Synergistic effects of Gd and Zr on grain refinement and eutectic Si modification of Al-Si cast alloy

The effects of Gd and/or Zr additions on the grain refinement and eutectic Si modification of A356 cast alloys have been systematically investigated. The results showed that a separate addition of Gd or Zr had almost no effect on eutectic Si modification, while except for an appreciable grain refinement, a combined addition of Gd and Zr significantly refined the eutectic Si and changed its morphology from plate-like to fibrous. As compared with Gd, Zr and Sr separate modification, the Gd and Zr combined addition produced fewer twins but more nanosized precipitates within eutectic Si. The excellent modification effect was attributed to the increase of constitutional undercooling and nanosized precipitates inhibited the growth of eutectic Si during solidification. The Gd and Zr combined addition obviously improved the mechanical properties of A356 cast alloy due to grain refinement and eutectic Si modification. Optimal tensile properties were achieved in the 0.4Gd+0.5Zr modified alloy. A high amount of Zr addition resulted in the formation of coarse Al5Si2Zr3 phase and the decrease of ductility.