At low temperatures relative to the melting point of crystalline solids, the dominant deformation mechanisms are slip and twinningat intermediate and high temperatures, other mechanisms become increasingly important and dominate material response under certain conditionsAt low stresses and high temperatures, where the creep rate varies with applied stress, the creep process was controlled by stress-directed atomic diffusion- involve the migration of vacancies along a gradient from grain boundaries experiencing tensile stresses to boundaries undergoing compression- simultaneously atoms would be moving in the opposite direction, leading to elongation of the grains and the test bar- This gradient is produced by a stress-induced decrease in energy to create vacancies when tensile stresses are present and a corresponding energy increase for vacancy formation along compressed grain boundaries.