First differential of temperature dependent ultrasonic parameters as an effective tool for identifying precipitation reactions in a slow heat-treated 8090 Al-Li alloy

Abstract

Ultrasonic velocities and attenuation measurements have been carried out to characterise the fine scale precipitation events in a slow heat-treated 8090 Al-Li alloy. The alloy was subjected to a slow heating (at the rate of 10 K h⁻¹) to the temperature of 463 K, before and after solution annealing at 803 K for 1 h. The changes in bulk modulus, Young's modulus and Poisson's ratio have been obtained from the ultrasonic velocity data. The variation in hardness and density has been found to be consistent with the changes in velocities and elastic constant. The ultrasonic on-line measurements from 300 to 623 K reveal clearly all the four transitions, including the formation of GP zones at 368 K, formation of δ' at 470 K, dissolution of δ' at 532 K and the formation of equilibrium S' and δ phases at 590 K. The first differential of temperature dependent ultrasonic parameters is found to be particularly effective in clearly diagnosing all the fine scale precipitation/dissolution events. Differentiation between the precipitation and dissolution type reactions is also possible from careful analysis of the first differential plots.