Phase Transformations

Introductory remarks

Much of materials science and engineering is concerned with microstructures in materials; therefore, much of materials curriculum is devoted to studying processes that are used for creating and stabilizing desirable microstuctures, and avoiding undesirable ones. Solid-solid structural transformations, which form the core of this course, are a good example of processes that produce a wide and interesting variety of microstructures; other examples of microstructure-producing processes would include: solidification, recrystallization and grain growth and mechanical deformation.

In this course, the emphasis is on solid-solid structural transformations: those transformations that involve a change in crystal structure; as we will see, they lead to the formation of microstructures. What, then, are 'non-structural' transformations? These are transformations that arise due to changes in the material at a sub-atomic level (for example, in the electronic structure) without a significant change in crystal structure; for example, $\alpha$-iron retains its bcc structure in both paramagnetic and ferromagnetic states. Other examples would include normal -> superconducting, and paraelectric -> ferroelectric transitions.

Some of these other transitions are often accompanied by a significant change in crystal structure; for example, the ferroelectric transition in barium titanate does produce a significant change in the crystal structure; the product is tetragonal, while the parent phase is cubic. Even in such cases, the microstructural features that are produced can be analyzed as due to a structural transition. For example, the the microstructure in the ferroelectric phase can be rationalized as being produced as a result of a martensitic tranformation involving a parent cubic phase and a product tetragonal phase. One can gain at least a zeroeth order understanding of microstructures, which can be refined (if at all it is needed!) by taking into account other features that arise from the fact that the product phase is ferroelectric by including, for example, the domain interactions through long-range electric fields.

Similarly, we will not deal with solidification; there is a separate course for studying solidification-related phenomena.

Central theme

In this course, we take the viewpoint that structural phase transformations in the solid state constitute a non-trivial application of three foundational domains of materials science: crystal structures, thermodynamics and kinetics (diffusion). This 'application' is non-trivial in another sense as well: solid-solid transformations demand that we view interfaces as fundamental entities whose structure and thermodynamic and kinetic properties (which are all intimately related) largely determine the microstructures arising from phase transformations. In a way, this course may as well be titled 'Interfaces in Solids'!

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