Preparation And Strengthening-Toughening Mechanisms Of Ti-Zr Based Alloys With High Strength And Ductility

TiZr based alloys have attracted intense attention from researchers because of their high specific strength,outstanding corrosion resistance and good biocompatibility.Harsh and complex service environments have a higher requirement for the properties of TiZr based alloys and as structural materials,strength and ductility are two indicators measuring the mechanical properties of materials.However,the TiZr based alloys developed before,with a major α phase of hexagonal close-packed lattice,usually behave higher strength and lower ductility.This restricts the application range of TiZr based alloys.To solve these problems above,this thesis aims at developing TiZr based alloys with high strength and ductility and studies the phase transformation,microstructure and mechanical properties of TiZrAlSn alloys and Ti-30Zr-5Al-3V alloy under thermal deformation and heat treatment processes by using laboratory and industrial preparation techniques.The related deformation mechanisms were analyzed and summarized and new approaches and design ideas for TiZr based alloys with high strength and ductility were also proposed.A series of TiZrAlSn alloys were prepared by non-consumable vacuum arc furnace by substituting Zr element for Ti in the classical Ti-5Al-2.5Sn alloy.As Zr content increased,the β transformation temperatures of alloys decreased.Under the same rolling temperature,the hot-rolled TiZrAlSn alloys covered α,α+β and β regions.Among them,Ti-35Zr-5Al-2.5Sn alloy with the major phase of α’ martensite obtained the extremely high yield strength of 1316±11MPa and the ultimate tensile strength(UTS)of 1609±12MPa,but the total elongation was only 3.4±0.3%.On the basis of this experiment,the characteristics of low strength and high plasticity of primary α phase and high strength and low plasticity of secondary α’ phase were studied.To achieve the purpose of TiZrAlSn alloys with high strength and ductility,we have tried to synthesize two characteristics of these two phases.In the subsequent experimental processes,Ti-30Zr-5Al-2.5Sn alloy was used as an example to achieve the property combination of primary α and secondary α’ phases by hot-rolling and water quenching process.It was found that the yield strength of alloys did not increase with the increase of α’ phase content and the main factor is the recovery and recrystallization degree of primary α phase.On this basis,the supersaturated solid solution was obtained by adding Al element into TiZrAlSn alloys and Ti-30Zr-7Al-2.5Sn alloy with high strength and ductility was obtained.The yield strength,uniform elongation and total elongation were1004±4MPa,6.3±0.1% and 22.3±0.1%,respectively.As the TiZr based alloys with a major α phase cooled from β region at a slower rate(air cooling or annealing),basket-weave lamellar microstructure was always obtained.This microstructure can be got by the processes of ordinary casting or direct cooling from the βregion.Due to the special distribution of grain boundary and influence of texture,this lamellar microstructure usually behaves higher strength but poor ductility.We have tried two ways to solve this problem.One is to develop TiZrAlSn alloys with high strength and ductility of this microstructure by introducing stress-induced deformation twins by means of reducing fault energy.The corresponding yield strength,uniform elongation and total elongation were 973±10MPa,9.2±0.2% and 17.4±0.5%,respectively.The other is to try to improve the ductility of alloy by introducing metastable β phase of transformation induced plasticity.Through this idea,we have improved the uniform elongation of forged Ti-30Zr-5Al-3V alloy from 5.7±0.2% to 13.5±0.5% and the true ultimate tensile strength(UTS)was also improved.This method achieved the improvement of both strength and ductility and the yield strength was only sacrificed ～100MPa.Furthermore,due to the lower β transformation temperature of Ti-30Zr-5Al-3V alloy,the deformation of alloy and equiaxed grain recrystallization of primary α phase is more complex.Thermal compression deformation of Ti-30Zr-5Al-3V alloy at the lower α+βregion was performed on Gleeble-3500.The recovery and recrystallization of these samples with different temperatures were determined to provide reference for other hot deformation processes such as rolling and forging.Finally,the mechanisms of strength and ductility/toughness of TiZr based alloys were analyzed and discussed synthetically and this provides reference for further development of TiZr based alloys with high strength and ductility.