Institute of Metals Division - Thermodynamics of Dislocation Mobility and the Third-Law Analysis of the Activation Process

The thermodynamics of the set of functions including the activation enthalpy. the activation free energy, and the activation entropy for the motion of dislocations at low temperatures is formulated without a specific model or a mechanism. The behniiior near absolute zero of all these functions and of other relevant quantities related to dislocation mobility Is examined in the light of the third law of thermodynamics. The velocity measurements of Stein and Low for edge dislocations in Si-Fe are used .for illustrotion. In the plastic deformation of crystalline materials, a fundamental assumption, which is supported by direct measurements using etch-pit techniques,'-4 is that the average velocity of a certain mobile dislocation is a single-valued function of temperature and the effective shear stress exerted on the mobile dislocation: Sometimes under certain assumptions, even without direct dislocation-velocity measurements, an activation volume can be obtained from a change in strain rate in a simple tension test. Similarly an ac tivation enthalpy can be obtained by changing the temperature in a creep test. However, the assumptions involved should be studied carefully before using the results of these tests. Consider, for example, a tensile specimen at some stage of deformation with a distribution of mobile dislocations. Let the total length of dislocations of type i be pi per unit volume. The Burgers vector of these dislocations is hi which makes an angle ?i with the tensile axis. With respect to the same tensile axis, let the normal of the slip plane of these dislocations