The mechanical behavior of the novel fiber CeraFib75 after various thermal exposures is examined. This fully crystalline mullite fiber was developed to exceed the thermal stability of commercially available oxide fibers. Therefore, heat treatments at temperatures ranging from 1000°C to 1400°C for 25 h were performed and results compared to the well-established Nextel TM 720 fibers. Mechanical characterization was realized with bundle tensile tests using acoustic emission sensors to determinate the fiber failure distributions. Investigations showed that the initial fiber microstructure of mullite grains with traces of alumina transforms starting at 1200°C. Changes include dissociation of the alumina-rich mullite phase and grain growth. Thus, strength reduction is measured as a result of these microstructure transformations. Remarkably, at 1400°C, fibers become more fragile and Weibull statistics can no longer describe the failure distribution. A relation between the distribution shape and the load redistribution capability of fibers is suggested. This is more pronounced for Nextel TM 720 fibers, which present much bigger grains and retain only 10% of their original strength. However, CeraFib75 fibers are more stable and exhibit a strength retention of 50% at the same conditions, which is attributed to the higher amount of mullite phase.
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Two different heat treatments have been carried out on similar Mn1−xNi1−xFe2xSi0.95Al0.05 compositions with magnetostructural transitions between hexagonal and orthorhombic crystal structures around room temperature. The samples were analyzed concerning their structural, microstructural, magnetic, and caloric properties. The results show that the introduction of a high-temperature step, before the heat treatment (1073 K/7 days) usually used in the literature for such compounds modifies the microstructure, leading to sharper transitions with shorter transition widths, and stronger latent heat peaks. Magnetic field-assisted calorimetry and vibrating sample magnetometry provide methods to assess the effect of magnetic field on the broad transitions for the sample with x = 0.32 and the sharp transitions seen in the sample with x = 0.31.
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