Modification of multi-component Al–Si casting piston alloys by addition of rare earth yttrium

Abstract

The effects of yttrium (Y) additions (0.2, 0.4, 0.6, 0.8 and 1.0 wt%) on the microstructures and mechanical properties of multi-component Al–Si casting piston alloys were studied in this work. The as-cast specimens were characterized and analyzed using different techniques (SEM, EDS, DSC, XRD, EBSD and TEM) to determine the modification capabilities and better understand the underlying mechanism of adding rare earth Y. The results indicate that the morphology of the α-Al phase could be refined from coarse dendrites to relatively fine cell-like structures and uniform equiaxed grains when adding Y at a concentration of 0.8 wt%. The corresponding mean secondary dendrite arm spacing decreased by 64.6% from 12.7 μm to 4.5 μm. Meanwhile, the rare earth Y effectively modified the eutectic Si structure from coarse flake and needle to fine fibrous structure and partial particles. The average roundness of the eutectic Si decreased by 52% from 9.8 to 4.7. In addition, the fish-bone-like γ-Al7Cu4Ni and δ-Al3CuNi phases gradually transformed into the block-shaped AlCuNiY-Aluminide phase with the addition of rare earth Y. The nanoscale Y-containing compounds adsorbed at the front of the Si phase growth interface to restrain further growth, which changed its growth direction and preferentially promoted the formation of a twin structure along the {111}〈112〉 orientation. With the modification and homogenization of the eutectic structure, the comprehensive mechanical properties were evaluated by the Q-factor increased by 52.6% from 173 MPa to 264 MPa. The fracture model of the multi-component Al–Si alloy gradually transformed from a classical, brittle, trans-granular fracture mode to a mixed ductile-brittle fracture style.