Positron annihilation study on repeated deformation/precipitation aging in Fe–Cu–B–N alloys

We have investigated the influence of deformation-induced defects on the isothermal Cu precipitation at 550 °C in an as-quenched (solute supersaturated) and annealed (solute depleted) Fe–Cu–B–N alloy by positron annihilation spectroscopy. Preferred precipitation of supersaturated Cu at defects sites was recently proposed as a self-healing mechanism for deformation damage. Repeated deformation/aging cycles result in a significant increase in tensile strength for as-quenched samples. Positron Annihilation Lifetime Spectroscopy revealed a larger positron lifetime for the annealed samples compared to the as-quenched samples caused by the additional open-volume defects at incoherent interfaces between Cu precipitates and the matrix. For both as-quenched and annealed samples deformation generates new defects (mainly dislocations), while subsequent aging leads to a reduction in the density of these defects. Simultaneous Coincidence Doppler Broadening measurements monitored the evolution of local environment at the positron annihilation sites (defects, Cu precipitates and matrix) during the repeated deformation/aging cycles. For the as-quenched samples, aging after deformation results in a sharp decrease in the defect contribution combined with a strong increase in the copper signature, which can be attributed to preferential copper precipitation at the defects introduced by plastic deformation. For the annealed samples, aging after deformation leads to a much weaker evolution of the Cu signature.