Early Stage Hardening in AlCuMg Alloy Studied by <sup>63</sup>Cu Nuclear Magnetic Resonance and Positron Annihilation

Abstract

Nuclear magnetic resonance (NMR) is shown to be a sensitive metallurgical characterization technique for metastable phase development during early stage aging of the high purity experimental alloy Al(1.05Cu 1.7Mg) (at.%). It is shown, using 63Cu NMR and positron annihilation lifetime spectroscopy (PALS), how room temperature structural evolution proceeds in Al(1.05Cu 1.7Mg) prepared in the form of supersaturated solid solution. The combination of NMR and PALS allows identification of solute aggregate chemistry and defect kinetics during natural aging. Guinier-Preston-Bagaryatsky (GPB) zone formation is detected by NMR within 0.5 h of quench, and the percentage of Cu atoms in GPB zones increases to ~80% within 50 h at room temperature, with the residual 20% of Cu atoms remaining in solid solution. The formation of GPB zones corresponds with the Vickers hardness, in fact the hardness is shown to directly depend on the amount of total Cu partitioned to GPB zones. The vacancy kinetics, as measured by PALS, show an exponential decay in time following the quench with the majority of decay complete within 50 h supporting the notion that early stage hardening by GPB zone formation is controlled by defect concentration and availability.