Upon consideration of the SiO2–Al
2O3 phase diagram, for each pair of compositions listed in (a) & (b) below, which would you judge to be the more desirable refractory if there may be a propensity for the furnace temperature (they are being used in) to shoot up (by ~100°C) above 1600°C? Justify your choices by considering liquidus temperatures and fractions of liquid present. Note, a lower fraction of the liquid phase present will lead to a greater mechanical integrity for a high temperature refractory application. (a) 20 wt% Al2O3-80 wt% SiO2 and 25 wt% Al2O3-75 wt% SiO2 Considering that a lower fraction of the liquid phase will lead to greater mechanical integrity, the 25 wt% Al2O3-75 wt% SiO2 will be more desirable for higher temperature refractory applications because of the higher liquidus temperature, and thus will have lower liquid phase fraction at any temperature within the mullite+liquid region. (b) 70 wt% Al2O3-30 wt% SiO2 and 80 wt% Al2O3-20 wt% SiO2 In this case, the 80 wt% Al2O3-20 wt% SiO2 will be more desirable because at 80 wt% alumina, the temperature at which liquid will form is much higher compared to at 70 wt% and therefore will have lower liquid phase fraction when there is 80 wt% alumina.
