Thesis (Ph. D.)--Virginia Polytechnic Institute and State University, 1993. / Vita. Abstract. Includes bibliographical references (leaves 192-196). Also available via the Internet.

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X-ray polycrystalline diffraction techniques are used to determined the defect structure in a commercial Cu-l1.55 at. %Be-O.23 at. %Co alloy during low temperature aging. The analysis of the X-ray diffraction patterns is aided by the computer modeling of the defects inside the crystals using an elastic model. The research is to provide more detailed understanding of the precipitate structures in the process of aging and the age hardening mechanism of the alloy. The diffraction profiles from samples aged for different times at 315°C are collected using a position sensitive proportional counter with eu 1(0:1 radiation. The hardness values of the samples are also measured. An elastic model for the coherent precipitates in anisotrpic matrix is developed according to Eshelby's treatment of transformed regions in an elastic continuum. The displacement fields generated by the precipitates in the surrounding matrix, obtained through the elastic model, are used to explain the (110) streaks near the Bragg reflection, and to calculate the powder diffraction patterns from the aged alloys. For the latter purpose, a general X-ray diffraction theory is developed, combining the work of Krivoglaz and Dederichs. The X-ray diffraction patterns are compared with existing TEM observations in the literature. The analysis of the diffraction patterns suggests that the precipitates in the early stage of aging (GP zones) can be the matrix constrained version of 7, the equilibrium phase, similar to an observation by Khatchaturyan and Laughlin on 7 and 7'. The quantitative calculation based on this model shows excellent agreement with experimental data for samples aged at 315°C. The averaged inter-precipitate spacing in the crystals for the optimally aged sample at 315°C is determined to be 200-300A. A simplified method of evaluating the thermal diffuse scattering and a method for calculating the diffuse scattering from polycrystalline materials with textures are also presented. / Ph. D.

Le travail proposé s’inscrit dans le contexte général de la maîtrise des propriétés des matériaux en relation avec les conditions d’usinage. Cette étude s'intéresse en particulier aux relations entre Microstructure et Usinage. La problématique développée dans ce travail concerne la mise en forme des alliages cuivre-béryllium à usinabilité améliorée (C17300) qui ont la réputation d’être versatiles à l'usinage, en dépit de propriétés mécaniques macroscopiques comparables. La mise en évidence de ces variations d'usinabilité et la recherche des causes et des solutions est l'objectif principal. Après la description du matériau d'étude et des méthodes de caractérisation, l'usinage des alliages C17300 est conduit en réalisant le suivi des efforts et des températures au cours d'une opération de coupe orthogonale stricte. Les résultats expérimentaux, à la fois de la caractérisation multi-échelle et de l'usinage des nuances industrielles dans un état de livraison et de remise en solution, sont présentés. Ils permettent de mettre en œuvre et de valider un modèle analytique de coupe. La question de l'influence de l'usinage sur la microstructure de la pièce usinée est complétée par le développement d'alliages modèles à dureté contrôlée élaborés en interne. Une caractérisation très précise de ces alliages (en surface, en sous-surface et dans les copeaux) permet d’évaluer l’impact de la microstructure initiale sur leur usinabilité. Enfin, une discussion confronte l'ensemble des résultats obtenus. Grâce à l'utilisation du modèle analytique validé, des informations essentielles (temps de contact, temps de transit et température au sein des bandes de cisaillement) permettent de mettre en évidence des phénomènes diffusionnels en cours d'usinage. L'adaptation de différents modèles de diffusion assistée permet d'expliquer les phénomènes observés. Pour clôturer l'étude, de nouveaux chemins de mise en forme des C17300 sont proposés, afin d'assurer l'obtention de propriétés mécaniques excellentes avec une usinabilité améliorée / The document presented hereby study the relationship existing between machining and materials behaviors, especially from a microstructure's perspective. The main purpose of this study is to understand the variability of cutting process of copper-beryllium alloys with improved machining capabilities, despite the fact that their macroscopic mechanical properties are even. After a description of those alloys and the characterization methods used in the present work, the machining of different materials are presented. The stresses and temperatures reached during orthogonal cutting are recorded and will describe the machinability. Experimental results of machining alloys which are in different metallurgical states are then presented. Those results will help the implementation of an analytical model of chips formation. The influence of machining process on work pieces and chips produced by orthogonal cutting of copper-beryllium alloys are then completed by a study of model materials developed in this work. Those alloys presenting well defined microstructure still possess the same hardness despite different thermo-mechanical treatments including mechanical deformation and ageing. A very detailed microstructure analysis of those model materials shows the influence of initial microstructure on their machinability. Diffusion mechanisms are put in light thanks to the information delivered by the analytical model (interaction time, transit time and temperature obtained through the different shear bands). Those assisted diffusion mechanisms explain the material behaviour during the orthogonal cutting. At last, better ways of material forming are exposed in order to have simultaneously excellent mechanical properties and excellent machinability for copper-beryllium alloys