UC Santa Barbara

Multi-phase metal composites have garnered increased attention in recent years. While the mechanical properties of these materials under high strain rate loadings have been found to depend heavily on microstructural heterogeneities, the current understanding of the roles of constituent properties and microstructure on dynamic properties is lacking. The current study addresses the outstanding issues through a combination of experimental measurements under both low and high strain rates, characterization of microstructural changes associated with dynamic loading, and simulations of stress wave propagation in a family of idealized two-phase metal composites.

The experimental work focuses on two classes of materials: a Cu/Nb composite made by equal-channel angular extrusion and a lamellar two-phase Zr-2.5wt% Nb. The Cu/Nb composite was found to exhibit plastic anisotropy similar to that of pure Cu made by the same process, but with greater strength and with compressive failure occurring through a kink banding mechanism. The strengths and localization behavior at low and high strain rates were similar to those obtained in a Cu/Nb nanolaminate. The Zr-2.5wt% Nb material was studied in two conditions, distinguished by a dynamic compression step following heat treatment and upon cooling. The materials were assessed in tension and compression at quasi-static and high-strain rates, and during incipient spall via flyer plate impact at rates >10^5 /s. Material failure in all cases was found to be dominated by prior β grain boundaries. In compression, boundaries normal to the loading direction promote shear banding; in tension, including incipient spallation, they are preferred locations for void nucleation.

Finite element simulations were used to study elastic wave propagation and stress distributions in idealized models of aligned two-phase metal composites. The properties relevant to wave propagation – wave speed and acoustic impedance – were varied through selection of Young’s moduli and densities of the constituent phases. Three regimes of wave behavior, characterized by the ratio of the wave speeds of the two phases, were identified. Shear waves were found to arise from purely axial loading, and their magnitude during late-time propagation were found to be reasonably approximated by a shear lag-type model for a subset of the composite configurations examined.

This work provides valuable insight into the role of microstructure on the performance of multi-phase metal composites, including the characteristics necessary for bi-phase interfaces to be a primary strengthening mechanism. It also shows that other microstructural features can dictate response over bi-phase interfaces. The finite element simulations of wave propagation provide a foundational base on which to build a more complete understanding of the evolution of stress distributions during dynamic loading.