Influence of Oxidation on Boron Sgregation to Grain Boundaries of In-Situ Fractured Ni 3 Al Alloys
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INFLUENCE OF OXIDATION ON BORON SGREGATION TO GRAIN BOUNDARIES OF IN-SITU FRACTURED Ni3Al ALLOYS S. A. Koch, D. T. L van Agterveld, G. Palasantzas, J. Th. M. De Hosson* Department of Applied Physics, Materials Science Center and Netherlands Institute for Metals Research, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands.
ABSTRACT Scanning electron and scanning Auger microscopy studies were performed on in-situ fractured B-doped hypostoichiometric Ni 3 Al alloys. The Auger measurements on the fracture surface showed a very small amount or the total absence of B. Further, B segregated to the grain boundaries during subsequent exposure to the ambient system ultra-high vacuum environment at room temperature. The B segregation appeared to be driven by a mechanism of electronic nature related to Ni enrichment and O supplied from the environment. Ni-oxidation at room temperature is in accordance with model predictions for small beam sizes (≤10 µm) based on the premise that the electron beam creates additional nucleation sites around of which oxide growth occurs. With increasing the size of the e-beam the oxidation process becomes slower and chemisorption of oxygen plays a significant role. As a result the Ni-oxide depth decreases drastically with increasing spot size and offers an alternative route for monitoring the thickness of NiO in a nanometer range.
INTRODUCTION In the last two decades the study of Boron (B) segregation to grain boundaries (GBs) in polycrystalline Ni 3 Al alloys attracted considerable research activities [1-8] because of its beneficial effect on the fracture behavior of intermetallic compounds, e.g. Ni 3 Al . It bears an excellent resistance to oxidation because an adherent surface oxide film is formed that protects the base metal from excessive attack [2]. Although, Ni 3 Al single crystals are highly ductile, its polycrystalline form fails intergranularly in a brittle fashion at ambient temperatures. However, the addition of B apeared to improve the ductility of hypostoichiometric (Ni-rich) Ni 3 Al alloys (Ni-24 at % Al) [1-10] where the size of grains plays a significant role [11-12]. A common theme in all studies of B-segregation to GBs in Ni 3 Al alloys is the fact that B segregates more strongly to GBs than to free surfaces, whereas B segregation was observed onto sputtered surfaces that were annealed below 800 °C [1]. Such a segregation behavior was attributed to the fact that sputtering induces a damage that is annealed out at elevated temperatures [1]. This is also supported by the fact that long-range order in Ni 3 Al single crystals is destroyed by sputtering and is recovered by heating at 1000 °C [1]. However, despite the vast number of studies in this alloy system, no precise attention has been paid to the dynamics of B segregation on intergranularly fractured surfaces in the absence of any annealing and/or sputtering treatments, In addition, possible effects of the electron beam (e-beam) induced oxidation after prolonged exposure have been largely ignored. 1 AA3
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