Preparation And Physical Metallurgy Of Titanium Alloys Containing Scandium

As important structural materials,titanium and its alloys had attracted much attention and remained long time hot topics since 1950s.One way to solve problems such as low room temperature plasticity,poor high temperature strength and service temperature is to develop new alloying system with better properties.In this paper,the alloys of Ti-Sc,Ti-Al-Sc and Ti-Al-V-Sc systems were prepared by vacuum arc melting and vacuum self consumable arc melting techniques.The influence of Sc on phase transformation,phase component,microstructure evolution and high temperature plastic deformation and its mechanism of titanium and titanium alloys were investigated by optical microscope,scanning electron microscope,transmission electron microscope,differential scanning calorimeter,X-ray diffraction instrument, microhardness tester and thermal simulation machine.The main contents and conclusions are as followings:(1) The influence of Sc on phase transformation,microstructure,microhardness and lattice constant of pure Ti showed that,the addition of 2wt%Sc markedly refined the microstructure of as-cast pure Ti and improved its microhardness from about 190HV_0.01 to 308 HV_0.01.With the increase of Sc content,the microstructures of alloys changed from bulk and acicularαphase(Ti-2Sc and Ti-5Sc) toαcolony along priorβgrain boundary(Ti-25Sc) and spiral helicine grains(Ti-37Sc).The lattice constants ofα,c and c/a values change with Sc content inαTi(＜7wt%Sc) phase region,but remain constant in(αTi+αSc)(＞25wt%Sc) two-phase region with little difference.The addition of Sc lowered the temperature ofα→βphase transformation, T_α/β,furthermore,based on thermodynamic analysis Sc was functioned as aβ-phase stabilizer.(2) The influence of Sc on phase component,microstructure evolution and mcirohardness of Ti-Al binary alloys and Ti-6Al-4V alloy were investigated.Sc helped to refine as-cast microstructure remarkably of Ti-6Al and Ti-10Al alloys,the phase component of as-cast Ti-Al binary alloys was mainlyαTi and Ti₃Al,AlSc and Al₃Sc phases appeared in the alloys after the addition of Sc.After vacuum annealing inβphase region,nearα+βphase region andαphase region,Ti₃Al phase disappeared in Ti-6Al alloy,Al₃Sc phase disappeared in Ti-3Al-3Sc and Ti-8Al-2Sc alloy, secondary Ti₃Al phase formed in Ti-10Al alloy.The microhardness of annealed Ti-Al and Ti-Al-Sc alloys were higher than that of as-cast alloys and increased with annealing temperature.The addition of 0.3wt%Sc refinedαcolony and had little effect on size of priorβgrain of Ti-6Al-4V alloy,however,the addition of 0.5wt%Sc not only refinedαcolony but also priorβgrain.Sc improved the microhardness of lamellarαphase and had little effect on the microhardness of bulkαphase.During the forging process of Ti-6Al-4V alloy,Sc formed oxide Sc₂O₃ due to the internal oxidation.After forging,the volume fraction of equiaxedαdecreased and lamellarαphase increased respectively with the increase of Sc content.(3) The high temperature deformation characteristics of Ti-6Al-4V, Ti-6Al-4V-0.3wt%Sc and Ti-6Al-4V-0.5wt%Sc alloy were investigated.Flow stress of alloys decreased with the increase of deformation temperature and the decrease of strain rate.Deformed at 850℃,the flow stress showed soften feature after peak state. With the increase of deformation temperature,the flow stress rapidly arrive peak followed by a steady state,some cases appeared strain hardening.The addition of Sc improved the peak stress value of Ti-6Al-4V alloy under the deformation conditions of(?)=1s^-1/850℃,(?)=0.1^s-1/850℃and(?)=1s^-1/1050℃.On the basis of hyperbolic sine constitutive model containing activation energy Q and temperature T proposed by Sellars and Tegart,the materials constant and flow stress constitutive equations of high temperature deformation were obtained.The deformation activation energy of Ti-6Al-4V,Ti-6Al-4V-0.3Sc and Ti-6Al-4V-0.5Sc alloys inβphase region were 134.014kJ/mol,121.643kJ/mol and 101.367kJ/mol respectively,inα+βphase region were 670.474kJ/mol,542.273kJ/mol and 679.783kJ/mol.When the deformation temperature was determined,the flow localization parameterφwhich depended on strain rate decreased with the strain rate.The parameterφvalues inβphase region were higher than that inα+βphase region,Sc content had little effect on parameterφ.(4) Based on the principle of thermal processing map,high temperature compressed deformation datas were used to obtain the processing map of Ti-6Al-4V, Ti-6Al-4V-0.3Sc and Ti-6Al-4V-0.5Sc alloys under the true stain of 0.05,0.2,0.4 and 0.6,depending on the maps the deformation stability of alloys was discussed.At four true strains,there were two or three instable regions in high and low temperature range.The range of instable regions enlarged with the increase of true strain and Sc content,especially of strain rate range.Under some instable conditions,adiabatic shear band.flow localization and cracks on the surface were observed in compressed samples.The peak power dissipation efficiency of alloys decreased with the Sc content.The peak power dissipation efficiency of equiaxed microstructure was higher than that of acicular microstructure.The alloys had dynamic recrystallization condition in a large range of deformation conditions.(5) The influence of Sc on deformation mechanism of Ti-6Al-4V alloy was studied by optical microscope and transmission electron microscope.Inβphase region,large grain superplasticity deformation controlled by self-diffusion happened at low strain rates,the priorβgrain boundary was clear andβphase transformed to acicular martensiteαphase;the static recrystallization appeared at edge part of deformed samples and prior grains were prolonged under the compressed pressure.The addition of Sc lowered the strain rate at which recrystallization can happen,and refined priorβgrain after low strain rate deformation,but had little effect on the size of priorβgrain after high strain rate deformation.Inα+βphase region,dynamic recrystallization happened inαphase under low and high strain rate deformation;adiabatic shear band appeared in Ti-6Al-4V alloy with equiaxed recrystallized and prolonged grains;after the addition of Sc to Ti-6Al-4V alloy,the dynamic recrystallization was more obvious and has larger grain size.There was no evident adiabatic shear band in alloy with Sc, only flow localization appeared.The deformation microstructure of alloy was more homogeneous at higher temperature,and the adiabatic temperature rise was not the main reason for inhomogeneous microstructure.Under the deformation conditions of low temperature and high strain rate,the addition of Sc improved the peak stress value of Ti-6Al-4V alloy,the transmission electron microscope observation show the mechanism,such as lamellarαphase showing twinning characteristic,dislocation pile up near the grain boundary,dislocation disappeared inside equiaxed grains and fine precipitations.