Chapter-10: Phase Transformations in Metals

10.1 Introduction
The goal is to obtain specific microstructures that will improve the mechanical properties of a metal, in addition to grain-size refinement, solid-solution strengthening, and strain-hardening.

10.2 Basic Concepts
Phase transformations that involve a change in the microstructure can occur through:
Diffusion
Maintaining the type and number of phases (e.g., solidification of a pure metal, allotropic transformation, recrystallization, grain growth.
Alteration of phase composition (e.g., eutectoid reactions, see 10.5)
Diffusionless
Production of metastable phases (e.g., martensitic transformation, see 10.5)

10.3 The Kinetics of Solid-State Reactions
Change in composition implies atomic rearrangement, which requires diffusion. Atoms are displaced by random walk. The displacement of a given atom, d, is not linear in time t (as would be for a straight trajectory) but is proportional to the square root of time, due to the tortuous path: d = c(Dt) 1/2 where c is a constant and D the diffusion constant. This time-dependence of the rate at which the reaction (phase transformation) occurs is what is meant by the term reaction kinetics.
D is called a constant because it does not depend on time, but it depends on temperature as we have seen in Ch. 5. Diffusion occurs faster at high temperatures.
Phase transformation requires two processes: nucleation and growth. Nucleation involves the formation of very small particles, or nuclei (e.g., grain boundaries, defects). This is similar to rain happening when water molecules condensed around dust particles. During growth, the nuclei grow in size at the expense of the surrounding material.
The kinetic behavior often has the S-shape form of Fig. 10.1, when plotting percent of material transformed vs. the logarithm of time. The nucleation phase is seen as an incubation period, where nothing seems to happen. Usually the transformation rate has the form r = A e-Q/RT (similar to the temperature dependence of the diffusion constant), in which case it is said to be thermally activated.

10.4 Multiphase Transformations
To describe phase transformations that occur during cooling, equilibrium phase diagrams are inadequate if the transformation rate is slow compared to the cooling rate. This is usually the case in practice, so that equilibrium microstructures are seldom obtained. This means that the transformations are delayed (e.g., case of supercooling), and metastable states are formed. We then need to know the effect of time on phase transformations