Magnesium and magnesium alloys are highly susceptible to reaction and combustion during the smelting process, resulting in gas adsorption and oxidation inclusions, resulting in a decrease in alloy quality. In addition, the corrosion resistance and high temperature creep resistance of magnesium alloys also make the application range to a certain extent limit.
Rare earth elements have special extranuclear electron arrangement and physical and chemical properties. The binding force with hydrogen and oxygen is greater than that of magnesium and hydrogen and oxygen. The rare earth oxide has a higher density coefficient than magnesium oxide, and the rare earth has good Alloying characteristics, so in the process of magnesium alloy smelting, adding appropriate amount of rare earth elements not only has the functions of removing hydrogen, reducing oxide inclusions, preventing melt combustion, but also refining magnesium alloy structure, improving its high temperature performance and improving resistance. The role of eclipse [1]. In this paper, the mechanism and application of rare earth in magnesium alloy are introduced from the aspects of purifying and removing impurities, flame retardant, refining microstructure, improving high temperature performance and corrosion resistance.
1. Purification of magnesium alloy melt by rare earth
1, in addition to hydrogen purification
The chemical nature of magnesium is active. During the smelting process, magnesium reacts with water vapor to make magnesium alloy have a strong tendency to hydrogen evolution. Hydrogen has a large solubility in magnesium alloy liquid, which leads to casting of pores, pinholes and shrinkage in castings. defect. In the magnesium alloy smelting process, rare earth elements are added, and the rare earth element has strong binding force with hydrogen, which can adsorb water vapor and hydrogen dissolved in the magnesium liquid, and generate high melting point rare earth hydride and rare earth oxide, rare earth hydride and rare earth oxidation. The specific gravity of the material is lighter than that of the melt, so that it floats up to form solid slag, thereby achieving the purpose of hydrogen removal [2]. The reaction formula of rare earth hydrogen removal is: 3H2O(g) + 5[ RE] = 3REH2 + RE2O3 (1) Roche Equality [3] The thermodynamics of the reaction of the above formula are calculated. The result is that at T = 1033K, for different For the rare earth elements, the reaction free energy varies from - 754121 J/mol to - 984571 J/mol, indicating that there is a strong reaction driving force between rare earth and hydrogen, so the addition of rare earth in the magnesium alloy smelting process can eliminate The role of hydrogen and oxygen.
2, in addition to the role of oxidized inclusions
Magnesium has a strong affinity with oxygen, and reacts with H2O and O2 to form stable MgO, which leads to a certain amount of oxidized inclusions in the magnesium alloy, thereby reducing the quality and performance of the magnesium alloy parts. The inclusions generally remain in the form of a film, a particle or a tuft on the matrix or grain boundary of the magnesium alloy casting. The presence of inclusions tends to cause fatigue cracking in the alloy, and the mechanical properties and corrosion resistance are reduced [4, 5]. The results of Bakke et al. [6] show that the particulate and film-like MgO in magnesium alloy accounts for more than 80% of the inclusions. Since the affinity of the rare earth element with oxygen is greater than the affinity of Mg to oxygen, the rare earth is added to the magnesium alloy liquid, and the rare earth is contacted with the oxide in the alloy liquid to capture oxygen in the MgO molecule to form a rare earth oxide. It can be seen that the rare earth easily interacts with the oxide inclusions in the magnesium solution to form rare earth oxides to remove oxide inclusions.
Guo Xutao et al [7] found that RE can reduce the mass fraction of inclusions larger than 10 μm in regenerated magnesium alloy from 20% to 10%. When 0.70% RE is added, the volume fraction of inclusions in the regenerated magnesium alloy can be reduced from 0.51% to 0.18%, which is reduced by 65.9%.
In addition to the purpose of removing hydrogen and oxygen, rare earth elements have strong interaction with elements such as sulfur, nitrogen and halogen, and can generate corresponding RE2S3, RES, RES2, RE3S4, REN, REX3. (X is a halogen element), etc., to remove non-metallic residual impurities caused by the use of gas protection, flux, etc.; at high temperatures, rare earth elements react with carbon, silicon, boron to form rare earth carbides, rare earth silicides, rare earth borides, etc. [8 ] . At the same time, the addition of rare earth will improve the physical and chemical properties of magnesium alloy liquid and slag, such as surface tension, fluidity, viscosity, inclusion solubility, etc., which is beneficial to the spheroidization of non-metallic inclusions and improve the impurity removal effect of magnesium alloy liquid.
It can be seen that rare earth as a purifying agent for the smelting process of magnesium and magnesium alloy can effectively reduce the impurity content and improve the quality and performance of the magnesium alloy.
Second, the flame retardant effect of rare earth
During the process of melting and casting magnesium and magnesium alloys under atmospheric conditions, magnesium reacts with oxygen to form stable MgO and emits a large amount of heat. Since the surface MgO film has a density coefficient αMg<1, it is porous and cannot effectively prevent oxygen from penetrating the oxide film, so that the oxidation of magnesium continues. In addition, the thermal conductivity of MgO is small, which is not conducive to the diffusion of heat released by oxidation. Aggravate the oxidation and combustion of magnesium. Rare earth is added as an alloying element to the magnesium alloy, which can effectively improve the ignition point of the magnesium alloy. Because the affinity of RE and oxygen is much greater than the affinity of Mg to oxygen, the rare earth reacts with the infiltrated oxygen or undergoes a displacement reaction with O in MgO to form rare earth oxide RE2O3. The density coefficient of rare earth oxide is α>1, and a small amount A1 will also react with oxygen to form Al2O3, which will form a dense composite protective film composed mainly of MgO, Al2O3, RE2O3, etc., which can effectively prevent the contact of O with the alloy liquid and prevent the combustion of the magnesium alloy.
Long Jinming et al. [9] studied the effect of Ce-rich mixed rare earth on the light-off temperature of Mg-8Al-0.8Zn-0.2Mn magnesium alloy. The results show that the light-off temperature is 689 °C after adding 0.8% mixed rare earth. When it is raised to 864 °C, the ignition point is increased by 175 °C. The ignition temperature of the 1% C-rich rare earth added to the AZ91D magnesium alloy reported by Huang Xiaofeng et al. [10] is close to 170 °C. Zou Yongliang et al [11] studied the effect of mixed rare earth on the light-off temperature of ZM5 magnesium alloy, and found that the addition of 0.12% mixed rare earth can increase the light-off temperature of ZM5 from 654.5 °C to 820 °C, and the light-off temperature increases. 165.5 ° C; However, when the amount of the mixed rare earth added is too high, the rare earth oxidation rate is too fast, the formed oxide film is too thick, the stress in the film is increased, and the film layer is damaged, and the light-off temperature of the magnesium alloy is lowered.
The above studies show that rare earth has a significant effect on the flame retardancy of magnesium alloys. The effect of rare earth on flame retardant and flame retardant of magnesium alloy is the same as that of Be and Ca. It is mainly to form dense oxide composite membrane. The optimum addition amount varies according to different alloy matrix and different alloying elements. The flame retardant effect depends on surface compounding. The ability of the film to block the passage of reactive species.
Third, the effect of rare earth on the microstructure of magnesium alloy
1. Effect of rare earth on as-cast microstructure of magnesium alloy
For the most widely used Mg-Al magnesium alloys, the typical as-cast microstructure consists mainly of the α2Mg matrix phase and the intermetallic compound β phase (Mg17Al12) precipitated by the divorced eutectic. The β phase is discontinuously distributed along the grain boundary, and a small part is distributed inside the grain [12]. In the smelting process, rare earth is added, and the rare earth element forms a needle-like or strip-like aluminum rare earth new phase with Al in the alloy. Most of these compounds are segregated on the grain boundary, which hinders the further growth of the crystal grains, thereby refining. Grain. Since the formation of the rare earth aluminum phase captures the Al in the alloy and affects the formation of the β phase, the networked Mg17Al12 phase gradually becomes an intermittent, diffusely distributed skeleton [13]. On the other hand, the addition of rare earths significantly refines the microstructure inside the grains of the magnesium alloy. The as-cast microstructure of the alloy depends on the solidification process. The solidification process of the alloy is usually carried out from the surface to the center. The final morphology of the grains (equal or columnar) depends on the liquid conditions at the leading edge of the solidification interface. A certain amount of rare earth is added to the magnesium alloy. Since the solid solubility of RE in the alloy is extremely low, the equilibrium partition coefficient k<1[14], the rare earth is easily enriched at the front of the solid/liquid interface, and the composition of the alloy is too cold. The branching process is intensified, the secondary dendrites are increased, and finally the dendrite spacing is reduced, and the internal structure of the grains is refined.
Zheng Weichao et al [15] found that the average grain size of AZ91D alloy was reduced from 51.6μm to 35.1μm when the rare earth element grain refinement of AZ91D alloy was studied. Yu et al [16] also found that after adding trace rare earth element Ce to pure magnesium, the grains are remarkably refined, and the columnar crystals are all converted into equiaxed crystals. Adding a trace amount of rare earth element Ce to the Mg-Al-Zn system AZ31 alloy can refine the grain size of the alloy, and the grain size is reduced from about 300 μm before the refinement to about 30 μm.
2. Effect of rare earth on solid solution and aging treatment of magnesium alloy
When the common magnesium alloy is solid solution, it forms a supersaturated α2Mg solid solution. In the subsequent aging process, the supersaturated α solid solution directly precipitates the non-coherent equilibrium phase without any intermediate stage, and there is no pre-precipitation or transition phase. However, there are two types of Mg17Al12 phase formation, namely continuous precipitation and discontinuous precipitation [17]. Most of the discontinuous precipitation starts from the grain boundary or dislocation, and the phase grows in the form of flaky Mg17Al12 in a certain orientation. The nearby α solid solution reaches the equilibrium concentration at the same time, and the discontinuous precipitation from the grain boundary proceeds to a certain extent. Continuous precipitation occurs inside. If rare earth is present in the alloy, the rare earth combines with Al during solidification to form a stable aluminum-rich rare earth phase in the form of strips and needles. The solution is not dissolved when it is dissolved and dissolved in time, and the form in the α-Mg matrix does not follow. The temperature changes and changes. The atomic diffusion caused by the dissolution-precipitation process at a high temperature is reduced, so that the thermal stability is high. During the aging process, due to the “pinning†effect of aluminum rare earth relative to Al, the concentration of Al in α-Mg is reduced, delaying the effect of age hardening, and lags the appearance time of hardness peak [13].
In summary, the rare earth can refine the grain of the magnesium alloy and refine the internal structure of the grain. In addition, since a stable aluminum rare earth phase is formed, atomic diffusion due to the dissolution-precipitation process during solution aging is reduced, but the age hardening effect is delayed.
Fourth, the effect of rare earth on the mechanical properties of magnesium alloy
Generally, magnesium alloys have the disadvantage of poor heat resistance. When the temperature is raised, the strength and creep resistance of the magnesium alloy are significantly reduced, so that it is difficult to use the parts as high temperature for a long time. The addition of rare earth elements can significantly improve the high temperature creep properties of magnesium alloys, and can significantly improve the hardness and strength of magnesium alloys at room temperature and high temperature. It is generally believed that the mechanism of adding rare earth elements to improve the mechanical properties of magnesium alloys is mainly fine grain strengthening, solid solution strengthening and grain boundary strengthening [18].
1, fine crystal strengthening
The presence of surface tension causes a hard-to-deformation zone at the grain boundary that hinders crystal deformation. The larger the surface tension, the larger the hard-to-deformation zone, and the greater the force (deformation resistance) required to cause slippage. The addition of rare earth to the magnesium alloy can refine the grain of the alloy and reduce the grain size. When the grain size decreases, the surface tension increases, thereby increasing the deformation resistance, so the mechanical properties such as strength and hardness are correspondingly improved. However, it should be noted that too fine grain structure may reduce the creep properties of the material.
2, solid solution strengthening
After the rare earth is added to the magnesium alloy, part of the solid solution is dissolved in α-Mg to form a solid solution, and the other part is formed into a magnesium 2 rare earth compound. They are relatively stable at high temperatures and are not easily precipitated, and these compounds also have high thermosetting properties. The so-called solid solution strengthening. Utilizing the high solid solubility of Y in Mg (12.5%, mass fraction) and the age hardening ability of Mg-Y alloy, it has been successfully developed, such as Mg-Y-Nd-Zr alloy, which is excellent at 300 °C. Creep resistance [20].
3. Grain boundary strengthening
The thermal stability of the beta phase has been considered to be the key to the thermal resistance of magnesium alloys. The melting point of the β-Mg17Al12 phase in the Mg-Al alloy is only 437 °C. When the temperature exceeds 120-130 °C, the Mg17Al12 phase on the grain boundary begins to soften, which can not pin the grain boundary and inhibit the rotation of the high temperature grain boundary. The effect is that the permanent strength and creep properties of the alloy are drastically reduced [21]. By adding rare earth to the magnesium alloy, an Al-RE-rich phase can be formed at the grain boundary. The phase has a high melting point and good thermal stability. Their presence can prevent the growth of the magnesium alloy grains and the grain boundary at high temperatures. Slip, which acts as a grain boundary strengthening, thereby significantly improving the high temperature performance and creep resistance of the magnesium alloy. However, the acicular aluminum-rich rare earth has a significant splitting effect on the matrix, which lowers the room temperature elongation and impact toughness of the alloy [13].
Huang Xiaofeng [18] studied the effects of Y and La-rich rare earth on the microstructure, as-cast mechanical properties and creep properties of magnesium alloys. The results show that the addition of rare earth can make the room temperature and high temperature hardness and tensile strength of magnesium alloys The yield strength is improved. The second stage creep rate of the 1% Y magnesium alloy is only 1/4 of that of the original magnesium alloy.
V. Effect of Rare Earth on Corrosion Resistance of Magnesium Alloys
Magnesium alloys form a thin oxide film on the surface of the environmental medium. This porous oxide film can not effectively prevent the environmental media, especially the oxidative and corrosive media from corroding the magnesium alloy matrix, affecting the magnesium alloy. Use performance. The addition of rare earth elements in magnesium alloy can effectively change the corrosion layer structure of the alloy, strengthen the cathode phase control, affect the electrochemical process of alloy corrosion, and thus improve the corrosion resistance of magnesium alloy [22].
1. Rare earth changes the structure of the corrosion layer of magnesium alloy
Due to the high activity of the RE element, the RE added to the magnesium alloy easily reacts with oxygen to form a rare earth oxide, and a composite oxide layer of Mg, Al, and RE is formed on the surface of the magnesium alloy. The rare earth oxide has low chemical activity and is insensitive to NaCl corrosive medium. It is difficult to form hydroxide with water in a solution containing Cl - , thereby maintaining the integrity of the composite oxide film and functioning as a passivation film. In addition, the rare earth can react with the impurity elements in the surface layer of the magnesium alloy to purify the surface, so that the surface active point of the magnesium alloy is reduced or disappeared, thereby improving the corrosion resistance of the alloy.
2. Rare earth strengthened cathode phase control of magnesium alloy
The microstructure of the Mg-Al magnesium alloy consists of an α phase and a Mg17Al12 phase. When α and Mg17 Al12 constitute a galvanic cell, α is an anode, and Mg17Al12 is a cathode. The addition of rare earth elements causes the Mg17Al12 phase in the alloy to become intermittent and dispersed, and because the rare earth captures part of Al to form an Al-RE phase, the amount of corrosion of the cathode phase Mg17Al12 is reduced. On the other hand, a part of Al and RE in the alloy form a new Al4RE compound, and the difference between the phase and the self-corrosion potential of magnesium is smaller than the difference between the self-corrosion potential of Mg17Al12 and magnesium, which reduces the effective active cathode area. Thereby it is advantageous to improve the corrosion resistance of the alloy.
3. Electrochemical process of rare earth affecting corrosion of magnesium alloy
The magnesium alloy undergoes a corrosion electrochemical reaction in a NaCl solution system, and a hydrogen evolution reaction occurs at the cathode, and a magnesium dissolution reaction occurs at the anode, that is, the magnesium alloy is corroded at the cathode. Electrons move from the anode to the cathode and accumulate on the cathode, causing the potential of the cathode to move in a negative direction. The degree to which the cathode potential becomes negative is related to the current density. The larger the current density, the more negative the cathode potential, and the more severe the corrosion of the magnesium alloy. The rare earth causes the corrosion current of the magnesium alloy to decrease, the polarization resistance increases, and the capacitive reactance decreases, thereby making the hydrogen evolution process more difficult and the corrosion resistance of the alloy is improved.
Wang Xifeng et al [23] studied the corrosion behavior of AZ91 magnesium alloy in NaCl solution after adding rare earth. The results show that the corrosion rate of alloy with 1% RE is only 1/7 of that of the original alloy at 20 °C. Xu Yue et al [24] used a treatment solution with Ce (NO3)3 as the main component to form a rare earth lanthanum conversion coating on the surface of AZ91 magnesium alloy, which effectively improved the corrosion resistance of magnesium alloy. Zhang Yong et al [25] studied the effect of Ce on the corrosion resistance of AZ91 magnesium alloy by gas phase diffusion method. The results show that Ce exists in the surface layer in the form of compound state, and plays a role in purifying the surface and microalloying of the alloy. The corrosion resistance is increased by nearly one time, the uniform corrosion rate is reduced from 1.850 mg/(m2·s) to 0.876 mg/(m2·s), and the corresponding corrosion current density at the same potential is significantly reduced.
Conclusion
China is rich in magnesium and rare earth resources. It is not only a major producer of magnesium and magnesium alloys, but also a major producer of rare earths. It has unique advantages in strengthening the development and research of rare earths in magnesium and magnesium alloys. Make full use of the unique physical and chemical properties of rare earth elements to further improve and improve the overall performance of magnesium alloys, especially the development of high temperature resistant and high strength magnesium alloys, so that the excellent properties of magnesium alloys can be fully utilized to meet the automotive industry and communication electronics. The demand for high quality and high performance magnesium alloys in the fields of industry, aerospace and other fields expands the application range of magnesium alloys.
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