Chapter 5: Diffusional Transformations in Solids

Diffusional Transformations in Solids begins by classifying the major types of phase transformations, including precipitation, eutectoid, ordering, massive, and polymorphic changes, emphasizing that most solid-state changes require thermally activated atomic movements. The text rigorously explores the thermodynamics and kinetics of nucleation, distinguishing between homogeneous nucleation, where embryos form randomly in the matrix, and the far more common heterogeneous nucleation, which occurs on high-energy defects such as grain boundaries, dislocations, stacking faults, and inclusions to reduce the activation energy barrier. The influence of interfacial energy, volume free energy, and misfit strain energy on critical nucleus size and nucleation rates is analyzed, alongside the role of undercooling. Following nucleation, the chapter details precipitate growth mechanisms, contrasting diffusion-controlled lengthening of plates and needles with the thickening of plates via ledge migration, while also addressing the Zener approximation for growth rates. The kinetics of overall transformations are mathematically described using the Johnson-Mehl-Avrami equation and visualized through Time-Temperature-Transformation (TTT) diagrams, which illustrate the competition between nucleation and growth rates. A significant portion of the chapter is dedicated to precipitation in age-hardening alloys, specifically using the Aluminum-Copper and Aluminum-Silver systems to illustrate the sequence from supersaturated solid solution to Guinier-Preston (GP) zones, intermediate transition phases, and finally the equilibrium phase. This section also covers the crucial role of quenched-in vacancies and the formation of precipitate-free zones (PFZs). The text further distinguishes between nucleation and growth processes and spinodal decomposition, a mechanism driven by negative diffusion coefficients where no nucleation barrier exists within the coherent spinodal. Mechanisms of microstructural instability, such as particle coarsening or Ostwald ripening, are explained through the Gibbs-Thomson effect. The chapter then shifts focus to ferrous metallurgy, analyzing the decomposition of austenite into proeutectoid ferrite (forming grain boundary allotriomorphs or Widmanstatten side-plates depending on undercooling) and the cooperative growth of ferrite and cementite in the pearlite reaction. It differentiates these diffusive processes from the formation of bainite (upper and lower) and the diffusionless, civilian massive transformations observed in systems like Copper-Zinc. Finally, the chapter concludes with discussions on ordering transformations, where atoms arrange into superlattices, and applies these theoretical concepts to practical case studies, including the heat treatment of Titanium-Aluminum-Vanadium forging alloys and the weldability of low-carbon microalloyed steels, highlighting the impact of cooling rates and phase stability on mechanical properties.