High-temperature deformation of mullite and analysis of creep curves
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Olagnon and G. Fantozzi Groupe d’Etudes de Me´tallurgie Physique et Physique des Mate´riaux (GEMPPM), URA CNRS 341, Bat. 502, INSA de Lyon, 69621 Villeurbanne, France
A. Azim Unite´ de Formation et de Recherche (UFR) Matériaux et Énergies Renouvelables, Université Chouaib Doukkali, Faculté des Sciences, BP 20 El Jadida, Morocco (Received 28 November 2002; accepted 28 April 2003)
The creep behavior of mullite was studied under different stresses and in the temperature range 1200–1450 °C, and an analysis of creep curves was proposed. The study of creep behavior of mullite at high temperatures clearly indicates that this material exhibits concurrent creep and slow crack growth. An effective transition stress exists at each temperature. The analysis takes account of the total creep curve; in particular, the primary and stationary stages. It is now possible to determine by extrapolation the steady-state creep rate for specimens that break in the transient domain during tests. Thus, one can verify the influence of the stress on the steady-state creep rate over a wide stress range. On the other hand, this analysis clearly indicates the existence of two values of the activation energy around 1300 °C; this suggests a change of creep mechanism at this temperature. I. INTRODUCTION
The increasing use of materials in high-temperature structural applications requires a good knowledge of mechanical properties such as fracture strength, toughness, and, even of more importance, the long-term duration controlled by creep and subcritical crack growth. The recent development of high-purity mullite with little or nearly no glassy phase and high creep resistance in addition to its low thermal expansion1,2 made this material a serious candidate for new application as an engineering material at high temperature. The mullite seems well suited for use in ceramic motors and gas turbines. These applications require good knowledge of creep behavior and mechanisms that control deformation during the creep in these conditions. Information concerning the deformation rate is essential to the lifetime prediction of a given material when it is used under particular conditions of temperature and stress. Nevertheless, little information on thermomechanic behavior of this particular formulation of mullite is available in the literature. It is this reason that led us to study and analyze the creep behavior of mullite. This analysis takes account of the total creep curve: primary and secondary domains. In this study we neglect the tertiary domain, which is not experimentally observed. By extrapolation, the secondary region will permit us to determine strain rates for specimens that broke in the transient domain during J. Mater. Res., Vol. 18, No. 8, Aug 2003
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creep tests. Thus, one will be able to verify the influence of the stress on the stationary deformation rate for higher stresses. II. EXPERIMENTAL The mullite used in this work was made by ITMA (Aragon, Spain) from a Baikowski powder (CA 193 C
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