Isothermal low cycle fatigue of a lost foam cast Al-Si-Cu alloy: study of the damage mechanisms with synchrotron X-ray tomography and Digital Volume Correlation

International audienceAl-Si-Cu alloys manufactured by the Lost Foam Casting process have a complex microstructure. In order to study its role on damage under loading conditions representative of the in-service conditions of automotive cylinder heads, in situ Low Cycle Fatigue tests monitored by synchrotron X-ray tomography have been performed at room and high temperature (250°C). The initiation and the first propagation stages of short fatigue cracks have been observed in the interior of the samples. Those observations were correlated with the 3D strain fields obtained from Digital Volume Correlation. Cracks nucleate usually at the first loading cycle at the sharp edges of subsurface pores due to their notch effect that induces a strain localization but also at hard inclusions located in the pores neighbourhood. Then, crack propagation proceeds through hard inclusions where the von Mises cumulated strain increases. This scenario is however temperature dependent. At room temperature, crack initiation is porosity driven while propagation is correlated with the presence of hard phases in the interdendritic space. At high temperature, massive fracture of eutectic Silicon was observed. Crack propagation is then dictated by the coalescence of these microcracks in the most strained area of the specimen, i.e. between or close to large pores