Study on Process and Properties of Quasicrystal Reinforced Mg-Y-Zn Magnesium Alloy by Squeeze Casting

Author:Wang Bing

Supervisor:hou hua


Degree Year:2019





Magnesium alloy has the advantages of small density,large modulus of elasticity,good heat dissipation,good shock absorption,good corrosion resistance to organic matter and alkali.However,its application is limited because of its low strength.Therefore,magnesium quasicrystals containing rare earth elements have attracted more and more attention due to their high strength and creep resistance.Most rare earth elements have relatively high solid solubility in magnesium,and have good solid solution strengthening and precipitation strengthening effects.Squeeze casting is an advanced near net-shape forming method.Its characteristic is that the liquid metal solidifies under pressure and produces certain plastic deformation during the forming process.Squeeze casting can effectively reduce shrinkage and porosity defects in the castings,and obtain castings with fine grains,compact structure and high mechanical properties.Therefore,it has great prospects in the fields of national defense,aeronautics and astronautics and automobile industry.Mg-xY-6x Zn and Mg-Y-xZn alloys were prepared by squeeze casting with Mg-Y-Zn alloy as the research object.The effects of squeeze casting parameters and alloy composition on the microstructure,mechanical properties and formation of quasicrystalline I phase of the alloy were studied.The main contents of this paper are as follows:(1)Mg95.1Y0.7Zn4.2 and Mg93Y1Zn6 alloys were prepared by squeeze casting method.The mold temperature and holding time of squeeze casting were 200℃and 20s,respectively.The squeeze casting pressure of 50 MPa,100 MPa and 150 MPa were studied under the pouring temperature of 700℃.The pouring temperature of 680℃,700℃and 720℃were studied under the squeeze casting pressure of 100MPa.It was found that with the change of applied pressure and pouring temperature,the composition of phases in Mg95.1Y0.7Zn4.2 and Mg93Y1Zn6 alloys was unchanged,all of which were consist ofα-Mg matrix and quasicrystal I-phase.Changing the appiled pressure and pouring temperature will change the size of grain and its distribution.With the increase of pouring temperature,the microstructure and grains of the alloys become larger,which changes from continuous network structure to discontinuous distribution.With the increase of appiled pressure and pouring temperature,the hardness,tensile strength and elongation of the alloys increase first and then decrease.The hardness,tensile strength and elongation of the alloys reach the best when casting temperature was 700℃and the appiled pressure was 100 MPa.Which were 76.5 HV,215.7 MPa,6.7%and 79.3 HV,221.9 MPa and5.2%,respectively.Fracture morphology of the alloys include cleavage surface and tear edge,which shows the characteristics of cleavage fracture.Therefore,in this experiment,the suitable parameters of squeeze casting should be 700℃and 100 MPa.(2)The Y/Zn ratio of Mg-Y-Zn alloy is 1:6,the squeeze casting parameters are 100 MPa and 700℃.The microstructure of the alloy is mainly composed of grayα-Mg matrix and black-gray lamellar eutectic grain boundary.With the increase of Y and Zn content,the eutectic structure(a-Mg+I-phase)in the alloy increases,the tensile strength and elongation of the alloy increase first and then decrease.When Y content was 1 at%,the tensile strength and hardness of the alloy reach the best,which were 221.9 MPa and 79.3 HV.When Y content was 0.7 at%,the maximum elongation of the alloy is 6.7%.The ratio of Y to Zn of eutectic structure in the alloy is close to 1:6,which is confirmed asα-Mg and I-phase.The petal-like structure in the alloy has five rotational symmetry,and its composition is 32.69 at%Mg,9.85 at%Y and 57.46at%Zn.The atomic ratio of Mg,Y and Zn is close to 3:1:6,which approximate to quasicrystal I-phase,the petal-like structure was identified as quasicrystal I-Mg3Y1Zn6 phase.The fracture morphology of Mg93Y1Zn6 alloy show quasi-cleavage fracture characteristics,while those of the other three alloys show cleavage fracture characteristics.(3)The formation of phases in Mg-Y-xZn alloys is determined by the ratio of Y to Zn atoms.When the ratio of Y to Zn was 1:5,the composition of the alloy wereα-Mg,I-phase and Mg-Y;when the ratio of Y to Zn was 1:6,the composition of the alloys wereα-Mg and I-phase;when the ratio of Y to Zn was1:7 or 1:8,the composition of the alloy wereα-Mg,I-phase and Mg7Zn3 phase.With the increase of Zn content,the tensile strength and elongation of the alloy increase first and then decrease.When the content of Zn was 6at.%,the hardness,tensile strength and elongation were 79.3HV,221.9MPa and5.2%.Fracture morphology of Mg94Y1Zn5and Mg93Y1Zn6 alloys show quasi-cleavage fracture characteristics,while those of Mg92Y1Zn7and Mg91Y1Zn8 alloys show brittle fracture characteristics.(4)The squeeze casting Mg93Y1Zn6 alloy was heated for 20 hours at 400℃and then for 8hours at 200℃.The microstructure of the alloy was composed ofα-Mg phase and quasicrystal I-phase.The grain boundary becomes more continuous and the grain size of the alloy becomes larger.The hardness of the alloy decreases,while the tensile strength and elongation increase,the hardness,tensile strength and elongation were 78.1HV,225.1MPa and 7.4%,respectively.(5)The average grain size of Mg93Y1Zn6 alloy by squeeze casting significantly increased after heated for 4 hours at 500℃and 2 hours at 550℃.Nearly spherical I-phase appeared inα-Mg matrix.The mechanical properties were 228.9MPa and 10.6%at 500℃×4h and 225.1MPa and 11.1%at 550℃×2h,respectively.The elongation of the alloy significantly increased.The hardness of Mg93Y1Zn6 alloy increased significantly after solid solution treatment at 500℃for4h and aging treatment 200℃for 10h.The tensile strength and elongation of Mg93Y1Zn6 alloy were 279.6MPa and 6.9%,respectively,and the mechanical properties of Mg93Y1Zn6 alloy were improved by 26.0%and 32.7%,respectively.