Transformation superplasticity of titanium by reversible hydrogen cycling

Commercially-pure titanium was reversibly alloyed and dealloyed with hydrogen at 860°C, thus repeatedly triggering the transformation between hydrogen-free α-Ti and hydrogen-alloyed β-Ti. Under an externally applied tensile stress, the internal mismatch stresses produced by the α-β transformation are biased, resulting in a strain increment accumulated after each chemical cycle in the direction of the applied stress. The use of low partial pressures of hydrogen eliminates any additional effect due to swelling mismatch in β-Ti. These strain increments are linearly proportional to the applied stress at small stress levels (σ < ∼2 MPa), as previously reported for transformation superplasticity achieved by thermal cycling of hydrogen-free α-Ti. A tensile strain of 100% is achieved without fracture upon long-term cycling, with an average strain rate of 1 × 10^-5 s^-1. The present experimental study investigates systematically the effect of hydrogen partial pressure, cycle time, and external stress upon the value of the superplastic strain increments, as well as the concurrent contribution of creep as a deformation mechanism.