Positron Annihilation Spectroscopy in the Evaluation of Microstructure of Cast Copper and Copper-Aluminum Alloys during Isochronal Annealing

Abstract

Isochronal annealing was carried out for cast Cu, Cu-5wt.%Al, Cu-10wt.%Al, and Cu-20wt.%Al alloys in the range of 423-1173 K. The annealing behavior was followed by positron annihilation lifetime spectroscopy (PALS). Microhardness (Hv) has been measured and used for comparison. The microstructure variations were observed by optical and scanning electron microscopy. Three stages were distinguished for all samples in the behavior of average lifetime ( av) and Hv. In the first stage (room temperature to 573 K), av and Hv reach extremely high values for Cu-20wt.%Al compared with other alloys. The analysis of the first stage indicates anneal softening for cast Cu and Cu-5wt.%Al and anneal hardening for Cu-10wt.%Al and Cu-20wt.%Al. The second stage (573-873 K) is characterized by a maximum of av at 673 K followed by a decrease up to the end of the stage for cast Cu, Cu-5wt.%Al (the highest maximum), and Cu-10wt.%Al; a slight change is observed for Cu-20wt.%Al. The change in Hv is notable only for Cu-10wt.%Al and Cu-20wt.%Al, and indicates that av is sensitive to the change that occurred in the bulk material, while Hv shows the general behavior of the surface of the material. The third stage (873-1173 K) is characterized by near saturation for both av and Hv, indicating complete recrystallization for cast Cu, Cu-5wt.%Al, and Cu-10wt.%Al and complete softening for Cu-20wt.%Al. Recrystallization is shifted to higher temperatures by increasing the amount of Al in Cu. The analysis of positron lifetime presents evidence of the existence of three-dimensional (3D) defects in addition to one-dimensional defects, so a modification to the simple trapping model is needed when considering inhomogeneous trapping.