Development of Microstructural Resettable Alloy Via Segregation Engineering

Although there is a limitation to reduce greenhouse gas emissions only through reduction of materials, reusing of materials could be one of the best method for reducing the total amount of greenhouse emission. Therefore, this study focused on developing a novel reusable alloy by resetting the mechanical properties after certain period of using. We suggest a unique alloy design method and resetting heat treatment which could save huge amount of energy cost and greenhouse gas emissions when it is commercialized. Mn was selected as a key alloying element, considered the segregation tendency and effect on phase stability in 9Cr steel system, which is widely used as a structural material. In the segregation engineering concept, the segregation tendency and the effect of the segregation to defects are most important factors which should be considered. In 9Cr steel, segregation of Mn to grain boundary could increase the local stability of the austenite phase. This could lead to partial reversion of grain boundary austenite when the temperature is precisely controlled. Since Mn segregation tendency in iron solvent is largest among 3d-transition metal, Mn is the best candidate to do the segregation engineering in 9Cr steel. After homogenization and air cooling, microstructure was constituted of fully martensite. During the subsequent annealing which is called reversion process, partial reversion occurs at the grain boundary by the Mn element which has been segregated and enhances the local reversion kinetics. Furthermore, we find out that this metastable austenite exhibit transformation induced plasticity (TRIP) behavior during deformation. As a result, the ductility of the 9Cr steel greatly increased with no significant reduction in strength. Since fresh martensite at the grain boundary which is transformed during deformation has high concentration of Mn, it could be partially transformed to austenite at relatively low temperature. Through precise control of this microstructural features and annealing condition, it was able to reset the microstructure after deformation. It was possible to implement a repetitive deformation and resetting process for resetting mechanical properties by controlling the reversible local phase transformation. Furthermore, we optimized the resetting process which is dependent on pre-strains that represent damage accumulated level. This results could suggest a novel approach method to develop a reusable metallic material which is becoming more important for the sustainable development.