Comparison of the atomic structure, composition, kinetics and mechanisms of interfacial motion in martensitic, bainitic, massive and precipitation face-centered cubic–hexagonal close-packed phase transformations

This investigation examines the atomic-level structural, compositional, kinetic and mechanistic aspects of phase transformations that occur during growth of product phases in face-centered cubic to hexagonal close-packed martensitic, bainitic, massive and precipitation reactions. A number of different alloy systems are examined, including Co–32Ni (numbers indicate at.%) for martensite (displacive), Ti–H and Zr–24N for bainite (or diffusional-displacive), Ti–46Al for massive (short-range diffusional) and Al–4Ag, Ti–48Al(–2Ta), Zr–34N and Al–2Cu(–Mg–Ag) for precipitation (long-range diffusional). These alloys are crystallographically similar, but the Ti–H and Zr–N systems involve interstitial solute and the Ti–46Al and Zr–34N systems do not have a low-index orientation relationship between the phases. It was found that all four transformations occur by the motion of ledges at the interfaces, although massive interfaces can also move by a continuous growth mechanism. The compositional changes involved in the transformations in Al–4Ag, Ti–H and Zr–N alloys are complete to within a few atom spacings at the ledges, and the growth rates of the interfaces are consistent with long-range diffusion of either interstitial or substitutional atoms. In the case of Co–32Ni alloy, the growth kinetics are clearly those of a diffusionless transformation. The similarities and differences among these various transformation interfaces and important questions in our understanding related to similar transformations in other crystal systems such as face-centered cubic to body-centered cubic are discussed.