Microalloying Effects Of Rare Element Of Nd,Y On The Mg-6Zn-1Mn Magnesium Alloy

Magnesium alloys, with the advantages properties of light weight, high specific strength, good thermalconductivity, damping, electromagnetic shielding etc., exhibit a wide applications in transportation, telecommunications, aerospace and other fields and have become one of the green engineering material in the twenty-first Century. However, due to the relatively low strength, poor corrosion resistances, it extremely limits their widely use as structural materials. Therefore, the development of high strength with good corrosion resistance of novel magnesium alloys has become a hot topic in materials research field.In the present study, the microalloying effect of Rare Element of Nd、Y on the high strength Mg-6Zn-1Mn(ZM61) alloy, which has been developed in our group recently. In addition, the effects of different cooling rate on the ZM61 alloy prepared by rapid solidification with copper mold casting has also been investigated. By using the optical microscope(OM), X-ray diffraction(XRD), scanning electron microscopy(SEM) equipped with Energy Dispersive Spectrometer(EDS), transmission electron microscopy(TEM), differential scanning calorimetry(DSC), material testing machine, electrochemical workstation and other experimental methods, different contents of rare earth Nd or Y on the microstructure, thermal stability, mechanical properties,corrosion resistance of ZM61 magnesium alloy have been systematically investigated. The main results were concluded as follows:(1) We found that a small amount of rare earth Nd(less than 0.4%) have an obviously grain refinemen effect on the as-cast ZM61 alloy. With the Nd content increasing, the mechanical properties of the alloy reduced due to the coarse second phase particles precipitates on the grain boundary. In addition, this study confirmed that the main effect of Nd was influence on the dynamic recrystallization rate rather than the texture of extruded ZM61 alloy. The dynamic recrystallization rate of extruded alloy with little Nd content is low, mainly generated the dispersed fine second phase. The alloys exhibit completely dynamic recrystallization with the increasement of Nd. However, due to the coarse T phase precipitation on the grain boundary, lead to a destorying mechanical properties of the alloys.(2) Our results demonstrated that the grain size of the solution treatment ZM61-Nd alloys with low Nd content decreases obviously and the majority of second phase dissolved totally on the magnesium alloy matrix. After the subsequent aging process, these fine phases were precipitated and dispersed in plays the role of dispersion strengthening. Therefore, these alloys show good mechanical properties. However, with the increasement of Nd content, the retention of T phase precipitated on the residual matrix were increased. Because the second phase precipitation will reduce after the subsequent aging treatment, the room temperature strength of these solid solution aging ZM61-Nd alloy will decrease. TEM results confirmed that the reinforced Mg Zn2 phase show β’1 rod-like phase and β’2 plate-like phase during aging. The orientation relationship between β’1 rod-like phase and magnesium matrix is: [112 ?0]_Mg 〖∕∕[0001]〗_Mg Zn2,(0002)_Mg∕∕(112 ?0)_Mg Zn2. The orientation relationship between β’2 plate-like phase and magnesium matrix is: [112 ?0]_Mg∕∕[101 ?0] Mg Zn2,(0002)_Mg∕∕(0002)_Mg Zn2. By adding of Nd element, it can accelerate the β’2 plate-like phase precipitation in the ZM61 alloys. Meanwhile, with the increase of aging time, some β’1 rod-like phase can change to β’2 plate-like phase.(3) In this study, the phase evolution with the Y content of as-cast ZM61 magnesium alloys was α-Mg+ Mg7Zn3�'α-Mg+I�'α-Mg+W�'α-Mg+X, which is mainly attributed to the different atomic ratio of Zn/Y. The I phase mainly segregated on dendrite, the W phase display a fishbone shape and the X phase always show lamellar form.(4) when the Y content was lower than 2%, the extruded ZM61 magnesium alloys show a high strength and good ductility due to the precipitation of fine second phase under extrusion. In addition, dimple size is larger from the fracture surface. With the increasement of Y, the second phase particles, mainly precipiting on the grain boundary, become larger, which is difficult to be crushed and refined under the hot extrusion process. Therefore, the comprehensive mechanicalproperties of extruded alloys decrease due to the weakening interfacial energy of the precipitates and matrix. At the same time, a small amount of rare earth Y(less than 2%) doped ZM61 alloys show good mechanical properties after solution and aging. This is attributed to the majority of second phase particles into the magnesium matrix during the solid solution process. Under the subsequent process of artificial aging, the second particles precipitated and dispersed in grain boundary and matrix, resulting in the improvement of the dispersion strengthening effect. With the increasement of Y, the second phase particles dissolve into the magnesium matrix decrease during the solid solution process, and reduce the strengthening effect. Meanwhile, the grain size of these alloys greatly raised, declining in the overall performance of alloys.(5) Our results demonstrated that after 515 ℃ /2h+90 ℃ /24h+180 ℃ /24 h solid solution and double aging treatment, the Mg-6Zn-Mn-2Y alloy with the yield strength up to 340 MPa exhibit a good comprehensive mechanical properties. The aging-hardness curves display a “lacking aging-peak aging-over aging” feature. At the same time, the alloy could hold a high hardness state for a long time in the process of ageing, exhibiting a strong age hardening ability. This could provide a possible technical route for the development of high strength ZM61-Y alloys.(6) When the ZM61 magnesium alloy was prepared by the rapid solidification(RS) technology, the RS ZM61 exhibit more uniform microstructure and more fine precipitates. The RS ZM61 magnesium alloy shows ultrahigh strength with 460 MPa, which is about 2 times as much as conventional casting magnesium alloy. This is mainly attributed to the precipitation of higher young’s modulus of Mg Zn2 phase than that of the Mg matrix. In addition, the potentiodynamic polarization experiment results show that the RS ZM61 magnesium alloy, with the low corrosion rate about 11.2 m/year, has a lower corrosion current density, the higher the pitting potential and wider passive region and shows excellent corrosion resistance.