An ultrasonic study of relaxation processes in pure and mechanically alloyed tungsten

摘要

High-temperature internal friction in a pure tungsten single crystal, polycrystals with different grain sizes, and mechanically alloyed tungsten polycrystals was studied. Positron annihilation spectroscopy was used to prove that all studied materials contain a detectable amount of dislocations. Then, resonant ultrasound spectroscopy was applied to determine the internal friction evolution with temperature from room temperature up to 740 degrees C. For the pure tungsten samples, a sharp increase of internal friction was observed for temperatures above 470 degrees C (for the single crystal) or above 400 degrees C (for the polycrystals); the activation energy corresponding to this increase was the same as the activation energy for the ductile-to-brittle transition in tungsten reported in the literature. For the alloyed materials, direct observation of the onset of this relaxation mechanism was impossible due to additional effects resulting from the secondary phases in the material.