TABLE OF CONTENTS The quenching to room temperature of austenite in a steel can lead to the formation of martensite, a very hard phase in which the carbon, formerly in solid solution in the austenite, remains in solution in the new phase. Unlike ferrite or pearlite, martensite forms by a deformation of the austenite lattice without any diffusion of atoms. The deformation causes a change in the shape of the transformed region, consisting of a large shear and a volume expansion. Martensite is, therefore, often referred to as a diffusionless, shear transformation, which is highly crystallographic in character because it is generated by a specific deformation of the austenite. When the formation of martensite is constrained by its surroundings, it forms as thin plates or laths in order to minimize the strain energy due to the deformation.
The martensite reaction in steels normally occurs athermally, i.e. the fraction transformed depends on the undercooling below a martensite-start temperature, M s. The extent of transformation does not seem to depend on time, as expressed in the Koistenen and Marburger equation [1] which describes the progress of transformation below M s:
V ?? is the fraction of martensite and T q the temperature below M s, to which the sample is cooled. This athermal character is a consequence of very rapid nucleation and growth, so rapid that the time taken can be neglected. Instead, the fraction transformed depends only on the number of nucleation sites...
