Creep is the high-temperature deformation of a material as a function of time. Other high-temperature properties related to creep are stress and modulus relaxation, internal friction, and grain boundary relaxation. The creep rate increases strongly with temperature, and is often proportional to the applied stress. Microstructure (grain size and porosity) influences the creep rate; other influences are lattice defects, stoichiometry, and environment. Thus, creep rates are strongly dependent on sample history and the specific experimental method used to measure them, so the only meaningful quantitative comparison of creep rates can be made for samples with the same histories and measurement method. Some torsional creep rates of different oxides are given in Table 11 to show the wide variability of creep values. Compared with some other high temperature materials such as mullite (3Al2O3^2SiO2), alumina has a higher creep rate, which sometimes limits its application at high temperatures (above about 1,500°C). See  for a review of creep in ceramics and  for a review of creep in ceramic-matrix composites.