中文摘要：

考察了Cr2Nb-20Ti（原子分数,下同）合金在5,10,20,100μm/s 4种定向凝固速率下的组织演变,研究结果表明：各凝固速率下合金的组织均由C15-Cr2Nb枝晶和枝晶间β-Ti构成,高温相β-Ti的形成是由Cr起到稳定化作用所致。随着凝固速率的增加,C15-Cr2Nb板条枝晶逐渐分裂,形貌由粗大板条转变为不规则多边形,同时凝固速率越大,扰动波长越小,枝晶组织越细,枝晶生长方向越偏向择优取向。实验中C15-Cr2Nb的择优生长晶面为（220）,因该晶面有较大的裸露键能,生长时处于优势而淘汰其它生长晶面。

英文摘要：

The Cr2Nb-20Ti（atomic fraction） prepared by directional solidification with the withdrawal rates of 5, 10, 20, 100 μm/s were investigated to understand the microstructure evolution of the Laves phase Cr2Nb/Ti alloy. OM, XRD, and SEM apparatus were employed to determine the solidified phases and microstructure. The results indicate that the microstructures of directionally solidified Cr2Nb-20 Ti alloy are all consisted of C15-Cr2 Nb dendrite and interdendritic β-Ti. The formation of metastable β-Ti instead of stable α-Ti is induced by the stabilization of Cr element. With the increase in withdrawal rate and perturbation at the liquid/solid interface, the C15-Cr2 Nb dendrites are refined and the dendritic morphology evolves from the petal-like pattern at 5 μm/s to the polygon-like pattern at 100 μm/s;simultaneously, the growth direction of the C15-Cr2 Nb dendrite gradually deviates from the heat flow direction and transits to the preferred growth direction. From the XRD experimental result, it shows that the C15-Cr2 Nb dendrite exhibits the（220） preferred growth plane due to the fact that the plane（220） has the maximal bare bond energy, leading to the fast growth kinetic and elimination of other growth planes.