Calorimetric studies of 7000 series aluminum alloys: I. Matrix precipitate characterization of 7075

Abstract

Both differential scanning calorimetry (DSC) and hot stage transmission electron microscopy were used to characterize the solid state reactions accompanying heating of the highest strength (T651) and overaged (T7351) tempers of 7075 aluminum alloy. Each of the observed endothermic or exothermic reactions that occurs over the 20 to 500°C temperature range has been ascribed to the dissolution or formation of a specific precipitate. The dissolution of each matrix phase,i.e., GP zones, ή and ή, can be characterized by a distinguishable dissolution temperature, dissolution enthalpy, activation energy, and activation entropy. The values of activation energy and activation entropy for the dissolution of the initial matrix precipitate of these phases indicate that the relative stability of the matrix precipitates is primarily influenced by the entropy rather than the energy term. This investigation provides a basis for the use of DSC analysis in the rapid, quantitative identification of the matrix microstructure of 7075 aluminum alloy as well as in the characterization of the matrix microstructure of other 7000 series aluminum alloys.