Diffusionlose

Diffusionlose, or diffusionless transformations, are phase changes in solids that occur with negligible long-range atomic diffusion. Instead, the atoms move cooperatively through displacive mechanisms such as lattice shears and shuffles, producing a new crystal structure while largely preserving chemical order.

These transformations are typically rapid and can be driven by temperature changes or mechanical stress. They often produce characteristic microstructures, such as twinned variants or needle-like plates, resulting from the crystallographic relationships between the parent and product phases. Important kinetic and thermodynamic markers include start and finish temperatures for the transformation, often denoted Ms and Mf for martensitic transformations and As and Af for the reverse austenite transformation in alloys.

The most well-known example occurs in steel: quenching austenite transforms it into martensite, a hard, brittle phase formed without diffusion. Diffusionless processes also occur in shape memory alloys such as nickel-titanium (NiTi), where temperature or stress can induce a reversible, diffusionless phase change between austenite and martensite, giving rise to shape memory and superelasticity. Diffusionless transformations are also observed in certain intermetallics and ceramics, though the specific crystallography and resultant properties vary.

In contrast, diffusion-controlled transformations involve redistribution of elements over long ranges (for example, the formation of pearlite or bainite in steel), which typically occur more slowly and rely on atomic diffusion. Diffusionless transformations are valued for enabling rapid microstructural changes and distinctive mechanical behaviors without substantial compositional rearrangement.

See also: martensite, shape memory alloys, diffusion-controlled transformations, bainite.