Abstract
It is a well known and important problem in the aircraft engine industry that alloy Ti-6242 shows a significant reduction in fatigue life, termed dwell debit, if a stress dwell is included in the fatigue cycle, whereas Ti-6246 does not; the mechanistic explanation for the differing dwell debit of these alloys has remained elusive for decades. In this work, crystal plasticity modelling has been utilised to extract the thermal activation energies for pinned dislocation escape for both Ti alloys based on independent experimental data. This then allows the markedly different cold creep responses of the two alloys to be captured accurately and demonstrates why the observed near-identical rate sensitivity under non-dwell loading is entirely consistent with the dwell behaviour. The activation energies determined are then utilised within a recently developed thermally-activated discrete dislocation plasticity model to predict the strain rate sensitivities of the two alloys associated with nano-indentation into basal and prism planes. It is shown that Ti-6242 experiences a strong crystallographic orientation-dependent rate sensitivity while Ti-6246 does not which is shown to agree with recently published independent measurements; the dependence of rate sensitivity on indentation slip plane is also well captured. The thermally-activated discrete dislocation plasticity model shows that the incorporation of a stress dwell in fatigue loading leads to remarkable stress redistribution from soft to hard grains in the classical cold dwell fatigue rogue grain combination in alloy Ti-6242, but that no such load shedding occurs in alloy Ti-6246. The key property controlling the behaviour is the time constant of the thermal activation process relative to that of the loading. This work provides the first mechanistic basis to explain why alloy Ti-6242 shows a dwell debit but Ti-6246 does not.
Highlights
Titanium alloys are widely used in gas turbine engines, often under extremes of loading; components include discs and blades
We focus on consequences in terms of the load shedding at hard-soft grain combinations in oligocrystals which has been argued to be a key factor in controlling the dwell fatigue debit in these alloys (Dunne and Rugg, 2008; Dunne et al, 2007a; Hasija et al, 2003; Zhang et al, 2015)
A dislocation-based crystal plasticity approach has been utilised to extract thermal activation energies for pinned dislocation escape for the titanium alloys Ti-6242 and Ti-6246 (9.8×10−20 J and 10.6×10−20 J respectively) such that the remarkably different cold creep responses resulting from the inclusion of a stress dwell within the fatigue loading cycle are captured accurately
Summary
Titanium alloys are widely used in gas turbine engines, often under extremes of loading; components include discs and blades. The rate-sensitive behaviour of titanium alloys at room temperature is believed to be fundamentally related to the dwell fatigue debit during service because it leads to the establishment of load shedding and high basal stresses which are argued to be crucial for facet crack nucleation (Anahid et al, 2011; Dunne and Rugg, 2008; Kirane and Ghosh, 2008).
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