Monitoring the precipitation reactions in a cold-rolled Al–Mn–Mg–Cu alloy using thermoelectric power and electrical resistivity measurements

Abstract

The aim of the present paper is to monitor multiple precipitation reactions in a normal Al alloy AA 3104 and a Mg- and Cu-rich alloy 3104 during isothermal annealing. Special emphasis is placed on the influence of dislocations on the precipitation kinetics and therefore different cold-rolling reductions are applied prior to the annealing treatment. Using both thermoelectric power (TEP) and electrical resistivity measurements it is possible to split concurrent and sequential precipitation reactions and to reduce the uncertainty about the actual precipitation reactions taking place. It is found that Al₂CuMg precipitates form at low annealing temperatures, irrespective of the extent of pre-deformation. During annealing at temperatures from 350°C and higher, precipitation of Mn-containing dispersoids is determined as the only process occurring. The rate of this precipitation process is strongly influenced by the degree of cold work prior to the heat treatment. The qualitative information derived from the combined TEP and electrical resistivity experiments is supported by transmission electron microscopy observations. It is also possible to calculate the weight fractions of the elements precipitated semiquantitatively. These calculated fractions are compared with equilibrium thermodynamic data and show good agreement for the precipitation of Al₂MgCu and the precipitation of Mn at temperatures up to 400°C.