冷却速率对N06625合金凝固过程微观偏析的影响

高温合金铸锭在凝固过程中各部分冷却条件不同,中心部分冷却速率缓慢。为了模拟铸锭内部的缓慢冷却速率,通过超高温激光共聚焦扫描显微镜(HT-CLSM)、钨灯丝扫描电子显微镜(SEM)、光学显微镜(OM)并结合相关计算,研究了N06625合金在不同冷却速率(10、20、30 ℃/min)下的凝固偏析和微观组织演变状况。结果表明,随着冷却速率升高,析出相的数量、尺寸和面积分数降低,且聚集程度降低,形状由大尺寸长条状和大块状转变为小条状和小块状;Laves相呈块状和条状在枝晶间析出,随着冷却速率降低,Laves相的尺寸和数量增大,聚集程度增加。枝晶干芯部Nb和Mo元素的质量分数以及Nb的有效分配系数(k_eNb])和Mo的有效分配系数(k_eMo])随冷却速率增大而增大,在所研究冷速范围内都表现出很强烈的偏析倾向,k_eNb]分别为0.46、0.55和0.62,k_eMo]分别为0.73、0.77和0.80,且k_eNb]的变化趋势更加明显。采用Clyne-Kurz偏析模型并结合试验中的数据,对Nb和Mo元素偏析程度的具体变化趋势进行定量表征,结果表明在N06625合金凝固过程中,冷却速率越低或固相分数越大,残余液相中Nb和Mo元素的质量分数上升速率越快。

Effect of cooling rate on microsegregation during solidification of N06625 alloy

In the solidification process of superalloy ingot, the cooling conditions of each part are different, and the cooling rate of the central part is slow. In order to simulate the slow cooling rate inside the ingot, the solidification segregation and microstructure evolution of N06625 alloy at three cooling rates (10, 20, 30 ℃/min) were investigated by high temperature confocal laser scanning microscopy (HT-CLSM), tungsten filament scanning electron microscopy (SEM), optical microscopy (OM) and related calculations. The results indicate that as the cooling rate increases, the number, size and the area fraction of precipitated phase decrease, the degree of aggregation decreases, and the shape of precipitated phase changes from large size long strips and large lumps to small strips and small lumps. Laves phase is precipitated between dendrites in the form of blocks and bars. With the decrease of cooling rate, the size and number of Laves phase increase, and the aggregation degree increases. The mass fraction of Nb and Mo elements at the core of dendrite stem and the effective partition coefficients of Nb(k_eNb]) and Mo(k_eMo]) increase with increasing cooling rate, Nb and Mo show strong segregation tendency in the range of cooling rate studied. k_eNb] is 0.46, 0.55 and 0.62 respectively, k_eMo] is 0.73, 0.77 and 0.80 respectively, and the change trend of k_eNb] is more obvious. The Clyne-Kurz segregation model was used to quantitatively characterize the specific change trend of Nb and Mo segregation degree in combination with the experimental data. The results show that during the solidification process of N06625 alloy, the lower the cooling rate or the larger the solid fraction, the faster the increasing rate of the mass fraction of Nb and Mo in the residual liquid phase.