Internal Stress Plasticity Due to Chemical Stresses

Internal stress plasticity occurs when a small external stress biases internal mismatch strains produced by, e.g., phase transformation or thermal expansion mismatch. At small applied stresses, this deformation mechanism is characterized by a deformation rate which is proportional to the applied stress and is higher than for conventional creep mechanisms. In this work, we demonstrate the operation of internal stress plasticity due to internal chemical stresses produced by chemical composition gradients. We subject specimens of β-phase Ti-6Al-4V to cyclic charging/discharging with hydrogen (by cyclic exposure of specimens to gaseous H2), under a small external tensile stress. As expected for internal stress plasticity, the average strain rate during chemical cycles at 1030°C is larger than for creep at constant composition (hydrogen-free or -saturated), and a linear stress dependence is observed at small applied stresses. Additionally, we present an analytical model which couples elastic and creep deformation with a transient diffusion problem, wherein the diffusant species induces swelling of the host lattice. Without the use of any adjustable parameters, the model accurately predicts both the observed strain evolution during hydrogen cycling of Ti-6Al-4V and the measured stress dependence of the deformation.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.