Pergamon 09567151(9400343-2 Printed in Great Britain HIGH TEMPERATURE DEFORMATION OF AN ALUMINA COMPOSITE REINFORCED WITH SILICON CARBIDE WHISKERS KENONG XIA and TERENCE G. LANGDON Department of Mechanical and Manufacturing Engineering, University of Melbourne, Parkville, Victoria, Australia 3052 and"Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA90089-1453, U.S.A (Received I February 1994; in revised form 20 July 1994) Abstract-Four-point bending creep tests we rried out in air on an alumina matrix composite reinforced with 9.3 vol. of silicon carbide whiskers. Typical three-stage creep was observed. In the temperature range of 1673-1823 K, the composite exhibited an average stress exponent of 3.8. The activation energy for creep was estimated as -820-830 kJ mol-. microstructure of the composite wa haracterized before and after deformation, dislocation networks and other configurations were observed It is concluded that the deformation mechanism consists of intragranular dislocation movement controlled by the lattice diffusion of oxygen ions 1 INTRODUCTION characteristics of an Al,O3 matrix composite ein forced with 9.3 vol. of sic whiskers Ceramic materials are becoming increasingly attrac tive for a wide range of engineering applications because they are generally harder, stronger and 2 EXPERIMENTAL MATERIAL AND PROCEDURES lighter than metals. More importantly, they maintain their high strength at very high temperatures and they The test material, designated Al, O9.3 vol% are more resistant to severe environments. Neverthe- SiC(w), consisted of an alumina matrix (a-Al,O,) less, the potential applications of ceramic materials and 9.3 vol. of silicon carbide whisker reinfor are often limited by their inherent brittleness, suscep- ment (equivalent to 7. 5 wt%). The composite was ibility to sudden catastrophic failure and low ther- fabricated by hot pressing a mixture of high purity mal shock resistance. Recent investigations have Al, O, powder and Sic whiskers without additives demonstrated that it is possible to toughen ceramic The material was produced as hot-pressed discs hav- matrices by adding various composite components: ing a thickness of 3 mm, and rectangular bars we for example, the toughness of composites such as cut from these discs with cross-sections of 2x 3 mm Al,O-SiC [1-3], Si, N SiC [4, 5], Al,O-Zro2[6], and lengths of -50 mm mullite-SiC [7] and SiC-TiC [8] are generally higher Prior to testing, the as-received specimens were han the monolithic matrices. characterized by scanning and transmission electron The creep behavior of ceramics becomes important microscopy in order to determine the grain size of when considering their use in structural applications the matrix, the distribution of the whiskers and the at elevated temperatures. In general, only limited nature of the interfaces between the matrix and the experimental data are available to characterize the whiskers. The grain boundaries of the matrix were creep behavior of composites, and this deficiency revealed by polishing on a series of diamond paste tends to inhibit the establishment of design criteria and then etching in phosphoric acid at 453K for nd the improvement of processing methods >30 min. The whiskers were revealed by etching the There are numerous reports of the high tempera- unpolished surfaces in phosphoric acid at 553K ture creep behavior of monolithic alumina and these for 5-10 min. The samples were coated with gold data are tabulated (9) and analyzed [10] elsewhere. and then examined in a Cambridge Stereoscan S There are also several reports of the creep behavior IV-10 scanning electron microscope operating with of alumina composites [11-18] but there has been no an accelerating voltage of 10 kV. The sample stage systematic investigation of the creep properties and was set at the horizontal position deformation mechanisms occurring in a well For observations of the whisker/ matrix interface, terized alumina composite. Accordingly, the samples were prepared for transmission electron investigation was conducted to determine the microscopy. A slice of 300-500 um thickness was 1421Pergamon Acta metall, mater. Vol. 43, No. 4, pp. 1421 1427, 1995 Copyright ~ 1995 Elsevier Science Ltd 0956-7151(94)00343-2 Printed in Great Britain. All rights reserved 0956-7151/95 $9.50 + 0.00 HIGH TEMPERATURE DEFORMATION OF AN ALUMINA COMPOSITE REINFORCED WITH SILICON CARBIDE WHISKERS KENONG XIA l and TERENCE G. LANGDON 2 ~Department of Mechanical and Manufacturing Engineering, University of Melbourne, Parkville, Victoria, Australia 3052 and 2Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90089-1453, U.S.A. (Received 1 February 1994; in revised Jorm 20 July 1994) Abstract--Four-point bending creep tests were carried out in air on an alumina matrix composite reinforced with 9.3 vol.% of silicon carbide whiskers. Typical three-stage creep was observed. In the temperature range of 1673-1823 K, the composite exhibited an average stress exponent of 3.8. The activation energy for creep was estimated as ~820--830 kJ mol-~. Microstructure of the composite was characterized before and after deformation. Dislocation networks and other configurations were observed in samples deformed to large strains. It is concluded that the deformation mechanism consists of intragranular dislocation movement controlled by the lattice diffusion of oxygen ions. 1. INTRODUCTION Ceramic materials are becoming increasingly attrac￾tive for a wide range of engineering applications because they are generally harder, stronger and lighter than metals. More importantly, they maintain their high strength at very high temperatures and they are more resistant to severe environments. Neverthe￾less, the potential applications of ceramic materials are often limited by their inherent brittleness, suscep￾tibility to sudden catastrophic failure and low ther￾mal shock resistance. Recent investigations have demonstrated that it is possible to toughen ceramic matrices by adding various composite components; for example, the toughness of composites such as A1203-SiC [1-3], Si3N4-SiC [4, 5], A1203-ZrO 2 [6], muilite-SiC [7] and SiC-TiC [8] are generally higher than the monolithic matrices. The creep behavior of ceramics becomes important when considering their use in structural applications at elevated temperatures. In general, only limited experimental data are available to characterize the creep behavior of composites, and this deficiency tends to inhibit the establishment of design criteria and the improvement of processing methods. There are numerous reports of the high tempera￾ture creep behavior of monolithic alumina and these data are tabulated [9] and analyzed [10] elsewhere. There are also several reports of the creep behavior of alumina composites [11-18] but there has been no systematic investigation of the creep properties and deformation mechanisms occurring in a well charac￾terized alumina composite. Accordingly, the present investigation was conducted to determine the creep characteristics of an A1203 matrix composite reinforced with 9.3 vol.% of SiC whiskers. 2. EXPERIMENTAL MATERIAL AND PROCEDURES The test material, designated A1203-9.3vol.% SiC(w), consisted of an alumina matrix (~-A1203) and 9.3 vol.% of silicon carbide whisker reinforce￾ment (equivalent to 7.5 wt%). The composite was fabricated by hot pressing a mixture of high purity A1203 powder and SiC whiskers without additives. The material was produced as hot-pressed discs hav￾ing a thickness of 3 mm, and rectangular bars were cut from these discs with cross-sections of 2 x 3 mm and lengths of ~ J0 ram. Prior to testing, the as-received specimens were characterized by scanning and transmission electron microscopy in order to determine the grain size of the matrix, the distribution of the whiskers and the nature of the interfaces between the matrix and the whiskers. The grain boundaries of the matrix were revealed by polishing on a series of diamond pastes and then etching in phosphoric acid at ~453 K for > 30 min. The whiskers were revealed by etching the unpolished surfaces in phosphoric acid at ~553 K for 5 10min. The samples were coated with gold and then examined in a Cambridge Stereoscan S IV-10 scanning electron microscope operating with an accelerating voltage of 10 kV. The sample stage was set at the horizontal position. For observations of the whisker/matrix interface, samples were prepared for transmission electron microscopy. A slice of ~ 300-500 pm thickness was 1421