CRITICAL-ASSESSMENT OF THE MECHANISTIC ASPECTS IN HAYNES 188 DURING LOW-CYCLE FATIGUE IN THE RANGE 25-DEGREES-C TO 1000 DEGREES-C

The law-cycle fatigue (LCF) behavior of a wrought cobalt-base superall oy, Haynes 188, has been investigated over a range of temperatures bet ween 25 degrees C and 1000 degrees C employing a triangular waveform a nd a constant strain amplitude of +/-0.4 pet. Correlations between mac roscopic cyclic deformation and fatigue life with the various microstr uctural phenomena were enabled through scanning electron microscopy (S EM) and transmission electron microscopy (TEM), detailing the crack in itiation and propagation modes, deformation substructure, and carbide precipitation. Cyclic stress response varied as a complex function of temperature. Dynamic strain aging (DSA) was found to occur over a wide temperature range between 300 degrees C and 750 degrees C. In the DSA domain, the alloy exhibited marked cyclic hardening with a pronounced maximum at 650 degrees C. Dynamic strain aging has been documented th rough the occurrence of serrated yielding, inverse temperature depende nce of maximum cyclic stress, and cyclic inelastic strain; developed a t half of the fatigue: life. Additionally, the alloy also displayed a negative strain rate sensitivity of cyclic Stress in the DSA regime. T hese macroscopic features in the DSA domain were accompanied by the su bstructure comprised of coplanar distribution of dislocations associat ed with the formation of pileups, stacking faults, and very high dislo cation density. Toward the end of the DSA domain, dislocation pinning by M(23)C(6) precipitates occurred predominantly. The deformation beha vior below and above the DSA domain has also. been investigated in det ail. The temperature dependence of LCF life showed a maximum at approx imate to 300 degrees C. The drastic reduction in life between 300 degr ees C and 850 degrees C has been ascribed primarily to the deleterious effects of DSA on crack initiation and propagation, while the lower l ife at temperatures less than 200 degrees C has been attributed to the combined influence of low ductility and larger cyclic response stress .