On the fcc → hcp transformation in a cobalt-base superalloy (Haynes alloy No. 25)

Abstract

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

An assessment of static recrystallization in L-605 Cobalt-based superalloy

“…This also has been reported that [26], formation of ε-HCP does not occur in nickel stabilized γ phase after heat treatment, but the material can undergo the martensite transformation during room-temperature plastic deformation. According to the results that obtained for hardness in this study, it is highly likely to form HCP phase in FCC matrix of L-605 alloy after cold rolling and heat treatment.…”

Effect of cold-rolling on mechanical properties of Haynes 25 cobalt-based superalloy

“…Moreover, it has been reported [8,14,15] that in Co-Cr-Mo superalloys martensitic transformation after heat treatment of cold-worked alloys can occur so that an hcp phase can be formed in fcc matrix.…”

“…Koizumi et al [8] and Tawancy et al [9] have reported, it is highly likely that the transformation of fcc to hcp happens in alloys with low stacking fault energy. According to these findings, there is a strong likelihood that the stacking faults on every {111} atomic planes of γ-fcc will be transformed to {0001} plane of the ε-hcp phase; by changing the stacking sequence on the {111} planes of fcc phase from …ABCABC… type to …ABAB… type related to {0001} planes of hcp phase.…”

Temperature evolution and life prediction in fatigue of superalloys

“…[9] into Eq. [10] gives [11] Subsequently, n can be determined from Eq. [11]; e.g., for the hysteresis loop in Figure 7, n is equal to 0.1658.…”

“…[6][7][8][9][10] The nominal chemical composition of ULTIMET alloy, developed by Haynes International, Inc., is shown in Table I. [11] It is typically used in the solution-annealed condition, and possesses high tensile strength combined with excellent impact toughness and ductility.…”