材料期刊网

The influence of addition of boron (B) on eutectic solidification in a near-eutectic Al-13.0%Si alloy modified with strontium (Sr) was investigated using thermal analysis and macro/microstructure observation. Addition of B in the Sr-modified alloy leads to a considerable increase in nucleation temperature (T_N, the minimum temperature prior to recalescence (T_M) and growth temperature (T_G). In the Sr-modified alloy, nucleation of eutectic might originate at the heterogeneous sites on the mold wall or in the melt near the wall, and eutectic solidification proceeds gradually towards the center, controlled by undercooling of melt. However, with addition of B in the Sr-modified alloy, undercooling required for eutectic nucleation became small, and hence eutectic solidification might occur almost simultaneously within whole casting, controlled by amount of heterogeneous sites. With excessive addition of B in the Sr-modified alloy, nucleation of eutectic grains was explosive within the whole casting and the power of Sr on eutectic solidification was completely poisoned.