Effects of heat treatment routes on microstructure and damping properties of Mg–Zn–Y–Mn alloys

The effects of different heat treatment processes on the morphology and damping properties of long-period stacking order (LPSO) phase in Mg-4.9Zn-8.9Y-xMn alloys were studied. The microstructure analysis shows that the as-cast second phase presents a vein-shaped distribution at the grain boundary, and the heat treatment temperature has a significant effect on the morphology of the second phase. After heat treatment at 540 °C, the LPSO phase at the grain boundary changes into rod-like; When the heat treatment temperature is 550 °C, the LPSO phase changes into bulk or flocculent, and part of the LPSO phase melts into the matrix.In this process, the addition of Mn also has an important influence on the morphology transformation of LPSO phase. An appropriate amount of Mn can divide the bulk LPSO into several small parts during heat treatment, thus forming a large number of dispersed LPSO phases. In general, heat treatment and Mn elements can affect the mechanical and damping properties of the alloy. Heat treatment can reduce the mechanical properties of the alloy, but effectively improve the damping properti es of the alloy. With the addition of Mn into the alloy, the mechanical properties of the alloy can be improved without significantly reducing the damping.


Introduction
With the rapid development of automobiles, aerospace, and transportation, effective control of vibration and noise is very important. Metal-based damping material is a structure-function integrated material that can reduce these problems [1]. In precision structures without dampers, metal-based damping materials show their uniqueness. Magnesium alloys have low density and high damping properties, and their development in damping materials has attracted much attention [2,3].
Pure magnesium has high damping property but low mechanical property. After alloying, the mechanical property is improved, but the damping property will be reduced [4,5]. It is also found that the addition of Zr, Ni [6] and Mn [7] elements not significantly reduce the damping of pure magnesium, and is suitable for the development of Mg-based damping alloys. Among them, Mg-Zr and Mg-Ni alloys have been used in aerospace. In the research of Mg-Mn alloy [7], the as-cast alloy shows a relatively high strain-dependent damping value. This is related to the fact that Mn can introduce a large number of movable dislocations, which can improve the energy dissipation during alloy vibration.
LPSO phase is a hot spot in the research of magnesium alloys. In the study of the influence of LPSO on the damping of magnesium alloy [8][9][10][11], it is found that the mechanical properties and damping properties were improved with the increase of LPSO content. In addition, the microstructure of LPSO phase in magnesium alloy can be changed by heat treatment to improve the damping and mechanical properties of the alloy. The rod-like LPSO phase can significantly improve the damping capacity while maintaining high yield strength [12][13][14]. In Mg-Ni-Y system [15], a large number of LPSO phases were also obtained. It was found that the appearance of LPSO phase can improve the damping of the alloy, and the damping of the alloy is affected by many factors such as alloy elements, second phase morphology, solid solution atoms, etc. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
In this paper, based on the Mg-Zn-Y system, the Mn element is introduced to reduce impurities in the casting process and improve alloy structure. The interaction between the Mn element and LPSO phase during heat treatment was explored, and the effect of Mn content on the size and morphology of the LPSO phase during heat treatment was studied. Promote the development and design of damping magnesium alloy containing LPSO phase.

Experiments
In order to obtain Mg-Zn-Y-Mn alloy (table 1), pure Mg, pure Zn, Mg-30wt% Y, and Mg-3% Mn were used as raw materials. All raw materials were mixed and melted in the electromagnetic furnace of low carbon steel crucible under an 820°C argon protective atmosphere. The ingot for the experiment was obtained by cooling it in a metal mold. Table 1 lists the nominal and actual chemical composition. The previous study [9,12] found that the LPSO phase in Mg-Zn-Y alloy has high temperature stability, and its dissolution temperature is about 547°C. It was found that the LPSO phase with rod-like structure can be obtained after heat treatment at 540°C for 4 h. In this experiment, 540°C and 550°C were selected as the experimental temperature to analyze the effect of Mn on the structure of LPSO during heat treatment.
In the experiment, a XRF fluorescence analyzer (XRF, EDX1800b) was used to detect the composition of casting alloy. a Vega II LMU scanning electron microscope and energy dispersive spectrometer were used to observe the microstructure of the samples. The phase composition of different parts of the interior was analyzed by energy-dispersive spectrometer (EDS, Vega II). The Zeiss Libra transmission electron microscope 200FE was used to conduct a more detailed microstructure analysis under the acceleration voltage of 200 kV. The thickness of the sample was required to be less than 50 μm, and the sample was prepared by ion thinning.
A Shimadzu CMT-5105 material testing machine was used to analyze the sample's mechanical properties, the tensile test specimen size was 18 mm × 4 mm × 2.5 mm, and the tensile test rate was 3 mm min −1 . The damping capacity was studied in single cantilever vibration mode using a dynamic mechanical analyzer (DMA Q800-TA) and evaluated using Q −1 = tanθ. Use the electric spark cutting machine to prepare the specification of 45 mm × 5 mm × 1.2 mm damping test samples. The test parameters are as follows: at room temperature, the strain amplitude is 10 −5 to 10 −3 , and the vibration frequency ( f ) is set to 1 Hz.

Effect of different heat-treatment process on microstructure
Microstructures of the as-cast Mg-Zn-Y-Mn alloys were investigated, as seen in figure 1. The second phase in Alloy I shows the vein-shaped distribution at the grain boundary, and no other second phase can be observed in the Mg matrix. Its average grain size is about 20 μm. With the addition of Mn content, it is refined in alloy II, and spot foreign matters begin to appear locally in the alloy. In alloy III, this kind of dot cluster is more obvious. When the content of Mn is not less than 1%, it can be seen in alloy IV and V that the grain is obviously refined and more uniform, and the grain size is about 12 μm. At the grain boundary, the second phase becomes thin and uniform.
SEM images using backscattered electrons of Mg-Zn-Y-Mn alloys after heat treatment at 540°C for 4 h were shown in figure 2. Table 2 shows theEDS elemental analysis of the Mg-Zn-Y-Mn alloys specified in figure 2(c). It can be found that the Y/Zn ratio in the second phase (point A) at the grain boundary is about 1.53. In combination with previous studies [9][10][11][12], the LPSO phase can be determined. Compared with as-cast alloys, the net-shaped LPSO phase at the grain boundary in alloy I am transformed into rod or small block, with some white Y-rich particles (point B) attached on it. However, with Mn addition, the result shows some difference. The rod-like phase in figure 2.2 is significantly smaller than that of the alloy without Mn. Moreover, many coarse LPSO phases begin to break into small rods or small pieces, although the grain size shows no obvious change. With increasing Mn further, in alloy IV and alloy V, the LPSO phase was refined but not completely shaken to  Figure 3 shows SEM images using backscattered electrons of these Mg-Zn-Y-Mn alloys after heat treatment at 550°C for 4 h. It is observed that heat treated at a higher temperature could significantly affect the  microstructure. Under this condition, grains will grow, obviously, and LPSO will start to dissolve. In Mg-Zn-Y-Mn alloys, different Mn content usually affects the microstructure during high-temperature heat treatment. On the whole, the second phase in the alloy changes from a net-shaped LPSO phase to a discontinuous bulk or rodlike second phase and precipitates some white second phases, with a significant increase in grain size. With the addition of Mn, large amounts of small gray bulks (as arrowed in spot B in figure 3(f)) appear, which are indexed as also as LPSO phase by EDS in table 3.
In addition, it can be found that a large number of white particles are distributed homogeneously in these alloys, such as D and E points. According to EDS, Zn, Y, and Mn elements were gathered, implying a mixture of α-Mn and Mg-Zn-Y eutectic phases. With the increase of Mn content, the dispersed white particles in the matrix also increase, and more dispersed and small LPSO appear. These small LPSO phases are distributed in the   Figure 4 shows the comparative analysis of the mechanical properties of Mg-Zn-Y-Mn alloys in three states. It can be seen that the mechanical properties of as-cast alloy are slightly higher than those of heat-treated alloy, and the higher the heat-treatment temperature, the lower the mechanical properties of the alloy, which is related to the change of second phase features (e.g. amount and refinement). With the addition of Mn content, the mechanical changes of the samples in the three states are relatively consistent, showing a monotonic upward trend, which is related to the fine grain strengthening of Mn and the refinement of the LPSO phase [9]. When the Mn content is low, there is no gain effect on the plasticity of the alloy. With the increase of the Mn content, the fine grain effect can effectively improve the plasticity of the alloy. After heat treatment, Mn particles can divide the large LPSO phase into a small LPSO phase because the bulk LPSO phase is not easy to coordinate the deformation between the grains and the second phase during the deformation process, which leads to uneven stress distribution and excessive concentration of local stress and induces the early initiation of micro cracks, which will deteriorate the plasticity of the alloy [16,17]. Therefore, the refining LPSO effect of Mn can effectively improve the plasticity of the alloy. Figure 5 shows the strain-damping curve of the Mg-Zn-Y-Mn alloys. It can be seen that the damping of the alloy can be divided into two parts. At the low strain stage, the strain has little effect on the damping value. When the strain increases to a certain stage, the damping of the alloy will be reflected rapidly. It is also a typical dislocationtype damping feature [18,19]. The addition of Mn content did not make a significant difference in damping performance. For the convenience of observation and analysis, the damping value when the strain is 10 −3 is selected for comparison, as shown in figure 6. It can be found that with the increase of Mn content in as-cast and 540°C heat-treated samples, the damping value increases and decreases. After heat treatment at 550°C, the damping value decreases first and then increases. This is related to many factors, such as grain refinement, second-phase morphology, and solid solution atom content, and is the result of a complex factor. On the other hand, in magnesium alloys, the addition of alloying elements tends to improve the mechanics and reduce the damping property [3]. In this experiment, adding Mn can improve the mechanical properties and ensure the damping performance remains relatively high.

Effect of different heat-treatment process on damping capacity
The damping capacity of alloy V with different process parameters is selected for specific analysis, as shown in figure 7. The heat-treated alloys usually show better damping properties than as-cast alloys. Furthermore, the higher the heat treatment temperature, the higher the alloy's damping. Like alloy V, at a strain of 10 −3 , the sample could get a Q −1 of 0.03 after heat treatment at 540°C for 4 h, which was significantly higher than with no heat-treating (0.02). After heat treatment at 550°C for 4 h. The damping properties have further improved, with a Q −1 of nearly 0.05 at a strain of 10 −3 .
On the other hand, According to previous studies, high damping capacity and high yield strength can be obtained by changing the morphology of the LPSO phase in the alloy. In both heat treat alloys at 540°C and 550°C (Seen in figures 2 and 3), Mn particles could divide the bulk LPSO phase into fine particles. The change in the

Effect of LPSO and Mn interactions during heat-treat process
In Mg-Zn-Y-Mn alloys, both the LPSO phases and Mn particles play roles in damping capacities. For discussion of the interactions between them, TEM was tested. Figure 8 shows a bright-field TEM image of the Mg-Zn-Y-Mn alloys from the direction of [1120] after heat treatment at 550°C for 4 h. Gray LPSO phases show a lamershaped morphology. The length of the phases ranged from several to several hundred nanometers, with different spacing between them. Some LPSO phases show a spacing of over 200 nm, while some are connected up to each other. The length of this phase was nearly 10 μm. There are two kinds of relationships between the Mn and LPSO phases. Some Mn particles existed around the gray lamer LPSO (seen in figure 8(a)), and some adhered to the massive phases. J D Robson reported that Mn could affect the recrystallization behavior of Mg alloys [20]. Mn could solute into α-Mg matrix while heat treated at 550 ∼ 600°C, and then precipitate annealed at 300°C. The reprecipitated Mn particles have regular morphology but no orientation relationship with the matrix. So in these Mg-Zn-Y-Mn alloys, α-Mn particles show no-coherent with α-Mg and LPSO phases. During heat treatment, these fine α-Mn particles could pin and impede recrystallization behavior, which would influent the damping capacity and weaken the positive action of the movable dislocation induced by α-Mn.
On the other hand, α-Mn could interfere with the growth of the LPSO phase. As the direction of LPSO is (0001) and (10-10), more and more dislocations formed at the no-coherent interface of the Mn and LPSO phase, with the increasing stress field. Then, the growing bulk LPSO phase would be cut into a large number of fine and dispersed ones, which should gather together into poles, as seen in figure 8. These small gray blocks show the same general direction with the rod-shaped around them. In summary, Mn could divide the coarse LPSO phase to form a fine LPSO phase in the matrix in the heat treatment process.

Mechanism of damping mechasm on Mg-Zn-Y-Mn alloys
In Mg-Zn-Y-Mn alloys, both LPSO phases and α-Mn particles have obvious effects on dislocations' slipping, leading to complicated internal friction. According to a previous study [7,9], as-cast Mg-Mn alloys present high strain-dependent damping values. Mn could provide long dislocation into the alloys, which can be explained using the Granato-Lücke dislocation damping theory [18,19]. As-cast alloys, alloy III shows better damping capacities in low-strain areas. Slight Mn could have a positive influence, while with more additions, damping values show a reduction. At that time, most LPSO phases show net-shaped morphology and a matrix of equiaxed dendrites along the grain boundaries, with no other secondary phase existing inside the grain. Also, the snowflakes-shaped α-Mn particles aggregated at the grain boundaries, similar to that in Mg-Mn alloys. It can be suggested that the Mn element in as-cast Mg-Zn-Y-Mn alloys show the same function as the as-cast Mg-Mn alloy in the previous study.
However, after heat treatment, the effect of Mn nearly disappeared. Upon treating the solute at 540°C, Y and Zn atoms could diffuse into an α-Mn matrix. Besides this, the net-shaped LPSO phase is transformed into a rod, not a secondary phase at the grain boundary. In another way, for LPSO phases, the mechanical properties and damping capacities are enhanced. It has an intent relationship with morphology, meaning that the G-L theory could not be perfectly explained. In that circumstance, the Mn element shows a complicated effect. At first, for a lot of solute atoms in the matrix, which could effectively make pinning on moving of dislocation, the spacing of the strong pinning point among the movable dislocation is really short. And the addition of Mn could not influent the distance. Secondly, the dislocations induced by Mn would still be pinned by the grain boundary, solute atoms, and the second phase. Also, there are no grains for the dislocations slipping, as the morphology was changed to rod-shaped. Additionally, Mn would increase the required activation energy that the dislocations crossing the barrier when the internal friction peak appears, resulting in a higher temperature at the peak appeared at and lower peak intensity. All in all, Mn shows no significant positive effect on damping behavior in heat-treated alloy at 540°C.
In Mg alloy, heat treatment could be used to improve the performance, for the dislocation, void, and other micro-defects would have redistribution, and the chaotic area would be eliminated [3,21]. Then, the movement of the movable dislocation could restart, and the strong pinning points within the grains could be reduced. At last, the damping capacity shows a sharp enhancement. It can be found that the damping of the alloy is significantly improved after heat treatment at 540°C, but the high temperature stability of LPSO will hinder the redistribution of micro-defects during heat treatment. When the heat treatment temperature reaches 550°C, with the LPSO phase melting into the matrix, a large number of casting defects are eliminated, and the damping characteristics of the alloy are greatly improved.
In general, there are not many studies on damping magnesium alloy containing LPSO phase. This paper discusses the influence of Mn content and heat treatment process on the microstructure and mechanics of Mg-Zn-Y-Mn alloy. Damping can be greatly improved by appropriate heat treatment process, and the mechanics of alloy can be improved by Mn content. However, it is still a difficult problem in this field to further obtain magnesium alloys with higher strength and damping to meet their application in aerospace engineering.

Conclusions
In this study, Mg-Zn-Y-Mn series alloys were prepared, and the microstructures and damping performance were tested. The effects of heat treatment process and Mn content on the morphology of LPSO phase are discusse. The main results are summarized as follows: (1)After heat treatment at 540°C, the massive LPSO phase in the alloy transforms into a rod-like structure; When the heat treatment temperature rises to 550°C, the LPSO phase begins to blend into the matrix, and a small point LPSO phase appears.
(2)Heat treatment has a significant impact on the properties of the alloy. With the increase of heat treatment temperature, the mechanical property of the alloy decreases, but the damping property increases significantly.
(3)Mn shows no significant effect on the phase and morphology of the secondary phase in as-cast alloys but could divide the LPSO phase during the heat treatment process. With increasing Mn, the divided LPSO is more dispersed. The addition of Mn is beneficial to the mechanical properties of the alloy and does not significantly reduce the damping of the alloy.