Effect of Pr on the microstructure and hardness of hypereutectic Al-25%Si alloy

The aluminum-silicon alloy (Si: 25wt.%) is modified by the Pr modifier. The results show that Pr has a remarkable refining role on primary silicon in aluminum-silicon alloy (Si: 25wt.%). When the additional amount of Pr is 0.6%, the grain size is refined from 180-220 μm without modification to 60-80 μm after modification. The shape of primary silicon has changed from coarse five-petal star-shaped and other complex shapes to fine polyhedra and plate-like, while the size of long needle-shaped eutectic silicon decreases and the distribution density remarkably decreases. The X-ray diffraction analysis results indicate that no new phases are generated in the modified alloy. The scanning electron microscopy analysis results show that after Pr modification, a white trapezoid structure AlSixPry ternary compound appears in the aluminum-silicon alloy (Si: 25wt.%) sample. The hardness testing results show that the Pr modification additional amount is 0.6%, and the hardness value of aluminum-silicon alloy (Si: 25wt.%) has been improved by about 32.5% from 56.3 HB without modification to 74.6 HB after modification.