The sulfidation and oxidation behavior of sputter-deposited Al-Ta alloys at high temperatures

Abstract

As a part of a systematic study to elucidate oxidation and sulfidation resistance of Al-refractory metal alloys at high temperatures, the behavior of sputter-deposited Al-(33–80) at.%Ta alloys has been examined at temperatures ranging from 1073 K to 1273 K in He-S 2 atmosphere and in Ar-O 2 atmosphere. The sulfidation kinetics of these alloys follow a parabolic rate law in an early sulfidation stage, although, in some cases, the sulfidation rates are decreased after prolonged sulfidation. The sulfidation resistance of these alloys is comparable to that of high purity tantalum and remarkably higher than those of typical high temperature alloys. The sulfide scales on these alloys comprise an outer aluminum-rich layer and an inner tantalum-rich layer. The formation of a protective inner tantalum sulfide layer is responsible for the excellent resistance to high temperature sulfidation. The oxidation kinetics of the Al-Ta alloys change with alloy composition and temperature. The oxidation of Al-33Ta initially follows a parabolic rate law, but after a particular period of oxidation, rapid oxidation is observed at high temperatures above 1173 K. In contrast, the oxidation rates of higher tantalum alloys decrease with oxidation time, although the oxidation rates in the early stage are higher than those of Al-33Ta. At the temperatures below 1123 K a rapid weight loss during oxidation was observed for the Al-Ta alloys. This seems to result from disintegration of these alloys due to the pest phenomenon.