Fatigue behaviour and lifing of two single crystal superalloys

Abstract

A model has been developed to predict the high temperature cyclic life of single crystal superalloys RR2000 and CMSX‐4 under conditions of creep and fatigue. A combined creep–fatigue model is used, although it is found that failure always occurs by creep or fatigue separately, and that creep–fatigue interaction has a minor influence. Microstructural investigation of a series of interrupted high‐ and low‐frequency tests are presented, these are combined with the results of a series of interrupted creep tests to identify the separate and interactive mechanisms of creep and fatigue. When creep damage is present the material behaves homogeneously. Under these conditions crack growth is initiation controlled, the mechanism of failure is surface or casting pore‐initiated planar crack growth followed by shear on crystallographic planes. As the temperature is lowered or the cyclic frequency increased, the material behaves less homogeneously and shear bands are formed during cycling. Crack growth under these conditions is again initiation controlled and failure is by rapid crystallographic crack growth along shear bands. Such a failure is a distinct fatigue failure and occurs when little creep damage is present. Under certain cyclic conditions, mainly those where the crystallographic failure mechanism is dominant, the material shows an anomalous increase in fatigue resistance with temperature up to approximately 950 °C. This behaviour has been quantified by relating it to the effect of strain rate and temperature on the yield strength of the material.

The Effect of Crystal Orientation and Temperature on Fatigue Crack Growth of Ni-based Single Crystal Superalloy

“…The criterion of the transition between stage-I and stage-II cracking were also studied [6,17,18,20,23]. Stage-I cracking prefers low temperature, high frequency, high SIF and vacuum condition, while stage-II cracking prefers high temperature, low frequency, low SIF and oxidizing atmosphere.…”

“…Many reports were already published describing anisotropic behavior of single crystal superalloys: tensile strength [2,3], fatigue strength [4][5][6], crack propagation behavior [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24]. In regard to crack propagation behavior of single crystal superalloys, it is well known that cracks tend to propagate on {111} slip planes at low temperature, while cracks usually propagate normal to loading direction independent of crystal orientation at high temperature [7][8][9].…”

Influence of Frequency on High-Temperature Fatigue Behavior of 17-4 PH Stainless Steels

“…6,7) That is to say, the high-temperature fatigue resistance of alloys was generally reduced with increasing temperature and decreasing frequency due to an interaction between creep and fatigue. 6,7) For this reason, it was suggested that, in the present work, the testing temperatures, 573 and 673 K, were still too low or the loading frequencies, 20 and 2 Hz, were too high to generate any detrimental creep damage during cyclic loading. Hence, time-dependent mechanisms such as creep or others did not significantly contribute to the fatigue failures for the given alloys at 573 and 673 K except for H900 condition tested at 673 K.…”

“…The occurrence of timedependent creep mechanism in high-temperature fatigue has been known to be a function of temperature and loading frequency. 6,7) At higher temperatures and lower loading frequencies, creep damage would readily take place and cause premature fatigue failure as compared with timeindependent fatigue failure mechanisms. 6,7) That is to say, the high-temperature fatigue resistance of alloys was generally reduced with increasing temperature and decreasing frequency due to an interaction between creep and fatigue.…”

“…6,7) At higher temperatures and lower loading frequencies, creep damage would readily take place and cause premature fatigue failure as compared with timeindependent fatigue failure mechanisms. 6,7) That is to say, the high-temperature fatigue resistance of alloys was generally reduced with increasing temperature and decreasing frequency due to an interaction between creep and fatigue. 6,7) For this reason, it was suggested that, in the present work, the testing temperatures, 573 and 673 K, were still too low or the loading frequencies, 20 and 2 Hz, were too high to generate any detrimental creep damage during cyclic loading.…”