Rationalisation of the Micromechanisms Behind the High-Temperature Strength Limit in Single-Crystal Nickel-Based Superalloys

Abstract

The peculiar atomic structure of γ′ precipitates [Ni3(Al/Ti)-L12] in Ni-based superalloys produces high-energy faults when dislocations glide them, giving their significant strength at high temperatures. The mechanisms behind the strength failure of these alloys above 700–800 ∘C are still controversial. Recent advances in atomic resolution microscopy have allowed to study these mechanisms with unprecedented detail. In our study, we have characterised in a careful systematic study a SX-[001] superalloy from RT to 1000 ∘C. Multiscale microscopy (TEM and SEM) has been combined with physical metallurgy and atomistic modelling to fully understand the correlation between the strength drop and the observed changes in the γ′ shearing mechanism. Our results show that, far from previous beliefs, the initial failing of alloy strength is not a consequence of the activation of dislocation climbing. Instead, there is a transition between three different mechanisms: (T<750∘C) continuous planar stacking faults below, (T = 750 ∘C) APB shearing at the strength peak anomaly and (T>800∘C) extensive twin deformation after the yield drop. Local chemical changes around the γ′ shearing dislocations boost these changes, thus producing the sudden drop of strength.