Hi, this is the second class of microstructure and phase transformation in steels. In this class, we will think about the evolution of non-equilibrium microstructure and related phase transformation phenomena in iron and steel. At first, let’s consider an iron-carbon alloy having these carbon concentrations. At this temperature, the microstructure of the alloy will be an austenite. Then, let’s think about what happens when we cool down the alloy to room temperatures. As we learned in the previous class, if the cooling rate is slow enough, the microstructure at room temperature will be close to that expected from the equilibrium diagram. It tells us that the microstructure of the alloy will consist of ferrite and pearlite as show in micrograph B. However, if the cooling rate is sufficiently fast, characteristic microstructure appears which is not predicted from the equilibrium phase diagram as shown in this micrograph C Crystallographic analysis reveals that this non-equilibrium microstructure has a nearly body centered cubic structure, more precisely, carbon super-saturated body centered cubic or body centered tetragonal structure. The evolution of non-equilibrium microstructure is led by the suppression of the diffusional reconstruction of lattice structure and the chemical redistribution because of the rapid cooling. This non-equilibrium microstructure is created by displacive transformation, which is totally different from the reconstructive mechanism. In this class, we will consider the nature of displacive transformation and the resultant microstructure.