Superplastic deformation induced by cyclic hydrogen charging

Deformation under the combined action of external stress and cyclic hydrogen charging/discharging is studied in a model material, titanium. Cyclic charging with hydrogen is carried out at 860 °C, which repeatedly triggers the transformation between hydrogen-lean α-Ti and hydrogen-rich Β-Ti. Due to bias from the externally applied tensile stress, the internal mismatch strains produced by this isothermal α-Β transformation accumulate preferentially along the loading axis. These strain increments are linearly proportional to the applied stress, i.e., flow is ideally Newtonian, at small stress levels (below ∼2 MPa). Therefore, after multiple chemical cycles, a tensile engineering strain of 100% is achieved without fracture, with an average strain rate of 10^-5 s^-1, which demonstrates for the first time that superplastic elongations can be achieved by chemical cycling. The effect of hydrogen partial pressure, cycle time, and external stress on the value of the superplastic strain increments is experimentally measured and discussed in light of a diffusional phase transformation model. Special attention is paid to understanding the two contributions to the internal mismatch strains from the phase transformation and lattice swelling.