Effects Of Heat Treatment On The Microsructure And Mechanical Property Of TiAl Alloy

Gamma-based TiAl alloys have become a newly material because of their low density, acceptable oxidation resistance and good high-temperature strength, which can be used to replace nickel based alloys. At the same time, casting is widely used in industry due to its near-net-shape and low cost, but the grain size of casting alloy is coarse which results in poor ductility and toughness at ambient temperature. A lot of efforts have been made to enhance their properties, such as adding alloying elements, ordinary heat treatment and cyclic heat treatment, in order to widen their application in the industry. Meanwhile the relationship between process, microstructure and property of TiAl alloys is rare which limits their application. In this paper, we chose the second generation TiAl alloy Ti-48Al-2Cr-2Nb(at%) as the canditate to investigate how the ordinary heat treatment regime, βstabilizing element V and the cyclic heat treatment regime influence the microstructure and mechanical property of TiAl based alloy. And we found a heat treatment regime to refine the cast Ti-48Al-2Cr-2Nb alloy and clarified the impact of the V on the microstructure and property of the cast Ti-48Al-2Cr-2Nb alloy. Meanwhile we made a breakthrough on the mutual restrictiom between strength and ductility of Ti-48Al-2Cr-2Nb alloy. Finally, we established the relationship between process, microstructure and property of Ti-48Al-2Cr-2Nb alloy.The experimental results showed that as the cooling rate raises, the vickers hardness increases and the amount of (3 phase increases, while that of (α2 phase reduces. Because furnace cooling(FC) refines the microstructure successfully, we chose it as the cooling mode. And 1306℃-1320℃/10min/FC could refine the microstructure to some extent, while 1330℃-1340℃/10min/FC can not make it.1310℃/10min/FC+1310-1320℃/10min/FC could obtain well-distributed microstructure with grain size between 20 and 30 um for Ti-48Al-2Cr-2Nb alloy, while the grain size of the master alloy is about 1000 um.V increases the amount of βphase, but decrease the growth rate of a phase during the βâ†'α phase transformation, which increases the stability of a phase and hinders the growth of a phase at high temperature, which is benefical for refining the grain. Meanwhile V reduces the lamellar spacing of TiAl alloy via substitutional solid solution. Therefore V significantly enhances the hardness of the alloy. Meanwhile V favors the massive structure and raises the quantity of the cracks of TiAl alloy, which limits the full use of quenching in heat treatment. And 1310℃/10min/FC+1310℃/10min/FC could obtain well-distributed microstructure with grain size between 30 and 40 um for Ti-48Al-2Cr-2Nb-1V alloy, while the annealing temperature for Ti-48Al-2Cr-2Nb-2V alloy and Ti-48Al-2Cr-2Nb-3V alloy should be below 1310℃.The microstucture of TiAl alloy is sensitive to the content of aluminium. For Ti-48Al-2Cr-2Nb alloy, we could obtain fine duplex microstructure with better dutility through 1330℃-1350℃/FC heat treatment. Raising the heat treatment temperature beyond 1370℃, we could acquire near lamellar and full lamellar with higher hardness and strength. We invented the cyclic heat treatment for Ti-48Al-2Cr-2Nb alloy, which can get well-distributed and fine near lamellar with higher strength and ductility. Ultimately, we set the relationship between process, microstructure and property of Ti-48Al-2Cr-2Nb alloy.