PSI - Issue 13

Kai Suzuki et al. / Procedia Structural Integrity 13 (2018) 1065–1070 Kai Suzuki et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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ductility and toughness (Gludovatz et al., (2014)). In this context, another crucial mechanical property, namely, fatigue resistance, may also show unexpected characteristics in equiatomic HEAs. In regard to fatigue, fatigue crack propagation is particularly important as a practical issue because such cracks are easily initiated at the stress concentration source in structural components. In a recent study (Thurston et al., (2017)), using compact tension testing, the authors evaluated the resistance of an HEA to mechanically long crack growth and found it to be superior to that of other austenitic steels. Microstructurally small fatigue crack growth, however, has not been examined yet. The characteristics of such growth are (1) the occurrence of microstructure-dependent mechanism and (2) the significant degree of scatter in fatigue crack growth rates (Goto, (1993)) (Ritchie et al., (1986)). In particular, the scatter in crack growth rates is practically important because it is a key to predicting the scatter in fatigue life. Hence, the purpose of this research was to elucidate the characteristics of microstructurally small cracks in an equiatomic HEA. More specifically, we evaluated the scatter in fatigue crack growth rates of an HEA in comparison with that of a ternary austenitic stainless steel having relatively low entropy and similar stacking fault energy. We prepared two solid solution-treated fully FCC alloys: Fe-18Cr-14Ni and Fe-20Cr-20Ni-20Mn-20Co (at.%). In this paper, these two alloys are referred to as low-entropy alloy (LEA) and HEA, respectively. Table 1 lists their chemical compositions. The detailed production processes of the LEA and HEA are presented elsewhere. The average grain sizes of the LEA and HEA were 86 and 77 μm, respectively. The stacking fault energies of these alloys have been reported to be 30 (Okamoto et al., (2016)) and 35 mJ m -2 (Habib et al., (2017)), respectively. Figure 1 shows the specimen configurations for the tensile and fatigue tests. The tensile and fatigue specimens were produced by electro discharge machining and lathing, respectively, from the solid solution-treated bars. 2.2. Tensile and fatigue tests Tensile tests were carried out at room temperature and at an initial strain rate of 10 -4 s -1 . The specimens were electrochemically polished before the tests. Rotary bending fatigue tests were conducted at a frequency of 30 Hz, a stress ratio of − 1, and room temperature. The specimen surface was mechanically and electrochemically polished. Therefore, the final diameter of the specimens was reduced by approximately 50 μm. In this experiment, the nominal stress was defined as the bending stress of the test piece minimum radial part, and stress concentration due to the shape of the test specimen was not considered. The specimen surface during the fatigue tests was obtained by using a replica method to measure the crack length. The replication was carried out under no-load conditions after immersing the replica sheet in methyl acetate. The replica images were taken by optical microscopy. 2. Experimental procedure 2.1. Sample materials and shape of the test specimen

Table 1 Chemical compositions of the alloys used [mass%].

Alloy

Fe

Mn 19.8

Ni

Co

Cr

C

S

P

Al

O

N

Si

HEA Bal

20.2 14.6

20.9

18.2 18.8

0.002 0.002

0.006 0.001

0.002 0.018 0.007 0.0087

LEA

Bal

0.001

0.001

<0.001

Fig. 1. Specimen geometry of the (a) tensile and (b) fatigue tests.