Hot-stage transmission electron microscopy study of phase transformations in hexacelsian (BaAl2Si2O8)
Phase transformations in synthetic hexacelsian were investigated by hot-stage transmission electron microscopy. A second phase transformation from an orthorhombic to hexagonal structure was identified in the synthetic hexacelsian at approximately 700 °C. The hexacelsian was found to exhibit a sequence of phase transformations on heating of hexagonal (P6 3 /mcm)-orthorhombic (Immm)-hexagonal (P6/mmm). Antiphase domain boundaries, which were observed in P6 3 /mcm and Immm phases, were absent in the P6/mmm phase. Crystal symmetries of the three phases were determined by convergent beam electron diffraction, and space group symmetries were derived by comparison of experimental selected area electron diffraction patterns with computer-simulated patterns.
Advances in Ceramic-Matrix Composites-IX
Fine ceramic fibers used as reinforcement of structures for thermomechanical applications are reviewed. The fibers have to show sufficient flexibility, high strength and thermomechanical stability in corrosive environment. These requirements impose specific fabrication routes and a strict control of their chemistry and microstructures. The relationship between fabrication process, high temperature mechanical behavior and microstructure evolution under load is described for SiC and alumina based fibers. It is shown that fibers of close chemical compositions but differing from dopant additions exhibit distinct creep behaviors. Favorable microstructural evolutions under load controlled by appropriate intergranular chemistry are proposed to reduce creep rates. The methods used to test and charcterize fibers of 10 urn in diameter are reviewed. INTRODUCTION Compared to organic, glass or carbon fibers, ceramic fibers can appear as anecdotal in terms of sales volumes, however the development of advanced materials is closely bound to the availability of high performance ceramic fibers. Applications, both actual and envisaged, are for aeronautical and ground based gas turbines, combustion chambers, heat exchangers... For all of these applications the structures have to withstand very high temperatures in oxidative and corrosive environments, so excluding the use of organic-, glassor carbon fibers. Ceramic fibers are often required to resist static or dynamic mechanical loading at high temperature, which can only be achieved by a close control of their microstructures. Ceramic fibers have to show sufficient flexibility so that preforms can be made by weaving and subsequently infiltrated by the matrix material. Flexibility is related to ~E/D where E is the stiffness and D the fiber diameter. A diameter of aroundlO urn is therefore required if Young's modulus exceeds 300 GPa (CC-AI2O3 To the extent authorized under the laws of the United States of America, all copyright interests in this publication are the property of The American Ceramic Society. Any duplication, reproduction, or republication of mis publication or any part thereof, without the express written consent of The American Ceramic Society or fee paid to the Copyright Qearance Center, is prohibited. Advances in Ceramic Matrix Composites IX 3 or SiC fibers). A strain to failure of around 1% is also required to facilitate the weaving process. Long term chemical, microstructural and mechanical stability and creep resistance above at least 1200°C are needed. A low reactivity with matrix and interface is necessary to stop or deflect matrix cracks, which is the basis of CMC tenacity. Alumina and silicon carbide bulk form are widely used for their high stiffness, and good high temperature mechanical properties in air. The relatively low strength of these bulk materials would be significantly increased if they could be transformed into small diameter fibers. Fine fibers must possess sub-micron grains. Moreover, for the same length, the probability of finding a critical defect in a specimen of a large cross section is much greater than in a fine fiber. The effect of any such defect will be confined to the few adjacent fibers within a composite structure. However the requirement of small diameters imposes production processes resulting in specific microstructures and properties for the fibers compared to conventional of bulk alumina and silicon carbide ceramics. Conventional ceramic processes, which include powder compaction and sintering, cannot be extended to fine ceramic fibers. Organic or glass fibers are obtained by the spinning of a bundle from a melt of the desired final composition, a process which is precluded for fine ceramic fibers as their melting points usually exceed 2000°C. The production of fine ceramic fibers requires the spinning of an organic or mineral precursor fiber, which is then heat-treated and pyrolysed for a very short time. This process has to yield not only the desired global chemistry but also a microstructure on a scale which is much finer than the fiber diameter and remain stable at high temperature and resistant to flaws and creep. The strong relations between the fabrication processes, microstructures and macroscopic properties have to be understood and controlled if high performance fibers are to be obtained. This paper will first present the specific test and observation methods for fine ceramic fibers and then follow the evolution in the last decades of both oxide and non-oxide ceramic fibers. The impact of small variations in the fabrication processes on the microstructure and mechanical behavior of these fibers will be reported. FIBER TEST METHODS Bundle tests In order to design structures in ceramic matrix composites it is necessary to be able to quantify the strengths and creep behavior of the fibers. The failure stresses of ceramic fibers show considerable scatter so that a statistical approach is necessary to predict the failure stresses of the fibers used in structures. Bundles of several hundreds of fibers are produced. The stress distribution of the fibers can theoretically be extracted by testing the bundle directly. Creep test of bundles is also a rapid and simple way to determine creep lifetime and to evaluate the creep rate. However in both cases testing bundles may introduce some ambiguities. 4 Advances in Ceramic Matrix Composites IX Fibers inside the bundle are not perfectly aligned and identically stretched at the beginning of the tensile test. A progressive loading of unequally stretched fibers when deformation increases is superposed to the failure of the weakest fibers and this leads to a non-linearity in the first part of the curves, Fig. 1. The progressive failure of the fibers and the redistribution of the load on the surviving fibers give to the last part of the tensile curve its dome shape. Mathematical models 12 enable to extract the statistical variation of the fiber strength from a bundle tensile test but this requires to know the distribution of the fiber slack. For the same reason tensile tests on bundle are not appropriate for a precise determination of the Young's modulus. Progressive loading of the fibers (slope t ) plus failure of the weakest fibers (slope I) I I E deduced only if true number of loaded fibers known (from slack and stress distribution) III Dome shape: fiber failures and load redistribution c <v E <u o JS Q. (O b I : primary creep or progressive loading of the fibers ? I I : displacement rate for which applied stress ? I I I : fiber damage (microcracks, cavities...) or progressive failure? Ill IL
SR topography of plastically deformed Fe-Si bicrystals - observation in situ and post mortem
SR reflection topography was applied to study the interaction of slip dislocations with the grain boundaries in Fe-4at%Si bicrystals deformed by compression. Three experimental settings with either a white or monochromatic beam were used and the corresponding orientation contrast analysed. Dislocation transmission through the grain boundary was observed in bicrystals with favourable geometry of the slip systems.