Conditions for the appearance of the anomalous dislocation damping peaks in Al-0.5% Cu alloy were further studied with a low frequency torsion pendulum. Experimental results showed that a suitable amount of cold working is necessary for a fully annealed specimen but not for specimens quenched from an elevated temperature.Previously observed internal friction peaks versus strain aging time and versus temperature which exhibit an anomalous amplitude effect were elucidated qualitatively by the simple dislocation atmosphere model. However, in making quantitative explanations, this simple model meets the following difficulties: (i) Under the action of the alternating stress used in internal friction measurements, the distance that the atmosphere of solute atoms can be dragged along with the dislocation is too small, (ii) For the atmosphere to catch up with the dislocation, the diffusion coefficient of the solute atoms constituting the atmosphere has to be many orders of magnitude larger than the ordinary value, (iii) The critical stress necessary to move the dislocation dragging the atmosphere along with a critical speed as calculated theoretically is several hundred times greater than the observed value.A modified dislocation atmosphere model in which the solute atoms are assumed to diffuse along dislocation kinks was proposed. This model takes into account the above mentioned difficulties and can explain the experimental results obtained in a qualitative manner. The basic assumption of this model is that in order to be able to observe the anomalous internal friction phenomena, a certain number of kinks must exist on the dislocation line. Such kinks can be formed through cold working or quenching the specimens from an elevated temperature. The motion of a dislocation line in a direction perpendicular to itself can be achieved through the sidewise motion of the kinks along the dislocation line. It is assumed that atmospheres of solute atoms are formed only on the straight portions at the ends of the kink, and not on the kinks themselves. In the process of the to and fro sidewise motion of the kinks, a to and fro short range diffusion of the solute atoms segregated at the ends of the kinks can carry out along the dislocation pipe. The diffusion coefficient for diffusion along a dislocation pipe is known to be much greater than that for the diffusion in the regular crystal lattice.
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