On rate-dependent polycrystal deformation: the temperature sensitivity of cold dwell fatigue

Abstract

A temperature and rate-dependent crystal plasticity framework has been used to examine the temperature sensitivity of stress relaxation, creep and load shedding in model Ti-6Al polycrystal behaviour under dwell fatigue conditions. A temperature close to 120°C is found to lead to the strongest stress redistribution and load shedding, resulting from the coupling between crystallographic slip rate and slip system dislocation hardening. For temperatures in excess of about 230°C, grain-level load shedding from soft to hard grains diminishes because of the more rapid stress relaxation, leading ultimately to the diminution of the load shedding and hence, it is argued, the elimination of the dwell debit. Under conditions of cyclic stress dwell, at temperatures between 20°C and 230°C for which load shedding occurs, the rate-dependent accumulation of local slip by ratcheting is shown to lead to the progressive cycle-by-cycle redistribution of stress from soft to hard grains. This phenomenon is termed cyclic load shedding since it also depends on the material's creep response, but develops over and above the well-known dwell load shedding, thus providing an additional rationale for the incubation of facet nucleation.

Cold Creep of Titanium: Analysis of Stress Relaxation Using Synchrotron Diffraction and Crystal Plasticity Simulations

“…where the elastic modulus along c-axis of the -titanium unit cell is about 1.4 times the value of that along a-axis [28,29,34]. Applying the same method to the transverse lattice strain, the transverse stiffness was calculated (Figure 9d).…”

The effect of the beta phase on the micromechanical response of dual-phase titanium alloys

“…Previous work has assessed several key factors in dwell, including microstructure morphology [3], temperature [42] and material hydrogen content [43]. In relation to the α−β phase, Venkataramani et al [44] implemented a CPFE model, with a homogenised α−β phase [7], to study the effect of various microstructural features on creep-induced load shedding in Ti-6242.…”

The role of molybdenum in suppressing cold dwell fatigue in titanium alloys

“…Zhang et al [12] developed an empirically based crystal plasticity model that reproduces this temperature dependence of CDF. The model assumes dislocations become pinned and released by some unspecified thermally activated process with an activation energy H. Their model incorporates experimentally determined temperature dependencies of the critical resolved shear stresses on the prismatic, basal and pyramidal planes, and of the anisotropic elastic constants.…”