Micromechanism involved in ultrafine grained ferrite/martensite dual phase steels strengthened by nanoscale Cu-rich precipitates

Abstract

The strengthening effects of grain refinement and nano-sized coherent precipitation are effective strengthening mechanisms in the development of low-cost advanced high-strength steels. In the present work, low carbon dual phase steels with ultrafine grains and nanoscale copper rich precipitates were produced by a combination of warm deformation and isothermal aging processes. Then, the microstructure and plastic deformation behavior were explored using the EBSD technique and crystal plasticity simulations, and the precipitation mechanism of nanoscale Cu-rich particles was investigated by HRTEM, atom-probe tomography in combination with first-principles calculations. The results indicate that the formation and compositional evolution of the Cu-rich particles are associated with the mixing enthalpy of alloying elements and the attractive interaction between the solute atoms. Besides the precipitation hardening effect, grain refinement improve both strength and ductility of the steels significantly, while more severe deformation will result in the heterogeneous distribution of martensite, then intensify the plastic heterogeneities and early hardening of ferrite grains during tension deformation, thus give rise to a decrease in ductility.