THE SINGLE CRYSTAL ELASTIC CONSTANTS OF THE TANTALUM - TUNGSTEN ALLOY SYSTEM FROM 4.2 TO 300 K

The three single crystal independent elastic constants c(,11), c(,12), and c(,44) were measured over the temperature range 4.2 K to 300 K for tantalum, tungsten, and five intermediate alloys. Anomalies were found for the shear constant c(,44) as functions of both temperature and composition. These appear to be interrelated. A similar, but less pronounced effect was found for the shear constant C' as a function of temperature at about the same composition as for c(,44). These anomalies are centered at approximately 50 percent tungsten. An evaluation by a model of Peter et. al. indicates that these anomalies are due to topology changes in the Fermi surface. The interatomic force constants for nearest and second nearest neighbors were calculated by two experimentally independent methods. Only the central forces between nearest and second nearest neighbors were considered for the first method. The elastic constants measured by this investigation were used for this calculation. The second method considered interactions from the third to the seventh nearest neighbors and used phonon dispersion data from Oakridge National Laboratories on the same crystals. The nearest neighbor force constants differed between the two methods. The Debye Temperatures were calculated using the Kubic Harmonics method. The dependence upon c(,44) for the Debye Temperature calculations is reflected upon the results. No calorimetric data was available for comparison. The Ta-W system was found to be isotropic to within about 5% from 50% tungsten to pure tungsten. The system also shows a remarkable adherence to the Cauchy relation if the long-range forces are considered