包晶Nd-Fe-B合金定向凝固与形核有关的组织选择

Microstructure Selection Involving Nucleation in Directionally Solidified Nd-Fe-B Peritectic Alloys

摘要:

对Nd14Fe79B7合金进行了定向凝固试验与形核有关的凝固组织形成研究。发现凝固速率为200和500 μm/s试样中局部区域有粗大等轴Fe枝晶形成；并且在凝固速率500 μm/s的试样中存在α-Fe枝晶与Nd2Fe14B枝晶交替生长的带状组织。分析表明，当凝固速率为200 μm/s时, 定向生长的Fe枝晶界面前沿液相中的温度分布相对于γ-Fe平衡液相线是过冷的; 当凝固速率为500 μm/s时, Fe枝晶生长界面前沿相对于γ-Fe和Nd2Fe14B平衡液相线均会产生过冷。理论分析可以解释定向凝固试样中等轴晶和枝晶带状组织的形成

Abstract:

Directional solidification experiments were performed on Nd14Fe79B7 alloys to investigate microstructure formation related to nucleation. It was found that a small amount of α-Fe dendrites with equiaxed morphology existed in specimens grown at 200 and 500 μm/s. A banded structure showing alternate growth of α-Fe and Nd2Fe14B dendrites, was formed in the specimen at 500 μm/s. The formation of the equiaxed morphology microstructure or the banded structure is attributed to the phase nucleation ahead of the solidifying interface, since an undercooled zone corresponding to γ-Fe liquidus would appear ahead of the growing γ-Fe interface at the growth velocity of 200 and 500 μm/s. However, at growth velocity of 500 μm/s, only an undercooled zone corresponding to Nd2Fe14B liquidus would appear. These undercooled zones cause the nucleation of γ-Fe and Nd2Fe14B phases, resulting in the different microstructure patterns. The theoretical predictions are in good agreement with the experimental results.