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

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

Study of the interphases in SiC/LAS composites by HREM and SIMS

The strength and toughness of the SiC Nicalon 202 fibers/LAS-matrix composites originates from the carbon richlayer which develops at the interfaces fibers/matrix during the process, To reach a better understanding of the formation of this carbon interphase in the SiC Nicalon/LAS, we have studied the interfacial region in several SiC/LAS composites which differ by i) the composition of the matrix, ii) the time of the hot pressing and iii) the composition of the reinforcement. As the interfacial region (IR) is about 100 nm thick, its characterization needs localised analyses. They have been performed by TEM, HREM and SIMS. In the composites SiC Nicalon fibers/LAS glass matrix and SiC Nicalon fibers/LAS + Nb2O5 glass matrix hot pressed during 10 minutes,the reaction between the fibers and the matrix results in:- a carbon layer whose thickness is increased by addition of Nb205 as previously found. We have shown that the microstructure of the carbon depends on the oxydant character of the matrix : it is mainly turbostratic in Nicalon/LAS (fig.l), whereas it is mainly amorphous in Nicalon/LAS + Nb205(fig.2).</jats:p