The experiments on creep mainly are performed at constant load. Thus the true stress during creep process increases due to the cross-section reduction (necking) and by reducing the cross-section area due to the formation of defects. In the first case, we can assume that deformation occurs by a mechanism of viscous flow, when the volume of the sample remains constant. In the second case the volume and density of the specimen change, due to the complex physical-mechanical processes promoting to the formation and development of damage. These two processes take place simultaneously, so it is impossible to separate them completely. When the fundamental study of the problem of high-temperature creep and long-term strength are developed it is necessary to carry out the experiments at constant stress.