Recent Developments in Solid State Acoustics

Abstract

Dislocations in metals are the source of internal friction and elastic dispersion. Depending on the strain amplitude, they can act by several different mechanisms. At the highest strain amplitudes, the Frank-Read source is the predominant mechanism and leads to an anelastic region and a fatigue plastic region. In the fatigue region dislocations can cut through other dislocations, producing jogs and vacancies. These vacancies deposited on incipient cracks can cause them to grow and produce the fatigue cracks observed in metals. At lower strains, breakaway from pinning atoms can cause an amplitude-dependent internal friction. This type of loss can be closely correlated to the density of dislocations, concentrations of impurity atoms, etc. For strains so small that breakaway cannot occur, an internal friction occurs which is amplitude independent. For zigzag dislocations not lying along minimum energy positions, the internal friction mechanism proposed by Koehler appears to account for this loss in the megacycle range but not in the kilocycle range. For moderately strained materials, dislocations can lie along minimum energy positions and give rise to Bordoni relaxations whose internal friction values peak at definite temperatures for definite frequencies. These appear to be connected with Peierls forces.