Positron annihilation study of ageing precipitation in deformed Fe–Cu–B–N–C

Abstract

The role of deformation-induced defects and carbon addition on copper precipitation during ageing at 550 °C is investigated in high-purity Fe–Cu–B–N–C alloy samples by positron annihilation spectroscopy. Complementary small-angle neutron scattering (SANS) and hardness tests are utilized to characterize the size distribution of the Cu precipitates formed and their influence on the mechanical properties. Samples with 0 and 8% cold pre-strain are utilized to study the influence of prior tensile deformation on the precipitation kinetics of copper. The time evolution of the coincidence Doppler-broadening spectra indicates that deformation-induced defects enhance the Cu precipitation kinetics, which is confirmed by the SANS results. In the S–W plot, a clear reduction in open volume defects is accompanied by a strong increase of Cu signature during the initial stage of ageing, implying that the open volume defects (mainly dislocations) act as nucleation sites for Cu precipitation. A comparison between the precipitation behaviour of Fe–Cu, Fe–Cu–B–N and Fe–Cu–B–N–C indicates that the addition of carbon does not alter the Cu precipitation mechanism but decelerates the kinetics. Hardness results confirm that carbon counteracts the acceleration of Cu precipitation caused by the addition of B and N.