Cr-induced morphology change of primary Mn-rich phase in Al-Si-Cu-Mn heat resistant aluminum alloys and its contribution to high temperature strength

Effect of Cr addition on the microstructure and mechanical properties of Al-12 wt%Si-3.5 wt%Cu-2 wt%Mn heat-resistant alloy were studied by OM, XRD and SEM microstructure observation and tension test at different temperature. Results indicate that Cr addition can obviously change the morphology of the primary Mn-rich phase in Al-12 wt%Si-3.5 wt. Cu-2 wt%Mn, from slender rods to dendrites first and then to star-type particles. XRD and SEM-EDS results suggest that no new Cr-rich phase is formed in the studied alloy, and Cr is mainly dissolved in the Mn-rich phase, besides partially in the matrix. Addition of Cr can significantly increases strength and elongation at room temperature and high temperature. When Cr addition level is at 1.0%, the strength at 350 °C reaches to the highest value of 106 MPa, about 28% higher than that without addition of Cr. In the alloys with or without Cr addition, the internal cracks originate from and propagate along the primary Mn-rich phase during tension at 350 °C.