Strain ratchetting of titanium upon reversible alloying with hydrogen

During cyclic hydrogen charging (e.g., in metal–hydride systems), internal stresses and strains can be developed due to lattice swelling and/or phase transformation (e.g., allotropic transformation or hydride precipitation). We examine macroscopic plastic deformation due to such internal stresses (strain ratctetting) in the Ti–H system, where gaseous hydrogen is alloyed with Ti, causing the Ti α–β allotropic transformation, and subsequently removed, producing the β–α transformation. Cyclic hydrogen charging is found to cause macroscopic plastic shrinkage strains in directions normal to the hydrogen concentration gradient. Furthermore, increasing the charging time leads to larger ratchetting strains. A simple adaptation of diffusion theory is used to describe the kinetics of strain evolution, and the contributions to total ratchetting from both the α–β phase transformation and the lattice swelling strains are quantified.