The effect of mesoscopic spatial heterogeneity on the plastic deformation of Al-Cu alloys

Abstract

<p>This work concerns the effect of manipulating the mesoscopic spatial arrangement of coarse, hard particles in a ductile metallic matrix on the overall macroscopic deformation behaviour of the bulk solid. The hypothesis that the spatial distribution of the harder phase influences the onset of yielding and strain hardening in a particle hardened ductile solid is examined by way of experiment on a well characterized metallic system containing two phases. Rapidly solidified hypoeutectic binary Al-Cu granules, with nominal Cu compositions of 5%, 10%, 17% and 24% by weight were chosen as a model system. At room temperature, the binary system consists of two terminal equilibrium phases: the intermetallic compound CuAl2 , and the ductile Al solid solution. At room temperature the intermetallic is approximately seven times harder than the matrix and intrinsically brittle. Materials possessing either a uniform spatial distribution or a bimodal spatial distribution of the CuAl 2 particles are generated through a combination of hot-pressing and high temperature forging. Compression tests and complimentary experiments were performed on the materials in order to determine the magnitude and distribution of plasticity and damage in the materials as a function of the local heterogeneity and applied strain. The experimental flow curves were then compared to simulations obtained from two non-linear self-consistent continuum models of particle hardened, power law solids developed from the Eshelby "Equivalent Inclusion" Method. The flow curves obtained experimentally for the spatially uniform materials are in good agreement with a self-consistent method in which the matrix is assumed to uniformly coat the elastic particles to form a continuous network. In contrast, a model which assumes a random disordered morphology of both the particles and the matrix underestimates the plastic compliance of the uniform materials when the concentration of the particles is non-dilute. At small strains, the hardening rate observed experimentally is enhanced by inhomogeneous spatial distribution of the second phase when the contrast between the properties of the hard and soft regions of the microstructure is strong and the volume occupied by the hard regions is high. A simple continuum deformation model which accounts for clustering is in good agreement with the flow curves of the clustered materials.</p>