range of 0.5–1.5%. Thermo-mechanical fatigue lives and stress-strain hysteresis loops are investigated by the experiments. In general, the thermo-mechanical fatigue is affected by various factors such as thermal expansion/contraction, elasto-plastic/creep deformation, softening by overaging, dynamic recovery, damaging and cracking. Multi-step finite element simulation techniques have provided an effective way of assessing the local damaging behavior of silicon particles, along with extracting the contribution from creep. Although the size and shape of the silicon particles in the material with a higher solidification rate are similar to those of a slowly cooled material, it exhibits superior thermo-mechanical fatigue property together with a smaller secondary dendrite arm spacing (SDAS, hereinafter). Since the difference in the stress-strain hysteresis loops between the two materials has vanished when the softening by overaging is almost finished, the observed difference in the thermo-mechanical properties is attributed to age-hardenability rather than the SDAS. The effects of damaging at the silicon particles to this difference are also suggested. In fact, damaged silicon particles have been observed extensively from an early stage of the thermo-mechanical loading, then forming fatigue cracks by the linkage of the damages. It has been clarified by the simulation that interfacial debonding is likely to occur rather than particle cracking in the materials used. The simulation provides valuable insights into the understanding of the spatial distribution of damage in the eutectic region. The simulation has also enabled to assess the contribution of the creep deformation, indicating that medium to high cycle fatigues are significantly affected by it.
