| Magnesium and its alloys have a series of advantages such as low density,high specific strength and specific stiffness,good thermal conductivity,good damping and shock absorption,excellent machinability and so on.They have been used in aerospace,automotive,communications,electronic computers and home appliances field.However,the poor high-temperature-mechanical properties and corrosion resistance of magnesium and its alloys make it greatly restricted in the field of structural materials compared to other light metals such as aluminum alloy and titanium alloy.In recent years,Mg-AI-Ca alloys have become a hot spot because they contain thermally stable intermetallic compounds and small grain sizes.Since the rare earth element Y has a unique nuclear electronic arrangement,it can improve the casting process of magnesium alloys,purify the melt,increase the strength and high temperature creep properties of the alloy,and improve the corrosion resistance.It is widely used in metallurgy,materials and other fields.At present,there are few researches on magnesium alloy with high Ca/A1 ratio.In this work,a small amount of rare earth element Y was added on the basis of high Ca/A1 ratio of AC55 magnesium alloy by semi-continuous water-cooling casting.Microstructure,mechanical properties and corrosion resistance of AC55.AC55-0.5Y.AC55-1.0Y magnesium alloy were tested Combined with the tensile test,creep test,electrochemical test and SEM analysis.Research indicates:(1)The micro structure of the as-cast AC55 alloy is mainly composed of a-Mg phase and divorced eutectic phase.The grains are coarse and the eutectic phase is mainly distributed in a continuous network along the grain boundary.And increase the content of Y element,the grain structure in the alloy is obviously refined,the eutectic phase changes from continuous to a semi-continuous network structure,AC55-0.5Y and AC55-1.0Y alloy structure in addition to a-Mg and A12Ca,but also Contains Al2Y intermetallics.(2)After extrusion,the alloy shows the typical fibrous structure.And in this process,dynamic recrystallization occurs takes place.A large number of equiaxed grains appear and the grains are obviously refined.The distribution of A12Ca and A12Y phases along the grain boundaries significantly improves the tensile strength and yield strength of the alloy.Compared with AC55 alloy,when the Y content increased to 1.0,the tensile strength and yield strength of the alloy reached the maximum of 361.33MPa and 345.97MPa,increased by 19.12%and 17.23%,respectively.(3)Three kinds of samples were tested at 175℃,200℃,225℃ and 250℃,respectively.The results showed that AC55-1.0Y alloy shows the highest tensile strength and yield strength,which were 199MPa，164MPa,126MPa and 108MPa,respectively.Creep tests were performed at 175℃，60MPa.The AC55-1.0Y alloy exhibited a minimum of 0.75%creep elongation and a minimum steady-state creep rate of 1.76 x 10-8 s-1.The stress index value of AC55-1.0Y alloy is calculated to be 5.94,which shows that the main creep mechanism of AC55-1.0Y alloy is dislocation climbing.(4)At 400’C,all the three alloys show higher superplasticity.AC55-1.0Y alloy reached the maximum elongation of 472.36%at a strain rate of 3.6×10-4s-1,showing a remarkable superplasticity.Using the constitutive equation,the strain rate sensitivity index(m)and deformation activation energy(Q)of the three alloys were obtained.(5)With the increase of Y content,the corrosion potential of the alloy gradually shifts and the corrosion current density gradually decreases.The corrosion potential and corrosion current density of AC55-1.0Y alloy are-1.56IV(vs.SCE)and 10.19(μA/cm2.The dense and stable corrosion product film formed during the corrosion process plays a crucial role in improving the corrosion resistance of the alloy.Small grain size produces more grain boundaries can also serve as a corrosion barrier to improve the alloy’s corrosion resistance.The corrosion resistance of the three alloys in the extruded state is:AC55<AC55-0.5Y<AC55-1.0Y. |
PDF Full Text Request