Correlation of microstructure with high-temperature hardness of (TiC,TiN)/Ti-6A1-4V surface composites fabricated by high-energy electron-beam irradiation

Correlation of microstructure with high-temperature hardness of (TiC,TiN)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. TiC, TiN and TiC+TiN powder mixtures containing 50% CaF2 flux were deposited on the surface of a Ti-6Al-4V alloy substrate, and irradiated by high-energy electron beam to form 1-mm-thick, defect-free surface composite layers. The surface composite layers contained a large amount (over 30 vol.%) of precipitates such as TiC, TiN, (TixAl1-x)N and Ti(CxN1-x) in the martensitic or N-rich acicular alpha-Ti matrix. This microstructural modification including the formation of hard precipitates and hardened matrices in the surface composite layers improved hardness and high-temperature hardness two to four times greater than that of the substrate. In particular, the surface composite fabricated with TiN powders had the highest hardness because of the highest volume fraction of TiN and (TixAl1-x,)N distributed in the hardened N-rich acicular alpha-Ti matrix. These findings suggest that the (TiC,TiN)/Ti-6Al-4V surface composites can be used for structural materials requiring excellent thermal resistance. (C) 2004 Elsevier B.V. All rights reserved.