Internal stress plasticity due to chemical stresses

Internal stress plasticity occurs when a small external stress biases inter nal mismatch strains produced by, e.g., phase transformation or thermal exp ansion mismatch. At small applied stresses, this deformation mechanism is c haracterized by a deformation rate which is proportional to the applied str ess and is higher than for conventional creep mechanisms. In this work. we demonstrate the operation of internal stress plasticity due to internal che mical stresses produced by chemical composition gradients. We subject speci mens of beta -phase Ti-6Al-4V to cyclic charging/discharging with hydrogen (by cyclic exposure of specimens to gaseous H-2), under a small external te nsile stress. As expected for internal stress plasticity, the average strai n rate during chemical cycles at 1030 degreesC is larger than for creep at constant composition hydrogen-free or -saturated), and a linear stress depe ndence is observed at small applied stresses. Additionally, we present an a nalytical model which couples elastic and creep deformation with a transien t diffusion problem, wherein the diffusant species induces swelling of the host lattice. Without the use of any adjustable parameters, the model accur ately predicts both the observed strain evolution during hydrogen cycling o f Ti-6Al-4V and the measured stress dependence of the deformation. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserve d.