Materials Science and Engineering, A I50 (1992) 75-85 75 Electron microscopy and diffraction of ordering in a Ni-25wt.%Mo alloy N. S. Mishra Research and Development Centre for Iron and Steel, Steel Authority of lndia Limited, Ranchi 834002 (India) S. Ranganathan Centre For Advanced Study, Department of Metallurgy, Indian Institute of Science, Bangalore 560012 (India) (Received October 16, 1990; in revised form August 23, 1991 ) Abstract Electron microscopy and diffraction were employed to study the disorder-order change in a Ni-25wt.%Mo alloy in the temperature range 680-880 °C. In addition to confirmation about the presence of microstructure and clusters in the state of short-range order and the occurrence of metastable Ni2Mo and Ni4Mo phase, the present study has enabled the con- struction of a time-temperature-transformation curve. It is also shown that the change from f.c.c, solid solution to ordered h.c.p. Ni3Mo proceeds through nucleation and growth and not by shear. 1. Introduction Nearly three decades after the classical study by Ellinger [1], Yamamoto et al. [2] investigated struc- tural changes during annealing at 860 °C of quenched Ni3Mo alloy and reported the occurrence of metastable Ni2Mo and Ni4Mo. Das and coworkers [3-5] studied ordering at 650 °C and confirmed these observations. This was followed by the very extensive and elegant studies of Van Tendeloo [6]. Enlarging the early observations of Ruedl and Amelinckx [7], he mapped the chronology of occurrence of metastable phases, characterized a new defect known as the dissociated antiphase boundary, measured the lattice relaxation at non-conservative growth and conservative glide anti- phase boundaries (APBs), and stacking faults and applied the cluster model of short-range order (SRO). The results were arrived at by ordering studies in the range 800-870 °C in an impressive series of papers [8-14]. There has been one isolated field ion micro- scopy study of Ni3Mo, emphasizing image contrast and computer simulation [ 15]. Martin and Williams [16] have considered ordering in a ternary alloy of Ni3Mo composition substituting aluminium, tantalum, vanadium or tungsten partially for molybdenum. The effect of these additions on the ordered structure by imparting isothermal treatments was studied. Ni3Mo long-range order (LRO) structures ordered via the formation of DI~,-, D022- and Pt2Mo- type superstructures. Finer diffraction effects were explained on the basis of spinodal ordering mechan- isms. An interesting aspect of progress in the ordering reaction by irradiating specimens with high energy electrons at different temperatures has been investi- gated recently by Banerjee and coworkers [17-19]. Concentration modulation and special points in the k space pairwise interaction potential function V (k) orig- inally formulated by De Fontaine [20] were invoked by these workers to explain the sequence of events prior to the formation of metastable D1 a- and Pt2Mo-type superstructures. In the light of this previous work, it was considered useful to extend the studies of ordering to a wider tem- perature range (680-880°C). Attention was focused on the nature of SRO, the sequence of phase formation during ordering and the characterization of defects. 2. Experimental details Nickel and molybdenum powder in 64.5:35.5 pro- portions by weight were mixed and melted in an arc furnace in an argon environment. The strips were cut into small pieces and melted several times to ensure homogeneity in composition. Specimens for electron microscopy were rolled to 250 # m thickness and cut into pieces of 10 mm x 10 mm size. The specimens 0921-5093/92/$5.00 © 1992 - Elsevier Sequoia. All rights reserved