| TiAl alloys exhibit high melting point,high specific strength and good corrosion resistance,but their application is limited due to complex solidification process and difficulty in processig.In this paper,the gas-atomizing powder combined with spark plasma sintering method was used to prepare Ti-46.5Al-2.15Cr-1.90 Nb alloy and heat treatment was also used to optimize the microstructure.The effect of different heat treatment processes on the microstructure and performance of the alloy at room temperature were studied.The TiAl alloy was further subjected to thermal deformation experiments and high temperature oxidation experiments for four types of microstructures with better overall mechanical properties,and the thermal deformation and oxidation resistance of these four typical samples were investigated.With a certain sintering parameter,the Ti-46.5Al-2.15Cr-1.90 Nb bulk alloy was prepared by spark plasma sintering.The room temperature structure was near γ and it contained large fraction of α2 phase,i.e.about 40.9%.The maximum true stress at room temperature was 1822.84 MPa,and the plastic deformation was 30.27%.Single-temperature and dual-temperature heat treatment were used to treat the as-sintered bulk alloy and its microstructure and room temperature performance were analyzed.The microstructures of four typical TiAl alloys were obtained,near gamma,duplex,near-lamellar structure and full-lamellar structure.After a single temperature heat treatment at 1250°C/15min/FC,a small and uniform duplex microstructure was obtained.The maximum true stress and plastic deformation were 1873.22 MPa and 33.53% at room temperature,respectively.Compared to the sintered near gamma microstructures,the maximum true stress and plastic strain were improved.After the single-temperature heat treatment at 1290°C/15min/FC,the near-lamellar microstructure was obtained.The maximum true stress and plastic deformation were 1644.95 MPa and 23.58% at room temperature,respectively.After 1310°C/15min/FC+1230°C/30min/FC dual temperature heat treatment,the full lamellar structure was obtained.The maximum true stress and plastic deformation were 1465.61 MPa and 20.75% at room temperature,respectively.The samples of the four typical Ti-46.5Al-2.15Cr-1.90 Nb alloys were subjected to thermal compression experiments.The deformation temperatures were 1100°C and 1200°C,respectively,with the strain rate of 0.01/s and the heating rate of 10°C/s.Moreover,the true stress-strain curves were obtained,and the microstructure evolution of the four sets of hot deformed samples was analyzed in combination with SEM and EBSD to compare their thermal deformation abilities.It was found that the deformation temperature and the microstructure of the alloy had a great influence on its thermal deformation ability.When the strain rate was 0.01/s,the dynamic recrystallization was insufficient at 1100°C,and the structure after deformation was not uniform.When the deformation temperature was increased to 1200°C,a uniform equiaxial duplex structure was obtained and the deformed structure had better uniformity.At the same time,the deformability of near-gamma and the duplex structure are significantly better than those of the near lamellar and full lamellar structures.The four typical samples of Ti-46.5Al-2.15Cr-1.90 Nb alloy were heated from room temperature to 750°C and 800°C respectively at 15°C/min in the air,preserved for 10 h and air-cooled to room temperature.Then the samples were weighed and experiments were repeated 10 times to establish the oxidation kinetic curve.With the SEM analysis of the surface morphology and the characteristics of the oxidation profile,the high temperature oxidation resistance of the four typical TiAl alloys was compared.It was found that the increase of mass in oxidation of the alloy at 750°C was significantly lower than that at 800°C.At the same time,when the cycle oxidation temperature was 800°C,the increase of mass in oxidation of the near gamma structure and duplex structure was significantly higher than those of the near lamellar and full lamellar structures.The increase of mass for the near gamma structure was approximately 1.26 mg.cm-2,while that of the full lamellar structure was about 0.77 mg.cm-2,which was about 60% of the former and indicated that the full lamellar structure had a stronger oxidation resistance. |
PDF Full Text Request