Paderborn, Universiẗat, Diss., 2000.

Située sur le piémont du massif montagneux du Trans-Ili Alataou, la ville d’Almaty est soumise à de nombreux risques naturels. Même si le risque de laves torrentielles, comparativement au risque sismique, est beaucoup moins diffus et plus localisé dans l’espace, il représente d’importants enjeux économiques et sociaux pour la ville d’Almaty. La ville poursuit son développement dans les fonds de vallées montagneuses et même dans des zones qui ne sont pas sous la protection des barrages. La question principale de cette recherche peut être formulée de la manière suivante : quelles sont les limites de la gestion du risque de laves torrentielles à Almaty en vue d’y apporter de possibles améliorations ? Ce questionnement implique de s’interroger sur les multiples facettes du problème de la gestion du risque de laves torrentielles, mais aussi de parvenir à évaluer le risque lui-même. Le parti-pris de cette recherche est donc d’interroger les causes et les solutions au risque de laves torrentielles dans un contexte de croissance urbaine et du réchauffement climatique. Ce travail est une contribution à la recherche en géographie - aménagement sur le Kazakhstan et plus précisément sur l’étude du risque de laves torrentielles à Almaty. Elle suppose d’adopter une approche théorique au carrefour de la géographie physique, de la géographie humaine et de l’aménagement urbain. / Located on the foothills of the Trans-Ili Alatau mountains, the city of Almaty is subject to many natural hazards. Although the debris flow risk, compared to seismic risk, is less diffuse and more localized in space, it represents important economic and social issues for the city of Almaty. The city continues its development in mountain valley bottoms and even in areas that are not protected by dams. The main question of this research can be formulated as follows: what are the limits of debris flow risk management in Almaty and the possible improvements? This questioning involves analyzing the multiple facets of the debris flow risk management, but also assessing the risk itself. The research objective is to examine the causes and solutions to the debris flow risk in the context of urban growth and climate change. This work is a contribution to research in geography and regional planning on Kazakhstan and more specifically on the debris flow risk in Almaty. It involves adopting a theoretical approach at the crossroads of physical geography, human geography and urban planning.

University, Diss., 2002--Mainz.

Universiẗat, Diss., 2002--Bonn.

No description available.

Estudos anteriores de precipitação, em outros aços superferríticos, constataram a ocorrência de precipitação das fases sigma, qui, de Laves e até de austenita, que no posterior resfriamento pode se transformar em martensita. O objetivo principal do presente trabalho foi estudar a precipitação de fases de Laves no aço inoxidável superferrítico X 1 CrNiMoNb 28 4 2 (Werkstoff-Nr. 1.4575). Para isto foram utilizadas algumas técnicas complementares de análise microestrutural, tais como microscopia óptica, eletrônica de varredura (MEV) e eletrônica de transmissão (MET). A composição química das fases foi estudada por análise de raios X por dispersão de energia tanto no MEV como no MET. A estrutura cristalina das fases foi estudada por difração de elétrons em área selecionada no MET. A fase que se precipita em maior quantidade no aço 1.4575 é a fase sigma. No entanto, também foram identificadas de maneira inequívoca, nos contornos de grãos, após envelhecimento a 850 ºC, pequenas quantidades de fase de Laves do tipo MgZn2, com estrutura hexagonal compacta e composição química (Fe,Cr,Ni)2(Nb,Mo,Si). O crescimento da fase de Laves é inibido pelo envolvimento da fase sigma, que também precipita nos contornos de grãos, mas em maior quantidade. Não foram detectadas as presenças das fases qui e austenita na temperatura estudada. / Previous precipitation studies in other superferritic stainless steels observed the occurrence of precipitations of sigma, chi and Laves phases and even of austenite, which on subsequent cooling may transform into martensite. The main objective of the present work was to study the Laves phase precipitation in a X 1 CrNiMoNb 28 4 2 (Werkstoff- Nr. 1.4575) superferritic stainless steel. Therefore, several complementary techniques have been employed such as optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The chemical composition of the phases has been studied using X-ray energy dispersive analysis both in SEM and TEM. Crystal structure of the phases has been analyzed using selected area electron diffraction in the TEM. The phase that precipitates in larger quantity in the 1.4575 steel is the sigma phase. However, it has been identified, in an unequivocal manner, on grain boundaries, after aging at 850 ºC, small quantities of Laves phase of the MgZn2 type, with a compact hexagonal structure and a chemical composition of (Fe,Cr,Ni)2(Nb,Mo,Si). Growth of the Laves phases is inhibited through the involvement of the sigma phase, which precipitates in higher quantities on grain boundaries. No chi and austenite phases have been detected in the studied temperature.

Intermetallic compounds are solid phases containing two or more metallic elements, whose crystal structure differs from that of its constituents [1]. The largest group among these compounds with more than 1400 binary and ternary representatives are the so-called Laves phases. The classification of an intermetallic compound as a Laves phase is solely based on the atomic configuration and the component ratio in the crystal structure. With the ideal composition AB2, the Laves phases crystallize in three closely related structure types which are named after their representatives, MgCu2 (C15, cubic), MgZn2 (C14, hexagonal) and MgNi2 (C36, hexagonal). Laves phases are built by almost all metals of the periodic system of the elements. A significant feature of many of these is the formation of broad homogeneity ranges by mutual substitution of atoms in combination with composition or temperature dependent phase transformations between the different Laves phase polytypes. Laves phases have received considerable attention in recent years as potential structural and functional materials. They combine high melting points with considerable creep resistance, high strength and fracture toughness and good corrosion and oxidation resistance. Some Laves phases like NbFe2 [2] or TaFe2 [3] show intriguing magnetic and electronic properties which provide a deeper inside into phenomenons like quantum criticality. Especially transition-metal based Laves phases like NbCr2 [4] and ZrCr2 [5] are promising candidates for the development of new high-temperature structural materials. The major drawback of the Laves phases, however, is their low-temperature brittleness. Many experimental and theoretical investigations have shown, that the low-temperature ductility can be improved by controlling the crystal structure with the help of phase transformations, by mechanical twinning or the addition of third elements. The addition of ternary alloying elements can alter the physical and electronic properties of the Laves phases and plays an important role in the composition dependent stability of the different polytypes. Some ternary Laves phases show an interesting phenomenon called site occupation reversal. It describes a composition dependent behavior of the alloying elements which prefer to occupy different crystallographic sites at different concentrations. The understanding of the point defect structure/mechanism and the site occupation of the alloying elements is thus of critical importance for the proper description of phase stability. The basis for the broad application of any metallic material is the knowledge of the corresponding phase diagram. The experimental determination of phase diagrams however, is tedious, time consuming and expensive work and the huge abundance of Laves phase makes this an impractical task. Thus, the time it takes to discover new advanced materials and to move them from the laboratory to the commercial market place is fairly long today. A cheap and fast enhancement for the development of new materials is the calculation of phase diagrams and physical properties using techniques like CALPHAD (CALculation of PHAse Diagrams) and DFT (Density Functional Theory). Very recently the Office of Science and Technology Policy of the United States White House announced to provide a budget of $100 million to launch the Materials Genome Initiative [6, 7]. The aim of this initiative is to provide the infrastructure and training needed to discover, develop, manufacture, and deploy advanced materials in a more expeditious and economical way [8]. One of the project’s three supporting legs is the calculation and prediction of crystal structures and physical properties using advanced Computational tools. "An early benchmark will be the ability to incorporate improved predictive modeling algorithms of materials behavior into existing product design tools. For example, the crystal structure and physical properties of the materials [. . . ]." [8]. Their computational tools of choice are the same as used in this work to predict crystal structures and site occupation factors. Contents of this work is the investigation of the substitutional disorder in binary and ternary Laves phases. This includes the experimental determination of the composition dependent stability of the Laves phase polytypes and the distribution of the substitution atoms in the crystal lattice of the respective phases, i.e., the site occupation factors (s.o.f.). For this purpose, detailed experimental studies on the two systems Cr–Co–Nb and Fe–Ta–V were performed and the Laves phase polytypes, their homogeneity ranges, the lattice parameters and the site occupation factors were determined. The experimental results are compared with the results obtained from quantum mechanical calculations. DFT is used to determine the composition dependent enthalpies of formation which serve as a measure for the stability of the different Laves phase polytypes. Additionally, the applicability of various approximations and their influence on the results has been checked. This study is thus also supposed to develop and improve the tools necessary for the calculation of phase stability and homogeneity ranges in ternary phases. Chapter two in the first part of this work describes the crystal structures of the Laves phases in detail with focus on the polytype stability, the site occupation and the c/a-ratio of hexagonal C14 Laves phases. Subsequently, the phase diagrams of the investigated systems and the occurring Laves phases are discussed. Chapter three briefly describes the experimental and theoretical methods used in this work. The last section of part one gives a detailed explanation of how the phase stability, the lattice parameters and the site occupation factors are calculated. The second part "Results and discussion" contains the discussion of the experimental and theoretical results for the intensively investigated systems Co–Cr–Nb (chapter five) and Fe–Ta–V (chapter six). Several other ternary C14 Laves phases and their site occupation behavior are studied in chapter seven. The thesis is concluded with a summary in chapter eight. Several additional information is contained in the appendix.

Estudos anteriores de precipitação, em outros aços superferríticos, constataram a ocorrência de precipitação das fases sigma, qui, de Laves e até de austenita, que no posterior resfriamento pode se transformar em martensita. O objetivo principal do presente trabalho foi estudar a precipitação de fases de Laves no aço inoxidável superferrítico X 1 CrNiMoNb 28 4 2 (Werkstoff-Nr. 1.4575). Para isto foram utilizadas algumas técnicas complementares de análise microestrutural, tais como microscopia óptica, eletrônica de varredura (MEV) e eletrônica de transmissão (MET). A composição química das fases foi estudada por análise de raios X por dispersão de energia tanto no MEV como no MET. A estrutura cristalina das fases foi estudada por difração de elétrons em área selecionada no MET. A fase que se precipita em maior quantidade no aço 1.4575 é a fase sigma. No entanto, também foram identificadas de maneira inequívoca, nos contornos de grãos, após envelhecimento a 850 ºC, pequenas quantidades de fase de Laves do tipo MgZn2, com estrutura hexagonal compacta e composição química (Fe,Cr,Ni)2(Nb,Mo,Si). O crescimento da fase de Laves é inibido pelo envolvimento da fase sigma, que também precipita nos contornos de grãos, mas em maior quantidade. Não foram detectadas as presenças das fases qui e austenita na temperatura estudada. / Previous precipitation studies in other superferritic stainless steels observed the occurrence of precipitations of sigma, chi and Laves phases and even of austenite, which on subsequent cooling may transform into martensite. The main objective of the present work was to study the Laves phase precipitation in a X 1 CrNiMoNb 28 4 2 (Werkstoff- Nr. 1.4575) superferritic stainless steel. Therefore, several complementary techniques have been employed such as optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The chemical composition of the phases has been studied using X-ray energy dispersive analysis both in SEM and TEM. Crystal structure of the phases has been analyzed using selected area electron diffraction in the TEM. The phase that precipitates in larger quantity in the 1.4575 steel is the sigma phase. However, it has been identified, in an unequivocal manner, on grain boundaries, after aging at 850 ºC, small quantities of Laves phase of the MgZn2 type, with a compact hexagonal structure and a chemical composition of (Fe,Cr,Ni)2(Nb,Mo,Si). Growth of the Laves phases is inhibited through the involvement of the sigma phase, which precipitates in higher quantities on grain boundaries. No chi and austenite phases have been detected in the studied temperature.

Intermetallic compounds are solid phases containing two or more metallic elements, whose crystal structure differs from that of its constituents [1]. The largest group among these compounds with more than 1400 binary and ternary representatives are the so-called Laves phases. The classification of an intermetallic compound as a Laves phase is solely based on the atomic configuration and the component ratio in the crystal structure. With the ideal composition AB2, the Laves phases crystallize in three closely related structure types which are named after their representatives, MgCu2 (C15, cubic), MgZn2 (C14, hexagonal) and MgNi2 (C36, hexagonal). Laves phases are built by almost all metals of the periodic system of the elements. A significant feature of many of these is the formation of broad homogeneity ranges by mutual substitution of atoms in combination with composition or temperature dependent phase transformations between the different Laves phase polytypes. Laves phases have received considerable attention in recent years as potential structural and functional materials. They combine high melting points with considerable creep resistance, high strength and fracture toughness and good corrosion and oxidation resistance. Some Laves phases like NbFe2 [2] or TaFe2 [3] show intriguing magnetic and electronic properties which provide a deeper inside into phenomenons like quantum criticality. Especially transition-metal based Laves phases like NbCr2 [4] and ZrCr2 [5] are promising candidates for the development of new high-temperature structural materials. The major drawback of the Laves phases, however, is their low-temperature brittleness. Many experimental and theoretical investigations have shown, that the low-temperature ductility can be improved by controlling the crystal structure with the help of phase transformations, by mechanical twinning or the addition of third elements. The addition of ternary alloying elements can alter the physical and electronic properties of the Laves phases and plays an important role in the composition dependent stability of the different polytypes. Some ternary Laves phases show an interesting phenomenon called site occupation reversal. It describes a composition dependent behavior of the alloying elements which prefer to occupy different crystallographic sites at different concentrations. The understanding of the point defect structure/mechanism and the site occupation of the alloying elements is thus of critical importance for the proper description of phase stability. The basis for the broad application of any metallic material is the knowledge of the corresponding phase diagram. The experimental determination of phase diagrams however, is tedious, time consuming and expensive work and the huge abundance of Laves phase makes this an impractical task. Thus, the time it takes to discover new advanced materials and to move them from the laboratory to the commercial market place is fairly long today. A cheap and fast enhancement for the development of new materials is the calculation of phase diagrams and physical properties using techniques like CALPHAD (CALculation of PHAse Diagrams) and DFT (Density Functional Theory). Very recently the Office of Science and Technology Policy of the United States White House announced to provide a budget of $100 million to launch the Materials Genome Initiative [6, 7]. The aim of this initiative is to provide the infrastructure and training needed to discover, develop, manufacture, and deploy advanced materials in a more expeditious and economical way [8]. One of the project’s three supporting legs is the calculation and prediction of crystal structures and physical properties using advanced Computational tools. "An early benchmark will be the ability to incorporate improved predictive modeling algorithms of materials behavior into existing product design tools. For example, the crystal structure and physical properties of the materials [. . . ]." [8]. Their computational tools of choice are the same as used in this work to predict crystal structures and site occupation factors. Contents of this work is the investigation of the substitutional disorder in binary and ternary Laves phases. This includes the experimental determination of the composition dependent stability of the Laves phase polytypes and the distribution of the substitution atoms in the crystal lattice of the respective phases, i.e., the site occupation factors (s.o.f.). For this purpose, detailed experimental studies on the two systems Cr–Co–Nb and Fe–Ta–V were performed and the Laves phase polytypes, their homogeneity ranges, the lattice parameters and the site occupation factors were determined. The experimental results are compared with the results obtained from quantum mechanical calculations. DFT is used to determine the composition dependent enthalpies of formation which serve as a measure for the stability of the different Laves phase polytypes. Additionally, the applicability of various approximations and their influence on the results has been checked. This study is thus also supposed to develop and improve the tools necessary for the calculation of phase stability and homogeneity ranges in ternary phases. Chapter two in the first part of this work describes the crystal structures of the Laves phases in detail with focus on the polytype stability, the site occupation and the c/a-ratio of hexagonal C14 Laves phases. Subsequently, the phase diagrams of the investigated systems and the occurring Laves phases are discussed. Chapter three briefly describes the experimental and theoretical methods used in this work. The last section of part one gives a detailed explanation of how the phase stability, the lattice parameters and the site occupation factors are calculated. The second part "Results and discussion" contains the discussion of the experimental and theoretical results for the intensively investigated systems Co–Cr–Nb (chapter five) and Fe–Ta–V (chapter six). Several other ternary C14 Laves phases and their site occupation behavior are studied in chapter seven. The thesis is concluded with a summary in chapter eight. Several additional information is contained in the appendix.

No description available.