高温度梯度定向凝固冷却速率对DZ4125合金γ′相的影响

EFFECT OF COOLING RATE ON γ' PRECIPITATE OF DZ4125 ALLOY UNDER HIGH THERMAL GRADIENT DIRECTIONAL SOLIDIFICATION

研究了高温度梯度定向凝固条件下冷却速率对DZ4125合金中γ′相形态、分布和尺寸的影响.结果发现,随着冷却速率的增大,合金的枝晶逐渐细化.当冷却速率达到36.4 K/s时,合金的凝固组织由粗枝晶变为超细枝晶.在此过程中,γ′相从立方形逐渐球形化,且枝晶干γ′相的球化速度比枝晶间的γ′相球化速度快.同时,枝晶干和枝晶间γ′相尺寸逐渐减小,枝晶干和枝晶间γ′相的均匀化程度增加.枝晶间γ′相比枝晶干γ′相的尺寸大,这种差别随冷却速率的增大而减小.造成γ′相在形貌和尺寸上变化的根本原因是冷却速率的变化引起γ固溶体中溶质的过饱和度ΔX,过饱和γ固溶体的过冷度ΔT,γ′相脱溶析出的临界形核功ΔG~＊和溶质在γ固溶体中扩散系数D的改变.

The mechanical properties of Ni-based superalloy are mainly determined by quantity, shape, size and distribution of γ′ precipitates. For a given alloy, the quantity, shape, size and distribution of γ′ precipitates are mostly affected by solidification parameters during solidification process. In present, the influence of solidification parameters on γ′ precipitates has been extensively reported. But unfortunately, these researches on γ′ precipitates under as cast condition are mostly intuitive description of solidification phenomena and the in-depth research on the influence of solidification parameters on the morphology, distribution and size of γ′ precipitates is little, especially under high thermal gradient directional solidification. In this paper, the influence of cooling rates on the morphology, size and distribution of γ′ precipitates in dendrite core and interdendritic areas of directionally solidified nickelbased superalloy DZ4125 was investigated under high thermal gradient about 500 K/cm. The cooling rates used in this experiment were 2.525, 5.15, 13.17, 25.875 and 36.4 K/s, respectively. The relative mechanisms of changes in the morphology, distribution and size of γ′ precipitates were discussed. These results show that, with cooling rate increasing, the dendrite microstructure of DZ4125 alloy becomes fine. At the solidification rate of 36.4 K/s, the dendrite morphology changes to a superfine dendrite solidification mode. In this process, the morphology of γ′ precipitates changes from cubic to spheric shape and the sphericized speed of γ′ precipitates in dendrite core quicker than in interdendritic. Furthermore, the average size of γ′ precipitates decreases gradually so that they become well-distributed in both dendrite core and interdendritic. This average size of γ′ precipitates in dendrite core is smaller than in interdendritic, however the difference in size decreases with cooling rate increasing. These changes in the morphology, distribution and size of γ′ precipitates result mainly from the degree of supersaturation of solute in γ solid solution AX, degree of undercooling in γsolid solution ΔT, critical precipitation nucleation work of γ′ precipitate ΔG^＊ and solution diffusion coefficient in γ solid solution D caused by altering cooling rate.