Effect of temperature on hydrogen absorption characteristic and microstructural evolution of TC21 alloy

Abstract

TC21 alloy was hydrogenated at different temperatures, and its hydrogen absorption characteristic, hydrogen content, hydrogen absorption kinetics, microstructural evolution and mechanism of hydride formation were investigated. Results show that under the same initial hydrogen pressure, hydrogen absorption rate of TC21 alloy is very slow and hydrogen content of the alloy is close to zero when hydrogenation temperature is lower than 500 °C. The initial hydrogen absorption rate achieves the maximum value at 600 °C. Hydrogen content increases first, then increases up to a maximum at 600 °C and decreases last with the increase of temperature. Hydrogen absorption reactions reach equilibrium faster when temperature increases. Hydrogen absorption reaction occurred during hydrogen absorption process of TC21 alloy is a complicated process which contains different stages according to hydrogen absorption kinetics. Hydrogenation temperature has a significant effect on microstructural evolution of TC21 alloy. With the addition of 0.68 wt.%H in the alloy, β transus temperature drops below 750 °C but still higher than 650 °C. HCP α2 phase and FCC δ hydride appear in the specimens hydrogenated at 550 °C and 650 °C according to XRD and TEM analyses. The morphologies of δ hydrides in the alloy hydrogenated at different temperatures are not identical, acicular δ hydrides exist in the alloy hydrogenated at 550 °C, while large amount of lamellar δ hydrides exist in the alloy hydrogenated at 650 °C. Furthermore, the formation mechanism of δ hydride is discussed, results reveal that δ hydrides precipitate directly in β phase at high temperature, and the most appropriate temperature for the precipitation of δ hydrides in β phase is 600 °C.