Phase transformation in the Niobium Hydrogen system: Effects of elasto-plastic deformations on phase stability predicted by a thermodynamic model

Constraint conditions and elasto-plastic deformation alter phase stabilities of metal-hydrogen systems. Experiments on niobium-hydrogen thin films reveal, that elastically deforming films suppress hydride formation at 300 K, while elasto-plastically deforming films can form a hydride phase. Building upon a thermodynamic model studying the coupling of elastic deformations, constraint conditions and phase separation, elasto-plastic deformations are incorporated to investigate hydride formation. The stress state for each constraint condition for both elastically and elasto-plastically deforming Nb is specified. The monotony of the resulting chemical potential reveals hydride formation to be possible in elasto-plastically deforming niobium-hydrogen films, while it is suppressed by large stresses in elastically deforming films. Critical temperatures of hydride formation and miscibility gaps for both elastically and plastically deforming niobium are computed. The critical temperature is way below 300 K in elastically deforming films, while it is close to that of a stress free system in strongly elasto-plastically deforming films.