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An Atomistic Modeling Study of Alloying Element Impurity Element, and Transmutation Products on the cohesion of A Nickel E5 {l_brace}001{r_brace} Twist Grain Boundary

Description: Atomistic modeling methods were employed to investigate the effects of impurity elements on the metallurgy, irradiation embrittlement, and environmentally assisted cracking of nickel-base alloys exposed to nuclear environments. Calculations were performed via ab initio atomistic modeling methods to ensure the accuracy and reliability of the results. A Griffith-type fracture criterion was used to quantitatively assess the effect of elements or element pairs on the grain boundary cohesive strength. In order of most embrittling to most strengthening, the elements are ranked as: He, Li, S, H, C, Zr, P, Fe, Mn, Nb, Cr, and B. Helium is strongly embrittling (-2.04 eV/atom lowering of the Griffith energy), phosphorus has little effect on the grain boundary (0.1 eV/atom), and boron offers appreciable strengthening (1.03 eV/atom increase in the Griffith energy). Calculations for pairs of elements (H-Li, H-B, H-C, H-P, and H-S) show little interaction on the grain boundary cohesive energy, so that for the conditions studied, linear superposition of elemental effects is a good approximation. These calculations help explain metallurgical effects (e.g. why boron can strengthen grain boundaries), irradiation embrittlement (e.g. how boron transmutation results in grain boundary embrittlement), as well as how grain boundary impurity elements can affect environmentally assisted cracking (i.e. low temperature crack propagation and stress corrosion cracking) of nickel-base alloys.
Date: June 16, 2003
Creator: Jr., G.A. Young; Najafabadi, R.; Strohmayer, W.; Baldrey, D.G.; Hamm, B.; Harris, J. et al.
Partner: UNT Libraries Government Documents Department

Free energy simulation of grain boundary segregation and thermodynamics in Ni sub 3-x Al sub 1+x

Description: The free energy simulation method is employed to study segregation to {Sigma}5 and {Sigma}13 (001) twist grain boundaries and their free energies in ordered Ni{sub 3-x}Al{sub 1+x}. In the temperature range studied (300--900K), it is shown that there is almost no segregation, strong Al segregation, and weak Ni segregation to the grain boundary for the stoichiometric, Al-rich, and Ni-rich bulk compositions respectively. It is also shown that the segregation is limited to a few (002) planes around the grain boundary and its magnitude decreases with increasing temperature. For Al-rich bulk composition, it is demonstrated that segregation at low temperature substantially lowers the grain boundary free energy. 8 refs., 7 figs.
Date: January 1, 1990
Creator: Najafabadi, R.; Wang, H.Y.; Srolovitz, D.J. (Michigan Univ., Ann Arbor, MI (USA). Dept. of Materials Science and Engineering) & LeSar, R. (Los Alamos National Lab., NM (USA))
Partner: UNT Libraries Government Documents Department