Temperature dependence of Young's modulus and the occurrence of an elastic anomaly in porous alumina-mullite composites prepared by starch consolidation casting

The temperature dependence of Young's modulus has been measured by impulse excitation up to 1400 degree celsius (heating and cooling) for alumina-mullite composites with 40-60 wt% alumina prepared by starch consolidation casting, after firing at 1570 degree celsius, resulting in materials with high porosities (43-57 %). Using previously determined temperature master curves, both analytical predictions and numerical computations on computer-generated microstructures are compared with experimental data, resulting in excellent agreement for the alumina-mullite composites. Apart from the overall decrease of Young's modulus during heating, an elastic anomaly starting at around 950 degree celsius and cumulating at around 1050 degree celsius is observed in all cases during heating (but not during cooling), and is shown to be stronger with increasing alumina content and porosity. The results are discussed in terms of a "glass phase hypothesis" and a "phase transition hypothesis", although neither of them provides a satisfactory explanation of the observed phenomenon.