SEM-DIC characterization of the damage mechanism of an AlSi10Fe0.7 casting alloy on the microstructure scale

Various brittle phases are present in commercial cast aluminum alloys, which strongly influence their mechanical behavior. Among these, silicon precipitates are nearly omnipresent, as Si is a common alloying element. In secondary alloys, usually Fe-containing phases cannot be avoided, and they tend to degrade the mechanical properties. The interaction between the silicon phase and the failure-critical intermetallic phase in the Al-Si-Fe phase system (β-AI₅FeSi) is studied in this paper in high resolution. A model alloy AlSi10Fe0.7 was defined, which is composed of a large grain Al-matrix, Si-precipitates and the plate-like β-AI₅FeSi phase. The goal of the study was to identify “hot spots” in the microstructure from which cracks may initiate under mechanical loading. The main tool was a deformation analysis via digital image correlation in the SEM (SEM-DIC). This allows the identification and tracking of developing strain localizations at different potential crack initiation sites with a high resolution as well as capturing an overview over the whole specimen. An adapted frame averaging script minimized measurement errors induced by drift. The SEM-DIC results show that the deformation field is governed by the elastic incompatibility of the microstructural constituents. Crack initiation occurs because of the detachment of the Si + β-AI₅FeSi phase boundary. Cracks then cross the phase boundary and propagate along twin boundaries in the β-AI₅FeSi phase. Final failure is caused by linking fractured brittle plate-like particles.