亚共晶Ni-Si合金的凝固组织特征研究∗

摘要

In this paper the Ni-Ni3 Si hypoeutectic in situ composites are prepared with bridgman directional solid-ification technique.Microstructure characteristic of Ni-Ni3 Si hypoeutectic in situ composites are systematically investigated.The alloy with hypoeutectic composition can totally transform to eutectic form under the solidifica-tion rate R=3μm/s.When the solidification rate gets to 8μm/s,shallow cellular microstructure can be formed beside the Ni3 Si phase.The cellular microstructure was more obvious when the solidification rate was 25μm/s. When the solidification rate reached 40μm/s,classic dendritic microstructure was observed due to the increase of the constitutional supercooling in front of the solid/liquid interface.The supercooling was evaluated according to the supercooling criterion,and the theory calculation was consistent with the experimental results well.Mo-reover,the interface growth temperature of theα-Ni phase and eutectic phase are calculated in terms of BH model.It shows that the entirely coupled eutectics could not be obtained.The theoretical calculation results were consistent with the experiment results.With the increase of the solidification rate,the primary dendrite arm spacing was decreased and the microstructure was refined.%采用布里奇曼定向凝固技术制备了 Ni-Ni3 Si亚共晶复合材料，系统地研究了Ni-Ni3 Si亚共晶的定向凝固组织特征。在较低的凝固速率R＝3μm/s时亚共晶成分的合金为规则的层片共晶组织。随着凝固速率的增大，当R＝8μm/s 时，平界面失稳，在第二相的旁边出现浅胞状组织。当R＝25μm/s 时在析出相的旁边出现了突起的胞状组织。当R＝40μm/s 时由于固液界面前沿的成分过冷逐渐增大，凝固组织生长成为典型的树枝晶组织。并根据“成分过冷”判据，评估了固液界面前沿的“成分过冷”的大小，理论计算与实验结果基本吻合。此外，根据 BH 模型计算和比较了α-Ni相的界面生长温度和共晶界面生长温度，证明较高速定向凝固下不太可能制备出全耦合生长的共晶组织。随凝固速率的增大，一次枝晶间距减小，组织细化。