Acoustic emission from dislocation motion in precipitation-strengthened alloys

A model is proposed to explain the effect of precipitation on acoustic emission from dislocation motion in age-hardening systems. The model is based on the hypothesis that a substantial number of dislocations must move rapidly and nearly simultaneously within a small volume of material in order to create detectable acoustic emission. In alloy systems where cross slip is difficult, precipitates, if they are not too strong, can serve as breakable pins and lead to the formation of dislocation avalanches and thereby increase acoustic emission near the onset of plastic flow. In easy cross slip systems, dislocations tend to cross slip around precipitates rather than form pile-ups, so the precipitates do not lead to increased acoustic emission. These considerations do not apply if the pins are Cottrell atmospheres, and high levels of acoustic emission may be observed in either easy or difficult cross slip systems when solute atmospheres provide the pinning. Acoustic emission measurements have been made as a function of heat treatment in several age-hardening systems with various stacking fault energies, elastic moduli, and precipitate strengths. For easy cross slip systems, including 7075 aluminium, 6061 aluminium, 2219 aluminium, and sterling silver, the acoustic emission was greatest in the solution-treated condition and was lowered by precipitation. For more difficult cross slip systems, including JBK–75, Incoloy 903, and an experimental beryllium-hardened austenitic stainless steel, the acoustic emission increased with increased aging times, for relatively short aging times, and then declined for longer aging times. No acoustic emission peak was observed in 17–10P, a stainless steel hardened by M23C6carbides. A relatively high acoustic emission peak was observed in 5083–0 and also in solution-treated 7075 and 6061, where serrated yielding indicated pinning by solute atmospheres.