Study on thermal expansion of intermetallics in multicomponent Al-Si alloys by high temperature X-ray diffraction

The coefficients of thermal expansion of intermetallic phases in multicomponent Al-Si casting alloys were measured by high temperature X-ray diffraction. The measured coefficient of thermal expansion (CTE) values ranged from 10 to 23 ppm/°C for the intermetallic phases. The large thermal expansion mismatch between the Al matrix and the Si particles resulted in cracks being observed in all alloys after quenching. The lattice parameters generally changed linearly with increasing temperature for all phases apart from the monoclinic Al9FeNi phase, along the a- and c-axes, which also demonstrated anisotropic behaviour and microcracking. The α-AlFeMnSi particle cracking is mainly dominated by its intrinsic brittleness and is not due to large thermal expansion misfit. No cracks were observed in Al2Cu particles due to a combined effect of reduced brittleness and a small thermal misfit.