Effect Of Heat Treatment On The Phase Composition And Mechanical Property Of Ti-7.5Nb-4Mo-2Sn Alloy

The meta-stable beta-type titanium alloys have low elastic modulus, highcorrosion resistance, excellent biocompatibility, good shape memory capacity andsuper elasticity, which show potential use as plant biomaterials. The deepunderstanding the effect and mechanical of alloying and microstructure on the shapememory and super elasticity is very important for the improvement of mechanicalproperties and shape memory effect of the alloys.In this thesis, a Ti-7.5Nb-4Mo-2Sn alloy was designed by adding Mo and Snelements into Ti-Nb alloy, based on the d-electron orbit theory, and the effect of heattreatment, including solution treatment and flash aging, on the microstructure andmechanical properties of the alloy were studied. The main conclusions are as follows:(1) The Ti-7.5Nb-4Mo-2Sn alloy was designed and the microstructure andmechanical were researched. The d-electron orbit theory was used for the alloydesigning. The result show that the microstructure of the as-cast Ti-7.5Nb-4Mo-2Snalloy mainly consisted of single β phase. The as-cast alloy after solution treatmentexhibited good shape memory property and super elasticity at room temperature. Thestar temperature of the martensite transformation is about-12°C, and the finishingtemperature for the austenite transform is about0°C. After deformation for8%atroom temperature, the total recovered strain of the alloy is about3.9%.(2) The effect of solution treatment on the microstructure and mechanical for theTi-7.5Nb-4Mo-2Sn alloy. After cold rolled for80%at room temperature, the alloy swere solution treated at700°C for different time in order to research the effect ofsolution treatment on the microstructure and mechanical. The results show that thesolution treatment has an important effect on the microstructure and mechanicalproperties of the cold rolled alloy. When the CR alloy was heated to700°C fordifferent durations, the grain size grew as the heating durations increase beforequenching into the water. After solution treatment, the alloy consisted of β phase anda little α phase and α" phase, insteading of β phase and α" phase before solutiontreatment. Solution treatment duration has also important effect on the mechanicalproperty of the Ti-7.5Nb-4Mo-2Sn alloy. The work hardening and deformationtexture vanished after solution treatment. The yield stress declined and the plasticityinclined after the solution treatment. At the same time, the critical stress of themartensite transformation declined after solution treatment for different durations. The alloy exhibited the best super elasticity after the solution treatment for10min at700°C. The grain size of the alloy has an important effect on the critical stress of themartensite transformation. The relationship between critical stress of the martensitetransformation and the grain size can be described by the Hall-Petch relation.(3) The effect of flash treatment on the microstructure and mechanical of thesolution treated alloy. After solution treated at700°C for10min, the alloys wereflash treated for different time at250°C to research the effect of flash treatment onthe microstructure and mechanical. The results show that the flash treatment canintroduce ω phase. The ω phase increased along with the flash treatment process. Thepresence of the ω phases can enhace the critical stress of the martensitetransformation, and it also can improve the super elasticity of the alloy. After treatedfor10min, the critical stress of the martensite incline to500MPa, and the elasticrecovery will reach46%.