Papers by Author: Yong-Hee Lee

Abstract: We have investigated athermal and isothermal martensitic transformations (typical displacive transformations) in Fe–Ni, Fe–Ni–Cr, and Ni-Co-Mn-In alloys under magnetic fields and hydrostatic pressures in order to understand the time-dependent nature of martensitic transformation, that is, the kinetics of martensitic transformation. We have confirmed that the two transformation processes are closely related to each other, that is, the athermal process changes to the isothermal process and the isothermal process changes to the athermal one under a hydrostatic pressure or a magnetic field. These findings can be explained by the phenomenological theory, which gives a unified explanation for the two transformation processes previously proposed by our group.

Abstract: The SPS process is a synthetic technique which enables sinter-bonding to occur at low temperature and within a short sintering time. Furthermore, it produces finer grain size than conventional sintering methods. Fe-TiC system is a relatively light material and is one third the size of tungsten carbide and less than half weight of tool steel. It is cost effective when used in Fe-TiC system due to high mass effect with low density and relatively low cost of changing tools and bearings. Fe-TiC system can be machined easily with conventional equipment. In the present work, SPS was conducted at the temperatures of 1200, 1250 and 1300°C at the sintering time of 3, 5 and 10min. As the sintering temperature and holding time increased, the relative density decreased and the hardness reached the lowest at 1250°C. These are attributed to the more pore being generated by liquid Pool at 1250°C and 1300°C. The result of the microstracture observation showed more M6C and MC carbide by liquid Pool as sintering temperature and holding time increased.