Chapter 3: Crystal Interfaces & Microstructure

Crystal Interfaces & Microstructure begins by defining interfacial free energy and surface tension using the broken-bond model, explaining how the coordination number of surface atoms influences the energy of solid/vapor interfaces. The text details the Wulff construction as a method for determining the equilibrium shape of a crystal by minimizing total surface energy based on the gamma-plot of orientation-dependent energies. The discussion transitions to grain boundaries within single-phase solids, distinguishing between low-angle tilt and twist boundaries—modeled as arrays of edge and screw dislocations—and high-angle random boundaries that exhibit more disordered structures. Key concepts in microstructural evolution are explored, including the kinetics of grain growth driven by boundary curvature and the reduction of total interfacial energy, as well as the phenomenon of secondary recrystallization or abnormal grain growth. The chapter also addresses the retarding effect of second-phase particles on boundary migration, known as Zener drag. A significant portion of the chapter classifies interphase interfaces into fully coherent, semicoherent, and incoherent types based on atomic matching and lattice parameters. It examines how lattice misfit and elastic strain energy compete with interfacial energy to dictate the equilibrium morphology of precipitates, leading to shapes ranging from spherical Guinier-Preston (GP) zones to Widmanstatten plates and needles. The mechanisms of coherency loss are analyzed, describing how growing precipitates transition from coherent to semicoherent states through the introduction of misfit dislocations or vacancy condensation once a critical radius is exceeded. Furthermore, the text defines glissile interfaces, such as those involving Shockley partial dislocations in martensitic transformations, and contrasts them with non-glissile interfaces found in civilian transformations. The chapter concludes by differentiating between atomically smooth and diffuse solid/liquid interfaces and analyzing interface migration kinetics, specifically distinguishing between diffusion-controlled growth and interface-controlled growth mediated by the ledge mechanism.