Studies on the β-ε Transformation in Cobalt-Nickel Alloys. II : Microstructure of Transformation Relief

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The dislocation model for the mechanism of diffusion-less transformation of the f. c. c. structure β into the c. p. h. ε in cobalt-nickel alloys was studied from electron-microscopic observation of relief markings on the crystal surface formed by the transformation. It can be considered that the transformation of β into ε proceeds through the motion of half-dislocations (α/6) [121]_β, (α/6)[211]_β, and (α/6) [112]_β in the (111) plane of a f. c. c. crystal. When these half-dislocations move in all directions over the crystal surface, tilts having three different inclinations to the surface are formed, which can be estimated to be 19°28', -10°2' and -10°2', respectively, if the surface is parallel to the (112)_β plane, while under an external stress only one type of half-dislocations similar in direction to the stress can be considered to move, and then only one sort of the tilts is preferred. These surface tilts result in the relief markings on the crystal surface. The above results predicted theoretically were confirmed by electron-microscopic observation of markings, and it was found that the width of the zone of homogeneous displacement in marking (about 0.1〜1μ for 25 per cent nickel alloy) decreased slightly with decreasing nickel content, but in alloys containing less than a few per cent of nickel it decreased abruptly and in pure cobalt it was of the order of 100A.
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