Environmental Embrittlement of Binary and Zr-Doped Ni 3 Al
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ENVIRONMENTAL EMBRITTLEMENT OF BINARY AND Zr-DOPED NI3 AI E. P. GEORGE,'
2 1 C. T. LIU, and D. P. Pope
2'Metals
and CeramicsDivision, Oak Ridge NationalLaboratory,Oak Ridge, TN 37831-6093 Dept.of MaterialsScience and Engineering, Univ. of Pennsylvania,Philadelphia,PA 19104-62 72
ABSTRACT This paper summarizes results of our recent work on moisture-induced environmental embrittlement in Ni 3AI. We took the unconventional approach of starting with single crystals of B-free Ni3 AI, which were cold rolled and recrystallized to produce crack-free polycrystalline material. Our results show that the intrinsic ductility (-16%) of Ni 3 A1 (23.4 at.% Al) is considerably higher than previously thought; however, it is severely embrittled by moisture in air (ductility dropping from a high of -16% when tested in oxygen to a low of -3% in air). Since Bdoped Ni 3 AI does not show such embrittlement (i.e., ductilities high in both air and oxygen), we conclude that a significant part of the beneficial effect of B must be related to suppression of this environmental effect. However, B must also improve grain boundary (GB) cohesion in Ni 3AI, since our B-free alloy fractures intergranularly whereas B-doped alloys in general fracture transgranularly. Addition of a small amount (0.26 at.%) of Zr to Ni3AI significantly improves its ductility: to 11-13% in air, and 48-51% in oxygen. The ductilities observed in oxygen are comparable to the highest ever ductility observed in B-doped Ni 3 AI, indicating that the GBs in this Zr-doped alloy are not intrinsically brittle (rather, environmental embrittlement is the main reason for its brittleness). Zr dramatically increases the resistance of Ni 3AI to GB fractureperhaps by increasing GB cohesion. However, Auger analysis shows little or no Zr segregation on the GBs of Ni 3 AI, making it unclear how it might actually affect GB cohesion. Zr does not significantly increase the resistance of Ni 3AI to environmental embrittlement; nor does it suppress intergranular fracture. In both these respects Zr behaves differently than B.
INTRODUCTION Polycrystalline Ni 3Al fractures intergranularly with limited tensile ductility [e.g., 1-4], in contrast to single crystals which undergo extensive plastic deformation before fracture [e.g., 57]. This behavior persists even in high-purity Ni3AI in which the GBs are quite clean and free of potentially harmful impurities [4,8,9]. It has been commonly believed, therefore, that the GBs in Ni3AI are "intrinsically" brittle-unlike GBs in conventional metals which rarely constitute a serious source of weakness unless embrittled by (extrinsic) impurities. We were prompted to revisit the subject of brittle fracture in Ni3 AI because of mounting recent evidence that the moisture in ordinary ambient air can severely embrittle many ordered intermetallics [10-24] [in the absence of such (extrinsic) embrittlement some of these intermetallics can actually exhibit extensive ductilities]. The proposed mechanism involves the reduction of H2 0 in air by reactive elements in the
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