Martensitic Transformation: Understanding Steel Microstructure

This lecture discusses the martensitic transformation in steel, focusing on the growth rate of martensite and the energy dynamics involved. The instructor explains that martensite forms rapidly once it germinates, driven by the stability of ferrite at low temperatures compared to austenite. The driving force for this transformation is the difference in free energy between imperfect martensite and austenite, which increases as temperature decreases. However, the formation of martensite incurs an energy cost due to the deformation that accompanies the transformation, leading to internal stresses and stored elastic energy within the alloy. The lecture also covers the phase diagram of iron-carbon alloys, highlighting the solubility of carbon in austenite versus ferrite and the significance of the eutectoid composition. The instructor emphasizes the importance of understanding these transformations for material properties and applications, particularly in the context of steel's mechanical characteristics and the implications of internal stresses during phase changes.