ELASTIC PROPERTIES OF MULLITE AND MULLITE-CONTAINING CERAMICS PART 1: THEORETICAL ASPECTS AND REVIEW OF MONOCRYSTAL DATA

In this first part of a two-paper set on the elastic properties of mullite and mullite ceramics the current literature is reviewed and, based on the available monocrystal data, a mutually consistent average set of elastic constants for polycrystalline mullite materials at room temperature is calculated and optimized via minimization of the total average relative root mean square (RMS) deviation. Based on the results of this paper, the recommended values of adiabatic elastic constants are 224.9 GPa for the Young’s modulus, 87.8 GPa for the shear modulus, 170.9 GPa for the bulk modulus and 0.281 for the Poisson ratio. The RMS deviation of this set is lower than for any other set, and thus these values exhibit a high degree of mutual consistency with respect to the elasticity standard relations. Micromechanical upper bounds and model relations are summarized for describing the porosity dependence of elastic moduli, and a simple relation is used to estimate the difference between adiabatic and isothermal elastic constants. The results show that this difference increases slightly with temperature, but is not larger than 4 % even at 1500°C. In particular, for the Young’s modulus the difference remains below 0.5 %. That means, the difference between adiabatic and isothermal Young’s moduli will usually be unmeasurably small and negligible in practice.