【Citation Format】
Zhi Xuanle, Ding Zhibing, Zhang Shuai, et al. Effect of Heat Treatment on Microstructure and Properties of Mg-Gd-Y-Zn-Ti Alloy[J]. Special Casting & Nonferrous Alloys,2024,44(3):392-397.
Citation:ZHI X L， DING Z B， ZHANG S， et al. Effects of heat treatment on microstructure and properties of Mg-Gd-Y-Zn-Ti alloy［J］.Special Casting & Nonferrous Alloys，2024，44（3）：392-397.
Magnesium alloy has attracted the attention of researchers due to its light weight, high specific strength, good hot forming performance and easy recycling, but it is weak in plasticity, strength and corrosion resistance, which limits its application in engineering. Therefore, researchers have used a variety of methods to change the mechanical properties of materials, such as alloying, large plastic deformation, and the introduction of new strengthening phases. Among them, rare earth-containing magnesium alloys have attracted extensive attention because of their excellent comprehensive performance in mechanical properties. When the rare earth elements and transition elements in magnesium alloy are in a certain proportion, a long period stacking ordered (LPSO) structure can be formed, and its unique strengthening mechanism and special atomic structure have attracted the attention of researchers. In the Mg-Y-Zn-Zr alloy with LPSO phase structure, the molar ratio of Y and Zn is considered to be the key factor affecting the formation of LPSO structure, and this second phase can hinder the dislocation motion and strengthen the matrix, so the more LPSO structure, the higher the strength. In the as-cast Mg-Zn-Y alloy with high content of Y and Zn, the mechanical properties of the alloy are improved by preventing the slip of the Mg matrix due to the large LPSO structure. Further research is needed to strike a balance between strength and plasticity for magnesium alloys containing LPSO structures.
Considering the needs of practical application, in order to better develop the Mg-RE alloy containing LPSO, it is necessary to conduct in-depth research on its mechanical properties. The previous preparation methods mainly focused on improving the strength by adding a large number of rare earth elements and using the large plastic deformation (SPD) process, but there were the following drawbacks: the high strength of magnesium alloys with high RE content did not match the low plasticity, especially under as-cast conditions, the density advantage caused by the addition of a large number of rare earths was sacrificed, and it was difficult to determine the SPD metamorphic parameters due to the unsatisfactory initial plasticity of as-cast alloys.
The research team of Hunan Provincial Key Laboratory of Intelligent Manufacturing of Efficient Power Systems, School of Mechanical and Energy Engineering, Shaoyang University, published an article entitled "Effect of Heat Treatment on the Microstructure and Properties of Mg-Gd-Y-Zn-Ti Alloys" in the journal "Special Casting and Nonferrous Alloys", Volume 44, Issue 3, 2024. Scanning electron microscopy (SEM) and electron tensile testing machine were used to analyze the effects of heat treatment on the microstructure, microstructure, phase composition and mechanical properties of Mg-12Gd-xY-1Zn-0.6Ti (mass fraction, %) alloy (x=0.4, 0.8, 1.2,%). The results show that there is an LPSO phase in the as-cast Mg-12Gd-xY-1Zn-0.6Ti alloy, and the LPSO phase is Mg12Gd5(Zn,Y). The mechanical properties of the alloy with 0.8% Y were the best, with a tensile strength of 222 MPa, a yield strength of 149 MPa, and an elongation of 3.76%. After solution treatment, the mechanical properties were improved to a certain extent, the tensile strength reached 272 MPa, the yield strength reached 188 MPa, and the elongation reached 5.4%. The peak aging response of the Y-added alloy was strong, and the peak aging state of Mg-12Gd-1Zn-0.6Ti-0.8Y alloy had the best mechanical properties, with a tensile strength of 388 MPa, a yield strength of 267 MPa, and an elongation of 6.44%.
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【Methodology】
The test alloy was Mg-12Gd-xY-1Zn-0.6Ti alloy (x=0, 0.4, 0.8, 1.2, mass fraction, %), and the design composition of the alloy is shown in Table 1. The test materials were pure magnesium (99.90%), pure zinc (99.99%), pure titanium powder (99.99%), Mg-30Gd and Mg-30Y master alloys, all raw materials were dried and surface oxides were removed before smelting, and the intermediate frequency induction furnace was used to melt at a temperature of 750 °C. A columnar stainless steel crucible was used, and argon gas was introduced for protection at the same time, and the alloy was completely melted and then kept warm for 5 min, and then taken out after natural cooling. Combined with the previous research results, the heat treatment processes were as follows: 500 °C×8 h solid solution, 500 °C×8 h solid solution+220 °C× 100 h aging.
The phase composition of the alloy was analyzed by AL-2700B X-ray diffractometer (XRD), and the block specimens with dimensions of 10 mm× 10 mm× 10 mm were used to polish the surface of the test samples with sandpaper of No. 500, 800, 1000, 1500 and 2000 before testing, and then polished the detection surface on the polishing machine. The tube voltage was 40 kV, the tube current was 30 mA, the scanning angle was 20°~90°, the step angle was 0.02°, and the sampling time was 0.5 h. The built-in energy dispersive spectrometer (EDS) was used to analyze the chemical composition of the intragranular and grain boundaries and the second phase to determine the type of the second phase in the sample, the hardness of the as-cast state, the solid solution state and the aging state was measured by the DHV-1000ZCCD microcomputer Vickers hardness tester, and the WDW-100D microcomputer-controlled electronic universal testing machine was used to test the mechanical properties of the alloy at room temperature, each group was tested 3 times, the average value was taken, and the tensile rate was set to 0.2 mm/min.
【Research Results】
The as-cast Mg-12Gd-xY-1Zn-0.6Ti alloy structure is mainly composed of α-Mg matrix, continuous hollow reticulated facies, gray fishbone facies and polygonal facies. With the increase of Y content, the gray-white second phase began to increase and was reticulated, and the black α-Mg phase began to decrease. A large number of secondary phases are aggregated together to create a blocky distribution. Due to the increasing Y content, the structure of the alloy becomes denser and denser, and the α-Mg phase becomes less and less. The LPSO phase distributed at the grain boundary mainly grows from the grain boundary to the grain, so the LPSO phase is formed at the grain boundary and extends into the grain, but there are still some places where the phenomenon of elemental segregation occurs. The conventional casting process used has a shorter cooling time, and more solutes Y and Zn will appear in the melt as the α-Mg matrix grows. In the final stage, the concentration ratios of x(Y)/x(Zn) and LPSO phases in the liquid phase are very close and the LPSO phase precipitates. In addition, as the solidification process progresses, the solute diffusion rate decreases due to the decrease in temperature. Locally, the concentrations of Y and Zn also change accordingly, and the molar ratio is close to that of the W phase. During this cooling process, there are not enough Mg atoms to participate in the precipitation of the LPSO phase, which leads to a possible decrease in LPSO phase formation with the increase of x(Y)/x(Zn).
图1 铸态Mg-12Gd-1Zn-0.6Ti-xY合金的SEM图
Fig.1 SEM images of as-cast Mg-12Gd-1Zn-0.6Ti-xY alloys
The continuous hollow reticulated phase, gray fishbone phase, and polygonal phase in Figure 1 are W[(Mg,Zn)3Gd], LPSO phase, and Mg5Gd, respectively. The LPSO phase usually exhibits an 18R-LPSO structure in as-cast alloys. Due to the high energy and chaos at the grain boundary, and the serious segregation of Y and Zn elements, it is conducive to the formation of lamination faults, which provides a basis for the formation of LPSO phase and thus promotes the formation of LPSO phase. A SIMILAR SITUATION WAS FOUND IN STUDIES SUCH AS BIAN LP.
图2 铸态Mg-12Gd-xY-1Zn-0.6Ti合金的高倍SEM图
图3 铸态Mg-12Gd-xY-1Zn-0.6Ti合金的XRD图
With the increase of Y content, the overall hardness of the alloy does not change much. When 0.8% Y is added, the mechanical properties of the alloy are ideal, and its tensile strength (σb), yield strength (σ0.2) and elongation (δ) are 222 MPa, 149 MPa and 3.76%, respectively, which may be due to the formation of the 18R-LPSO phase with high elastic modulus and high hardness. When 1.2% Y is added, there will be more segregation of rare earth elements at the grain boundary, which also leads to the poor strengthening effect of the α-Mg solid solution, and its tensile strength, yield strength and elongation are only 205 MPa, 136 MPa and 3.09%, respectively. Previous research results have shown that alloys with LPSO structure have higher strength and hardness than pure magnesium, which is very beneficial for improving the mechanical properties of magnesium alloys.
图4 铸态Mg-12Gd-xY-1Zn-0.6Ti合金应力-应变图
图5 铸态Mg-12Gd-xY-1Zn-0.6Ti合金的硬度
图6 铸态Mg-12Gd-xY-1Zn-0.6Ti合金的力学性能
Compared with as-cast alloys, the grain boundaries in the alloy microstructure after solution treatment become obvious, a wide range of eutectic phases disappear, and only some phases mainly exist in granular or square form. Figure 8 shows the SEM high-magnification image of the solution-treated alloy. It can be clearly observed that it is difficult to eliminate the second phase aggregation caused by the casting process only by conventional heat treatment processes. With the increase of Y content, the layered LPSO phase can be seen from Fig. 8b~Fig. 8d, which has good mechanical properties, but the appearance of LPSO phase is also accompanied by the appearance of some bulk phases, and the number of second phases in the alloy is increased. A similar phenomenon was found in the microstructure evolution of Mg-Zn-Y-Gd alloy studied by Liao H et al. The bulk phase is the second product of the LPSO phase in the change, the main reason is that the content of rare earth elements is too high or too low in some places in order to compete for rare earth elements in the formation process of the two adjacent phases, and the nucleation of the bulk phase of rare earth elements is greater than the speed of diffusion into the crystalline, so the massive phase is formed. There are two aspects of LPSO phase in the alloy after solution treatment, the first is because the solid solubility of Gd decreases after the addition of Y element to the alloy, and the second is because the lamination fault energy decreases after the addition of Y element to the alloy. The LPSO phase also appears after heat treatment of alloys containing the Y element, because the LPSO phase is also a lamination fault.
图7 固溶态Mg-12Gd-xY-1Zn-0.6Ti合金的SEM图
图8 固溶态Mg-12Gd-xY-1Zn-0.6Ti合金的高倍SEM图
Compared with as-cast alloys, the tensile strength and yield strength of the alloy after solution treatment are significantly improved, especially when 0.8% Y is added, because the LPSO phase can effectively improve the strength and toughness of the alloy. The layered LPSO phase improved the mechanical properties of the alloy, but the properties of the alloy decreased when 1.2% Y was added, which was due to the increase of bulk phase and rod phase due to solution treatment, which was consistent with the research conclusion of LU R P et al.
Fig.9. Stress-strain curves of Mg-12Gd-xY-1Zn-0.6Ti alloy in solid solution
图10 固溶态Mg-12Gd-xY-1Zn-0.6Ti合金的硬度
图11 固溶态Mg-12Gd-xY-1Zn-0.6Ti合金的力学性能
Fig.12. Stress-strain curves of Mg-12Gd-xY-1Zn-0.6Ti alloy in peak timeliness
图13 峰时效态Mg-12Gd-xY-1Zn-0.6Ti合金的力学性能
Compared with the as-cast state and the solid solution state, the properties of the alloy were greatly improved, and the mechanical properties of the alloy with 0.8% Y added to the peak aging state were the best. This is because after low-temperature aging treatment, a large number of rare-earth-rich phases are precipitated inside the alloy, and the alloy also contains a large number of LPSO phases and undissolved second phases with high temperature stability, and the existence of these strengthened phases improves the comprehensive properties of the alloy.
Fig.14 Aging hardness of Mg-12Gd-xY-1Zn-0.6Ti alloy at 220 °C
【Conclusions】
(1) The as-cast structure of Mg-12Gd-xY-1Zn-0.6Ti alloy appeared coarse dendritic and discontinuous network distribution at the same time, most of the dendrites dissolved into the magnesium matrix after solid solution treatment, and only a small number of granular and square phases were not dissolved.
(2) After 500 °C×10 h solid solution, 500 °C×10 h solid solution + 225 °C×80 h aging, Mg-12Gd-0.8Y-1Zn-0.6Ti alloy exhibited ideal mechanical properties.
(3) The mechanical properties of Mg-12Gd-0.8Y-1Zn-0.6Ti alloy were significantly improved after solution aging, and the tensile strength, yield strength and elongation increased from 222 MPa, 149 MPa and 3.76% in the as-cast state to 388 MPa, 267 MPa and 6.44%, respectively, which were increased by 74%, 79% and 71%, respectively. At the same time, the solid solution strengthening and precipitation strengthening effects of the alloy are obvious.