High-temperature mechanical properties of hot-pressed TiN with fine grain size

Abstract

Mechanical properties of fine-grained TiN were studied by compression tests at temperatures ranging from 1073–1823 K and strain rates from 2×10-5 to 5×10-3 s-1. The temperature dependence of maximum (fracture or peak) stress of TiN reveals three regions with different activation energy and strain-rate sensitivity. At lower temperatures, brittle fracture takes place without plastic deformation. Fracture stress is independent of temperature, and greater than 1 Gpa. In the intermediate temperature region, specimens fracture in a quasi-brittle manner after a few per cent plastic deformation. Fracture stress decreases above 1300 K (in the intermediate temperature region), due to the deformation-assisted fracture. TiN becomes fully ductile at further higher temperatures, at which five independent slip systems are available. This ductile to brittle transition characteristic of TiN is similar to MgO (ionic) but different from TiC (covalent), though all three materials take the same NaCl type of lattice structure. In the high-temperature region, the activation energy for plastic deformation is close to that for diffusion of nitrogen in TiN. Strain-rate sensitivity in this region is typical of superplasticity, suggesting the possibility of superplastic forming. © 1998 Chapman and Hall