Simultaneous breakaway of a dislocation from several pinning points

The amplitude-dependent internal friction (ADIF) of very pure aluminum (99.9999%) and Al dilute alloys (20–100 ppm) has been measured at temperatures between 2 and 240 K. Two universal properties have been found in the temperature dependence of the required stress amplitude for a constant decrement. At relatively low temperatures (or high stress amplitude), the decrease of the stress with increasing temperature is proportional to T2/3 down to 65% of the stress for 0 K, and then deviates from the T2/3 dependence. It is explained that the elementary process is due to unpinning of a dislocation from a single solute atom. The interaction potential has been determined successfully from this region (above 40% of the stress at 0 K). At relatively high temperatures (or stress less than 40% of that for 0 K), we find for the first time that the stress changes as proportional to T−1 for all dilute Al alloys but for a pure Al crystal. It is explained that the elementary process is due to simultaneous unpinning of a dislocation from several solute atoms for dilute alloys.