Kinetics of Domain Growth in Ordered Ni 4 Mo
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INTRODUCTION
THE order-disorder
transformation in the Ni4Mo (Ni20 at. pct Mo) alloy has been extensively studied in the past. This alloy exists as a short-range ordered (SRO) facecentered cubic (fcc) structure (c0 above 868 °C, but transforms 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 transformation to six unique orientational variants in the lattice orientation of the LRO domains relative to the SRO lattice.1'2 This gives rise to three types of domain interfaces: antiphase boundary (APB), antiparallel twin boundary (APTB), and perpendicular twin boundary (PTB), in increasing 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 separates 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 extensively by various techniques, which include X-ray diffraction,4'5'6 transmission electron microscopy, 3'7-~5 optical metallography,11'14-17and electrical resistivity. 14.18Aging 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 Technology, Lung-Tan, P.O. Box 1-26, Taiwan, R. O.C. CHARLIE R. BROOKS, Professor, and E. EUGENE STANSBURY, Professor Emeritus, are with the Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996. Manuscript submitted July 11, 1986. METALLURGICALTRANSACTIONS A
mechanism, depending on the temperature of transformation. 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 simultaneous presence of distinct superlattice and SRO spots. In continuous ordering, the alloy exists in a single-phased transitional state, and electron diffraction patterns would show bridges of diffuse intensity linking the superlattice and SRO spots. 13 With continued aging, the initially n
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