Chapter 6: Diffusionless Transformations

The discussion distinguishes martensitic transformations from diffusion-controlled reactions by describing the atomic movement as a "military" transformation, where atoms shift in a disciplined, coordinated manner across a glissile interface, often traversing grains at speeds approaching the speed of sound. A significant portion of the chapter is dedicated to the crystallography of the phase change, specifically the Bain Strain model, which explains how the face-centered cubic austenite lattice is distorted into a body-centered tetragonal martensite structure. Because the pure Bain deformation alters the lattice dimensions in a way that disrupts the interface, the text explains the requirement for a lattice invariant shear—accomplished through internal mechanisms such as twinning or dislocation slip—to ensure the habit plane remains macroscopically undistorted (an invariant plane strain). The summary explores how alloy composition, particularly carbon content, dictates the morphology of the resulting microstructure, distinguishing between the lath martensite found in low-carbon steels and the acicular, plate-like twinned martensite observed in high-carbon or high-nickel alloys. It also addresses the thermodynamics of the transformation, including the concept of the Martensite Start (Ms) and Martensite Finish (Mf) temperatures, and provides evidence that nucleation is a heterogeneous process occurring at lattice defects rather than a homogeneous one. Furthermore, the chapter details the tempering process, where the metastable, supersaturated martensite decomposes upon heating into stable phases through sequences involving carbon segregation, the precipitation of transition carbides like epsilon-carbide, and the formation of cementite or alloy carbides which lead to secondary hardening. Finally, the concepts are applied to advanced material systems, including Transformed Induced Plasticity (TRIP) steels, maraging steels, and the unique reversible shape-memory effect seen in equiatomic nickel-titanium alloys known as Nitinol.