Effect of residual stresses on dynamic mechanical behaviour of metal matrix composite material: A numerical study

Abstract

Metal matrix composites (MMCs) are hard, abrasion and impact resistant structural materials containing reinforcement constituents, like carbides embedded in a ductile metal matrix. The fabrication of an MMC involves a high temperature process, which produces an internal stress state within the material without any external boundary loads. The vastly different mechanical and thermal properties, like elasticity and coefficient of thermal expansion of the constituent phases, causes thermal strains to develop between them during the subsequent cooling process. Consequently, the metal phase is left in mean tensile stress and the reinforcement in mean compression, as the metal shrinks more in comparison. Measuring this stress with destructive or non-destructive methods is either difficult and costly or gives information only as an average from the measured area restricted to surface layers. That is why accurate simulations can give additional information on how the stresses distribute in the microstructure. In addition, the effect of these residual stresses is not well known on the dynamic behaviour of MMC materials, mainly how the internal stress state influences strength and fracture characteristics. In this thesis, the fabrication process of a carbide reinforced MMC was simulated with finite element method using a crystal plasticity material model for a 3D representative volume element mimicking a real microstructure. The physical basis of the model should yield significant detail on the distribution of stresses within the microstructure. This simulation was attempted to be validated using X-ray diffraction measurements and an analytical formula. The effect of this simulated stress state on the dynamic behaviour of the MMC was evaluated by uniaxial compression simulations with various strain rates and was compared to non-stressed cases. The simulated fabrication process revealed that the residual stress state was mostly hydrostatic in both phases, with carbides in mean compressive stress and metal matrix in mean tensile stress. The compressive stress in the carbides were highest in magnitude at carbide-carbide interfaces, lowering towards middle regions of the carbides. Tensile stresses were also present in the carbides at metal-carbide interfaces. The experimental study revealed that the surface layers of the MMC were prone to local differences in phase distributions, which was observed as vastly different stress levels depending on the measurement area. Additionally, the metal matrix was textured at the surface, making stress measurements impossible using X-ray diffraction. These issues made quantitative validation of the simulations unsuccessful with XRD, although all methods predicted a mean compressive and mean tensile stress in the carbide and matrix respectively. The results from the analytical formula with respect to simulated results coincided better, however no definite validation could be made. The uniaxial compression simulations gave qualitative information on the effect of residual stresses. The simulations showed that the residual pressure in the carbides elevated the yield strength of the material observed as slightly delayed fracture and higher compressive strength. It was observed that the fracture process during loading was mostly driven by stress concentrations due to geometrical factors and residual stresses were not observed to alter the main fracture characteristics. This study laid premises to study and understand effects of residual stresses on more complex loading modes on MMC materials.