Internal-stress plasticity in titanium by cyclic alloying/dealloying with hydrogen

Internal-stress plasticity is a deformation mechanism where an externally applied stress biases internal mismatch stresses. We report here a novel method to induce internal-stress plasticity in titanium: a reversible change of chemical composition (at constant temperature) is produced by cyclic alloying/dealloying with hydrogen, rather than by thermal cycling (at constant composition) which induces the well-known phenomenon of transformation-mismatch plasticity in titanium. We demonstrate that chemical cycling with hydrogen, which induces internal stresses by both swelling mismatch and transformation mismatch, results in large, reproducible strain increments in the direction of the applied stress. We systematically explore different processing variables (applied stress, temperature, as well as hydrogen concentration, cycling rate and flow rate) and discuss our results in the light of previous studies on internal-stress plasticity induced by thermal cycling.