Study of Fracture Mechanisms of Ultrahigh Strength Steels with Nanometer Scale Twins

Synthesizing high-strength and high-ductility materials is always the driving force for materialists to explore the strengthening and toughening mechanism of materials and to modify the compositions and microstructures for extreme performance. Among the various means to reach the ideal properties, the nanostructured material has long been recognized as an effective solution. However, nanocrystalline (NC) materials suffer from low ductility which hinders their applications because the strengthening mechanisms used basically increase grain boundary volume fraction by reducing the grain size down to the nanometer size. The experimental results reported recently in SCIENCE [1, 7-8] on NT copper seem to suggest that denser twins could render higher strength as well as higher ductility. These results demonstrated that materials with nanometer twins are materials with high potential to reach the dual properties of high ductility and high strength. The goal of the present project is to develop a new route for producing high density nano-twin steels with ultrahigh strength (Yield stress > 2 GPa) by maintaining very good ductility (> 20% elongation in tension). Based on our preliminary study, different high C (from 0.45-1%) and Mn contents (from 17% to 23%), which exhibits exceptional twin induced plasticity capacities with very good combination of strength and ductility, will be selected as a family of case study materials. We will investigate the fundamental mechanisms of such exceptional performances. The proposed project includes three scientific tasks as follows:To investigate the optimized processing conditions by simulation to obtain the high nano-twin density in high strength steelTo produce a steel with the dual properties of ultrahigh yield strength (2 GPa) and high ductility with an elongation higher than 20%To develop the physical models for describing the strength and ductility of nano-twinned materials and metals with dual phases of nano-grain and nano-twins