Refractory materials, especially at high temperatures, will undergo plastic deformation under a certain stress lower than their ultimate strength, and the amount of deformation increases with the extension of the load time, and even leads to material damage. This phenomenon that the deformation caused by external force increases with time is called creep. The creep of refractory materials is represented by the compressive creep produced by the action of compressive stress at a certain temperature and after a certain period of time. Sometimes it is often expressed by the time to reach a certain amount of strain, or it can be directly described by the curve of deformation or strain rate and time. Because the result of the strain measured by compressive stress is not easy to reflect the phase change inside the material, it is often expressed by its bending creep or torsional creep (shear creep).
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Creep of refractory materials is a function of temperature, stress, time and material structure. The higher the temperature of the material, the greater the stress, the longer the time, the greater the creep rate. When the refractory material is completely composed of crystals, the creep is mainly controlled by the diffusion of vacancies in the crystal, the movement of dislocations, the slip of grain boundaries and the bonding state between the grains, in addition to the influence of the bond strength of the crystals related to the elasticity of the crystals. Fewer crystal defects, fewer grain boundaries and stronger inter-grain interspersed bonding are less likely to cause severe creep.
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When the material contains a glass phase, especially when the glass phase is a continuous phase, the creep of the material is controlled by the glass phase. The greater the amount of glass phase and the lower the viscosity, the material can produce viscous flow under low stress, so the creep is more serious at high temperature. When the refractory material contains impurities that may form liquid phase at high temperature, the more liquid phase formed by the impurities, the faster the viscosity decreases with the increase of temperature, and the more serious the creep will be.
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The pores or cracks at the grain boundary in the refractory material can directly cause the slip of the grain boundary. In addition, the liquid phase in the material can penetrate into the pores between the solid phases at high temperature, especially into the medium and small pores below 25 μm, so the existence of pores and the migration of the liquid phase also affect the creep. Therefore, the porosity of the material has a great influence on the creep. Increasing the porosity of a material actually increases its creep rate.
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