GRAIN BOUNDARY SEGREGATION IN NONSTOICHIOMETRIC NiAl POLYCRYSTALS P. Lejc ˘ek, S. Hofmann* and V. Paidar Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, CZ-180 40 Praha 8, Czech Republic * Max-Planck-Institut fu ¨r Metallforschung, Institut fu ¨r Werkstoffwissenschaft, Seestraße 92, D-70174 Stuttgart, Germany (Received March 30, 1996) (Accepted August 26, 1997) 1. Introduction NiAl base intermetallic is one of the prospective materials for high temperature applications. Techno- logical use, however, requires knowledge of the behavior of all structural constituents in order to avoid the weak ones. Since grain boundaries in these intermetallics represent such weak links, it is necessary to study their properties and to improve them appropriately. One of the most important properties of grain boundaries is their chemical composition at elevated temperatures, i.e., solute and impurity segregation (1). So far, the segregation of various impurities such as boron (2–5), beryllium (2,5,6), carbon (6,7), nitrogen (8), iron and gallium (9), zirconium (9 –11) and molybdenum (10) has been studied. Among these elements, only boron (2–5) and zirconium (9 –11) were found to segregate at the boundaries of NiAl base intermetallics. Less information is given in the literature about segregation tendencies of the component elements, nickel and aluminum. Ni enrichment was observed at grain boundaries of Ni-rich NiAl (12,13) and in stoichiometric NiAl (2). Additions of carbon or boron to the stoichiometric NiAl lead to depletion or enrichment of Ni, respectively. In contrast to Ni 3 Al intermetallics, no significant Ni-B cosegregation was detected in NiAl (2). Recent computer simulations of the atomic structure of the =5, {012} symmetrical grain boundary suggested a low tendency of Ni to segregate at the grain boundary plane, whereas it can be concentrated at the layers adjacent to this plane. On the other hand, aluminum segregates to both the grain boundary plane and its vicinity. It means that Al should segregate easily in Al-rich NiAl (14). To contribute to the disclosure of interfacial chemistry in NiAl base intermetallics, the chemical composition of grain boundaries was studied in nonstoichiometric (both Ni-rich and Al-rich) NiAl alloys at different temperatures. The results are discussed from the viewpoint of segregation thermo- dynamics. 2. Experimental Two NiAl base alloys were prepared by induction melting of pure Ni (99.99 analysis, the Al-rich alloy contained 50.4at.%Al and the Ni-rich alloy contained 48.1at.%Al. The ingots were annealed for 4 h at 1500 K in argon atmosphere and furnace cooled. After this treatment, the ingots of both alloys contained Pergamon Scripta Materialia, Vol. 38, No. 1, pp. 137–143, 1998 Elsevier Science Ltd Copyright © 1998 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 1359-6462/98 $19.00 + .00 PII S1359-6462(97)00442-9 137