The attenuation of sound oscillations was studied in zone-melted niobium containing 350 ppm of 02 and ~ 500 ppm of H 2. Measurements were made by the method described in Ref. 1. The resonator was a plate 0.3 mm thick and of dimensions 10 x 15 mm with two slits, so that it had the shape of a capital E. Two edges were fastened in a massive metallic block, while the middle was a resonator proper. The frequency of the first mode of twisting oscillations was 1.3 kHz; measurements were made at the maximum amplitude of deformation from 5 x 10-9 to 10-7. Oscillations were excited by a fiat electrode placed at a distance of 0.03-0.05 mm from the surface of the specimen to which constant polarizing and alternating exciting voltages were applied. Oscillations were detected by the same electrode which was involved in the oscillating circuit of the high-frequency generator. An instrument measuring frequencies and an amplifier with a phase rotator composed the feedback circuit in which, at the correctly established phase, oscillations occurred at the specimen resonance frequency.
Attenuation was measured as a function of a small static stress o~, which was applied due to the constant voltage up to 400 V supplied to the electrode. In such a case the maximum elastic deformation was ~ 10-6
The dependence of attenuation on static stress shows maxima of resonance character (Fig. 1). The attenuation drops with increasing amplitude of the alternating stress tr~ according to the law (1 + Aa 2)t {A is a constant), i.e., proportionally to the coefficient of transition saturation.
Near the maximum of attenuation at a static deformation of ~ 106 we observed the appearance of slow periodic oscillations of the amplitude of the resonator vibrations after the external resonator excitation circuit was switched off (Fig. 2).