磷(P)在Ni3Al中的溶解度很低, 凝固过程中向剩余液体中偏聚, 最终在晶界上析出富P相. 凝固后期, P在剩余液体中的偏聚程度升高, 降低形核率, 抑制试样心部等轴晶的形成, 促使试样外缘晶粒向心部延伸生长,增加柱状晶长度. P降低合金终凝温度, 增加糊状区长度, 阻碍液相补缩,增加形成疏松的倾向. P产生上述作用的根本原因是其降低固、液两相体积自由能差和原子跃迁穿过液/固界面的激活能.
Phosphorus is hardly dissolved in the solidified Ni3Al solids,and tends to be segregated in the residual liquids of Ni3Al. The phosphorus--bearing phase is precipitated at the end of solidification. The segregation of phosphorus in the residual liquids inhibits the nucleation of Ni3Al grains and the formation of the equiaxed grains at the central part of the sample, and favors the growth of the columnar grains towards the central part of the sample. The reason for phosphorus to decrease the nucleation rate of Ni3Al is that it lowers the difference of free energy between solids and liquids, and increases the activation energy for the atoms to transit through the liquid/solid interface. Phosphorus lowers the final solidification temperature of Ni3Al, increases the length of mushy zone, inhibits the feeding of casting, and hence increases the tendency of micropores formation.
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