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The creep and fatigue properties of the directionally solidified nickel base superalloy CM247LC DS have been investigated. Constant creep tests have been carried out on specimens with different orientations in the temperature range of 700-1OOO'C at different loads to obtain times to rupture up to 35000 h. The comparison of the creep properties with the IN738LC conventionally cast alloy and with the oxide dispersion strengthened MA6000 alloy has shown the better creep performance of the alloy CM247LC DS in the temperature range of interest for application in land based gas turbines. A series of cyclic load creep tests has allowed to study the effect of load variations on the creep. The effect of cycling stress is to increase the strain rate, compared with the constant load creep tests and then to reduce the rupture life of the alloy. The LCF tests, performed at the temperatures of 85O'C and 950°C in longitudinal strain controlled conditions, have evidenced a fairly stable cyclic response. Basquin and Coffin-Manson relationships can adequately predict the fatigue life of the alloy. The CM247LC DS alloy exhibits a better fatigue life than IN738LC and MA6000 alloys.

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Abstract. The thermo-mechanical fatigue (TMF) behaviour of the Nimonic 90 Nickel base superalloy has been investigated within two laboratories. In-phase-tests (IP) where the maximum mechanical strain occurs at the maximum temperature (850°C), and 180°-out-of-phase-tests (180° OP) where the maximum mechanical strain coincides with the minimum temperature (400°C) have been applied. All tests were carried out at varying mechanical strain ranges with a constant strain ratio of R ε = -1. A temperature rate of 5 K/s was used throughout the whole cycle without any additional cooling system during decreasing temperature. The fatigue life of 180° OP tests is longer compared to identical IP tests. The stress / mechanical strain hysteresis loops are completely different and some characteristic values are compared to each other. The fracture surfaces observed show that fatigue crack (or cracks) starts on the external surface and propagates inwards. The fractures of 180° OP tests are transgranular showing the presence of fatigue striations, while the fractures of IP tests are mixed transgranular and intergranular with no fatigue striations. IntroductionSince the seventies, materials subjected to cyclic stresses and temperatures have been studied by isothermal low cycle fatigue testing (LCF) and the results have been produced by considering a reference temperature that corresponds to the maximum value of the thermal cycling. After the introduction of thermo-mechanical fatigue (TMF) as a diagnostic device for the material study [1], the comparison of LCF and TMF testing has determined opposite conclusions. Partly LCF and TMF results were found to be comparable [2,3]. Partly it was found that the stress history of a thermomechanical cycle leads to results completely different from those produced by isothermal testing [4, 5,]. The TMF procedure is particularly recommended for those new materials whose high temperature properties are not known. There is clear evidence that the TMF test procedure does represent material loading of hot components in e.g. power stations and aero engines much better than LCF testing. In order to customise the TMF testing procedure a European project has recently concluded [6] and a Code of Practice (CoP) has produced [7]. The TMF tests were performed on the Nickel base superalloy Nimonic 90 at defined experimental conditions and the results contributed to the definition of the test procedure. The scope of the present work is to extend the comparison of TMF results performed on Ni90 alloy according to the CoP procedure and to verify the material life in different experimental conditions.

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