phasetransformation

Phase transformation is a process in which a material changes from one phase or crystal structure to another, typically in response to changes in temperature, pressure, composition, or external fields. These changes alter properties such as density, symmetry, or mechanical behavior.

In solids, common transformations include solid–solid transitions between polymorphs or allotropes, as well as diffusion-based transformations such as precipitation or eutectoid decomposition that produce distinct phases in a material.

Mechanisms can be diffusion-controlled, where atoms rearrange by long-range diffusion, or diffusionless (displacive), where atoms shift cooperatively with little or no diffusion, as in martensitic transformations.

Driving forces are governed by thermodynamics: a lower Gibbs free energy phase is favored at a given temperature and pressure; undercooling or supersaturation, elastic strain, and chemical driving forces can promote transformations. Hysteresis often accompanies first-order transformations.

Kinetics involve nucleation of a new phase and subsequent growth, or rapid cooperative rearrangements in diffusionless cases. Models include nucleation-and-growth theories and phase-field simulations.

Characterization uses differential scanning calorimetry, X-ray diffraction, electron microscopy, and in-situ techniques to identify phases, measure latent heat, and monitor microstructural evolution.

Examples include water–ice, austenite–martensite in steels, alpha–beta titanium or zirconia phase changes, and diffusion-controlled eutectoid or precipitation transformations in alloys.

Applications span heat treatment of metals to tailor hardness and toughness, shape-memory alloys, ceramics, and thin-film devices, where control of phase assemblages determines performance.