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A phenomenological calculation of phase equilibria by combining the cluster variation method (CVM) with the Lennard-Jones-type (L-J-type) pairwise atomic interaction energies has been attempted for the Fe-Pd-Ni ternary system in order to examine the possibility of Ni as the substituting element for Pd. The calculated Ni content in an Fe-Pd-based L10 ordered phase at 863 K shows fairly good agreement with the experimental analysis performed for Fe-5Pd-5Ni alloy by energy-dispersive X-ray spectroscopy of a transmission electron microscope. The detailed calculation indicates that Ni is preferentially substituted in a Pd site in the L10-ordered phase, which can be interpreted by the Lennard-Jones-type pairwise potentials employed for the present study. It is concluded from the thermodynamic point of view that Ni is a promising candidate as the substituting element for Pd in an Fe-Pd-based L10 ordered phase.

The effect of Zr on the formation of Nb/Nb5Si3 lamellar microstructure by eutectoid decomposition reaction of Nb3Si is investigated. It has been shown that the kinetics of the eutectoid decomposition of high-temperature Nb3Si phase into Nb and Nb5Si3 phases are sluggish in the binary Nb-Si system and that they are enhanced by Zr additions. The time-temperature-transformation (TTT) diagram for the decomposition is experimentally determined and the acceleration of the reaction by small Zr addition of 1.5 at. pct is confirmed by comparison with the reported TTT curves of binary and ternary alloys containing Ti. The role of the ternary element on the decomposition kinetics is discussed in terms of crystallographic orientation relationships (ORs) and Zr distribution in the parent Nb3Si phase during solidification.

vol. 83, pp. 593-600. 5. W.C. Jeong and K. Cho: J. Kor. Inst. Met. Mater., 1999, vol. 37, pp. 672-79. 6. N. Yoshinaga, K. Ushioda, and O. Akisue: Proc. Symp. on High Strength Sheet Steels for Automotive Industry, R. Pradhan, ed., ISS, Warrendale, PA, 1994, pp. 149-58. 7. T. Matsumoto, S. Hamanaka, T. Yamada, and T. Tanaka: Int. Forum for Physical Metallurgy of IF Steels, ISIJ, Tokyo, 1994, pp. 269-73. 8. Bainite Committee of Iron and Steel Institute of Japan, Atlas for Bainitic Microstructures, T. Araki et al., eds., ISIJ, Tokyo, 1992. 9. G. Krauss and S.W. Thompson: Iron Steel Inst. Jpn. Int., 1995, vol. 35, pp. 937-45.

In-situ observation of the formation and disappearance of the surface relief associated with the twinning during the order-disorder transitions among CuAu-I (L10), CuAu-II (PAP) and disordered fcc phases was conducted using Confocal Scanning Laser Microscopy equipped with a gold image furnace. The Retro effect was confirmed in poly-crystal samples, however no evidence was found in single-crystal samples. Also observed in poly-crystal samples are that the disordering temperature detected by the disappearing of relieves is different from grain to grain, and that grain boundary cracking alleviates the Retro effect. The observed phenomena were explained based on the crystallographic orientation relationship among grains investigated by FESEM/EBSD in terms of the elastic strain effect around grain boundaries induced by transition. It was confirmed that in each grain the surface relieves correspond to a set of two {011} planes having a axis perpendicular to both planes in common. It was also found that the larger the average strain of two neighboring grains is, the lower the transition temperature. This observation was explained by the stress effect on the stability of a phase.

The authors have reported in the previous study that the sluggish decomposition of Nb3Si phase is effectively accelerated by Zr addition [1]. This is obvious at lower temperature range than the nose temperature of the TTT curve. In the present study a eutectic alloy containing 1.5 % of Zr was investigated. The crystallographic orientation relationships among phases, such as eutectic Nb and product phases formed by eutectoid decomposition of Nb3Si (eutectoid Nb and Nb5Si3phases) in the Zr-containing sample which was heat treated at 1300°C were investigated by FESEM/EBSD for further understanding of the decomposition process in alloy with a different microstructure.