Precipitate dissolution during deformation induced twin thickening in a CoNi-base superalloy subject to creep

TL;DR

This study investigates how deformation-induced microtwin formation in a CoNi-base superalloy leads to precipitate dissolution during creep at high temperature, revealing microstructural changes affecting alloy performance.

Contribution

It provides new insights into the mechanism of precipitate dissolution during twin thickening in superalloys under creep conditions.

Findings

Microtwins are depleted of gamma prime elements across their width.

Precipitate dissolution occurs during twin thickening, affecting microstructure.

The Kolbe reordering mechanism is proposed as the dissolution process.

Abstract

The tensile creep performance of a polycrystalline Co/Ni-base superalloy with a multimodal gamma prime distribution has been examined at 800C and 300MPa. The rupture life of the alloy is comparable to that of RR1000 tested under similar conditions. Microstructural examination of the alloy after testing revealed the presence of continuous gamma prime precipitates and M23C6 carbides along the grain boundaries. Intragranularly, coarsening of the secondary gamma prime precipitates occurred at the expense of the fine tertiary gamma prime. Long planar deformation bands, free of gamma prime, were also observed to traverse individual grains ending in steps at the grain boundaries. Examination of the deformation bands confirmed that they were microtwins. Long sections of the microtwins examined were depleted of gamma prime stabilising elements across their entire width, suggesting that certain alloy compositions are susceptible to precipitate dissolution during twin thickening. A mechanism for the dissolution of the precipitates is suggested based on the Kolbe reordering mechanism.