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5 years ago

2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

temperature difference

temperature difference between a reference sample at Tref and the sample (Tsample) was recorded. Reference and sample were under the same atmosphere conditions. The reference sample of the current measurements consisted of a 20 mg alumina block. In the range of 100°C to 800°C, the test was run at a heating rate of 20°C/min using samples of 20 mg. The samples were placed in an alumina crucible and the test was run using a dynamic nitrogen atmosphere achieved by a constant flux of 50 ml/min of high purity nitrogen (>99.9999% N2). The microstructures of the MMC after the SDTA were investigated using an optical microscope. 3.5.2. Non-destructive testing The MMC bodies were non-destructively tested using a Wällischmiller Rayscan 200 3D-X- ray computer tomography system shown schematically in Figure 3.13. The procedure has previously been described by Nagel et al. (164) . 1 Figure 3.13 Wällischmiller Rayscan 200 3-D CT X-ray scanning system: The micro-focus X-ray-tube (1), component to be tested (2), and area detector (3) (164) . The infiltrated bodies were fixed onto the manipulator (2) and scanned with a resolution of 53 µm edge lengths per cubic volume-pixel (voxel) and therefore only macro pores could be 87 2 3

detected. This operation took approximately 80 min using a microfocus X-ray source (1). An area detector (3) with a 1024x1024 pixel matrix was used. The subsequent 20 minute reconstruction step led to a 3D-database which was used for making the virtual 2D-cuts. 3.5.3. Compression of preforms during infiltration For determination of the preform compression, MMC samples were cut along the longitudinal axis and fine ground in several stages using graded SiC papers down to a 1200 grit size. The cross-sectional area of the MMC was measured using digital images. The borderline of the MMC was detected by using a grey value threshold and the area was evaluated using image analysis software. A reproducibility test on one sample showed a scatter of 0.2 mm² for a typical area of 400 mm². The relative preform compression, cpr , was calculated as the ratio of the MMC cross-sectional area to the initial cross-sectional area of the preform. When using the die casting technique for preform infiltration, a minimum of three samples per parameter set were measured. 3.5.4. Microstructure and Detailed Image Analysis Metallographic preparation of the composite samples was achieved by grinding and polishing on a Struers Planopol automatic polisher. Qualitative and detailed image analyses were performed using a Zeiss Axiophot optical microscope fitted with a computer-controlled XYZ- stage. For determination of local saturation of the preform with metal, Zeiss KS400 image analysis software in combination with the macro presented by Bernthaler et al. (165) were used. High resolution microstructures were captured using a LEO Gemini 1525 SEM. 3.5.5. Mechanical properties The elastic modulus, bending strength and fracture toughness, KIC , of the alloy IS and the MMCS were determined using bend test bars of dimensions 42 mm x 4 mm x 3 mm. The elastic modulus was calculated from the first resonant frequency of the bar in accordance with ASTM-E 1876-99 (166) . The bending strength was measured in the four-point mode taking into 88