Formation of a TiN/Ti 5Si 3 nano-grain composite by a non-equilibrium PM process

Abstract

In the present paper, the authors have proposed a non-equilibrium powder metallurgy (PM) process which enables improved formability and the production of a (TiN+Ti 5Si 3) composite. The microstructure and high-temperature deformation behavior of Ti–Si 3N 4 mechanically alloyed (MA) powder compacts were investigated. Powders of the elements Ti and Si 3N 4 of composition Ti–20 mass%Si 3N 4 were blended for MA. The MA process for 720 ks resulted in the formation of amorphous and α-Ti phases. These phases changed to TiN, Ti 2N and Ti 5Si 3 phases after heat treatment at elevated temperatures. A (TiN+Ti 5Si 3) ultra-fine microduplex structure was obtained after heat treatment at 1473 K for 3.6 ks. The amorphous and α-Ti phases were stable even after vacuum hot pressed (VHP) at 803 K, while they disappeared and α-Ti, Ti 2N and Ti 5Si 3 phases appeared in 813 and 823 K VHP specimens. Compression tests revealed that the 803 K VHP specimen with the non-equilibrium phases had the lowest flow stress at 993 K and an initial strain rate of 4.2×10 −4 s −1 in these three VHP specimens. The slower strain rate test produced a larger amount of harder phases such as TiN, Ti 2N and Ti 5Si 3. Therefore, there exists an appropriate condition for a low temperature and high strain rate forming process. A (TiN+Ti 5Si 3) microduplex structure was also obtained in the specimen compressed to 25% after annealing at 1473 K for 3.6 ks.