Refractory Metals

It has already been pointed out earlier that high melting point temperature materials could be advantageously put to use at high temperature. There are several metals, known as refractory metals, that have quite high melting point. Those which have been commercially exploited include, Mo, W, Nb and Ta. Unfortunately the oxidation resistance of these metals is very poor and any exposure to air at such temperature would result in high corrosion. All these metals have very low room temperature ductility which make them susceptible to damage at ambient temperatures. The only way in which these metals may be used at elevated temperature is to have them coated by a thin layer which would make them impervious to oxygen. A coating which would cut off oxygen completely has not yet been developed because a small hole in the coating could be very dangerous. Contrary to it satisfactory coating for Ni base superalloys has been developed.