Pulse Current Sintering And Formability Of γ-TiAl Based Alloy

γ-TiAl based alloys are promising candidates for elevated temperature structural materials due to their superior characteristics, such as low density, high young's modulus, excellent strength and oxidation resistance at elevated temperature. However, they suffer from room temperature brittleness and poor workability, hindering their applications. The grain size has a very important effect on the properties of material. However,it is difficult to prepare TiAl alloy with fine and homogeneous microstructure by traditional casting and mechanical working processes. Pulse Current Auxiliary Sintering(PCAS), because of its quick heating rate, short sintering time, controllable microstructure, high material density, is videly used to produce TiAl alloys with refined microstructure.In this paper, TiAl based alloys from the prealloyed powder of were prepared by pulse current auxiliary sintering. In the synthesis process, influences of different parameters, including heating-up rate, sintering temperature and duration time, on obtained microstructures in sintered products were studied. Effect of microstructure on properties at elevated temperature was tested. High tensil properties were obtained at relatively low temperature. Based on the tensil properties, superplastic extrusion of blade was conducted.Pulse Current Auxiliary Sintering: When powders were synthesized in the oven at 1250℃for 5min with heating rates of 4.5, 3.8, 3, 2.5, 2℃, near gamma, duplex, nearly lamellar, fine fully lamellar and coarse fully lamellar microstructures were obtained, correspondingly. Fine grain size was abtained. The grain size of nearγmicrostructure has reached 4μm。Tensile testing at elevated temperature: Tensile tests of different microstructure were conducted in temperature range of 950℃~1000℃. The results showed that one with equi-axed fine grains possessed more superior elevated temperature tensile properties. When the strain rate range was 1.040×10-4s-1~2.083×10-4s-1, the obtained elongation was over 240%. The maximum elongation got to 437%. High superplastic behavior at relatively low temperatures realized. Cavities were observed in the sample after tensil tests. It is analyzed that cavities wre helpful to elongation at first. But they lead to the fracture of alloy eventually. The morphologic of the cross section showed that a majority of high temperature tensile fracture was ductile.Imitated extrusion of blade: Based on the high temperature superplasic properties, with nearγmicrostructure and fully lamellar microstructure, models of blade were extrued at 1250 ~1350℃. The results show that when the sample with nearγ microstructures was extruded at 1250℃with the extrusion rate 0.05mm/min, good quality and improved mechanical properties were obtained. Whenγ-TiAl alloys with fully lamellar microstructures was extruded at 1250℃with the extrusion rate 0.02mm/min, dynamic recrystallization happened. The microstructure transferred to equiaxial and uniformγ-TiAl with nearγmicrostructures. The properties of the material were improved.