Microstructure Evolution of Mg-2Y-0.6Nd-0.6Zr Alloy Rolled at Room and Liquid Nitrogen Temperature

: The microstructure evolution, texture, mechanical behavior and twin deformation of the ECAPed Mg-2Y-0.6Nd-0.6Zr alloy at liquid nitrogen temperature were investigated by rolling samples. The ECAP processed material appeared the texture of 45 ° to the extrusion direction and its yield strength reached 93.6 MPa. The results showed that cryorolling encourages twinning in Mg-2Y-0.6Nd-0.6Zr alloy, enhancing the tensile strength and texture. Activation of {10-12} twinning during rolling was found to be more pronounced in the cryorolled samples than in the cold rolled samples owing to a lower temperature. As a result, the cryorolled samples had more twins than and cold rolled ones, the proportion of twin areas of room temperature rolling and ultra-low temperature rolling were: 2.445% and 4.234%.


Introduction
Magnesium (Mg) alloys are currently the lightest commercial metal structural material. At the same time, it has high specific strength, good damping and vibration reduction, good thermal conductivity and good electronic shielding effects [1][2][3][4]. Therefore, Mg alloys have attracted increasing attention in aerospace, pharmaceutical, chemical and automotive fields in recent years [5][6][7]. However, the limited number of available slipping systems at room temperatures due to its hexagonal close-packed (HCP) structure leads to poor deformability [8][9][10]. Therefore, it is of great significance to study the plastic deformation mechanism and mechanical properties of magnesium alloys at low temperature. Still further work is needed in this area since, for example, the evolution of texture and twinning in Mg alloys materials during cryorolling is still not thoroughly understood.
In recent years, some researchers have also conducted a series of studies on the plastic deformation of Mg and Mg alloys at room temperature. Volkov [11] et al. used cold hydrostatic extrusion and low-temperature annealing to improve the mechanical properties of pure magnesium. The results of the study found that the magnesium was wrapped to prevent the material surface from cracking, constrained and improved performance, and the pure magnesium was deformed at a lower temperature to produce ultrafine-grained pure magnesium with 3 m and high plasticity. O.V. Antonova [12] et al. performed severe plastic deformation extrusion on three pure magnesium samples with different columnar structure orientations at room temperature. The study showed that activation of twins during deformation changed the orientation of the original grains, which was more conducive to sliding. In most regions, substructures such as 100 nm cells are found. Under the condition of high load value, it may have provided the necessary stresses for activation of non-basal plane slip systems with high Schmitt factor, and the grain size is significantly refined, reaching more than two orders of value to the initial size. SQ Zhu et al. [13] showed that dynamic twin boundaries and stacking faults are conducive to the formation of low-angle grain boundaries, which can be transformed into high-angle grain boundaries and form ultra-fine dynamic recrystallized grains. Wang et al. [14] showed that for the grains of about 6-22 m, the dislocation slip is mainly dominated during the deformation process. However, for the grains of about 22-41 m, the twins are mainly dominated by the deformation process.
Mg alloys exhibit different deformation mechanisms and mechanical behaviors at different temperatures. There is no doubt that temperature is an important factor that affecting the deformation mechanism of magnesium alloys with different properties. In addition, many studies have reported the deformation temperatures of magnesium alloys are at room temperature and above, but few studies have reported mechanical behavior and deformation mechanisms of a rolled Mg alloys at low temperatures, especially ultra-low temperatures (liquid nitrogen temperature). Therefore, the present study systematically investigates the evolution and mechanical properties of the fine-grained Mg-2Y-0.6Nd-0.6Zr alloy under low temperature rolling deformation, such as microstructure, twinning, texture, and explores the plastic deformation mechanism and twinning behavior of Mg alloy at room temperature and ultra-low temperature.

Materials and Methods
The as-received material used in this study was as-cast Mg-2Y-0.6Nd-0.6Zr alloy, which was smelted by lowpressure casting and introduced with SF6 and CO2 gas protection. Rod-shaped magnesium alloys with a size of 12 mm⨉80 mm were cut out of the initial material for ECAP. The samples were homogenized at 450 °C for 6 h and then cooled in air. After that, the Mg-2Y-0.6Nd-0.6Zr alloy bar was placed into an ECAP die and pressed at 400 °C for four passes under a compressive speed of 0.4 mm/s. The ECAP die structure used is shown in Figure 1, where φ = 120 °, Ψ = 30 °. Finally, the ECAPed Mg alloys were rolled in two passes from 12 to 10 mm in diameter at ultra-low temperature (liquid nitrogen immersion) and room temperature, respectively. The total deformation of rolling was 16.7%. In the ultra-low temperature rolling process, the samples were immersed in liquid nitrogen for 10 to 15 minutes before each rolling.
The microstructures and textures of the processed specimens were observed on the ND-ED(RD) plane using optical microscopy (OM), electron backscatter diffraction (EBSD). ND and ED (RD) represent the normal direction and extrusion direction of the extruded sample. Additionally, for OM the grain structure was revealed by subsequent etching using a solution of ethanol (14 ml), water (2ml), glacial acetic acid (4 ml) and picric acid (0.82g). The grain size was measured and calculated using Image-Pro Plus image analysis software. Electron back-scattered diffraction (EBSD) analysis were performed on a S-3400N scanning electron microscope using Channel 5and Mtex software to acquire and analysis data. The tensile samples were machined along the pressing direction, tensile testing was carried out at room temperature (RT) at an initial strain rate of 3×10 -3 s -1 using an electronic universal Instron 8501 testing machine.  Figure 2 shows the microstructure of Mg-2Y-0.6Nd-0.6Zr alloy by using the OM techniques. The average grain size for as-annealed Mg alloy was calculated to be 52 m. After 4 passes equal channel angular pressing, as shown in Figure  2 (b), the effect of shear stress and dynamic recrystallization make the grains significantly refined, Consequently, the average grain size decreases from 52 m to 10 m. After rolling at room temperature and rolling with liquid nitrogen, different degrees of twinning appeared, and the grains were also elongated. Comparing Figure 2 (c) with Figure 2 (d), it can be found that the cryorolled samples had more twins than and cold rolled ones, but there is no obvious twins in the samples rolled at room temperature. In short, when the size of the magnesium alloy is 10 m, the lower the deformation temperature, the easier the twins appear. Figure 3 shows the EBSD inverse pole figure map of the Mg-2Y-0.6Nd-0.6Zr alloy in different states, the observation surface is ED (RD) -ND. As shown in Figure 3 (a), the large grains in the original homogenized structure are directly broken into tens of substructures, and there are many sub-structures inside the large grains after 4 ECAP passes processing, and the accumulated strain and deformation storage energy increase, the effect of dynamic recrystallization is obvious, so that more fine dynamic recrystallized grains are formed and grown, and the grain size is refined to 10 m [15]. However, for rolling at different temperatures, the grain size of the sample does not change significantly, and twins appear inside the grains, which are mirror-like, as shown in Figure 3 (b) and Figure 3 (c), it was observed that the number of twins appeared in ultra-low temperature rolling compared to room temperature rolling. 1) planes are slightly weaker than the rolled texture. With decreasing the deformation temperature, the strongest pole was gradually pulled back to the center, and the typical basal surface wire texture appeared, and the pole density increasing from 8.64 to 10.78. Thus, it can be concluded that ultra-low temperature rolling treatment can increase the texture.    Figure 5 shows the distribution of orientation difference of Mg-2Y-0.6Nd-0.6Zr alloy at different rolling temperatures after ECAP. It can be seen that the proportion of low angle grain boundaries (LAGBs) is 63.4% after 4 passes of equal channel angular pressing. After rolling, the proportion of high angle grain boundaries (HAGBs) decreases, and the proportion of LAGBs increase to about 72%. Of course, it can also be seen from the figure that the peak value at 86 ° increases obviously, which may also be due to the appearance of cone twin. basically exist in relatively large grains when rolled at room temperature, while the twin crystals of small grains are rare. The microstructure of the samples rolled at room temperature is not very uniform, and the large grains are stretched. Figure 6 (c) shows the rolling deformation treatment at ultra-low temperature. A large number of twins are observed in the figure. Compared with the samples deformed at room temperature and high temperature, 86º<112 -0> tensile twins appear more frequently. With the decrease of deformation temperature, the critical shear stress (CRSS) of the basal slip increases continuously, and the basal slip is blocked. At this time, twinings play a vital role and are more likely to occur, and the crystal rotation also adds deformation to change the crystal orientation. At room temperature, the CRSS required to start the non-basal slip system of magnesium and its alloys is about 100 times that of the base18dicated that twins occupy a large part of the deformation at the state of ultra-low temperature, and twins are more likely to be produced at lower temperature than rolling at room temperature. This reflects that twinning is strongly influenced by temperature.

Microstructure
At low temperature, the twin critical shear stress of magnesium alloy is lower than that of pyramidal <a> and pyramidal <c + a> slips . The {101 -2} <101 -1> of the twins has the smallest CRSS value, which is only greater than the basal < a > slip. According to the research, at room temperature, the CRSS value of basal slip of pure magnesium is 0.5-0.7 MPa, and has little relationship with temperature. The CRSS values of cylindrical slip and conical slip are as high as 40MPa or more , but it decreases significantly with increasing temperature. In Mg alloys {101 -1}, the CRSS required for compression twins is 76-153MPa , which is much greater than the CRSS (2-3MPa) required for {101 -2} tensile twins . It is easy to expand from its twin boundary, which is the most important for plastic deformation in magnesium alloys. In conclusion, the above reasons can also explain that 86º <112 -0> tensile twins are more likely to appear during ultra-low temperature rolling. Figure 7 shows the change of Schmid of rare earth magnesium alloy at different deformation temperatures after ECAP. As we all know, the larger the Schmid is, the easier the deformation will be. After 4 passes ECAP deformations, the average Schmid factor is 0.35, which can provide favorable conditions for subsequent deformation, but after room temperature rolling and liquid nitrogen rolling, the Schmid factor decreases significantly, and even tends to 0, so that the basal slip is difficult to carry out. After rolling in liquid nitrogen, the increase of basal texture also reduces the slip deformation of magnesium alloy, which is dominated by twinning and grain rotation. Figure 8 shows the original grain regions R1, R2 selected in the room temperature rolled Mg-2Y-0.6Nd-0.6Zr alloy (Figure 6 (a)). Twins are marked in Figure 8  In Figure 8 (c), it can be found that twins "I", "II" and "III" also have very similar orientations in the polar diagram. Of course, it can be observed from Figure 8 (b) that their 3D orientation diagrams are extremely similar, only rotated a little.

Twin variants of magnesium alloys at low temperature
However, the twin "IV" is different from the previous one, belonging to another twin variant of {101 -2} stretching twin. In grain R2, the number of twins is 2, but it is observed from Figure 8    -1> stretching twins in grain R1 during ultra-low temperature rolling deformation, which have occupied the majority of the grain. The {0001} pole figures corresponding to grain R1 and Figure 9(b) show that the twin "I", "II" and "III" are almost identical in crystal orientation. In addition, there are two twin variants in grain R2. Twins "II" and "III" are independent of each other in grain R2, and there is no delivery. In the grain R3, the twins "I" and "II" are cut, and the angle between the two is about 15°. At least 3 twins exist in the grain. Through observation and analysis of Figure 9(h) and Figure 9(i), it is found that the twins "I", "II", and "III" all have different orientations, but the orientations of the twins "II" and "III" are not much different. The difference in the threedimensional schematic diagram is also small. The twin "I" is different from the other two twins. Not only is the distribution on the (0001) pole figure very different from the others, but the difference can also be observed on the threedimensional stereogram. In short, it can be obtained that there are at least two variants of {101 -2} tensile twins in grain R3.

Analysis of mechanical properties
The corresponding tensile properties are shown in Table 1. It can be observed in the figure that after rolling at different temperatures, the strength and plasticity of the sample have changed greatly. After 4 passes of equal channel corner extrusion, the tensile strength of the material is 181MPa, the yielding strength is 93.67MPa and the elongation is 28.5%. After rolling, the properties of the materials have been improved to varying degrees. The tensile strength of the sample rolled at room temperature is 218.04MPa and the yield strength is 174.79MPa. The tensile strength of the sample deformed by ultra-low temperature rolling has reached 245.89MPa, which is about 12.7% higher than that at room temperature and the yield strength is 216.21 MPa, an increase of about 23.69% compared to room temperature rolling. The main reason is that at low temperature and room temperature, Mg-2Y-0.6Nd-0.6Zr alloy has a large number of small grains formed inside the grains. At the same time, due to the work hardening, texture strengthening and twinning, the material is in during room temperature tensile deformation, twins hinder the plastic deformation behavior, so that the plasticity of the material is reduced, but the strength is improved.
Mg-2Y-0.6Nd-0.6Zr alloy has a strong basal texture after cold rolling and ultra-low temperature rolling. It can be seen from Figure 4 that the basal surface texture strength is 9.38 and 10.78, respectively. Stretching along its ED (RD) direction, in polycrystalline materials, the relationship between yield strength and critical shear stress is: Where σ is the yield strength, τCRSS is the critical shear stress, and m = cosφ * cos represents the orientation factor, also known as the Schmidt factor. The orientation factor has a great influence on the tensile yield strength of magnesium alloy materials. Earlier, EBSD was used to calculate the Schmid factor of the micro-basal slip. The Schmid factor of the basal slip decreases with the decrease of the deformation temperature. After rolling at room temperature and ultra-low temperature rolling, the Schmid factor is relatively small as 0.33 and 0.28, respectively. The larger the Schmid factor, the easier the basal slip is activated, but the opposite is not easy. When deformed at room temperature, the slip system is mainly based on the basal slip. For Mg-2Y-0.6Nd-0.6Zr alloy samples with a strong base texture, it will cause the phenomenon that the yield strength increases and the plasticity decreases in the tensile test, which is also the effect of texture strengthening. Therefore, the tensile strength of ultra-low temperature rolling increased to 245.89MPa, and the plasticity decreased compared with the four-pass deformation of equal channel angular extrusion and room temperature rolling deformation.

Conclusions
(1) The proportion of twins in room temperature rolling is 2.445%, and that in ultra-low temperature rolling is 4.234%. At the same amount of deformation, the lower the temperature, the more twins can be excited. Compared with cold rolling, there are more twins in liquid nitrogen rolling. At the same time, there are more variants of {101 -2} stretching twins during ultra-low temperature deformation, and the appearance of variants is mainly related to the Schmid factor.
(2) After four passes of equal channel angular pressing, the texture of (0001) plane is 45 ° to the extrusion direction, and the texture gradually changed after ultra-low temperature rolling, thus forming a typical basal texture after ultralow temperature rolling, and its maximum polar density is 10.78. After four passes of equal channel angular pressing, the Schmid factor is 0.35. After room temperature rolling and ultra-low temperature rolling, the Schmid factor is relatively small, which is 0.33 and 0.28 respectively, and the base slip is not easy to start.
(3) Comparing the tensile properties of room temperature rolled samples and ultra-low temperature rolled samples, their yield strengths were 174.79 MPa and 216.21 MPa, respectively, and ultra-low temperature rolling increased about 23.69% compared to room temperature rolling. However, the rolled samples showed increased strength and decreased plasticity. This is the result of the combined effects of work hardening, texture strengthening and changes in twin content.
Author Contributions: Y.T., X.S., conceived and designed the experiment; A.L. and X.S. contributed to retrieve documents; Y.T. collected the data; Y.T. analyzed the data and wrote the paper. This article was checked and revised by W.L.