Title

A coupled, physics-based matrix-grain boundary model for creep of carbide strengthened nickel-based superalloys - I. Concepts and formulation

Document Type

Article

Date of Original Version

1-2-2020

Abstract

At elevated temperatures and moderate to high stresses, dislocation creep is considered the dominating mechanism of deformation governed by the motion of dislocations through the matrix and along the grain boundary. A local increase in dislocation activity within the matrix has been observed to accelerate grain boundary sliding along boundaries intersecting the active slip system. The increase in rate is considered here as a function of the increase in supply of intragranular dislocations to the grain boundary, thereby facilitating grain boundary sliding as the dislocations glide under the applied shear stress. As such, the rate at which dislocations arrive to the boundary from the matrix controls the rate at which the grain boundary can slide. One means of suppressing dislocation mobility through both the matrix and along the grain boundary is the precipitation of secondary phases. Materials which precipitate secondary phases in intergranular regions suppress grain boundary sliding directly, acting as pinning points, while materials which precipitate in intragranular regions suppress grain boundary sliding indirectly by limiting the rate at which dislocations arrive to the boundary. To consider the rate controlling properties of both intra- and intergranular carbides on grain boundary sliding. A physics-based model for creep is presented, which considers the microstructural rate controlling features of both the matrix and grain boundary as it pertains to dislocation mobility and the effect on the overall grain boundary sliding rate. Mobility of dislocations through the matrix and along the grain boundary is modeled as a function of microstructural condition – namely M23C6 carbide size, spacing, and volume fraction. The motion of dislocations within the grain boundary is modeled as a viscous flow process. It is considered that grain boundary sliding requires a supply of extrinsic dislocations from the matrix which, upon absorption into the grain boundary, glide and subsequently pile-up at irregularities (triple points, carbides, etc.) under the influence of an applied shear stress (or traction). With increasing number of dislocations in the pile-up, back stress accumulates at the interface, reducing the effective stress and subsequent sliding rate. Dislocation mobility through the matrix as it relates intragranular microstructure has been simulated, from which the rate controlling properties of intragranular particles is investigated.

Publication Title

Materials Science and Engineering A

Volume

769

Share

COinS