A Study On The Microstructure And Mechanical Properties Of A Ti-Al-Fe-V-Cr-Zr Alloy With High Strength And Toughness

The constantly increasing demand for aircraft components has led to the development of a class of structural titanium alloys with high strength and toughness. It is important to design a new type of titanium alloy which can achieve excellent strength to density and good strength to toughness combinations. Among the developed high strength and high toughness titanium alloys, the Ti-1023, Timetal555 and VT22 are widely used for their great overall properties. The new Ti-Al-Fe-V (Cr, Zr) alloy has been designed through decreasing the content of ferrum by adding chromium and zirconium, based on molybdenum equivalency and Bo-Md molecular orbital method, to aim at developing a new type titanium alloy with high strength and high toughness. After primary design computation, the new alloy was optimized finally as Ti-3Al-4.5Cr-1Fe-4V-1Zr. The microstructure and mechanical properties of a new near β alloy subjected to different heat treatments were studied by using the conventional post-heat treatment characterization tools (Optical microscopy, SEM, TEM). The following conclusions are drawn from this work:The β-ransus temperature of new Ti-Al-Cr-Fe-V-Zr alloy is measured to be among 810～820℃ by a metallography method. Large amount of Widmanstatten a laths precipitate under the solution treatment below β-transus. Additionally, it is revealed that the volume fraction of a phase decreases with the increase of solution temperature, also, the strength of the alloy increases while the elongation and area of reduction decreases. When the solution treatment is undertaken above the β-ransus, the P grains are seen from the optical micrograph without any acicular a phase remains in it. The alloy under this condition reveals little plasticity corresponding to the large amount of finely and homogeneously distributed ω phase occurred on the β matrix.The microstructure of the alloy solution solution treated in the α+β region followed by aging consists of the primary a phase, second a phase and the metastable p matrix. With the rise of solution temperature in the range of a+P field, the secondary a phase reveals a more intensive precipitation, at the same time, the volume fraction of it is increasing. As a result, the strength of the alloy increases while the elongation and area of reduction decreases. Under the same solution conditions, with the increase of aging temperature, the secondary a phase becomes coarser and decreases in amount. At the same time, the strength of the alloy decreases while the elongation and area of reduction increases. When the aging time expanded, the secondary a phase grew up with a bigger length-width ratio, the strength of the alloy decreases while the elongation and area of reduction increases too. When the specimen is solution treated above the P-ransus and followed by aging, the microstructure of the alloy consists of a large number of secondary a phase with punctiform morphology. It is revealed that alloy treated under this condition reveals little ductility while achieving relatively high strength.A bizarre phenomenon occurs that the alloy aged at 500℃ reveals little ductility. Thus a phase diagram calculation is undertaken to find out the possible second phase precipitated in the aging process. Simultaneously, the TEM observation is taken to seek out the reason that leads to the little plastic of the specimens aged at 500℃. The selected area electron diffraction pattern has demonstrated the occurrence of the new phase precipitated in the specimens aged at 500℃. The calculated phase diagram has demonstrated that the alloy treated at 500℃ would lead to the precipitates of phase such as the TiFe, TiFe2, TiCr, TiCr2 phase. The determination of the new phase precipitated in the specimens aged at 500℃ reveals difficult because of the little volume fraction of it and randomness of experiment.After the optimization of heat treatment, the alloy can achieve a good combination of strength-ductility-toughness. The tensile strength of the alloy ranges from 1048～1273 MPa, with the fracture toughness in the range of 83.8～99.8 MPa·m1/2. When the alloy is solution treated at 790℃ followed by aging at 550℃ for 2h, the tensile strength of it can reach 1273 MPa, with an elongation of 11.0%, meanwhile, the fracture toughness is tested to be 83.8 MPa·m1/2. As a result, the newly designed alloy can achieve a good combination of tensile strength and plasticity through appropriate heat treatment.