4.2. FCG behavior of welding joint
In current study, it was found that significant inhomogeneity of FCG rate existed in different regions of 7085Sc FSWed joint (Fig. 11). As widely known that FCG behavior is significantly associated with the microstructure of alloys. For heat-treatable alloys, previous works have proved that grain structure and hardening phases are the main microstructure factors on FCG rate [31, 39, 40]. The BESD analysis and TEM observation shown in Fig. 6 and Fig. 7 revealed that the grain structures in BM and AS-HAZ were approximately similar, while the morphology of hardening phases in these two regions were quite different (Fig. 8 and Fig. 9). Thus, it can be reasonably concluded that the difference of FCG behavior between BM and AS-HAZ was mainly induced by hardening phases, while the effect of grain structure could be negligible. As compared with BM/AS-HAZ, grain structure and hardening phases in WNZ had more distinctive characteristic, therefore, the effects of both two factors on the FCG behavior should be highlighted. According to early researches [25, 31, 41, 42], hardening phases played a noticeable role to intervene FCG process by interaction between dislocations. During the FCG process, dislocation nucleation and movement were promoted at the crack tip, undoubtedly, the interaction between hardening phase and movable dislocation turn to be frequent in this region. For coherent or semi-coherent phase, dislocation could shear it and glide reversibly with in a persistent slip band during cyclic loading. This kind of kinematical reversibility of cyclic slip would lead to a more deflective, tortuous and bifurcate fatigue crack path as well as inhomogeneous deformation on the crack tip, contributing to enhancement of FCG resistance. On the contrary, dislocation tends to bypass non-coherent phases rather than shear. In this situation, irreversible deformation turns to be dominated and increased damage accumulation on the crack tip, and finally degrade the FCG resistance. It should be noted that whether in shear or bypass mechanism, the radius (r) and the volume fraction (f) of hardening phases are two important microstructural parameters. Wen et al. [25] summarized and proposed succinct equations to express the relationship as listed below: shear mechanism:\(\frac{\text{da}}{\text{dN}}\propto\left(r\bullet f\right)^{-1/2}\)(1) bypass mechanism:\(\frac{\text{da}}{\text{dN}}\propto{r\bullet f}^{-1/2}\) (2) Obviously, the FCG rate is negatively related to r and f of shearable precipitates according to Eq. (1), and the FCG rate is positively related to r of non-shearable precipitates while negatively related to its faccording to Eq. (2). These relationships can be used to explain the FCG inhomogeneity of 7085Sc FSWed joint. As can be seen from TEM images, the dominated hardening phases in both BM and AS-HAZ samples consisted mostly of coherent GPII and semi-coherent πœ‚β€², indicating that shear mechanism was the main interaction mode in these two regions during cyclic loading. However, the average size and volume fraction of precipitates in AS-HAZ are much higher than that of BM. According to Eq. (1), the FCG resistant of AS-HAZ should theoretically be higher than BM. This theoretical deduction is entirely consistent with experimental data shown in Fig. 11. For WNZ sample, precipitates in matrix consisted of a considerably amount of reprecipitated coherent phase (πœ‚β€² and GPII) as well as some coarse non-coherent phase (πœ‚). According to shear mechanism mentioned above, high volume fraction of coherent phases would be beneficial to enhancement of FCG resistance. The supporting evidences could be found in Bray’s work demonstrating that naturally aged Al-Cu-Mg alloys had excellent fatigue properties [41]. According to Eq. (2), bypass mechanism of πœ‚ phases with large r and low f in WNZ also contributed to reduction of FCG rate. In addition to the effects of precipitates, another considerable factor on FCG rate of WNZ is the grain structure. It has been known that due to small angle of preferential slip planes between adjacent grains, fatigue crack tends to expand across multiple grains at a time in alloy with high texture intensities, resulting low FCG resistance [43, 44]. As compared with BM and AS-HAZ, WNZ possessed recrystallized grains with smaller size and random crystal orientation (Fig. 6 and 7), in this situation, a higher force was required for the propagation of fatigue crack as the impeding effect caused by high angle grain boundary. Therefore, the deflection, torsion and bifurcation of fatigue crack occurred more frequently (Fig. 12), leading to a significant decrease of FCG rate. Due to the combined effects of precipitates and grain structures, FCG property of WNZ was better than other welding regions as well as BM. In addition, the influence of grains structure on FCG is associated with the cyclic plastic zone (CPZ) at the crack tip [25]. The CPZ size could be estimated by\(\delta_{f}=\frac{1}{8\pi}\left(\frac{K}{\sigma_{y}}\right)^{2}\), here (Οƒy) accurately would be the strength of the plastic zones near the fatigue crack tip. Obviously, the CPZ rises with increasing of Ξ”K. When CPZ size is larger than a grain scale, the effects of grain structure on the FCG would turn to be dominant [45]. Because BM and AS-HAZ processed similar grain structure, thus, the crack in BM and AS-HAZ samples showed roughly the same FCG rate at high Ξ”K (>20 MPa m1/2).