The microstructure of ordered (Co 0.78Fe 0.22) 3V alloy

Abstract

The (Co 0.78Fe 0.22) 3V alloy belongs to a class of long-range-ordered alloys that are being developed for elevated-temperature applications. The microstructure after quenching and after, subsequent aging at temperatures between 973 and 1073 K has been characterized by analytical electron microscopy. Short-range order (SRO) and small VC matrix precipitate particles were observed in the as-quenched material. At 973 K VC precipitated discontinuously along grain boundaries and on extrinsic stacking faults. Aging at 1073 K precipitated VC along grain boundaries and on extrinsic stacking faults, and produced intrinsic stacking faults that were precipitate-free. Ordered domains grew during aging at rates proportional to t 1 2 ; the activation energy for growth was 222 ± 20 kJ/mol. Thermal antiphase boundaries (APBs) had isotropic energies and ( a 2 )〈110〉 displacement vectors. Intrinsic and extrinsic stacking faults also serve as APBs, with displacement vectors of ( a 6 )〈112〉 and ( a 3 )〈111〉 . respectively. Intrinsic faults had relatively high APB energies and interacted strongly with thermal APBs, while the reverse was true for extrinsic faults. Extrinsic stacking faults nucleated at VC particles in the matrix by punching out ( a 2 )[011] dislocations, which dissociated into Frank and Shockley dislocations. The Frank partial climbed away from the particle, and growth proceeded by the Silcock-Tunstall mechanism of alternate precipitation and climb of the Frank partial. Intrinsic stacking faults also nucleated at VC particles, but the perfect dislocation around the particle dissociated into two Shockley partials: ( a 2 )[ 1 ̄ 01] → ( a 6 )[ 1 ̄ 1 ̄ 2] + ( a 6 )[ 2 ̄ 11] . Then the intrinsic fault grew by glide of the outer Shockley partial.