OXIDATION RESISTANCE OF TIAL SIGNIFICANTLY IMPROVED BY COMBINATION OFPREOXIDATION AND HF ADDITION

The cyclic-oxidation resistance of a TiAl coupon in a flow of purified oxygen at 1300 K has been significantly improved by preoxidation at 1 200 K for 100 ks in a pack of a mixture of 70 mass% chromia and 30 mas s%, metallic chromium powders under an Ar atmosphere. The excellent ox idation resistance obtained is attributable to the formation of alumin a scales by the preoxidation, where oxygen partial pressure was though t to be maintained very low. The addition of 0.24 mass% Hf to TiAl imp roved the oxidation resistance by forming scales rich in alumina. A co mbination of the preoxidation and the Hf addition further improved the oxidation resistance. After 10 cycles (200 h) of oxidation at 1300 K the scale thickness was still less than 1 mu m.