Effect of Al on the Microstructure and Mechanical Properties of Mg-6Zn Alloy

In this work, the effect of trace Al on the microstructure and mechanical properties of Mg-6Zn alloy was studied. The results show that the as-cast ZM60 alloy consists of α-Mg phase and Mg51Zn20 phase. After the addition of Al, fine Al8Mn5 phase increases in the microstructure of the experimental alloy, and the number of the second phase increases. Dynamic recrystallization occurred in all the experimental alloys during extrusion deformation, the grain size of Al alloy is significantly smaller than that of ZM60 alloy, and the average grain size of ZAM610 alloy is the smallest, which is 5.6µm. ZAM610 alloy has the best mechanical properties, with tensile strength, yield strength, elongation after fracture, U-notch impact work and non-notch impact work of 302.7MPa, 216MPa, 19.7%, 10.0 J/cm2 and 56.0 J/cm2, respectively. The fracture of the impact sample shows that there are many tearing edges in the ZM60 alloy, which shows the characteristics of quasi-cleavage fracture. The ZAM600 and ZAM610 alloys have a large number of dimples, which are characterized by dimple fracture. The improved properties of magnesium alloys after microalloying can be attributed to the formation of fine Al8Mn5 phase, which has good Zener blocking effect, refines the dynamic recrystallized grains, and improves the strength and impact toughness of the alloy.


Introduction
Magnesium alloy has several advantages, including a low density, high specific strength, and ease of recycling, etc.It has a broad range of applications in aerospace, railway transportation, automobile industry and other fields due to its good energy-saving and emission reduction effect [1].In addition to having the requirement of strength for new generation of metal structure materials, magnesium alloys also have the aspect of impact toughness as an important performance index, which directly affects the use of the material performance as well as structural safety and reliability.Mg-Zn alloys are one of the main research directions for high strength and high toughness magnesium alloys, such as ZK60 alloy, which has tensile strength in excess of 300MPa, yield strength exceeding 250MPa, and elongation of more than 10%.Since Zr in ZK60 alloy is prone to burn out in the melting process, and the potential difference between the ZR-rich phase and the α-Mg matrix is larger than that between other second phases and matrix, the corrosion resistance of magnesium alloy is reduced [2], and therefore the application of ZK60 alloy is greatly limited.
As an replacement for Zr, Xin Guangshan et al [3] developed a new high-strength Mg-Zn-Mn alloy with high zinc content, namely ZM61 alloy, on the basis of ZM21.Mg-xZn-1Mn (x=4, 5,6,7,8,9) alloys were also examined in Chongqing University [4,5].By adding 1-3% Al to ZM61 alloy, S. S. Park et al. found that the addition of Al element could refine the microstructure of the alloy and lead to changes in the types of dispersion particles [6], however, when a large amount of Al was added to ZM61 alloy, the coarse Mg21(Al, Zn)17 phase is formed in the microstructure of ZM61-3%Al alloy, which greatly reduces the toughness of the alloy.
Squeeze casting is a process in which pressure is applied in the solidification process of molten metal to make it crystallize and solidify under high pressure, so as to obtain castings with fine, dense structure and no internal shrinkage [7].Magnesium alloys can benefit from squeeze casting by improving their mechanical properties.For example, squeeze casting the Mg-6Zn-0.1Ca-0.5Mnalloy under 95MPa pressure and T6 heat treatment resulted in a tensile strength of 300 MPa, a yield strength of 180 MPa, and an elongation of 10%, which were 122%, 62% and 32% higher than gravity casting respectively [8].Since the Hall-Petch constant of magnesium is 280MPa•m 1/2 , which is more than 4 times that of aluminum (68 MPa•m 1/2 ) [9], fine crystal has a very significant strengthening effect on Mg-Zn alloy.In this paper, the evolution of microstructure and mechanical properties of Mg-6Zn alloy microalloyed with different contents of Al element during squeeze casting and extrusion deformation were examined to explore the mechanism of high strength and high toughness of magnesium alloy, so as to provide a technical solution for producing highly efficient magnesium alloys at a reasonable cost.

Alloy preparation
Mg-6Zn-0.5Mnalloy is used as the matrix alloy, three kinds of experimental alloys with Al content 0, 0.5% and 1.0% were prepared by using high purity Mg, Al, Zn and Mg-10% Mn as matrix alloy.Firstly, pure Mg, pure Al and pure Zn were melted in the crucible of resistance furnace under the mixed gas atmosphere of CO2 and SF6 (mix ratio: 100:1), and then Mg-10% Mn was added.When the temperature rose to 730℃, Ar2 was used for refining, and the mixture was kept for 30min before pouring into the squeeze casting mold.The pressure of squeeze casting was held 30s at 100 MPa.The mold temperature was 250℃ and the size of cylindrical casting ingots was Φ150 mm × 100 mm.The chemical composition of the alloys is summarized in table 1.The ingots were homogenized at 335 °C for 8h, and then extruded at 360 °C with the extrusion ratio of 16:1 and an extrusion rate of 1.0mm/s.

Experimental method
All samples were cut from the billets along the extrusion direction, cleaned and polished.The samples for microstructure observation were etched in a solution of 10ml deionized water, 6g picric acid, 5ml acetic acid and 100ml of ethanol.Thin foil specimens for the transmission electron microscopy (TEM) observation were prepared using a standard mechanical grounding followed by the ion-beam milling techniques.The TEM observation was carried out at 5.0kV.Through inductively coupled plasma (ICAP6300) analysis, the chemical composition of the tested alloys was determined according to the standard GB/T 13748.20-2009.Optical microscopy (OM, Olympus) was used to observe the microstructure of the extruded alloys.Scanning electron microscopy (SEM, Quanta Feg 250) and transmission electron microscopy (TEM, FEI 5022/22 Tecnai G2 20 S-TWIN) were used to observe the refined microstructure and the distribution of second phases in detail.The second phases in the as-cast alloys were characterized by a x-ray diffractometer (XRD, Rigaku D/max-2500/PC).Tensile and impact tests were performed at ambient temperature following the ASTM standard A370-03A and GB/ T 229-2020 at a crosshead speed of 1mm min -1 .Cylindrical tensile samples of 36mm in gauge length and 6mm in gauge diameter were machined from as-extruded bars.Impact samples of 55mm in gauge length and 5mm in gauge square were machined from as-extruded bars.

Microstructure of as-cast alloy
Figure 1 shows the X-ray diffraction curves (XRD) of the three experimental alloys as cast.The microstructure of ZM60, ZAM600 and ZAM610 alloys is composed of α-Mg phase and Mg51Zn20 phase.

Figure 1. The XRD curvers of the as-cast alloys
In order to understand the morphology of the second phase precipitated from the experimental alloy after microalloying, the as-cast microstructure of the experimental alloy was further observed by scanning electron microscope.Figure 2 shows the as-cast SEM photo of the experimental alloy.
A small amount of the second phase is present in the matrix ZM60 and is mainly distributed at the grain boundary.Since the Zn content in the alloy is close to the maximum solid solubility, the second phase exhibits a typical dissociated eutectic structure.EDS analysis shows that the second phase in the alloy is composed of Mg and Zn elements, and combined with relevant data [10], the second phase is determined to be Mg51Zn20 phase.With the addition of Al, the Al8Mn5 phase is formed in the matrix, resulting in bright color and fine granularity.In the ZAM600 alloy, the Al8Mn5 phase is less, while the Al8Mn5 phase is more in the ZAM610 alloy.SEM images show that the eutectic morphology of the alloy does not change significantly after Al addition, but the amount of the second phase in the alloy increases and the distribution is more uniform.The figure shows that all the experimental alloys underwent complete dynamic recrystallization.After extrusion deformation, the matrix ZM60 alloy only has a few small particles, the size is less than 10μm, which is the undissolved second phase particles broken in the extrusion deformation.With the addition of Al element, the number of undissolved second phase particles in the alloy increased, but the size of the broken particles did not change significantly.The metallographic pictures show that all the experimental alloys undergo dynamic recrystallization after extrusion deformation.With the increase of Al content, the recrystallized grain size of the experimental alloy decreases obviously, and the average grain size of ZM60, ZAM600 and ZAM610 are 20μm, 7.2μm and 5.6μm, respectively.
Figure 4 shows TEM images of the experimental alloy after extrusion.As shown in the figure, there is almost no micrometer or submicrometer particle phase in the ZM60 alloy.However, some submicron particles appear in ZAM600 and ZAM610 alloys, which are more abundant in ZAM610 alloy than ZAM600 alloy.
Figure 5 shows EDS analysis of Al8Mn5 phase in ZAM600 and ZAM610 alloys.As can be seen from the figure, Al8Mn5 phase by SEAD, these sub-micron particle phases can be identified as Al8Mn5 phase.

Tensile properties
Table 2 shows the tensile mechanical properties of ZM60, ZAM600 and ZAM610 alloy after extrusion deformation at ambient temperature.The data in the table shows that the tensile properties of matrix ZM60 alloy are low.When a small amount of Al is added, the strength and elongation of the alloy have obviously increased.Compared with ZM60 alloy, the tensile strength, yield strength and elongation after fracture of ZAM600 alloy are increased by 39.6MPa, 35.7MPa and 4.4% respectively.With the increase of Al content, the strength and toughness of the alloy are further improved, and ZAM610 alloy has the best strength and toughness.The results show that the Al content of ZM60 alloy is positively proportional to the strength and plasticity of the alloy when a trace amount of Al (≤1%) is added.

Impact performance and fracture morphology
The ambient temperature impact properties of ZM60, ZAM600 and ZAM610 alloys after extrusion deformation were tested, and the results are shown in table 3. The fracture section area of the U-notch sample is 0.15 cm 2 , and that of the non-notch sample is 0.25 cm 2 .Note: N is the notch sensitivity coefficient, which is the ratio of the impact toughness of the unnotched sample and the U-notched sample.
As can be seen from the data in the table, the impact absorption energy of the matrix alloy at room temperature is the lowest.After adding Al, the impact absorption energy of the alloy is increased, and the impact absorption energy of the non-notched sample of ZAM610 alloy is the highest.It can be seen from the change of impact data of non-notched samples that the Al content is obviously proportional to the impact absorption energy of the alloy.The change of notch sensitivity coefficient shows that the notch sensitivity of alloy increases with the increase of Al content.
Figure 6 shows the macro morphology photos of the fracture specimens of ZM60, ZAM600 and ZAM610 alloy in the extrusion state.As can be seen from the figure, the U-notch samples of all the experimental alloys have flat fractures, small degree of deformation, no fiber area and shear lip on the surface, and almost all of them are radioactive areas.The fracture of the non-notched sample is uneven, and the deformation of the sample is obvious along the impact direction.The higher the Al content in the alloy, the greater the longitudinal bending degree of the sample after impact.The relationship between fracture morphology and Al content is not obvious.
Compared with non-notched specimens, U-notched specimens show obvious stress concentration in the impact test, but the fracture modes of both specimens are the same.Therefore, this paper focuses on the comparative study of the fracture microstructure of non-notched specimens.Figure 7 shows SEM images of fracture morphology at the center of unnotched ZM60, ZAM600 and ZAM610 alloys.It could be seen from the figure that the cleavage surface of ZM60 alloy was composed of a large number of cleavage planes and tearing edges, belonging to cleavage fracture.The fracture surface of ZAM600 and ZAM610 alloys is composed of a large number of dimples, which is a ductile fracture.The dimples of ZAM610 alloy are deeper than those of ZAM600 alloy.In addition, there are a large number of fractured granular phases in the fracture dimples of Al alloy.

Effects of trace elements and squeeze casting on as-cast microstructure of alloy
Figure 8 shows the phase diagram of MG-Zn-Mn-Al alloy with 6% Zn content and 1% Mn content calculated by the FactSage [5] .According to the Mg-Zn binary phase diagram, eutectic reaction occurs at 613K(340°C), the liquid phase solidities and forms α-Mg and primary eutectic compounds [11,12].It can be seen from the phase diagram that α-Mg, MgZn and Al8Mn5 phases are precipitated during solidification when 0.5%-1.0%Al is added.it has been suggested [6] that since the diffusion of Zn in Mg matrix (1.22×10 -11 cm 2 /s at 310℃) and the solid solubility of Zn are much higher than that of Mn, after adding Al to Mg-Zn alloy, the size of Al8Mn5 phase formed by adding Al to Mg-Zn alloy is much smaller than that of MgZn phase, which is consistent with the scale of Al8Mn5 phase and MgZn phase observed in the experiment.The squeeze casting has good microstructure refinement effect [7].In common cases, due to the solidification and shrinkage of the metal and coating on the inner wall of the mold, there is a gap exists between the casting and the inner wall of the mold [13,14].This gap cannot be eliminated during the gravity casting, resulting in low heat conduction efficiency between the casting and the mold, and a low cooling rate of the casting.Squeeze casting can eliminate this gap, make the casting and mold wall completely contact, improve the heat conduction efficiency, the cooling rate of the casting rapidly increased.At this time, the diffusion of solute is inhibited [15], and the amount of solute discharged to the liquid phase during metal solidification is reduced by diffusion back into the solid phase, resulting in the increase of the concentration of solute in the liquid metal and the final increase of the number of the second phase.However, the fine Al8Mn5 phase in the alloy provides heterogeneous nucleation, increases the number of nucleation, and finally refines the as-cast microstructure of the alloy containing Al.Since ZAM610 alloy contains a higher amount of Al ZAM600 alloy, more Al8Mn5 phase can be formed, resulting in more secondary phases precipitating from ZAM610 alloy than ZAM600 alloy.

Effects of trace elements on extrusion microstructure of alloy
It can be seen from Figure 3 that there are broken particle phases in the experimental alloys after extrusion deformation, distributed along the extrusion direction, where the number of particle phases in the Al-containing alloys is more than that in the ZM60 alloy without Al.According to the results of EDS analysis, these particle phases are the undissolved MgZn/MgZnAl phases in the experimental alloys.In the process of extrusion deformation, these granular phases hinder dislocation movement and cause dislocation jam near the granular phase, resulting in the area around the particle phase is suitable for dynamic recrystallization grain nucleation [16].Figure 9 shows an enlarged extruded microstructure of ZAM610 alloy.The figure shows that there are more second phase particles on the recrystallized grain boundaries, while there are fewer particles on the recrystallized grain boundaries of the ZM60 alloy, which indicates that the undissolved particles promote the grain refinement of the alloy.

Figure 9. Optical images of the as-extruded alloys
As can be seen from Figure 8, Al8Mn5 phase has a high melting point and is difficult to dissolve into the matrix after homogenization treatment at 335℃.During the extrusion process, the Al8Mn5 phase can pin the sliding of grain boundaries, which is conducive to grain refinement.In addition, the residual undissolved second phase in the alloy after homogenization treatment is broken under extrusion pressure to form fine particles.The existence of these particles provides a nucleation position for the dynamic recrystallization of the alloy, which is conducive to grain refinement.According to Zener blocking theory [17], fine precipitates have a strong Zener blocking (Zener pinning) effect, that is, limiting grain boundary migration will prevent grain growth.For a certain size of grain, the grain growth will stop when the grain growth driving force is balanced with the grain induced pinning force.The grain diameter (Dz) of Zener pinning can be expressed as follows: Dz=4r /3f (1) Where f and r are the volume fraction and average radius of pinned particles.
The dynamic recrystallization process of the alloy is divided into nucleation process and grain growth.When the grain grows, Zener pinning plays an important role when the grain diameter D > Dz, but it does not play role when the grain diameter D < Dz.The extruded ZAM610 and ZAM600 alloys have MgZn phase and Al8Mn5 phase, while the ZM60 alloy has only MgZn phase.Due to the small size of Al8Mn5 phase, it belongs to micron or submicron particles.The ZAM610 alloy has a good Zener blocking effect, that is, it has the strongest pinning effect on the recrystallized grain growth, which leads to the smallest dynamic recrystallized grain size.

Effects of trace elements on the strength of alloys
Fine grain strengthening is an effective strengthening method for magnesium alloys.According to the Hall-Petch relationship, the finer the grain size, the better the toughening effect.The Hall-Petch formula is: σ=σ0+Kd -1/2 (2) In the above equation, σ is the yield strength of the polycrystal; σ0 stands for yield strength of single crystal; K is the constant related to the grain boundary structure, magnesium and its alloy is 280MPa•m 1/2 , d is the grain size.
The average grain size of the extruded ZM60 alloy is 20μm, and the average grain size of the extruded ZAM600 alloy and ZAM610 alloy decreases by 64% and 72% after the addition of trace Al element, respectively.According to Equation (2), the strength increase caused by fine crystals is Δσ=σ-σ0=Kd -1/2 , corresponding to the theoretical strength increase of fine crystals in ZAM600 alloy is 167% of that of ZM60 alloy, and the theoretical strength increase of fine crystals in ZAM610 alloy is 189% of that of ZM60 alloy.Thus, adding trace Al element can obtain better fine grain strengthening effect.However, the average yield strength of ZAM600 alloy and ZAM610 alloy is 26.3% and 56.5% higher than that of ZM60 alloy, respectively.The reason for this is that the addition of Al to ZM60 alloy not only has the effect of fine grain strengthening, but also has the effect of solution strengthening.Solid solution strengthening is an important part of improving the strength of alloys.Solid solution strengthening effect will be produced when Al atoms are dissolved in magnesium matrix.
Solid solution strengthening can improve the strength of the alloy through the atomic size effect, modulus effect, shortrange order effect and lattice distortion effect.The advantages of solution strengthening are as follows: ① significantly increase the hardness, strength and toughness of the alloy without noticeably decreasing the plasticity of the material; ② Solid solution in the intermetallic compound strengthening phase, strengthen and toughen the strengthening phase, improve the plasticity and toughness of the alloy.③ The morphology and distribution of the reinforcing phase can be improved by adding the solution-strengthened alloy elements, so as to improve the properties of the material.Solution strengthening is caused by atomic dislocation or lattice distortion in the alloy.The diameter of Mg atom is 0.320nm, that of Al atom is 0.286nm, that of Zn atom is 0.268nm, and that of Mn atom is 0.254nm.Large differences in atomic size and chemical properties enhance the distortion, result of the strength, hardness and resistance of the alloy are increased, while the plasticity and toughness of the alloy are decreased.The more alloying element atoms are dissolved, the greater the lattice distortion is occured, and the higher the strengthening effect is obtained.

Effects of trace elements on impact properties of alloys
In polycrystals, under the additional shear stress along the slip direction, the soft oriented grains with large Schmid factor start to slip and undergo plastic deformation, while the hard oriented grains with small Schmid factor are difficult to deform.Due to the different orientation of each grain, the shear stress value of the external force on each grain varies greatly, which makes the grain rotate and the grain orientation change accordingly.As a result, the Schmid factor of the hard oriented grains is increased, and the sliding process is easy to be carried out.As a result, the Schmid factor of the previously hard-oriented grains increases, which makes the slip process easier to proceed.Therefore, compared with the magnesium alloy with large grain, the fine grain magnesium alloy has stronger plastic deformation ability.
The grain size of Al-containing alloy is significantly smaller than that of ZM60 alloy, and the elongation after fracture and plastic deformation capacity are also significantly higher than those of ZM60 alloy.It can be seen from FIG. 6 (d) that under the impact load, the bending deformation degree of ZM60 alloy sample is the smallest, while that of ZAM610 alloy sample is the largest.It can be seen from the micro-fracture morphology of the impact sample that there are many tearing edges in ZM60 alloy, which indicates that its toughness is poor.However, the fracture of ZAM600 and ZAM610 alloys has a large number of dimples, indicating that their toughness is good.
J. Kuike [18,19] studied the plastic deformation mechanism of fine crystal magnesium alloy and found that the α dislocation could occur cross slip from the base plane to the prism plane even at room temperature.It is concluded that non-basal slip tends to occur near the grain boundary.When the grain size is below 100μm, the range of non-basal slip is about 10μm away from the grain boundary.Therefore, when the grain is refined to less than 10μm, non-basal slip can run through the whole grain.As evident from this, the addition of Al to ZM60 alloy refines the grain size of the alloy, enhances the dislocation slip, improves the plastic deformation ability of the alloy, and thus improves the impact toughness of the alloy.

Conclusion
(1) The as-cast microstructure of ZM60 alloy consists of α-Mg and Mg51Zn20 phases;after adding trace amount of Al, the microstructure of ZAM600 and ZAM610 alloys increases with the addition of fine Al8Mn5 phase, and the amount of second phase increases.
(2) Extrusion deformation promotes dynamic recrystallization, and the undissolved MgZn/MgZnAl phase promotes the nucleation of dynamic recrystallized grains and refines the recrystallized grains.The Al8Mn5 phase has a strong Zener blocking effect which inhibits the grain growth.ZAM6100 alloy has the smallest average grain size at 5.6μm.
(3) ZAM610 alloy has the best mechanical properties, and its tensile strength, yield strength, elongation after breaking, U-notch impact work and non-notch impact work are 302.7MPa,216MPa, 19.7%, 10.0 J/cm 2 and 56.0 J/cm 2 , respectively.(4) The fracture of the impact sample shows that there are many tearing edges in the ZM60 alloy, which shows the characteristics of quasi-cleavage fracture.The ZAM600 and ZAM610 alloys have a large number of dimples, which are characterized by dimple fracture.The improved properties of magnesium alloys after microalloying can be attributed to the formation of fine Al8Mn5 phase, which has better Zener blocking effect and refines the dynamic recrystallized grains, contributing to the improved strength and impact toughness of the alloy.

Figure 4 .Figure 5 .
Figure 4.The bright field TEM images of the as-extruded alloys

Table 1 .
Chemical composition of the as-cast alloys

Table 2 .
Tensile properties of the as-extruded experimental alloys

Table 3 .
Impact properties of the as-extruded experimental alloys