Deformation Mechanisms in Ni-Base Disk Superalloys at Higher Temperatures

Abstract

This paper presents results from a research initiative aimed at investigating high temperature creep deformation mechanisms in Ni-base superalloys through a combination of creep experiments, TEM deformation mechanism characterization, and state of the art modeling techniques. The effect of microstructure on dictating creep rate controlling deformation mechanisms was revealed for specimens with a bimodal γ′ size distribution that possessed different secondary γ′ size, tertiary γ′ volume fraction, and γ channel width spacing. It was found that the less creep resistant microstructure was the one with a greater secondary γ′ size, wider γ channel width, and higher volume fraction of tertiary γ′. Deformation in this microstructure commences by way of a/2<110> dislocations concentrated in the γ matrix at lower strains, which then transition to a SISF precipitate shearing mode at larger strains. The more creep resistant microstructure possessed a finer γ channel width spacing, which promoted a/2<110> dislocation dissociation into a/6<112> Shockley partials at lower strains and microtwinning at higher strains. Dislocation precipitate interaction was further explored using microscopic phase field modeling, which was able to capture key microstructural aspects that can favor dislocation dissociation and decorrelation since this appears to be a precursor to the microtwinning deformation mode. New viable diffusion pathways associated with the reordering processes in microtwinning have been explored at the atomistic level. All of the above activities have shed light onto the complex nature of creep deformation mechanisms at higher temperatures.

High-resolution electron microscopy of dislocation ribbons in a CMSX-4 superalloy single crystal

“…An example of this phenomenon is illustrated in Figure 9. This local ordering is an indication of localised atomic shuffles taking place to create the SESFs in the alloy [14][15][16].…”

“…Since these high-energy complex faults would be formed at both leading and trailing edges of the dissociated a 110 and a 112 superdislocations, they may have a significant effect on the mobility of these dislocations. It has also recently been proposed by Kovarik et al [14][15][16] that the movement of dislocation configurations involving net a 3 112 displacements through the γ phase may be controlled by local, diffusion-mediated reordering of the high energy faults, converting them into lower energy configurations. Thus, direct observation of the partial dislocation configurations at the atomic scale are sought to improve the understanding of the rate-controlling processes in these important alloys.…”

Shearing of γ′ precipitates by a〈112〉 dislocation ribbons in Ni-base superalloys: A phase field approach

“…However some form of atomic shuffle is needed to produce the lower energy SESF. In the work of Unocic et al [20], a possible mechanism of unzipping through vacancy migration is proposed. Alternatives to this could be other diffusional processes or thermally activated shear.…”

Prediction of Mechanical Behaviour in Ni-Base Superalloys Using the Phase Field Model of Dislocations

“…The Phase Field Model has proved a powerful tool for studying the deformation mechanisms in L1 2 ordered superalloys on the scale of the dislocation interactions with precipitates [1,2]. The method has also been successfully applied to modelling γ' precipitate shearing by dislocations on the {111} plane [3] in single crystal superalloys and to study the role of channel plasticity during rafting [4,5] at higher temperatures and lower stresses (> 1000°C and < 200 MPa).…”