Contribution of soft cu particles to tensile deformation behavior in iron

Dispersion strengthening is one of the most cffcctivc strengthening methods in metals. In particular, many kinds of hard particles, such as carbide and oxide, have been applied as dispersed particle in structural steel. On the other hand, precipitation of Cu, which is softer than the ferritc matrix, also draws attention as dispersed particle for strengthening, because nano-sized Cu particles arc easily dispersed uniformly by aging heat treatment within matrix, and besides, its Ostwald growth ripening is very slow. In this study, Cu and VC-dispcrsed ferritic steels with an identical yield strength were prepared, and then subjected to tensile testing and cold rolling in order to compare the effects of soft Cu and hard carbide (VC) particles on tensile deformation behavior of iron. As a result, Cu-dispcrscd steel exhibited lower work hardening rate and larger elongation. In particular, the local elongation is significantly larger than VC-dispersed steel. TEM observation reveals that the shape of VC within ferritc matrix is unchanged even after severe cold deformation and that tangled dislocation structure develops around VC panicles. This leads to the large work hardening rate is formed in carbide-dispersed steel. On the contrary. Cu particles were clearly elongated along the deformation direction with own plastic deformation in high strain region. For example, the average aspect ratio of the deformed Cu particles was elongated up to 7.2 after 70% cold rolling. In addition, tangled dislocation was hardly observed around the particles. Therefore, it is concluded that the large local elongation of Cu-dispcrscd steel is derived from the plastic deformation of Cu particles resulting in the reduction of the stress concentration at (ferritc / particle) interface and the formation of a void there.