Dwell Fatigue I : Damage Mechanisms

Abstract

The mechanisms controlling deformation and failure under high temperature creep-fatigue conditions of materials are examined in this paper. The materials studied are solder alloys, copper alloys, low alloy steels, stainless steels, titanium alloys, and Ni-based alloys. The deformation and failure mechanisms were different (fatigue, creep, oxidation and their interactions) depending upon test and material parameters employed. Deformation mechanisms, such as cavity formation, grain boundary damage, intergranular (IG) and transgranular (TG) damage, oxidation, internal damage, dislocation cell concentration, and oxide mechanisms are very important in order to gain more knowledge of fatigue behavior of materials. The observed mechanisms can be categorized 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-fatigue interaction by mixed (TG) and/or (IG) creep damage by cavity formation, and oxidation striated surface damage. Oxidation damage was found to depend upon a critical temperature and compression and tension dwell periods in a cycle. ○ Dwell sensitivity was effective only below a certain strain range; once this threshold was exceeded NCR value was not affected by further increase in dwell time. Microstructure changed depending upon test temperature, dwell period, and strain range. Triple point cracking favored mechanisms such as cavitation. New metal precipitates formed depending upon the temperature, strain range and dwell time. Some precipitates were beneficial in blocking the grain boundary damage from creep, whereas other precipitates changed the dislocation sub- structure, promoting more damage. New cells formed during tests that coarsened with longer dwell times. In some cases dynamic strain aging occurred enhancing fatigue behavior of materials. ○ Depleted regions developed due to high temperature exposure, which was a function of dwell time applied in a cycle, and material composition that aided in the formation and/or propagation of (IG) cracks. ○ Dwell cycles evolved mean stresses in tension and compression directions. Mean stresses in tension were more deleterious and caused dwell sensitivity. ○ Dwell sensitivity was also dependent on material conditions, and discontinuities 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.