Development Of Mg-Bi And Mg-Bi-Ca Alloys And Analysis Of Their Microstructures And Mechanical Properties

Magnesium alloy has good biological compatibility. Currently, biological materials applied to medicinal field are mainly titanium alloy and non-corrosive steel, and this two kinds of materials have some limitation, titanium alloy is hard to process and costly, non-corrosive steel has high density and contain element that is harmful to human body such as Ni and Cr. Considering the above factors, it is very important to develop new kind of magnesium alloy as biological material.In this paper, Mg-Bi and Mg-Bi-Ca alloys prepared from casting and deformation process which had good ductility were developed. The mass percent of Bi in the alloy are 2%, 5%, 9% respectively and the mass percent of Ca in the alloy is 1.5%. The microstructures and phase change of the alloys were analyzed by using optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and differential thermal analysis. Mechanical properties were also tested for each of the alloys in order to do some exploratory experiment for the development of new kind of magnesium alloy as biological material.The results show that the microstructures of Mg-Bi binary alloys were consisted of a-Mg matrix and precipitate phase Mg₃Bi₂. The grain size of the alloys was between 80μm and 270μm which reduced with the increasing of the content of Bi. Bacillary precipitate Mg₃Bi₂ was aligned at a 120°angle to each other. The microstructure of Mg-2Bi was consisted of a-Mg and granular Mg₃Bi₂. The microstructure of Mg-5Bi was consisted of a-Mg, granular and bacillary Mg₃Bi₂ and reticular Mg₃Bi₂ which discontinuously distributed in the grain boundary. The microstructure of Mg-9Bi was consisted of a-Mg, granular and bacillary Mg₃Bi₂ and reticular Mg₃Bi₂ which continuously distributed in the grain boundary. The actual density of the three alloys was 1.799 g/cm³, 1.820 g/cm³ and 1.886 g/cm³ respectively. The microstructures of Mg-Bi-Ca ternary alloys were consisted of a-Mg matrix, precipitate phase Mg₃Bi₂ and CaMg₂Bi₂ phase. Compared with the Mg-Bi binary alloys, the microstructures of the ternary alloys were significantly refined because of the adding of Ca.Because of the dynamic recrystallization, the microstructures of the EX-Mg-Bi alloys were consisted of refined equiaxed or approximate equiaxed grains. The grain size of the alloys was between 80μm and 270μm. The ultimate tensile strength of the EX-Mg-2Bi, EX-Mg-5Bi and EX-Mg-9Bi alloy obtained under the extrusion ratio of 25 were 220.45MPa, 244.14MPa, 258.03MPa respectively, while the yield tensile strength of them were 202.94MPa, 225.08MPa, 253.53MPa, and the elongations were 6.67%, 11%, 9.20% respectively.Because of the dynamic recrystallization, the microstructures of the EX-Mg-Bi-Ca alloys were consisted of refined equiaxed grains. The grain size of the alloys was within 80μm. The ultimate tensile strength of the EX-Mg-5Bi-1.5Ca and EX-Mg-9Bi-1.5Ca alloy obtained under the extrusion ratio of 17.36 were 197.80MPa, 208.78MPa respectively, while the yield tensile strength of them were 159.46MPa, 182.57MPa, and the elongations were 7.13%, 5.27% respectively.The grain size of RE-2-EX alloy and RE-4-EX alloy was about 10μm, while the grain size of RE-6-EX alloy was about 16μm. The ultimate tensile strength of the RE-2-EX, RE-4-EX and RE-6-EX alloy obtained under the reciprocating extrusion ratio of 12.76 and extrusion ratio of 17.36 were 205.84MPa, 219.50MPa, 220.25MPa respectively, while the yield tensile strength of them were 167.64MPa, 184.79MPa, 185.86MPa, and the elongations were 16.33%, 18.67%, 10.33% respectively. RE-4-EX alloy had the optimal mechanical properties.