3.4 Observation on fracture surfaces
After ultrasonic fatigue tests, the scanning electron microscope was applied to characterize the fracture morphologies of failed specimens.Figure 6 shows typical SEM images of two representatives failed specimens at different magnifications. It can be clearly seen that fatigue crack nucleated and initiated from the specimen surface and then continue to expand, which resulting in the ultimate fatigue failure, as presented in Figures 6a and 6e . Meanwhile, the crack growth direction can be determined according to the radial ridge of the material fracture.
Figures 6b and 6f display the enlarged images of the regions near the crack initiation sites of the two failed specimens, respectively. Obviously, along the growth direction of the crack, the fracture morphology varies greatly with the crack grows deeper, which is quite analogous to that of high-strength steels and titanium alloys [17, 33, 34]. Firstly, the regions near the crack initiation sites, with a distance of about 100 μm, present a relatively smooth morphology. As the region went further away from the crack initiation sites, the fracture morphology became rougher than Zone I. Meanwhile, lots of radial ridges can be seen in Zone II. As the distance goes further, no characteristic morphology but radial streaks can be observed in the regions far away from the crack initiation sites. In summary, according to the difference in morphology, we utilized dash lines to divide the fracture surface into three distinct regions: Zone I, Zone II, and Zone III. These regions corresponding to three stages of crack initiation and propagation, Zone (I): crack initiation region; Zone (II): short crack propagation region; (III): long-crack propagation region.
High-magnification images of Zone I and Zone II are presented inFigures 6c and 6g . It is obvious that there are much more striped crystallographic profiles in Zone II comparing to Zone I. It can be seen more clearly in Figures 6d and 6h . By comparing the striped profiles with the α-Mg grains in the metallographic diagram of the material, it was found that they are highly similar in appearance as well as area proportion. Namely, these striped crystallographic profiles result from the fracture of the α-Mg grains near the crack initiation site. It has been reported in some previous researches that there exist strong and weak phases in duplex stainless steel [35-38]. The crack initiation and propagation behaviors are closely related to the coordinated plastic deformation of the two phases (γ-austenite phase with faced centered cubic structure and α-ferrite phase with body centered cubic structure). On one hand, when the hardness of the two phases differs greatly, the crack tends to initiate in the weaker phase of the material. In this case, the phase with much higher hardness is a hard phase in the material. For instance, M.W. Tofique et al [35]. and R. Strubbia et al. [37] found that since the hardness of ferrite grains is much lower than that of austenite grains, the plastic damage accumulation mainly in the ferrite grains. On the other hand, the crack nucleation will be highly affected by the microstructure (grain orientation, grain boundary, etc.) of the material when the hardness of these two phases differs slightly [35, 38]. As a structural alloy with a dual-phase structure, the LZ91 alloy may possess hard and soft phases as well. Therefore, we investigated the microstructure of two phases of the LZ91 alloy in the discussion section and attempted to associate it with crack initiation and propagation.