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.