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2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

a) contact heating

a) contact heating Al-alloy b) heating in tube Al-alloy Al 2O 3-tube Al 2 O 3 -sheet T> Tliq substrate substrate substrate h = 10 mm T= 750°C + impulse 67 substrate Oxide skin Figure 3.2 Schematic of the different set-ups of droplet generation in the sessile drop apparatus. a) contact heating method and b) heating and melting in a tube prior to contact with the substrate. Temperature was measured using thermocouples directly mounted to the top of the substrate. The thermocouples were also used for control of heating intensity fitted with a PID-controller. The furnace was heated to 200°C prior to the test. After evacuation to a pressure lower than 10 -4 Pa, the inside of the furnace was flushed with 99.9995% purity argon (BOC 2311470) with a typical oxygen and moisture contents of less than 1 volume part per million (vpm). This procedure was repeated three times. Once completed, the chamber was evacuated to the minimum pressure attainable, which was in the range of 2·10 -5 Pa to 7·10 -5 Pa. The system was heated to the measuring temperature of 750°C at a rate of 600°C/h. Once the droplet formed, it was held for 30 minutes during which a sequence of 10 images was recorded. The images were processed using image analyzing software (ImageAccess of Imagic Bildverarbeitung AG, Glattbrugg/ Switzerland). The droplet profile was digitized and a binary image was generated. The coordinates of the boundary in the region of the triple point were calculated as shown schematically in Figure 3.3. The line which was fitted using the least-

squares fit function within the MAPLE (Waterloo Maple Inc., Canada) mathematical software to a cubic function: y + 3 = a + bx dx Equation 39 The inflection point and the slope of the tangent at this point were calculated to evaluate the quasi-static contact angle θst . After the tests, cross-sections through the centre of the droplet were prepared metallographically to investigate the microstructure of the areas near the interface. droplet boundary θ st tangent boundary spline 68 baseline Figure 3.3 Schematic of the contact angle measurement extracted from digital images. The dashed line represents the spline in the vicinity of the triple line. 3.3. MMC constituents In the preparation of preform-MMCs, a liquid metal is forced under pressure into a porous ceramic preform. The chemical compositions of the alloy were presented in Table 3.2. The target ceramic volume fraction in the preforms was between 0.30 and 40. The preform fabrication can be subdivided into three steps: liquid powder processing, drying and shaping, and finally preform sintering. The resulting pore structures and permeabilities as well as the compressibility of the preforms were evaluated. For reference purposes, two types of preforms were purchased. The fabrication route of preforms and their constituents as well as the properties of the purchased preforms are outlined in the following sections.

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