NiO-MgO Isomorphous System
From the phase diagram for the NiO-MgO binary system [Figure 10-8(b)], describe a composition that can melt at 2600 °C but will not melt when placed into service at 2300 °C.
The material must have a liquidus temperature below 2600 °C, but a solidus temperature above 2300 °C. The NiO-MgO phase diagram [Figure 10-8(b)] permits us to choose an appropriate composition.
To identify a composition with a liquidus temperature below 2600 °C, there must be less than 60 mol% MgO in the refractory. To identify a composition with a solidus temperature above 2300 °C, there must be at least 50 mol% MgO present. Consequently, we can use any composition between 50 mol% MgO and 60 mol% MgO.