Effect And Mechanism Of In-situ Quasicrystal On Strength And Damping Capacity Of High Zinc-magnesium Alloy

As the lightest structural metal in industrial application at present.Magnesium alloys,which own good casting and processing properties,high specific stiffness,good damping and recyclability,have been widely used in the transportation,aerospace and 3C products.However,the high temperature mechanical properties of the AZ magnesium alloys are poor because of the presence of the lower stability of the?-Mg₁₇Al₁₂ phase,which limits the promotion and development of the alloys.Recently,some new high-strength magnesium alloys have been developed,such as rare earth magnesium alloys,but rare earths are expensive so that it is hard to apply in civilian and industry largely.By adjusting the Zn/Al ratio,the quasi-crystalline phases?-Mg₃₂?Al,Zn?₄₉ and?-Al₂Mg₅Zn₂ were obtained by ordinary casting,which has a positive effect on the improvement of the alloy performance for the Mg-Zn-Al alloys.The faster cooling rate promotes the formation of non-equilibrium states and is beneficial to the formation of quasicrystal phase.and the high-temperature mechanical properties of existing high damping magnesium alloys are not ideal.Therefore,there is an important research significance in developing a high-strength and high-damping magnesium alloy with an inexpensive alloying of Zn and Al.In this study,high zinc-magnesium alloys as experimental alloys were selected and a copper mold for generating in-situ autogenic quasicrystals by rapid solidification was designed.Based on the results of numerical simulations and comparative tests,the best process for smelting is obtained.The effects of cooling rate on crystal morphology and yield were explored.Additionally,the effect of different Zn content on the microstructure and mechanical properties of high studied.Then,study of the effects which Zn content exerts over the damping properties of high zinc-magnesium alloys was investugated at different frequencies with the increase of temperature.The microstructure and phase composition of the alloy were analyzed by optical microscopy?OM?,X-ray diffraction?XRD?,and scanning electron microscopy?SEM?.The mechanical properties of the alloy were characterized by Vickers microhardness and tensile properties at room temperature.And the damping properties of the alloy were detected by using a multi-functional internal pressure.Main results of this study are listed as following:?1?Compared with traditional steel molds,the new copper mold can significantly refine the cast alloy structure,improve the microstructure,increase the tensile strength at room temperature by 7.3%,and increase the elongation by35.4%.When the content of Zn is less than 8%,most of the skeletal second phases in the alloy are uniformly distributed at the grain boundaries,and a small amount is distributed in the grain boundary and dendrite in the form of grains or lumps,with a typical as-cast microstructure.With the increase of Zn content,the grains are refined and the second phase distribution is more uniform,forming a spatial network structure.When the Zn content further increases,the grains become abnormally coarse,the dendrite structure becomes rough,and the grain boundaries are thicker.?2?With the increase of Zn content,the tensile strength and elongation of the test alloy increase first and then decrease,while the yield strength gradually increases.When the content of Zn is 11%,the high zinc magnesium alloy gets the best comprehensive mechanical properties,and the tensile strength,yield strength and elongation at room temperature are 186MPa,120MPa and 6.1%,respectively.The fractures of magnesium alloys with Zn content less than 11%are dominated by transgranular fractures.The morphology features are disorderly and finely cleaved facets.In which there are a few river-like patterns and few torn edges formed by tearing.It is a hybrid fracture consisting of cleaved steps and dimples,which named a quasi-cleavage fracture.The fractures of magnesium alloys with Zn content higher than 11%are dominated by intergranular fractures.The fracture morphology is characterized by smaller cleavage facets and developed river-like patterns formed by cleavage steps,which is a cleavage farcture.?3?With the increase of temperature,the damping value of Mg-Zn-Al alloy increases gradually,and the damping peaks of the alloy are all around 170?.Below 170?,the internal friction values of alloys with different Zn content do not change significantly with increasing temperature.The higher the content of Zn,the lower the internal friction value,and the higher frequency damping is greater than the low frequency damping;and While the temperature is above170?,the results are completely contrary to the previous.As the temperature increases,the internal friction value has a significant increase,the low-frequency damping increases considerably,exceeding the high-frequency damping.This is the combined effect of dislocation damping and grain boundary damping.