The Microchemistry of Grain Boundaries in Ni 3 Al
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THE MICROCHEMISTRY OF GRAIN BOUNDARIES IN Ni 3AI D. N. SIELOFF*, S. S. BRENNER AND HUA MING-JIAN Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261 *Current Address - IBM Corporation, Burlington, VT ABSTRACT Grain boundary regions in B-doped as well as B-free Ni 3AI were studied by field-ion microscopy and atom probe microanalysis. In the ductile, recrystallized, Ni-rich alloys the segregation of boron was often accompanied by an enrichment of nickel. Such an enrichment was not observed at boundaries in B-free alloys. Boron was also observed to segregate to the boundaries in a 25.2A1 - IB alloy which was reported to contain boron clusters. Such clusters were not observed, instead a high concentration of boron pairs were found. INTRODUCTION Boron markedly decreases the brittleness of polycrystalline Ni3 A1 [1,21. The ductilization effect is clearly associated with changes in the grain boundary regions since single crystals of the aluminide are quite ductile. The beneficial effect of boron is, however, only observed in Nirich alloys, i.e. boron doped Al-rich alloys remain brittle. Other solutes, including carbon, have been tried but none have been found to be as effective as boron. Extensive efforts have been made to clarify the ductilization affect of boron. Auger spectroscopy [2,3] and atom probe analyses [4,5] have shown that boron is segregated to the grain boundaries in B-doped Ni 3AI. At a concentration of 500 wt ppm the boron at the boundaries is enriched by a factor of between 50 and 100. Two major models have been proposed to explain the boron effect. The first suggested [6] that the segregated boron changes the electronic character of the boundary regions, thereby enhancing the cohesion of the boundaries. The second model, primarily due to Schulson and coworkers [7], suggests that boron eases the transmission of slip across boundaries, reducing the incidence of dislocation pile-ups and subsequent crack initiation. Preliminary results by the author (SSB) and coworkers [8] showed that in Nickel-rich alloys the segregation of boron may be accompanied by the cosegregation of nickel. Foiles [9] using embedded atom calculations predicted such an effect even in B-free alloys although the calculated magnitude was smaller than the observed one. The observed nickel enrichment raised the possibility that the grain boundary regions in the ductile materials are disordered supporting Schulson et al.'s model of increased slip accomodation. The failure of boron to ductilize Al-rich alloys was attributed by Dasgupta et al. [10] to the formation of boron clusters which reduces the concentration of boron available to the boundaries. It was suggested by these authors that the clusters nucleate at constitutional vacancies. The concentration of these vacancies was estimated by positron annihilation studies to be approximately 10-6 or 1017 per cm 3 . In this paper we present more fully the results of atom probe analyses of grain boundaries in both B-doped and B-free Ni-rich alloys. Res
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