Investigation On Microstructure, Strengthening And Toughening Mechanisms Of Mg-Zn-Al-RE Magnesium Alloys

Mg alloys, as the lightest metallic structural materials, have great potential application in automotive industries due to their low density, high specific strength and specific stiffness, good damping characteristics and excellent machinability etc. The most commonly used Mg alloys are Mg-Al system, which offers outstanding combination of die castability, mechanical properties, and corrosion resistance. However, because of the rapid decrease of strength and poor creep resistance, the application of these alloys is restricted at the temperature lower than 120℃. The deterioration of high temperature mechanical properties in the Mg-Al alloys is attributed to the coarsening and softening of discontinuousβ-Mg17Al12 phase. To date many efforts have been dedicated to improve the high temperature mechanical properties of Mg-Al based alloy, accordingly a series of commercial alloys have been explored, such as Mg-Al-Si, Mg-Al-RE, Mg-Al-Sr, Mg-Al-Ca, and Mg-Al-Ca-Sr systems. The aims of these efforts are mainly based on the decrease of Al content and the suppression ofβphase by forming the high melting point intermetallics, however the low castability, high cost and/or poor recyclable of the developed alloys are still problems concerned. Due to the disadvantage of Mg-Al alloy, the Mg-Zn-Al (ZA) alloy with more Zn content than Al is proposed as a low-cost, creep-resistant and diecastable alloy to bridge the gap between AZ91D and some creep-resistant but costly alloy. However, the ZA alloy exhibits low ductility due to the formation of a large amount of network phase at the grain boundaries, moreover the alloy is prone to hot tearing, and the die castability is sensitive to the alloy composition. Until now, except for using as decorative material, there is still limited commercial application of this alloy system.Rare earths is one of the most useful alloying element for Mg alloy, which not only can purify melt and improve casability, but also plays a role in refining the microstructure and improving the heat resistance. More recently, many efforts have been engaged in the development of Mg-Al-RE alloys, whereas there is still limited report on Mg-Zn-Al-RE system. Therefore, the influences of RE content on the microstructure and mechanical properties were systematically studied in this work. According to the above findings, high content of RE-containing was firstly proposed to improve the strength and ductility of the Mg-Zn-Al alloy. The microstructure and mechanical properties of the Mg-Zn-Al-RE alloys with 4 wt.% Ce-rich mischmetal (RE) and variable Zn and Al contents were investigated, and one of the alloys was found to have good strength and ductility. On the one hand, the Ce-rich mischmetal in the alloy was changed into other RE with high solubility inα-Mg matrix, and different REs with same content were further added. The effects of different types of RE in the alloy were studied. On the other hand, the partial substitution of Sn for Ce-rich mischmetal was introduced to the alloy, and the effects of Sn and RE elements on the improvement of aging hardening effect and heat resistance of the alloy were firstly studied. Then, the die castability, microstructure and mechanical properties of the Mg-Zn-Al-RE alloy system were firstly investigated in this work. The thermal stability and transformation of the RE-containing phases were studied, and the relationship between alloy composition and die castability, strength and ductility, and creep resistance were revealed, which gave a foundation for the further development of novel Mg-Zn-Al-RE heat resistant alloy. The major efforts in present research are as follows:(1) The influence of low RE content on the microstructure and mechanical properties.The castability, phase composition, strength and ductility of Mg-Zn-Al alloy are related to the Zn and Al contents. In this work, two types of alloys with composition of Mg-20Zn-8Al and Mg-7Zn-5Al (wt.%) were selected to be the master alloys. The effects of RE content on the microstructure and mechanical properties of the master alloys were investigated. The results show that the rod-like Al4Ce phase forms due to the Ce addition, and the content increases with increasing Ce content. The phase compositions of the as-cast Mg-7Zn-5Al alloy consist ofα-Mg,τ-Mg32(Al, Zn)49, and a small quantity of theβ-Mg17Al12 phase. The addition of Ce-rich mischmetal can suppress the formation ofβphase, meanwhile the rod-like Al2REZn2 phase will be formed. When increased the RE content, the rod-like Al2REZn2 particles will get together, which seems like branch-like morphology.It is indicated that a small amount of RE addition will increase the constitutional undercooling at the head of the solid/liquid interface, as a result, refine the grain size as well as the shape of the network secondary phase. However, the high RE addition increases the formation of RE-containing compounds, decreasing the growth inhibitory effect of Zn and Al elements, result in the coarsening of grain size. Both the grain size and the content of RE-containing compounds influence the mechanical properties of the Mg-Zn-Al alloys. Generally, a small amount of RE addition can improve the room temperature and elevated temperature mechanical properties of the Mg-Zn-Al alloys.(2) The microstructure and mechanical properties of high RE-containing Mg-Zn-Al-RE alloys.Mg-Zn-Al alloy has low ductility due to plenty of the network eutectic phase distributed at the grain boundaries. Although low content of RE addition at some extent can refine the grain size and secondary phase, there is still not much improvement on the ductility. The aims of following investigation are to introduce high RE-containing to suppress or decrease the formation of network eutectic phase. The microstructure and mechanical properties of 4wt.% RE-containing Mg-Zn-Al-RE alloys with variable Zn and Al contents were studied. It is indicated that the content of Mg-Zn-Al phase will be remarkably reduced due to the formation of high content of Al2REZn2 phase. However, the high content of the Al2REZn2 phase will decrease the aging hardening effect of the alloy. The distribution of RE-containing compound changes with the contents of Zn and Al solutes. When increasing the solute, the distribution of the rod like RE-containing compounds will be changed from branch-like to dispersed, and the content ofτ-Mg32(Al, Zn)49 phase gradually increase. The distribution of the rod-like phase and the content ofβ(orτ) phase are critical to the mechanical properties of the Mg-Zn-Al-RE alloys. The Mg-6Zn-5Al-4RE alloy, with branch-like Al2REZn2 phase and low content ofβphase, offers good strength and ductility.(3) Investigation on the microstructure and mechanical properties of Mg-6Zn-5Al-4Gd-(Ce or Y) alloys.RE elements containing the lanthanum series and also yttrium and scandium occupy IIIA subgroup of the periodic table of elements, therefore having a number of similar features. However, each RE possesses different crystalline structure, and the solid solubility inα-Mg has great difference, thus the influence of each RE element on Mg alloys would be different. Previous investigation shows that the types of RE-containing compounds in the Mg-Zn-Al-RE alloy may be related to the RE elements. In this work, the Gd element was used to fully substitute Ce-rich mischmetal in the Mg-6Zn-5Al-4RE. The microstructure and mechanical properties of Mg-6Zn-5Al-4Gd alloys were studied. On the other hand, the Ce and Y elements with same contents were further added to the Mg-6Zn-5Al-4Gd alloy, respectively. The alloying mechanisms of different RE elements were studied. The results show that most of the RE-containing compounds in the Mg-6Zn-5Al-4Gd alloy are rod-like Al2GdZn2, meanwhile a small amount of polygonal Al2Gd phase can also be found. When 1wt.%Ce was added to the alloy, the content of the semi-continuous network Mg-Zn-Al eutectic phase decreases, while the content of rod-like RE-containing increases, and the branch-like distribution becomes more obvious. When 1wt.%Y was added to the Mg-6Zn-5Al-4Gd alloy, the content of polygonal Al2RE phase evidently increases, and the Mg-Zn-Al eutectic phase distributes as band-like morphology.The addition of Ce or Y obviously refines the grain size for the Mg-6Zn-5Al-4Gd based alloy, while the Y element has better refining effect. The Ce and Y show different grain-refining mechanisms: the Ce addition mostly promotes the growth of secondary dendrite, while the Y addition mainly increases the heterogeneous nucleation sites. Compared with the Mg-6Zn-5Al-4RE alloy, both the Gd substitution and the further addition of Ce and Y elements can obviously improve the aging hardening effect of the alloy. The Y addition is beneficial to improve the strength and ductility of Mg-6Zn-5Al-4Gd alloy, whereas the Ce addition decreases the ultimate tensile strength and elongation of the alloy due to the formation of a lot of shrinkage porosities. The peak-aged Mg-6Zn-5Al-4Gd-1Y alloy shows optimal strength and ductility.(4) The effects of mischmetal and tin on the microstructure and mechanical properties of Mg-Zn-Al alloy.The Mg-6Zn-5Al-4RE alloy exhibits acceptable mechanical properties at room temperature, but the strength obviously decreases when the testing temperature increases. The high content of RE-containing increases the cost of the alloy. Moreover, the formation of plenty of RE-containing compounds decreases the solution hardening effect and aging hardening effect of Zn and Al solutes. Based on the above, the partial substitution of Sn for RE on the microstructure and heat resistance of Mg-6Zn-5Al-xRE-ySn alloy was studied. The results show that, with increasing the Sn content, the content of Al2REZn2 phase decreases, and the network Mg-Zn-Al eutectic phase gradually increases, meanwhile the Mg2Sn phase can be found. Furthermore, the increase of Sn content results in the distribution of branch-like RE-containing compounds changing into dispersed morphology, and the Mg2Sn and Mg-Zn-Al phases become coarsening compared with the low Sn-containing alloy.Mg-Zn-Al-RE-Sn alloy has high aging hardening effect. The hardness-versus-aging time curves of the studied alloys exhibit two peaks at different aging temperature and the two corresponding aging processes contain theτand Mg2Sn precipitates. The presence of double aging peaks is attributed to the different peaks. Compared with the Mg-6Zn-5Al-4RE alloy, the partial substitution of Sn element decreases the strength and ductility but improve the heat resistance of the as-cast alloy. The room temperature and elevated temperature strength can be evidently improved after peak-aged treatment. The room temperature strength for each alloy is almost the same at both peak-aged states, whereas the elevated temperature strength of the first peak-aged state is higher than that of the second peak-aged state.(5) Investigation on the die-casting defects, microstructure and mechanical properties of Mg-Zn-Al-RE alloys.Mg alloy die casting has a lot of advantages, such as low production cost, high productivity, and low thermal shock damage to the mold etc. At present, most of casting Mg produces is manufactured by die cast process. However, the die castability of Mg-Zn-Al alloy is sensitive to the alloy composition. Based on the previous results in our work, the high RE-containing was selected to decrease or suppress the formation of low melting point eutectic phase, therefore improve the hot tearing resistance. Aiming to design the novel Mg-Zn-Al-RE die cast alloy, the influences of alloy composition on the die cast defects, microstructure and mechanical properties were studied. Compared with the results of Mg-Zn-Al and Mg-Al-RE systems, the relationship between alloy composition and die castability, and mechanical properties was discussed. The results show that the high content of RE-containing can shorten the solidification range, and decrease or suppress the formation of low melting point eutectic phase. Because of RE-containing, the hot tearing susceptibility as well as the externally solidified crystals of the Mg-Zn-Al-RE alloy is obviously reduced compared with the Mg-Zn-Al alloy with same Zn and Al contents.Mg-1Zn-4Al-4RE alloy is composed ofα-Mg, Al4RE, Al2RE and Al11RE3 phases. With increasing the Zn content, the polygonal Al2RE compounds decreases, while the Al-RE phases are gradually replaced by Al2REZn2 phase. Almost no polygonal RE-containing phase can be formed when the Zn content up to 4wt.%, and the RE-containing phases in the alloys are mainly Al2REZn2 phase, and a small quantity of Al11RE3 phase. In the Mg-Zn-Al-RE alloy system, the Al2REZn2 phase shows high thermal stability. When over-aged the alloy at 150℃for 720h, the Al11RE3 phase will be transformed into Al4RE phase in the low Zn-containing alloys, while both Al11RE3 and Al4RE phases will finally be changed into Al2REZn2 phase in the high Zn-containing alloys. However, when over-aged at 320℃, the RE-containing compounds do not decompose from the observation of morphology, however it can be found that the Al4RE phase is changed into the Al11RE3 phase in the Mg-1Zn-4Al-4RE alloy, while the Al11RE3 phase is transformed into the Al2REZn2 phase in the Mg-4Zn-4Al-4RE alloy.Both the room temperature and elevated temperature yield strength of Mg-Zn-Al-RE alloy increase with increasing Zn or Al content. However, the content of solutes has less effect on the yield strength at 175℃than that at room temperature and 150℃. The ultimate tensile strength and the ductility are critic to die castability, the Mg-4Zn-4Al-4RE alloy with low casting defects exhibits optimal comprehensive tensile properties. Compared with the AE44 alloy, the Zn addition arouses solute segregation and supersaturation, resulting in the formation of Mg-Zn-Al precipitates during creep, as a result obviously decreases the creep resistance of the alloy. The creep resistance of Mg-Zn-Al-RE alloy is related to Zn and Al contents. It is revealed that, in the 4wt.% Al-containing alloys, the minimum creep rate increases with increasing the Zn content, while decreases with increasing the Zn content in the 6wt.% Al-containing alloys. The creep resistance of the studied alloys in this work is: ZAE144 > ZAE244 > ZAE444 > ZAE644 > ZAE1064 > ZAE864 > ZAE664.(6) Preliminary study on the thermodynamic phase diagram calculation of Mg-Zn-Al-Ce alloy system.Due to lack of the phase diagram of Mg-Zn-Al-RE system, to date we cannot predict the solidification process and phase composition for the definite alloy. In order to further design alloy composition and improve mechanical properties of the alloy, it is crucially necessary to understand knowledge of phase diagrams and thermodynamic data of the involved alloy systems. According to reported data, the related thermodynamic descriptions of Mg-Zn-Al-Ce system were evaluated and partly optimized in this work. Using present database, the binary phase diagrams, and the isothermal sections and liquidus surfaces of Mg-Zn-Al, Mg-Zn-Ce as well as Mg-Al-Ce ternary system can be calculated. However, because of limited experimental reports on Al-Ce-Zn system, the phase diagram calculation for this system is not accurate. Based on the experimental results of the Mg-Zn-Al-RE alloys in this work, combining with the solidification process calculation by using Schiel model, the Al-Ce-Zn liquidus surface was roughly back-evaluated. The extrapolated vertical sections of the Mg-Zn-Al-Ce system can give a reference for the further development of the Mg-Zn-Al-RE alloy.