THE CHOICE OF THE PROPER REFRACTORY FOR THE CASTING OF HIGHMELTING ELECTROPOSITIVE METALS

As titanium, zirconium, and other of the high melting electropositive metals become more important, the problem of using suitable refractory materials for their casting becomes more important. This paper discusses the method of choosing and testing possible container materials. To make the discussion more specific, titanium is used as an example. As titanium melt at 2000 {+-} 10 K, it is immediately clear that one is restricted to refractory materials melting considerably above 2000 K. This greatly limits the possible materials that might be considered. The possibility of using any pure high melting element can be quickly eliminated as titanium reacts quite vigorously with non-metals such as carbon and due to its high boiling point and therefore high internal pressure, one can predict that it dissolves even the most refractory metals. Examination of phase diagrams confirms that even metals such as tantalum, tungsten, and rhenium would not be able to resist attack by titanium. One is thus limited to high melting compounds such as the oxides, sulfides, nitrides, carbides, silicides, and borides. The first consideration is that, if possible, one would use a compound which is thermodynamically stable in the presence of titanium metal at 2000 K. Titanium should not be able to react with the refractory to form a titanium compound. Thus all compounds less stable than the titanium compound which can exist in equilibrium with titanium metal are excluded