MICROMECHANICAL PROPERTIES OF MELTED BORIDES OF RARE-EARTH METALS

The compounds of boron with metals are characterized by a high strength of the interatomic bonds and a low relaxation ability of the internal stress. As a result they are very fragile materials. The value of the microfragility of these compounds may be used for a comparative estimation of their strength and of their possible practical application. The structure of the surface disturbance and the relative microfragility (microelasticity) of a number of alkaline and rare earth metal hexaborides obtained by melting (sometimes accompanied by zone purification) were determined by electron microscopy and measurements of the microhardness and microfragility. The modulus of elasticity and the effective surface energy of a number of materials were determined by the method of continuous indentation. It was established that, if we place these compounds in the sequence in which their microelasticity decreases, we have the following order: SmB6→BaB6→EuB6→CaB6→ YbB6→LaB6→PrB6→NdB6→GdB6. In this order there is a noticeable increase in the length of the metal-metal and metal-B bonds which have a metallic character and are weakened when passing from samarium to gadolinium. The ability of the metal atoms to transfer some of their electrons to the boron atoms increases in the same direction. This also increases the fragility of the compounds because of the strong acceptor ability of the boron atoms and their tendency to form a stable sp3 electron state. © 1979.