Issue 48

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 48 (2019) 545-553; DOI: 10.3221/IGF-ESIS.48.52 546 I NTRODUCTION ustenitic stainless steel has been widely used in various engineering fields because of its high heat resistance, high corrosion resistance, and excellent formability [1]. Recent years have seen a rise in the demand for improvement in the mechanical properties of structural steels, including the present austenitic stainless steel. Thus, increasing the structural reliability of steels has become important. The microstructures and mechanical properties of stainless steels can be controlled by surface modification [2-7], the addition of different alloying elements [7, 8], and grain refinement [2, 5, 9, 10]. Grain refinement through severe plastic deformation is particularly effective in strengthening metallic materials; however, it leads to a decrease in ductility [9, 11, 12]. In order to suppress the decrease in ductility due to the formation of a homogeneous fine-grained structure, new microstructural designs were proposed [11-17]. Our research group has designed a harmonic structure using powder metallurgy to sinter mechanically milled stainless steel powders [18-23], which improve both their strength and ductility by suppressing necking during tensile deformation [20]. In particular, we have focused on the fatigue properties under four- point bending [24-27] and near-threshold fatigue propagation of long cracks [28-30] in titanium-based materials with a bimodal harmonic structure. To achieve sufficient performance with the newly developed harmonic structured stainless steels, their fatigue properties and fatigue crack propagation behavior need to be examined. The purpose of the present study is to elucidate the mechanism of fatigue fracture in bimodal harmonic-structured austenitic stainless steel under four-point bending, and to examine the effects of the bimodal harmonic structure on the fatigue crack propagation in austenitic stainless steel. E XPERIMENTAL PROCEDURES Material and specimens his study employed austenitic stainless steel (JIS-SUS304L) containing 19.35% Cr, 9.18% Ni, 1.83% Mn, 0.25% Si, and 0.02% C (all by mass, with the balance being Fe). This material was made into a powder (particle diameter: 120 µm) through a plasma rotating electrode process that can be used to fabricate spherical particles with negligible contamination by impurities such as oxygen or nitrogen gas [31]. A bimodal microstructural design using mechanical milling (MM) and spark plasma sintering (SPS) was introduced for the formation of the harmonic-structured SUS304L. MM was performed for 180 ks in Ar at room temperature for the SUS304L powders using a planetary ball mill (Fritch P-5) with a tungsten carbide vessel and steel ball bearings to form fine grains on the particle surfaces. The rotation speed was 200 rpm, and the ball-to-powder mass ratio was 2:1. The powders were subsequently consolidated by SPS at 1223 K for 3.6 ks under vacuum (less than 15 Pa) and applied pressure (50 MPa) using a 25-mm internal diameter graphite die to produce the specimens, hereafter referred to as the “MM series.” A second set of specimens was prepared by sintering the as-received powders (hereafter, the “untreated series”) for comparison. The tensile strength of the MM series was higher than that of the untreated series, but the elongations of the two series were almost the same [19, 20]. The MM series has a Vickers hardness of 169.6±2.5 HV, as measured for a polished surface with an indentation force of 1.961 N and a load holding time of 10 s ( n = 30). Hardness value of the MM series is higher than that of the untreated series (125.2±3.6 HV). The sintered materials were sliced into disks approximately 1.5 mm thick and machined into a blunt-notched specimen for four-point bending fatigue tests [26, 27]. After machining, the specimen surface was polished with emery paper (#240 to #4000) to a thickness of 1 mm and polished in a SiO 2 suspension to obtain a mirror finish. The notch roots of the specimen were also polished with emery paper (#240) to remove the electro-discharge machined layer. Testing Four-point bending fatigue tests were performed in an electrodynamic fatigue testing system under a stress ratio R of 0.1. The frequency of stress cycling was 10 Hz, and the tests were conducted in ambient conditions. Fatigue tests were interrupted after a certain number of cycles, and acetyl cellulose films were placed on the specimen surface using the replica method to examine fatigue crack initiation and propagation. Once the crack length was measured using optical microscopy, the stress intensity range,  K , was calculated [27, 28, 32]; the aspect ratio, c / a , for small cracks was estimated as follows: c / a = 1 - 1.607( a / t ) + 1.080( a / t ) 2 - 0.2149( a / t ) 3 for a / t < 1 and (1) c / a = 0.259 for a / t > 1, (2) A T