Abstract
This paper examines microscale crystal slip accumulation, cold creep, and stress redistribution (load shedding) related to dwell fatigue in a range of α–β Ti alloy microstructures. The role of basal slip and prism slip is evaluated in load shedding in a rogue grain combination. The results enrich the Stroh dislocation pile up interpretation of dwell by accounting for the anisotropic rate dependence of differing slip systems together with morphology.Microstructural morphology has been found to play an essential role in cold creep and load shedding in dwell fatigue. Basketweave structures with multiple α variants have been shown to give the lowest load shedding for which the mechanistic explanation is that the β lath structures provide multiple, small-scale α variants which inhibit creep and hence stress relaxation, thus producing more uniform, diffuse stress distributions across the microstructure through microscale kinematic confinement, imposed by multi (α)-to-single (β) BOR relations (i.e. multiple α variants sharing the same parent β grain). The critical consequence of this is that alloys typically having multi-variant basketweave structure (e.g. Ti-6246), remain free of dwell fatigue debit whereas those alloys associated with globular colony structures (e.g. Ti-6242) suffer significant dwell debit. This understanding is important in microstructural design of titanium alloys for resisting cold dwell fatigue.
Highlights
Component failure due to cold dwell fatigue was first recognized from in-service behaviour in RR RB211 engine Ti-alloy discs on Lockheed Tristar aircraft in the 1970s [42]
A consequence of the dramatic inhibition of cold creep occurring during the stress dwell in the basketweave (g3) grain, which is wellorientated for α slip, is likely that the stress distributions borne by this grain are different than would be the case if it were pure α or α–β colony
The findings from this study are potentially important in guiding choice of material microstructures to achieve better cold creep resistance and dwell fatigue life
Summary
Component failure due to cold dwell fatigue was first recognized from in-service behaviour in RR RB211 engine Ti-alloy discs on Lockheed Tristar aircraft in the 1970s [42]. The mechanisms of fatigue facet nucleation in titanium alloys have been studied by several authors including Dunne and Rugg [14] In the latter, the load shedding within a rogue grain (soft-hard) combination was thought to play a significant role in facet fracture near basal planes [47,56], and which potentially leads to extreme values in fatigue indicator parameters [40,41]. Recent pillar tests show that thicker β laths produce more impedance to slip transfer through αβ-α phase boundaries than thinner laths [59] and the basketweave structure is less rate sensitive in α-β titanium [57] than that for colonies. The colony structure has sandwiched α-β phase laths with each α-β interface having the same one (α)-to-one (β) BOR relation These findings suggest an explanation for the long standing problem in dwell debit difference between alloys Ti6242 and Ti-6246. The methodology is to employ the dislocation based crystal plasticity modelling method in which basic phase property data has been established from direct micro-pillar testing to give intrinsic slip strengths and phase strain rate sensitivities
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