Phase transformation strengthening of high-temperature superalloys

Abstract

Decades of research has been focused on improving the high-temperature properties of nickel-based superalloys, an essential class of materials used in the hot section of jet turbine engines, allowing increased engine efficiency and reduced CO2 emissions. Here we introduce a new ‘phase-transformation strengthening' mechanism that resists high-temperature creep deformation in nickel-based superalloys, where specific alloying elements inhibit the deleterious deformation mode of nanotwinning at temperatures above 700 °C. Ultra-high-resolution structure and composition analysis via scanning transmission electron microscopy, combined with density functional theory calculations, reveals that a superalloy with higher concentrations of the elements titanium, tantalum and niobium encourage a shear-induced solid-state transformation from the γ′ to η phase along stacking faults in γ′ precipitates, which would normally be the precursors of deformation twins. This nanoscale η phase creates a low-energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties.

Segregation-Assisted Plasticity in Ni-Based Superalloys

“…The existence of these faults is a consistent proof of the high activity of partial dislocations shearing at these testing conditions (800°C-650 MPa). This high density of faults contrasts with the relatively cleaner microstructures reported by Smith et al [2] presumably explained by the higher level of strain imposed here to the samples studied. The complex dislocation structures extend also to the c-channels where dislocation pile-ups can be observed.…”

“…From these results it arises that shearing by Shockley partial dislocations promotes significantly the plastic flow of the alloy at high temperatures. Therefore, mechanisms that limit the plasticity to just APB shearing are desirable for strengthening the alloy as observed experimentally by Smith et al [2] Additionally, it is noteworthy the higher level of segregation is observed for APB, which may be linked The concentrations at the faults are averaged-integrated assuming the cut-off point to be half of the peak. When several faults of the same type were available (for twins and SESFs), the concentration values are standard-averaged values between the same type of faults.. with the necessity of higher amounts of c-stabilizers in order to lower the energy of the fault.…”

“…[35][36][37] As a result, the strong effect of the local chemical changes on the creep and plastic strength must be included in future models. One of the first experimental attempts to accomplish with this purpose has been presented by Smith et al [2] proposing the addition of certain elements (Ti, Ta, and Nb) to block the effect of the c-stabilizers and eventually strengthening the alloy. In order to push forward the temperature capabilities of these alloys, these segregation processes need to be fully integrated in multi-physics continuum models which potentially will allow to capture the dependence of the strength of the alloy on its chemical composition.…”

“…Barba et al [14] and Smith et al [2] have proposed a model to rationalize this phenomenon. In their models there is a local shift of chemistry from c 0 !…”

“…[1][2][3] This is because dislocations, even when dissociated, cannot pass through the ordered lattice without their creation in one form or another. A corollary is a range of interesting but practically important plastic phenomena: anomalous yielding, a substantial strain hardening effect and an anisotropy of tensile/compressive behavior which is non-Schmidian.…”