X-ray and neutron diffraction studies of syntactic metal foams and metal matrix composites

Synchrotron x-ray and neutron diffraction can provide both the in-situ elastic phase strains and the phases present in metal matrix composites subjected to thermo-mechanical loading by measuring the lattice spacings parallel and perpendicular to the loading axis, as well as changes in the crystalline structure of the composite constituents. Such measurements can give insight into load transfer between phases, the onset of matrix or reinforcement plasticity or damage, and thermally or mechanically induced phase transformations.; Four composite systems are presented: (a) bulk metallic glass composites containing low volume fractions of tungsten and tantalum particles, (b) bulk metallic composites containing low volume fractions of both tantalum particles and crystallized matrix inclusions, (c) copper composites containing high volume fractions of particles of the negative thermal expansion ceramic zirconium tungstate, and (d) aluminum matrix syntactic foams containing high volume fractions of hollow ceramic spheres. In the bulk metallic glass composites, plasticity of the metallic reinforcement was observed during mechanical cycling, leading to residual stresses that may alter the subsequent composite behavior. The zirconium tungstate present in the low thermal expansion copper composites was observed to undergo both thermal and stress induced transformations during thermal cycling, confirming the interpretation of ex-situ thermal expansion measurements. In the aluminum syntactic foams, matrix plasticity and ceramic microsphere damage were seen, as well as relative unloading of the matrix during mechanical testing and an improvement in elastic properties due to presence of the hollow spheres. For all systems, continuum mechanical modeling using the Eshelby method was performed, with good agreement found between predictions and measurements.