Investigation of creep deformation mechanisms at intermediate temperatures in René 88 DT

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Department of Materials Science and Engineering


Creep deformation substructures in the superalloy René 88 DT have been investigated after small-strain (0.2-0.5%) creep at 650 °C using conventional and high resolution transmission electron microscopy. Clear differences in creep strength and deformation mechanisms have been observed as a function of applied stress and precipitate microstructure. Both coarse and fine bimodal precipitate microstructures have been tested, produced by relatively slow and fast cooling from the supersolvus solutionizing temperature. The finer γ′ microstructure exhibited significantly lower creep rates. It has been established that microtwinning caused by the passage of Shockley partial dislocations on successive {1 1 1} planes is the dominant deformation process at low applied stress, and changes to shearing by 1/2[1 1 0] dislocations and Orowan looping around the larger secondary precipitates at higher applied stress. In the coarser microstructure, the dominant deformation mode is isolated faulting where 1/2[1 1 0] dislocations shear the matrix while superlattice extrinsic stacking faults are created in the secondary γ′ particles. The detailed mechanisms by which these deformation modes proceed are discussed, leading to the proposition that the thermally activated process for both microtwinning and isolated faulting is similar, involving diffusion-mediated re-ordering within the γ′ particles in the wake of shearing 1/6〈1 1 2〉 Shockley partials. Based on the present evidence, it is proposed that the tertiary γ′ volume fraction is crucial in dictating the transition in mechanism and the creep strength of these alloys.

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Acta Materialia