Abstract
Oxidation behaviour of vanadium-aluminum alloys containing 5 and 10 at% aluminum has been investigated at temperatures ranging from 800 to 993 K in oxygen pressures of 1.33 and 133 Pa by means of a thermo-electrobalance, an X-ray diffractometer, an electron probe microanalyzer, a micro-hardness tester as well as metallographic techniques. Whereas mass gain due to oxidation initially followed the parabolic rate law, it then turned into linear kinetics, resulting in paralinear-like behaviour in most cases. The improvement of oxidation resistance of vanadium due to alloying aluminum became higher with decreasing temperature, while the effects of alloy composition and oxygen pressure were nearly negligible. The alloys of both compositions exhibited similar oxidation kinetics and scale structures to each other. The typical oxide scale was composed of three layers: an outer layer of nearly pure VO2, an intermediate VO2+Al2O3 layer and an inner V2O3 layer. The amount of oxygen dissolved into substrates of these alloys was reduced at temperatures up to 897 K, while it increased at 993 K compared with that for pure vanadium. This decrease in oxygen dissolution can be ascribed to the decrease in oxygen diffusivity due to addition of aluminum, whilst the increase in oxygen dissolution is attributable to internal oxidation of aluminum. The activity of aluminum in vanadium-aluminum alloys was found to be remarkably low, which affected the oxidation behaviour of these alloys.
