Refractories - Microstructure
Refractories are complex materials with a microstructure which is generally complex and heterogeneous (grains with a wide range of size and with a multiplicity of morphologies, porosity and cracks, etc). At room temperature, refractories generally exhibit microscopic cracks due to thermal expansion coefficient mismatches of their various phases caused by thermal stresses at the cooling stage of their fabrication. The analysis of their thermo-mechanical properties is crucial to improve their high temperature performance.
The Young’s modulus is a mechanical property relatively sensitive to the evolution of the microstructure and can be characterized by the impulse excitation technique (IET) as function of the temperature.
Microstructural characterization by high temperature Young’s modulus analysis
Pabst et al. studied the temperature dependence of the Young’s modulus of silica brick materials. They consist almost entirely of tridymite (64 %) and cristobalite (36 %). The rectangular shaped samples had a length of 160 mm, a width of 20 mm and a thickness of 10 mm. The Young’s modulus at elevated temperatures have been measured by the IET according to the ASTM E1876 standard in the IMCE RFDA HT1600 furnace. Resonant frequencies have been measured continuously from room temperature to 1200 °C (first heating cycle), down to room temperature (first cooling cycle) and followed by two similar heating and cooling cycles with a controlled heating and cooling rate of 5 °C/min.
Fig. 1 and 2 show the temperature dependence of the Young’s modulus of a silica brick material with 19,4 % porosity from room temperature to 1200 °C/300 °C measured with the impulse excitation technique. Based on the conclusions of Pabst et al., it is clear that the temperature dependence of the Young’s modulus during the first heating cycle is different from the subsequent cycles. In the intermediate temperature range between approximately 50 and 250 °C, these materials become very flexible with stiffness minima of around 60 % of the room temperature values. The sudden changes in this temperature regime are clearly related to phase transitions between subpolymorphs of tridymite and cristobalite.
Also a clear hysteresis effect i.e. shifts of the phase transition temperature (of around 30 °C) is shown in this temperature regime. The volume contraction opens existing microcracks and lowers the E-modulus of the material. Therefore the abrupt volume changes during heating and cooling over phase transitions are responsible for microcrack opening and closing and lead to an indirect enhancing effect. Furthermore, it is shown that the Young’s modulus at around 800 °C can be more than three times as high as the room temperature. Finally, after a complete heating-cooling cycle, the initial room temperature Young’s modulus is obtained again which means that a temperature of 1200 °C does not induce any irreversible changes in the microstructure. In other words, the elastic modulus of these materials is not a unique function of the temperature but due to the thermal memory, it is dependent on the thermal history.
From the results, it is clear that the hysteresis loops can be reproduced many times and that the microstructure affects the absolute values of the Young’s moduli, while the transition temperatures are generic features of the solid phases, essentially unaffected by microstructural details. Therefore impulse excitation measurements can be considered as a highly sensitive tool for the investigation of phase transistions.
- W. Pabst et al., Elastic anomalies in tridymite- and cristobalitebased silica materials. Ceramics International 40 (2014) 4207-4211