Hysteresis in the Palladium—Hydrogen System

When hydrogen is dissolved in palladium, the α phase forms first; the β phase nucleates and grows in this. Because of the large volume change which accompanies the α-to-β transition, plastic deformation of the α phase occurs as it is stretched beyond its elastic limit. Hence, absorption is accompanied by growth of the β phase under compressive stress of the α phase, but desorption take place from the β phase of the plastically deformed solid. Since this is not a thermodynamically reversible process, hysteresis results. It is shown that the absorption loop of the isotherm can be calculated from the desorption loop and the yield strength of the metal. Some reinterpretation of earlier theories is presented.