Effects of Cerium on the Mechanical Properties of Al–20Si Alloy

The chemical modification of Al–20Si alloy in the case of incorporation of rare earth cerium (Ce) is investigated in this study, focusing on the resulting microstructure evolution and changes in mechanical properties and hardness before and after cerium modification. The microstructure of the cerium-modified alloy is composed of the aluminum matrix, eutectic silicon, and primary silicon phases, as well as the Al2Si2Ce phase, a cerium-containing solid solution, and the α-Al11Ce3 phase. It is worth noting that this study reports for the first time an unusual phenomenon in which the addition of cerium raises tensile strength of the alloy while concurrently decreasing its surface hardness. The increase in tensile strength is mainly ascribed to the refinement of primary silicon. On the contrary, the reduction of hardness is explained by the competition between Ce-induced toughening and solid solution strengthening, as well as the generation of the relatively soft α-Al11Ce3 phase. Mechanical properties at room temperature show that the greatest tensile strength of 153 ± 7 MPa is reached at a cerium concentration of 0.7 wt.%, while the corresponding elongation of the mechanical specimen is 1.36 ± 0.287. The fracture mechanism changes from cleavage fracture in the unmodified alloy to ductile fracture after Ce addition.