Elastic constants of mullite containing alumina platelets

Using dynamic methods, we measured the elastic constants of a composite com prising alumina platelets (0.2 volume fraction) in a 3:2 mullite matrix. In stead of the expected elastic stiffening, we found an elastic softening. Fo r 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 con sidered voids, cracks., poor particle-matrix-interface bonding, and an enve loping third phase, either around the platelets or around the mullite parti cles. We concluded that the mullite particles must be enveloped by silica, an elastically soft phase. Solid-mechanics modeling considering a three-pha se composite supports our conjecture. Our conjecture received further confi rmation from careful SEM and STEM observations. (C) 2001 Elsevier Science L td. All rights reserved.