Kinetics of Domain Growth in Ordered Ni4Mo VIJAY K. VASUDEVAN, HWANG P. KAO, CHARLIE R. BROOKS, and E. EUGENE STANSBURY The kinetics of domain growth in Ni4Mo in the temperature range of 600 to 850 °C were investigated using transmission electron microscopy. It was found that domain growth in Ni4Mo is analogous to metallurgical grain growth and can be described by the expression D" = kt, where D is the average domain size, t is the aging time, k is a constant, and the exponent n is the reciprocal of the slope of the log D vs log t plot. The value of n changed with temperature from 2.0 at 850 and 800 °C to 2.9 at 700 and 600 °C. This change was explained in terms of relative domain orientation effects. The activation energy for domain growth was obtained as 69 Kcal/mole (2.9 × 105 Joules/mole) in the temperature range of 800 to 850 °C and as 92 Kcal/mole (3.85 x 105 Joules/mole) in the temperature range of 600 to 700 °C, which on comparison with available diffusion data established that the growth process was interface-controlled at the higher temperatures and bulk diffusion-controlled at the lower temperatures. I. INTRODUCTION THE order-disorder transformation in the Ni4Mo (Ni- 20 at. pct Mo) alloy has been extensively studied in the past. This alloy exists as a short-range ordered (SRO) face- centered cubic (fcc) structure (c0 above 868 °C, but trans- forms below this temperature to a long-range ordered (LRO) body-centered tetragonal (bct) structure (/3) with c/a = 0.623. J,2 The crystallography of the two forms allows trans- formation to six unique orientational variants in the lattice orientation of the LRO domains relative to the SRO lat- tice.1'2 This gives rise to three types of domain interfaces: antiphase boundary (APB), antiparallel twin boundary (APTB), and perpendicular twin boundary (PTB), in in- creasing order of surface energy. 3 The APB occurs when two neighboring domains having parallel c-axes are related by a translation mismatch. The displacement vector, R, for the APB can be of four types: R1 = l[010], R2 = ½[110], R3 = 1[01 --+ 1], R4 = 3110 --- 1], and their negatives, referred to the fcc cell, or equivalently ½[130], ½1210], ~[13 -+ 5], ~013] --- 5], and their negatives, respectively, in terms of the tetragonal superlattice cell.3 The APTB sepa- rates two domains for which rows of molybdenum atoms enclose an angle of 2 arctan (1/3). The structures are mirror images, that is, twin related, and the c-axes are antiparallel. The PTB results when the c-axes in adjacent domains are perpendicular.3 The structural changes, kinetics, and mechanisms of the SRO to LRO transformation have been studied exten- sively by various techniques, which include X-ray diffrac- tion, 4'5'6 transmission electron microscopy, 3'7-~5 optical metallography,11'14-17and electrical resistivity. 14.18 Aging the as-quenched alloy below the critical ordering temperature results in formation of NiaMo domains either by a nucleation and growth process or by a continuous (spinodal) ordering VIJAY K. VASUDEVAN is Visiting Research Assistant Professor, Department of Materials Science and Engineering, University of Illinois, Urbana, IL 61801. HWANG P. KAO is Assistant Research Associate, Materials R & D Center, The Chung-Shan Institute of Science and Tech- nology, Lung-Tan, P.O. Box 1-26, Taiwan, R. O.C. CHARLIE R. BROOKS, Professor, and E. EUGENE STANSBURY, Professor Emer- itus, are with the Department of Materials Science and Engineering, Uni- versity of Tennessee, Knoxville, TN 37996. Manuscript submitted July 11, 1986. mechanism, depending on the temperature of trans- formation. 1°'~2-15'19Evidence for both mechanisms can be obtained by electron diffraction. In ordering by nucleation and growth, the alloy initially exists in a two-phased state consisting of Ni4Mo domains in a SRO matrix; electron diffraction patterns would be characterized by the simulta- neous presence of distinct superlattice and SRO spots. In continuous ordering, the alloy exists in a single-phased tran- sitional state, and electron diffraction patterns would show bridges of diffuse intensity linking the superlattice and SRO spots. 13 With continued aging, the initially nucleated domains grow. Other than the reports of Ling and Starke, 5'6 who used X-ray diffraction to determine the change in domain size as a function of time in the temperature range of 650 to 700 °C, there have been no systematic studies of domain growth in NiaMo over a wide range of temperatures and times. The purpose of this paper is to report on transmission electron microscopy studies of domain growth in Ni4Mo aged in the temperature range of 600 to 850 °C for times to 4500 hours. II. EXPERIMENTAL METHODS The alloy used in this study was prepared by induction remelting a Ni-Mo master alloy, with nickel corrections to obtain the NiaMo composition, and casting into water- cooled copper molds to produce cylindrical ingots about 20 mm in diameter and 64 mm long; the total impurity content in the alloy was 0.29 wt pct. The cast alloys were homogenized at 1150 °C for 100 hours in dry hydrogen, furnace cooled, reheated to 950 °C for 1 hour, and then water quenched to retain the high-temperature fcc a phase. Subsequent swaging with intermediate anneals in the tem- perature range 950 to 1000 °C were used to produce rods approximately 3 mm in diameter. Samples 100 mm long were cut from the swaged rods, encapsulated in rough vacu- um in Vycor, solution-treated at 950 °C for 2 hours, and water quenched upon breaking the capsules to retain the fcc a phase. Smaller samples were cut from the solution-treated rods, sealed in Vycor under rough vacuum, and aged at 600, 700, 800, and 850 °C. After aging for times to 4500 hours, the capsules were broken and the specimens water quenched to arrest further transformation. METALLURGICAL TRANSACTIONS A VOLUME 19A, APRIL 1988--941