Issue34

S. Kikuchi et alii, Frattura ed Integrità Strutturale, 34 (2015) 261 - 270; DOI: 10.3221/IGF-ESIS.34.28 266 Effects of grain size on the fatigue crack propagation of Ti-6Al-4V alloy To clarify the reason for decreasing  K th values of Ti-6Al-4V alloy by creating a harmonic structure as mentioned in the previous section, effects of grain size on the fatigue crack propagation were investigated for the bulk homogeneous material. Fig. 7 shows the relation of the crack growth rate, d a /d N ( R = 0.1), against the stress intensity range,  K , in the bulk homogeneous Ti-6Al-4V alloys with different grain size [19]. The  K th value of the coarse-grained bulk homogeneous material ( d =5.7  m) was high compared to the fine-grained one ( d =2.2  m). In contrast, as results of calculating the effective threshold stress intensity range,  K eff,th , the effect of grain size on the value of  K th disappears. The values of K cl of the bulk homogeneous Ti-6Al-4V alloys are shown in Fig. 8. The K cl value of the coarse-grained bulk material ( d =5.7  m) was higher than that of the fine-grained one ( d =2.2  m). In the case of the fine-grained material ( d =2.2  m), the value of K cl decreased with increasing  K and was saturated to approximately 0.6 MPam 1/2 . The grain size strongly influences the behaviors of fatigue crack propagation and crack closure of Ti-6Al-4V alloy; the fine grains reduce the value of K cl . Grain size  K  K eff d = 2.2  m d = 5.7  m 2 3 4 5 6 10 -11 10 -10 10 -9 10 -8 Stress intensity range  K ,  K eff , MPam 1/2 Crack growth rate d a /d N , m/cycle R = 0.1 3 4 5 6 0.4 0.6 0.8 1.0 1.2 1.4  K , MPam 1/2 K cl , MPam 1/2 : d = 2.2  m : d = 5.7  m R = 0.1 Figure 7 : Relationship between crack growth rate and stress intensity range for the bulk specimen with homogeneous microstructure Figure 8 : Relationship between crack closure intensity and stress intensity range for the bulk specimen with homogeneous microstructure. Fractography and crack profiles The role of crack closure at near-threshold levels is generally attributed to the roughness-induced mechanism [20]. To characterize the surface topography of fracture surfaces, the three dimensional axonometric drawing was produced. Examples of axonometric drawings for the (a) IP and (b) Harmonic series are shown in Fig. 9. The surface topography of the IP series was rougher compared to the Harmonic series. Nalla et al. [15] has reported that the structure sensitivity of fatigue crack growth changes depending on the microstructure of Ti-6Al-4V alloy. Moreover, the coarser microstructure showed a more tortuous and deflected crack path than the finer microstructure, resulting in increasing the fatigue crack growth resistance due to the roughness-induced crack closure. The present study exhibited the same trend of the previous study [15]. Fig. 10 shows the inverse pole figure (IPF) maps obtained by EBSD at specimen’s surface. In this figure, crack profiles are represented by black lines. In Fig. 10(a), crack profile of the Harmonic series showed very smooth and a fatigue crack was arrested at the coarse-grained structure, represented by the arrow mark. However, in some areas, a crack profile was influenced by the microstructure of the Harmonic series. Fig. 10(b) showed that a fatigue crack avoided propagating the coarse-grained structure of the Harmonic series, and propagated across the fine-grained structure.