Precipitate Shearing, Fault Energies, and Solute Segregation to Planar Faults in Ni-, CoNi-, and Co-Base Superalloys

Abstract

The mechanical properties of superalloys are strongly governed by the resistance to shearing of ordered precipitates by dislocations. In the operating environments of superalloys, the stresses and temperatures present during thermomechanical loading influence the dislocation shearing dynamics, which involve diffusion and segregation processes that result in a diverse array of planar defects in the ordered L12 γ′ precipitate phase. This review discusses the current understanding of high-temperature deformation mechanisms of γ′ precipitates in two-phase Ni-, Co-, and CoNi-base superalloys. The sensitivity of planar fault energies to chemical composition results in a variety of unique deformation mechanisms, and methods to determine fault energies are therefore reviewed. The degree of chemical segregation in the vicinity of planar defects reveals an apparent phase transformation within the parent γ′ phase. The kinetics of segregation to linear and planar defects play a significant role in high-temperature properties. Understanding and controlling fault energies and the associated dislocation dynamics provide a new pathway for the design of superalloys with exceptional properties.

Dislocation Arrangements and Cyclic Microplasticity Surrounding Stress Concentration in a Ni‐Based Single‐Crystal Superalloy

“…This indicates that even the high local stresses were initially not sufficient to shear the strong γ' precipitates right away; instead, γ' shearing occurred subsequently as a considerable pile up of dislocations in the channels, which locally raised the stresses enough to induce a slip transmission into the precipitates. [48,49] It has been known that the deposition and subsequent reactions of dislocations at the γ/γ' interface in the plasticity-assisted creep of nickel superalloys at high temperatures create superdislocations that shear γ' driven by the increase in stress field induced by the following dislocations of the same slip system. [49][50][51] Extended faults (that are typically observed when superdislocations shear γ' [52,53] ) were not seen in our observations due to the orientation of observations similar to what was shown in another study.…”

“…[48,49] It has been known that the deposition and subsequent reactions of dislocations at the γ/γ' interface in the plasticity-assisted creep of nickel superalloys at high temperatures create superdislocations that shear γ' driven by the increase in stress field induced by the following dislocations of the same slip system. [49][50][51] Extended faults (that are typically observed when superdislocations shear γ' [52,53] ) were not seen in our observations due to the orientation of observations similar to what was shown in another study. [51] It is likely that the same dislocation reaction as seen in ref.…”