Study On Microstructure Evolution And Properties Of Rare Earth-bearing Magnesium-calcium Alloys

Due to its low density and high melting point,Ca can often be used as a new alloying element to strengthen the magnesium matrix.The quality of the alloy can be further reduced in magnesium alloy.The second phase Mg2Ca formed by Ca and Mghas betterhigh-temperature resistance,which can effectively improve thehigh-temperature resistance and retain the characteristics of light-weight magnesium alloy.In addition,Ca is a rich metal element inhuman body.Therefore,Mg-Ca based alloyshave certain application prospects for the future development of medical materials.In this paper,the microstructure,phase composition and element distribution of alloys in different states are analyzed by means of metallographic microscope,X-ray diffractometer and Sigma scanning electron microscope.The influence of different content of Ca on thehypoeutectic structure of Mg-Ca alloy and the influence of different content of rare-earth element Dy on the structure of Mg-Ca alloy are studied,and the differences are observed by means of Sub-Rapid solidification.Dy content of the component influences the structure of Mg-Ca-Dy alloy and particulates some of the alloys.The results show that:Thehypoeutectic structure of Mg-Ca is composed of white ?-Mg matrix and eutectic structure Mg2Ca distributed along grain boundary.With the increase of Ca content,grain refinement and eutectic structure between crystals increase and become thicker.After adding rare-earth element Dy,the primary alpha-Mg phase is refined obviously by rare-earth element.When Dy content reaches 4%,the equiaxing effect is most obvious.?-Mg changes from columnar dendrite to fine equiaxed crystal;Dy content continues to increase and refinement is not obvious;when Dy content reaches 10%,the primary alpha-Mg phase coarsens again.Dispersion and intergranular distribution of Dy elements,rare-earth phase MgDy2 formed with Mg elements is occasionally distributed along the cell wall of grain boundaries in angular dots.The structure of Mg-Ca-Dy alloy obtained by Sub-Rapid solidification is finer to a certain extent than that of metal mold casting.The solid solubility of Ca in Mg is improved by the cooling rate,and the distribution of alloy components is more uniform,which slows down the segregation of components.The tensile properties of as-cast Mg-Ca-Dy alloys are generally nothigh.With the increase of rare earth Dy elements,the mechanical properties of Mg-Ca-Dy alloys are better.The tensile strength of the alloys cast with copper molds of 12 mm size is improved from 82 MPa of Mg-4%Ca alloys without rare earth elements to 173 MPa of Mg-4%Ca-5%Dy,and the enhancement effect is obvious.The tensile strength of 8 mm size copper mold was increased from 90 MPa of Mg-4%Ca alloy without rare earth element to 194 MPa of Mg-4%Ca-4%Dy,and it was verified that the faster the cooling rate,the better the mechanical properties of the alloy.After heat treatment of Mg-4%Ca and Mg-4%Ca-4%Dy alloys,the eutectic structure of the two alloys gradually transformed to granular with the increase of temperature and the extension of holding time.The addition of rare earth Dy element will shorten the granulation time of the internal structure of Mg-4%Ca alloy during heat treatment,and Mg-4%Ca-4%Dy maintains the macroscopic integrity of grain boundary as a whole during granulation.The eutectic structure of Mg-4%Ca alloy was most uniformly distributed on the matrix after 24 h ofheat treatment at 480?.After 16 h of heat treatment at 480? for Mg-4%Ca-4%Dy alloy,the eutectic structure was maintained as granular distribution among the crystals.