LOW-CYCLE FATIGUE DWELL EFFECTS AND DAMAGE MECHANISMS

This study reviews and elucidates the dwell sensitivity behavior and m echanisms controlling deformation and failure under high-temperature l ow cycle fatigue (HTLCF) of a range of materials. Dwell sensitivity ma ps were constructed utilizing normalized cycle ratio (NCR) and strain levels. The trends identified are summarized as follows: Dwell cycles were beneficial to the creep-fatigue resistance only in isolated cases for copper alloys; AMZIRC and NARaloy-Z, and superalloys; PWA 1480 an d MA 754 an (ODS) alloy. Solders (96.5 Pb-3.5 Sn and 37 Pb-63 Sn), cop per alloys; AMZIRC and NARaloy-Z, low alloy steels; 1 Cr-Mo-V, 1.25 Cr -Mo and 9 Cr-l Mo, stainless steels; SS 304, SS 304L, SS 316, and SS 3 16L, superalloys; Mar M 002, Rene 80, Inconel 617, IN 100, PWA 1480 an d MA 754 were observed to be tensile dwell sensitive. Low alloy steel 2.25 Cr-Mo, titanium alloys Ti-6 Al-4V and IMI 829 and superalloys Mar M 002 below 1040 degrees C, Waspaloy and Rene 95 were found to be com pressive dwell sensitive. An attempt has been made to predict the dwel l sensitivity fatigue behavior empirically relating the strength ratio s with ductility ratios. It was proposed that when the ductility ratio was equal to the strength ratio, compressive dwell sensitivity occurr ed and for unequal conditions, tensile dwell sensitivity occurred. The se factors were determined and dwell sensitivity predicted. The mechan isms controlling deformation and failure are categorized as follows:Ea ch cycle type produced deformation in either transgranular (TG), mixed , or intergranular (IG) mode. Cyclic softening resulted in IG deformat ion as the stresses reduced. Grain boundary sliding, cavity formation and oxidation damage interact and lower life faster than TG modes, in which striations were observed. Depending upon the cycle time, stresse s, and temperature, deformation in terms of precipitation, slip patter ns, carbides, depletion of chromium carbides, Cr-Mo clusters etc. occu rred. These resulted in IG corrosion, oxidation and creep-fatigue inte ractions causing additional damage. Dynamic strain aging occurred depe nding upon the microstructure, temperature and material composition. P recipitates developed which enhanced HTLCF resistance, however, other competition mechanisms under dwell conditions are not known.The dwell sensitivity behavior and mechanisms controlling deformation and failur e of numerous materials are summarized in this paper. (C) 1998 Elsevie r Science Ltd. All rights reserved.