The effect of Ca and P interaction on the Al-Si eutectic in a hypoeutectic Al-Si alloy

The influence of combined additions of Ca and P on the nucleation of the Al-Si eutectic of a high purity Al-7 wt pct Si alloy was investigated. Thermal analysis and interrupted solidification experiments were undertaken to obtain characteristic temperatures of the eutectic reaction and to study the macroscopic growth mode of the Al-Si eutectic. Additions of P are known to increase the nucleation frequency of eutectic grains due to the formation of AlP particles acting as nucleation sites for eutectic silicon. It could be shown that the addition of Ca to melts containing 5 and 20 ppm P leads to a dramatic decrease of the nucleation frequency resulting in a change of the growth mode of the eutectic cells when the P:Ca weight ratio was in the range of 0.02-0.07. Al2Si2Ca intermetallics precipitated when the Ca content exceeded 100 ppm and were predominantly P-enriched. In addition, the possibility of Ca3P2 formation is discussed. It is suggested that the principal effect of Ca is poisoning of AlP particles and deactivation of residual P in the melt leading to a higher recalescence undercooling. Thereby the coarse flake-like Si was refined into fine needle-like Si morphology. Modification in terms of a definite flake-to-fibrous transition cannot be obtained with Ca addition of up to 300 ppm in a high purity alloy. (C) 2013 Elsevier B.V. All rights reserved.