| Zn has a promising prospect in the field of biodegradable materials as an essential elements of the human body.But the low strength and poor plasticity of pure Zn fail to provide sufficient mechanical support for human tissue reconstruction.As reported by literature,Li improves the mechanical strength of Zn alloys significantly.However,before the present study,the study of Zn-Li alloys faces three major problems:(Ⅰ)The solid phase transformation process and the characteristic of phases in Zn-Li alloys have not been reported,resulting in lack of in-depth understanding of the microstructure and strengthening mechanism of the alloy system;(Ⅱ)The addition of Li increases the strength of the pure Zn but decreases its ductility,especially that hot rolled hypereutectic Zn-Li alloys show extremely brittleness,which inhabits their industrial application of it;(Ⅲ)Corrosion behavior,corrosion products morphology and degradation mechanism of hypereutectic Zn-Li alloy are unknown,especially degradation mechanism of theβ-LiZn4(β)phase in the alloy system has not been revealed.These problems hinder the further optimization and performance improvement of Zn-Li alloys.Aiming to solve these problems above,advanced material characterization methods and analytical testing methods,e.g.,TEM,3D-APT and SKPFM,were used in this thesis to study Zn-Li alloys.Results are summarized as follows:(1)The solid phase transformation characteristics of the Zn precipitation in βand the precipitation of β in Zn were first revealed.When the content of Li in an alloy is higher than the eutectic component(Li＝0.44wt.%),the alloy is composed of the coarse primary β dendrite and Zn+β eutectic.TEM results show that dense needle-like Zn phase precipitates in primary β dendrite,resulting in the formation of(3/Zn lamellar structure with an average inter-spacings of 800 nm.The reproducible orientation relationship(OR)between the two phases can be described as[1213]β//[2110]Zn,(1010)β 4.50 from(0002)Zn,denoted as OR1.The thermodynamic reason for the formation of β/Zn lamellar structure is attributed to the decrease of solid solubility of Zn in β phase with decreasing temperature,which decreases from 92 at.%at the ectectic temperature to 88 at.%at 100℃.Thus,the solid phase transiformation of β→Zn occurs in the as-cast alloy during solidification,resulting in the transformation from brttile β phase to β/Zn lamellar with high strength and high ductility.Meanwhile,with the decrease of temperature,the solid solubility of Li in Zn decreased by 1 at.%,resulting in the formation of a nano network of P phase in Zn grains.And the OR between these two phases is[1213]β//[2110]Zn,(1010)β//(0002)Zn,denoted as OR2.There are three crystallographically equivalent variants of OR2,and the angles between them are calculated to be 123.26° or 113.49°,resulting in the formation of a β phase network.(2)A novel hot-warm rolling process was developed to improve the ductility of hypereutectic Zn-Li alloys.Previous studies have revealed the unbalanced mechanical properties of hypereutectic Zn-Li alloys to the low plasticity.Therefore,based on the known phase transition mechanism and microstructure characteristics of Zn-Li alloy,a suitable hot-warm rolling process was designed for hypereutectic alloy to gain high toughness.The high-temperature rolling includes four passes at 350℃ and followed by water quenching,and the total deformation was 75%.The low temperature rolling includes one pass at 100℃,and the total deformation was 90%.Hot rolling ensures the plastic deformation of the hard and brittle β phase in the alloy,as well as refines the coarse as-cast microstructure.In the process of warm rolling,the high solid solubility of solute atoms in different phases decreases with decreasing temperature.Consequently,the alloy was strengthened by the dual-phase precipitation,e.g.,the hard brittle β phase was transformed into β/Zn lamellar structure with high strength and high plasticity,and the network of β phase or the high-density nano α-Li2Zn3 precipitates strengthen Zn grain.Finally,the near-eutectic and hyper-eutectic Zn-Li alloys with high strength and high plastic properties(YS>240 MPa,UTS>390 MPa and El>45%)were prepared.At the same time,the dynamic recrystallization occurs during the warm rolling process in Zn-Li alloy,which reduces the dislocation density and internal stress and refines the grain size.Therefore,the Zn-Li alloys gain high mechanical performance through precipitation strengthening,fine grain strengthening and dynamic recrystallization softening.(3)A two-stage degradation process of Zn-Li alloy has been revealed in simulated body fluid(SBF)with time,and Li alloying has been proved to inhibit pitting corrosion by changing the microstructure and chemical composition of the corrosion layer.For the first time,the SKPFM test confirmed that the surface potential of β phase is lower than that of Zn phase,which acts as the anode in the galvanic couple and corrodes preferentially.This indicates that the medical implants prepared by Zn-Li alloy can be completely degraded in human body.The preferential degradation of β phase leads to the supersaturation of Li+ions in the solution near alloy surface,and the high chemical potential gradient leads to the aggregation of Li atoms to the surface.Therefore,during immersion stage 1,the corrosion products on the alloy surface mainly are composed of dense granular Li2CO3 corrosion products,which inhibits the nucleation and expansion of pitting on the alloy surface effectively.Thus,the average corrosion rate of the immersed alloy at 30 days is 9.8 μm/y,which is only 1/7 of that of pure Zn.TEM and XPS results show that the Li-rich corrosion layer on the surface of the alloy at the initial stage consists of two sublayers.The dense and defectless outer layer with a thickness of 55 nm is composed of Li2CO3 grains,which inhibits pitting corrosion by weakening the infiltration and erosion of Cl-ions in SBF.The inner layer is a natural oxide layer on the alloy surface,which promotes the formation of the Li2CO3 grains layer by providing heterogeneous nucleation sites.However,with the increase of immersion time,the Li-rich corrosion layer is gradually replaced by the Zn-rich corrosion layer,which leads to local corrosion on the alloy surface.In summary,a hot-warm rolling process suitable for hypereutectic Zn-Li alloy was developed in this thesis by understanding the solid phase transition of alloy.Thus,Zn-Li alloys with high strength,high plasticity and uniform corrosion were prepared,and the mechanism of strengthening and uniform corrosion of Zn-Li alloy was revealed,which provides basis for the further optimization of Li-containing Zn alloys as biodegradable materials. |
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