914 J. Appl. Cryst. (1997). 30, 914-9 i 7 An Anomalous Small-Angle X-ray Scattering Study on Phase Decomposition in a Model Superalloy NiCrAi F. BLEY, a F. LIVET, a O. LYON, b H. OKUDA c* AND J. P. SIMON a ~' Laboratoire de Thermodynamique et Physicochimie Metallurgiques, UMR-CNRS et INP Grenoble, BP75, 38402 Saint Martin d'Heres, France, h LURE Bat. 209D 91405 Orsay, France, and C Department of Materials Science and Engineering, Kyoto University, Kyoto 606-01, Japan (Received 14 February 1997; accepted 21 February 1997) Abstract The precipitation structure in a model nickel-based superalloy was examined by using anomalous X-ray small-angle scattering. Preliminary results are presented on the temporal power law of the precipitation growth and the partitioning of the solute atoms between the precipitates and the matrix. In the present experiment, the concentration of nickel was the same in the matrix and the precipitates, giving no contrast change near the nickel K edge. elements (Simon & Lyon, 1994) and on superlattice reflections at wide angles (AWAXS) to measure the long-range order parameters (Marty, Bessiere, Bley, Calvayrac & Lefebvre, 1990). These two sets of mea- surements were made on the same samples, for different heat treatments. Since the experiments were also a test of the reliability of a newly built beamline at ESRF, some observations remain not fully understood. This communication presents the ASAXS results and sum- marizes the analysis. 1. Introduction Because of their importance in the aerospace industry, nickel-based superalloys have become of increasing interest over the last decade. Their excellent mechanical properties are due to the presence of NisAl-type (LI2) ordered precipitates 7' dispersed in the solid solution 7. From the phase diagram (Taylor & Floyd, 1952), the tie- line of Ni3(AlxCr l_x) consists of an enrichment of AI in 7' with the corresponding depletion of Cr. Because of the steep slope of the 7 solubility limit, Al-rich alloys should be homogenized at high temperature and then they easily decompose again during the quench. The early stages can be observed for quenches from about 1123 K for alloys off-centre with respect to the miscibility gap. In this part of the miscibility gap, there is strong experimental and theoretical evidence (Kampmann & Wagner, 1984) that simultaneous nucleation-and-growth and coarsening take place, the latter becoming dominant as the system tends towards equilibrium. Such a study is in progress by tomographic atom probe (Schmuck & Blavette, private communication); the ordering occurs for sizes less than 2 nm and coarsening begins to be predominant with respect to growth for sizes greater than 4nm. This local technique is complementary to averaging techniques such as X-ray scattering. In order to differentiate the elements, anomalous scattering was used near the Cr and Ni edges. Experiments were carried out at small angles (ASAXS) to measure the variation in composition between precipitates and matrix for three (i 1997 International Union of Crystallography Printed in Great Britain - all rights reserved 2. Experimental The alloy was melted in a semi-levitation furnace from pure elements and cast in a copper water-cooled cylindrical mould. The composition was Ni-17.8 at.% Cr-5.5 at.% AI. Slices were cold-rolled down to 80 lam, recrystallized by thermal treatment for 8 h at 1073 K in an evacuated furnace and cooled by pulling out of the furnace. Samples were electrochemically thinned down to about 20 l.tm, a thickness suited for X-ray measure- ments in transmission. The process was controlled such that the ingot was macroscopically homogeneous, single phase (7 phase, i.e. disordered f.c.c, solid solution) and that the as-quenched sample gave no detectable SAS signal and no sign of segregation during the quench. The grain size was very small, about 1 I.tm, for the coupled powder diffraction measurements. Synchrotron SAXS measurements were performed at ESRF (Grenoble, France) on the SAXS camera of the D2AM beamline of the French Cooperative Research Group. This line has been optimized for anomalous scattering with easy changes to the X-ray wavelength (energy) near an edge. In our case, the Cr edge, 5990 eV, and the Ni edge, 8333 eV, were used. The D2AM optics give a focused beam near the sample, as briefly descri- bed in the ESRF Beamline Handbook. Since a linear vertical detector was used, the vertical focusing by mirrors, in principle, limits the resolution at small angles. In practice, the resolution is limited by parasitic diffuse scattering, but can extend down to the scattering Journal of Applied Crystallography ISSN 0021-8898 :~' 1997