A study of a cellular phase transformation in the ternary Ni- Ai- Mo alloy system
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INTRODUCTION
RECENTLY, there has been considerable attention paid to superalloys based on the Ni-A1-Mo ternary system for potential applications as advanced alloys for gas turbine components. Various processing techniques have been used in the development of these materials. Eutectic compositions have been directionally solidified to form composites consisting of ductile c~-Mo fibers within a 3' + 3" matrix. ~.2 Single crystals of certain compositions have also been fabricated. 3 Alloys containing increasing Mo content (compared with conventional alloys) have been developed, primarily through rapid solidification processing (RSP)J These materials typically contain refractory additions such as Ta, W, V, and Nb, together with the more conventional alloying elements (C, B, Hf, Zr, and Cr). These alloys exhibit excellent mechanical properties at both high (900 to 1150 ~ and intermediate (600 to 850 ~ temperatures. 3"4 At the intermediate temperatures, the property enhancement may be attributed to the precipitation of a fine dispersion of NixMo phases in the 3' matrix as shown in Figure 1. These phases consist of one or more of, first, a metastable form of Ni3Mo possessing the D022 structure, second, Ni2Mo which is also metastable and has the orthorhombic Pt2Mo crystal structure, and third, Ni4Mo with the Dla structure. In the binary Ni-Mo system, for compositions bordered by the NiaMo and NiMo phase boundaries, these various phases have been observed together with the stable form of Ni3Mo which possesses the orthorhombic Cu3Ti crystal structure, s The structure of this phase may also be considered as pseudo-hexagonal, derived by shearing on the (112) planes of the ordered D022 Ni3Mo. This implies an orientation relationship in which the (010) planes of orthorhombic Ni3Mo* are parallel to the (112) D022 planes. Therefore it *The indexing of Reference 5 is used. Thus, a = 0.5064 nm, b = 0.4224 nm, and c = 0.4448 nm. M. J. KAUFMAN, J. A. EADES, and H. L. FRASER are, respectively, Graduate Research Assistant, Principal Research Scientist, and Associate Professor in the Department of Metallurgy and the Materials Research Laboratory at the University of Illinois, 1304 West Green Street, Urbana, IL 61801. M.H. LORETTO is Professor in the Department of Physical Metallurgy, University of Birmingham, Birmingham, United Kingdom. Manuscript submitted September 28, 1982. METALLURGICALTRANSACTIONS A
would be tempting to speculate that the orthorhombic form of Ni3Mo might be formed directly from the D022 phase by a shear transformation. However, the actual transition is somewhat more complex and may be summarized as consisting of the following reactions: 6 disordered fcc ~ short range order --~ 9022 --~ Ni2Mo + Ni4Mo + fcc shear ~-- NilvMo5 ~ Ni3Mo transformation The resulting structure is initially heavily faulted; it contains both stacking faults, from the shear transformation, on the (010) Ni3Mo planes and nonconservative antiphase boundaries (APB's) on the (100) Ni3Mo planes. The APB's are present immediately after the shear tr
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