Effect of Trace Elements on the Crystallization Temperature Interval and Properties of 5xxx Series Aluminum Alloys

The influence of alloying elements Er, Zr, Cu, Si and Zn on the crystallization temperature interval, microstructure, mechanical properties and corrosion behavior of Al-Mg-Mn alloy were studied by differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), tensile testing, electrochemical measurements and nitric acid mass loss test (NAMLT). The results show that the crystallization temperature range of Al-Mg-Mn alloy with addition of Zn decreased 4.7 °C. Cold rolled alloys mainly contain S texture, Copper texture, Brass texture, and Goss texture; the content of the S texture is the highest. With the addition of trace elements, the second phase Al3Er, Al3Zr, Al2CuMg, Mg2Si and MgZn2 can be formed, which can improve the tensile strength and yield strength of Al-Mg-Mn alloy. The addition of the alloying element Zn can also improve the intergranular corrosion resistance of the Al-Mg-Mn alloy.

50Cu (wt.%) master alloy. Appropriate amounts of the starting materials were melted in a crucible by heating the surrounding furnace to 760 °C. Subsequently, the liquid metal was poured in a cast iron mold at about 720 °C. The nominal compositions of the alloys are shown in Table 1. The as-cast alloys were homogenized at 460 °C for 24 h to eliminate segregation, then cold rolled to plates with 2 mm thick. The Al-3Mg-0.5Mn-0.2Cu ingot is extremely high in hardness, which leads to a decrease in the plastic deformation ability of the alloy during rolling. In this experiment, the Al-3Mg-0.5Mn-0.2Cu ingot is annealed at 400 °C for 1 h, cold rolling is performed after air cooling to room temperature. Table 1. Nominal chemical compositions of studied alloys (wt. %).

Alloys
Mg The cold rolled sample was mechanically and electrolytically polished to prepare a sample for EBSD analysis. EBSD analysis was carried by Zeiss Auriga FIB SEM (Zeiss, Jena, Germany). The electrolyte composition is 10% perchloric acid and 90% absolute ethanol. Due to the large deformation of the cold-rolled alloy and the severe stress concentration, it will affect the EBSD signal collection. To this end, we use the Ion Beam Milling System Leica EM RES102 (Leica, Wetzlar, Germany) to surface-treat the polished alloy.
X-ray diffraction (XRD) analysis was carried by Panalytical Empyrean XRD system (Panalytical, Almelo, The Netherlands) using Cu Kα radiation for identifying the phase composition in cold rolled alloys. The scan was made between the angular range of 5°~90°, with the X-ray generator power set at 45 kV and 40 mA.
Thermal analysis was conducted by means of differential scanning calorimetry using a calorimeter STA 449F3 from Netzsch produced by Germany. The range of temperature rise was 25~800°C, and the heating rate was 10 k/min. The tensile specimens were machined according to GB/T 16865-2013 size specifications [33]. The alloys were subjected to a tensile test at room temperature using a SHIMADZU AG-IC 50KN tensile testing machine (Shimadzu, Kyoto, Japan) to determine its mechanical properties. The tensile direction was parallel to the rolling direction, tensile specimens with a gauge length of 25mm and a width of 6 mm, and the strain rate was 2.5 × 10 −4 s −1 . Three samples were taken from each alloy, and its average value was taken as the experimental result.
The polarization curve was measured on the RST5200F electrochemical workstation (Shiruisi, Zhengzhou, China), and the samples were polished by sandpaper, mechanical polishing, deionized water cleaning, alcohol ultrasonic cleaning and blow-drying with a blower. The effective working area of the sample was 0.64 cm 2 . The three-electrode test system was used, the working electrode was the exposed surface of the sample, the counter electrode was a platinum electrode, and the reference electrode was a saturated calomel electrode. The medium solution used in the experiment was 3.5wt.% NaCl solution, and the scanning rate was 1 mv/s. Nitric acid mass loss test (NAMLT) was used to evaluate the susceptibility to IGC in terms of the degree of sensitization (DoS) as guided by ASTM G67 standard practice [34].

Microstructure Analysis
EBSD technology is mainly to determine the crystal orientation of a certain area of the material and cannot directly obtain the microstructure and structure information of the material. The orientation information of the obtained material can be reconstructed into the orientation imaging image (OIM) by using Channel 5 software (HKL Channel 5, Oxford Instruments, Abingdon, Oxfordshire, UK). Different orientations correspond to the corresponding colors, which can clearly reflect the microstructure and texture of the material. After plastic deformation, as the degree of deformation increases, the original equiaxed grains will gradually elongate in the direction of their deformation. When the amount of deformation is large, the crystal grains become blurred, and the crystal grains are hard to distinguish and present a piece of fiber-like streaks, which is called fiber structure. The direction in which the fibers are distributed is the direction in which the material flows and expands. Figure 1 is the EBSD diagram of the cold-rolled alloy. It can be seen from Figure 1 that after the cold-rolled alloy is deformed by a large deformation (90%), the grains of the alloy are severely elongated, broken and distributed in a fiber-like manner along the RD direction. In Figure  1a-c,f, mesh-shaped shear bands were formed, the shear bands marked by black arrows. It is at an angle of 30°~45° with the rolling direction. Compared with the control Al-3Mg-0.5Mn alloy, the addition of the alloying elements resulted in the refinement of the fiber structure after cold rolling. The distribution of the misorientation angles for cold rolled alloys are shown in Figure 2. The abscissa indicates the grain orientation difference, and the ordinate indicates the frequency of occurrence. The black and gray lines in Figure 1 represent high angle grain boundaries (HAGBs, misorientation angles greater than 15°) and low angle grain boundaries (LAGBs, misorientation angles between 2° and 15°), respectively. After the 90% deformation of the cold-rolled alloy, the grain orientation difference is mainly concentrated in the range of 2°~10°, and the small-angle grain boundary accounts for a large proportion. This is due to the fact that the rolling deformation causes the grain splitting to result in a small angle grain boundary.  Figure 3 is a recrystallized region distribution map of a cold rolled alloy. The blue region represents recrystallized grains, the red regions represent deformed grains, and the yellow regions represent substructures. The frequency of the three different types of grains is plotted in Figure 4. After the cold-rolled alloy is deformed by 90% deformation, the grain type is mainly the deformation grain (frequency greater than 90%) of the small-angle grain boundary, and there are also some deformation substructures and a small amount of recrystallized grains. The recrystallization of the cold rolled alloy is due to the statistical error of the analytical software and the friction between the rolls and the sheet during rolling, and the heat generated causes partial recrystallization of the alloy. Mapping of different types of grains: blue-recrystallized, yellow-substructured, and reddeformed. Grain boundaries with misorientations larger than 15° were superimposed as black lines while subgrain boundaries with misorientations smaller than 15° but larger than 5° were superimposed as gray lines: (a) Al-3Mg-0.5Mn; (b) Al-3Mg-0.5Mn-0.2Er; (c) Al-3Mg-0.5Mn-0.2Zr; (d) Al-3Mg-0.5Mn-0.2Cu; (e) Al-3Mg-0.5Mn-0.2Si; (f) Al-3Mg-0.5Mn-0.2Zn.  The orientation imaging of the cold rolled alloys are shown in Figure 6. Different colors represent different texture types. The content of five typical textures was determined in this paper, and other texture types were classified as random textures. As can be seen from Figure 6, the S texture has the highest content. The S texture content of the Al-3Mg-0.5Mn-0.2Cu alloy is not much changed compared with the Al-3Mg-0.5Mn alloy, and the S texture of other alloys is increased.  Table 2 shows the texture statistics for different cold rolled alloys. This result is consistent with the texture strength in the pole figure of Figure 5.  Figure 7 shows X-ray diffraction (XRD) patterns of cold rolled alloys. As can be seen from Figure  7, in the cold-rolled alloys, in addition to the Al6Mn phase, it is mainly the matrix phase Al. The addition of trace elements forms the second phase Al3Er, Al3Zr, Al2CuMg, Mg2Si, MgZn2, and further contains an AlFeSi impurity phase. These second phases can significantly affect the properties of the cold rolled alloys. Fe and Si are common impurity elements in aluminum and its alloys. Although high-purity aluminum and industrial pure magnesium are used to melt-cast alloys, they may contain trace amounts of Fe, Si elements, and may also be introduced during the smelting process. Since the content of trace elements added in the alloy is only 0.2wt.%, only weak peaks are found in the diffraction pattern of the alloy, which indicates that the content of each phase in the cold rolled alloy is small, and it is difficult to completely determine by XRD.

As-Cast DSC Analysis
The DSC curve of the as-cast alloys are shown in Figure 8. It can be seen from Figure 8 that with the addition of alloying elements, the DSC curve characteristics of the alloy change, and the peak shape appears to be different. There is a distinct endothermic peak on the heating curve, and there is also a distinct exothermic peak on the cooling curve. These two distinct peaks correspond to the melting and solidification of the alloy.  According to the DSC heating curve, the starting point of the endothermic peak is the solidus temperature, and the starting point of the exothermic peak on the cooling curve is the liquidus temperature [35]. The temperature difference between the two is the crystallization temperature range of the as-cast alloy. The results are shown in Table 3. It can be seen from Table 3 that the Al-3Mg-0.5Mn-0.2Zn alloy with Zn element has the lowest crystallization temperature range. Compared with the Al-3Mg-0.5Mn alloy, the crystallization temperature interval is lowered by 4.7 °C. Thus, the casting properties of the alloy are improved. This is because when Zn is added to the Al-3Mg-0.5Mn alloy, the MgZn2 strengthening phase is formed, and a certain amount of Mg content is consumed, which lowers the crystallization temperature range of the alloy.

Analysis of Mechanical Properties
Tensile test of the cold-rolled alloy sheet was carried out at room temperature, and the initial strain rate was 2.5 × 10 −4 s −1 , the stress-strain curves are shown in Figure 9, and the results are summarized in Figure 10. As can be observed in figure 9, the measured stress-strain curve of the sample drifts in the initial stage (elastic deformation stage) since the extensometer is not equipped. However, as all samples are tested with same machine, the influence can be ignored. It can be seen from Figure 10 that the tensile strength and yield strength of the Al-3Mg-0.5Mn alloy are improved after adding a small amount of alloying elements Er, Zr, Cu, Si and Zn. Among them, the strength of Cu-containing alloy is the highest, followed by Zr-containing alloy. The tensile strength are 363 and 359 MPa, which are 31 and 27 MPa higher than the control alloy, respectively.The addition of 0.2wt.% of the Er element cannot refine the crystal grains of the alloy, while the Al3Er second-phase particles formed by it can produce second-phase strengthening. These particles can strongly pin dislocations, increase the tensile strength, and yield strength of the alloy and decrease the elongation. The addition of the Zr element increases its strength and slightly decreases its elongation. When Zr is added to the Al-3Mg-0.5Mn alloy, the grains are refined and the strength is improved. At the same time, due to the refinement of the grains of the alloy, plastic deformation can be carried out in more grains, so that the plasticity of the Al-3Mg-0.5Mn-0.2Zr alloy is improved. In addition, Al3Zr second-phase particles formed in the alloy can cause second-phase strengthening, which can also increase the strength of the alloy and reduce the plasticity of the alloy. The addition of Cu element makes the strength of Al-3Mg-0.5Mn alloy increase most significantly, and its elongation has also improved. This can be explained from the following two aspects: First, add 0.2wt.% Cu element to Al-3Mg-0.5Mn alloy refines the crystal grains of the alloy, which can make the plastic deformation of the cold-rolled alloy more uniform and increase the elongation of the alloy. Besides, Al2CuMg second-phase particles formed in the alloy can strongly pin dislocations, increasing the strength of the alloy and reducing its plasticity. Therefore, the increase of the elongation of Al-3Mg-0.5Mn-0.2Cu alloy is the result of the combined effect of the two. The increase of the strength of the alloy after adding Si element is also due to the fine grain strengthening and the second phase strengthening, with a slight increase in the elongation, but it is not obvious. It is also due to the formation of the second phase particles of Mg2Si, which improves the strength and reduces the plasticity of the alloy at the same time, while the refinement of the grains improves the plasticity of the alloy. The joint action of the two makes the elongation of Al-3Mg-0.5Mn-0.2Si alloy slightly increased, but not significantly. After the addition of Zn, the strength and elongation of the alloy are improved, which is mainly due to the fact that the MgZn2 particles can strongly pin dislocations during the plastic deformation of the alloy, resulting in secondary phase strengthening in Al-3Mg-0.5Mn-0.2Zn alloy.  Figure 11 is the polarization curve of the experimental alloys in a 3.5wt.% NaCl solution. Using the software provided by the electrochemical workstation, the self-corrosion potential and corrosion current density of the experimental alloys were fitted according to the Tafel curve extrapolation method as shown in Table 4. It can be seen from Table 4 that in addition to the Si element, the addition of other alloying elements increases the self-corrosion potential of the Al-Mg-Mn alloy and lowers the corrosion current density, and the corrosion resistance is improved. Among them, the alloy with Cu added has a self-corrosion potential of −0.79 v, which is improved in corrosion resistance compared with the control alloy. Most of the previous studies have shown that the addition of Cu in the Al-Mg alloy leads to a decrease in corrosion resistance, which is inconsistent with the experimental conclusions obtained in this paper. At present, there is still debate about the influence of Cu on the corrosion performance of Al-Mg alloy [36]. The Mg2Si particles formed by adding Si to the Al-3Mg-0.5Mn alloy are anodes with respect to the substrate, which accelerate the corrosion rate of the alloy [31], resulting in reduced corrosion resistance. The addition of Zn element precipitates MgZn2 phase at the grain boundary, and its self-corrosion potential is closer to that of aluminum matrix than that of β (Al3Mg2) phase, so that the potential difference between MgZn2 phase and matrix is smaller, and it is not easy to form a corrosion cell, so that the self-corrosion potential of Al-3Mg-0.5Mn-0.2Zn alloy is 80 mv higher than that of Al-3Mg-0.5Mn alloy, and the corrosion current density is one order lower than that of Al-3Mg-0.5Mn alloy. Thus, the corrosion resistance of the alloy is improved. Figure 11. Polarization curve of cold rolled alloys in 3.5wt.% NaCl solution. Table 4. Electrochemical parameters of cold rolled alloys in 3.5wt.% NaCl solution.

Analysis of Intergranular Corrosion Performance
Intergranular corrosion is a locally corroded corrosion phenomenon in which a metal material corrodes along a grain boundary or a grain boundary of a material in a specific corrosive medium, causing a loss of bonding force between the crystal grains. In the homogenization process, the Al3Mg2 phase is precipitated in the alloy. The phase tends to grow on the grain boundary. The electrode potential is lower than that of the matrix, which forms a corrosive galvanic cell with the matrix. The Al3Mg2 phase acts as an anode and preferentially dissolves, causing grain boundaries. corrosion. Alloys resistant to intergranular corrosion have a mass loss of about 1 to 15 mg/cm 2 in the NAMLT test. The results of intergranular corrosion of the cold rolled alloys are shown in Figure 12. It can be seen from Figure 12 that after the addition of trace elements, the intergranular corrosion resistance of the alloys does not change much, both being in the range of 1~2mg/cm 2 , both less than 15mg/cm 2 , belonging to the intergranular corrosion resistant alloy. With the addition of the Er element, the precipitation of Al3Mg2 phase increases accordingly, which increases the grain boundary corrosion current, accelerates the corrosion rate, and deepens the intergranular corrosion depth. The addition of Cu element can significantly change the electrode potential in the solid solution, and the selfcorrosion potential of Al2CuMg containing Cu phase is low. The primary cell can be formed between the copper poor area of the grain boundary and the precipitated phase, precipitated phase and matrix of the grain boundary, which reduces the intergranular corrosion resistance of the alloy. The addition of Si can lead to the Mg2Si phase precipitate on the grain boundaries. The self-corrosion potential is lower than that of the β (Al3Mg2) phase. The potential difference between the Mg2Si phase and the matrix becomes larger, and it is easy to form a primary cell. Therefore, the intergranular corrosion resistance of the alloy is reduced. The addition of Zn element precipitated the MgZn2 phase at the grain boundary, and its self-corrosion potential was closer to the self-corrosion potential of the matrix than the β (Al3Mg2) phase. The potential difference between the MgZn2 phase and the matrix is small, and it is not easy to form a galvanic cell. The precipitation of the MgZn2 phase at the grain boundary also suppresses the continuous precipitation of the β (Al3Mg2) phase at the grain boundary, which improves the alloy Intergranular corrosion performance.

Conclusions
By studying the influence of Er, Zr, Cu, Si, Zn on the crystallization temperature interval, mechanical properties and corrosion behavior of Al-3Mg-0.5Mn alloy, the following conclusions can be drawn: 1. The grains of the cold rolled alloys are severely stretched, broken and distributed fibers along the RD direction, and a large number of fine grains are sandwiched between the fibrous structures. In the Al-3Mg-0.5Mn alloy, Al-3Mg-0.5Mn-0.2Er alloy, Al-3Mg-0.5Mn-0.2Zr alloy and Al-3Mg-0.5Mn-0.2Zn alloy, a mesh-like shear band is formed. It is distributed at an angle of 30°~45° to the rolling direction. The grain types are mainly deformed grains with a small angle grain boundary (frequency is greater than 90%), and there are some deformed substructures and a small amount of recrystallized grains. The cold rolled alloys mainly contains S texture {123} <634>, Copper texture {112} <111>, Brass texture {110} <112> and Goss texture {110} <001>, of which S texture extreme density is the strongest. The addition of alloying elements increases the content of deformed texture.
2. Al-3Mg-0.5Mn-0.2Zn alloy has the lowest crystallization temperature range. Compared with Al-3Mg-0.5Mn alloy, the crystallization temperature range is reduced by 4.7 °C. The addition of Er, Zr, Cu and Si to Al-3Mg-0.5Mn alloy did not decrease the crystallization temperature range of the alloy but increased it.
3. The addition of trace elements Er, Zr, Cu, Si and Zn all improved the tensile strength and yield strength of Al-3Mg-0.5Mn alloy. Among them, the strength of Cu-containing alloy increased most significantly, followed by Zr-containing alloy. The tensile strength are 363 and 359 MPa, respectively, increased by 31 and 27 MPa. The strengthening mechanism of the alloy containing Er and Zn elements is mainly the second phase strengthening, and the strengthening mechanism of the alloy containing Zr, Cu and Si elements is mainly the fine grain strengthening and the second phase strengthening.
4. In addition to Si, the addition of Er, Zr, Cu, and Zn elements increases the self-corrosion potential of Al-3Mg-0.5Mn alloy, reduces the corrosion current density, and improves the corrosion resistance. After the addition of trace elements, the alloy's resistance to intergranular corrosion changes little, and the mass loss is in the range of 1~2mg / cm 2 , all less than 15mg/cm 2 , which belongs to the intergranular corrosion resistant alloy. The addition of the Zn can improve the intergranular corrosion property of the Al-3Mg-0.5Mn alloy. 5. In the experimental range, the Al-3Mg-0.5Mn-0.2Zn alloy has a lower crystallization temperature range and has both good mechanical properties and corrosion resistance, and comprehensive performance is good.