Segregation-driven cross-slip mechanism of shockley partials in the gamma prime phase of CoNi-based superalloys

TL;DR

This study reveals a novel Shockley partial dislocation cross-slip mechanism in the gamma prime phase of CoNi-based superalloys, influenced by elemental segregation and local stress, which enhances high-temperature deformation resistance.

Contribution

It provides the first evidence of Shockley partial dislocation cross-slip within the gamma prime phase, elucidating the role of elemental segregation and stress in this process.

Findings

Elemental segregation reduces activation energy for cross-slip.

Cross-slip facilitates stair-rod dislocation formation.

Enhanced deformation resistance at high temperatures.

Abstract

In general, the cross-slip of superdislocations (a/2<011>) from {111} planes to {001} planes has been frequently observed in superalloys, accompanied by the formation of an antiphase boundary (APB) and driven by thermal activation. However, no prior studies have evidenced the occurrence of Shockley partial dislocation (a/6<112>) cross-slip within the gamma prime phase of superalloys. In this work, we present a newly observed cross-slip phenomenon: the Shockley partial dislocations cross-slip from one {111} plane to another {111} conjugate plane, facilitated by the formation of a stair-rod dislocation in the ordered gamma prime phase of a CoNi-based superalloy. Compression tests were conducted at 850 degrees Celsius with a strain rate of 10^-4 s^-1. Defects such as stacking faults and dislocations, along with the associated chemical fluctuations, were characterized using high-resolution scanning transmission electron microscopy (HRSTEM) and energy-dispersive X-ray spectroscopy (EDS). Elemental segregation was found to reduce the activation energy required for cross-slip by decreasing the energies of stacking faults and dislocations. In addition to elemental segregation, local stress concentrations, arising from the combined effects of applied stress, shearing dislocations within the gamma prime phase, and dislocation pile-ups, also play a critical role in triggering cross-slip. The formation of sessile stair-rod dislocations via this newly identified Shockley partial cross-slip in the gamma prime phase is beneficial for enhancing the high-temperature deformation resistance of the alloy by increasing the critical resolved shear stress required for further plastic deformation.