Influence of interstitial carbon on the formation of monohydride and dihydride of Ti25V35Cr40 alloys

The presence of interstitial carbon atoms was found to affect the hydrogenation properties of Ti25V35Cr40Cx (x = 0, 0.1, and 0.5) alloys over the range T = 30-300 °C. Heat treatment at T = 1200 °C was used to homogenize the distribution of carbon atoms throughout the specimens. X-ray diffraction, P-C isotherm curves, and thermodynamic measurements, including van't Hoff plots and in-situ temperature-programmed desorption spectra, were used to study the destabilization of the β-phase monohydride and γ-phase dihydride in annealed Ti25V35Cr40Cx as a result of the presence of interstitial carbon atoms. The experimental results showed that the interstitial carbon atoms introduced both obstacles and carbon-induced microstrain into the structure, which significantly destabilized the γ-phase dihydrides to enhance the corresponding plateau pressures at T = 30-80 °C. On the other hand, destabilization of the β-phase monohydride was less pronounced in the presence of interstitial C atoms at T = 150-300 °C. This effect was ascribed to a lower H content in the saturated β-phase monohydrides as compared with the saturated γ-phase dihydrides, as well as microstrain relaxation at higher temperatures.