The Influence of Pre-oxidation in a Silica-powder Pack on the Cyclic Oxidation Behavior of High Nb-TiAl Alloys

The lightweight TiAl alloys exhibit a large number of outstanding properties at high temperature. However, the actual service of TiAl alloy has been hindered by their poor oxidation resistance above 700 degrees C. Nb addition is benefit to the oxidation resistance of TiAl, while pre-oxidation under low oxygen pressure at high temperature is an effective measure for improving the oxidation resistance of TiAl alloys. Considering the differences of oxidation behavior between high Nb-TiAl and TiAl alloys, the influence of pre-oxidation on the cyclic oxidation behaviors of high Nb-TiAl alloys was investigated in air at 900 degrees C. The pre-oxidation was performed by heating the specimens, buried in silica powder and encapsulated in a silica tube under a vacuum of 1. 3 X 10(-3)Pa, at 900-1050 degrees C for 16 hours. The scales formed by pre-oxidation were very adherent and protective. After cyclic oxidation for 200 cycles (200 hours), the mass gain of pre-oxidized alloy was about 10%-40% of that of untreated alloy. The least mass gain had been got at the 1050 degrees C. This excellent oxidation resistance is attributed to the formation of dense Al2O3-rich layer and the extremely dense rutile layer.