Properties And Related Mechanisms:in-Situ Nano-MgO Particles Reinforced Mg-Zn-Ca Alloy

Duo to its excellent biosafety,Mg-Zn-Ca alloys are the preferred material for in vivo degradation.However,the mechanical properties and degradation rate of the alloys cannot match the clinical operating requirement at the same time.Its application is limited.The strength and corrosion resistance of the Mg-Zn-Ca alloys can be improved effectively by adding Mg O reinforced particles.However,agglomeration,which affects the interface bonding between the reinforced particles and the matrix,is easily formed by mechanical stirring during smelting.The properties of composites cannot be effectively improved.The purpose of this paper is to improve the interface stability between Mg O reinforced particles and Mg matrix and obtain Mg-based degradable composites with excellent mechanical properties and corrosion resistance.Nano-Ca O particles with different contents were added into Mg-1Zn melt by high shear stirring and ultrasonic stirring technology.Mg-Zn-Ca/MgO composites of nano-particle reinforced were prepared by in-situ reaction of nano-Ca O and Mg melt.Subsequently,the materials were subjected to hot extrusion deformation.The effects of in-situ nano-Mg O on microstructures,mechanical properties and degradation properties of Mg-Zn-Ca alloy were investigated from the first principles calculation and experiment.The main research results are as follows:（1）First principles calculations indicated that the Mg（0001）/XO（111）（X=Ca or Mg）interface had lower mismatch than that of the Mg（0001）/XO（110）and Mg（0001）/XO（100）interfaces.The interfacial adhesion energy and interfacial energy of Mg/XO-O was higher than that of Mg/XO-X in the same stacking order,indicating that the interface stability of Mg/XO-O was higher than that of Mg/XO-X.In addition,the atomic charges on the Mg（0001）surface would transfer to the oxygen atoms on the o-termination of XO（111）surface to form ionic bond.The ionic bond strength at the Mg/Mg O-O interface was higher than ionic bond strength in Mg/Ca O-O interface.Therefore,Mg O can be used as a more effective nucleation substrate in Mg melt than Ca O.Ca O and Mg may react in situ to form Mg O（2）Experimental results showed that nano-MgO particles could be formed by the reaction of Ca O with molten Mg,and the Ca could be replaced to obtain Mg-Zn-Ca/Mg O composites.The addition of nano-Ca O significantly refined the grain size and distinct improved mechanical properties of the Mg-1Zn alloy.At the same time,corrosion products of Mg（OH）₂ compactness was enhanced and corrosion rate of composites were reduced due to nano-Mg O formation.The Mg-1Zn-0.5Ca O composite had the best corrosion resistance with a weight loss of 9.875 mg·y^-1·mm^-2.The Mg-1Zn-1.0Ca O composite had the best mechanical properties.The hardness,yield strength,tensile strength and elongation of Mg-1Zn-1.0Ca O composite reached 55.3HV,70.0MPa,167.5MPa,7.5%,respectively.（3）Experimental results showed that the addition of nano-Ca O effectively reduce the recrystallization grain size after hot extrusion.Ca₂Mg₆Zn₃ and Mg₂Ca phases were precipitated from Mg-1Zn-0.5Ca O and Mg-1Zn-1.0Ca O composites after extrusion,respectively.With the increasing of Ca O content,the basal texture strength increased from 3.97 of Mg-1Zn alloy to 12.05 of Mg-1Zn-1.0Ca O composite.The yield strength of the material was also increased with the Ca O increasing.The extruded Mg-1Zn-0.5Ca O had better comprehensive properties.Its yield strength,elongation and annual corrosion rate of long-term immersion（21 d）were 334.9 MPa,8.0%and 1.06 mm/y,respectively.After extrusion annealed,the grain size of the materials increased and the strength decreased,but the elongation increased to 2-3 times than that of the extruded materials.（4）First principles calculations indicated that the difference of work function between surfaces was decreased when O was doped with Mg-Zn-Ca alloy.The（0001）base work function of Mg1Zn1Ca2O（Z1X2O）was the highest,reaching 3.789 e V.Compared with Cl^-,O atom and O₂ were more easily adsorbed on the surface of Z1X2O（0001）.The Mg O passive film was priority formed under monolayer O atoms covering Z1X2O（0001）surface.The addition of Cl^-destroyed the protective film and elongated the Mg-O bond,causing the inner surface layer was oxidized by O atom.The binding energy between corrosive layer and uncorroded layer in Z1X2O（0001）surface decreased with the increasing of Cl^-.The corroded tendency of uncorroded layer Mg atomic layer was increased.The Mg-O interaction was interfered by Cl^-after the passive film breakdown,forming Cl-O-Mg structure,which reduced the protection of oxidation layer and promoted further oxidation of Z1X2O（0001）.