ABSTRACT
This chapter describes the transformations observed in different close-packed structures and then discusses the investigation of the mechnism of transformation by X-ray diffraction methods. A metastable state exists at a local free energy minimum and faces an activation barrier for a phase transition to the equilibrium state. The transition of a metastable state to the equilibrium state may involve additional intermediate metastable states if access to phase transition requires a smaller activation energy than that required for the direct transition to the equilibrium state. The polytype structures occurring in cobalt-based alloys often contain large concentrations of random distribution of stacking faults giving rise to diffuse intensity on the X-ray diffraction photographs. In cases the diffraction effects cannot be computed using the conventional theories of X-ray scattering from one-dimensionally disordered crystals developed on the assumption of random faulting. A suitable model of non-random faulting based on the mechanism of the transformation has therefore to be employed to interpret the experimental results.
