Thermally-induced face-centered cubic to hexagonal close-packed martensitic phase transformations were studied in single crystals. Transmission electron microscopic  in situ  experiments, lattice fringe and high-resolution transmission electron microscopic images were analyzed. At room temperature, the as-grown monocrystals were in the hexagonal close-packed low-temperature phase, and hexagonal close-packed lamellae with an average thickness of 10 to 11nm were observed which were separated by very thin face-centered cubic lamellae of retained austenite. The shape of the transformation fronts, and a lack of long-range strain, indicated that the Shockley Burgers vectors of the partial dislocations which occurred at the fronts largely compensated each other. The reverse hexagonal close-packed to face-centered cubic transformation was studied during  in situ  heating. High-resolution transmission electron microscopic images were used to analyze the partials, which were located at the transformation fronts, by drawing Burger’s circuits around them. In the case of high-resolution transmission electron microscopic images for a [12¯•0]hcp  [1¯10]fcc beam direction, the characteristics of the partials were analyzed as being 30, 90, -30, 30, 90, -30, 30, 90, -30. This regular sequence of the 3 different Burgers vectors led to compensation at the atomic scale. It was concluded that transformation models that were based upon a polar dislocation mechanism or upon overlapping stacking faults could be excluded.

T.Waitz, H.P.Karnthaler: Philosophical Magazine A, 1996, 73[2], 365-86