Dwell Effects on High Temperature Fatigue Damage Mechanisms: Part II

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The mechanisms controlling deformation and failure under high temperature creep-fatigue conditions of materials are examined in this paper. The materials studied were pure alloys, solder alloys, copper alloys, low alloy steels, stainless steels, titanium alloys, tantalum alloys, and Ni-based alloys. The deformation and failure mechanisms were different (fatigue, creep, oxidation and their interactions) depending on test and material parameters employed. Deformation mechanisms, such as cavity formation, grain boundary sliding, intergranular (IG) and transgranular (TG) damage, oxidation, internal damage, dislocation cell formation, and other damage mechanisms are very important in order to gain knowledge of fatigue behavior of materials. The observed mechanisms can be categorized as follows: (a) depending on the test parameters employed, a high NCR resulted in high strain levels. The damage was due to creep-fatigue interaction by mixed TG and IG cracking, creep damage by cavity formation and surface damage by oxidation. Oxidation damage was found to depend on a critical temperature and compression and tension dwell periods in a cycle. (b) Dwell sensitivity was effective only below a certain strain range, and once this threshold was exceeded NCR value was not affected with a further increase in dwell time. (c) Microstructures changed depending on test temperature, dwell time, and strain range. Triple point cracking and cavities were formed as a result. New precipitation occurred depending on temperature, strain range and dwell time. Some precipitates were beneficial in blocking the grain boundary damage by creep, whereas other precipitates changed the dislocation substructure promoting more damage. (d) Depleted regions on the grain boundaries developed due to exposure at high temperatures resulting in the formation or propagation of IG cracks. (e) Dwell cycles evolved mean stresses in tension and compression directions. Mean stress in tension was more deleterious and caused dwell sensitivity. (f) Dwell sensitivity was also dependent on material condition and discontinuities present in a material. These mechanisms are summarized in this paper.



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