Precipitation and Phase Transformation of Copper Particles in Low Alloy Ferritic and Martensitic Steels

Abstract

In order to make clear the influence of matrix microstructures on age-hardening behavior in copper containing steels, the precipitation and the phase transformation of copper particles during isothermal aging were investigated by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and atom-probe field ion microscopy (AP-FIM). In ferrite, it was found from the DSC analysis and the TEM observations that copper clusters, bcc and 9R structures exist prior to the formation of the stable fcc copper phase, and that the main structure of the particles in a maximum hardness specimen is a bcc structure. The bcc→9R transformation was found to occur during the particle coarsening by the qualitative analysis of the DSC curves and the kinetics was discussed by the free energy calculation in Fe-matrix/Cu-precipitate system. In martensite, most of the copper precipitated on laths and on dislocations during isothermal aging. This resulted in the decrease of the amount of bcc particles and the considerable decrease of maximum age-hardness of the specimen. On the other hand, the age-hardening behavior of 80%-deformed ferrite was almost the same as that of ferrite, although the dislocation density of the deformed ferrite was larger than that of ferrite. In this paper, the factors controlling the precipitation and the phase transformation of copper particles in association with the hardening behavior are discussed with focus on the effect of defects (dislocation, vacancy) in steels. Moreover, the effect of manganese addition on the precipitation of copper particles is discussed.