The effect of hydrogen on the elastic constants of tantalum single crystals
Stewart, William LeRoy
Roberts, John M.
Master of Science
The elastic constants of tantalum were measured in the pure state and at several concentrations of hydrogen doping. These measurements were carried out in the temperature range between 23°C and -127°C on four different crystals, two each of the (llO and LOO orientations. This study set out to determine whether or not a significant Snoek effect existed, i.e., if C* (where C* = 1/2(^ - C12)) decreased significantly with increasing hydrogen content. In carrying out this study a more accurate and extensive check could be made on data previously determined by the inverted torsion pendulum method; ultrasonic measurements at room temperature. McSkimin’s technique^ was used for measuring the acoustic wave velocity in solids. This velocity was then used with the respective densities to calculate the elastic constants. This study yielded a decrease in C of .11 percent per atomic percent hydrogen in the alpha phase region. In addition, C-Q was found to increase by .16 percent per atomic percent hydrogen, increased .16 percent per atomic percent hydrogen, and (where = 1/2 (C^ + C^2 + 2C^) increased by .68 percent per atomic percent hydrogen at room temperature. The Voigt averaged Young's modulus, E, was found to increase by .69 percent per atomic percent hydrogen and the bulk modulus, K, was found to increase by .72 percent per atomic percent hydrogen. The temperature coefficients of the elastic constants exhibited no appreciable change due to hydrogen addition Analysis of the data shows that the tetragonal distortion due to hydrogen in the tantalum lattice is very slight and the distortion parameter X^ - X^ is evaluated as 2.74 x 1 per atomic percent hydrogen. Correlation of the current results with lattice parameter and dilation changes of tantalum due to hydrogen reveals hydrogen to produce a dipole distortion of strength A = 3.88 eV and B = 3.5 eV 3 based upon the Seeger, Mann and Jan Notation.