| Due to the high specific strength, high specific stiffness and high temperature properties including oxidation resistance, creep resistance and good high temperature strength, TiAl-based alloys are considered as potential applied materials of aeroengine blades. However, in the period of service life the aeroengine blades suffer from a variety of cyclic loadings, resulting in a great requirement of the strength and fatigue properties of materials. Therefore it makes a remarkable sense to improve the microstructure of TiAl alloys in order to enhance the strength and high cyclic fatigue property.In the paper, the Ti-47Al-2Cr-2Nb billets with a fine directionally solidified structure are prepared by electromagnetic cold crucible technology. The study of macrostructure and microstructure of Ti-47Al-2Cr-2Nb billets is by control of one influential factors, the withdrawal velocity. It shows that the S/L interface is inclined to concave deeply with the increase of withdrawal velocity, meanwhile the deviate angles of columnar grains to longitudinal are increased, the grain size and interlamellar spacing become smaller. In addition, the secondary dendritic grains become stronger and the arm spacing gets larger with the growth rate increasing. It is clarified that the interlamellar spacing and the growth rate follow this relationship:λ=126.8×ν-1.5.Through cold crucible directionally solidification procedure to refine microstructure of Ti-47Al-2Cr-2Nb,the comprehensive properties are notably promoted. As a result, the fracture toughness of Ti-47Al-2Cr-2Nb reaches20.33MPa-m1/2, and the bending strength attains600MPa. The tensile properties are enhanced remarkably, with the tensile strength reaching650MPa at room temperature and the thermally tensile strength attaining490MPa. At800℃, the tensile elongation is promoted to5%. It points that the fracture toughness and the tensile strength at RT and800℃increase gradually with the interlamellar spacing decreasing. The relationship between fracture toughness and interlamellar spacing are as follows:KIC=9.93+0.0456λ-0.5.Meanwhile the relationship between thermally tensile strength and interlamellar spacing are as follows: σb=413.9+0.332λ-0.5. The fractographs of tensile fracture specimens reveals that: the propagation of cracks is perpendicular to the lamellar or parallel to the lamellar. In addition, at room temperature, the tensile fracture mode is totally brittle fracture. And at800℃the fracture mode on the macro is brittle fracture, while the microscopic fracture mode is a mixture of brittle fracture and ductile fracture mode.The result of high cycle fatigue tests of directionally solidified Ti-47Al-2Cr-2Nb reveals that the fatigue limit is300MPa occupied60%of tensile strength for a withdrawal velocity of1.0mm/min, while for the withdrawal velocity of1.2mm/min the fatigue limit attains247MPa occupied45%of tensile strength. The results indicate that the fatigue resistance of1.0mm/min is superior to that of1.2mm/min. In order tomake a draft of S-N curve, the equations of S-N curve are calculated, based on thefatigue data. The study of fractographs of fatigue fracture specimens shows that thecracks initiation are adjacent the interlamellar zone or near the B2phase. It was alsofound that the propagation of cracks is perpendicular to the lamellar or parallel to thelamellar on the macro. While on the micro, the cracks propagate with the extension ofriver patterns or in the direction of fatigue striations’ normal direction. Referring to theplastic passivation theory, the mode of fatigue crack propagation is explained properly.Finally, the simple model of fatigue crack propagation is established. |
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