The effect of thermal history on intergranular boron segregation and fracture morphology of substoichiometric Ni/sub 3/Al
While it has attractive mechanical properties and good corrosion resistance, the usefulness of polycrystalline Ni/sub 3/Al has been restricted because of its propensity for brittle intergranular fracture. While this intergranular brittleness can be aggravated by the intergranular segregation of certain impurities, particularly sulfur, the grain boundaries of Ni/sub 3/Al are intrinsically brittle and Ni/sub 3/Al will fail intergranularly in the absence of detectable impurity segregation. Addition of boron resulted in the fracture morphology changing from primarily intergranular to largely transgranular; and more importantly, the intergranular segregation of boron was conclusively demonstrated. The range of boron concentrations over which these beneficial effects are observed is well within the solubility limit, which has been estimated to be 1.5 at. % (4,5). Rice (6) developed a relationship between equilibrium intergranular segregation and grain boundary cohesion. According to this theory, the potential for intergranular embrittlement by a solute is related to the relative intensity of segregation of the solute to free surfaces as compared to segregation to grain boundaries. Rices theory allowed for the case of a solute segregating more strongly to grain boundaries than to free surfaces. If this difference is sufficiently large (approximately a factor of two), Rice's theory predicts an enhancement of grain boudary cohesion. White and coworkers (4,7) noted the rather unusual phenomenon of boron segregating much more strongly to grain boundaries of Ni/sub 3/Al than to free surfaces, while sulfur (an embrittling impurity) was shown to exhibit the opposite effect.
- Research Organization:
- Materials Science and Engineering, Univ. of Tennessee, Knoxville, TN 37996-2200
- OSTI ID:
- 5246560
- Journal Information:
- Scr. Metall.; (United States), Journal Name: Scr. Metall.; (United States) Vol. 20:7; ISSN SCRMB
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
360103* -- Metals & Alloys-- Mechanical Properties
ALLOYS
ALUMINIUM ALLOYS
BORON ADDITIONS
BORON ALLOYS
BRITTLENESS
CORROSION RESISTANCE
CRYSTAL STRUCTURE
CRYSTALS
ELEMENTS
FRACTURE PROPERTIES
GRAIN BOUNDARIES
IMPURITIES
INTERMETALLIC COMPOUNDS
MECHANICAL PROPERTIES
METALLURGICAL EFFECTS
MICROSTRUCTURE
NICKEL ALLOYS
NICKEL BASE ALLOYS
NONMETALS
POLYCRYSTALS
SEGREGATION
SULFUR