Dwell fatigue, I: Damage mechanisms

The mechanisms controlling deformation and failure under high temperature c reep-fatigue conditions of materials are examined in this paper. The materi als studied are solder alloys, copper alloys, low alloy steels, stainless s teels, titanium alloys, and Ni-based alloys. The deformation and failure me chanisms were different (fatigue, creep, oxidation and their interactions) depending upon test and material parameters employed. Deformation mechanism s, such as cavity formation, grain boundary damage, intergranular (IG) and transgranular (TG) damage, oxidation, internal damage, dislocation cell con centration, and oxide mechanisms are very important in order to gain more k nowledge of fatigue behavior of materials. The observed mechanisms can be c ategorized as follows: Depending upon the test temperature, higher NCR resulted with higher strain range, dwell time and lower strain rates. The damage was due to creep-fati gue interaction by mixed (TG) and/or (IG) creep damage by cavity formation, and oxidation striated surface damage. Oxidation damage was found to depen d upon a critical temperature and compression and tension dwell periods in a cycle. Dwell sensitivity was effective only below a eel-rain strain range; once th is threshold was exceeded NCR value was not affected,by further increase in dwell time. Microstructure changed depending upon test temperature, dwell period, and s train range. Triple point cracking favored mechanisms such as cavitation. N ew metal precipitates formed depending upon the temperature, strain range a nd dwell time. Some precipitates were beneficial ih blocking the grain boun dary damage from creep, whereas other precipitates changed the dislocation sub- structure, promoting more damage. New cells formed during tests that c oarsened with longer dwell times. in some cases dynamic strain aging occurr ed enhancing fatigue behavior of materials. Depleted regions developed due to high temperature exposure, which was a fu nction bf dwell time applied in a cycle, and material composition that aide d in the formation and/or propagation of(IG) cracks. Dwell cycles evolved mean stresses in tension and compression directions. M ean stresses in tension were more deleterious and caused dwell sensitivity. Dwell sensitivity was also dependent on material conditions, and discontinu ities present in a material. These parameters together with test parameters produce damage interactions in a particular fashion that evolve different micro-mechanisms. The dwell sensitivity micro-mechanisms are summarized in this paper.