Study on the Microstructure of Mg-4Zn-4Sn-1Mn-xAl As-Cast Alloys

In this study, the microstructure of the Mg-4Zn-4Sn-1Mn-xAl (x = 0, 0.3 wt.%, denoted as ZTM441 and ZTM441-0.3Al) as-cast alloys was investigated using scanning electron microscopy (SEM), focused-ion/electron-beam (FIB) micromachining, transmission electron microscopy (TEM), and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The analysis results revealed that the microstructure of the ZTM441 and ZTM441-0.3Al as-cast alloys both mainly consist of the α-Mg matrix, skeleton-shaped MgZn2 eutectic texture, block-shaped Mg2Sn, and Zn/Sn-rich nanoscale precipitate bands along the grain boundary and the interdendrite. Nanoscale α-Mn dispersoids formed in the grain in the ZTM441 alloy, while no α-Mn formed in the ZTM441-0.3Al alloy instead of nanoscale Al3Mn2 particles. In the ZTM441 as-cast alloy, part of the Zn element is dissolved into the α-Mn phase, and part of the Mn element is dissolved into the MgZn2 phase, but in the ZTM441-0.3Al alloy, there are no such characteristics of mutual solubility. Zn and Mn elements are easy to combine in ZTM441 as-cast alloy, while Al and Mn are easy to combine in ZTM441-0.3Al as-cast alloy. The Mg-Zn phases have not only MgZn2-type crystal structure but also Mg4Zn7- and Mg149Zn-type crystal structure in the ZTM441-0.3Al as-cast alloy. The addition of Al changes the combination of Mn and Zn, promotes the formation of Al3Mn2, and the growth of the grain.


Introduction
Magnesium (Mg) alloys have been widely used in transportation, aerospace, and electronics industries because of their low density, high specific strength, good damping performance, and suitability for surface treatments [1][2][3][4][5].However, the adoption of magnesium alloys remains limited for low absolute strength at room temperature and elevated temperature.Although Mg-RE (rare earth) alloys have better comprehensive properties such as high strength and heat resistance [6], the RE elements are heavy, expensive, and scarce.It is necessary to select free-RE elements to substitute RE elements to decrease the cost and the weight of high strength, heat-resistant magnesium alloys.
Studies on the microstructure of Mg-6Zn-4Sn-1Mn-xAl (wt.%) alloys have been reported [46][47][48].These studies focused on the second phase and properties of the extruded and aged states of the Mg-6Zn-4Sn-1Mn-xAl series alloys after homogenization heat treatment.The results show that the grain sizes decrease and then increase with the increase in Al element content, and the addition of Al elements can promote the formation of second phases such as Al 8 Mn 5 , Al 11 Mn 4 , Mg 32 (Al,Zn) 49 , Al 2 Mg 5 Zn 2 , and MgZn.However, in this study, the Al-Mn phases possess an Al 3 Mn 2 structure, which is not consistent with that reported in the literature [46][47][48][49].And the combination of Mn and Zn would be changed for the Al addition.Therefore, it can be deduced that the formation of the second phase is sensitive to the contents of the alloying elements.It is necessary to investigate the second phase in the Mg-4Zn-4Sn-1Mn-xAl (x = 0, 0.3 wt.%) alloys.
In the present study, the microstructure of the Mg-4Zn-4Sn-1Mn-xAl (x = 0, 0.3 wt.%) as-cast alloys was investigated using Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM), focused-ion beam (FIB), and transmission electron microscopy (TEM) techniques.The distribution, composition, and crystal structure of the complex second phase in the as-cast alloy were systematically analyzed in the two alloys.The effect of the addition of Al element on the formation law of the second phases in the Mg-4Zn-4Sn-xAl (x = 0, 0.3 wt.%) as-cast alloys was explored.This study will provide a basis for the design of subsequent heat treatment and hot working processes.
SEM samples were prepared via mechanical polishing with grinding and polishing machines.Labopol-30 and Tagramin-30 EBSD samples were electrochemically polished, followed by mechanical polishing, in a solution AC2 cooled at −30 • C, using a voltage of 20 V and a current of 0.5 mA for 40 s.TEM samples were mechanically ground down to 50 µm and then cut into 3 mm diameter thin foils, and then prepared via twin-jet electropolishing (MTP-1A Double-Sprayer, JIAODA, Jiading, Shanghai, China) in an electrolyte containing 85 vol.%C 2 H 5 OH, 5 vol.%HClO 4 , and 10 vol.%C 6 H 14 O 2 at −30 • C and the subsequent ion thinning (695.0PIPS II ion thinning apparatus, Gatan, Pleasanton, CA, USA) at −140 • C with a voltage of 2 KeV.TEM samples for the gray contrast band and site-specific intergranular second phase observation were prepared using a focused-ion/electron dual beam system (Helios 5 CX, Thermo Fisher Scientific, Waltham, MA, USA).

XRD Results
Figure 1 shows the XRD patterns of the two as-cast alloys.The XRD results were identified by the data from the Inorganic Crystal Structure Database (ICSD) with reference numbers 170902 for α-Mg, 642855 for Mg 2 Sn, and 46006 for MgZn 2 , respectively.The analyses revealed that ZTM441 and ZTM441-0.3Alas-cast alloys mainly consist of α-Mg matrix, Mg 2 Sn, and MgZn 2 .No Mg 17 Al 12 phases appeared in the ZTM441-0.3Alas-cast alloy, which can be attributed to the low Al/Zn ratio [16,50].electron backscatter diffractometer.Bright-field (BF) images, high-angle annular darkfield scanning transmission electron microscopy (HAADF-STEM) images, high-resolution transmission electron microscopy (HRTEM) images, and the selected area electron diffraction (SAED) patterns were obtained using Talos F200X TEM equipped with Super X fourdetector energy spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) at 200 kV.SEM samples were prepared via mechanical polishing with grinding and polishing machines.Labopol-30 and Tagramin-30 EBSD samples were electrochemically polished, followed by mechanical polishing, in a solution AC2 cooled at −30 °C, using a voltage of 20 V and a current of 0.5 mA for 40 s.TEM samples were mechanically ground down to 50 µm and then cut into 3 mm diameter thin foils, and then prepared via twin-jet electropolishing (MTP-1A Double-Sprayer, JIAODA, Jiading, Shanghai, China) in an electrolyte containing 85 vol.%C2H5OH, 5 vol.%HClO4, and 10 vol.%C6H14O2 at −30 °C and the subsequent ion thinning (695.0PIPS II ion thinning apparatus, Gatan, Pleasanton, CA, USA) at −140 °C with a voltage of 2 KeV.TEM samples for the gray contrast band and sitespecific intergranular second phase observation were prepared using a focused-ion/electron dual beam system (Helios 5 CX, Thermo Fisher Scientific, Waltham, MA, USA).

XRD Results
Figure 1 shows the XRD patterns of the two as-cast alloys.The XRD results were identified by the data from the Inorganic Crystal Structure Database (ICSD) with reference numbers 170902 for α-Mg, 642855 for Mg2Sn, and 46006 for MgZn2, respectively.The analyses revealed that ZTM441 and ZTM441-0.3Alas-cast alloys mainly consist of α-Mg matrix, Mg2Sn, and MgZn2.No Mg17Al12 phases appeared in the ZTM441-0.3Alas-cast alloy, which can be attributed to the low Al/Zn ratio [16,50].

SEM Results
The EBSD results were analyzed using the OIM 8 software.Figure 2a-c,g-i show the inverse pole figure (IPF) maps, image quality (IQ) maps with grain boundaries (GBs), and grain size distribution histograms of ZTM441 and ZTM441-0.3Alas-cast alloys.The misorientation angles between the adjacent grains are used to identify the low-angle grain boundaries (LAGBs, 2° ≤ θ ≤ 15°) and high-angle grain boundaries (HAGBs, θ > 15°).The average grain size weighted by area (denoted as the average grain size) was calculated by the formula which is as follows:

SEM Results
The EBSD results were analyzed using the OIM 8 software.Figure 2a-c,g-i show the inverse pole figure (IPF) maps, image quality (IQ) maps with grain boundaries (GBs), and grain size distribution histograms of ZTM441 and ZTM441-0.3Alas-cast alloys.The misorientation angles between the adjacent grains are used to identify the low-angle grain boundaries (LAGBs, 2 • ≤ θ ≤ 15 • ) and high-angle grain boundaries (HAGBs, θ > 15 • ).The average grain size weighted by area (denoted as the average grain size) was calculated by the formula which is as follows: where A i is the area of grain i and d i is the equivalent diameter of grain i.The equivalent diameter of a particular grain is calculated by determining the area of a grain and then assuming the grain is a circle.The average grain size is 215.632 µm (d 1 ), the fraction of HAGBs is 67.8%, and the fraction of LAGBs is 27.5% in the ZTM441 alloy, while the average grain size is 398.797µm (d 2 ), the fraction of HAGBs is 50.7%, and the fraction of LAGBs is 40.7% in ZM441-0.3Alalloy.Adding Al makes the grain grow, and the fraction of HAGBs decreases.There are a large number of dendritic crystals formed during the solidification processes in ZTM441 and ZTM441-0.3Alas-cast alloys, as shown in Figure 2b,h. ̅ ∑   ∑ where Ai is the area of grain i and di is the equivalent diameter of grain i.The equivalent diameter of a particular grain is calculated by determining the area of a grain and then assuming the grain is a circle.The average grain size is 215.632 µm (d1), the fraction of HAGBs is 67.8%, and the fraction of LAGBs is 27.5% in the ZTM441 alloy, while the average grain size is 398.797µm (d2), the fraction of HAGBs is 50.7%, and the fraction of LAGBs is 40.7% in ZM441-0.3Alalloy.Adding Al makes the grain grow, and the fraction of HAGBs decreases.There are a large number of dendritic crystals formed during the solidification processes in ZTM441 and ZTM441-0.3Alas-cast alloys, as shown in Figure 2b,h.Figures 3 and 4 show BSE images of the ZTM441 and ZTM441-0.3Alas-cast alloys, respectively.And the corresponding EDS analysis results of the two alloys are shown in Tables 2 and 3, respectively.The microstructure of the two alloys mainly consists of three contrasts (Figures 3a and 4a): dark gray-contrast, gray-contrast, and bright-contrast.According to BSE images and EDS results, it can be seen that ( 1) the phase with dark gray contrast is an α-Mg matrix in the two alloys; and (2) the phases with bright-contrast are skeletal Zn-rich phase (marked by yellow arrows in Figure 3), block-like Sn-rich phase (marked by blue arrows in Figure 3), and angular Mn-rich phase (marked by green arrows in Figure 3) in the ZTM441 alloy, while they are skeletal Zn-rich eutectic phases (marked by yellow arrows in Figure 4); block-like Sn-rich phases (marked by blue arrows in Figure 4); and angular Al-Mn-rich phases (marked by azure arrows in Figure 4) in the ZTM441-0.3Alalloy.
Figure 3 and Figure 4 show BSE images of the ZTM441 and ZTM441-0.3Alas-cast alloys, respectively.And the corresponding EDS analysis results of the two alloys are shown in Table 2 and Table 3, respectively.The microstructure of the two alloys mainly consists of three contrasts (Figures 3a and 4a): dark gray-contrast, gray-contrast, and bright-contrast.According to BSE images and EDS results, it can be seen that ( 1) the phase with dark gray contrast is an α-Mg matrix in the two alloys; and (2) the phases with brightcontrast are skeletal Zn-rich phase (marked by yellow arrows in Figure 3), block-like Snrich phase (marked by blue arrows in Figure 3), and angular Mn-rich phase (marked by green arrows in Figure 3) in the ZTM441 alloy, while they are skeletal Zn-rich eutectic phases (marked by yellow arrows in Figure 4); block-like Sn-rich phases (marked by blue arrows in Figure 4); and angular Al-Mn-rich phases (marked by azure arrows in Figure 4) in the ZTM441-0.3Alalloy.The high magnification BSE images of the gray-contrast bands in ZTM441 and ZTM441-0.3Alas-cast alloys are shown in Figure 5a,b, respectively.The EDS line scanning results of the Zn, Sn, Mn, and Al profile along the red line (labeled A in Figure 5a and B in Figure 5b) in the gray-contrast bands are shown in Figure 5c,d.It shows that the gray contrast bands are enriched with Zn and Sn elements.The gray-contrast band similar to zone labeled B in ZTM441-0.3Alwas sampled by FIB as the object, and its composition and structure were analyzed via TEM in Section 3.3 below.The high magnification BSE images of the gray-contrast bands in ZTM441 and ZTM441-0.3Alas-cast alloys are shown in Figure 5a,b, respectively.The EDS line scanning results of the Zn, Sn, Mn, and Al profile along the red line (labeled A in Figure 5a and B in Figure 5b) in the gray-contrast bands are shown in Figure 5c,d.It shows that the gray contrast bands are enriched with Zn and Sn elements.The gray-contrast band similar to zone labeled B in ZTM441-0.3Alwas sampled by FIB as the object, and its composition and structure were analyzed via TEM in Section 3.3 below.

ZTM441
Figure 6 shows the HAADF image and EDS maps for the second phases in ZTM441 as-cast alloy, and Table 4 shows the corresponding composition of the labeled second phases in Figure 6a.The Zn, Sn, and Mn contents (at.%) in the α-Mg matrix at position C in Figure 6a

ZTM441
Figure 6 shows the HAADF image and EDS maps for the second phases in ZTM441 as-cast alloy, and Table 4 shows the corresponding composition of the labeled second phases in Figure 6a.The Zn, Sn, and Mn contents (at.%) in the α-Mg matrix at position C in Figure 6a are 3.02, 1.26, and 0.31, respectively.The contents (at.%) of Mg, Zn, Sn, and Mn for position A are 27.98,66.24, 0.05, and 5.73, and for position B are 68.94,0.20, 30.33, and 0.53, which are close to the composition of MgZn 2 and Mg 2 Sn phase, respectively.The content of Mn in MgZn 2 was detected to be 5.73 at.%, which is lower than the maximum solubility of Mn in MgZn 2 , 14.0 at.% [51].Meanwhile, nanoscale Mn-rich particles occurred in the grain, which is beneficial to refining grain [52].
Materials 2023, 16, x FOR PEER REVIEW 8 of 21 solubility of Mn in MgZn2, 14.0 at.% [51].Meanwhile, nanoscale Mn-rich particles occurred in the grain, which is beneficial to refining grain [52].The SAED patterns of the Zn-rich phase and Sn-rich phase shown in Figure 6a along three different zone axes were acquired via a series of tilting (Figure 7).Combining the EDS results and the magnitude of the reciprocal vector and the angle between the vectors, Figure 7a-c     The SAED patterns of the Zn-rich phase and Sn-rich phase shown in Figure 6a along three different zone axes were acquired via a series of tilting (Figure 7).Combining the EDS results and the magnitude of the reciprocal vector and the angle between the vectors, Figure 7a-c     Figure 9 shows the HAADF image and corresponding EDS results for nanoscale Mnrich particles in the grain, which is similar to the phases in the red box marked as D in Figure 6a.The contents (at.%) of Mg, Zn, Sn, and Mn at position A in Figure 9a   Figure 9 shows the HAADF image and corresponding EDS results for nanoscale Mnrich particles in the grain, which is similar to the phases in the red box marked as D in Figure 6a.The contents (at.%) of Mg, Zn, Sn, and Mn at position A in Figure 9a  Figure 9 shows the HAADF image and corresponding EDS results for nanoscale Mn-rich particles in the grain, which is similar to the phases in the red box marked as D in Figure 6a.The contents (at.%) of Mg, Zn, Sn, and Mn at position A in Figure 9a

ZTM441-0.3Al
Figure 11 shows the HAADF image and EDS maps for the second phases in ZTM441-0.3Alas-cast alloy, and Table 5 shows the corresponding composition of the labeled second phases in Figure 11a.According to EDS quantitative results, the contents (at.%) of Mg, Zn, Sn, Mn, and Al at position A in Figure 11a

ZTM441-0.3Al
Figure 11 shows the HAADF image and EDS maps for the second phases in ZTM441-0.3Alas-cast alloy, and Table 5 shows the corresponding composition of the labeled second phases in Figure 11a.According to EDS quantitative results, the contents (at.%) of Mg, Zn, Sn, Mn, and Al at position A in Figure 11a     In order to determine the crystal structures of the phases at positions A, B, C, D, and E in Figure 11a, the SAED patterns along different zone axes were obtained via a series of tilting.Combining the EDS results, the SAED patterns shown in Figure 12a-c 14b.Mg4Zn7 and MgZn2 usually co-exist because of their similar formation energy [59,60].MgZn2 and Mg4Zn7 phases usually precipitate after heat treatment or deformation for certain magnesium alloys containing Zn [58,[61][62][63]; however, these phases have rarely been reported in the as-cast alloy.[59,60].MgZn 2 and Mg 4 Zn 7 phases usually precipitate after heat treatment or deformation for certain magnesium alloys containing Zn [58,[61][62][63]; however, these phases have rarely been reported in the as-cast alloy.
Figure 15 shows the HAADF image and corresponding EDS results for the squareshaped Al-Mn-rich phase.The contents (at.%) of Mg, Zn, Sn, Mn, and Al are 42.60,1.35, 0.09, 37.36, and 18.60 at position A in Figure 15a, with the atomic ratios Mn: Al = 2:1 and Mn: (Al, Mg) = 2:3.Figure 16 shows the SAED pattern, BF image, and HRTEM images of the square-shaped Al-Mn-rich phase taken along the [001] Al 3 Mn 2 zone axis.It was determined that the Al-Mn-rich phase almost conforms to the crystal structure of Al 3 Mn 2 , which has a cubic structure with a space group of P4-13 _ 2. The calculated lattice parameters are a = b = c = 6.6287Å, slightly different from the theoretical value (a = b = c = 6.3951Å [64]).The Al-Mn phases are dominated by Al 8 Mn 5 and Al 11 Mn 4 in the magnesium alloys [42,[65][66][67].The formation of Al 3 Mn 2 in magnesium alloys has rarely been reported.Figure 15 shows the HAADF image and corresponding EDS results for the squareshaped Al-Mn-rich phase.The contents (at.%) of Mg, Zn, Sn, Mn, and Al are 42.60,1.35, 0.09, 37.36, and 18.60 at position A in Figure 15a, with the atomic ratios Mn: Al = 2:1 and Mn: (Al, Mg) = 2:3.Figure 16 shows the SAED pattern, BF image, and HRTEM images of the square-shaped Al-Mn-rich phase taken along the [001]Al Mn zone axis.It was determined that the Al-Mn-rich phase almost conforms to the crystal structure of Al3Mn2, which has a cubic structure with a space group of P4-132 _ .The calculated lattice parameters are a = b = c = 6.6287Å, slightly different from the theoretical value (a = b = c = 6.3951Å [64]).The Al-Mn phases are dominated by Al8Mn5 and Al11Mn4 in the magnesium alloys [42,[65][66][67].The formation of Al3Mn2 in magnesium alloys has rarely been reported.Figure 15 shows the HAADF image and corresponding EDS results for the squareshaped Al-Mn-rich phase.The contents (at.%) of Mg, Zn, Sn, Mn, and Al are 42.60,1.35, 0.09, 37.36, and 18.60 at position A in Figure 15a, with the atomic ratios Mn: Al = 2:1 and Mn: (Al, Mg) = 2:3.Figure 16 shows the SAED pattern, BF image, and HRTEM images of the square-shaped Al-Mn-rich phase taken along the [001]Al Mn zone axis.It was determined that the Al-Mn-rich phase almost conforms to the crystal structure of Al3Mn2, which has a cubic structure with a space group of P4-132 _ .The calculated lattice parameters are a = b = c = 6.6287Å, slightly different from the theoretical value (a = b = c = 6.3951Å [64]).The Al-Mn phases are dominated by Al8Mn5 and Al11Mn4 in the magnesium alloys [42,[65][66][67].The formation of Al3Mn2 in magnesium alloys has rarely been reported.The TEM samples containing the gray contrast bands were obtained via FIB sitespecific lifting in the ZTM441-0.3Alas-cast alloy.Figure 17 shows the HAADF images and the corresponding EDS results for the Zn and Sn elements in the nanoscale phase in the gray contrast bands.The EDS quantitative results are similar to those of the α-Mg matrix (see Figure 6a) due to the beam extension.The corresponding EDS maps of Zn and Sn elements show that the gray contrast band mainly consists of the Zn-rich phase, contradicting those of the EDS line scanning results in Figure 5, which show Zn and Sn elements are rich in the gray contrast bands.It is possible that the Sn element escapes due to electron/ion beam irradiation during FIB sample preparation or TEM observation.Therefore, it is considered that the gray contrast may consist of both Zn-rich and Sn-rich phases or Zn-Sn-rich phases.The TEM samples containing the gray contrast bands were obtained via FIB site-specific lifting in the ZTM441-0.3Alas-cast alloy.Figure 17 shows the HAADF images and the corresponding EDS results for the Zn and Sn elements in the nanoscale phase in the gray contrast bands.The EDS quantitative results are similar to those of the α-Mg matrix (see Figure 6a) due to the beam extension.The corresponding EDS maps of Zn and Sn elements show that the gray contrast band mainly consists of the Zn-rich phase, contradicting those of the EDS line scanning results in Figure 5, which show Zn and Sn elements are rich in the gray contrast bands.It is possible that the Sn element escapes due to electron/ion beam irradiation during FIB sample preparation or TEM observation.Therefore, it is considered that the gray contrast may consist of both Zn-rich and Sn-rich phases or Zn-Sn-rich phases.The TEM samples containing the gray contrast bands were obtained via FIB site-spe cific lifting in the ZTM441-0.3Alas-cast alloy.Figure 17 shows the HAADF images and the corresponding EDS results for the Zn and Sn elements in the nanoscale phase in th gray contrast bands.The EDS quantitative results are similar to those of the α-Mg matrix (see Figure 6a) due to the beam extension.The corresponding EDS maps of Zn and Sn elements show that the gray contrast band mainly consists of the Zn-rich phase, contra dicting those of the EDS line scanning results in Figure 5, which show Zn and Sn element are rich in the gray contrast bands.It is possible that the Sn element escapes due to elec tron/ion beam irradiation during FIB sample preparation or TEM observation.Therefore it is considered that the gray contrast may consist of both Zn-rich and Sn-rich phases o Zn-Sn-rich phases.

Discussion
In this study, the microstructure of Mg-4Zn-4Sn-1Mn-xAl (x = 0, 0.3 wt.%) as-cast alloys was comparatively analyzed using a combination of SEM, EBSD, FIB, and TEM techniques.The intergranular second phases and the gray-contrast zones were especially fixed-point lifted out by FIB.Then, the types of the second phases were determined using SAED, HRTEM, HAADF-STEM, and EDS.The results show that the intergranular second phases in the ZTM441 as-cast alloy are skeleton-shaped α-Mg+MgZn 2 eutectic texture and blockshaped Mg 2 Sn and are skeleton-shaped α-Mg+Mg-Zn eutectic texture consisted of MgZn 2 , Mg 4 Zn 7 , and Mg 149 Zn, and block-shaped Mg 2 Sn in the ZTM441-0.3Alalloy.Among them, the Mg 149 Zn was not reported.Therefore, the distribution and crystal structures of the intergranular second phases are complex.The gray-contrast zones along the grain boundary and the interdendrite are composed of the nanoscale Zn/Sn-rich precipitates.Otherwise, the dispersoids, α-Mn and Al 3 Mn 2 , formed in the grain in the ZTM441 and ZTM441-0.3Alalloys, respectively.
With the addition of the Al element, firstly, the grain sizes increase from 215.632 µm in the ZTM441 to 398.797 µm in the ZTM441-0.3Al.It is considered that the addition of Al element inhibits the formation of α-Mn and weakens the refining grain effect of dispersoid.Although Al 3 Mn 2 may also play the role of α-Mn in refining grains, via extensive observation, the size of Al 3 Mn 2 is larger than that of α-Mn, and the number density is lower than that of α-Mn.Secondly, it can be found that the α-Mn dispersoids disappear and are replaced by Al 3 Mn 2 particles.Ye, Hou, and Deng reported the Al 8 Mn 5 and Al 11 Mn 4 in the Mg-6Zn-4Sn-1Mn-xAl alloys [46][47][48][49].However, no studies have been reported on the formation of the Al 3 Mn 2 phase in Mg-Zn-Sn-Mn-Al alloy.Therefore, the structure of the Al-Mn phase is very sensitive to the content of alloying elements.In addition, in ZTM441 alloy without Al, the Mg-Zn phase contains a certain amount of Mn (5.73 at.%), and a certain amount of Zn (3.35 at.%) is dissolved in α-Mn at the same time.However, in ZTM441-0.3Alalloy, the Mg-Zn phase nearly does not contain Mn, and the Mn element will combine with Al to form the Al-Mn phase.Therefore, it is believed that Al changes the combination of alloying elements.
In this study, the lattice constants of the second phase were determined using the SAED patterns.When excluding the measurement error of the SAED patterns, the lattice constants of some of the second phases still deviate significantly from the theoretical values, which are mainly attributed to the solid solution of other alloying elements in the second phase.The EDS results show that the content of Mn in the MgZn 2 in the ZTM441 alloy is In the alloys, the eutectic microstructure, Mg-Zn phase, and Mg-Sn phase will dissolve in the subsequent heat treatment, while α-Mn and Al 3 Mn 2 will still exist in the matrix.These dispersoids can provide nucleation particles for dynamic recrystallization in the subsequent hot processing process, thus playing a role in refining grains and increasing the strength of the alloy [68][69][70].The nanoscale Zn/Sn-rich phase in the gray-contrast zone is likely to precipitate during the slow cooling of cast ingot.Therefore, during the solid solution treatment process, re-dissolution will occur so as to obtain a high saturation solid solution, providing conditions for subsequent aging treatment.Two precipitation sequences can be achieved via subsequent appropriate aging processes, such as doublestage aging.The strength and toughness of the alloy can be improved by the MgZn 2 and Mg 2 Sn precipitates formed along the prismic plane and base plane, respectively [71,72].To sum up, via the study of the microstructure in the as-cast alloys, it can be found that a variety of phases can be formed in the alloy system, and via subsequent processing and heat treatment, multiphase composite strengthening and toughening can be achieved.The Mn element can also enhance the corrosion resistance of the alloy.Therefore, the alloy is expected to develop into a new magnesium alloy with excellent comprehensive properties.

Conclusions
In this study, the crystal structures, composition, and distribution characteristics of the intergranular second phases and intragranular dispersoids in ZTM441 and ZTM441-
Figure 2d,j show BSE images of the two alloys, and corresponding IPF maps and phase distribution maps are shown in Figures 2e,f and 2k,l, respectively.The results indicate that the second phases are distributed at the grain boundary and the interdendrite in the two alloys, and the second phases are mainly Mg 2 Sn and MgZn 2 in ZTM441 and Mg 2 Sn in ZTM441-0.3Al.
Figure 2d,j show BSE images of the two alloys, and corresponding IPF maps and phase distribution maps are shown in Figures 2e,f and 2k,l, respectively.The results indicate that the second phases are distributed at the grain boundary and the interdendrite in the two alloys, and the second phases are mainly Mg2Sn and MgZn2 in ZTM441 and Mg2Sn in ZTM441-0.3Al.

Figure 3 .
Figure 3. BSE images of ZTM441 as-cast alloy.(a) Low magnification BSE image, (b) the high magnification image of the red area in (a), (c) BSE image of the co-distributed block-shaped Sn-rich and skeletal Zn-rich phases of the red area in (b), and the BSE images of separately distributed (d) Znrich particles at position C in (b), (g) skeletal Zn-rich phase, (e) Mn-rich particle, (f) angular Mn-rich phase, and (h) block-shaped Sn-rich phase.

Figure 3 .
Figure 3. BSE images of ZTM441 as-cast alloy.(a) Low magnification BSE image, (b) the high magnification image of the red area in (a), (c) BSE image of the co-distributed block-shaped Sn-rich and skeletal Zn-rich phases of the red area in (b), and the BSE images of separately distributed (d) Zn-rich particles at position C in (b), (g) skeletal Zn-rich phase, (e) Mn-rich particle, (f) angular Mn-rich phase, and (h) block-shaped Sn-rich phase.

Figure 4 .
Figure 4. BSE images of ZTM44-0.3Alas-cast alloy, as follows: (a) low magnification BSE image, (b) the high magnification image of the red area in (a), and the BSE images of (c) the co-distributed block-shaped Sn-rich and Al-Mn-rich phases in the red area in (b), (f,g) the co-distributed blockshaped Sn-rich and skeletal Zn-rich phases, and the BSE images of separately distributed, (d) skeletal Zn-rich phase, (e) Al-Mn-rich particles, and the (h) angular Al-Mn-rich phase.

Figure 4 .
Figure 4. BSE images of ZTM44-0.3Alas-cast alloy, as follows: (a) low magnification BSE image, (b) the high magnification image of the red area in (a), and the BSE images of (c) the co-distributed block-shaped Sn-rich and Al-Mn-rich phases in the red area in (b), (f,g) the co-distributed blockshaped Sn-rich and skeletal Zn-rich phases, and the BSE images of separately distributed, (d) skeletal Zn-rich phase, (e) Al-Mn-rich particles, and the (h) angular Al-Mn-rich phase.

Figure 5 .
Figure 5. (a,b) BSE images of ZTM441 and ZTM441-0.3Alas-cast alloys; (c) EDS line scanning results of Zn, Sn, and Mn profile along the red line in the gray area (B region in Figure 3b) of ZTM441alloy and (d) the EDS line scanning results of Zn, Sn, Mn, and Al profile along the red line in the gray area (B region in Figure 4b) of ZTM441-0.3Alalloy.
Figure 6 shows the HAADF image and EDS maps for the second phases in ZTM441 as-cast alloy, and Table 4 shows the corresponding composition of the labeled second phases in Figure 6a.The Zn, Sn, and Mn contents (at.%) in the α-Mg matrix at position C in Figure 6a are 3.02, 1.26, and 0.31, respectively.The contents (at.%) of Mg, Zn, Sn, and Mn for position A are 27.98,66.24, 0.05, and 5.73, and for position B are 68.94,0.20, 30.33, and 0.53, which are close to the composition of MgZn2 and Mg2Sn phase, respectively.The content of Mn in MgZn2 was detected to be 5.73 at.%, which is lower than the maximum

Figure 5 .
Figure 5. (a,b) BSE images of ZTM441 and ZTM441-0.3Alas-cast alloys; (c) EDS line scanning results of Zn, Sn, and Mn profile along the red line in the gray area (B region in Figure 3b) of ZTM441alloy and (d) the EDS line scanning results of Zn, Sn, Mn, and Al profile along the red line in the gray area (B region in Figure 4b) of ZTM441-0.3Alalloy.

Figure 6 .
Figure 6.(a) HAADF image of the second phase in ZTM441 as-cast alloy, (b) mixed map of HAADF image and the EDS maps of (c) Mg, (d) Zn, (e) Sn, and (f) Mn.

21 _ 1 _ 0 ]
Figure 8   shows the BF image, SAED pattern, and the HRTEM image of the MgZn2 phase along the [MgZn zone axis.Stacking faults can be observed in the MgZn2 particle.

Figure 6 .
Figure 6.(a) HAADF image of the second phase in ZTM441 as-cast alloy, (b) mixed map of HAADF image and the EDS maps of (c) Mg, (d) Zn, (e) Sn, and (f) Mn.

3 ] 1 _ 11 ]
were indexed as the SAED patterns along [MgZn 2 for the Zn-rich phase, and Figure 7d,e along [Mg 2 Sn , [011] Mg 2 Sn , and [ _ 112] Mg 2 Sn for the Sn-rich phase.The Zn-rich phase at position A in Figure 6a almost conforms to the crystal structure of MgZn 2 phase, which has a close-packed hexagonal structure with a space group of P63/mmc.The calculated lattice parameters are a = b = 5.985 Å and c = 9.284 Å, slightly different from the theoretical value (a = b = 5.223 Å and c = 8.566 Å [53]), which may be caused by the solid solution of Mn element in the MgZn 2 phase and the measurement error of the reciprocal vector.The Sn-rich phase at position B in Figure 6a almost conforms to the crystal structure of the Mg 2 Sn phase, which has a face-centered cubic (FCC) structure with a space group of Fm _ 3 m.The calculated lattice parameters are a = b = c = 7.381 Å, slightly different from the theoretical value (a = b = c = 6.759Å [54]).

1 _ 10 ]
Figure 8 shows the BF image, SAED pattern, and the HRTEM image of the MgZn 2 phase along the [2 _ MgZn 2 zone axis.Stacking faults can be observed in the MgZn 2 particle.

Figure 7 . 21 Figure 7 . 11 _ 1 ] 1 _ 12 ]
Figure9shows the HAADF image and corresponding EDS results for nanoscale Mnrich particles in the grain, which is similar to the phases in the red box marked as D in Figure6a.The contents (at.%) of Mg, Zn, Sn, and Mn at position A in Figure9aare 16.63, 3.35, 0.38 and 79.63, close to the α-Mn phase.Figure 10a-c shows the SAED patterns along [112 _ ]α-Mn, [1 _ 33]α-Mn, and [111]α-Mn of the Mn-rich phase at position A (Figure9a) acquired via a series of tilting.The Mn-rich phase almost conforms to the crystal structure of the α-Mn phase, which has a body-centered cubic (BCC) crystal structure with a space group of I4 _

Figure 8 . 2 _ 1 _ 10 ]
Figure9shows the HAADF image and corresponding EDS results for nanoscale Mnrich particles in the grain, which is similar to the phases in the red box marked as D in Figure6a.The contents (at.%) of Mg, Zn, Sn, and Mn at position A in Figure9aare 16.63, 3.35, 0.38 and 79.63, close to the α-Mn phase.Figure 10a-c shows the SAED patterns along [112 _ ]α-Mn, [1_33]α-Mn, and [111]α-Mn of the Mn-rich phase at position A (Figure9a) acquired via a series of tilting.The Mn-rich phase almost conforms to the crystal structure of the α-Mn phase, which has a body-centered cubic (BCC) crystal structure with a space group of I4 _

14 _ 5 _ 3 ] 3 _
Figure 9 shows the HAADF image and corresponding EDS results for nanoscale Mn-rich particles in the grain, which is similar to the phases in the red box marked as D in Figure 6a.The contents (at.%) of Mg, Zn, Sn, and Mn at position A in Figure 9a are 16.63, 3.35, 0.38 and 79.63, close to the α-Mn phase.Figure 10a-c shows the SAED patterns along [11 _ 2] α-Mn , [

Figure 9 .
Figure 9. (a) HAADF image and the EDS quantitative results of nanoscale Mn-rich phase at position A in ZTM441 as-cast alloy, and the (b) mixed map of HAADF image and the EDS maps of (c) Mg, (d) Zn, (e) Sn, and (f) Mn.

Figure 9 .
Figure 9. (a) HAADF image and the EDS quantitative results of nanoscale Mn-rich phase at position A in ZTM441 as-cast alloy, and the (b) mixed map of HAADF image and the EDS maps of (c) Mg, (d) Zn, (e) Sn, and (f) Mn.
Figure11shows the HAADF image and EDS maps for the second phases in ZTM441-0.3Alas-cast alloy, and Table5shows the corresponding composition of the labeled second phases in Figure11a.According to EDS quantitative results, the contents (at.%) of Mg, Zn, Sn, Mn, and Al at position A in Figure11aare 59.49, 0.25, 39.97, 0.11, and 0.18, close to Mg 2 Sn phase.The contents (at.%) at position B are 20.84,78.81, 0.07, 0.16, and 0.12; at position C are 37.69, 61.39, 0.05, 0.08, and 0.79; at position D are 29.34,69.52, 0.08, 0.12, and 0.80; and at position E are 35.75,63.52, 0.07, 0.07, and 0.59.Mn is almost non-existent in the Zn-rich phase due to the Mn mainly forming the Al-Mn-rich phase in ZTM441-0.3Alas-cast alloy, which is different from that in the ZTM441 as-cast alloy.
Figure11shows the HAADF image and EDS maps for the second phases in ZTM441-0.3Alas-cast alloy, and Table5shows the corresponding composition of the labeled second phases in Figure11a.According to EDS quantitative results, the contents (at.%) of Mg, Zn, Sn, Mn, and Al at position A in Figure11aare 59.49, 0.25, 39.97, 0.11, and 0.18, close to Mg2Sn phase.The contents (at.%) at position B are 20.84,78.81, 0.07, 0.16, and 0.12; at position C are 37.69, 61.39, 0.05, 0.08, and 0.79; at position D are 29.34,69.52, 0.08, 0.12, and 0.80; and at position E are 35.75,63.52, 0.07, 0.07, and 0.59.Mn is almost non-existent in the Zn-rich phase due to the Mn mainly forming the Al-Mn-rich phase in ZTM441-0.3Alas-cast alloy , which is different from that in the ZTM441 as-cast alloy.
were, respectively, indexed as along [013] Mg 2 Sn , [011] Mg 2 Sn , and [ _ 112] Mg 2 Sn for the Sn-rich phase at position A. The crystal structure of the Sn-rich phase almost conforms to Mg 2 Sn phase, which has an FCC structure with a space group of Fm _ 3 m.The calculated lattice parameters are a = b = c = 7.175 Å, close to those in ZTM441 as-cast alloy (a = b = c = 7.381 Å) in Figure 6, slightly different from the theoretical value (a = b = c = 6.759Å). Figure 12d-f shows the HRTEM images of the Mg 2 Sn phase taken along the [011] Mg 2 Sn , [013] Mg 2 Sn , and [ _ 112] Mg 2 Sn zone axis, which further indicate that the Sn-rich phase mainly has an Mg 2 Sn-type structure, and certain regions were damaged due to electron beam irradiation during FIB sample preparation or TEM observation (see Figure11a).

Figure 14 1 _ 0 ] 1 _ 0 ]
Figure 14 shows the SAED pattern, HRTEM images, and FFT electron diffraction patterns of the area containing position D and E along the [21 _ 1 _ 0]MgZn //[011]Mg Zn zone axis.Based on the FFT electron diffraction patterns (inset in Figure 14b,c), it was confirmed that the diffraction spots in Figure 14a marked with red and yellow circles are contributed by positions E and D, respectively.The Zn-rich phase at position D almost conforms to the crystal structure of the Mg4Zn7 phase, which has a monoclinic structure with a space group of C2/m.The calculated lattice parameters are a = 25.820Å, slightly different from the theoretical value (a = 25.96Å [57,58]).The Zn-rich phase at position E also has the MgZn2 structure as that at position B. The coherent interfaces along [011]Mg Zn , and [21 _ 1 _ 0]MgZn 2 are shown in Figure 14b.Mg4Zn7 and MgZn2 usually co-exist because of their similar formation energy[59,60].MgZn2 and Mg4Zn7 phases usually precipitate after heat treatment or deformation for certain magnesium alloys containing Zn[58,[61][62][63]; however, these phases have rarely been reported in the as-cast alloy.

Figure 15 .
Figure 15.(a) HAADF image and the EDS quantitative results (inset) of the Al-Mn-rich phase in ZTM441-0.3Alcast alloy at position A and the EDS maps of (b) Mg, (c) Zn, (d) Sn, (e) Mn, and (f) Al.

Figure 15 .
Figure 15.(a) HAADF image and the EDS quantitative results (inset) of the Al-Mn-rich phase in ZTM441-0.3Alcast alloy at position A and the EDS maps of (b) Mg, (c) Zn, (d) Sn, (e) Mn, and (f) Al.

Figure 15 .
Figure 15.(a) HAADF image and the EDS quantitative results (inset) of the Al-Mn-rich phase in ZTM441-0.3Alcast alloy at position A and the EDS maps of (b) Mg, (c) Zn, (d) Sn, (e) Mn, and (f) Al.

Materials 2023 , 21 Figure 16 .
Figure 16.(a) BF image and the SAED pattern of the Al-Mn-rich phase in the ZTM441-0.3Alalloy taken along [001]Al Mn zone axis in region A in Figure 15a and the HRTEM images (b) at the edge of the phase along [001]Al Mn zone axis and (c) in the phase along [001]Al Mn zone axis.

Figure 17 .
Figure 17.(a,b) HAADF images and corresponding EDS maps of (c) Zn element and (d) Sn element for the phases in gray contrast in ZTM441-0.3Alas-cast alloy.

Figure 16 .
Figure 16.(a) BF image and the SAED pattern of the Al-Mn-rich phase in the ZTM441-0.3Alalloy taken along [001] Al 3 Mn 2 zone axis in region A in Figure 15a and the HRTEM images (b) at the edge of the phase along [001] Al 3 Mn 2 zone axis and (c) in the phase along [001] Al 3 Mn 2 zone axis.

Figure 16 .
Figure 16.(a) BF image and the SAED pattern of the Al-Mn-rich phase in the ZTM441-0.3Alallo taken along [001]Al Mn zone axis in region A in Figure 15a and the HRTEM images (b) at the edge o the phase along [001]Al Mn zone axis and (c) in the phase along [001]Al Mn zone axis.

Figure 17 .
Figure 17.(a,b) HAADF images and corresponding EDS maps of (c) Zn element and (d) Sn elemen for the phases in gray contrast in ZTM441-0.3Alas-cast alloy.

Figure 17 .
Figure 17.(a,b) HAADF images and corresponding EDS maps of (c) Zn element and (d) Sn element for the phases in gray contrast in ZTM441-0.3Alas-cast alloy.

5 .
73 at.%.The measured lattice parameters of the MgZn 2 are a = b = 5.985 Å, c = 9.284 Å, obviously larger than the theoretical values a = b = 5.223 Å and c = 8.566 Å, which may be caused by the solid solution of Mn element in the MgZn 2 phase.Similarly, the content of Zn in the α-Mn in the ZTM441 alloy is 3.35 at.%.The measured lattice parameters are a = b = c = 10.108Å, obviously larger than the theoretical values a = b = c = 8.913 Å, which may be caused by the solid solution of Zn element in the α-Mn phase.

Table 1 .
Actual compositions of the as-cast alloys in this work (wt.%).

Table 2 .
Composition of the labeled second phases in ZTM441 as-cast alloy in Figure3(at.%).

Table 4 .
Composition of the labeled second phases in ZTM441 as-cast alloy in Figure6a(at.%).

Table 4 .
Composition of the labeled second phases in ZTM441 as-cast alloy in Figure6a(at.%).