Effect Of Hf, B Alloying And Heat Treatment On The Microstructure Of TiAl-Based Alloy

In order to improve comprehensive mechanical properties of Ti-46Al alloy and expand its application range, Hf and B alloying and heat treatment were used to investigate the refinement technology of TiAl-based alloy Ti-46Al.Firstly, the effect of Hf on the microstructure of as-cast Ti-46Al alloy was observed. The results indicated that Hf can refine the grain size from 1mm in Ti-46Al binary alloy to 400μm with the increasing Hf content of 7at.%. Hf also had the effect of grain refinement on Ti-46Al-0.2B triple alloy, but the grain size increased to some extent when the addition of Hf was above 5.0at.%. The effect of B on the grain refinement of Ti-46Al-7Hf alloy was outstanding, and the grain size decreased to 100μm with the increasing B addition of 1.0at.%. The addition of Hf and B can yield compostion supercooling and refine the dentritic stem and first dentritic arm on one hand, and can promote B2 phase to generate on the other hand. The increased B2 phase can inhibite the growth of grain in the peritectic transformation and refine the grain as a result.Secondly, the effect of the parameters of quenching-tempering heat treatment on the structure of Ti-46Al-5Hf was studied, and the results indicated that the structure of alloy was changed from columnar crystal to uniform massive strucuture after solution inαsimple phase field and then being quenched, and then the near fully lamellar structure was obtained by tempering and air cooling. The uniform and fine structure can be produced while discontinuous coarsening of structure can be prevented by raising solution and tempering temperature and shortening solution time and tempering time. The optimum heat treatment process was solution at 1400℃for 1h and quenching in 50℃hot water, tempered at 1280℃for 3h and air cooling.Fanally, the effect of Hf and B content on the quenching-tempering structure was analyzed by employing the optimium heat treatment process, the results indicated that the microstructure was gradually refined with the increasing Hf content without B. The quenching-tempering structure of alloys was changed from near fully lamellar structure to duplex structure with the increasing Hf content when the B content was fixed while the untransformed lamellar structure in quenching-tempering structure of alloys was increased with the increasing B content when the Hf content was fixed. Hf and B can stabilize the as-cast lamellar structure and inhibite the formation of lamellar structure during tempering and air cooling in alloy.