PSI - Issue 13

Akira Maenosono et al. / Procedia Structural Integrity 13 (2018) 694–699 Akira Maenosono et al. / Structural Integrity Procedia 00 (2018) 000 – 000

695

2

1. Introduction Alloys based on Ti – 6Al – 4V possess advantageous properties, such as high specific strength, thermal resistance, and corrosion resistance, and have therefore been widely used. However, the Ti – 6Al – 4V alloy has a large scatter in fatigue life (Nalla et al., 2002). One of the factors causing this scatter is a statistical scatter of uncontrollable material characteristics. This complicates the evaluation and prediction of the fatigue life. To ensure the safety of structural components, it is necessary to estimate an excessive safety factor. In other words, the fatigue performance of the Ti – 6Al – 4V alloy is underrated, because of the high safety factor. Therefore, it is necessary to clarify the underlying factors controlling the scatter of the fatigue property. In practice, fatigue crack propagation life is important, because most components have many stress concentration sources that assist fatigue crack initiation. Fatigue crack propagation life is divided into two sections: microstructurally small crack and microstructurally large crack regions (Ritchie and Lankford, 1986; Omura et al., 2017). In particular, propagation behaviors of a microstructurally small crack strongly depend on material factors, such as crystallographic orientation and grain boundary, which cause the scatter in fatigue life (Chowdhury et al., 2016; Koyama et al., 2017). In terms of material factors, the initial microstructure of the Ti – 6Al – 4V alloy in industrial practice is a fully lamellar structure that consists of α and β phases (Koyama et al., 2017). The α phase is the primary phase whose crystal structure is hexagonal close packed (HCP). HCP has a large plastic anisotropy derived from its fewer slip planes than body-centered cubic (BCC) and face-centered cubic (FCC) structures. In addition, a microstructurally small crack tends to propagate along the specific slip plane by a mode II mechanism. In terms of mechanical factors, shear stress on the slip plane is important in microstructurally small cracks. In this study, low-cycle fatigue tests of Ti – 6Al – 4V alloy with microstructurally small artificial notches were performed. The crack growth behavior was observed by in situ scanning electron microscopy (SEM) coupled with electron backscattered diffraction (EBSD) analysis. Hence, the dominant material/mechanical factors that affect microstructurally small fatigue crack growth behaviors are presented, and the underlying mechanisms of the fatigue crack propagation life scatter are clarified.

2. Experimental procedures 2.1. Material

Alloy Ti – 6Al – 4V consisting of a fully laminated microstructure, shown in Fig. 1(a), was prepared. Table 1 shows the details of the chemical composition. Figure 1(b) shows the specimen geometry used for a monotonic tensile test and fatigue tests. Specimens were produced by electric discharged machining. Prior to the tests, the specimen surface was mechanically ground and polished. The 0.2% proof stress and tensile strength at ambient temperature at an initial strain rate of 6.3 × 10 -4 s -1 were 910 and 980 MPa, respectively.

Fig. 1. (a) Initial microstructure of the Ti – 6Al – 4V alloy; (b) specimen geometry for the tensile test and in situ fatigue experiments.