THE EFFECT OF INTERSTITIAL N ON GRAIN BOUNDARY COHESIVE STRENGTH IN Fe

Technical Report · Mon Sep 22 04:00:00 EDT 2003

DOI:https://doi.org/10.2172/940235· OSTI ID:940235

Increased nitrogen levels have been correlated with decreased ductility and elevated ductile-to-brittle transition temperature in pressure vessel steels [1]. However, the exact role played by nitrogen in the embrittlement of steels remains unclear. Miller and Burke have reported atom probe ion microscopy findings from neutron-irradiated low-alloy pressure vessel steel showing the presence of a 1 to 2 ruonolayer thick film of Mo, N, and C at prior austenitic grain boundaries (GB's) [2], suggesting a role for nitrogen as an intergranular embrittler. It is of interest for the development of mitigation strategies whether nitrogen must combine with other impurities to form nitride precipitates in order to exert an embrittling effect. Briant et al [1] have associated the embrittling effect of N in steels exclusively with intergranular nitride formation. This association suggests that high nitrogen levels may be acceptable if nitride precipitation at grain boundaries is suppressed. To address whether precipitate formation is indeed essential to the N embrittlement process in pressure vessel steel, a computational study was undertaken to ascertain whether the presence of interstitial nitrogen alone could embrittle an Fe GB. If so, nitrogen in any form must be kept completely away from the grain boundaries, if not out of the material altogether. The effect of interstitial N on the cohesion of an Fe {Sigma}3[110](111) grain boundary (GB) was investigated by ab-initio electronic structure calculations to reveal that free interstitial N produces a large strengthening energy, reduces the magnetic moments of the GB Fe atoms and is embrittling at the GB's.

• Research Organization: Bettis Atomic Power Laboratory (BAPL), West Mifflin, PA
• Sponsoring Organization: USDOE
• DOE Contract Number: AC11-98PN38206
• OSTI ID: 940235
• Report Number(s): B-T-3517
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ATOMS
DUCTILITY
ELECTRONIC STRUCTURE
EMBRITTLEMENT
GRAIN BOUNDARIES
IMPURITIES
INTERSTITIALS
ION MICROSCOPY
MAGNETIC MOMENTS
MITIGATION
NITRIDES
NITROGEN
PRECIPITATION
PRESSURE VESSELS
PROBES
STEELS
TRANSITION TEMPERATURE