Properties of in-situ nitride reinforced titanium-aluminide layers formed by reactive low pressure plasma spraying with nitrogen gas

In-situ nitride dispersed TiAl layers were produced from pre-alloyed TiAl powder by a reactive low pressure plasma spraying (RLPPS) process with nitrogen as a powder carrier gas. RLPPS TiAl layers had fine grain structures whose grain sizes ranged from approximately 50 to 150 nm and included ternary nitride, Ti2AlN. The volume fraction of nitride decreased with increasing spray distance from 300 to 600 mm, whereas no significant change in the nitrogen contents of sprayed layers occurred. Nitride precipitation may be suppressed by rapid cooling in sprayed layers formed at a long spray distance. The maximum hardness of 709 Hv was obtained at room temperature in a RLPPS TiAl layer formed with an appropriate spray distance, 400 mm in the present case. However, a RLPPS TiAl layer formed with shorter spray distance, 300 mm, was superior in hardness at temperatures above 873 K because of higher volume fraction of nitride. Although the hardness values of RLPPS TiAl layers dropped to approximately 450 Hv after being exposed to a temperature of 1273 K for 24 h, the hardness value was higher by approximately 100 Hv than that of a LPPS TiAl layer formed with argon carrier gas. This is caused by effects of both the fine grain structure and nitride precipitation hardening.