Relationship between the microstructure of the interface and the mechanical behaviour of composites materials

The mechanical behaviour of glass, ceramic or metal matrix composites reinforced by long fibers depend on many parameters. As reminded at the beginning of this paper, the most important are the fiber strength and the resistance to debonding and/or to sliding at the interfaces. The aim of the “tailoring” of the interfaces, whose principle is briefly described, is to control these parameters. To predict its efficiency is still uncertain. Coupled microstructural and mechanical characterizations of the composites, but also of their constituents, have to be performed to reach a better understanding of the mechanical behaviour of these complex materials. The efficiency of such an approach is demonstrated by the studies of the SiC Nicalon fiber/LAS glass matrix and the SCS-6 fiber/Ta-6AI-4V matrix composites described in this paper. From the comparison of these studies to those previously performed in glass or ceramic glass matrix composites, it is possible to discuss the interest of a key point in the “tailoring” of the interfaces : the introduction of a carbon interphase by fiber-matrix reaction or by fiber coating before the process.La tenue mécanique des composites à fibres longues et matrice verre, céramique ou métal dépend de nombreuses variables. Comme il est rappelé au début de cet article, les plus importantes sont la résistance des fibres et la résistance à la décohésion et/ou au frottement aux interfaces. Le “génie” des interfaces, dont le principe est brièvement décrit ici, est destiné à contrôler ces paramètres. Prévoir son efficacité est encore prématuré. Il faut développer des études couplées de la microstructure et des propriétés mécaniques des composites, mais aussi de leurs constituants, pour mieux comprendre l'origine de la tenue mécanique de ces matériaux complexes. C'est ce que montrent les études des composites à fibres SiC Nicalon/matrices verre LAS et à fibres SCS-6/matrice Ti-6AI-4V qui sont décrites dans cet article. La comparaison de ces résultats avec ceux obtenus précédemment dans les composites à matrice verre ou verre céramique permet de discuter l'intérêt d'un point clé du génie des interfaces : l'introduction d'une interphase de carbone par réaction fibre-matrice ou par revêtement des fibres avant élaboration

Analysis of local deformations in heterostructures containing short period superlattices by high-resolution transmission electron microscopy

This work describes the application of the Lattice Fringe Spacing Measurement (LFSM) method to the study of complex multiquantum well heterostructures containing both low-misfit and strain compensated short period superlattices in barriers and wells, respectively. 90º-wedge cross-sectional samples have been used. The adequate choice of both experimental conditions and digitized sampling allows the whole heterostructure to be visualized and studied in a single High Resolution Transmission Electron Microscopy (HRTEM) image. Sample preparation and image processing technique are simple and inexpensive, resulting a fast procedure particularly suited for the analysis of large areas. By this way, in a single HRTEM image we have measured, in the growth direction, the lattice spacings at either side of the multiple grown interfaces as well as the period variations of both types of superlattices; in addition, we have measure on the same image the lattice strain in a direction perpendicular to the growth direction by using the LFSM and the Cumulative Sum methods. We have observed local lateral variations within the wells, with regions tensile or compressively strained, while a vestige of the grown SL remains, indicating the occurrence of a strain induced lateral composition modulation process spontaneously produced during the growth of strain compensated short-period superlattices. This is further confirmed in cross-section prepared by the tripod mechanical polisher method.The authors express their sincere thanks to A. Guerrero for his support in the CUSUM program, to J. Thibault who invited them to take benefit of the 4000EX microscope available in the CEN-Grenoble, and to A. Sanz-Hervás, from Depto. Tecnología Electrónica, E.T.S.I.T.-U.P.M., Madrid, for his support in XRD measurements.Peer reviewe

PHASE TRANSFORMATION IN BORON DOPED SiC : HREM STUDY OF A TWIN INTERFACE

Dans les matériaux étudiés, la transformation de phase β → α donne lieu à la formation de plumes ou fers de lance qui peuvent être décrites comme des macles de transformation. Cet article décrit la structure d'un joint de macle obtenue par microscope électronique à haute résolution (MEHR). La macle étudiée a une structure 15R et un joint de macle à grand angle. Nous avançons une hypothèse pour décrire la formation d'une telle macle.In the materials studied, the phase transformation β → α involves the formation of feathers which can be described as penetration twins. In this investigation the structure of a twin interface is studied by high resolution electron microscopy (HREM). The twin studied has a 15R structure and a high angle twin boundary. An attempt is made to explain the formation of such a twin