铸造TiAl合金表面细晶层的组织形貌及其形成

Microstructural Morphologies and Formation of Surface Fine-Grain Layer of Cast TiAl Alloy

采用三种具有不同蓄热系数和热导率的铸型制备具有不同表面细晶层组织形貌的TiAl合金试样,并结合凝固理论分析组织形貌及其形成。结果发现,采用冷却速率更大的石墨和铸钢铸型,因快速冷却导致非平衡凝固,在表面细晶层形成初生γ相,而冷却能力更小的陶瓷型试样则无初生γ相;同时,石墨型和铸钢型试样表面细晶层厚度、覆盖率、层片团和层片间距比陶瓷型试样小;但由于铸型冷却速率不同,只有石墨型和陶瓷型迎流面和背流面表面细晶层形貌相同,而铸钢型试样迎流面和背流面形貌不同。铸型的蓄热系数和热导率是影响铸造TiAl合金表面细晶层组织形貌的主要因素。

The TiAl specimens with surface fine-grain layers exhibiting different microstructural morphologies were prepared by using of three different cast moulds with different heat storage coefficient and thermal conductivity coefficient, and the microstructural morphologies and the formation of surface fine-grain layers were analyzed. The results indicate that there are primary gamma phases in the surface fine-grain layer of the specimens prepared in graphite mould and cast steel mould because of non-equilibrium solidification at a higher cooling rate during casting, but there is no primary gamma phase in that of the ceramic mould specimen. And the thickness, coverage ratio, colony size and the lamellar spacing of the surface fine-grain layer of the graphite mould and cast steel mould specimens are much smaller than those of the ceramic mould specimen. Due to the cooling rate difference of different cast moulds, the microstructural morphologies of the upstream surface and the lee surface of the graphite mould and ceramic mould specimens are similar, but they are different in that of the cast steel mould specimen. The heat storage coefficient and thermal conductivity coefficient are the main factors which influence the microstructural morphologies of the surface fine-grain layer in cast TiAl alloys