Thermally activated dislocation motion including inertial effects in solid solutions
Isaac, Randall Duane
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https://hdl.handle.net/2142/25626
Description
Title
Thermally activated dislocation motion including inertial effects in solid solutions
Author(s)
Isaac, Randall Duane
Issue Date
1977
Doctoral Committee Chair(s)
Granato, A.V.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Date of Ingest
2011-07-01T18:03:43Z
Keyword(s)
thermally activated dislocation motion
inertial effects
solid solutions
potential energy
Brownian motion
Language
en
Abstract
Dislocation motion through an array of obstacles is considered in
terms of the potential energy of the dislocation as it moves through the
array. The obstacles form a series of potential wells and barriers which can trap the dislocations. The effect of thermal fluctuations and of a
viscous drag on the motion of the dislocation is investigated by analogy with Brownian motion in a field of force. The rate of escape of a trapped dislocation is found to depend on the damping coefficient only for a large viscous drag. The probability that a dislocation will be trapped by a well or barrier is found to depend on the damping coefficient for a small
viscous drag. This inertial effect determines how far a dislocation will travel after breaking away from an obstacle. The stress required to break away a dislocation does not change
during the normal-superconducting transition when the damping coefficient
decreases~ However, inertial effects in the superconducting state cause
an increase in the amount of plastic strain for each breakaway process. The apparent decrease in the strength of a superconductor is due to the distribution of the breakaway stress of the dislocations. At a constant rate of deformation, fewer dislocations need to be broken away in the superconducting state than in the normal state.
Internal friction measurements depend on the distance traveled by a dislocation after breakaway. The logarithmic decrement of an ultrasonic wave would therefore be expected to depend on inertial effects unless the
obstacles are segregated around the dislocation.
Recent dynamic bias stress experiments have been interpreted as a change in dislocation segment lengths assuming segregated obstacles. However, it is shown that a hysteresis loss which occurs for both segregated and nonsegregated obstacles also contributes to the bias stress effect. Bias stress data are obtained that indicate the significance of the hysteresis loss contribution to the bias stress effect.
Internal friction measurements are obtained on pure Pb samples and on dilute Pb-Sn alloys. The dependence on the damping coefficient is observed during the normal-superconducting transition. From these data we obtain the distribution of the breakaway stress and determine the value of the damping parameter. The value of the critical damping and the average dislocation loop length are also calculated for these samples. These results are combined to obtain the electronic damping coefficient.
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