Vacancy-solute interaction in the WE54 alloy during artificial ageing: a positron annihilation spectroscopy study

The vacancy–solute interaction during artificial ageing at 250oC was studied in the commercial alloy WE54 by coincidence Doppler broadening of the positron annihilation radiation. The results how that in the initial state of ageing positron trapping is weak. Under these conditions, the faint signal comes predominantly from small Zr particles, with minor contributions from vacancy–Nd complexes. Positron trapping increases concomitantly with the formation of the b´- and b1- phases at the onset of precipitation hardening, and becomes very intense at later ageing stages when the b1-phase is formed. The chemical composition in contact with open-volume defects changes after 4 h of ageing with a progressive enrichment in Mg, concomitant with a hardness increase. There is evidence for a prevalence of Nd over Y up to 24 h of ageing and then a prevalence of Y over Nd for 96 h (Y/Nd = 3) when the b-phase is dominant.