【摘要】 设计了不同厚度台阶铸板以实现冷却速率梯度,采用离心熔模铸造制备了不同冷却速率的β凝固含Bγ-TiAl合金样品,研究了冷却速率对硼化物和室温拉伸性能的影响。结果表明,硼化物分布在晶界,其长径比随冷却速率的提高而增大,而形貌由短棒状转变为丝带状。慢冷样品中短棒状TiB为B27结构,而快冷样品中丝带状TiB为B_f结构。2种结构TiB均存在生长各向异性,[010]和[100]分别为B_f和B27结构的最慢生长方向,前者更为显著,这可能与上述方向Ti、B原子周期性间隔排列,原子短程重排更为困难有关。随着冷却速率提高,材料屈服强度提高,但室温塑性下降,这与快冷样品中的细长丝带状硼化物容易萌生裂纹并迅速扩展有关;慢冷样品中的短棒状硼化物不易萌生裂纹,相应室温塑性较好。更多还原

【Abstract】 solidifying γ-TiAl alloys have attracted much attention for their higher specific strength and better mechanical properties at elevated temperature. They usually need some boron addition to refine the lamellar grain size, which is believed to improve their poor room temperature ductility. However,the boron addition may cause some side effects on mechanical properties for the formation of borides with unfavorable morphology and crystal structure, which is severely influenced by the alloy composition and cooling rate during casting. The components of γ-TiAl applied usually have complex structure, such as different thicknesses, which leads to different cooling rates and therefore different microstructures and mechanical properties. To evaluate the influence of cooling rate on the microstructure and mechanical properties of γ-TiAl investment casting, plate with step thicknesses was designed to achieve different cooling rates. Step plates of β-solidifying boron-containing TiAl alloy were fabricated by centrifugal casting in Y2 O3 facing coating ceramic moulds. It was found that boride mainly distributed on grain boundary, and its aspect ratio increased with increasing cooling rate, with its morphology varying from short, flat plate to long, curvy ribbon. The short plate and curvy ribbon borides were Ti B with B27 and Bfstructure, respectively. Both types of boride exhibit anisotropic growth characteristics(especially for Bfstructure), with the slowest growth rate along [100] and [010] for B27 structure and Bfstructure, respectively. This is attributed to the difficulty of atomic rearrangement along corresponding directions during solidification. The cooling rate increase caused the increase of yield strength but the decrease of room temperature ductility, the former results from the decreasing of grain size and lamellar spacing, while the latter results from the easy cracking nucleation and propagation of the long curvy boride, leaving smooth curvy surfaces on the fracture surface. Samples containing short flat plate boride showed better ductility, and no smooth curvy surface was observed.更多还原