HIGH TEMPERATURE CYCLIC DEFORMATION OF A DIRECTIONALLY SOLIDIFIED Ni‐BASE SUPERALLOY

Abstract

The high temperature low cycle fatigue behaviour of a directionally solidified Ni-base superalloy hardened by about 65% volume fraction of y'-precipitates was investigated in order to determine the fatigue life parameters for longitudinal (L) and longitudinal transverse (LT) grain orientations. The fatigue resistance was compared with that of two oxide dispersion strengthened (ODS) Ni-base superalloys with a similar elongated grain structure.The fatigue life of the alloy can be adequately predicted by Basquin and Cof6n-Manson empirical relationships and the fatigue ductility parameters in these relationships show a similar trend with the tensile ductility properties.The studied alloy exhibits a fairly stable cyclic stress response, with only a slight stress softening. Fatigue crack initiation occurs mainly at shrinkage pores on the surface or sub-surface of the specimens. The crack growth direction is predominantly perpendicular to the applied load. The fracture mode in the LTdirection is transgranular and fatigue life is shorter by a factor of about six compared to the L-direction. The fatigue life of the alloy is longer than that of the ODS Ni-base superalloys with which it is compared. NOMENCLATURE N = number of cycles to failure n = progressive number of cycles E = Young's modulus A% = total strain range L\E, = elastic strain component A E~ = plastic strain component Au, = stress amplitude Q = failure ductility coefficient by= fatigue strength coefficient

Creep fatigue of a directionally solidified Ni-base superalloy - smooth and cylindrically notched specimens

“…LCF at 850 and 950 °C does not result in a significant coarsening of the γ′ particles during fatigue testing 8 . Because only a small fraction of precipitates are sheared under conditions of strain‐controlled LCF tests at these temperatures suggests the dislocation networks that form at the γ‐γ′ interface impede other dislocations from penetrating the precipitates, thereby resulting in a stable cyclic stress response.…”

“…CM247LC DS exhibits superior fatigue resistance and creep properties when compared to IN738LC and oxide‐dispersion‐strengthened (ODS) superalloys, likely due to its higher ductility 1,8,9 . Under LCF and thermomechanical fatigue, life is also highly dependent on strain rate because of an interaction of cyclic plasticity with time‐dependent processes such as creep and oxidation 9 …”

“…The origin of failure under LCF is mainly related to the geometrical discontinuities on the specimen surface including shrinkage pores, crevices created by oxidation effects, fracture of carbides located at the specimen surface and indentations arising from sources like machining marks 8 …”

“…CM247LC DS exhibits superior fatigue resistance and creep properties when compared to IN738LC and oxidedispersion-strengthened (ODS) superalloys, likely due to its higher ductility. 1,8,9 Under LCF and thermomechanical fatigue, life is also highly dependent on strain rate because of an interaction of cyclic plasticity with time-dependent processes such as creep and oxidation. 9 The origin of failure under LCF is mainly related to the geometrical discontinuities on the specimen surface including shrinkage pores, crevices created by oxidation effects, fracture of carbides located at the specimen surface and indentations arising from sources like machining marks.…”

Creep crack growth data reduction of directionally solidified Ni base superalloy usingCtestimation scheme for anisotropic materials

“…at elevated temperature. [11][12][13][14] However, there have been limited numerical and experimental studies on the crack tip deformation mechanics for anisotropic creeping materials at elevated temperatures. [15][16][17][18][19] Gardner et al 15 discussed the applicability of the C t parameter to a DS Ni based superalloy.…”