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 T iAl powder by a reactive low pressure plasma spraying (RLPPS) process with nitrogen as a powder carrier gas. RLPPS TiAl layers had fine grai n structures whose grain sizes ranged from approximately 50 to 150 nm and included ternary nitride, Ti2AlN. The volume fraction of nitride d ecreased with increasing spray distance from 300 to 600 mm, whereas no significant change in the nitrogen contents of sprayed layers occurre d. Nitride precipitation may be suppressed by rapid cooling in sprayed layers formed at a long spray distance. The maximum hardness of 709 H v was obtained at room temperature in a RLPPS TiAl layer formed with a n appropriate spray distance, 400 mm in the present case. However, a R LPPS TiAl layer formed with shorter spray distance, 300 mm, was superi or in hardness at temperatures above 873 K because of higher volume fr action of nitride. Although the hardness values of RLPPS TiAl layers d ropped to approximately 450 Hv after being exposed to a temperature of 1273 K for 24 h, the hardness value was higher by approximately 100 H v than that of a LPPS TiAl layer formed with argon carrier gas. This i s caused by effects of both the fine grain structure and nitride preci pitation hardening.