Ultrasonic study of alpha -complex peaks in single crystals and polycrystals of pure molybdenum at megahertz frequencies

Abstract

Measurements of the temperature dependence of the ultrasonic attenuation at 5, 10, and 28 MHz in molybdenum single crystals of 99.995% purity, oriented along (111), (110) and (100), have provided a detailed study of the alpha complex of dislocation relaxation peaks following compressive stressing at room temperature. At 5 MHz the peaks were at 148, 167 and 206 K, independent of the orientation of the crystal. As expected, each of the peaks moved to higher temperatures as the frequency of measurement was increased and from these data the activation enthalpies and attempt frequencies were 0.18 eV and 6.6*1011 Hz, respectively, for alpha 1, 0.22 eV and 1.84*1013 Hz, respectively for alpha 2, and 0.20 eV and 4.95*1011 Hz, respectively, for alpha 3. Similar measurements at 10 MHz on polycrystals of the same purity showed the effects of increasing the applied stress from 0.2 to 29 MPa and of irradiation with doses Dy of gamma -rays up to 8.6*108 gamma vt and doses Dn of neutrons up to 10.8*109 nvt. It was found that the peaks' strength Qmax-1 varied as D-m, where m is a parameter for each irradiated pack.