Microstructure And Properties Of Biomedical MgZnCa Amorphous Composites

Mg-Zn-Ca amorphous alloys are one kind of potential biodegradable biomedical materials due to their low density,close elastic modulus to that of human bone,good biocompatibility and degradability.However,the limited inherent ductility at room temperature makes it difficult to predict the failure of the material and to be processed and formed.Therefore,at present they are hardly to meet the clinical medical requirements for mechanical properties,and this seriously hinders their applications.Introduction of the second phase to the amorphous alloy matrix could effectively hinder the rapid propagation of single shear band,and promote the nucleation,initiation and interlacing of multiple shear bands,thereby improve the plasticity of the bulk amorphous materials.The in-situ crystalline phases exhibit a good interface with the amorphous matrix and a dispersive distribution,which can significantly improve the plasticity of the amorphous material.In this paper,quaternary Mg₆₅Zn₃₀Ca₄Sr₁and ternary Mg₇₀Zn₂₅Ca₅alloys were taken as research objects.?=2 mm rod amorphous sample of Mg₆₅Zn₃₀Ca₄Sr₁alloy and amorphous composite sample of Mg₇₀Zn₂₅Ca₅alloy were fabricated using industrial raw materials by copper mold casting under argon protective atmosphere.Proper heat treatments were conducted to manipulate the internal structure of the samples.XRD,SEM,TEM,compression test and electrochemical measurement were used to investigate the effects of different heat treatment processes on the microstructure,mechanical properties and corrosion properties of Mg-Zn-Ca?-Sr?alloys to obtain a new degradable medical magnesium alloy material with good mechanical properties and corrosion resistance.The detail research results are as follows:?1?The microstructure of?=2 mm rod sample of Mg₆₅Zn₃₀Ca₄Sr₁alloy is amorphous before heat treatment.After heating treatment at 110?for 40 min,equiaxed dendrite Mg phase and oval Mg₂Ca phase were precipitated in the amorphous matrix.With the increase of heat treatment temperature,the degree of amorphous crystallization and the volume fraction of crystalline phase increases.Meanwhile,the dendrite Mg phase gradually dissolves and the number of dendrites decreases.Dendrite Mg are completely dissolved and formed Zn rich with different morphologies such as hexagonal phase,rod phase,linear phase and block phase,but the content of elements in these phases shows a concentration gradient characteristic are formed after heat treatment at 300?for 60 min.Compression test shows that?=2 mm rod amorphous sample of Mg₆₅Zn₃₀Ca₄Sr₁alloy demonstrates a brittle fracture mode.With the increase of heat treatment temperature,the plasticity of Mg₆₅Zn₃₀Ca₄Sr₁alloy goes better and the fracture strength decreases first and then increases.After heat treated at 300?for 40 min,the specimens show obviously macro plastic deformation behavior,and the compressive plasticity reaches to 2.0%.When holding time is extended to 60 min at 300?,the macroscopic plastic deformation of the sample is further increased to 5.8%,and the yield strength and fracture strength are up to 620 MPa and 790.17 MPa,respectively.Electrochemical test results show that the corrosion potential is-1.18 V and the corrosion current density is 53.46?A/cm²of?=2mm rod amorphous sample of Mg₆₅Zn₃₀Ca₄Sr₁alloy.After heat treatment at 300?×60 min,the coarse dendrite Mg phases in the structure is complete dissolved,and a fine and uniformly distributed Zn-rich structure is formed on the surface of the sample.This microstructure is beneficial to form an effective protective layer of product film mainly composed of stable Zn?OH?₂precipitates on the surface of the alloy,which accounts for the best corrosion resistance of the sample close to that of the amorphous alloy.?2?The?=2 mm rod sample of Mg₇₀Zn₂₅Ca₅alloy is composed of dendrite Mg phase and the reticular quaiscrystal phase Ca_6.93Mg_20.58Zn_72.49distributed around the dendrites before heat-treatment.After heating treatment at temperature interval from 150?to 310?,as the heating treatment temperature increases,the dendrite Mg phase first coarsens?150?,220??,and then the continuity of large-scale dendrites phase decrease and granulate gradually?280??.The sample is mainly composed of granular Mg phase,small white particle phases Mg Zn₂and rod-shaped Ca₂Mg₅Zn₁₃distributed in the vicinity of Mg grain boundaries after heating treatment at 310?for 60 min.The compression curve of Mg₇₀Zn₂₅Ca₅alloy exhibits a certain plastic deformation ability for the sample before heat treatment.The plastic strain reaches 2.8%and the compressive strength is as high as 711.64 MPa.After heating treatment,all the samples show a significant work-hardening phenomenon.With the increase of heat treatment temperature,the plastic strain and the fracture strength decreases first and then increases.When heat-treated temperature is 310?,the fracture strength is 651.60 MPa,but its plastic deformation is up to7%.According to the results of corrosion test,it is found that the?=2 mm rod sample of Mg₇₀Zn₂₅Ca₅alloy shows the corrosion potential of-1.24 V and the current density of 173.41?A/cm²before heat treatment.After heat treatment,the corrosion resistance first decreases and then increases with increasing of heat-treated temperature.The sample heat treated at 310?shows the best corrosion resistance,the corrosion potential is-1.23 V,the corrosion current density is 118.92?A/cm²,and it is better than the corrosion resistance of the sample before heat treatment.