A critical assessment of the mechanistic aspects in HAYNES 188 during low-cycle fatigue in the range 25 °C to 1000 °C

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 1997-02, Vol.28 (2), p.347-361
• Main Authors: BHANU SANKARA RAO, K, CASTELLI, M. G, ALLEN, G. P, ELLIS, J. R
• Format: Article
• Language: English
• Subjects: Applied sciences; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Fatigue, corrosion fatigue, embrittlement, cracking, fracture and failure; Fatigue, embrittlement, and fracture; Materials science; Metals. Metallurgy; Physics; Treatment of materials and its effects on microstructure and properties
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-997-0137-z

The low-cycle fatigue (LCF) behavior of a wrought cobalt-base superalloy, Haynes 188, has been investigated over a range of temperatures between 25 and 1000 C employing a triangular waveform and a constant strain amplitude of +/- 0.4 percent. Correlations between macroscopic cyclic deformation and fatigue life with the various microstructural phenomena were enabled through SEM and TEM, detailing crack initiation 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 and 750 C. In the DSA domain, the alloy exhibited marked cyclic hardening with a pronounced maximum at 650 C. Dynamic strain aging has been documented through the occurrence of serrated yielding, inverse temperature dependence of maximum cyclic stress, and cyclic inelastic strain developed at half of the fatigue life. Additionally, the alloy also displayed a negative strain rate sensitivity of cyclic stress in the DSA regime. These macroscopic features in the DSA domain were accompanied by a substructure comprised of coplanar distribution of dislocations associated with the formation of pileups, stacking faults, and very high dislocation density. Toward the end of the DSA domain, dislocation pinning by M23C6 precipitates occurred. (Author)