China Science

Surface intermixing behavior during thin-film deposition in the Co–Al system was investigated on the atomic scale by three-dimensional classical molecular dynamics simulation. Asymmetry of the surface intermixing was observed: Al deposition on a Co substrate resulted in an Al thin-film with an atomically sharp interface, while a Co thin-film deposited on an Al substrate had an interfacial intermixing layer of B2 structure. This phenomenon is discussed in terms of the kinetics of atomic intermixing on the surface. A kinetic criterion for the atomic intermixing is whether the increased kinetic energy of the deposited atom near the surface is larger than the energy barrier to atomic intermixing on the surface. Local acceleration of the deposited atoms near the surface provides an explanation of the puzzling phenomenon of the significant intermixing under low-energy deposition conditions such as thermal evaporation or molecular beam epitaxy.

Sang-Pil Kima?Seung-Cheol Leea?Kwang-Ryeol LeeaEmail:krlee@kist.re.kr?Yong-Chae Chungb
[a]Computational Science Center, Future Fusion Research Laboratory, Korea Institute of Science and Technology, Seoul 136–791, Republic of Korea;[b]Division of Materials Science Engineering, Hanyang University, Seoul 133–791, Republic of Korea

Simulations of a monatomic model amorphous matrix embedded with approximately 37% of a body-centered cubic phase demonstrate mechanisms by which nanocrystallites can alter the mechanical response of metallic glass. Three effects affect the resulting ductility: (i) the presence of weak amorphous–crystalline interfaces, (ii) the fraction of nanocrystallites oriented to prevent twinning relative to the loading stress, and (iii) the shear-induced growth and dissolution of the nanocrystallites when they are impinged by shear bands. While the first effect dominates in these simulations due to system size limitations, the third effect appears to be crucial for understanding the ductility of experimental samples. These simulations indicate that shear-induced growth of existing nanocrystallites, rather than nucleation of new crystalline regions, may account for the observed enhancement in ductility.

Yunfeng Shia?Michael L. FalkaEmail:mfalk@umich.edu
[a]Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136, USA

An empirically determined measure of the solute drag force called the drag factor is derived and defined. The drag factor is the derivative of mobility with respect to grain size, and describes well the drag effect of solute in the six different aluminas measured. A normalized drag factor allows direct comparison of different dopants, and validation of theoretically predicted trends. This construction is used to verify that the role of magnesia and rare-earth dopants in reducing the grain-growth rate is due to solute drag from the intrinsic mobility. These dopants segregate to the core of the grain-boundary, which differs from classical solute drag models that derive the drag effect from solute in the near-boundary lattice. The solute drag factor is also used to understand the role of drag in grain-boundaries that have mobilities that are enhanced relative to the pure material. This new approach for analyzing grain-growth advances the understanding of microstructural evolution and its relationship to properties.

Shen J. DillonaEmail:sjd6@lehigh.edu?Shantanu K. Beheraa?Martin P. Harmera
[a]Center for Advanced Materials; Nanotechnology, Lehigh University, Bethlehem, PA 18015, USA

This is the second of a two-part paper intended to develop a framework for collecting data, quantifying characteristics and subsequently representing microstructural information from polycrystalline materials. The framework is motivated by the need for incorporating accurate three-dimensional grain-level morphology and crystallography in computational analysis models that are currently gaining momentum. Following the quantification of microstructural features in the first part, this paper focuses on the development of models and codes for generating statistically equivalent synthetic microstructures. With input in the form of statistical characterization data obtained from serial-sectioning of the microstructures, this module is intended to provide computational modeling efforts with a microstructure representation that is statistically similar to the actual polycrystalline material.

Michael GroeberaEmail:groeber.9@osu.edu?Somnath GhoshbEmail:ghosh.5@osu.edu?Michael D. UchiccEmail:Michael.Uchic@wpafb.af.mil?Dennis M. DimidukcEmail:Dennis.Dimiduk@wpafb.af.mil
[a]Graduate Research Associate, Materials Science; Engineering, The Ohio State University, Columbus, OH 43210, USA;[b]Nordholt Professor, Mechanical Engineering; Materials Science; Engineering, The Ohio State University, Columbus, OH 43210, USA;[c]Air Force Research Laboratory, Materials; Manufacturing Directorate, AFRL/MLLMD, Wright-Patterson AFB, OH 45433, USA

Single pan thermal analyses (SPTA) have been performed on Cu–14.5 wt.% Sn, Cu–21.3 wt.% Sn and Cu–26.8 wt.% Sn peritectic alloys. For this purpose, a SPTA assembly has been built and calibrated. As the latent heat is a function of temperature and composition during solidification of alloys, a new heat flow model coupled to a Cu–Sn thermodynamic database has been defined for the calculation of the corresponding evolutions of the solid mass fraction, fs(T). To verify the accuracy of this model, a close comparison with a microsegregation model that includes back-diffusion in the primary -solid phase has also been conducted successfully. The thermal analyses have finally shown that the Cu–Sn phase diagram recently assessed in the review of Liu et al. is the most reliable.

F. Kohlera?T. Campanella1?S. Nakanishi2?M. Rappaz aEmail:michel.rappaz@epfl.ch

Three glass-forming alloy compositions were chosen for ribbon production and subsequent electron microscopy studies. In situ tensile testing with transmission electron microscopy (TEM), followed by ex situ TEM and ex situ scanning electron microscopy (SEM), allowed the deformation processes in tensile fracture of metallic glasses to be analysed. In situ shear band propagation was found to be jump-like, with the jump sites correlating with the formation of secondary shear bands. The effect of structural relaxation by in situ heating is also discussed. Nanocrystallization near the fracture surface was observed; however, no crystallization was also reported in the same sample and the reasons for this are discussed. Both the TEM and the SEM observations confirmed the presence of a liquid-like layer on or near the fracture surface of the ribbons. The formation of a liquid-like layer was characterized by the vein geometries and vein densities on the fracture surfaces and its dependence on shear displacement, ?, is discussed. A simple model is adapted to relate the temperature rise during shear banding to the glass transition and melting temperatures and this is used to explain the variety of fracture surfaces which are developed for macroscopically identical tensile testing of metallic glasses together with features which exhibit local melting.

D.T.A. Matthewsa?V. Ocelíka?P.M. Bronsvelda?J.Th.M. De Hosson aEmail:j.t.m.de.hosson@rug.nl
[a]Department of Applied Physics, Netherlands Institute for Metals Research; Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Constitutive equations are constructed for single-crystal nickel-based superalloys. Account is taken of dislocation glide in the channels of the matrix phase (referred to as ?) of the face-centred cubic (fcc) type, dislocation climb at the interfaces with the reinforcing L12 precipitates (referred to as ??) and the processes leading to cutting of the interfaces by dislocation ribbons via stacking fault shear of the type. A treatment of ribbons produced by the combination of channel dislocations by an appropriate set of dislocation reactions is developed. The model allows the following features of superalloy creep to be recovered: dependence upon microstructure and its scale, effect of lattice misfit, internal stress relaxation, incubation phenomena, the interrelationship of tertiary and primary creep, and vacancy condensation leading to damage accumulation. Using the model, the creep deformation behaviour of the single-crystal superalloy CMSX-4 is studied, with emphasis on the interrelationship between primary and tertiary creep. It is shown that the values for the various parameters used in the modelling are physically reasonable and are related to the microstructure and its evolution during creep. The creep anisotropy prevalent in these materials due to primary creep is recovered correctly.

A. MaaEmail:a.ma@imperial.ac.uk?D. Dyea?R.C. Reedb Email:r.reed@birmingham.ac.uk
[a]Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK;[b]Department of Metallurgy; Materials, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK

During fusion welding, the presence of sulfur in steel often affects heat and fluid flow in the weld pool and its geometry. While the role of sulfur during welding of stainless steel plates with the same sulfur content is well understood, welding of stainless steel plates containing different concentrations of sulfur has not yet received proper attention. Here we report an experimental and modeling investigation of gas tungsten arc butt welding of stainless steel plates containing different sulfur concentrations. The main variables studied were sulfur concentrations in the two plates, welding current and welding speed. The results show significant shift of the fusion zone toward the low sulfur steel. The asymmetric fusion zone profile with respect to the original joint interface could be quantitatively explained through numerical modeling of heat transfer and fluid flow considering a bead shift observed experimentally.

S. Mishraa?T.J. Lienertb?M.Q. Johnsonb?T. DebRoyc Email:debroy@psu.edu
[a]Department of Metallurgical Engineering; Materials Science, Indian Institute of Technology Bombay, Mumbai, India;[b]Materials Science; Technology: Metallurgy Group, Los Alamos National Laboratory, Los Alamos, NM, USA;[c]Department of Materials Science; Engineering, The Pennsylvania State University, University Park, PA, USA