On the coarsening mechanism and deformation of borides under annealing and creep in a polycrystalline superalloy

TL;DR

This study investigates the coarsening mechanisms and deformation behaviors of intergranular Cr-rich borides in a nickel-based superalloy under creep and annealing, revealing structural changes, fault formations, and implications for creep performance.

Contribution

It uncovers the coarsening process of borides under creep, identifies structural coexistence, and links deformation features to microstructural evolution in superalloys.

Findings

Borides coarsen more under creep than annealing.

Coarsened borides exhibit tetragonal and orthorhombic structures coexisting.

High density of planar faults correlates with chemical fluctuations.

Abstract

We have investigated the coarsening mechanism of intergranular Cr-rich M2B borides after creep and annealing at 850C for approximately 3000 hours in a polycrystalline nickel-based superalloy. Borides were found to be coarser after creep, with measured thicknesses in the range of 800-1100nm, compared to borides annealed in the absence of external applied load (400-600nm). The borides had a thickness of 100-200nm before exposure at 850C. Transmission electron microscopy revealed that coarsened borides have either the tetragonal I4/mcm structure, or the orthorhombic Fddd, with those two structures coexisting in a single particle. The presence of a very high density of planar faults is systematically observed within the coarsened borides. The faults were correlated with chemical fluctuations of B and Cr, revealed by atom probe tomography. Our results allow us to suggest that borides coarsen by an epitaxy-like mechanism. In addition, partitioning of Ni and Co was observed at dislocations within the borides after creep providing insights into the deformation of borides. Consequences of coarsened intergranular borides on the creep performance of polycrystalline superalloys are discussed.