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Session Chairperson: Jeff Rickman, Dept. of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18105-3195
1:30 pm INVITED
ESTIMATION OF GRAIN BOUNDARY ENERGETICS FROM MICROSTRUCTURAL DATA: Brent Adams, W.W. Mullins, Dept. of Materials Science and Engineering, Carnegie-Mellon University, Pittsburgh, PA 15213-3890; David Kinderleherer, Mathematics Department, Carnegie-Mellon University, Pittsburgh, PA 15213
Abstract not available.
THE RELATIONSHIP BETWEEN GB SEGREGATION AND FRACTURE STRENGTH: R.G. Faulkner, L.S. Shvindlerman, Institute of Polymer Technology and Materials Engineering, Loughborough University, Loughborough, Leics LE11 3TU, Institute for Solid State Physics, Russian Academy of Science, Chernogolovka, 142432, Moscow, Russia
A thermodynamic analysis is performed of the effects of segregation and grain boundary structure on grain boundary free energy in metallic solids. This leads to a definition of the reduction in grain boundary free energy caused by a given level of segregant. In turn this allows the prediction of grain boundary fracture strength assuming a ductile material intergranular fracture mechanism as proposed by McMahon and Vitek. The net result is that intergranular fracture strengths can be predicted as a function of the segregation patterns existing for the material's specific thermal history, the grain boundary sigma value, and the interplanar spacing on the boundary plane. A knowledge of the binding energy of the segregant atoms to the boundary is required but this is shown to be calculable on the basis of atomic misfits for metallic lattices. The results are compared with available data from Lim and Watanabe on GB fracture in Al-Sn-Zn materials.
THE ROLE OF GRAIN BOUNDARY ARBON FILMS IN THE RATE-DEPENDENT, INTERGRANULAR FRACTURE OF NICKEL-COPPER: MarjorieAnn E. Natishan, Matthew Wagonhoffer, Mechanical Engineering Department, University of Maryland, College Park, MD 20742
As part of an ongoing investigation to bound the service conditions of intergranular failure in nickel-copper alloy K-500 containing grain boundary precipitates of carbon, previously tested stress-rupture specimens were examined to document and characterize the deformation associated with grain boundary failures. Of particular interest was defining the role of the grain boundary carbon precipitates in the rate-dependent intergranular fracture process under ambient conditions. The failed creep/stress rupture specimens were sectioned perpendicular to the fracture surface and prepared for optical and electron microscopy of the microstructure adjacent to the primary fracture. High resolution microscopy revealed voids opened along the grain boundaries at carbon precipitate/nickel grain boundary interfaces. It is postulated that voids nucleate along these interfaces due to the combined driving force of the applied local stress resulting from dislocation accumulation at grain boundary/carbon interfaces and vacancy diffusion to these high energy sites. The intergranular failure then results from crack propagation along intergranular paths weakened by void formation.
ATOMISTIC STUDIES OF SEGREGATION AND FRACTURE IN Al-Mg ALLOYS: X.Y. Liu, J.B. Adams, Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, 105 South Goodwin Avenue, Urbana, IL 61801
High Mg content (>5%) Al-Mg alloys are known to cause edge cracking during the hot working. We investigated one possible explanation, that Mg segregation to grain-boundaries can embrittle them. The extent of Mg segregation to a wide variety of grain-boundaries in Al alloys with 10% Mg is determined with the EAM. These results are consistent with our previous surface segregation studies, which are consistent with experimental studies of surface segregation. The Griffith ideal work of fracture is determined by rigidly cleaving both pure Al and Al-Mg alloys; the presence of 20-40% Mg at interface is found to lower the work of adhesion by 10%. Segregation of Mg to surface (for slow fracture, i.e., below the threshold for catastrophic failure) suggests a further reduction of 20% in the work of adhesion. Dynamic studies of fracture are also carried out with Molecular Dynamics, to determine the effect of plastic deformation.
3:10 pm BREAK
3:30 pm INVITED
ATOMISTIC SIMULATION OF NANOCRYSTALLINE MATERIALS: S.R. Phillpot, P. Keblinski, D. Wolf, Materials Science Division, Argonne National Laboratory, Argonne, IL 60439; H. Gleiter, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany
Molecular-dynamics simulations have been used to synthesize and characterize fully dense, three-dimensional, "relaxed" nanocrystalline fcc metals and silicon. The high-energy grain boundaries in these nanocrystalline materials are found to be highly disordered. In the case of silicon the structures of the highly-constrained grain boundaries, triple lines and point grain junctions were found to be highly disordered and similar to the structure of amorphous silicon. Computer simulations of the behavior of an idealized model nanocrystalline material expose important parallels that exist in the dynamical properties of nanocrystalline materials and glasses. Both types of heavily disordered, metastable microstructures exhibit low- and high-frequency lattice-vibrational modes not seen in the perfect crystal, giving rise to similar thermodynamic properties at low temperatures, most notably a pronounced anomaly in their specific heats and a free-energy based phase transition from the nanocrystalline state to the glass below a critical grain size. Based on a free-energy argument, we suggest that below a critical grain size nanocrystalline materials should be unstable with respect to the amorphous phase. Work supported by the U.S. Department of Energy, BES Materials Sciences, under Contract W-31-109-Eng-38 and by the A. v. Humboldt and Max-Planck Foundations (Max-Planck Research Award program).
THE EFFECT OF GRAIN BOUNDARY CHARACTER ON TIME DEPENDENT CRACK PROPAGATION IN A Ni-BASE SUPERALLOY: P.F. Browning1, M.F. Henry1, K. Rajan2, 1General Electric Corp. Res. & Dev., P.O. Box 8, Schenectady, NY 12301; 2Rensselaer Polytechnic Institute, Dept. of Materials Engrg., Troy, NY 12180
An investigation of the effect of grain boundary character in the Ni-base superalloy Rene 88DT on environmentally assisted crack propagation in high temperature, gaseous environments will be presented. When tested at elevated temperature under hold time fatigue or sustained loading conditions in gaseous environment, crack propagation can occur intergranularly in this alloy, and hence may be influenced by grain boundary character. Automated microanalytical techniques have been applied to the crack tip area, with the intent of determining whether intergranular cracking occurs more readily along certain types of grain boundaries. The Coincident Side Lattice approach has been taken to characterize the misorientation between adjacent grains.
DYNAMIC EMBRITTLEMENT IN BERYLLIUM-STRENGTHENED COPPER: Ranjani C. Muthiah, C.J. McMahon, Jr., University of Pennsylvania, Philadelphia, PA; Amitava Guha, Brush Wellman Inc., Cleveland, OH
Dynamic embrittlement refers to the quasi-static diffusion-controlled decohesion, usually along grain boundaries, that can occur when a high-strength alloy is stressed in the presence of surface-absorbed impurities. Oxygen-induced cracking of nickel-based alloys at high temperatures (relevant to gas turbine engines) is thought to be an example of this phenomenon. A precipitation-hardened Cu-0.26%Be, which can be heat-treated to a wide range of strengths, is being used as a low-temperature model material for dynamic embrittlement. This alloy was found to be resistant to cracking at 200°C in an inert atmosphere but was highly susceptible to grain boundary cracking in air. Preliminary tests indicate that the failure time is highly stress-dependent, but there is an incubation time for the cracking, presumably related to crack nucleation. Crack growth rate studies are being done to eliminate the effect of the variable nucleation time. The oxygen partial pressure and yield strength dependence of the cracking mechanism are also being studied.
ATOMIC-SCALE SIMULATIONS OF GRAIN-BOUNDARY FRACTURE: F. Cleri, ENEA, Divisione Materiali Avanzati, Centro Richerche Casaccia, CP 2400, 00100 Roma A.D., Italy; S.R. Phillpot, D. Wolf, Materials Science Division, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439; S. Yip, Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, MA 02138
The results of fully atomistic simulations of grain-boundary fracture are presented. Microcracks were placed in the interface plane of symmetric-tilt grain boundaries (STGBs) for an fcc system described by a smoothed Lennard-Jones potential uniaxially loaded in pure mode-I. Proper treatment of boundary conditions was ensured, as demonstrated by the good agreement of crack propagation energy and stress fields with the continuum-elasticity solutions for the bulk reference crystal. The ductile vs. Brittle response of different STGBs was investigated as a function of the grain-boundary structure. A directional toughness effect was observed and compared to available experimental data and continuum-elasticity-based theoretical treatments. Extensive comparison with the Peierls-Nabaro-Rice model of dislocation nucleation from a crack tip was carried out, showing that some of the assumptions of the model need to be reconsidered in the case of interfacial fracture. *Word of FC was partly supported by Italian National Research Council (CNR), grant AI93.02171.11. Work of SRP and DW was supported by the US Department of Energy, BES-Materials Sciences under Contract W-31-109-Eng-38.
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