ThSi~

Recent requi rements for higher and higher oper-at ing temperatures in the rocket and missile field, nuclear reactors, and many other appl ications have led to the considerat ion of the so-cal led "refractory hardmetals " for use as one class of s t ructura l mate-rials in the temperature ranges where today's super-alloys are no longer useful. The term refractory hardmetals includes com-pounds of the t rans i t ion metals with boron, carbon, nitrogen, silicon, and sulfur. These mater ia ls are characterized in general as hard, br i t t le interst i t ia l compounds which have very high melt ing points. They differ f rom the corresponding oxides main ly in their electrical and thermal behavior which is dis-t inct ly metall ic. They are good conductors of elec-tr ic ity and heat. Many are even better in this re-spect than austenit ic stainless steel and t i tan ium which are among the poorer conduct ing metals. Thus the term refractory hardmeta ls erves to dist inguish this class of mater ia ls from high melt ing metals such as tungsten and molybdenum and from the more re-f ractory oxides. Table I. Physical properties of some metallic silicides. Silicide Crystal structure Elec-trica