Interfacial diffusion in high-temperature deformation of composites: A discrete dislocation plasticity investigation

Abstract

We present a discrete dislocation plasticity (DDP) framework to analyse the high temperature deformation of multiphase materials (composites) comprising a matrix and inclusions. Deformation of the phases is by climb-assisted glide of the dislocations while the particles can also deform due to stress-driven interfacial diffusion. The general framework is used to analyse the uniaxial tensile deformation of a composite comprising elastic particles with dislocation plasticity only present in the matrix phase. When dislocation motion is restricted to only glide within the matrix a strong size effect of the composite strength is predicted with the strength increasing with decreasing unit cell size due to dislocations forming pileups against the matrix/particle interface. Interfacial diffusion decreases the composite strength as it enhances the elongation of the elastic particles along the loading direction. When dislocation motion occurs by climb-assisted glide within the matrix the size effect of the strength is reduced as dislocations no longer arrange high energy pileup structures but rather form lower energy dislocation cell networks. While interfacial diffusion again reduces the composite strength, in contrast to continuum plasticity predictions, the elongation of the particles is almost independent of the interfacial diffusion constant. Rather, in DDP the reduction in composite strength due to interfacial diffusion is a result of changes in the dislocation structures within the matrix and the associated enhanced dislocation climb rates in the matrix.

Discrete dislocation plasticity analysis of the high-temperature cyclic response of composites

“…inter-diffusion of Al in Ni for nickel-based superalloys) along the interface. The interfacial mass transport is modelled as a normal velocity discontinuity across the interface specified by [17,10]…”

“…where D (i) is the interfacial diffusion parameter and ξ is a local co-ordinate along the interface ( Fig. 1a); see Shishvan et al [10] for further details. Perfect bonding is assumed between the precipitates and the matrix so that there is no sliding across the interface and this DDP boundary value problem is solved using the superposition scheme described in [10].…”

“…The material parameters of the model are adapted from [10] and are representative of nickel-based superalloys. The elastic constants of the matrix and precipitates are E = 100 GPa and ν = 0.37.…”

“…Modelling of the high-temperature behaviour of such composites has received less attention but recent advances in DDP modelling approaches for the high-temperature deformation of crystalline metals [8,9] have aided such investigations. For example, Shishvan et al [10] have extended the high-temperature DDP frameworks and developed a DDP approach to model the effect of stress-driven diffusion at the interfaces of multi-phase materials. Their study demonstrated that dislocation climb and interfacial diffusion reduce the effect of precipitate size on the high-temperature strength of such composites via the formation of low energy dislocation structures in the matrix phase.…”

“…The same framework was then also used to demonstrate [11] that the formation of wavy γ/γ interfaces in nickel-based superalloys triggers tertiary creep in these materials. The high-temperature DDP framework of Shishvan et al [10] which explicitly models not only the climb of the dislocations but also stress-driven diffusion at the interfaces is ideally suited to test the various hypotheses for the mechanisms of stress softening during LCF. This is the focus of this study where we investigate the straincontrolled cyclic high-temperature response of composites akin to nickel-based superalloys.…”