Microstructure and high temperature deformation behavior of a TiN/Ti5Si3 nano-grain composite produced by non-equilibrium PM process

Microstructure and high temperature deformation behavior of Ti-Si ₃N₄ mechanically alloyed (MA) powder compacts were investigated. Powders of the elements Ti and Si₃N4 whose composition was Ti-20 mass% Si₃N₄ were blended for MA. A planetary ball mill was used for milling under an Ar gas atmosphere. The MA powder milled for 720 ks was heated at various temperatures. The MA powder was consolidated by vacuum hot pressing (VHP) at 200 MPa for 10.8 ks at 803 K. The specimens were provided for compression tests at 913 K-1073 K at various initial strain rates. The MA process of Ti and Si₃N₄ powders for 720 ks resulted in the formation of an amorphous and α-Ti phases. These phases changed to TiN, Ti₂N and Ti₅Si₃ phases after the heat treatment at elevated temperatures. A (TiN+Ti₅Si₃) ultrafine microduplex structure was obtained after the heat treatment at 1473 K for 3.6 ks. The compression tests revealed that the 803 K-VHP specimen with non-equilibrium phases show the lowest flow stress at 993 K at initial strain rate of 4.2×10^-4 s^-1 in the three VHP specimens. Furthermore, the 803 K VHP specimen indicated lower flow stress at 993 K at an initial strain rate of 4.2×10^-4 s^-1 rather than that at 2.1×10^-4 s^-1. Such a reverse of the flow stress between two different initial strain rates was attributed to the phase transformation during the deformation. The slower strain rate test produced larger amount of harder phases such as TiN, Ti₂N and Ti₅Si₃. The specimen compressed to 25% (ε=0.28) at 993 K at an initial strain rate of 4.2×10^-4 s^-1 consisted of an (α-Ti+Ti ₂N+Ti₅Si₃) microduplex structure with an average grain size of approximately 40 nm. Therefore, there exists an appropriate condition for a low temperature and high strain rate forming process. A (TiN+Ti₅Si₃) microduplex structure with an average grain size of approximately 250 nm was also obtained in the specimen compressed to 25% after annealing at 1473 K for 3.6 ks