Elastic constants of mullite containing alumina platelets

Using dynamic methods, we measured the elastic constants of a composite comprising alumina platelets (0.2 volume fraction) in a 3:2 mullite matrix. Instead of the expected elastic stiffening, we found an elastic softening. For example, the Young modulus was 222 GPa, below mullite's value of 228 GPa, and far below the value predicted by a solid-mechanics model assuming good alumina-mullite interfaces: 256 GPa. As possible softening sources, we considered voids, cracks, poor particle-matrix-interface bonding, and an enveloping third phase, either around the platelets or around the mullite particles. We concluded that the mullite particles must be enveloped by silica, an elastically soft phase. Solid-mechanics modeling considering a three-phase composite supports our conjecture. Our conjecture received further confirmation from careful SEM and STEM observations.