Cast Al-Si alloys are used in automotive components like pistons due to their relatively high strength-to-weight ratio as well as excellent castability and heat conductivity. Particularly, the piston bowl rim in modern diesel engines must be able to withstand thermo-mechanical fatigue (TMF) conditions in a temperature range between RT-380 °C with thermal cycles of a few seconds during service.
The TMF behaviour of Al-Si piston alloys with ~12 wt.% of Si and varying contents of Cu, Ni and Mg was investigated using the thermomechanical simulation system Gleeble. The temperature was cycled between 80-380°C keeping a constant strain of 0%.
3D characterization of the microstructure and the damage evolution were conducted by synchrotron tomography experiments at the beamline P05 at DESY/Petra III in Hamburg for specimens after a selected number of thermal cycles. Cracks through primary Si particles parallel to the sample length could be detected after 4 cycles and formation and accumulation of cracks oriented perpendicularly to the sample length could be observed with increasing number of TMF-cycles. Correlation between damage evolution and microstructural parameters indicate the influence of volume fraction, interconnectivity, interparticle distances of rigid phases, i.e. Si and intermetallics. Furthermore, the results obtained by the means of synchrotron tomography are complemented by neutron diffraction experiments on for the same selected cycle numbers as for the tomography in order to gain information about specific phase strains induced by TMF. Thermodynamic simulations provided further understanding on the impact of chemical composition on the formed microstructure and features affecting damage initiation, accumulation and retardation.