It is known that the cobalt-based family of Multiphase alloys (Multiphase is a registered trademark of SPS Technologies, Inc.) derive their strength, in part, from cold working and that additional strengthening can be obtained by subsequent ageing heat treatments [1][2][3][4][5][6][7][8][9]. In the case of alloy MP35N ( 3 5 C o -3 5 N i -2 0 C r10Mo, in wt %), the strengthening associated with cold working is attributed to a face-centred cubic (fc c) -~ hexagonal close-packed (h c p) transformation and/or formation of mechanical twins depending on the deformation temperature relative to the Md temperature [3,4] (Md is the temperature below which the f c c --* h c p transformation can be stressinduced). The strengthening which results from subsequent ageing heat treatments is correlated with the f c c ~ h c p transformation [4] or the formation of intermetallic compounds [6].Haynes alloy No. 25 (Haynes is a registered trademark of Cabot Corporation) is a cobalt-base superalloy with a nominal chemical composition of C o10Ni-20Cr-15W. Typically, 'the alloy is heat treated at 1205 ° C for 15 min followed by rapid air cooling. In this condition, the matrix is a fc c solid solution with a lattice constant of 0.357 nm. In view of the behaviour of alloy MP35N, it may be expected that alloy 25 is also susceptible to the fc c ~ h c p transformation particularly as it contains less nickel than alloy MP35N. Therefore, the present investigation was undertaken to determine the susceptibility of alloy 25 to the fc c ~ h c p transformation with emphasis on the role of cold working and ageing heat treatments.Sheet samples (1.3 mm thick) were cold reduced by up to 20%. Ageing heat treatments were conducted at temperatures in the range 370 to 760 ° C. The effect of cold working on the mechanical properties was determined from tensile tests. Light optical metallography and thin-foil transmission electron microscopy were used for the microstructural characterizations. Samples for light optical metallography were etched in a solution consisting of 95% HC1 and 5% H202 by volume. Thin foils for transmission electron microscopy and diffraction work were prepared by the jet polishing technique in a solution of 30vol % nitric acid in methanol at about -20 ° C. All foils were examined at 100 kV. Fig. I shows characteristic optical microstructures of alloy 25 in the annealed condition ( Fig. la) and after ageing for 48h (Fig. lb) and 500h (Fig. lc) at 760 ° C. It can be seen that after ageing at 760 ° C, the microstructure contains a phase with a widmanst~iten-type morphology. This phase was identified by electron diffraction as h c p s-phase with lattice constants of a = 0.25nm and c = 0.41 nm. The electron diffraction pattern of Fig. 2 illustrates the coexistence of

Argon or water atomized alloy powders for use in powder-metallurgy parts usually have a particle size in the range 20-200µ. Because of this small size and the accompanying difficulty in specimen preparation, studies of metal powders have usually excluded transmission techniques and knowledge of the microstructure is confined to scanning electron microscopy studies.Fig. 1 shows a scanning electron micrograph of an argon-atomized particle of Haynes alloy 6B. It is clear that the particle is a spherical “microcasting” showing typical dendritic structure. When such particles are mounted and sectioned, the dendritic morphology is still observed (Fig. 2) indicating that there is a solidification structure throughout the particle and not just at the surface.

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