چكيده به لاتين
Ordered intermetallic titanium aluminide alloys represent a unique category of advanced metallic materials, Due to low weight, high strength and suitable working temperature. These alloys can be used in jet engines, so that due to low weight, they can increase the jet efficiency by increasing the thrust to weight ratio. However, Due to the low workability of titanium aluminide alloys, it seems necessary to produce these type of alloys with improved workability via different chemical compositions. Therefore, in the present study, a new titanium aluminide alloy was designed and Produced before examining its hot deformation behavior. For this purpose, gamma titanium aluminide ingots with stoichiometric composition of Ti-44Al-5Nb-1.5Zr-1Cr-1B-0.17(La, Ce) (at%) were cast using an arc furnace in vacuum atmosphere (10-3mbar). Then, with the remelting of cast ingot, cylindrical pieces were produced. High pressure homogenization heat treatment of cylindrical pieces were performed by hot isostatic press for a period of 4hr at 1000°C under pressure of 100 MPa. Hot compression tests were carried out at temperature range of 900-1150°C using a range of strain rates between 0.001 and 0.1s-1 to strain level of 0.7. An artificial neural network was used to model flow curves. The microstructure of the samples was examined by optical microscopy (OM) and scanning electron microscopy (SEM). X-ray diffraction (XRD) and energy dispersive x-ray spectroscopy (EDS) were also used to identify phases and compounds within the microstructure. The results of microstructural studies showed that deformation process at 900°C not only caused a phase transformation of TiAl/Ti3Al to TiAl+TiAl3/Ti2AlNb phases, but also a conversion of lamellar equiaxed structure containing TiAl and Ti3Al phases into separate and elongated grains of TiAl3 and Ti2AlNb phases. By increasing the deformation temperature to 950°C and 1000°C temperatures, the Ti2AlNb phase was eliminated and volume fraction of TiAl3 phase reduced. At 1050°C, phase transformation of TiAl/Ti3Al to TiAl+Ti19Al6 phases occurred and the first recrystallized grains were formed in the microstructure. By increasing the deformation temperature to 1100°C and 1150°C, volume fraction of the recrystallized grains increased. On other hand by Increasing the strain rate at 1100°C and 1150°C resulted to a reduction in volume fraction of recrystallized grains. In addition, at temperature of 1100°C and strain rate of 0.1s-1, phase transformation of TiAl/Ti3Al to TiAl+Ti1.335Al2.665 phases and at temperature of 1150°C and strain rates of 0.001s-1 and 0.01s-1, phase transformation of TiAl/Ti3Al to TiAl+AlNb2 phases occurred. Results obtained from the flow curves shows that in most of the stress-strain curves, there exist of three sections, containing work hardening, softening and steady state sections. However, in some of the flow curves, the steady state section was not observed and in some curves containing steady state section, a secondary work hardening section was also observed. The studied alloy has positive strain rate sensitivity but a negative temperature sensitivity and the results of flow curves modeling by artificial neural network showed that an accuracy of 0.999 can be obtained.
