| The high temperature titanium alloy is widely used in aircraft engines for its lowdensity,high strength at high temperature and excellent corrosion resistance.In order toimprove thermal stability and creep performance for high temperature titaniumalloy,the rare earth elements alloying has become an important way to improve themicrostructure and properties of high temperature titanium alloy.In this thesis,new600℃high temperature titanium alloys including Er were designed,then the tensileproperties of alloys at room temperature and high temperature,thermal stability andcreep properties of alloys were tested.The influence of Er on the high temperaturestability and creep properties of alloys were investigated by analyzing theprecipitation behavior of second phase,as well as comparing the above properties withhigh temperature titanium alloy with Nd element.The alloys,e.g.Ti-6Al-2.5Sn-4Zr-0.3Mo-1Nb-0.35Si-xEr（x=0,0.1,0.3wt%） andTi-6Al-2.5Sn-4Zr-0.3Mo-1Nb-0.35Si-0.4Nd were melted using the vacuum consumable arc furnace.The microstructure of cast alloys was typical lamellar microstructure.The solubility of Er in the matrix is very low.Most of Er exist in the form of complex second phase containing Ti,O,Al,Zr,Sn and Er compound.Erhas refined the microstructure of alloys significantly.The alloys were forged in β phase firstly,then forged in α+β phase.After beenforged,the alloys were heat treated in the condition of1020℃/1h/AC+750℃/2h/AC.All four alloys has two type of microstructure after heat treatment,i.e. lamellarand duplex microstructure,respectively.There were no silicides precipitate aftersolution treatment,but the silicides increased after aging treatment.Less silicidesprecipitated in the original β layer,most of silicides precipitated in the boundary oflamellar α.After aging treatment, almost no α2phase precipitated in all four alloys.Er did not improve the strength and plasticity of alloys at room temperature and600℃.After600℃/100h thermol exposure, α2phase,which size is5nm to10nm,precipitated homogeneously in α phase.The spacing between the α2particleswas small,about10nm grade as well as silicides,precipitated both within grain and inthe grain boundary of α phase,both of the α2and silicides can obstacle the motion ofdislocation strongly.Er distributed in grain boundary in the form of oxides.Because ofthe low binding force,the second phase with Er detached from the matrix phase duringtensile test,which result in no improvement on strength and plasticity after thermolexposure compared with non-Er alloy.Alloys that contained Er has excellent creep properties in the condition of600℃/100h/150MPa.Silicides and α2phase,which pin up dislocations strongly,precipitated in all four alloys during creep.The second phase with Er reduced the amount ofβ-stabilizing element in surrounding matrix phase,thus increased the content of Si inthe matrix phase.Er refined grain,impeded dislocation movement,thus improved thecreep properties of alloys. |
