Research On Microstructures And Properties Of Mg-Zn-Y(-Gd) Alloys

Magnesium alloy is the lightest engineering metal materials with high electromagnetic shielding characteristics, damping capacity, thermal conductivity and electrical conductivity. However, the lower mechanical properties, corrosion resistance and high temperature resistant performance, the higher machining cost limit the wide application of magnesium alloys. Therefore, how to take advantage of the merits, make up for deficiencies and expand the application has become the focus of magnesium alloy research. In this paper, Mg-Zn-Y(-Gd) magnesium alloys with different atom ratio of Zn and RE were prepared by the near equilibrium solidification method, and the rare earth magnesium alloys with icosahedral quasicrystal phase (I-phase), Mg3Zn3Y2 (W) phase and long period stacking ordered (LPSO) structure phases were obtained. The microstructures and the second phases of Mg-Zn-Y(-Gd) alloys were determined by OM, SEM, EDS, XRD, TEM and SAED. And the formation mechanism of I and LPSO phases were investigated respectively. On this basis, the relationship between I-phase distribution and the cooling rate during solidification of alloy was established, the relationship between the corrosion resistance and the cooling rate was determined, and the corrosion mechanism of the Mg alloys with I-phase was revealed. The solidification process of LPSO enhanced Mg-Zn-Y(-Gd) magnesium alloy was explained. The effects of Y and Gd interaction on formation of LPSO have been discussed systematically. The enhancement mechanism, the corrosion resistance mechanism and the thermal conductive mechanism were discussed. In addition, the plastic deformation mechanism of LPSO enhanced grain-refining magnesium alloys was revealed.I-phase can be obtained in as-cast Mg68Zn29Y3 and Mg68Zn28Y4 alloys with high Zn/RE atom ratio. The as-cast Mg68Zn29Y3 and Mg68Zn28Y4 alloys exhibited different solidification process. I-phase in Mg68Zn28Y4 alloy can nucleate and grow directly in the melt, while in Mg68Zn29Y3 alloy W-phase was formed firstly, and then I-phase can be generated with the peritectic reaction. The effect of cooling rate on I-phase formation, size, volume fraction and distribution is remarkable. Experiment and simulation methods were used to analyse the influence of cooling rate on solidification microstmcture, the volume fraction and distribution of I-phase in as-cast Mg68Zn29Y3 magnesium alloy. And the results show that with the faster cooling rate, the kinetics undercooling is larger, and then the nucleation rate is higher, so as to promote the nucleation and growth of quasicrystals with a small plane type. While with cooling rate decreasing, the nucleation rate of I-phase was declined. I-phases changed from dispersion and fine to fragmentation and coarse, and the secondary dendrite arms of I-phase formed around the primary arms. By simulating the environment of ocean, I-phase exhibited higher corrosion resistance. The corrosion current density of as-cast Mg68Zn28Y4 alloy etching after 30 hours was reduced by two orders of magnitude than that of unetched Mg68Zn32 alloy which was up to 6×10-5 A/cm2, and the corrosion potential was -1.35 V. Though, high Zn content can obtain I phase with micron grade size and owing to its’strong corrosion resistance can improve Mg alloy corrosion resistance. However, the high potential difference will cause the matrix and the solid solution phase as anode to form rapid corrosion. In addition, the high stress can formed between the micron I phase and matrix phase, which the brittleness of Mg alloy will be increased and easy to form the stress corrosion cracking. Although, low Mg and high Zn/RE ratio of I enhanced magnesium alloy is not applicable for engineering materials, but the results of this study clearly elaborate the formation condition, forming process and the corrosion resistance performance of I-phase, and provide basis for the design of I enhanced Mg matrix engineering materials.Stable 14H-LPSO phase was obtained by as-cast method in low Zn/RE atom ratio (Zn/RE=1) Mg92Zn4YxGd4-x (x=4,3,2,1, at%) and Mg94Zn3YxGd3-x (x=3,2, 1.5,1, at%) magnesium alloys. In the two-dimensional space, LPSO phase grows in lamellar sturcture in Mg92Zn4YxGd4-x (x=4,3) and Mg94Zn3YxGd3-x (x=3,2,1.5,1) alloys. Good compressive mechanical properties of as-cast Mg-Zn-Y(-Gd) alloys attribute to the solid solution strengthing, the second phase strengthing and the dispersion strengthing etc. The better compressive mechanical properties will be achieved by enhancing the volume fraction of LPSO phase in Mg alloy. Adding amount of Zn and Gd alloy elements would weaken the corrosion resistance of as-cast Mg-Zn-Y(-Gd) alloys. When Gd≤1 at.%, as-cast Mg-Zn-Y(-Gd) alloys displayed a good corrosion resistance. But, the corrosion resistance of as-cast Mg-Zn-Y(-Gd) alloys decrease when Gd>1.5 at.%. And the corrosion resistance of Mg94Zn3YxGd3-x alloys is better than that of Mg92Zn4YxGd4-x alloys because of the lower Zn content. Mg94Zn3Y3 alloy has the best corrosion resistance with 3.988× 10-5 A/cm2 corrosion current density and -1.524 V corrosion potential, which is superior to the commercial AZ91D magnesium alloy. R(Q(R(QR))) was selected as the corrosion electrochemical equivalent circuit of LPSO reinforced Mg-Zn-Y(-Gd) alloy which analysed by ZSimpWin software. Heat conduction mechanism of the samples was studied that the thermal conductivity is affected by the distribution and volume fraction in the matrix, microstructure, interfacial morphology, solute elements of each phase, etc. Among them, the dislocation scattering and interface scatting of phonons are the main influence factors, and along with the influence of mictrostructure and phase distribution on the phonon transport. Stacking fault (SF) and grain boundaries of LPSO phase can impede diffusion movement of phonons and decline the thermal conducitivity of LPSO phase enhanced magnesium alloy.The segregation phenomenon of as-cast Mg92Zn4Y4 and Mg92Zn4Y3Gd1 alloys was eliminated by homogenization treatment at 500℃ for 12 h. A large number of lamellar 14H-LPSO phases precipitated from a-Mg matrix in as-homogenization Mg92Zn4Y3Gd1 alloy. Then, the 14H-LPSO reinforced grain-refining magnesium alloys were prepared by hot extrusion process at 300℃,340℃,370℃ and 400℃ with ectrusion ratio of 9:1. The deformation texture of extruded Mg92Zn4Y3Gd1 alloy is composed mainly of {0001} basal texture. That is to say, the {0001} plane parallels to the extrusion direction from 340℃ to 400℃ ectrusion process. Y can reduce the basal texture componment, and prompt the extruded Mg92Zn4Y4 alloy to actuate {1121} pyramidal plane slip systems. The mechanical properties of extruded Mg92Zn4Y4 alloy and Mg92Zn4Y3Gdi alloy at room temperature and high temperature are influenced by the microstructure of before and after extrusion, texture type, W and LPSO enhanced phase, elements and twinning. At room temperature, the ultimate tensile stress (UTS) and the elongation of extruded Mg92Zn4Y4 alloy is 409.7 MPa and 5.15%, respectively. However, extruded Mg92Zn4Y3Gd1 alloy shows excellent mechanical properties at 100℃-300℃. After 250℃ tensile testing, the UTS and the elongation of Mg92Zn4Y3Gd1 alloy are 297.8 MPa and 15.7%, respectively. Moreover, at 300℃, the UTS of as-extruded Mg92Zn4Y3Gd1 alloy is 225.3 MPa, and the elongation rises to 35.5%.