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

Abstract

The "phase field microelasticity theory of dislocations" has been used to study the propagation of dislocation ribbons with an overall Burgers vector of a 112 through a simulated Ni-base superalloy. The driving force for dislocation dissociation reactions and formation of planar faults is incorporated into the free energy using γ-surface functions specially fitted to ab initio data. The model shows that the mechanism of cutting of the γ precipitates by these ribbons exhibits significant dependence on stress magnitude, orientation and precipitate shape. In the case of mixed screw-edge ribbons a change of shearing mode is observed, from stacking fault shear to APB shear, when the applied stress approaches the yield of the material. This transition is absent in pure edge ribbons.

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

“…Vorontsov et al used a phase field model with this type of γ-surface for Ni 3 Al and Ni to investigate stacking fault shear in superalloys [3]. The study proved useful in identifying the effects of applied stress, dislocation character and precipitate geometry on the shearing mechanism.…”

“…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). However, the model, described in [3], had limited applicability to simulating γ' cutting at elevated temperatures, because it did not incorporate the formation of extrinsic stacking faults.…”

Shearing of γ′ precipitates in Ni-base superalloys: a phase field study incorporating the effective γ-surface

“…Using PFMD, it was found [1] that the shearing of γ precipitates was sensitive to the shape of the precipitate, the difference in resolved stresses experienced by the constituent dislocations of the a 112 ribbon and the overall magnitude of the applied stress. However, the experimentally observed formation of the SESF was not incorporated into the physics of the model used in [1].…”

“…In a preceding work, Vorontsov et al [1] used the Phase Field Model of Dislocations (PFMD) [2] to investigate the shearing of γ precipitates in Ni-base superalloys under primary creep conditions (750-850 • C and σ [001] >500 MPa) [3]. The study concentrated on the phenomenon of stacking fault shear, where the L1 2 ordered precipitates are cut by a 112 dislocations.…”

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

“…The structure of the latter can be thought of as SISF over CISF. Both atomistic simulations [7] and the Phase Field Model of Dislocations (PFMD) [8][9][10] have shown that the a 3 112 dislocations are indeed likely to be dissociated. Both complex faults have a very high fault energy [11], and as a result the bounding Shockley partial pair would have such a small separation that when observed using conventional TEM imaging, they would appear as a single a 3 112 dislocation.…”