Divergence of Mechanisms and the Effect on the Fatigue LIfe Variablity of Rene' 88 DT

Abstract

In safely and cost-effectively managing aging turbine engine components, it is important to understand the inherent variability in the fatigue behavior of superalloy systems. The variability in fatigue life was investigated in Rene` 88 DT, a superalloy used widely in a number of critical turbine engine components for which low cycle fatigue is typically the life-limiting factor. Loadcontrolled, axial fatigue testing was performed at 593 C and 10 Hz on specimens taken from a pancake forging. A stress-life (S-N) curve was established where numerous tests were performed at single stress levels in order to determine cumulative probability of failure. It is observed that the variability in cycles to failure increases with decreasing stress level. In addition, as the variability in life increases, a separation of individual populations can be identified within the full population of failed specimens, suggesting that different failure mechanisms control the fatigue performance of the separate populations. It is proposed that the application of separate probability functions based upon the divergent failure mechanisms provides more accurate predictions of the variability of fatigue performance and reduces unnecessary conservatism.

Microstructural Conditions Contributing to Fatigue Variability in P/M Nickel-Base Superalloys

“…At 593C, the René88DT has a 0.2% offset yield of 985 MPa and an elastic modulus of 176 GPa [3]. At 650C, the IN100 measured a 0.2% yield of 888 MPa, ultimate tensile stress of 1225 MPa, an elastic modulus of 175 GPa, and a strain at failure of 41%.…”

“…The IN100 specimens were tested in the LSG surface condition. To further minimize the effects of surface residual stresses due to machining, the gage sections of the René88DT specimens were electropolished using a procedure described in [3].…”

“…While extensive databases of long crack growth data exist for nickel-base superalloys, similar databases for small crack growth data are lacking, as is our general understanding of their behavior particularly under LCF conditions. Factors associated with differences in small crack behavior include local microstructural features including neighboring grain (mis)orientation, grain substructure (γ' precipitates), twins, and anomalies such as the presence of inclusions and pores [3][4][5][6][7].…”

Low cycle fatigue in rene 88DT at 650 °C: Crack nucleation mechanisms and modeling

“…Defects, arising from the powder metallurgy processing and including ceramic inclusion particles and pores and voids, have been observed to nucleate dominant cracks in several Ni-base superalloys in low cycle fatigue (LCF) conditions at elevated temperatures. [1][2][3][4][5] Crack nucleation due to plastic damage accumulation within a surface slip band has also been observed by several authors at room temperature and elevated temperature conditions. [5,6,7] There is seemingly a competition between crack nucleation from surface slip band cracking and crack nucleation from defects in the material.…”

Fatigue Crack Initiation in Nickel-Based Superalloy Ren� 88 DT at 593�C

“…• C [16]. Fatigue failures were observed in this alloy for lifetimes beyond 10 of cyclic stress and exceeds two orders of magnitude at σ max = 600MPa.…”