Boron is a unique and popular grain refiner element in cast titanium aluminide (TiAl) alloys, as it helps to improve mechanical properties if properly alloyed. However, the formation mechanism of different types of borides in cast TiAl alloys is not yet clearly understood. This study seeks to correlate the chemical composition and cooling rate during solidification of cast TiAl alloys, with the type of boride precipitated and the resulting microstructure. Several β-solidifying γ-TiAl alloys of the TNM family were cast, alloying boron to a starting Ti-44.5Al-4Nb-1Mo-0.1B (at.%) alloy. The alloys were manufactured with an induction skull melting furnace and poured into a stepped 2, 4, 8 and 16 mm thickness mold to achieve different cooling rates. On one hand, the results reveal that boron contents below 0.5 at.% and cooling rates during solidification above 10 K/s promote the formation of detrimental ribbon borides. On the other hand, boron contents above 0.5 at.% and cooling rates during solidification below 10 K/s promote the formation of a refined microstructure with blocky borides. Finally, the formation mechanisms of both ribbon and blocky borides are proposed.

中文翻译：

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硼含量和冷却速率对β-凝固γ-TiAlTNM合金组织和硼化物形成的影响

硼是铸造铝化钛（TiAl）合金中一种独特且流行的晶粒细化元素，因为如果适当地合金化，它有助于改善机械性能。但是，尚不清楚铸造的TiAl合金中不同类型的硼化物的形成机理。这项研究试图将铸造TiAl合金凝固过程中的化学成分和冷却速率与所析出的硼化物类型和所产生的微观结构联系起来。铸造了几种TNM系列的β凝固γ-TiAl合金，将硼合金化为初始Ti-44.5Al-4Nb-1Mo-0.1B（原子％）合金。这些合金是用感应颅骨熔化炉制造的，并倒入2、4、8和16毫米厚的阶梯状模具中以实现不同的冷却速度。一方面，结果表明硼含量低于0.5at。％和凝固期间高于10 K / s的冷却速率促进有害的带状硼化物的形成。另一方面，硼含量高于0.5 at。％且凝固过程中的冷却速率低于10 K / s会促进形成具有块状硼化物的细化微结构。最后，提出了带状和块状硼化物的形成机理。