Load partitioning in aluminum syntactic foams containing ceramic microspheres

Syntactic foams were fabricated by pressure-infiltrating liquid aluminum (commercial purity and 7075-Al) into a packed preform of silica–mullite hollow microspheres. These foams were subjected to a series of uniaxial compression stresses while neutron or synchrotron X-ray diffraction measurements of elastic strains in the matrix and the microspheres were obtained. As for metal matrix composites with monolithic ceramic reinforcement, load transfer in the pure aluminum foams is apparent between the two phases during elastic deformation, and is affected at higher stresses by matrix plasticity. Calculating effective stresses from the lattice strains shows that the microspheres unload the pure aluminum matrix by a factor of about 2. In the aluminum alloy foams, an in situ reaction between silica and the melt leads to the conversion of silica to alumina in the microsphere walls and the precipitation of silicon particles in the matrix. This affects the load transfer between the matrix and the reinforcement (microspheres and particles), and increases the macroscopic foam stiffness by over 40%, as compared to the pure aluminum foams. Composite micromechanical modeling provides good predictions of the elastic moduli of the syntactic foams, capturing the effects of load transfer and suggesting that significant stiffness improvements can be achieved in syntactic foams by the use of microspheres with stiff walls and/or by the incorporation of a stiff reinforcing phase within the metallic matrix.