PSI - Issue 7

Ludvík Kunz et al. / Procedia Structural Integrity 7 (2017) 44–49 Ludvík Kunz / Structural Integrity Procedia 00 ( 201 7) 000–000

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time to fracture until the amplitude reaches a certain threshold value. Then the time to fracture starts to decrease. The length of the plateau on the curves increases with the applied mean stress for both the alloys. For the highest mean stress of 600 MPa for IN 713LC the threshold stress amplitude which starts to decrease the lifetime is about 80 MPa, which makes 13 % of the mean stress. For the lowest applied mean stress, 450 MPa, the threshold stress amplitude is about one half of that at 600 MPa, i.e. 40 MPa which means less than 9 % of the mean stress. For the MAR-M 247 loaded at 900°C with the mean stress of 450 MPa, the threshold value is about 120 MPa, which makes 27 % of the mean stress. For the mean stress 300 MPa the threshold value of the stress amplitude is lower, nearly 60 MPa. Pure creep loading resulted in both the materials in an intergranular fracture. An example of the fracture surface of the IN 713LC specimen failed by pure creep at the mean stress 600 MPa is shown in Fig. 4(a). The surface clearly reflects the dendritic structure. Fig. 4(b) shows the fracture surface of a specimen loaded by the same mean stress but with superimposed stress amplitude 40 MPa. In addition to the characteristic creep fracture surface, transcrystalline facets as a result of fatigue crack initiation and growth start to appear. Examples are indicated in Fig. 4(b) by arrows. The area covered by fatigue fracture surface increases with increasing stress amplitude. Above the threshold the percentage of fatigue fracture surface rapidly increases. The behavior of MAR-M 247 was found to be qualitatively the same. Fig. 4(c) shows the facets on the fracture surface of a specimen failed at the mean stress 300 MPa and superimposed stress amplitude 40 MPa. 5. Discussion The experimental results for both the superalloys show that the stress amplitude with 120 Hz frequency superimposed on the mean stress does not influence the time to fracture until a certain threshold value is reached. This threshold increases with increasing mean stress. In the case of pure creep loading (the stress amplitude is zero) a number of micromechanisms is responsible for failure; see e.g. Sklenička (1999) . An important role plays an intergranular damage, which results in formation of intergranular cracks. An example of such cracks in IN 713LC is shown in Fig. 5. The fracture surface of specimens loaded by pure creep was for both the tested superalloys and all mean stresses characteristic by dendritic features. The strain at fracture of all the specimens tested in the region where the stress amplitude does not influence the time to fracture was nearly the same. For IN 713LC it was 3.4 ± 1.3 % and for MAR-M 247 8.3 ± 1.4 %. The data have quite large scatter. This is not surprising taking into account that the investigated cast alloys are very coarse grained and the specimen gauge length contains only a low number of grains. The average strain at fracture of specimens tested above the threshold decreases with increasing stress amplitude markedly. For instance, for the IN 713LC tested with the stress amplitude 120 MPa the strain at fracture was 1.0 ± 0.4 % and for MAR-M 247 for the stress amplitude 160 MPa it was 1.7 ± 0.6 %. The superimposed stress amplitude can generally influence the time to fracture in two ways: First, it can influence the creep deformation processes and thus according to the loading parameters and material either accelerate or decelerate the creep and thus determine the time to fracture. Sheffler (1972) observed the dramatic acceleration of the creep rate in the first and second creep stages by application of cyclic vibrations with kHz frequency in two strain-aging refractory alloys (Mo- and Ta-base). Creep rate acceleration was explained by the negative strain rate sensitivity which is associated with the strain aging phenomenon in these materials. On the other hand, Lukáš et al. (1997) observed serious deceleration of creep rate and higher time to fracture due to the superposition of small vibrations in the case of Ni-base CMSX-4 single crystals. The explanation of the observed effect is based on more difficult formation and/or easier annihilation of jogs on vibrating dislocations which results in less effective recovery. Creep curves determined in this work for creep/fatigue loading exhibit large scatter due to the very coarse grained structure. This makes impossible to draw unambiguous conclusion about the influence of superimposed stress amplitude on the creep rate. The second way how the stress amplitude influences the time to fracture is initiation and propagation of fatigue cracks. They generally initiate at places of highest stress Fig. 5. Intergranual cracks in IN 713LC specimen loaded at 800 °C by mean stress 500 MPa.