Study On Microstructure And Properties Of Mg-Zn-Gd-Zr Alloys With Different Zn/Gd Ratios

As we all know,magnesium alloy has many disadvantages,such as low strength and poor corrosion resistance,which has seriously hindered its application and development in various fields.Therefore,to decelop high strength and corrosion resistant of rare earth magnesium alloys has become a research hotspot in recent years.The materials used for this project are Mg-Zn-Gd-0.6Zr alloys.By adjusting the Zn/Gd ratio to study the influence of Zn/Gd ratio on the microstructure,second phase,mechanical properties and corrosion resistance of Mg-Zn-Gd-0.6Zr alloys.(1)Microstructures and phase compositions of as-cast Mg-Zn-Gd-Zr alloys' are related to Zn/Gd ratios.When the Zn/Gd ratio is 0.27,the lamellar-like X-Mg12ZnGd and a mount of bulk-like W-Mg3Zn3Gd2 are existed in the allloys;When the Zn/Gd ratio increases to 0.56,most of the second phase is W-Mg3Zn3Gd2;When the Zn/Gd ratio increases to 1.4,strip-like I-Mg3Zn6Gd phase appears in the alloy.After homogenized and extrusion,the secondary phase morphology and compositiong of the alloy do not changed,but the amount of X-Mg12ZnGd phase and I-Mg3Zn6Gd phase is scarely increased.T5 aging treatment does not change the phase kinds,and the content of W-Mg3Zn3Gd2 and I-Mg3Zn6Gd were not changed,either.However,the gray LPSO structure X-Mg12ZnGd phasechanges to bright white and trip-like structure;The volume fraction of the lamellar X-Mg12ZnGd phase and strip I-Mg3Zn6Gd phase in the T5 condition is increased compared with extrusion condition,but the volume fraction of the W-Mg3Zn3Gd2 is nearly unchanged.(2)The mechanical of extruded extruded Mg-Zn-Gd-Zr alloy with different kinds of phases properties at room temperature are different.With the increase of Zn/Gd ratios,the strength of the alloys decrease first and then increase,the elongations increases first and then decrease.Among these alloys,the I-Mg3Zn6Gd phase containing Mg-7Zn-5Gd-0.6Zr alloy owns the better mechanical properties,the ultimate tensile strength and tensile yield strength are 350MP and 285MPa,respectively.At the same time,elongation reaches to 14%.When the alloys are stretched at 250?,the strength gradually decreases and the elongation increases with the increase of Zn/Gd ratios.The mechanical properties of X-Mg12ZnGd phase containing Mg-3Zn-11Gd-0.5Zr alloy are better,its ultimate tensile strengthcis and yield strength are 205MPa,and 179MPa,and elongation is 11%.The elongation of the I-Mg3Zn6Gd phase containing alloy is 209%.(3)Through T5 treatment,the strength of the experimental alloys and the elongations are all decreased.With the increase of Zn/Gd ratios,the strength decreases first and then increases.The elongation the opposite.The tensile strength and yield strength of X-phase containing Mg-3Zn-11Gd-0.5Zr alloy increase from 338MPa and 230MPa to 390MPa and 295MPa after T5 heat treatment.When it is stretched at 200?and 250?,with the increase of Zn/Gd ratios,the strength decreases and the elongation increases.The X-phase containing Mg-3Zn-11Gd-0.5Zr extrusion bar has good tensile properties at 250?.When it is heat treated by T5,its tensile strength and yield strength reach to 231 MPa and 170MPa,respectively,and the elongation reaches 16%.Thetensile strength of I-phase containing Mg-7Zn-5Gd-0.6Zr extrusion bar was lower and the elongation reaches to 863%when it was extended at 250? after T5 heat treatment.(4)The corrosion resistance of Mg-Zn-Gd-Zr alloys with different Zn/Gd ratios are different.Among the as-cast alloys,the corrosion resistance of I-phase containing Mg-7Zn-5Gd-0.5Zr alloy is better;Among the homogenized and as-extruded alloys,the corrosion resistance of X-phase containing Mg-3Zn-11Gd-0.5Zr alloy is better.After heat treatment,the corrosion resistance of W-phase containing Mg-5Zn-9Gd-0.5Zr alloy is better.(5)The corrosion resistance of Mg-Zn-Gd-Zr alloys in different states are different.The as-extruded X-phase containing Mg-3Zn-11Gd-0.5Zr alloy has better corrosion resistance;The corrosion resistance of W-phase containing Mg-5Zn-9Gd-0.5Zr alloy under T5 heat treatment is better;The as-cast I-phase containing Mg-7Zn-5Gd-0.5Zr alloy has better corrosion resistance.(6)Under different states,the corrosion resistance of Mg-Zn-Gd-Zr alloys with different Zn/Gd ratios are different.The as-extruded X-phase containing Mg-3Zn-11Gd-0.5Zr alloy is better,the corrosion potential is-1.49V,the polarization resistance is 623?·cm2.