THERMODYNAMIC ASPECTS OF THE GRAIN BOUNDARY SEGREGATION IN Cu(Bi) ALLOYS L.-S. CHANG{ 1 , E. RABKIN 2 {, B. B. STRAUMAL 3 , B. BARETZKY 1 and W. GUST 1 1 Max-Planck-Institut fu¨r Metallforschung und Institut fu¨r Metallkunde, Seestr. 92, D-70174 Stuttgart, Germany, 2 Department of Materials Engineering, TECHNION–Israel Institute of Technology, 32000 Haifa, Israel and 3 Institute of Solid State Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow District, Russia Abstract—The grain boundary segregation of Bi in dilute polycrystalline Cu–Bi alloys was systematically studied as a function of temperature and composition. The temperature dependencies of the Gibbsian excess of Bi at the grain boundaries exhibited discontinuous changes at the temperatures close to, but dierent from the bulk solidus temperatures. The observed segregational phase transition was interpreted in terms of prewetting model. # 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Auger electron microscopy; Copper alloys; Grain boundaries; Segregation 1. INTRODUCTION The system Cu(Bi) represents a classical example for the strong segregation of oversized impurity atoms to the grain boundaries (GBs) in dilute alloys and has been the subject of detailed investigations for more than a hundred years [1]. The contempor- ary understanding of the GB segregation in the Cu(Bi) system is based on the results of Auger elec- tron spectroscopy (AES) [2], energy dispersive X- ray spectroscopy (EDS) in the scanning trans- mission electron microscope (STEM) [3], high resol- ution electron microscopy (HREM) [4] and computer simulation [5] studies. These works have shown that: . The segregation zone at the GB is only a few interatomic distances thick. . The enrichment ratio is very high and the GB core may be represented as a monolayer of Bi for Bi concentrations in Cu as low as 25–30 at. p.p.m. . Bi atoms occupy the Cu sites in the GB and decrease the free volume in the GB structure. . In some cases, a brittle-to-ductile fracture tran- sition is observed during the increase of tempera- ture [6]. . The Gibbsian excess of Bi atoms at the GBs depends on their crystallographic parameters and geometry. However, until now no proper attention has been paid to the eect of temperature on the GB segre- gation behaviour of Cu(Bi) alloys. It is a conven- tional wisdom that a high concentration of low- melting point impurities at the GB may cause a local melting there. The situation in which this local melting occurs at the same conditions as the bulk one corresponds to the perfect wetting of the GB by the liquid phase. The perfect wetting of GBs and free surfaces by the melt is the reason why solids cannot be overheated. If, however, the local melting at the GB occurs at the conditions at which the liquid phase is thermodynamically unstable in the bulk, the phenomenon of prewetting or premelting occurs [7]. Under premelting conditions the for- mation of a microscopically thin layer of the liquid phase with properties dierent from those of a bulk liquid is possible. Therefore, the term quasi-liquid is more appropriate in this case. The relationship between GB wetting/prewetting and GB segregation was first discovered by Kikuchi and Cahn [8]. Using the cluster variation method they have shown that the GB core becomes more disordered (melted) and its width increases logarithmically with increas- ing temperature. Though the occupancy of each in- dividual segregation site by the solute atoms decreases with increasing temperature, the value of the Gibbsian excess of impurity atoms increases because of the widening of the GB core. This inter- esting theoretical prediction has not been verified experimentally until now. In binary systems the stability of the liquid phase in the bulk is determined by the liquidus and soli- dus lines of the binary phase diagram. Recently, we have determined the solidus line of the Cu–Bi phase diagram with a high precision [9]. In this paper we will present the results of extensive AES study of the temperature and concentration dependence of Acta mater. Vol. 47, Nos 15, pp. 4041–4046, 1999 # 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Printed in Great Britain 1359-6454/99 $20.00 + 0.00 PII: S1359-6454(99)00264-5 {Present address: Institute of Materials Engineering, National Chung Hsing University, 250 Kuo Kuang Road, 402 Taichung, Taiwan, R.O.C. {To whom all correspondence should be addressed. 4041