Preparation And Properties Of Mg-Zn-Ca Glass Matrix Composites For Biomedical Materials

Compared with polycrystalline magnesium alloys,Mg-based amorphous alloys exhibit advantages such as the closest density and elastic modulus to those of human bones,higher strength,better corrosion resistance,degradability,biocompatibility and bio-safety.Among them,Mg-Zn-Ca amorphous alloys are considered to be the most promising implanted magnesium alloys whose constitutional elements are all essential nutrient elements for human body.Degradation rate and mechanical properties are two important issues that must be considered during the application of biomedical materials.Reported researches demonstrated Mg-Zn-Ca amorphous alloys possess higher strength and corrosion resistance than polycrystalline magnesium alloys.However,the inherent brittleness of amorphous materials restricts the application of Mg-Zn-Ca amorphous materials.Fabrication processes and alloying elements are key factors affecting the glass forming ability,microstructure,corrosion resistance and mechanical properties of Mg-based amorphous alloys.Mg-Zn-Ca based amorphous alloys and their composites containing different volume fractions of crystal phases and particle sizes were prepared by controlling the solidification rates and alloy compositions in the present work.The effect of solidification rates,alloying elements Mn and Sr on the microstructure,mechanical properties and corrosion resistance of Mg₆₆Zn₂₈Ca₆ alloys were investigated.The results are as following:The effects of different solidification rates on the microstructure and properties of amorphous alloys were investigated by preparing Mg₆₆Zn₂₈Ca₆ rod samples with different diameters??=2 mm,4 mm,6 mm?.The samples of Mg₆₆Zn₂₈Ca₆ alloys with?=2 mm are completely amorphous.While Mg,Mg₇Zn₃ and Ca₂Mg₆Zn₃ phases are formed in the rod samples with diameter?=4 mm and?=6 mm.Furthermore,the volume fractions of crystal phases are about 2%and 25%,respectively.The result of electrochemical tests show that?=4mm rod sample exhibits the minimum corrosion current density about 52.4?A/cm²,indicating that its corrosion resistance better than that of the?=2 mm fully amorphous rod sample and?=6mm rod sample.The results of compression tests at room temperature suggest that all the samples with different diameters exhibit a macroscopic brittle fracture.As the cooling rate decreases,the compressive strength and elastic strain of the samples decrease significantly and the fully amorphous sample with?=2 mm shows the highest compressive strength and elastic strain,which are about 501MPa and 3.96%respectively.The microstructure,mechanical properties and corrosion resistance of Mg₆₆Zn₂₈Ca_6-xMn_x?x=0,0.1,0.3,0.5?rod samples under the same solidification rate?all rod samples with 2 mm in diameter?were investigated.Nano-sized phases are precipitated with the addition of Mn in the rod samples,and the volume fractions of nanocrystals in Mg₆₆Zn₂₈Ca_6-xMn_x?x=0.1,0.3,0.5?alloy are 35%,45%and 41%,separately.The results of compression test at room temperature show that the addition of Mn improves the mechanical properties of the alloys.Mg₆₆Zn₂₈Ca_5.9Mn_0.1.1 alloy demonstrates the maximum compressive strength?about 627 MPa?and elastic strain?7.51%?.The results of electrochemical and immersion tests show that corrosion resistance of Mg₆₆Zn₂₈Ca_6-xMn_x?x=0,0.1,0.3,0.5?alloy rod samples vary with the trend of 0.3Mn>0Mn>0.1Mn>0.5Mn.The sample of Mg₆₆Zn₂₈Ca_5.7Mn_0.3.3 is provided with the highest nanophase volume fraction?45%?,but displays the lowest corrosion current density?about 52?A/cm²?,which represents the best corrosion resistance.The Mg-based glass matrix composite with chemical composition of Mg₆₆Zn₂₈Ca_6-xMn_0.3.3 displays the best corrosion resistance and the minimum corrosion current density?about 52?A/cm²?,which is smaller than the amorphous alloy without Mn?about 63.5?A/cm²?.The formation of a uniform and continuous product layer on the surface of the alloy resulted from the precipitation of large volume fractions of nanophases could hinder further corrosion and account for the improved corrosion resistance.The effects of different solidification rates??=2 mm,4 mm?and Sr element on the microstructure and properties of Mg₆₆Zn₂₈Ca_6-x-x Sr_x?x=0,0.4,0.6,0.8?alloys were investigated.It's suggested that the nano-sized Mg₇Zn₃ phases precipitated from Mg₆₆Zn₂₈Ca_6-xSr_x?x=0.4,0.6,0.8?rod samples with diameter of?=2 mm,and the volume fractions of precipitated nano-phases are 17%,49%and 13%,respectively.For the alloy sample with?=4 mm,there are more crystal phases of Mg,Mg₇Zn₃ and Ca₂Mg₆Zn₃ precipitated in the Sr-doped alloys than Sr-free alloys,in which the crystal phases are small and uniform distribution when the addition of Sr is 0.6at%.The compression test shows that the compressive strength and elastic strain of the alloys are improved because of the addition of Sr element.For the samples with?=2 mm,the compressive strength and elastic strain are the highest when the Sr content is 0.4at%,reaching 639 MPa and 7.39%,respectively.For the samples with?=4 mm,the compressive strength and elastic strain are the largest when the Sr content is 0.6at%,about 492 MPa and2.72%,respectively.The results of electrochemical tests show that the corrosion resistance of the Sr-doped alloy rod samples with diameter of 2 mm and 4 mm are lower than that of the Sr-free alloys.While the alloy rod samples containing 0.6at%Sr exhibits better corrosion resistance than 0.4Sr and 0.8Sr alloy rod samples owing to uniform distribution of crystal phases.