What is the role of rhenium in single crystal superalloys?

Abstract

Abstract. Rhenium plays a critical role in single-crystal superalloys -its addition to first generation alloys improves creep life by a factor of at least two, with further benefits for fatigue performance. Its use in alloys such as PWA1484, CMSX-4 and Rene N5 is now widespread, and many in this community regard Re as the "magic dust". In this paper, the latest thinking concerning the origins of the "rhenium-effect" is presented. We start by reviewing the hypothesis that rhenium clusters represent barriers to dislocation motion. Recent atom probe tomography experiments have shown that Re may instead form a solid solution with Ni at low concentrations (< 7 at.%). Density functional theory calculations indicate that, in the solid solution, short range ordering of Re may be expected. Finally, Re has been shown to diffuse slowly in the γ -Ni phase. Calculations using a semi-analytical dislocation climb/glide model based upon the work of McLean and Dyson have been used to rationalise the composition-dependence of creep deformation in these materials. All evidence points to two important factors: (i) the preferred partitioning of Re to the γ phase, where dislocation activity preferentially occurs during the tertiary creep regime and (ii) a retardation effect on dislocation segments at γ /γ interfaces, which require non-conservative climb and thus an associated vacancy flux.

On the Thermodynamics and Kinetics of TCP Phase Precipitation in Re‐ and Ru‐ Containing Ni‐base Superalloys

“…The detailed mechanism of the Re influence on creep strength is still under dispute. As the originally proposed effect of Re clustering, i.e., short range ordering within a γ‐matrix phase cannot be confirmed in modern alloy systems, its low diffusion coefficient has been proposed as cause for high strength. Anyway, the beneficial effect of Re is evident.…”

Modelling defects in Ni–Al with EAM and DFT calculations

“…The hypothesis of Rhenium clustering, originally proposed in pioneering atom probe experiments [4,5], has been dismissed after more refined experiments (three-dimensional atom probe technique [6,7] and extended x-ray absorption fine spectroscopy [8,9]) and theoretical atomistic investigation [10]. Slow-diffusing atoms in the matrix were thought to hinder dislocation climb processes at the interface [11]. However, the reductions of the vacancy diffusion coefficient predicted from first-principle calculations is not large enough to provide the creep strengthening effect experimentally observed [12].…”

Enabling QM-accurate simulation of dislocation motion in γ−Ni and α−Fe using a hybrid multiscale approach

“…The need to produce accurate dynamical representations of chemical processes involving defects in metallic systems has been steadily increasing in recent years. An accurate description of dislocations, including plastic deformation and the effects of impurity atoms, could provide crucial and currently missing insight for industrially relevant problems, for example helping to elucidate the effect of Rhenium in Ni-based superalloys [1], or the fundamental mechanisms underlying hydrogen embrittlement of steels [2,3]. Density functional theory (DFT) is a tremendously useful tool and nowadays a standard technique in many branches of physics, chemistry, and materials science [4].…”