PSI - Issue 7

6

R.D. Xu et al. / Procedia Structural Integrity 7 (2017) 84–91 R. D. Xu, et al. / Structural Integrity Procedia 00 (2017) 00 –000

89

(a)

350 400 450 500 550 600 650 700

(b)

350 400 450 500 550 600 650 700 σ max ,MPa

x-orientation x-orientation line z-orientation z-orientation line

x-orientation x-orientation line z-orientation z-orientation line

σ max ,MPa

10 4

10 5

10 6 Nf

10 7

10 8

10 4

10 5

10 6 Nf

10 7

10 8

Fig. 4. S-N curves of fatigue results at 400 º C (a)and at 600 º C (b)

The comparison between the fatigue stress vs. fatigue life curve at 400 º C for specimens in X-orientation and the one in Z-orientation is shown in Fig. 4 (a). It is obvious that the fatigue life in decline while the maximum stresses increase. At the same maximum stresses, the fatigue life of samples in Z-orientation is shorter than that in X orientation. When N f = 10 4 cycles, the gaps of fatigue lives of specimens in two building orientations are tiny. Nonetheless, as maximum stresses decreasing, the gaps enlarge and fatigue lives of the Z-orientation specimens are much shorter at N f = 10 5 ~10 6 cycles. The stress-life curve at 600 º C is shown in Fig. 4 (b). The fatigue lives decrease while the maximum stresses increase, which is similar to the ones at 400 º C. The distinction is that there is no significant difference between the fatigue lives of specimens in two building orientations at 600 º C 。 The fatigue lives of Z-orientation specimens are shorter than the ones of X-orientation specimens at N f = 10 4 cycles, while the fatigue lives of X-orientation specimens are shorter at N f = 10 6 cycles. The S-N curves of X-orientation and Z-orientation specimens cross at N f =2.8 × 10 5 cycles. According to the analysis for the coefficients of the fitting formula, the scatter in fatigue life data of X-orientation is lower than Z-orientation. 3.3. Failure mechanisms The fatigue fracture surface analysis with SEM reveals three crack initiation sites. The first kind of crack initiation begins at external surface where the defects, such as pores and un-melted zones, are observed. SEM images of the other crack initiation beginning at surface shows that there are not visible defects, with some abraded. The third kind of crack initiation located in the sub-surface of specimens, originated from a crystalline grain with flat crystal faces. All of the crack initiations are exhibited in Fig. 5, marked by arrowheads. The fracture surfaces of X-orientation and Z-orientation are in similar rules. In the short and middle life regimes, i.e. N f = 10 5 ~10 6 cycles, the crack initiation sites are attached to the surface. Since the surface defects are subjected to more plastic deformation, they lead to the nucleation generated early and accelerate the crack initiation process. When N f > 10 6 cycles, the crack initiation sites located away from the external surface, with crack fronts featuring the crystalline interface cracking. In addition, it is only observed in Z-orientation that the crack initiations begin from surface, which may lead to the difference of the scatter in fatigue lives data between the X-orientation and Z orientation specimens.