Thermodynamics of hydrogen absorption and desorption in TC21 alloy

In this paper, it was addressed a hydrogen absorbing and desorbing thermodynamics in α+β type TC21 titanium alloy with high strength and toughness based on thermodynamic experiments and calculation. The relationship between concentration (C), temperature (T), and pressure (P) of TC21 alloy is shown by P–C–T curves during hydrogen absorption and desorption process, which were measured by multistep hydrogenation/dehydrogenation methods from 625 to 750 °C. The P–C–T isotherms at a given temperature were separated into three regions. The partial thermodynamic functions of hydrogen reaction were evaluated by a modified form of Sievert’s law and P–C–T relation of different regions was expressed by the modified Sievert’s law. The results show that the enthalpy of hydrogen reaction in the first and third region relies on hydrogen content. According to Vant’s Hoff law, enthalpy and entropy of hydrogenation platform in TC21 alloys are −53.58 kJ·mol⁻¹ and −127.41 J·K·mol⁻¹, respectively. Compared with P–C–T curves of hydrogen absorption, that of hydrogen desorption exists hysteresis.