采用液态金属冷却(LMC)定向凝固结合液淬法, 在温度梯度(G)为250 K/cm, 凝固速率(V)为2-400 μm/s的条件下系统研究了镍基高温合金DZ125的固/液界面形态、胞/枝晶间距及MC碳化物形态. 随着V的增大, 界面形态经历了浅胞状→深胞状→粗枝晶→细枝晶的转变. 一次胞晶间距随着V的增大而增大, 当V=5 μm/s时, 固/液界面达到胞-枝转变点, 且一次枝晶间距达到最大值. 当V≧10 μm/s时, 一次枝晶间距和二次枝晶间距随着V的增大而减小. MC碳化物生长形态随着V的增大由胞-枝转变点的八面体依次向骨架状、汉字体状、细小枝晶转变. 分析表明, 一次枝晶间距与Trivedi模型和Ma模型吻合较好, 同时, 低速下(V≦50 μm/s)一次枝晶间距与Hunt-Lu模型吻合得很好, 但高速时发生了明显的偏差. 通过非线性拟合得到一次枝晶间距和二次枝晶间距与V分别满足λ1=314.6V-0.24±0.02和λ2=97.76V-0.33±0.01. MC碳化物在凝固中后期由液相析出, 其形态主要受到固/液界面形态和V的影响.
The Ni-based superalloy DZ125 was prepared by liquid metal cooling (LMC) directional solidification and quenching technology with withdrawal rate ($V$) range of 2-400 μm/s and temperature gradient up to 250 K/cm. The morphologies of solid/liquid (S/L) interface, cellular/dendritic arm spacings and the morphologies of MC carbide were studied systematically. The shallow cellular interface arised at V=2 μm/s. With increasing the withdrawal rate, the S/L interface turns into deep cellular (V=3 μm/s) and dendritic (V≧5 μm/s) interfaces successively. The cellular spacing is increased with increasing the withdrawal rate. However, the primary dendritic arm spacing is decreased with increasing the withdrawal rate. The maximum value of cellular/dendritic spacings appears at transition from cellular to dendritic interfaces (V=5 μm/s). Meanwhile, the morphology of MC carbide changes from octahedron to frame-like, Chinese-script and finally to fine dendrite with increasing the withdrawal rate. Compared with the theoretical models of primary dendrite spacing, the results are good in agreement with Trivedi's and Ma's models. Furthermore, they are also in agreement with Hunt-Lu model only at lower withdrawal rates (V≦50 μm/s). The relationships of primary and secondary dendritic arm spacings with withdrawal rates can be described as λ1=314.6V-0.24±0.02 and λ2=97.76V-0.33±0.01, respectively. MC carbide precipitated from the melt during solidification, and its morphology is dependent both on the withdrawal rate and the morphology of S/L interface.
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