LOW CYCLE FATIGUE BEHAVIOR OF THE DIRECTIONALLY SOLIDIFIED NICKEL BASE SUPERALLOY RENE 80

The Low Cycle Fatigue (LCF) behavior of a Directionally Solidified (DS) nickel base superalloy Rene-80 has been studied in the temperature range of 75-1600F. Three orientations viz., longitudinal, 45(DEGREES) and transverse to the growth axis, have been investigated.;A detailed LCF study at 1600F in the plastic strain range of 0.02-0.25% has shown that the DS alloy has an improved LCF life over the Conventionally Cast (CC) one, especially at low strain ranges. Longitudinal and 45(DEGREES) specimens have exhibited comparable LCF lives with the transverse specimens showing a minimum evidencing the detrimental effect of transverse boundaries.;In comparison with the CC alloy, the DS alloy has: (1) a Chinese Script morphology of MC carbides in the inter-dentritic regions which is more pronounced in the middle section and (2) a pronounced dentritic structure. The script morphology of MC carbides has been found to affect the fatigue life by affecting the early propagation of a crack. Two major substructural changes have been observed with increasing temperature. Up to 1400F (gamma)' retains the duplex structure while at 1600F and above it coarsens significantly. Also up to 1400F dislocations are found only in the inter-precipitate regions with their morphology consistent with thermally activated motion. Grain boundaries have been shown to greatly assist crack initiation and early propagation. However, as a result of a limited number of transverse grain boundaries (in the coarse grained DS alloy), the crack initiation competes between grain boundaries and interdentritic regions.;In the investigated strain range the study has shown that the Coffin-Manson relationship is not obeyed mainly as a result of varying modulus. Total strain and stress range are better parameters in characterizing the LCF behavior of this material.;The LCF life has been found to decrease with increasing temperature and at 1400F it decreases sharply to a minimum before increasing significantly at higher temperatures. This behavior has been attributed to: (1) environment, (2) property of (gamma)' and, (3) a relatively limited thermally activated dislocation motion. The increase in life at 1600F has been attributed to the coarsening of (gamma)' and an increase in the material's ductility.