Study on Hot Deformation Behavior and Microstructure Evolution of MgAlCuZnMnCe Alloy

The Mg79Al12.5Cu2.5Zn4MnCe multi-principal element alloys was prepared by induction melting under a high-purity argon. The true stress-strain curves at deformation temperatures of 250-350°C and strain rates of 0.001-1s−1 were used to establish the constitutive equations as well as the thermal processing map. The optimal hot processing parameters for the alloy were determined as a deformation temperature of 320-350°C and a strain rate of 0.1-1s−1. Further investigation into the impact of various deformation parameters on the microstructure and macrotexture of the alloy during hot compression was conducted. The findings indicate that at a deformation rate of 1s−1, the second phase of the alloy is broken and deformed. The texture intensity follows a pattern of initially decreasing and then increases as the temperature rises. The change in grain size is influenced by the deformation crushing effect, initiation of the non-substrate slip system, and grain growth. When the strain rate increases, the changes in texture intensity and distribution are not pronounced, indicating that the strain rate has minimal influence on the macro texture.


Introduction
Magnesium alloy is known as the green metal of the 21st century, due to its low density, high specific strength, excellent electromagnetic shielding effect, and recyclability, showing a wide spectrum of applications and development space [1][2][3][4][5] .Improving the performance of magnesium alloys and identifying a suitable magnesium alloy for engineering applications is a pressing issue.However, achieving this task is challenging using the existing magnesium alloy system.Simply increasing the content of alloying elements has limited effects on improving strength and can also reduce plasticity [6] .Therefore, to fully realize the potential of magnesium alloy products, it is essential to develop high-performance deformed magnesium alloys [7] .Multi-element alloying is the trend in the development of new high-performance magnesium alloys [8] .The multi-principal element alloy discussed in this work is a brand-new kind of magnesium alloy that was strengthened utilizing the multi-element alloying technique.It has high strength, hardness, and corrosion resistance.
The unique hexagonal crystal structure of magnesium alloys and their limited slip system at ambient temperatures restrict their processing.However, thermal processing can be used to improve the microstructure and formability of magnesium alloys by activating additional slip systems [9][10][11][12][13][14] .When magnesium alloys are deformed at high temperatures, the slip system of the base, prismatic, and conical surfaces is activated, leading to an increase in their plastic deformation capacity.However, the temperature range for deformation is narrow, and the process parameters need to be carefully controlled due to their high sensitivity [14][15][16][17] .Therefore, research to improve the processing of deformed magnesium alloys is critical for regulating organizational evolution and increasing magnesium alloy mechanical characteristics [18,19] .David et al. [20] devised an accurate formability model for the variation of flow stresses with strain rate and particle size in magnesium alloys at high temperatures.Huppmann et al. [21] investigated the plastic deformation and recrystallisation behaviour of AZ31 and ME21 and devised a double sine equation to simulate the relationship between flow stresses on temperature and strain rate.Wang et al. [22] examined the flow behavior, constitutive model, DRX kinetic model, and processing diagram of Mg-Al-Ca-Mn alloy under various hot compression settings and estimated the alloy's hot working parameters.
To gain a comprehensive understanding of novel magnesium based multi-principal element alloys, it is crucial to investigate their thermal deformation behavior, microstructure, and constitutive description of material flow [23] .This paper investigates the hot deformation behavior of Mg79Al12.5Cu2.5Zn4MnCealloy using the thermal processing diagram based on the dynamic material model (DMM) and the microstructure and macrotexture of the material during deformation to find the appropriate thermal processing parameters for Mg79Al12.5Cu2.5Zn4MnCealloy to optimize the plastic processing of the alloy [24,25] .

Experimental procedures
The Mg-Al-Cu-Zn-Mn-Ce alloy was smelted in a vacuum induction furnace utilizing 99.9% Mg, 99.9% Al, 99.9% Zn, Al-50% Cu, Al-25% Mn and Mg-30% Ce, and then cased into a graphite mold.The compositional mass fractions are presented in Table 1.
The first phase of the smelting process was to evacuate the smelting furnace.The furnace was then filled with high-purity argon gas, producing a continuous high-purity argon gas environment for the alloy.This atmosphere prevented the alloy from oxidizing during the smelting process.The ingots were machined into compression specimens with dimensions of Ø10×15 mm.Hot compression experiments were conducted using a Gleeble 3500 hot/force simulator at temperatures ranging from 250 to 350°C and a strain rate of 0.001 to 1s -1 .Prior to hot compression, the specimens were heated to a predetermined temperature and maintained for 5 minutes.During thermal deformation, the strain, temperature, strain rate, and nominal stress were automatically recorded.
The Rigaku X-ray diffractometer (Smartlab) with a scanning angle of 10°-90°and a scanning speed of 0.5°/min was used for the X-ray diffraction examination of the materials.The microstructure was observed in scanning electron microscopy (SEM) imaging using the Geminni SEM300.A Rigaku X-ray diffractometer (Smartlab) was selected to scan the surface of the measurement sample, which had a sample inclination of 0°-90°, followed by macrotexture measurements computed by JTEX software.followed by a subsequent continuous flow softening.The stress-strain curves produced at different temperatures and strain rates reveal that the peak strain during flow softening increases with deformation rate but decreases with deformation temperature.Peak stress gradually rises as deformation temperature decreases with a certain deformation rate.The prism and cone slip system is activated as the critical decomposition shear stress (CRSS) of the substrate system decreases due to the increase in deformation temperature [26][27][28] .Peak stress increases significantly as the deformation rate increases at a certain deformation temperature.The reason is that when the deformation rate increases, the density of dislocations in the crystal increases, which hinders the movement of the dislocation [29][30][31] .

Interpretation from hot flow curves
In Figure 1, each curve displays a distinct single-peaked feature with approximately the same trend, owing to the apparent DRX that has occurred.Due to the applied stress, the dislocation density within the alloy rapidly increases during the early phases of deformation.This increases the probability of pile-up and entanglement, which in turn impedes the movement of dislocations and leads to work hardening.As a consequence, the flow stress also exhibits a rapid increase as the strain increases.As the amount of deformation increases, dislocations rearrange and annihilate, and the stress growth rate gradually reduces.Once the dislocation density reaches a critical value, the stored energy from deformation triggers dynamic recrystallization (DRX), which results in recrystallized grains nucleating and growing.This process is accompanied by dynamic softening, and the alloy reaches its maximum stress level when the softening effects of work hardening and dynamic recrystallization are balanced.In the later stages of deformation, the flow softening caused by dynamic recrystallization becomes the dominant factor, leading to a gradual decrease in flow stress and eventual steady-state [32] .

Establishment of the constitutive equation
The material thermodynamic constitutive equation, which illustrates the association between strain rate, peak stress, and deformation temperature, can represent the relationship between material peak stresses and thermodynamic parameters.
At low stress levels, it may be characterized by an exponential equation [33] .ε =Aσ n 1 exp -Q/ RT (1) At high stress levels, it may be described by a power exponential equation [33] .
ε =Aexp βσ exp -Q/ RT (2) The hyperbolic sine law may be used to precisely characterize the relationship between flow stress, deformation temperature, and strain rate at all stress levels [34] .
ε =A sinh ασ n exp[-Q/(RT)] (3) In equations ( 1)-( 3), with ε [ s −1 ] the strain rate; With σ [MPa] the flow stress; With Q[J/mol] the deformation activation energy; With R[J/mol•K] the gas constant, the R is taken equal to 8.314 J/mol•K for this formula.;With T[K] the absolute temperature; K、A、α、β、n 1 and n are the material constants, where A is the structure factor and n is the stress index, the stress level parameter α=β/n 1 .
Figure 2 shows the lnε -lnσ relations derived from a lnε -lnσ linear fit to the peak stresses σ max generated under various deformation conditions.Figure 3 illustrates the lnε -σ relationship derived from a lnε -σ linear fit to the peak stresses σ max generated under varying deformation conditions.From the graph of relationship lnε -lnσ, the slope β of its fitted line can be obtained as 0.10541, and from the graph of relationship lnε -σ , the slope n 1 of its fitted line can be obtained as 10.2229, i.e. α=β/n 1 =0.010311.Figure 4 shows the ln[sinh(ασ)]-1000/T relationship, obtained from the graph, which has an average slope of K = 4.438.The activation energy of deformation is a crucial parameter in determining the ease of deformation of a material.To calculate this energy, one can take the logarithm of both sides of equation ( 1) simultaneously, resulting in equation ( 4): The examined material's deformation activation energy is much greater than the 134 KJ/mol deformation activation energy noted in the pure magnesium analyzed by Shi et al., at 274.14 KJ/mol [35] .This is because the alloy contains mixed phases that increase the material's deformation activation energy.A higher activation energy makes deformation more challenging.Additionally, dynamic precipitation and pining effects can hinder dislocation movement, further increasing the deformation activation energy [36] .
Figure 5 demonstrates the lnε -ln[sinh(ασ)] relationship obtained from the graph, which has an average slope of n=7.43.In order to accurately quantify the flow stresses σ during thermoplastic deformation of a material, it is necessary to include a temperature compensation factor Z that accounts for the effects of both deformation temperature T and strain rate ε [37] .This factor is crucial in accurately predicting the behavior of the material under various thermoplastic conditions.Z=ε exp Q/(RT) (6)   lnZ=lnA+nln[sinh(ασ)] Substituting Q 、 T 、 ε and σ max of the deformation condition into equation (7), the (ln[sinh(ασ)]，lnZ) coordinate point can be obtained.

Interpretation from processing maps
The thermal processing map is a useful tool for optimizing the process parameters of metal materials during thermoplastic deformation.And the thermal processing map enables us to understand the mechanism of alloy deformation and the evolution of microstructure.Additionally, the thermal processing map can help analyze the causes of plastic instability in the material, thereby avoiding the generation of defects.In this study, we will utilize the dynamic material model (DMM) theory to analyze the thermal simulation compression data and generate a thermal processing map of the alloy.The DMM is expressed by equation ( 8): In equation ( 8), P stands for the system's overall external energy input.G stands for dissipation, the energy lost during thermoplastic deformation.On the other hand, J represents the energy utilized for microstructure evolution during thermoplastic deformation, which is denoted by dissipation coefficients.
The energy required for the material's microstructure to change during deformation and the linear dissipation rate create a proportional relationship known as the power dissipation efficiency η, which is used to describe the power dissipation of a material.η= J J max = 2m m+1 (9)   In equation (9), with m the strain rate sensitivity coefficient, which is determined by the proportion of G and J at a certain deformation temperature and strain rate.
Figure 7 shows the energy dissipation diagram T-σ-η obtained from the variation of the power dissipation rate at various temperatures and strain rates.

Figure 7. Energy dissipation diagram of MgAlCuZnMnCe alloy
When analyzing the deformation mechanisms in the thermoplastic deformation process of materials, it is insufficient to solely rely on the energy dissipation diagram.It is necessary to consider the various defects that may occur during the process and their impact on the material.Additionally, determining the instability zone of thermal processing requires the use of a relevant instability criterion.The relevant instability criterion is [38] : In equation (10), the instability coefficient is represented by ξ(ε ).The material instability plot is a contour plot of the instability coefficient obtained at various deformation temperatures and strain rates.Figure 8 shows the instability diagram, with the dark region representing the instability zone (area ξ(ε ) ≤ 0) of the thermoplastic deformation process.This area is vulnerable to flow instability during processing and should be avoided during thermal processing.

Figure 8. Instability maps of MgAlCuZnMnCe alloy
The thermal processing maps of the material are derived by superimposing the corresponding energy dissipation and instability maps.These maps are obtained at deformation temperatures ranging from 250-350°C and strain rates of 0.001-1s -1 .The contour line in the figure represents the energy dissipation efficiency η, while the instability zone is shaded.
The processing safety zone is the processing interval at high strain rate, which is the strain rate at 0.001-0.1s-1 in Figure 9's thermal processing maps; The alloy instability zone refers to the processing range at low strain rates, typically occurring between 250-340°C and strain rates below 0.001s -1 .
Greater energy dissipation efficiency in the safety zone shows a lower energy dissipation state and excellent processability of the material.In summary, the deformation area with a processing temperature of 330-350°C and a processing rate of 0.1-1s -1 is preferred in the alloy.Figure 11 shows the X-ray diffraction of as-cast MgAlCuZnMnCe alloy, Figure 12 shows the analysis of the second phase composition of as-cast MgAlCuZnMnCe alloy, and Table 2 shows the atomic ratio of the second phase in Figure 12.The matrix is α-Mg, according to the results of XRD pattern analysis and complete EDS point analysis.The atomic fraction of the dark gray second phase in the reticular structure is Mg:Al:Zn≈42:34:24, which is recognized as Mg32(Al, Zn)49, combined with the XRD point results.The atomic fraction of the light gray second phase in the reticular structure Al:Mn:Ce≈64:26:10, which is recognized as Al8Mn4Ce, combined with the XRD point results.The brilliant white second phase is considered to be the CeCuAl3 phase due to X-ray diffraction results and atomic fraction inference for Al:Cu:Ce≈ 63:14:23.The horizontal sections of MgAlCuZnMnCe compressed with different strain rates at 350°C can be shown in the Figure 13, the organization has a clear directionality and the α-Mg extends perpendicular to the direction of compression.The convergence of deformation energy leads to the increase of the dislocation proliferation and plugging during high-speed extrusion.The number of dislocations passing through per unit time is reduced, increasing the density of second phase dislocations and enhancing the plastic deformation resistance of the alloy.The pressure on the α-Mg phase during compression deformation will increase, resulting in a more slender direction perpendicular to the compression axis [39] .
Figure 13 shows the distribution of the bright white CeCuAl3 phases along the flow direction.The size of the second phase's grain decreases due to fragmentation as the rate of strain increases.Besides, the reticulated structure Mg32(Al, Zn)49 and Al8Mn4Ce phases are also distributed along the flow orientation.The reticular structure phase exhibits obvious compression and aggregation with an increase in extrusion rate, and when compared to the reticular structure in the casting stage, the bulk reticulated structure phase exhibits obvious fracture.Figure 13.(d) shows no clear flow direction at a strain rate of 1s -1 , and the reticulated structure phases Mg32(Al, Zn)49 and Al8Mn4Ce remain unaffected by deformation.The CeCuAl3 phases are clearly aggregated in the form of agglomerates or stripes.The rupture-causing plastic deformation limit has been achieved in both the matrix and the reticulated structure phases when the strain rate is large, while the bright white phase aggregates due to its better plasticity compared to the matrix structure.
Figure 14 shows the SEM image of MgAlCuZnMnCe with different deformation temperature at the deformation rate 1s -1 .The network second phase and the white massive second phase both develop transverse fractures at a deformation temperature of 250 °C, as shown in Figure 14.(a).The phases of Mg32(Al, Zn)49 and Al8Mn4Ce clearly break at a deformation temperature of 280°C, while CeCuAl3 partially break, as shown in Figure 14.(b).The main body is surrounded by broken fragments, which are a result of the extrusion force that occurs during high-speed deformation.The Mg32(Al, Zn)49 and Al8Mn4Ce phases exhibit minimal deformation with just a few cracks at a deformation temperature of 320°C, as shown in Figure .14. (c).Transverse cracks may be seen in the larger CeCuAl3 phases, while the Mg32(Al, Zn)49 and Al8Mn4Ce phases both show fine rod-shaped while second-phase precipitates.
The insufficient deformation is attributed to the initiation of non-substrate slip systems at high temperatures, which helps relieve the dislocation density under high strain rates.The Mg32(Al, Zn)49 and Al8Mn4Ce phases transform into discontinuous flocculent and finely divided blocks at a deformation temperature of 350°C, as shown in Figure 14.(d).The reticular second phase dissolves back into the matrix structure due to the increase in solid solubility of the matrix alloy elements at high temperatures.Additionally, the CeCuAl3 phases experience significant growth at high temperatures.crystal plane textures of the α-Mg phase accumulate from the edge to the center along the TD direction and progressively disperse uniformly as the deformation temperature rises.At a deformation temperature of 350℃, the texture is distributed parallel to the transverse direction (TD).The strength of the texture initially of the {0002} 、 {1010} decreases before gradually increasing as the deformation temperature rises.At a deformation temperature of 320°C, the texture strength is observed to be the weakest, while the distribution of texture remains uniform.This suggests that the thermal processing at a deformation temperature of 320°C is susceptible to plastic deformation, indicating that the material exhibits high plasticity.These deformation conditions align with the optimal processing.
During hot deformation, the basal texture of the MgAlCuZnMnCe alloy weakens due to complete dynamic recrystallization, which weakens both the preferred orientation nucleation and growth, ultimately resulting in weakened texture.As the deformation temperature rises, the recrystallized grains grow and the coarse second phase increases, resulting in a slight increase in the texture strength of the alloy.The macrotexture distribution remains relatively constant across different deformation rates under the deformation temperature of 320℃.The macrotexture distribution is symmetrical along the TD axis and follows a typical basal texture pattern, diffusing and then gathering along the TD axis.At the deformation rate of 0.1 on the 0002 crystal plane, the maximum texture strength is 1.91.The macroscopic texture strength of the 0002 、 {1120} crystal plane is greater at the final deformation rate of 1 s -1 compare to the deformation rate of 0.001 s -1 , but the difference is not appreciably significant.
However, the degree of recrystallization in the alloy varies with different rates of deformation.It can be shown that a higher deformation rate results in a shorter duration of dynamic recrystallization and dynamic recrystallization grain growth.This results in a lower degree of dynamic recrystallization and less effect on the macrotexture of the MgAlCuZnMnCe alloy basal plane.As a result, the macrotexture

Conclusion
(1) The thermal compression stress-strain curves of MgAlCuZnMnCe alloy were obtained and a constitutive model based on the hyperbolic sine law was established.Additionally, the hot processing map of the alloy was established using the dynamic material model (DMM), and the thermal processing parameters for the alloy were determined.The range for the deformation temperature is 320-350℃, and the deformation rate should be within 0.1-1s -1 .
(2) The SEM image of the longitudinal section of the alloy microstructure at 350 °C reveals that the alloy structure undergoes flow direction deformation as a result of thermal compression.However, when the deformation rate is 1s -1 , the flow direction deformation is absent due to the plastic limit of the matrix structure being exceeded.At a deformation rate of 1s -1 , the second phase breaks at a large deformation rate.Additionally, the Mg32(Al, Zn)49 and Al8Mn4Ce phases dissolve into the matrix at a deformation temperature of 350°C.
(3) The crystal plane texture strength of Mg79Al12.5Cu2.5Zn4MnCedecreases initially and then increases with an increase in deformation temperature when the deformation rate is constant.At a temperature of 320℃, the texture distribution becomes uniform, resulting in the lowest intensity.This is indicated as beneficial for deformation processing.When the deformation temperature is kept constant, the texture strength initially decreases and then increases as the deformation rate increases.However, the overall change is not significant, and the texture distribution remains relatively unchanged.The deformation rate does not have a noticeable effect on the texture's strength or distribution.

Figure 1
depicts the hot compression flow stress-strain curves for the MgAlCuZnMnCe alloy.The curves show distinct properties at different temperatures, from 250-350°C.

Figure 6
show the (ln sinh ασ ,lnZ) relationship, its linear correlation coefficient R= 0.9825, lnA is the intercept of the (ln sinh ασ ,lnZ) relationship curve, we can get lnA = 53.228,so A = 1.31 × 10 23 .Therefore, the flow behavior of the alloy under high-temperature compression deformation can be described by the double sine curve of the Arrhenius-type constitutive model.

Figure 9 .
Figure 9. Thermal processing map of MgAlCuZnMnCe alloy

Figure 10 .
Figure 10.Microstructure of the precipitated phase of as-cast MgAlCuZnMnCe alloy (a)low magnification, (b)high magnification

Figure 15 shows
Figure15shows the texture pole figure of the MgAlCuZnMnCe alloy at various deformation temperatures, with a deformation rate of 1s -1 .The picture shows that the {0002}、{1010}、 {1120}

Table 2 .
EDS analysis of the chemical elements of the cast MgAlCuZnMnCe alloy in Figure12