Design And Research On The Microstructure And Properties Of New Titanium Alloys With High Strength And Low Elastic Modulus For Aerospace Applications

Titanium alloys have been widely used in aerospace field due to the high strength,low density and good corrosion resistance.In order to meet the demands of elastic materials for aerospace,design and development of new titanium alloys with higher strength and lower elastic modulus have become the research hotspot.In this paper,Ti-Nb-Zr-Mo-(Sn)alloys were designed according to the relevant design theories of titanium alloys.The effect of Zr,O contents on the Ti-Nb-Zr-Mo-O alloys was studied and the influence of cold rolling and aging treatment on the microstructure and properties of Ti-32Nb-4Zr-2.8Mo-0.3O(wt.%)alloy was systematically investigated.The main results are as follows:The Ti-Nb-Zr-Mo-(Sn)alloys were designed by d-electron alloy design method and average valance electron concentration theory.It is shown that the Ti-32Nb-4Zr-2.8Mo alloy has good mechanical properties after 90%cold rolling(Rm=879MPa,E=46GPa).The effect of Zr,O contents on the microstructure and properties of cold rolled Ti-32Nb-xZr-2.8Mo-yO alloys was investigated.The results show that the addition of Zr and O decreases the grain size.The Ti-32Nb-xZr-2.8Mo-0.3O(x=0～6%)alloys are all composed of single β phase.With the increase of Zr content,the elastic modulus of the alloy decreases first and then increases.However,Zr element has little effect on the tensile strength and hardness.In the Ti-32Nb-(2,4)Zr-2.8Mo-yO(y=0-0.45%)alloys,with the increase of oxygen content,the strength and elastic modulus increase while the ductility decreases.The cold rolled Ti-32Nb-4Zr-2.8Mo-0.3O alloy exhibits better mechanical properties.The solution treated Ti-32Nb-4Zr-2.8Mo-0.3O alloy is composed of equiaxed P grains.After cold rolling,the grains are elongated along the rolling direction.With the increase of cold reduction,the lattice parameter and residual strain increase,while the average grain size decreases.The increase of dislocation density leads to the increase in strength and the decrease in elastic modulus.The stress-induced phase transformation does not occur during cold deformation.The deformation mechanisms of cold rolling are related to dislocations slipping and deformation twins.The cold rolled Ti-32Nb-4Zr-2.8Mo-0.3O alloy with 90%reduction has high strength and low elastic modulus(Rm=1092MPa,E=51GPa,A=12.5%).The influence of aging treatment on the 90%cold rolled Ti-32Nb-4Zr-2.8Mo-0.3O alloy was investigated within the temperature range of 300℃～700℃.With the increase of aging temperature(the aging time is 30 min),the strength of the alloy increases first and then decreases,whereas the elastic modulus decreases first and then increases.Isothermal ω phase appears at 350℃,which leads to a slight decrease in elastic modulus.Then the ω phase grows and a phase precipitates at 450℃,resulting in the increase of strength.When aged at 350 ℃,the ω phase precipitates with increasing aging time.As a result,the strength increases,the elastic modulus decreases first(0～30 min)and then increases(30～60 min),while the ductility decreases.When aged at 350℃ for 30min,the Ti-32Nb-4Zr-2.8Mo-0.3O alloy achieves an optimal combination of high strength and low elastic modulus(Rm=1118MPa,E=48GPa,A=13.4%).