Low-Cycle Fatigue Behavior of Hydrostatic Extruded AZ80 Mg Alloy

The fatigue properties of conventional and hydrostatic extruded AZ80 Mg alloy were fully studied under strain-controlled mode. The hydrostatic extrusion can effectively increase the tensile strength, but decrease the elongation. Consequently, the low cycle fatigue lifespan of the hydrostatic extruded specimens was shorter than that of the conventional extruded ones. The Manson-Coffin-Basquin relationship was adopted to describe the fatigue lifespan of the extruded AZ80 alloys. Finally, the fracture surfaces were observed by scanning electron microscope. The fatigue crack stable propagation zone of the hydrostatic extruded sample has decreased significantly compared with that of the conventional extruded sample. The hydrostatic extruded specimen has small reverse plastic zone size, which leads to microscopically rough fatigue crack propagation zone.


Introduction
Because of superior specific strength and stiffness, Mg alloys are attractive for the transportation industry [1].Currently, most Mg alloys are processed by casting which introduces defects and decreases mechanical properties.As such, wrought Mg alloys are better choice for high strength.Most Mg alloys have hexagonal close-packed crystal structure which provides insufficient deformation modes [2].As such, a strong basal plane texture is easily formed during the conventional extrusion or rolling.Additionally, {10-12} tensile twin can be activated and causes the tension-compression yield strength asymmetry [3] which deteriorates the mechanical properties.For example, it could cause the tensile average stress during symmetric low-cycle fatigue (LCF) process and accelerate crack initiation and propagation.As such, it is important to suppress the tension-compression strength asymmetry.One appealing way is to refine the grains.After the grains could be refined, the twinning activity decreases and more non-basal deformation modes could be activated.Therefore the strength asymmetry is attenuated.Over recent years, severe plastic deformation has been adopted to refine the micro-structures.For example, equal-channel angular pressing (ECAP) processing is widely applied in metals to grain refinement [4,5].When the grain size of AZ31 Mg alloy was refined down to ~2 μm, the twinning activity was suppressed and the tension-compression strength asymmetry disappeared [6].However, its tensile strength was lower than that of conventional extruded ones because of the formation of 45º softening texture [6,7].Achieving higher strengths require non-conventional extrusion, such as hydrostatic extrusion.
Hydrostatic extrusion shows its mechanical advantages in the production of Mg alloys [8].Hydrostatic extrusion is similar to conventional extrusion, but the billet is enclosed by fluid which eliminates the frictional force between the billet and the container.As such, the shear stress is significantly decreased [8][9][10][11].Additionally, hydrostatic pressure can prevent the crack propagation and enhance the alloy's formability [9].However, the hydrostatic extruded Mg alloy showed higher basal plane texture strength than that of conventional extruded one [8].Recently, Zhang et al. [12] combined hydrostatic extrusion with circular ECAP to adjust the grain size and texture of AZ80 Mg alloy.This processing can refine the grain size by ECAP, but avoid the formation of softening texture by hydrostatic extrusion.The processed AZ80 Mg alloy exhibits high tensile strength with suitable plasticity.However, the fatigue properties of these alloys are unclear and need to be investigated for engineering application.
In the current investigation, an AZ80 alloy was produced by hydrostatic extrusion combined with circular ECAP.The LCF behaviors were fully struded under the strain-controlled mode.In order to demonstrate the fatigue properties of hydrostatic extrusion, a conventional extruded AZ80 alloy was chosen as a counterpart for comparison.Their microstructures and cyclic behaviors will be investigated detailed so as to discover their fatigue failure mechanisms.The result of this paper might provide a useful guide for the safe application of this newly developed Mg alloys.

Experimental Procedures
The chemical composition of AZ80 alloy is 8%Al, 0.5 %Zn with the balanced Mg.The AZ80 alloy was direct chill-cast into bars with 70 mm diameter.These bars were soluted at 415 ℃ for 32 h and then quenched in air.Conventional extrusion was conducted at 400 ℃ with an extrusion ratio of 16:1.The hydorstatic extrusion combined with circular ECAP was conducted at 290 ℃ with an extrusion ratio of 1.25:1.The detailed extrusion process was reported in an earlier report [12].
An Instron-8801machine was used for all mechanical tests which were completed in air at room temperature.The tensile specimen has a gauge length of 10 mm and rectangle section of 2×3 mm 2 .The compressive specimen has a length of 8 mm and rectangle section of 5×5 mm 2 .The initial strain rate was 5×10 -4 /s.LCF specimen has a gauge length of 14 mm and rectangle section of 6×5 mm 2 according to ASTM E606.All specimens were cut from the rod along the extrusion direction.The LCF tests were conducted under fully-reversed tension-compression loading of the strain rate of 1.2%/s.Prior to the fatigue test, the specimens were mechanically polished with silicon carbide abrasive paper to attain smooth surface.Subsequently, the residual stress of the smooth surface was eliminated by electropolishing with a solution containing 30 ml HClO4, 100 ml glycol, and 360 ml ethanol.The microstructures were examined using an optical microscope and a Quanta-200 scanning electron microscope.

Results and Discussion
Figure 1 gives the optical microstructures of the hydrostatic and conventional extruded AZ80 alloys observed on the longitudinal section.As seen, as-extruded microstructure was more homogeneous and composed of equiaxial grains with 21 μm mean grain size.The microstructure of hydrostatic extruded alloy was bimodal with coarse grains with ~ 60 μm mean grain size which were surround by small grains with ~ 1 μm mean grain size.The fine-grains were caused by dynamic recrystallization and the coarse grains were the unrecrystallized regions [12].The engineering tensile and compressive stress-strain curves of the conventional and hydrostatic extruded specimens are shown in figure 2. It notes that the work hardening rate and the yield strength showed an obvious different under tension and compression.The ratio of compressive to tensile yield strength was about 0.83 and 0.8 for hydrostatic and conventional extruded samples, respectively.That indicated the extruded AZ80 alloys showed the nearly same value of tension yield strength and compression one, which is significantly different with that of solution treated Mg alloys [13,14].The strain hardening curve under compression of the extruded alloys exhibited a concave shape.This type of hardening was attributed to the {10-12} extension twinning [15].These findings suggest that extension twinning during compression may be an important deformation mechanism, but its activity decreased significantly.Additionally, the hydrostatic extruded sample had higher strength levels.The hydrostatic extruded specimen yielded at approximately 280 MPa, while the conventional extruded one has a yield strength of nearly 190 MPa.The elongation of the hydrostatic extruded specimen was nearly 9%, lower than that (~15%) of the common extruded one.As such, it may be concluded that hydrostatic extrusion can significantly enhance the tensile strength, but decrease the elongation moderately.Figure 3 present the first cycle hysteresis curves at different total strain amplitudes ranging from 0.2% to 0.6%.The hysteresis loops were nearly symmetrical between the tensile and compressive cycle, which was unusual for the cyclic behaviors of wrought magnesium alloy.This symmetric shape may be attributed to reduced the tension-compression yield strength asymmetry which may be caused by the decreased twinning activity [16][17][18][19].
Figure 4 shows the variation of the stress amplitude with the number of loading cycles.The hydrostatic extruded samples showed higher cyclic stresses than the conventional extruded ones, which may be due to the higher tensile strength of the hydrostatic extruded samples.Additionally, the stress amplitude of conventional extruded AZ80 alloy decreases during the early cycles, manifesting cyclic softening.And then it increases during the rest of fatigue lifetime, manifesting cyclic hardening.However, the hydrostatic extruded AZ80 alloy shows cyclic stability during the whole fatigue process.Figure 5 shows the dependence of fatigue lifetime on the strain amplitude of the conventional and hydrostatic extruded specimens.Under the same total strain amplitude, the conventional extruded specimen showed longer LCF lifetime than the hydrostatic extruded one.But at the low strain amplitude, the hydrostatic extruded alloy showed superior high-cycle fatigue lifespan compared with the as-extruded one.This may be originated from the point that better ductility retards the crack propagation and improves the LCF properties, whereas high strength suppresses the crack initiation and increases the HCF properties.The cyclic stress vs. plastic strain amplitude curves shown in figure 6 are described by the Holloman relationship △σ/2=K ' (△εp/2) n' , where △σ/2 is the stress amplitude at half lifetime, Δεp/2 is the plastic strain amplitude at the half lifetime, n' is the cyclic strain hardening exponent, and K' is the cyclic strength coefficient.Table 1 gives the fitted results based on the dependence of stress amplitude on the plastic strain amplitude.The conventional extruded samples show higher cyclic strain hardening exponent which is 0.4 that the monotonic one which is 0.14.That indicates that the alloy underwent superior hardenability under fatigue loading.For the hydrostatic extruded samples, the similar result exists.The results indicated that cyclic deformation shows better hardenability than monotonic loading for both alloys.The fatigue lifespan is determined by the strain amplitude which can be split into its elastic and plastic amplitudes.Figure 7 shows the dependence of the fatigue lifespan on the elastic and plastic strain amplitude.This dependence could be manifested by the Coffin-Manson-Basquin rules, respectively.Table 1 lists   The fatigue strength coefficients by fitting were σf=391 MPa for hydrostatic extruded sample, and 278 MPa for conventional extruded one.This result was corresponding to the uniaxial tensile results.The hydrostatic extruded sample shows higher strength than the conventional extruded counterpart.The conventional extruded sample has a fatigue ductility coefficient εf of 7.3%, which is three times the hydrostatic extruded one (2.3%).It is corresponding to higher elongations of conventional extruded sample than that of hydrostatic one.The lower fatigue ductility coefficient, the shorter fatigue lifespan.As such, the LCF lifespan of the hydrostatic extruded alloy with low fatigue ductility coefficient is shorter than that that of the conventional extruded one with higher ductility coefficient.Figure 8 shows typical SEM fractographs for the hydrostatic and conventional extruded samples at 0.5% total strain amplitude, respectively.Two major zones were formed during fatigue process.A dashed line discriminated these two zones.The dark region is the fatigue crack stable propagation zone.The light region is the unstable crack propagation (final rupture) zone.In both cases, the fatigue crack initiated at the surface.And the hydrostatic extruded sample has small fatigue crack stable propagation zone, compared with the conventional extruded one.The larger crack stable zone, the higher material utility.As a result, the hydrostatic extruded sample has a short fatigue lifetime due to limited crack propagation zone.Figure 8 also shows microstructure feature at the fatigue crack stable propagation zone at a higher magnification.The conventional extruded AZ80 alloy shows flat fracture surface.While for the hydrostatic extruded sample, the fracture surface is rather rough.The different fracture surface of the two alloys may be caused by different reverse plastic zone size.When the reverse plastic zone size is limited, the interaction between the crack and a grain ahead could cause the grain cracking along the cleavage plane.As such, the fatigue crack advances, resulting in rough fracture surfaces.However, when the plastic zone is large enough, several grains could interact with the crack tip.As such, these grains would deformation at the same time, leading to the formation of planar slip bands.Because these planar slip bands provide the easy pathway for cracking, a flat fracture surface forms [20].According to the fracture mechanics, the reverse plastic zone size rp is defined as rp=(1/απ)(△K/2σys) 2 , where △σ is the stress range applied in fatigue test, ΔK is the stress intensity factor range (△K=Y△σa 1/2 ), σys is the yield strength of AZ80 alloy, Y is a geometry-related coefficient, a is the

Figure 2 .
Figure 2. Static tensile and compressive stress-strain curves of the AZ80 alloy.

Figure 3 .
Figure 3. Hysteresis loops at the first cycle.

Figure 4 .
Figure 4. Stress amplitude vs. the number of cycles at various total strain amplitudes.

Figure 5 .
Figure 5.Total strain amplitude as a function of the number of cycles to failure.

Figure 6 .
Figure 6.Cyclic stress and strain curves of AZ80 alloy.

Figure 9 .
Figure 9.Comparison of the yield stress and stress amplitude.

Table 1 .
Fitted fatigue parameters based on the fatigue life curves.