Improved thermochemical calculations for maraging steels and comparison with atom probe measurements of phase compositio
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With regard to (3), it was shown in Reference 1 that the A1-Co decagonal quasicrystal transformed to the monoclinic 72-Al13Co4 below 800 ~ and that the reverse transformation occurred between 800 ~ and 900 ~ Thus, it was concluded that the A1-Co decagonal quasicrystal was a stable phase within this temperature range. However, Drs. Grushko and Freiburg believed "no strong evidence in favor of this conclusion was presented" and they argued "to date, all quasicrystals classified as stable have been reported for ternary compositions." Furthermore, they maintained that the ternary AI-Co-Cu decagonal phase has a tendency to be stable at a higher temperature, with the decrease of the Cu content. More work should be done to prove this is true in very low Cu contents, or even with no Cu in the binary A1-Co decagonal phase. It was pointed out t2J that the C-inflated, monoclinic r2-Al~3Co4 phase is a better approximant to the A1-Co decagonal phase than All3CO4. Now we have found a z3-inflated ~-All3CO 4 phase with even larger lattice parameters: a = 6.4, b = 0.81, and c = 5.2 nm, and /3 ~ 108 deg. This is an even better approximant. In Figure 2, the [010] EDP taken along the b axis of this rational approximant r 3 is compared with the tenfold EDP of the decagonal quasicrystal (DQC) in a composite EDP. The 0 0 5, 0 0 8, 0 0 13, and 0 0 21 diffraction spots of the z3 phase coincide almost exactly with the strong spots of the DQC in ~--inflated ratios. Therefore, the spot rows of the DQC can be well extended into the phase, and the pentagons marked with arrows and dots in these two phases appear similar as far as strong spots are concerned. From the approximant point of view, the decagonal quasicrystal can be considered as the end member of this series of z-inflated phases with an infinitely large unit cell.
REFERENCES 1. X.L. Ma and K.H. Kuo: Metall. Mater. Trans. A, 1994, vol. 25A, pp. 47-56. 2. X.L. Ma and K.H. Kuo: Metall. Trans. A, 1992, vol. 23A, pp. 1121-28.
2538--VOLUME 25A, NOVEMBER 1994
Improved Thermochemical Calculations for Maraging Steels and Comparison with Atom Probe Measurements of Phase Composition W. SHA and G.D.W. SMITH A series of studies of phase transformations in VASCOMAX C-300 and T-300* maraging steels (for *VASCOMAX C-300 and T-300 are trademarks of Teledyne Vasco (formerly Vanadium Alloy Steel Company), Latrobe, PA.
composition, see Table I) has been published by the authors, based mainly on atom probe analysis and thermochemical calculations, t~-41 It was experimentally observed that decomposition started with the formation of Mo-enriched Ni3Ti phase in the martensite matrix and FeTMO6 phase formed at a later stage of aging. While generally good agreement was achieved between the experimental studies and the thermodynamical modeling, a significant drawback was that the Ni3Ti phase, which was invariably present in these aged maraging steels, was not included in the calculations. This was a result of the lack of information in the available databases at the time the work was done. This note reports
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