ELECTRICAL-RESISTIVITY AND POSITRON LIFETIME STUDIES OF PRECIPITATIONEFFECTS IN AL-CU ALLOY

Decomposition of the supersaturated solid solution of 2 at.% Cu in Al was investigated by means of electrical resistivity and high-resolutio n positron lifetime measurements. The phase composition of this alloy was determined by transmission electron microscopy. Electrical resisti vity measurements were performed by a classical four-point method. Pos ition lifetime spectra were measured by means of a spectrometer consis ting of two BaF2 detectors and a standard fast-slow coincidence system . The specimens were first exposed to the solution heat treatment at 7 83 K for 19 hours with a subsequent quenching. Then the specimens were isochronally annealed in the temperature range 293 divided by 573 K. Annealing responses of electrical resistivity and positron annihilatio n were studied. Resistivity measurements as well as transmission elect ron microscopy observations confirmed the well-known decomposition seq uence of the Al-Cu alloy. The decomposition of the alloy was manifeste d by the decrease in the intensity of positron lifetime component tau( 2) = (207 +/- 2) ps correlated with simultaneous appearance and increa se in the intensity of tau(3) = (180 +/- 5) ps component. Component ta u(2) originates from positron annihilation in vacancies trapped at the Guinier-Preston zones while tau(3) comes from annihilation of the pos itrons localized in the misfit dislocations at coherent precipitates o f the Al2Cu phase. The shortest observed component tau(1) apparently b elongs to annihilation of untrapped positrons.