Brittle fracture and grain boundary chemistry of microalloyed NiAl

Journal Article · Sun Apr 01 04:00:00 EDT 1990 · Journal of Materials Research; (USA)

DOI:https://doi.org/10.1557/JMR.1990.0754· OSTI ID:7061661

George, E P; Liu, C T ^[1]

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1. Metals and Ceramics Division, Oak Ridge National Laboratory, P. O. Box 2008, Oak Ridge, TN (USA)

The room-temperature tensile properties, fracture mode, and grain boundary chemistry of undoped stoichiometric NiAl, as well as NiAl doped with boron, carbon, and beryllium, have been investigated. Pure, stoichiometric NiAl fractures with limited tensile ductility in a predominantly intergranular manner. Auger analyses revealed that the grain boundaries in NiAl are extremely clean and free of any segregated impurities, indicating that they are intrinsically brittle. Boron, when added to stoichiometric NiAl at a bulk level of 300 wt. ppm, segregates to the grain boundaries and suppresses intergranular fracture. However, there is no attendant improvement in tensile ductility because boron is an extremely potent solid solution strengthener in NiAl, more than doubling its yield strength. As a result, any potential benefit of improving grain boundary strength is more than offset by the increase in yield strength. Unlike boron, both carbon (300 ppm) and beryllium (500 ppm) are ineffective in suppressing intergranular fracture in NiAl, and Auger analyses of the C-doped alloy revealed that carbon did not affect the fracture mode because it did not segregate to the grain boundaries. Although neither beryllium nor carbon suppressed grain boundary fracture, their effects on the tensile ductility of NiAl were quite different: the ductility of the Be-doped alloy was higher than that of the B-doped alloy because beryllium, unlike boron, has a rather modest strengthening effect in NiAl, whereas the C-doped alloy was brittle like the B-doped alloy, because carbon is a potent solid solution strengthener, just like boron. These observations were rationalized by considering a hard-sphere model for interstitial and substitutional sites in NiAl. It was concluded that boron and carbon occupy interstitial sites, whereas beryllium dissolves substitutionally.

• DOE Contract Number: AC05-84OR21400
• OSTI ID: 7061661
• Journal Information: Journal of Materials Research; (USA), Journal Name: Journal of Materials Research; (USA) Vol. 5:4; ISSN JMREE; ISSN 0884-2914
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360102 -- Metals & Alloys-- Structure & Phase Studies
360103* -- Metals & Alloys-- Mechanical Properties
ALLOYS
ALUMINIUM ALLOYS
AUGER EFFECT
BERYLLIUM ADDITIONS
BERYLLIUM ALLOYS
BORON ADDITIONS
BORON ALLOYS
CARBON ADDITIONS
CRYSTAL DEFECTS
CRYSTAL STRUCTURE
DUCTILITY
FAILURES
FRACTURES
GRAIN BOUNDARIES
INTERSTITIALS
MECHANICAL PROPERTIES
MEDIUM TEMPERATURE
MICROSTRUCTURE
NICKEL ALLOYS
POINT DEFECTS
SOLUBILITY
STOICHIOMETRY
TENSILE PROPERTIES