PSI - Issue 13

Tomoki MIZOGUCHI et al. / Procedia Structural Integrity 13 (2018) 1071–1075 Tomoki Mizoguch / Structural Integrity Procedia 00 (2018) 000 – 000

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crack planes, which causes RISS (R. Ballarini et al . 1987 and S. Hamada et al. 2018). The nanometer-scale roughness is referred to as nano-roughness. In advanced high strength steels such as TRIP-maraging steels, understanding the roughness effect is key to quantitatively design and predict the fatigue crack resistance. While the micro-roughness effect has been simulated quantitatively (J. Llorca 1992 and R. Pippan et al. 2004), the effect of nano-roughness on fatigue crack growth has not been well discussed yet. Therefore, we propose methods for the quantification of microstructural roughness. In this study, we focus on two types of roughness that trigger RICC and RISS in a TRIP-maraging steel: (i) as micro-roughness, we consider grain-size-scale roughness, which dominates the crack propagation direction in each grain, and (ii) as nano-roughness, we consider microstructural roughness, which is associated with the dislocation substructure and second phase interface in each grain interior. Hence, we attempt to quantify the two-scale fatigue crack roughness of a laminated TRIP-maraging steel and correlate the quantitative roughness values with the microstructures.

Fig. 1. Schematic representation of RICC associated with geometrical mismatch between fatigue crack surfaces in grain size scale (S. Suresh and R. O. Ritchie 1982). (a 1 ) Crack opening and (a 2 ) subsequent crack closing without mode II displacement. (a 3 ) Stress intensity factor range without mismatch. (b 1 ) Maximum crack opening displacement with mismatch. (b 2 ) Unloading with mismatch. (b 3 ) Stress intensity factor range with mismatch. θ micro is the micro-roughness deflection angle of a crack. K max is the maximum intensity factor. K min is the minimum intensity factor. K cl is the stress intensity factor to close crack. Δ K eff is the effective stress intensity factor range. 2. Experimental procedure A laminated Fe-9MN-3Ni-1.4Al-0.001C (mass%) TRIP-maraging steel was used in this study. The final annealing time was 8 h. Details of the production process have been presented elsewhere (J. Millán et al . 2014; M.M. Wang et al . 2014). The initial micro structure showed metastable austenite and maraging martensite. The prior austenite grain size and lath size were 29.7 μm and 0.5 μm, respectively. The retained austenite fraction was 36%. Further detailed microstructural characterizations have been presented elsewhere. We performed fatigue testing using a scanning electron microscope and obtained high-resolution images of crack morphologies after different numbers of cycles. Figure 2 shows the specimen geometry. The specimen was mechanically polished to a mirror finish using colloidal silica with a particle diameter of 60 nm. To achieve a fatigue crack tip in specific grains, artificial micro notches were introduced by focused-ion beam (FIB) machining. Furthermore, to accelerate crack initiation at the notch tip, a tetrahedron was created at one side of each notch tip, as shown in Figs. 1(b) and (c). Prior to the FIB machining, the specimen surface was chemically etched with 3% nital to determine the microstructural positions of the notch tips. The FIB machining was conducted using a Quanta 3D 200i system with an acceleration voltage of 30 kV and a beam current of 15 nA (major part of the notch) or 5.0 nA (tetrahedron part of the notch). After introducing the notches, the specimen was mechanically polished to a mirror finish again to eliminate microstructural-relief-induced stress concentration. The in situ fatigue test was conducted using a MTEST5000 system and JSM-IT300 microscope with the stress ratio R = 0 and maximum stress σ max = 800 MPa at a frequency of 0.06 Hz. Secondary electron (SE) images near the crack tip were taken every 500 cycles and the test was stopped before the most propagated crack reached the grain boundary.

Fig. 2. (a) Specimen configuration for in situ fatigue test. (b) SE image of micro-notch before the test. (c) Schematic of 3D configuration of micro-notch.

3. Quantification methods 3.1. Micro-Roughness

A crucial parameter associated with micro-roughness is deflection angle of a crack, θ micro . Figure 3(a) shows a schematic representation of micro-roughness. From the viewpoint of the RICC model shown in Fig. 1, the height of crack deflection is a directly important parameter, but θ micro is also closely related to the degree of RICC. Regarding friction stress on the crack planes, the effect of friction becomes larger as θ micro becomes smaller.