Beneficial effects of Sc/Zr addition on hypereutectic Al-Ce alloys: Modification of primary phases and precipitation hardening

Al-Ce alloys possess desirable elevated-temperature performance due to the high stability of Al11Ce3 phases. The morphology and size of the Al11Ce3 phases play a significant role in determining the strengthening effect in Al-Ce alloys. In this paper, the effect of Sc/Zr addition on the microstructure, mechanical properties, and thermal stability of a hypereutectic cast Al-Ce alloy was investigated. The results demonstrated that Sc and Zr atoms could adsorb on the Al11Ce3 phase surfaces and control the growth process in the 0.13 wt % Sc-0.06 wt % Zr addition alloy, along with the size of primary Al11Ce3 phases was refined from similar to 79 mu m to similar to 56 mu m. As the total Sc/Zr content increased, the formation of primary Al-3(Sc, Zr) phases could significantly refine the primary Al11Ce3 phases by acting as heterogeneous nucleation sites. Therefore, in 0.23 wt % Sc-0.16 wt % Zr and 0.49 wt % Sc-0.23 wt % Zr alloys, the modification effect on primary Al11Ce3 phases was based upon the interaction of adsorption effect and heterogeneous nucleation. The sizes of primary Al11Ce3 phases were refined to similar to 37 mu m and similar to 32 mu m, leading to a significant improvement in ultimate tensile strength (from similar to 117 MPa to similar to 182 MPa), yield strength (from similar to 75 MPa to similar to 145 MPa) and elongation (from similar to 1.4% to similar to 3.4%) in the 0.49 wt % Sc similar to 0.23 wt % Zr alloy compared to the unmodified alloy. The Sc and Zr atoms that were dissolved in the matrix as a decomposed solid solution precipitated the L1(2)-Al-3(Sc, Zr) phases after thermal exposure, which maintained full coherency with the Al matrix, leading to a hardness increase, rather than a decrease, after 300 degrees C and 400 degrees C thermal exposure. Therefore, Al-15Ce-(Sc-Zr) alloys open a new alloy system for engineering materials useable in high-temperature applications.