Thermomechanical behaviour of mullite
In the present work a complete analysis of the mechanical properties of dense mullite compacts and their relationship with microstructural features is made. The flexural strength curve shows three different regions corresponding to a low temperature region (20-600°C) with a transgranular fracture of mullite grains, a medium temperature region (600-1200°C) where an increase in σf is observed due to a lower stress intensity factor value at the tip of the critical defects due to a poor load transmission across the sample, and a high temperature range (T > 1200°C) where fracture appears at lower stress, first by non-catastrophic intergranular propagation up to a critical size where transgranular fracture takes place. Fracture toughness was found to be highly dependent on the deformation rate. The low value of the stress exponent in the creep law (n = 1 at 1200°C) is associated with diffusion assisted by a sliding process. The diffusional shape changes of the mullite grains are accommodated by grain boundary sliding assisted by viscous flow of the glassy phase. Due to the high sintering temperatures (> 1600°C) required to obtain dense mullite compacts, a silica rich glassy phase is present at grain boundaries leading to plastic deformation under load at high temperatures. The thermomechanical behaviour of mullite is controlled by the viscosity of such a grain boundary amorphous phase and has been determined by using the internal friction technique.
| Main Authors: | , , , |
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| Other Authors: | |
| Format: | artículo biblioteca |
| Published: |
Elsevier
1997
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| Online Access: | http://hdl.handle.net/10261/223257 http://dx.doi.org/10.13039/501100000780 |
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| Summary: | In the present work a complete analysis of the mechanical properties of dense mullite compacts and their relationship with microstructural features is made. The flexural strength curve shows three different regions corresponding to a low temperature region (20-600°C) with a transgranular fracture of mullite grains, a medium temperature region (600-1200°C) where an increase in σf is observed due to a lower stress intensity factor value at the tip of the critical defects due to a poor load transmission across the sample, and a high temperature range (T > 1200°C) where fracture appears at lower stress, first by non-catastrophic intergranular propagation up to a critical size where transgranular fracture takes place. Fracture toughness was found to be highly dependent on the deformation rate. The low value of the stress exponent in the creep law (n = 1 at 1200°C) is associated with diffusion assisted by a sliding process. The diffusional shape changes of the mullite grains are accommodated by grain boundary sliding assisted by viscous flow of the glassy phase. Due to the high sintering temperatures (> 1600°C) required to obtain dense mullite compacts, a silica rich glassy phase is present at grain boundaries leading to plastic deformation under load at high temperatures. The thermomechanical behaviour of mullite is controlled by the viscosity of such a grain boundary amorphous phase and has been determined by using the internal friction technique. |
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