Transformation strains in martensitic phase transitions of Co alloys

The thermally induced fcc-->dhcp martensitic phase transformation was inves tigated in two different CoFe alloys (Co-5.75 and 6.0 at. % Fe). Analysis b y transmission electron microscopy methods yields that in both alloys the t ransformation proceeds by the movement of transformation dislocations (part ials) that are correlated on an atomic scale; partials on adjacent close pa cked planes interact and combine to a paired partial. Two different and com petitive modes of the transformation were observed. In Co-6.0 at.% Fe all t he paired partials have the same Shockley partial Burgers vector adding up their long-range strain fields (transformation mode A). Contrary to this, i n Co-5.75 at.% Fe paired partials of different Burgers vectors are compensa ting their long-range strain fields on an atomic scale (transformation mode B). The mode of the transformation seems to depend on both the parameters of the material and the experimental conditions.