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Environmental Embrittlement of Binary and Zr-Doped NI3AI

Published online by Cambridge University Press:  01 January 1992

E. P. George ,
C. T. Liu  and
D. P. Pope
E. P. George
Affiliation:
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6093
C. T. Liu
Affiliation:
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6093
D. P. Pope
Affiliation:
Dept.of Materials Science and Engineering, Univ. of Pennsylvania, Philadelphia, PA 19104-6272
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Abstract

This paper summarizes results of our recent work on moisture-induced environmental embrittlement in Ni3Al. We took the unconventional approach of starting with single crystals of B-free Ni3Al, which were cold rolled and recrystallized to produce crack-free polycrystalline material. Our results show that the intrinsic ductility (∼16%) of Ni3Al (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 B-doped Ni3Al 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 Ni3Al, 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 Ni3Al 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 Ni3Al, 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 Ni3Al to GB fracture— perhaps by increasing GB cohesion. However, Auger analysis shows little or no Zr segregation on the GBs of Ni3Al, making it unclear how it might actually affect GB cohesion. Zr does not significantly increase the resistance of Ni3Al to environmental embrittlement; nor does it suppress intergranular fracture. In both these respects Zr behaves differently than B.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 288: Symposium L – High-Temperature Ordered Intermetallic Alloys V , 1992 , 941
Copyright
Copyright © Materials Research Society 1995

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