Atomistic Studies of Grain Boundaries and Heterophase Interfaces in Alloys and Compounds. Final Report, July 1987-August 1998 PDF Download
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Languages : en
Pages : 10
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Book Description
The overarching goal of the research supported by this grant was investigation of the structure and properties of interfaces in multicomponent systems by atomistic modeling. Initially, the research was devoted to studies of segregation to grain boundaries in binary disordered alloys. The next step was then studies of the structure and properties of grain boundaries in ordered compounds, specifically Ni3Al and NiAl, and grain boundary segregation in these compounds in the case of off-stoichiometry. Finally, the structure of Nb/sapphire interfaces, in particular the core configurations of the misfit dislocations, was studied.
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Languages : en
Pages : 10
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The overarching goal of the research supported by this grant was investigation of the structure and properties of interfaces in multicomponent systems by atomistic modeling. Initially, the research was devoted to studies of segregation to grain boundaries in binary disordered alloys. The next step was then studies of the structure and properties of grain boundaries in ordered compounds, specifically Ni3Al and NiAl, and grain boundary segregation in these compounds in the case of off-stoichiometry. Finally, the structure of Nb/sapphire interfaces, in particular the core configurations of the misfit dislocations, was studied.
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Languages : en
Pages : 13
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In this research project we carry out theoretical, computer modeling, studies of the atomic structure of grain boundaries in binary alloys. Both ordered and disordered alloys are investigated. The goal is to analyze those structural, chemical and electronic features that distinguish alloys from pure metals and are responsible for remarkably different intergranular fracture behavior of alloys when compared with pure metals. The most important phenomenon is, of course, segregation and related structural changes in the boundary region. When studying segregation phenomena copper-bismuth is a very suitable model system since bismuth segregation occurs readily, leads to boundary faceting and thus to remarkable changes in the boundary structure, as well as to a very strong embrittlement. Our recent research concentrated on the investigation of the structure of? = 3 (111)/(11{bar 1}) facets formed during segregation from boundaries which were originally curved.
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Languages : en
Pages : 34
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Atomic structure and atomic level properties of grain boundaries in binary alloys have been studied theoretically. Both ordered and disordered alloys were investigated with the aim responsible for the physical and mechanical properties differing from pure metals. An integral part is atomic interactions. The first are the many-body potentials akin to the embedded atom method. The most important potentials for Cu-Bi which combined empirical data and ab initio electronic structure calculations to construct reliable potentials. LMTO based tight-binding method were developed fully for pure metals with future application alloys. Structural studies were carried out for disordered Cu-Bi and ordered Ni3Al and Cu3Au alloys. In the former case the investigation was linked with HREM, and result is solution of structure of [Sigma] = 3 (111)/(11{bar 1}) facets containing Bi formed during segregation from originally curved boundaries. In the latter case, effect of ordering strength and stoichiometry deviations the boundary structure was investigated. The most interesting finding is a very different behavior of Ni rich and Al rich Ni3Al which may explain why only Ni rich compounds can be ductilized by boron. In both cases the surplus element segregates to the boundaries.
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Languages : en
Pages : 34
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Book Description
Atomic structure and atomic level properties of grain boundaries in binary alloys have been studied theoretically. Both ordered and disordered alloys were investigated with the aim responsible for the physical and mechanical properties differing from pure metals. An integral part is atomic interactions. The first are the many-body potentials akin to the embedded atom method. The most important potentials for Cu-Bi which combined empirical data and ab initio electronic structure calculations to construct reliable potentials. LMTO based tight-binding method were developed fully for pure metals with future application alloys. Structural studies were carried out for disordered Cu-Bi and ordered Ni3Al and Cu3Au alloys. In the former case the investigation was linked with HREM, and result is solution of structure of? = 3 (111)/(11{bar 1}) facets containing Bi formed during segregation from originally curved boundaries. In the latter case, effect of ordering strength and stoichiometry deviations the boundary structure was investigated. The most interesting finding is a very different behavior of Ni rich and Al rich Ni3Al which may explain why only Ni rich compounds can be ductilized by boron. In both cases the surplus element segregates to the boundaries.
Author: Min Yan
Publisher:
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Category :
Languages : en
Pages : 510
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Languages : en
Pages :
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The necessary precursor for any atomistic studies is a description of interatomic forces. Since no satisfactory description of interatomic forces in binary alloys could be found, the first stage of the research was devoted to this problem. The energy of the lattice was written as a sum of pair interactions and of a density dependent term. For pure metals the parameters in both the pair potential and the density dependent term were determined so as to fit the cohesive energy and elastic constants, and satisfy the condition of equilibrium for the experimentally known lattice parameter. The potentials and density dependent terms for pure metals were then used in constructing the corresponding description of interatomic interactions in alloys, using a modified method. The principle modifications are use of density dependent terms and fitting of the enthalpy of mixing. These procedures are described in detail. The grain boundary studies were started for the systems Cu-Ag, Cu-Bi and Au-Ag. However, since the density dependent part of the energy must now be taken into account explicitly a substantial modification of existing computer programs for grain boundary calculations is needed. This is at present in progress and thus only the directions of this work are reported. A limited effort was also devoted to the development of a theory of intergranular fracture in embrittled materials. The main purpose is to establish a relationship between the cohesive energy and the plastic work which would enable us to use the recently developed technique of measuring the local stress of fracture for direct studies of intergranular cohesion.
Author: P. E. J. Flewitt
Publisher: Wiley
ISBN: 9780471979517
Category : Technology & Engineering
Languages : en
Pages : 0
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Over recent years the understanding of grain boundaries, interphase boundaries and free surfaces has advanced greatly, leading to a clear recognition that these discontinuities make a significant contribution to the physical and mechanical properties of materials from multiphase metals and alloys to electronic materials. Grain Boundaries -Their Microstructure and Chemistry discusses the interrelationship between microstructure and chemistry of the grain boundary, with particular emphasis on the influence of the environment (air, liquid and liquid metal) and composition (bulk and impurity). This highly practical volume presents a brief background to interphase and grain boundaries, before considering in detail grain boundary composition and composition changes, and how grain boundary composition affects material properties. Very recent advances in techniques such as electron energy loss spectroscopy, high-resolution transmission electron microscopy and atom probe, and the facinating new insights into grain boundary, microstructure that they have revealed, are also discussed. Grain Boundaries - Their Microstructure and Chemistry is an indispensable text for design and safety engineers in many industries, including power and aerospace, as well as for materials scientists and engineers in academia and research institutes.
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Languages : en
Pages : 16
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Progresses in the structural and chemical characterization of grain boundaries in metals are reported. We have developed a solid state method for fabrication of a variety of important grain boundaries. This method is based on a sequence of heavy deformation of single crystals followed by controlled recrystallization. The structure of complex grain boundaries such as the quasiperiodic 45°[100] twist in pure aluminum or the periodic [Sigma]5[310] in aluminum-5%Mg alloy has been elucidated. We have found the structural unit (SU) model to be very powerful for the description of quasiperiodic interfaces. The applicability of the SU model to heterophase interfaces is verified for Si-Al interfaces. Further advances have been achieved in the understanding of the driving forces for grain boundary segregation including elastic and electronic effects. Chemical effects on grain boundary core structure are observed in the case of Al-Mg alloys and Sr doped Si.
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Languages : en
Pages :
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The aim of this research project is to carry out atomistic studies of grain boundaries with impurities. The emphasis of the investigation is on the effect of segregation upon the structure of grain boundaries and their properties. Since the necessary precursor of any atomistic study is a description of interatomic interactions an empirical scheme for the construction of interatomic potentials was first developed. In this scheme which was summarized in a previous report, the interatomic interactions are described by pair potentials together with a volume dependent term which represents a major contribution to the overall energy of the studied system. A new relaxation method had to be developed that includes the volume dependent term into the calculations which are then carried out at constant pressure rather than at constant volume. Furthermore, this method also includes the possibility of dealing with spatially varying concentration of impurities. It is described in more detail in the report.
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Languages : en
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