HIGH-TEMPERATURE BEHAVIOUR OF METALS

摘要

The term "creep " indicates a permanent and time-dependent deformation that occurs when a material is exposed at high temperature under a constant applied stress (or load). Creep is observed in all metals, provided that the operating temperature (T) is higher than 0.3-0.5 T_(M') where T_M is the absolute melting temperature. The strain vs time creep curve at a given temperature and stress consists of three regions: i. a primary stage or transient creep, during which, in general, the strain rate (epsilon = delta epsilon / delta t) decreases with time and the material experiences strain hardening; ii. a secondary stage or steady state, during which the strain rate is constant (epsilon = epsilon_(ss)) and strain hardening is balanced by recovery of the structure; iii. a tertiary stage, during which strain rate increases with time, leading to the fracture of the component. The increase in creep rate during this stage results from a number of different damaging phenomena, such as cavity nucleation and growth, cracks formation, extensive macroscopic necking.