Effect of rolling temperature on microstructure and mechanical properties of hot-rolled Mg-Al-Ca-Zn-Mn alloys

In order to address the problems of traditional magnesium alloys, such as the easy formation of basal texture and poor plasticity at room temperature, a multivariate micro alloyed Mg-1Al-0.9Ca-0.5Zn-0.4Mn material has been designed by taking advantage of the unique role of solute atoms such as Ca. The effects of its rolling process on grain size, second phase, texture, and tensile properties were compared and analyzed. The results show that the rolling temperature will significantly change the organization and properties: a small number of large-sized reticulate phases, linear phases, and granular phases will exist along the rolling direction inside the Mg-1Al-0.9Ca-0.5Zn-0.4Mn alloy. When the rolling temperature is raised, it will change the number and size of the second phase and microstructure morphology. For three plates rolled at 300°C, the best-combined strength-plasticity mechanical properties were presented in this work; elongation is 2.5%, tensile strength is 167 MPa, and yield strength is 146 MPa along the RD, respectively. Besides, some larger-sized phases still exist in this alloy, which affects its mechanical properties.


Introduction
Magnesium alloy has super-high potential for being lightweight, with high strength, high ductility, good machinability, and well damping properties.Magnesium and its alloys can be used in aerospace, rail transportation, electronic information, and automotive industries [1,2].Nevertheless, compared to steel and aluminum alloys, magnesium alloys have a large gap between their practical application and development potential.The low strength of magnesium alloy is the main reason for this phenomenon.Finding effective ways to promote the strength of magnesium alloy is a world-class problem.Adding rare earth elements is a valid way to promote the strength of magnesium alloys, but rare earths are very expensive and scarce, which limits their application [3,4].In recent years, the Mg-Ca system magnesium alloys, which do not contain rare earth elements and have higher strength, have attracted much attention.The reason is that the Ca element has high equilibrium solid solubility in the α-Mg matrix, and its solid solubility will decrease significantly with the decrease in temperature.Besides, Mg2Ca precipitated phase and α-Mg matrix has a similar crystal structure.The Ca element could accelerate the precipitation of the precipitated phase, so the content of the precipitated phase in Mg-Al-Ca alloys is larger [5].At the same time, adding trace Ca elements will also refine grain and weaken weave structure, so most of the Mg-Ca deformed magnesium alloys have excellent tensile properties [6].In addition to the enhancement of mechanical properties, studies have shown that alloying elements such as Ca and Mn can also play a role in weakening the texture.The texture weakening mechanism is related to the recrystallization nucleation mechanism (PSN) induced by the second phase particles containing Ca or Mn and the reduction of the lattice constant [7].
The study aims to find effective solutions for the bottleneck problems of low thermo-mechanical properties, poor formability, and low service strength.Based on what has been discussed above, a kind of Mg-1Al-0.9Ca-0.5Zn-0.4Mnalloy was designed using the micro-alloying method, and the organization and properties were regulated by warm-rolling.We systematically investigated the effect of the warm-rolling temperature on the microstructure evolution and tensile properties.The experimental results could provide a theoretical basis and guidance for the industrial production of these alloys.

Experimental procedures
The ingot Mg-1Al-0.9Ca-0.5Zn-0.4Mn(wt.%) was obtained by melting pure magnesium, pure zinc, pure manganese, Mg-30wt.%Camaster alloy, and Al-10wt.%Mnmaster alloy in steel crucibles.The molten material was carefully poured into a sand mold using low-pressure casting techniques and then allowed to cool naturally through air convection.The chemical makeup of the sample is summarized in Table 1, in which Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES) was then used to analyze the quantification.150 × 50 × 17 mm steel plates were cut from ingots.The homogenization process is 450°C × 12 hours and air cooling.The rolling temperature is 300°C, 350°C and 400°C.Before rolling, three plates were held in a resistance furnace for 30 minutes at the rolling temperature, then rolled to the target thickness of 12 mm.Each-pass reduction is 10-20% in the same direction, with 15 min annealing between two passes.After rolling, the plates were quenched in water.
The microstructure of the RD-TD planes was observed by scanning electron microscopes (SEM), with an electron backscatter diffractometer (EBSD) and an energy dispersion spectrometer (EDS).EBSD texture characterization was performed by Hitachi Se3400 SEM, operated at 20 kV.The crystal orientation between grains was conducted.The data was analyzed using TSL-OIM profiling software, and the scanning step size was 0.9 μm.The macro-texture of the RD-TD plane was analyzed using a Rigaku D/max-2400 XRD system, and measurement angles ranged from 0° to 70°, CuKα radiation.Magnesium powder was employed as a reference sample to optimize the data and utilize the DIFFRAC plus TEXEVAL software for data processing.For SEM observation, the polished specimens were corroded with a solution (30 ml distilled water, 12 ml acetic, 2 g bitter-sour, 10 ml ethanol).Samples for texture analysis were prepared through mechanical grinding and electrolytic polishing in a 10% perchloric acid and 90% ethanol electrolyte solution at 15 V.The "dog-bone" shaped tensile coupons were prepared for tensile properties test.The gauge thickness is 2 mm, the gauge is 4 mm wide, and 36 mm long.Stretching tests were performed using the universal tester MTS E45.305 at a fixed horizontal axis speed of 1.5 mm/min.

Microstructures
Figure 1 shows the SEM images of the morphology of the second phase in three alloy plates rolled at different temperatures, which are classified into three types: reticulate, linear, and granular.In Figure 1(d), a mass of elongated reticulated phases and some uniformly distributed granular second phases existed along the RD direction in the sample rolled at 300°C.Some mainly granular and linear second phases in the plate were rolled at 350°C, and only some granular and semicontinuous reticulated second phases in the organization of the plate were rolled at 400°C.In Figures 1(c) and 1(d), for the reticulate phase, the diameter is about 200-400 μm; for the linear phase, the length is about 200 μm.Based on the EDS analysis results, these phases are MgAlCa ternary phases, reticulate phases, linear phases, and granular phases, which are MgAl2Ca.

Texture
Figure 2 shows the (0002) texture of Mg-Al-Ca-Zn-Mn alloy plates rolled at three temperatures.When the plates rolled at 300℃, 350℃, and 400℃, the textures of the plates are non-basal texture, and their texture strengths are different; the texture strength tends to be stabilized with increasing temperature.
The sample rolled at 300℃ has the lowest texture strength of 2.72, while the texture strength was 9.44 for the 350°C rolled sheet and 8.18 for the 400°C rolled sheet, respectively.When the temperature rises to 400℃, the plates show non-basal texture with two texture peaks, which extend along the rolling direction, and one of the peaks has a relatively weak texture strength of 6.58.The formation of this texture may be related to the initiation of vast non-basal slip systems during the deformation of alloy plates rolled at the elevated temperature, which also weakened the strength of the basal texture to a certain extent.3 shows the inverse IPF map, KAM map, and microscopic pole diagram of three plates.From Figure 3, we can see that the microstructure of rolled plates consisted of many fine dynamically recrystallized grains with sizes of 5-10 μm, and coarse deformed grains.A mass of low-angle gain boundaries (LAGB), which has high stored strain energy, was also observed within the grain.The internal energy storage of the grain is highest in the sample rolled at 400℃ (Figure 3 (f)), which is 5.The sheet rolled at 350°C had the highest texture strength of 40.96.The microscopic texture is similar to the macroscopic texture.However, the intensity of the microscopic texture is significantly higher than that of the macroscopic texture, which may be related to the relative strength of the texture intensity on the polar map.At the same time, the scanning area of EBSD is much smaller than that of XRD.

Mechanical properties
Figure 4 shows the alloys stretching Engineering stress-strain curves for the RD directions of the samples.Table 2 shows the mechanical properties data.We can see that the rolled alloy exhibits high tensile strength (>114 MPa) and yield strength (>104 MPa) but has low elongation.This is mainly due to the density of dislocations during rolling, combined with the presence of a large size second phase in the alloy.This results in dislocation blockage and stronger work hardening during further deformation.Therefore, greater forces need to be applied to deform the alloy, resulting in higher yield strength and poorer plasticity.The curves show that the strength and ductility of the plates are well matched when rolled at 300°C, the elongation is 2.5% in the RD direction, the tensile strength is 167 MPa, and the yield strength is 146 MPa.When the rolling temperature is 350℃, the plates have the highest strength among the three different deformation temperatures, with a yield strength of 165 MPa and a tensile strength of 171 MPa.However, the elongation along the RD direction was 2.0%.The plate rolled at 400°C is not as strong or plastic.The tensile strength is 114 MPa, and the yield strength is 104 MPa.The elongation in the RD direction is 0.5%.

Conclusions
A Mg-1Al-0.9Ca-0.5Zn-0.4Mnalloy was designed by multi-component microalloying, and the influence of rolling temperatures on the second phase, texture, and mechanical properties of the plates was studied.The following two conclusions are reached: (1) Mg-1Al-0.9Ca-0.5Zn-0.4Mnalloy was at warm rolling.The deformed organization consists of a few fine dynamic recrystallization grains and large grains with a large number of grain boundaries at small angles, with double twins and compression twins.It is accompanied by a small number of larger-sized block phases and long strings along the rolling direction that remain from the casting process, as well as fine spherical and rod-like particle phases within the grains.
(2) The Mg-1Al-0.9Ca-0.5Zn-0.4Mnalloy sheet has low textile strength and some internal grain energy storage when rolled at 300°C.Therefore, the best strength-plasticity integrated mechanical properties are shown at this time: the tensile strength is 167 MPa, the yield strength is 146 MPa, and the elongation is 2.5% along the RD direction.Besides, there are still some larger-sized phases in this alloy that affect its mechanical properties, and the material has room for improvement.

Figure 2 .
Figure 2. Macro-texture of three rolled plates: (a)300℃, (b)350℃, (c) 400℃.Figure3shows the inverse IPF map, KAM map, and microscopic pole diagram of three plates.From Figure3, we can see that the microstructure of rolled plates consisted of many fine dynamically recrystallized grains with sizes of 5-10 μm, and coarse deformed grains.A mass of low-angle gain boundaries (LAGB), which has high stored strain energy, was also observed within the grain.The internal energy storage of the grain is highest in the sample rolled at 400℃ (Figure3(f)), which is 5.The sheet rolled at 350°C had the highest texture strength of 40.96.The microscopic texture is similar to the macroscopic texture.However, the intensity of the microscopic texture is significantly higher than that of the macroscopic texture, which may be related to the relative strength of the texture

Figure
Figure 2. Macro-texture of three rolled plates: (a)300℃, (b)350℃, (c) 400℃.Figure3shows the inverse IPF map, KAM map, and microscopic pole diagram of three plates.From Figure3, we can see that the microstructure of rolled plates consisted of many fine dynamically recrystallized grains with sizes of 5-10 μm, and coarse deformed grains.A mass of low-angle gain boundaries (LAGB), which has high stored strain energy, was also observed within the grain.The internal energy storage of the grain is highest in the sample rolled at 400℃ (Figure3(f)), which is 5.The sheet rolled at 350°C had the highest texture strength of 40.96.The microscopic texture is similar to the macroscopic texture.However, the intensity of the microscopic texture is significantly higher than that of the macroscopic texture, which may be related to the relative strength of the texture

Figure 4 .
Figure 4. Room temperature tensile mechanical properties of three plates.

Table 2 .
Room-temperature tensile properties of three plates.