Regulation Of Microstructure And Mechanical Properties Of Hot Deformed Zn-xLi Alloy

In recent years,medical metal materials have received widespread attention and a large number of applications because of their excellent comprehensive mechanical properties and easy processing.However,traditional medical metal materials have common problems,that is,they can not be degraded and need to be removed by secondary operation,causing secondary damage to patients.Among the biodegradable medical metal materials developed after that,the degradation rate of iron-based medical materials is too slow,and the degradation rate of magnesium-based medical metal materials is too fast,which can not meet the requirements of related medical materials.Due to the unique advantage of electrode potential of zinc-based medical metal materials,the degradation rate of zinc-based medical metal materials is very suitable.However,the biggest problem in the practical application of zinc-based medical metal materials is that the comprehensive mechanical properties are poor and can not meet the relevant requirements of medical implant materials.Therefore,in this paper,alloying with Li elements and reverse extrusion deformation are mainly used to control the microstructure and comprehensive mechanical properties of Zn alloy.The effects of Li content on the phase structure,grain structure and comprehensive mechanical properties of as-cast and extruded ZnxLi(x=0,0.3,0.6,0.9,1.5 mass%)alloys were studied by means of metallographic observation,SEM microstructure observation,XRD analysis,EBSD microstructure observation and room temperature tensile testing.The main results are as follows:(1)When the Li content is in the range of 0 mass%0.9 mass%,it exists mainly in the form of β-Zn4Li phase in the as-cast alloy,and when it increases to 1.5 mass%Li,the element exists in the form of α-Zn4Li phase in addition to β-Zn4Li phase.And the morphology and distribution of the second phase vary greatly with different Li content.When the content of Li is 0.6 mass%,the β-Zn4Li phase mainly exists at the grain boundary in the form of discontinuous bone,and a small amount is distributed inside the grain.When the content of Li is 0.9 mass%,β-Zn4Li phase and η-Zn phase mainly form a complex eutectic structure.When the content of Li reaches 1.5 mass%,α-Zn4Li phase is mainly formed,which exists in the alloy in the form of fine lamellar eutectoid structure with η-Zn phase.(2)With the increase of Li content,the strength of the as-cast alloy increases gradually,while the plasticity keeps at a low level.The tensile strength and elongation of as-cast pure Zn are 15.1 MPa and 0.15%respectively,and when the content of Li element increases to 1.5 mass%,the tensile strength of the alloy reaches 122.1 MPa,and the elongation is 0.21%.The main reason is the fine grain strengthening effect caused by the refinement of the microstructure of the alloy with the addition of Li element and the second phase strengthening effect caused by the formation of the second phase β-Zn4Li and α-Zn4Li phase.(3)After backward extrusion,the phase structure of Zn-xLi alloy changes obviously,which-is mainly composed of η-Zn,α-Zn4Li and β-Zn4Li.With the change of Li element content,the morphology and volume fraction of the second phase change obviously.At low Li content,such as 0.3 mass%and 0.9 mass%,the second phase in the alloy is mainly rod-like and irregular block,while when it is increased to 1.5 mass%,the second phase in the alloy is transformed into a tightly arranged lamellar eutectoid structure.(4)Dynamic recrystallization occurs in pure Zn during reverse extrusion deformation,the proportion of recrystallized grains is 85.8%,the average grain size is 29.60 μm,the grain boundary type is mainly large-angle grain boundary,and the texture type is non-basal plane texture.When the Li element content increases to 1.5 mass%,the recrystallization ratio decreases to 39.7%,the grain boundary is mainly small-angle grain boundary,the texture is transformed into base texture,and its strength is high.This is mainly due to the fact that with the addition of Li element,a large number of second phases are formed in the alloy,which strongly pinning dislocations,low-angle grain boundaries and large-angle grain boundaries,delaying dynamic recrystallization and promoting dynamic recovery in the alloy,resulting in a decrease in the proportion of recrystallization in the alloy,a gradual increase in the proportion of small-angle grain boundaries,a decrease in average grain size,and a change of texture type to basal texture.(5)With the increase of Li content,the tensile strength of the extruded alloy increases gradually,while the elongation decreases at first and then increases.The tensile strength of extruded pure Zn is 111.2 MPa,and the elongation is 49.6%.When 0.3 mass%Li element is added,the tensile strength of the alloy increases to 258.5 MPa,and the elongation decreases to 9.0%.When the content of Li element increases to 1.5 mass%,the tensile strength and elongation of the alloy are the highest,reaching 597.3 MPa and 41.5%respectively.The main strengthening mechanisms include grain boundary strengthening caused by large and small angular grain boundaries,second phase strengthening caused by α-Zn4Li and β-Zn4Li phases and texture strengthening caused by basal plane texture.