HVEM in situ Study of High-Temperature Deformation of Ceramic and Metallic Materials
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introduced into the design of specimen stages by the Osaka HVEM group [2, 3]. A doubletilting high-temperature deformation stage using the same principle for heating was designed for the HVEM in Halle implying a number of new features, which are described in some detail in [4]. Up to now, about 60 experiments have been performed in this stage. The present paper briefly discusses the principles of the design of the stage, the performance parameters, specimen preparation, and the limitations of the experiments. In the main part, examples are given of the high-temperature in situ deformation of some ceramic and metallic materials. EXPERIMENTAL High-Temperature Straining Stage for the HVEM The problem to be solved was the design of a straining stage for temperatures clearly above 1000 °C, which is suited to plastically deform structural ceramics. An estimation of the radiation and conduction losses showed that the power necessary for high temperatures was about 200 W. As mentioned above, it is difficult to produce this heat within the desired volume by indirect resistance heating. Accordingly, electron bombardment was chosen to heat the specimen grips. Electron bombardment of the specimen itself cannot be used for electrically insulating materials like ceramics. It was not tried to reduce the necessary heating power by well designed thermal shielding as in [5]. Instead, water cooling was used in order to reduce the drift and to reach the thermal steady state quickly. The geometry of the stage was prescribed by the top-entry specimen stage of the Halle HVEM with a pole piece bore of more than 30 mm and sufficient space above the pole piece. As a consequence, the drive mechanism had to be placed above the specimen level so that levers loaded in bending have to transmit the load or displacement, respectively, to the specimen, similarly to a room temperature straining stage designed earlier [6]. The hot zone near the specimen is then located at the tips of the levers, as shown in Fig. 1. The specimen grips (1) are made of compact W-27Re alloy. The specimen is mounted from below. The coil filaments for electron bombardment (2) are fixed to the heat shields (3) by W wires, one on each side electrically insulated with respect to the shields to introduce the filament current (4). The shields themselves are insulated against the specimen grips. The acceleration voltage is applied between the filaments and the grips. The temperatures of both grips are measured by W-Re 3/25 thermocouples (5).
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Fig. 1 Perspective view and cross section of the hot zone of the high-temperature straining stage. After [4].
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Fig. 2 Schematic drawing of the high-temperature tensile device with control equipment. After [4].
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Further features of the stage are: - Water cooling to lead away the heat radiated from the grips through water channels in the supporting cone. - Copper lamellae heat exchangers for leading away the heat losses by conduction through the W-Re levers. - Drive of the specimen deformation by t
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