Grain boundaries, trace elements, and fracture of intermetallic compounds

Conference · Sat Jul 01 04:00:00 EDT 1995

OSTI ID:72531

Lin, H; Pope, D P ^[1]; George, E P ^[2]

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1. Univ. of Pennsylvania, Philadelphia, PA (United States). Dept. of Materials Science and Engineering
2. Oak Ridge National Lab., TN (United States). Metals and Ceramics Division

Even though single crystals of Ni{sub 3}Al are quite ductile when tested in ambient conditions, polycrystalline material is very brittle due to grain boundary failure. The tendency for grain boundary failure is greatly exacerbated by atmospheric water vapor. A number of studies have shown that within the sensitivity of Auger spectroscopy, grain boundary failure is not accompanied by any measurable segregation of a third element, therefore grain boundaries in Ni{sub 3}Al appear to be intrinsically weaker than the bulk. A major breakthrough in this problem occurred in 1979 when Aoki and Izunii showed that small additions of B can greatly improve the ductility. This has led to a world-wide interest in how B strengthens the grain boundaries. It was later shown by Liu and coworkers that there is an optimal amount of B in Ni{sub 3}Al for the purpose of increasing the ductility. Polycrystalline Ni-rich, B-free Ni{sub 3}Al produced by the usual casting and working processes shows almost no ductility at room temperature, and failure is completely intergranular. With the addition of 0.02 wt. % B the tensile elongation increases dramatically to over 40% and failure becomes transgranular by microvoid coalescence for fine grained materials and stepped/faceted for large grained materials. Thus the central questions raised by these observations are: (1) Why is B-free Ni{sub 3}Al intergranularly weak and (2) how does B improve the grain boundary strength? In this paper the authors will describe the trends of the research on these two topics in recent years and concentrate on the intrinsic properties of the grain boundaries, correlating structure with brittleness, and also incorporating considerations of test environment and B addition.

• DOE Contract Number: AC05-84OR21400
• OSTI ID: 72531
• Report Number(s): CONF-941144--; ISBN 1-55899-265-0
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALUMINIUM ALLOYS
BORON
BORON ADDITIONS
EMBRITTLEMENT
FRACTURE PROPERTIES
GRAIN BOUNDARIES
INTERMETALLIC COMPOUNDS
MECHANICAL PROPERTIES
METALLURGICAL EFFECTS
NICKEL BASE ALLOYS
TENSILE PROPERTIES