.8.5 Nb-tial-based Alloy Preparation - Organization - Performance Relationship

Development of new structural materials used at high temperature is important and essential to take the first step in aerospace industry. High Nb-TiAl based alloys have become the major direction in this field. The deformation mechanism, hot processing, oxidation behavior at high temperature, and a new high performance 8.5Nb-TiAl based alloy with substantial fine grains were researched systematic in this thesis. The main contents and results are showed as the follows:1. The deformed mechanism of 8.5Nb-TiAl based alloys at high temperature during the compression tests was investigated.(1). At 600°C¹100°C, compression tests of Ti-45Al-8.5Nb-0.3W-0.05Y alloy, named as TAWY, were conducted by hot simulator. The results indicated that the alloy can be compressed as 70%⁸0% deformation at 1100°C, and broke down the as-cast microstructures. Therefore, 8.5Nb-TiAl based alloys can be forged unless the deformed rates is less than 10^-2s^-1 and the temperature is below 1100°C.(2). The yield strength of FL and DP microstructures exhibits the trend of decreasing, whereas the compression elongation rises with the deformed rate decreasing during the compression tests at high temperature. At 800°C¹000°C, both ultimate compression strength and yield strength for FL microstructure exhibits higher than that for DP microstructure, but the compression elongation is opposed.(3). The existence of twin and stacking faults in the deformed microstructure of TAWY alloy is coordinate with the hot processing. At 600°C⁸00°C, the hardworking ability is controlled by the dislocation sliding in y grains during the deformed processing. The softening phenomena result from the dynamic recrystallization at 900°C¹100°C. The deformation method is showed as the dislocations sliding and grain boundaries immigrating. And it is also with the complement of twinning. B₂ phase in TAWY alloy contributes to hot processing and coordinate the deformation with absorbing and emitting the dislocations.2. Hot processing of 8.5Nb-TiAl based alloys was investigated.The cladding forging in near a phase field can break down the as-cast microstructure, resulting in various plane and neat pancakes with perfect surface. Deformed in the (α₂+γ) phases, 8.5Nb-TiAl based alloys can be extruded into bar materials with the size of φ50mmx 400mm.3. The physics analysis for high temperature deformation of 8.5Nb-TiAl based alloys was studied.(1). Ti-45Al-8.5Nb-0.2W-0.2Mo-0.05Y (TAMWY) and Ti- 45A1 -8.5Nb -0.2W -0.2Mo -0.3B (TAMWB) exhibit the softening condition during the compression tests at 760°C¹000°C. The dual logarithm of true stress and true strain for these two alloys is linear better. At those temperatures, the strain sensitivity m, for TAMWY and TAMWB alloys is 0.06-0.09 and 0.06-0.08 respecting. The activation energy of high temperature deformation is 243 kJ/mol, 279 kJ/mol and 287 kJ/mol for TAWBY alloy;289 kJ/mol, 327 kJ/mol and 345 kJ/mol for TAWMY alloy. These two alloys deformed as the form ofatoms mutual diffusion at 760°C¹000°C.(2) The deformed constitution for TAMWY and TAMWB alloys is showed as the follows:TAMWB, ï¿¡ = 6.3o-137exp(-270000/i?7) TAMWY, s = 6.6o-I4Iexp(-320000/i?7) 4. The oxidation behavior of 8.5Nb-TiAl based alloys was studied at high temperature.(1) Oxidizing layer of TAWY alloy is merely l|im after 800°C-200h oxidation. During oxidizing at 900°C¹000°C, the oxidizing velocity enhances quickly with four separate layers. The first layer is discontinuous A12O3, the second layer is the mixture of AI2O3 and TiC>2, the third layer is enrich Nb, and the last layer is diffusion layer. This diffusion differs from the morphology of TAWY alloy matrix. The energy diffraction analysis indicates that other elements are similar to the TAWY alloy composition except Al content increasing.(2). Compared the oxidizing mass gain for TAWY alloy with other TiAl alloys, the curve of TAWY alloy is parallel to the time axis during the 800°C-200h air exposure. Whereas other TiAl alloys, such as K5, Ti-48A1, and Ti-48Al-2Cr-2Nb alloys, exhibit seriously oxidizing. Oxidation resistance of TAWY alloy amounts to 1000°C, which is 200°C higher than that for Ti-48A1 alloy after 1 OOh oxidizing.(3). 8.5Nb alloying formed a discontinuous AI2O3 layer in the TAWY oxidation surface. Between oxidation layer and diffusion layer, a rich Nb layer appears. Below the Nb rich layer, O content decreases quickly. T(4). The oxidizing dynamic curves of TAWY alloy are described as follows: 800°C. Am122= 1/493 t (0 phases appear as the phase orientation (110) ^Jl (0001)?, and [111]B2//[H 20]ra. Great many typical substructures were also observed in deformed TAWBY alloy, such as dislocations, twins, and stacking faults.(2). Tensile strength for DP microstructure is higher than FL microstructure of TAWBY alloy at ambient temperature, and the elongation exhibits the same rule. But the elongation for both microstructures is less than 1%. The fracture toughness of FL microstructure exhibits higher owing to the crack restrained and propagation changeable frequency after the crack nuclei. The lamellae interface, ordered domain boundary, and grain boundaries contributes to the strength at room temperature.(3). Compared the elevated temperature tensile strength for TAWBY alloy with Ti-46Al-5Nb-lW, Ti-48Al-8Nb-lB, Ti-44Al-4Nb-4Zr-lB, Ti-47Al-2Nb-2Cr-4Ta and K5 alloys, TAWBY alloy has already owned the best properties. Under the same condition, TAWBY alloy exhibits the perfect properties like K5 alloy and so on.