Thermally activated precipitation at deformation-induced defects in Fe-Cu and Fe-Cu-B-N alloys studied by positron annihilation spectroscopy

Abstract

We have investigated the influence of deformation-induced defects on the isothermal precipitation at $550\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ in as-quenched (solute-supersaturated) and annealed (solute-depleted) Fe-Cu and Fe-Cu-B-N alloys by positron annihilation spectroscopy and hardness tests. Using the coincidence Doppler broadening technique, the evolution of local environment at the positron annihilation sites (open-volume defects, Cu precipitates, and matrix) was monitored as a function of the aging time. For all samples, plastic deformation causes a pronounced change in $S$ and $W$ parameters signaling the formation of open-volume defects. For the as-quenched samples, aging results in a sharp decrease in the amount of open-volume defects combined with the rise of a strong copper signature, which can be attributed to preferential copper precipitation at the open-volume defects introduced by plastic deformation. In contrast, the open-volume defects of the annealed samples can only be reduced partially. Both the hardness tests and the positron annihilation spectroscopy indicate that the addition of B and N to the Fe-Cu alloy causes a significant acceleration of the precipitation in the as-quenched alloys.