Morphological modification of Mg2Si phase and strengthening mechanism in Mg2Si/Al composites by Eu addition and T6 heat treatment

Abstract

In order to improve mechanical properties of Mg2Si/Al composites, Eu element was added to modify Mg2Si morphology, and T6 heat treatment was used to control Mg2Si morphology and precipitate strengthening phase. Microstructure and morphology of Mg2Si were observed by synchrotron X-ray tomography and TEM, and first-principles calculations were also performed to testify the effect of Eu modification. Results show that the size of primary Mg2Si particles decreases and eutectic Mg2Si phase transforms from lamellar to fibrous by Eu addition. After heat treatment, sharp angles of primary Mg2Si particles passivate, and eutectic Mg2Si dissolves and appears to be short dot-like. Meanwhile, nano-sized β" phase precipitates in the matrix. For morphology of Mg2Si with Eu modification, TEM results show that Eu impedes the growing of Mg2Si, which is verified by first-principles calculations that Eu atom preferentially adsorbs on Mg2Si {100} facet. The adsorption and suppression growing of Mg2Si transform the morphology of Mg2Si and thus improve the elongation. UTS and elongation of the heat-treated Eu modified composites are 281 MPa and 8.4%, which improved 81% and 200% compared to the as-cast Al-15%Mg2Si composite. The strengthening mechanism mainly results from precipitation strengthening of nano-sized β" precipitates in the heat-treated composite.