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.
Highlights
Dwell sensitivity at 20◦C is well known in titanium alloys and mostly associated with Al-alloyed hexagonal close-packed (HCP) α-Ti or near-α alloys [1]
The results show the role of load shedding which causes the stresses at the boundary between the soft and hard grains to increase significantly during the load dwell; figure 12 shows the yy stresses along path A-A’ at the points for both stress dwell start and its end, shown in figure 11a
The temperature sensitivity of load shedding and cold dwell has been investigated in model alloy Ti-6Al, and it is shown that load shedding is crucially dependent on temperature from 20◦C up to rspa.royalsocietypublishing.org Proc
Summary
Dwell sensitivity at 20◦C is well known in titanium alloys and mostly associated with Al-alloyed hexagonal close-packed (HCP) α-Ti or near-α alloys [1]. Much work has been carried out in order to understand the mechanistic basis for the dwell debit [2,3] This has largely focused on the role of micro-texture, and the combination of very particular crystallographic orientation combinations, operating synergistically with the known creep response of Ti alloys, which occurs even at low temperatures. The mechanistic basis for the diminution of the cold dwell effect on the dwell debit as temperature increases up to about 200◦C has remained elusive, but continues to be of crucial importance in aero-engine applications where the same temperature dependence of facet nucleation has been observed (D Rugg 2014, personal communication). The response of oligocrystals under cyclic loading over the range of temperature is examined in order to quantify the stress relaxation and ratcheting behaviours expected to result
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