Microstructure And Creep Rupture Properties Of Cold Crucible Direction Ally Solidified Ti-47Al-2Cr-2Nb

Ti Al-based alloys possess high strength, large specific stiffness and high temperature oxidation resistance, creep resistance and other excellent characteristics, which make it become the potential application material of the aircraft engine blade. How to evaluate and improve high temperature mechanical properties of the Ti Al-based alloys have become an important content in the field of materials research. The creep rupture strength is one of the important security and reliability indicators about measuring structural materials at high temperature. However, the organization of Ti Al-based alloys on the performance of high temperature especially creep rupture properties performance impact study is not perfect, how reliable to predict the long-ter m high temperature creep rupture strength has important theoretical and engineering significance.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:.To measure ensile mechanical properties of the directional solidification Ti-47Al-2Cr-2Nb alloy at room and high temperature. Room temperature tensile strength up to 568 MPa, ther mally tensile strength up to 458 MPa at 800℃, the elongation can be up to 5.1%.When the withdrawal velocity is 1.0 mm/min, the function relation between the thermally tensile strength changes with temperature:σ₀=273.885714+1.742000（T-273）-0.001857（T-237）². The relationship between thermally tensile strength and interlamellar spacing are as follows: σ at 800℃. 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 at 800℃ the fracture mode on the macro is brittle fracture, while the microscopic fracture mode is a mixture of brittle fracture and ductile fracture mode.To determine the thermally creep rupture properties of directional solidification Ti-47Al-2Cr-2Nb alloy at two temperature under different stress states. The results show that the longest creep rupture life can reach 89 h when the withdrawal velocity is 1.0 mm/min at 650 ℃, under 175 MPa. The creep rupture life of directional solidification sample was much higher than the as-cast sample. The increase of the rise of temperature and stress are lower the creep rupture life. If the test conditions were same, the creep rupture life can be highest when the withdrawal velocity is 1.2 mm/min. Analysis of fracture morphology, the results showed that the fully lamellar-FL Ti A1-based alloys have good creep resistance, the stability of the lamella structure largely determines the creep rupture strength, the stability of dense lamella structure is higher, the more densely of the lamella structure, the hi gher of the creep rupture life. Directional solidification of creep rupture sample fracture mode shows that: the propagation of cracks is perpendicular to the lamellar or parallel to the lamellar, which is similar to thermally tensile fracture mode. The endurance plasticity was improved at 800℃.Through linear regression equation draw the stress and fracture time curve on the log-log plot, and get experience formula: lnσ=6.209912-0.232990 lnt, and according to this equation extrapolation for =116.98 MPa.Creep rupture strength prediction model is established through the Larson-Miller and Manson-Haferd parameter method. Simply by applying this model to test the directional solidification Ti-47Al-2Cr-2Nb thermally tensile strength can infer the creep rupture strength. Obtained by these two models are very close to the predicted and the measured values. Also found Larson-Miller and Manson-Haferd parameter values is not fixed, and is affected by temperature.