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

Abstract

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 ∼79 μm to ∼56 μm. As the total Sc/Zr content increased, the formation of primary Al3(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 ∼37 μm and ∼32 μm, leading to a significant improvement in ultimate tensile strength (from ∼117 MPa to ∼182 MPa), yield strength (from ∼75 MPa to ∼145 MPa) and elongation (from ∼1.4% to ∼3.4%) in the 0.49 wt % Sc-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 L12-Al3(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 °C and 400 °C thermal exposure. Therefore, Al–15Ce-(Sc–Zr) alloys open a new alloy system for engineering materials useable in high-temperature applications.