Creep of Al-Cu alloys during age hardening

High-purity polycrystalline alloys of aluminum with 1 to 4 pct Cu were tested in creep over a temperature range of 200° to 540°C. Hardness, X-ray, and tensile data supplemented the creep results. Starting with alloys in the quenched, single-phase condition, creep tests were performed both within the single-phase and two-phase region of the equilibrium diagram. Aging under stress was initiated by enclosing the specimen with the hot furnace at the same time that the load was applied. The effects of aging on the minimum creep rate were determined by comparison of two types of creep curves; 1) the curves obtained when creep is accompanied by aging, and 2) the curves obtained under identical circumstances but without aging. The first curves were determined experimentally, while the second were obtained by extrapolation of the high-temperature single-phase creep data into the two-phase field. At the intermediate stresses, precipitation resulted in stronger alloys. Inversions in the relative strengths of the two-phase and single-phase alloys were predicted at the higher and lower stresses, whereby the single-phase alloys became stronger than the two-phase alloys. Low stress (high temperature) inversions have been reported by other investigators.