ELASTIC PROPERTIES OF C40 TRANSITION-METAL DISILICIDES

Room-temperature and low temperature elastic properties of hexagonal C 40 transition metal disilicides, NbSi2 and TaSi2, have been studied us ing Resonant Ultrasound Spectroscopy (RUS). All five independent elast ic stiffness constants c(ij) for NbSi2 and TaSi2 single crystals have been obtained. The temperature dependence of the c(ij) is normal but n ot large. The orientation dependence of the Young's and shear moduli w as examined in comparison with other transition metal disilicides. The room temperature shear moduli in the {0001} plane, with values of 145 .3 and 143.7 GPa for NbSi2 and TaSi2 respectively, are low relative to those in the equivalent pseudo hexagonal {110} close-packed plane for tetragonal C11(b) MoSi2 and WSi2. The isotropic elastic constants for polycrystalline materials were also calculated. The results show that the various moduli are all much higher than those of the constituent elements. The room temperature Poisson's ratios of NbSi2 and TaSi2 are 0.18 and 0.19, respectively, which are smaller than those of the cons tituent elements and smaller than most materials. The Debye temperatur es, theta(D), were estimated to be 688 K for NbSi2 and 552 K for TaSi2 . The elastic properties of C40 VSi2, NbSi2, TaSi2, and CrSi2 and C11( b) MoSi2 and WSi2 are compared and the possible influence on mechanica l behavior discussed.