Issue 48

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 48 (2019) 545-553; DOI: 10.3221/IGF-ESIS.48.52 548 failure, for the MM series (260 MPa) with high hardness was higher than that for the untreated series (105 MPa). Sufficient data could not be obtained for the MM series because some MM series specimens did not exhibit fatigue failure at stress amplitudes above 280 MPa owing to plastic deformation. Fig. 3 shows SEM fractographs of the MM series ( N f = 3.4×10 5 cycles), which failed at  a = 280 MPa, and of the untreated series ( N f = 5.6×10 6 cycles), which failed at  a = 120 MPa. In all micrographs, the surface subjected to tensile stress is the upper surface. Macroscopic observation revealed only one fatigue crack near the specimen surface, which gradually propagated across the cross-section of the specimen. The fracture surface of the MM series specimen is divided into two regions by a clear boundary, as is shown in Fig. 3(a). In contrast, a characteristic, powder-like microstructure is observed on the surface of the untreated series specimen (see Fig. 3(b)). These results indicate that the MM suppresses the formation of pores in SUS304L during the subsequent SPS process owing to plastic deformation [34] of the SUS304L powder surface. The formation of pores tends to be suppressed with increasing SPS temperature [35]; thus, the SPS temperature (1223 K) is not high enough for the initial SUS304L powder in the present study. To elucidate the mechanism of fatigue crack initiation in the harmonic-structured SUS304L, EBSD analysis was conducted on the specimens after fatigue testing. Figs. 4 shows the inverse pole figure (IPF) map obtained by EBSD for the fracture surface near the crack initiation site of the MM series and a schematic in which the red square indicates the analyzed region in the MM series. A fatigue crack was initiated in the coarse-grained (> 10 µm) structure of the harmonic-structured MM series. In addition, the fatigue crack profile is not influenced by the harmonic structure, and propagates perpendicular to the loading direction. These same effects have also been observed with a Ti-6Al-4V alloy [24, 25] and CP titanium [27]. Furthermore, Zhang et al. [23] reported that the fatigue limit for the harmonic-structured JIS-SUS316L austenitic stainless steel tends to increase as the MM time increases under uniaxial stress loading, because the areal fraction of the fine-grained structure tends to increase as the MM time increases [25, 36]. The harmonic structure increases the fatigue limit of SUS304 owing to grain refinement and the suppression of pore formation during the SPS process. Figure 3 : SEM fractographs of the (a) MM series ( N f = 3.4x10 5 cycles) failed at  a = 280 MPa and (b) untreated series ( N f = 5.6x10 6 cycles) failed at  a = 120 MPa. Figure 4 : Inverse pole figure (IPF) map obtained by EBSD analysis for MM series that failed at a stress amplitude of 280 MPa ( N f = 3.4×10 5 cycles). Fatigue crack propagation behavior To examine the effect of pores on the fatigue properties of SUS304L, small fatigue crack propagation was examined by the replica method for the untreated series with pores. In the present study, small fatigue crack propagation in the MM series is not examined because the MM series have no pores. Fig. 5 shows optical micrographs of the surface of the untreated series