Short-range ordering of Cu 3 Au above T c
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Short-range ordering of Cu3 Au above Tc in the topmost 80 A of a (001) face M. Kimuraa) and J. B. Cohen Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208
S. Chandavarkar Department of Physics, University of Tennessee, Knoxville, Tennessee 37966
K. Liang Exxon Corporate Research Laboratory, Annandale, New Jersey 08801 (Received 2 October 1995; accepted 3 October 1996)
The short-range order in the near surface region of the Cu3 Au(001) face was investigated above the critical temperature by glancing-incidence x-ray diffraction, measuring the diffuse intensity throughout a two-dimensional region of reciprocal space. This intensity was analyzed quantitatively to obtain the two-dimensional Cowley –Warren short-range-order parameters and atomic displacements. Monte-Carlo simulation based on these values has revealed that the atomic configurations in the surface consist of ordered domains and clusters in a disordered matrix. There is a large number of h10j antiphase domain boundaries (APDB).
I. INTRODUCTION
The alloy Cu3 Au, which orders from the A1 to the L12 structure at the critical temperature Tc 663 K, is considered a classic example of an order-disorder phase transformation. In the L12 structure, Au atoms have 12 Cu atoms as nearest-neighbors (NN) and Cu atoms have 4 Au and 8 Cu atoms at NN sites. This alloy is one of the best for studying long-range order (LRO) and short-range order (SRO) near Tc by x-rays because of the high degree of scattering contrast between Cu and Au, the relatively small change in lattice constant during the transformation, its resistance against corrosion and oxidation, and the modest Tc . In fact, the atomic ordering of bulk Cu3 Au above Tc has been investigated extensively.1–4 The latest results show the existence of small ordered-domains of 1–10 unit cells surrounded by a disordered matrix at T 703 K; some of the larger domains consist of smaller ones separated by conservative APB on h100j planes.4 Surfaces or interfaces could show different behavior from the bulk in an order-disorder transformation, for example, because atoms at surfaces have lower coordination and are less tightly bound than those in the bulk, so that the strain energy might be less important. Several studies have already been made of long-range order in the Cu3 Au(001) surface5 ; both surface-induced disordering and surface-induced ordering have been reported. As to the former, the crystallographic structure has been a)
Current address: Advanced Materials & Technology Research Laboratories, Nippon Steel Co., 1618 Ida, Nakahara-ku, Kawasaki, 211 Japan. J. Mater. Res., Vol. 12, No. 1, Jan 1997
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investigated by several methods. During heating from below Tc , LRO in the top layer shows a more continuous transition to disorder than the bulk in LEED studies,6–8 SPLEED measurements,9 and glancing incidence x-ray diffraction (GI-XRD) techniques,10 and in all of these,
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