Microstructure dependence of fatigue crack propagation behavior in wrought magnesium alloy

This paper deals with the fatigue crack propagation behavior of rolled AZ31B magnesium alloy (grain size: approximately 40 m). Fatigue crack propagation tests were performed on single edge notched tension specimens at a stress ratio of R = 0.1 and a frequency of 10 Hz at room temperature. Loading axes were parallel to the rolling direction; fatigue cracks propagated parallel to the transverse direction (L-T specimen), parallel to the short transverse direction (L-S specimen). Loading axis was perpendicular to the rolling direction; fatigue cracks propagated parallel to the transverse direction (S-T specimen). The crack growth rate (da/dN) of the L-S specimen was several times lower than that of the L-T specimen in the examined stress intensity factor range (K). Fracture surfaces of the L-T and L-S specimens showed many steps parallel and perpendicular, respectively, to the macroscopic crack growth direction. The da/dN of the S-T specimen was higher than that of the L-T and L-S specimens in the examined K. The fracture surface was covered by quasi-cleavage facets independent of macroscopic crack growth direction, and the fracture surface roughness at low K was larger than that at high K.


INTRODUCTION
agnesium alloys are the lightest structural material with high specific strength and stiffness.These features make magnesium alloys attractive for applications in the automotive industry [1].Wrought Mg-Al-Zn system alloys, especially rolled and extruded magnesium alloys, are the candidates for structural parts.It is necessary to investigate the fatigue crack initiation and propagation behavior of the material used for structural parts.

M
It is well known that magnesium alloys have a hexagonal close-packed (HCP) structure, and strong textures are formed by rolling.In such textured magnesium alloys, basal planes are aligned parallel to the rolling direction by rolling.Depending on their texture, rolled magnesium alloys show unique deformation behavior such as mechanical anisotropy [2][3][4], pseudoelasticity in cyclic loading-unloading [4][5][6][7][8], and asymmetricity of stress-strain hysteresis loops in strain controlled low-cycle fatigue tests [9][10][11][12][13][14] and even in load controlled high-cycle fatigue tests [3,9], etc.The orientation dependence of fatigue crack propagation behavior of magnesium single crystals [15][16][17], and the effect of grain size [18][19][20][21] and texture [22][23][24][25] on fatigue properties of polycrystalline magnesium alloys have been reported in previous works.However, the effect of texture on the fatigue crack propagation behavior of magnesium alloy is still poorly understood.In the present study, the crystallographic orientation dependence of the fatigue crack propagation behavior of rolled AZ31B magnesium alloy is investigated.

EXPERIMENTAL PROCEDURE
ommercial rolled AZ31B magnesium alloy plate with a thickness of 60 mm was used in the present study.Chemical composition of the alloy is listed in Tab. 1.The alloy has equiaxed grains, and the average grain size was approximately 40 m.The alloy showed typical texture as shown in Fig. 1 and Fig. 2; basal planes were aligned parallel to the rolling direction.Three types of tensile and compressive specimens were machined with the loading axis parallel (L-specimen), perpendicular (T-specimen) and normal (S-specimen) to rolling direction.Compressive and tensile mechanical properties of the alloy is summarized in Tab. 2.   Single edge notched tension specimens with a width (W) of 12 mm, a thickness (B) of 4 mm, a initial crack length (a) of 1 mm, and a length (L) of 50 mm were machined from a rolled as shown in Fig. 3. Loading axes were parallel to the rolling direction; fatigue crack propagated parallel to the transverse direction (L-T specimen), parallel to the short transverse direction (L-S specimen).On the other hand, loading axis was normal to the rolling direction; fatigue cracks propagated parallel to the transverse direction (S-T specimen).Fatigue crack propagation tests were performed on an electro-hydraulic testing machine (capacity: 9.8 kN) at a stress ratio of R = P min /P max = 0.1 (tension-tension) and a frequency of 10 Hz at room temperature in air.Here, Pmax and Pmin are the maximum and minimum applied load, respectively.The fatigue crack length was determined using a microscope.The stress intensity factor range (K) was calculated using the following equation:

Al
where P is the applied load, and W, B and a are the width, thickness, and crack length, respectively.The fracture surface of the fatigue tested specimens were observed by SEM.  he relationship between the crack growth rate (da/dN) and the K is shown in Fig. 4. The figure shows that the da/dN of the L-S specimen was several times lower than that of the L-T and S-T specimens in the examined K.
The da/dN of the S-T specimen was higher than that of the L-T and L-S specimens in the examined K.The macroscopic fatigue crack propagation is parallel to the a-axis (perpendicular to basal plane) of each grain for the L-T specimen, and parallel to the c-axis (perpendicular to basal plane) of each grain for the L-S specimen.Therefore, it is inferred that the c-axis direction is unfavorable for the fatigue crack propagation in rolled AZ31B magnesium alloy.It is also found that the macroscopic fatigue crack propagation is various directions to the a-axis (parallel to basal plane) of each grain in the S-T specimen.Thus, it appears that the fatigue crack propagated easily in a direction parallel to the a-axis (parallel to basal plane) of each grain in the S-T specimen.
In addition, the deformation twinning of polycrystalline magnesium alloys plays an important role in the deformation process because of the limitation of slip system.When the specimens were subjected to a stress higher than the compressive and tensile yield stress in the load-controlled fatigue test, deformation twins were observed in the extruded magnesium alloys [3,4,26].In contrast, free deformation twins are observed around the fatigue crack path (within the plastic zone) in the L-T and L-S specimens.It appears that the fatigue crack growth is not controlled by the deformation twinning at a crack tip in textured polycrystalline magnesium alloys.

Figure 3 :
Figure 3: Shape and dimensions of SENT specimen (in mm).

Fig. 5
Fig.5shows the typical SEM images of the fatigue tested L-T, L-S, and S-T specimens.Many straight lines or steps were observed over several grains along with the macroscopic direction of fatigue crack growth in the L-T specimen (Fig.5(a) and (b)).In contrast, in the L-S specimen, many lines or steps perpendicular to the macroscopic direction of fatigue crack growth were observed (Fig.5(c) and (d)).The fracture surface of the L-T and L-S specimens had a similar morphology and did not depend on K.On the other hand, various quasi-cleavage facets independent of the macroscopic direction of fatigue crack growth were observed in the S-T specimen (Figs.5(e) and (f)).Moreover, the fracture surface roughness at low K, up to K = 7 MPa•m 1/2 , was larger than that at high K, above K > 7 MPa•m 1/2 .

Figure 4 :
Figure 4: Relationship between crack growth rate and stress intensity factor range.