Chapter 10: Phase Transformations: Development of Microstructure and Alteration of Mechanical Properties

Phase transformations are organized into three categories based on their dependence on atomic diffusion and compositional change: transformations without compositional shifts that require diffusion, transformations with compositional redistribution that depend on diffusion, and athermal diffusionless processes. The fundamental mechanism underlying all transformations involves nucleation and growth, wherein stable nuclei emerge only after overcoming critical free energy barriers and achieving a minimum critical radius. Homogeneous nucleation, occurring spontaneously within bulk material, demands substantial supercooling but is less common in practical systems. Heterogeneous nucleation at grain boundaries, surface defects, and existing interfaces substantially reduces energy requirements and represents the dominant transformation pathway in real materials. Kinetic behavior is described by the Avrami equation, which characterizes the fraction of material transformed as a function of time through a characteristic S-shaped progression. Time-temperature-transformation diagrams and continuous-cooling-transformation diagrams visualize the relationship between temperature, elapsed time, and resulting microstructure, serving as essential tools for designing heat treatment processes. The chapter details microstructural evolution in iron-carbon systems, including the eutectoid reaction that produces pearlite in laminar form, with coarser structures developing at higher temperatures and finer structures at lower temperatures. Bainite forms at moderate temperatures through a mixed-mode transformation generating needle or plate morphologies with excellent combinations of strength and ductility. Spheroidite develops through prolonged heating, yielding soft, easily machined structures. Martensite arises from rapid quenching and represents an ordered tetragonal iron structure that exhibits maximum hardness but minimal ductility. Tempering reheating of martensite below the eutectoid temperature partially restores ductility by allowing controlled cementite precipitation. The chapter concludes by connecting microstructure to mechanical behavior and introduces shape-memory alloys like Nitinol, which exploit martensitic transformations to enable recovery of original geometry upon heating, illustrating how transformation mechanisms enable engineering innovation.