Effect mechanisms of micro-alloying element La on microstructure and mechanical properties of hypoeutectic Al-Si alloys

Abstract

The rare earth influence on the as-cast microstructures and mechanical properties of aluminum alloys attracted great attentions in the last decades. But up to date no reports can be found on the effect of micro-alloying element La (La addition is below 0.1 wt.%) on the solidification of hypoeutectic Al-Si alloys. This study carried out solidification experiments with Al-6Si alloys micro-alloyed by element La. The α-Al grain refinement, the eutectic Si modification and the tensile properties improvement caused by micro-alloying element La were investigated. The effect mechanisms of La were discussed. It is demonstrated that the addition of La as low as 100 ppm can deprave the more effective heterogeneous nucleation conditions for the eutectic Si caused by the impurity P. The addition of 0.06 wt.% La is sufficient to achieve an ideal α-Al grain refinement, eutectic Si modification and ductility improvement of the alloys. LaAlSi phase forms in the Al-Si alloy with the additive amount of La higher than 0.06 wt.%. It has a tetragonal structure. Micro-alloying element La refines the α-Al grains by working as a surfactant and modifies the eutectic Si by promoting the formation of the significant multiple Si twins.