Abstract
Ta sheets were calorized in a quartz ampoule by using Al vapor generated from the powdered FeAl2 alloy at 1000°C for various durations of pretreatment. The Ta-Al alloy layer formed on the surface was examined by microscopy, X-ray diffraction, and electron probe micro-analysis. It was found that the calorizing process of Ta sheets obeyed a parabolic rate law and the rate constant was about one third of that for siliconizing (formation of TaSi2 layer) under the same conditions. The alloy layer formed was mainly of TaAl3 compound and the formation of its layer proceeded through only an inward diffusion of Al, but the transient initial process was found to occur relatively rapidly to form a low Al-Ta alloy layer through the mutual diffusion of both Ta and Al atoms, which was recognized by the existence of Ta oxide particles as the markers. After a long duration of valorizing process (25hr), the surface of the alloy layer was found to be converted gradually into a low Al-Ta alloy owing to the slight decrease of Al vapor from the powdered Al-source, though the overall kinetics of the process showed no alteration.
