# Effect of Heat-Treating on Microstructure and High Cycle Bending Fatigue Behavior of AZ91 and AZE911 Magnesium Alloys

Authors: Ahmad Yousefi Parchin Oliya; Mohammad Azadi; Mohammad Sadegh Aghareb Parast; Mehdi Mokhtarishirazabad
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

This article investigates the influences of heat-treating on the microstructures and the high cycle fatigue (HCF) properties of AZ91 and AZE911 (AZ91 + 1%RE) magnesium alloys. For such an objective, AZ91 and AZE911 alloys were used after solution treatment at 415°C for 5 hours aged at 215°C for 3 hours and at 215°C for 5 hours, respectively. To investigate the HCF behavior, a rotational bending fatigue test was performed with stress ratio (R) −1 and frequency of 100 Hz at room temperature. Optical microscopy (OM) study demonstrates that heat treatment leads to a reduction in grain size and transformation of continuous and discontinuous precipitations into needle-shaped precipitations, which are located on the grains of the alpha phase. Scanning electron microscopy (SEM) showed both marks of quasicleavage and cleavage on the fracture surface of specimens. These planes indicated the brittle behavior of the fracture. Moreover, in heat-treated specimens, the size of cleavage patterns was smaller, and microcracks were shorter. These behaviors affected the strength of the material and the fatigue lifetime. The results of mechanical tests show a negligible influence of heat-treating on the HCF behaviors of AZ91-T6 and AZE911-T6 alloys. Stress-lifetime curves (S-N) show an increase in fatigue strength in 3.8 × 105 fatigue cycles, from 95 MPa to 125 MPa for the AZ91-T6 alloy and from 125 MPa to 155 MPa for the AZE911-T6 alloy, after heat treatment.


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## Body

## 1. Introduction

Magnesium alloys are lightweight materials that are used in the automotive and aerospace industries due to their high strength-to-weight ratio and good vibration damping [1] and proper thermal conductivity [2,3]. Most of the components produced by these alloys are under cyclic loadings through a high cycle regime, which causes the fatigue phenomenon. The AZ91D alloy including 9 wt.% of aluminum and 1 wt.% of zinc is the most used magnesium alloy in the automotive industry, due to great casting ability, good corrosion resistance, and well room/low temperature mechanical performance. Nevertheless, the weak fatigue strength under high forces or stresses limited the applications of the AZ91D alloy [4]. Thus, the improvement of the fatigue properties of magnesium alloys is the most important objective of many researchers, which is discussed as a literature review, below.Wenwen et al. [5] investigated the weaknesses of the mechanical properties of the AZ91 alloy. They showed that reducing of mechanical properties in AZ91 under high-stress levels and temperature conditions was due to the softening behavior of the intermetallic phase (Mg17Al12). Mahmudi et al. [6] presented the effects of 2 wt.% rare earth elements (RE) additions and annealing process (T4) on the structure and shear strength of AZ91. They reported that stability and shear strength are improved by reducing the volume fraction of the β phase (Mg17Al12), which could inhibit grain growth during the annealing process (T4) at 420°C. Kuffova [7] depicted the influence of heat-treating on the HCF properties of magnesium alloys and reported that the rate of fatigue crack growth depends on the stress intensity factor, which increased with enhancing the asymmetry coefficient. The crack growth could be decreased because of ageing (T6) and could be enhanced after the dissolution of annealing (T4). Bag and Zhou [8] investigated the HCF behavior of the AZ91D-T6 alloy. They claimed that the high fatigue crack propagation resistance is more due to the larger deviation of the crack path from the maximum stress planes to many branches in the heat-treated material, which decreased the effective stress intensity at the crack tip, resulting in a lower crack growth rate. Zhang et al. [9] compared the strain-controlled behavior of NZ30K-T6 and AZ91D-T6 magnesium alloys. They demonstrated that increasing the strength of the matrix could increase the resistance of the matrix to the dislocation movement and the slip bands formation. Hassan and Lewandowski [10] evaluated the effect of adding Silicon Carbide-nanoparticles and T4/T6 heat treatments on the fatigue lifetime of AZ91D. They claimed that, for both heat treatments, the samples did not fail in the range of 106–107 cycles. For that region, the AZ91D alloy with T6 heat treatment showed better fatigue properties compared to T4. Dobrzanski et al. [11] characterized the influences of ageing and solution on the microstructure of MgAl9Zn1. They showed that the solution formed the β phase (Mg17Al12) in the form of planes that were mostly at the grain boundary and the ageing formed the precipitations of β phase to be evenly distributed in the needle shape on the grains of the alpha phase. Horstemeyer et al. [12] presented the effect of different T4/T6 treatments on the microstructure of the AZ91 alloy. They indicated that cast and dissolved samples had cyclic hardening, which can generally be due to increased dislocation density, and the aged samples exhibited nonuniform behavior during microplastic deformation. Govindaraju et al. [13] illustrated the effect of heat-treating and FSP on the AZ91D alloy. They indicated that crack growth at high-stress levels changed from continuous to discontinuous mode under the influence of heat-treating. FSP had better effects on fatigue lifetime compared to the heat-treating process. Leo Prakash et al. [14] characterized the effects of long-term annealing (T4) and ageing (T6) on precipitates in the microstructure of magnesium alloys. They showed that the aged Mg-RE and Mg-Zn alloys had cyclic softening behavior due to the presence of cohesive precipitates, which could be easily cut by dislocations. Conversely, the aged Mg-Al alloys formed inconsistent precipitates. Suman [15] evaluated the crack behavior in AZ91D-T6 and AM60B-T6 Mg alloys. They found that the rate of crack propagation significantly depends on the microstructure and microstructural constituents. In the AM60B-T6 alloy, the precipitates caused the decelerating influence on the crack propagation rate, while in the AZ91D-T6 alloy, the Mg17Al12 phase did so. Dimitrov and Shtarbakov [16] examined the HCF ultrasonic test of the AZ91 alloy. They reported that fatigue cracks were created from existing microcracks in the phase boundary between β phase precipitations (Mg17Al12) and the main α-Mg matrix. Heat-treating to resolve these precipitations could increase the strength of the AZ91 alloy. Mayer et al. [17] presented the strength limit, stress intensity of Mg and Al alloys and showed that the pores caused the cavities, blowholes, and microdeformations during heat-treating. Li et al. [18] assessed the fracture surface of AZ91D-T6. They observed that the crack growth area consisted mainly of fine facets and serrated fatigue striations inside the grains. Moreover, very small amounts of undissolved and cracked β phase particles (Mg17Al12) were observed on the fracture surface. In one research, Mokhtarishirazabad et al. [19] investigated the effect of rare earth elements (RE) on the HCF behavior of the AZ91 alloy. They found that the addition of 1% RE improved the fatigue crack propagation resistance and fracture toughness of the AZ91 alloy, significantly. Moreover, they claimed that the addition of 1% RE improved the strength limit of (none-heat-treated) AZ91 as much as A319-T6 and A356-T7 aluminum (cylinder head) alloys, at the same fatigue lifetime range (104–106 cycles). Rezanezhad et al. [20] studied the effect of heat-treating on HCF and fracture behaviors of aluminum-silicon alloys. The HCF lifetime improved by heat-treating, significantly under both the highest and lowest stress levels. Azadi et al. [21] investigated the thermomechanical fatigue behaviors of the AZE911 magnesium alloy, considering T6 heat-treating. They found an effective difference in the lifetime of the AZE911 alloy when the heat treatment was applied. Lee et al. [22] analyzed the effects of Y, Sr, and Nd additives and heat treatment on the hardness and microfracture mechanism of the AZ91 alloy. They found that the in-situ fracture of an AZ91-Sr-T6 alloy, coarse needle composed particles, and the Mg17Al12 phase at the grain boundary provided an easy intergranular fracture site under low-stress intensity factor levels, thereby reducing toughness. Moreover, in AZ91-Y-T6 and AZ91-Nd-T6 alloys, the fracture toughness improved. Khisheh et al. [23] analyzed the effects of the roughness and heat-treating on HCF behaviors of A380. Through cyclic loading, they claimed that, under the highest and lowest stress levels, heat-treating improved the HCF lifetime by 26% and 85%, respectively. Azadi et al. [24] depicted the effects of heat-treating on the microstructure of cylinder head material. They claimed that heat-treating could affect the microstructure and led to the uniform distribution for silicon and intermetallic phases.As a result of reviewing the literature, researchers have tested many approaches to enhance the mechanical and fatigue properties of magnesium alloy cylinders and cylinder heads such as different types of heat treatment [7–12,14,16–18,20–23], FSP [13], the addition of rare earth elements [4,6,19–23], the addition of nanoparticles [10,20,24], and so on. In addition, investigation of the HCF properties of heat-treated AZ91 and AZE911 alloys is still rare. Therefore, the use of heat treatment to improve the HCF behavior in these magnesium alloys could be a novelty of this research.


## 2. Tests and Materials

The studied materials included AZ91 and AZE911 (AZ91 + 1%RE) magnesium alloys. To improve the material properties of AZ91, RE elements were added to the melt. Therefore, the composition of the Ce-rich MM (Misch Metal) plus AZ91 was made by melting in the steel crucible. This job was done under the controlled environment of CO2 + 1%HFC- (Hydrofluorocarbon-) 134a at the temperature of 750 ± 5°C for 30 minutes, in the furnace. Moreover, RE elements included 67% Ce, 23% La, 7% Pr, and 2% Nd. To have a uniform distribution, the melt was stirred for 1 minute, by the mild steel rod. Then, the casting process was carried out in the preheated bottom-filled steel die, whose temperature was around 200°C. Finally, Table 1 illustrates the chemical composition of studied magnesium alloys.Table 1
Alloy chemical compositions (wt. %) in this research.
AlloyAlZnMnCeLaMgAZ919.430.930.19——BalanceAZE9118.840.810.140.640.33BalanceBased on the objective of metallographic studies, samples were etched in an acetic glycol solution. The etchant included 60 ml ethylene glycol, 20 ml acetic acid, 20 ml water, and 1 ml HNO3.After casting, the standard samples of fatigue testing were machined based on the ISO-1143 standard, as depicted in Figure1.Figure 1
(a) The equipment for fatigue testing and (b) the geometry of fatigue specimens.
(a)(b)Before the HCF tests, the machined cylindrical-shaped specimens were solution-treated at 415°C for 5 hours, and then the AZ91 and AZE911 alloys aged at 215°C for 3 hours and 215°C for 5 hours, respectively. The samples were solutionized and aged in an ATE1100L horizontal tubular furnace from the ATRA Company, under the CO2 gas-controlled atmosphere to prevent the alloys from surface oxidation. The hardness test was performed by the Brinell method (using the universal hardness tester, model UV1 from KOOPA Company) according to the ASTM-E10-14 standard with a steel ball indenter, whose diameter is 10 mm under a force of 1500 kg, repeated up to four times. HCF tests were conducted under four or five stress levels for AZ91-T6 and AZE911-T6 alloys with a STF-600 2-point rotational bending fatigue tester from the SANTAM Company, under fully reversed cyclic bending loads with the stress ratio of −1 and through 100 Hz of the loading frequency. The endurance limit of the sample was considered as 106 cycles or until the full fracture of the specimen.To evaluate the failure mechanisms for the fatigue crack, including both initiation and propagation, scanning electron microscopy (SEM), model VEGA3 from the TESCAN Company, fractography was used on the fracture surface of specimens, with aided energy-dispersive X-ray spectroscopy (EDS) to find elements. The changes of phases and precipitations in the microstructures of AZ91 and AZE911 alloys were observed before and after heat treatment by optical microscopy (OM). The microstructures of the samples have been observed in the optical microscopy, HM-LUX model from the Leitz Company.In order to have a summarized information on all testing, samples, and studied materials, Table2 is presented.Table 2
Summarized information for testing and samples in this research.
MaterialsNumber of samplesStress amplitude (MPa)AZ911164.6, 74.5, 84.7, 94.6, and 104.5AZE91111104.6, 114.5, 124.7, 134.6, and 144.4AZ91-T613114.5, 124.5, 134.6, and 144.7AZE911-T612155.3, 165.5, 176.0, and 185.7


## 3. Results and Discussion

Figure2 shows the effects of heat-treating on the microstructure of AZ91 and AZE911 alloys. The alloy microstructure consisted of the α-Mg solid solution (matrix or alpha phase), the grain boundary, the interdendritic eutectic β phase (Mg17Al12), and the needle-shaped precipitates due to the transformation of β (Mg17Al12) precipitates during the ageing process. Heat-treating led to two distinct influences on the microstructure of AZ91 and AZE911 alloys. The first effect was the transformation of continuous and discontinuous β phase (Mg17Al12) precipitations at the grain boundary to needle-shaped precipitations on α-Mg phase grains, which was the most effective factor in increasing the hardness. The second effect was the reduction of grain size from 300 μm to 130 μm for the AZ91 alloy and from 230 μm to 90 μm for the AZE911 alloy (using the ImageJ software), which caused a significant increase in the number of grain boundaries as well as the number of grains, which was the main reason for increasing the strength limit. Therefore, logical agreements could be seen compared to the literature for these microstructural changes by heat-treating [8,18,25,26].Figure 2
The microstructure of (a) AZ91, (b) AZE911, (c) AZ91-T6, and (d) AZE911-T6 alloys.
(a)(b)(c)(d)Bag and Zhou [8] examined optical micrographs of the cast, dissolved, and aged microstructures. In all casting, dissolved, and aged states, the β phase (Mg17Al12) can be seen at the grain boundary and the eutectic α phase is adjacent to the β phase. Zhang et al. [9] demonstrated that the discontinuous precipitation of the β phase (Mg17Al12) occurs at the ageing temperature of 200°C, where a layered structure is perpendicular to the base plane of the matrix, is nucleated at the grain boundary, and grew into the grain. Based on linear intercept, the average grain size of the AZ91-T6 alloy was about 135 μm (for AZ91-T6, no significant difference was observed in mean grain size and grain size distribution). Figure 3 shows the effect of heat treatment on the hardness of AZ91 and AZE911 alloys. Ageing at 215°C for 3 hours increased the hardness of the AZ91 alloy from 67 to 108 Vickers (60% improvement) and ageing at 215°C for 5 h increased the hardness of AZE911 from 76 to 115 Vickers (50% improvement). Figure 4(a) shows all the HCF experimental data and compares the stress levels and fatigue cycles until failures (including S-N curves) for AZ91 and AZE911, before and after heat-treating. As shown in Figure 4, AZ91-T6 and AZE911-T6 alloys showed a premature failure under high-stress levels. Moreover, fracture assessments demonstrated that large porosities were responsible and effective for such a behavior. Figure 4(b) shows the average HCF lifetimes with the corresponding trend lines. It can be seen from Figure 4(b) that heat-treating had two prominent influences on the stress-lifetime curve. The first influence was to increase the fatigue strength of the material at each stress level and the second one was the fatigue behavior of AZ91-T6 and AZE911-T6, which was affected by two mentioned factors. The first factor was the grain size and the pore distribution, and the second factor was the size and precipitations of the intermetallic Mg17Al12 phase.Figure 3
The effect of heat-treating on the hardness of AZ91 and AZE911 alloys.Figure 4
(a) All data, (b) averaged values, after and before heat treatment, and (c) the HCF behavior of AZ91-T6 in this research versus other research [7,9,16–18,20], (d) the HCF behavior of AZE911-T6 versus AZ91 containing rare earth Ce [27], Nd [4], MM [28], and RE, (e) the HCF behavior of AZ91-T6 versus A319-T6 [29], and (f) the HCF behavior of AZE911-T6 versus A356-T6 [25] and A380-T6 [26].
(a)(b)(c)(d)(e)(f)Enhancing the density of the grain boundary and distributing the tensile stress through more grains (in other words, lower stress concentrations per grain) results in a reduction of the grain size in the alloys. In addition, the propagation of fatigue cracks generally occurred along with the grain boundary or along with theβ phase in magnesium alloys. Thus, reducing the amount of interdendritic β particles had a significant influence on the propagation of fatigue cracks in the case of intergranular fracture [19]. Experimental data showed that, for example, in 3.8 × 105 cycles, the fatigue strength of the AZ91 alloy increases from 95 to 125 MPa (31% improvement) and fatigue strength of the AZE911 alloy from 125 to 155 MPa (24% improvement), after heat treatment. The feasibility of the replacement of the AZ91 alloy with the AZE911 alloy for the cylinder head in automotive engines by adding 1% RE was discussed [19]. Now, it is shown that the mechanical properties and strength limit of the AZ91-T6 alloy are the same as the AZE911 alloy. This means that heat treatment has approximately the same effect as the addition of 1% MM, 1% Ce, and 1% Nd on the HCF behavior of AZ91. Figure 4(c) shows the rotational bending fatigue curves of the AZ91-T6 alloy in this and other studies. In Figure 4(c), some researchers reported the fatigue strength of heat-treated AZ91 in the range of 104–106 cycles at 130–190 MPa [7,10,20], while some others reported it at 60–120 MPa [9,16–18]. Xu et al. [28] claimed that 1% MM addition has drastically reduced the grain size of beta phase (Mg17Al12) to 66 μm and alpha phase to 16 μm and reduced the size of dendritic cells, which has increased the fatigue limit by reducing the number of casting defects. Hassan and Lewandowski [10] reported similar results for grain size reduction after heat-treating of the AZ91-T6 alloy. Beside the grain size reduction, Dimitrov and Shtarbakov [16] claimed that heat treatment caused dissolving of precipitations on the α-Mg phase in terms of improving the fatigue strength of the AZ91D alloy. There are reasonable agreements in the literature to change grain size after heat treatment [8,9,18,30–33]. From Figure 4(d), the fatigue strengths of AZ91 + X% Nd, AZ91 + X% Ce, AZ91 + 1% MM, and AZE911 alloys can be seen in the 104–107 cycles range. Figure 4(d) shows that the strength limit of AZ91 + 1% MM and AZ91 + X% Nd alloys in the 104–107 cycle is 70–90 MPa, the strength limit of the AZ91 + X% Ce alloy in the 104–107 cycle is 100–120 MPa, and finally, the strength of the AZE911 alloy in the 104–106 cycle is 105–145 MPa, which increased to 155–185 MPa after heat treatment. It means that the strength limit of heat-treated AZ91 containing rare earth elements (AZE911-T6) alloys is much higher than the none-heat-treated (AZ91 + X% Ce, AZ91 + X% Nd and AZ91 + 1% MM and AZ91 + 1%RE) alloys. In this research, the feasibility of using AZ91-T6 instead of A319-T6 and the replacement of AZE911-T6 instead of A356-T6 and A380-T6 aluminum alloys for the application in the cylinder head of diesel engines were discussed. Although the mechanical properties and the HCF strength of the AZE911-T6 alloy were lower than A356-T6 and A380-T6, it was still in the range of required data for such an application, and similarly, the same claim is true for replacing the AZ91-T6 alloy instead of A319-T6. In Figure 4(e), the HCF behavior of the AZ91-T6 magnesium alloy is compared to the A319-T6 aluminum alloy, and in Figure 4(f), the HCF behavior of the AZE911-T6 magnesium alloy is compared to A356-T6 and A380-T6 aluminum alloys (the most commonly used aluminum alloys in the automotive industry) in S-N curve. From Figure 4(f), the AZE911-T6 fatigue strength was very close to A356-T6 and A380-T6 alloys and as shown in Figure 4(e), the fatigue strength is very close for AZ91-T6 and A319-T6 in the range of 104–106 cycles. On one hand, in the same cycle range, the difference between the fatigue strength of AZE911-T6 and both A356-T6 and A380-T6 alloys was much less than the difference in the HCF strength of AZE911, A356-T6, and A380-T6. On the other hand, the difference in the fatigue strength of AZ91-T6 and A319-T6 was less than the difference in the fatigue strength of AZ91 and A319-T6. This means that, in terms of the fatigue strength, A356-T6 and A380-T6 aluminum alloys can be replaced with AZE911-T6, and the A319-T6 aluminum alloy can be substituted by AZ91-T6.With this replacement, the weight of the cylinder head in diesel engines could be decreased by up to 50% [19]. Moreover, several factors must be considered before performing the replacement approach on the cylinder head. These factors are mechanical properties under high-stress levels, the fatigue properties at high-stress levels, the allowable plastic strain, the corrosion strength, etc., through designing the component. For example, by the energy-based technique, Azadi et al. [24] estimated the fatigue lifetime of the cylinder head of diesel engines. They demonstrated that the fatigue lifetime was almost similar when A356-T6 was substituted with AZ61A [19].Figure4(b) shows the logarithmic scale of the maximum stress versus the strength limit. By extrapolating the curve equation from different data and a comparison of this formulation with the Basquin formulation, it is as follows [19]:(1)σa=σf′2Nfb,where σa is stress amplitude, Nf is fatigue lifetime, σf′ is the fatigue strength coefficient, and b stands for the fatigue strength exponent, and the fatigue strength of alloys can be predicted in high fatigue cycles. As a note, the Basquin formulation can only be used for several cycles of less than 106 cycles [19]. Therefore, the relationship in the S-N curve, on a logarithmic scale, for alloys including AZ91, AZ91-T6, AZE911, and AZE911-T6 can be calculated, which is reported in Table 3.Table 3
Fatigue properties of studied cylinder head magnesium alloys.
AlloysUsing averaged valuesσf′ (MPa)bAZ91474.52−0.137AZ91-T6263.83−0.060AZE911568.12−0.124AZE911-T6306.1−0.053The fatigue strength of both AZ91 and AZE911 alloys in 3.8 × 105 cycles was equal to 95 and 125 MPa, respectively. Therefore, heat-treating on AZ91 and AZE911 resulted in a 31% (125 MPa) and 24% (155 MPa) increase in AZ91-T6 and AZE911-T6 alloys in 3.8 × 105 cycles, respectively.As known, the growth of microcracks depends on cyclic plasticity, accumulation (stacking) of the fault energy, slip barriers such as grain boundaries, precipitates, secondary phases, and solute atoms that can create a threshold value for the growth of cracks [19]. Fen et al. [30] agreed with Yan and Wang [34] that reducing accumulated (stacking) fault is one of the probable reasons to have the proper fatigue strength of AZ91 alloys during the heat treatment. The other factors that affect the fatigue behavior of these alloys are porosities and inclusions. The heat treatment can refine the grain size by three reasons. First, the heat treatment usually causes more protrusion, making stand out and increase in the amount/number of grain boundaries. Increasing the amount/number of grain boundaries means the presence of more grains and the presence of more grains means the presence of smaller grains [31,35]. Second is increasing the microhardness of both Beta (Mg17Al12) and Alpha phases (Mg matrix). Third is dissolving the precipitations of Beta (Mg17Al12) phase in the Mg matrix grains [32]. As an important result, refining or decreasing the grain size and increase in the amount/number of grain boundaries are the main reasons to decreasing the fatigue cracks initiation from porosities and inclusions [33].Moreover, AZ91 is the most susceptible Mg-Al alloy for shrinkage (contraction) porosity. Thus, enough fatigue crack nucleation sites could be seen for this alloy. In addition, the heat treatment did not have a significant effect on the amount and distribution of inclusions and porosities of AZ91 and AZ91 + X% + rare earth alloys, only better distribution and location of precipitations, which resulted in the reduction of fatigue cracking, the crack growth rate, from inclusions and porosities area (as fatigue crack nucleation sites) [34,36,37]. Therefore, any scientific report on the influence of heat-treating on the number of porosities and inclusions in AZ91 and AZ91 + X% + rare earth alloys and its influence on the fatigue lifetime requires a large statistical work. Therefore, commenting on this factor is not reliable. Moreover, the reduction of the grain size and the formation of new needle-shaped precipitations within the gains of alpha phase in the microstructure of AZ91 and AZE911 alloys, which leads to a decrease in the fatigue crack growth, is responsible for increasing the fatigue strength of AZ91 and AZE911, after heat-treating.Based on the microscopic evaluation on the fracture surface of samples, three distinct regions occur through the fatigue fracture, where there is a nucleus or crack initiation (region 1), crack propagation (region 2), and the final fracture (region 3), as shown in Figure5(a) for the AZ91-T6 alloy and in Figure 5(b) for the AZE911-T6 alloy. In these figures, the arrows indicate the direction of crack propagation. In almost all specimens, fatigue cracks initiate beneath the surface of the material, according to cast defects. Macroscopic images of the fatigue fracture surface of AZ91-T6 and AZE911-T6 alloys show two distinct fracture regions. It should be noted that Figures 5(a) and 5(c) show the surface fracture of the AZ91-T6 alloy with a stress level of 125 MPa and a fatigue lifetime of 3.8 × 105 cycles and in Figures 5(b) and 5(d), the surface fracture of the AZE911-T6 alloy is shown with a stress level of 175 MPa and a fatigue lifetime of 5 × 105 cycles. Fatigue surface fracture of AZ91-T6 and AZE911-T6 alloys has a completely brittle appearance. The presence of cleavages and quasicleavages, the more microcracks than the cracks, perpendicular to the direction of crack growth path to the direction of stress levels or force applied and silvery surface fracture caused a brittle fracture. Figures 5(c) and 5(d) show the cast defects area in the fracture surface of AZ91-T6 and AZE911-T6 alloys, respectively.Figure 5
The fracture surface analysis of (a) AZ91-T6 and (b) AZE911-T6 and cast defects area in (c) AZ91-T6 and (d) AZE911-T6.
(a)(b)(c)(d)In Figure6, further magnification of the fatigue crack area shows more structural details in region 2. As can be seen, the difference between this region for the AZE911-T6 alloy and the same region for AZ91-T6 is the presence of fine fatigue striations on the cleavages, and the presence of these striations can be observed in Figures 6(a) and 6(b).Figure 6
The morphology of tear-ridges and striations in the propagation region of fatigue cracks (region 2) of (a) AZ91-T6 and (b) AZE911-T6, cleavage and quasicleavage in the fracture region (region 3) of (c) AZ91-T6 and (d) AZE911-T6, and cracks and microcracks in the surface fracture (e) AZ91-T6 and (f) AZE911-T6.
(a)(b)(c)(d)(e)(f)Increasing the percentage of region 2 reduces the growth rate of cracks. Because none of the samples of the AZE911-T6 alloy failed at the stress level tested for the AZ91-T6 alloy, it was not possible to compare the failure levels of the samples tested at the same stress level. In fact, the step-by-step growth of cracks in the area of fatigue crack growth has led to the formation of bands or, in other words, parallel striations due to the growth of fatigue cracks in the preferred direction in each grain (depending on the crystalline orientation). The fatigue steps of AZ91-T6 and AZE911-T6 are in the propagation zone of fatigue cracks (region 2). It is observed that the fatigue striations of both alloys become rough, although they were smooth and flat [19,38,39]. In addition, the size of the fatigue steps of AZE911-T6 becomes finer and the striations have a more irregular pattern than the AZ91-T6 alloy. No significant differences in the morphology of the collapse fracture (region 3 in Figures 6(c) and 6(d)) of AZ91-T6 and AZE911-T6 were observed in this study. In other words, the prominent effect of heat-treating is related to the area of the propagation of fatigue cracks and has a negligible influence on the fracture morphology of the fatigue striations fracture zone [7],[18],[40–43], Figure 6(c) shows a brittle fracture due to the presence of fine fatigue striations with faceted surfaces, and Figure 6(f) shows serrated fatigue striations propagation patterns.It can be seen from the SEM analysis that AZ91-T6 showed a more brittle behavior than AZE911-T6 due to the following reasons:(i)
The size of cleavage marks in AZ91-T6 is much larger than that in AZE911-T6.(ii)
The values of quasicleavages in AZ91-T6 are much less than that in AZE911-T6.(iii)
The number of fatigue striations in AZ91-T6 is more than that in AZE911-T6.The fracture of samples of the AZE911-T6 alloy at high-stress levels can be attributed to the presence of continuous precipitations around the grains and a large number of grain boundaries (due to the decreasing grain size), which surrounded the grains like an outer wall and lead to deviation of the fatigue crack to some shorter cracks and even have a barrier role to the growth of fatigue cracks or microcracks, which results in a reduction of fatigue growth rate and long fatigue lifetime (at high-stress levels).As the last point, for the further works, the transmission electron microcopy (TEM) could be used for a better understanding of microstructural changes in the material, after the heat treatment.


## 4. Conclusions

In the present article, HCF properties in AZ91 and AZE911 (cylinder head) magnesium alloys with and without heat-treating were demonstrated. Then, obtained experimental data could be listed as follows:(i)
The hardness of the AZ91 alloy increased by 61% (67 to 108 Vickers) after ageing at 215°C for 3 hours and the hardness of AZE911 increased by 51% at 215°C for 5 hours (76 to 115 Vickers).(ii)
Optical microscopy (OM) examination showed that ageing treatments transform the continuous and discontinuous precipitations into needle-shaped precipitations.(iii)
Significant reduction occurs in grain size from 300μm to 130 μm for the AZ91 alloy and from 230 μm to 90 μm for the AZE911 alloy, after the heat treatment.(iv)
Scanning electron microscopy (SEM) showed brittle fracture behavior due to the presence of the cleavage and quasicleavage patterns on the specimen fracture surfaces. Besides, cleavage planes were smaller in heat-treated samples, which had influences on the fatigue strength and the lifetime of the alloy. However, the AZ91-T6 alloy shows a more brittle behavior than the AZE911-T6 alloy.(v)
Heat-treating significantly improved the strength limit (stress levels). Curves of stress-lifetime (S-N curves) show an enhancement in the fatigue strength at 3.8 × 105 cycles, from 95 MPa to 125 MPa for the AZ91-T6 alloy and from 125 MPa to 155 MPa for the AZE911-T6 alloy.



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*Source: 4030062-2022-03-03.xml*