| TiAl alloys are one of the most promising light-weight structural materials for making structures and parts working at elevated temperatures in aerospace field, due to their low density, high specific strength, good high-temperature burning resistance and oxidation resistance, excellent creep and fatigue properties. However, their applications have been limited due to poor hot deformability, low ductility at room temperature, and insufficient oxidation resistance at temperatures above 750℃. By adding a balanced concentration of β-stabilizers Nb and Mo elements to the TiAl alloys in this paper, not only possess a large amount of disordered BCC β-phases which improves hot workability and wide hot processing condition window at elevated temperatures, but also improves the oxidation resistance. Currently, the research on sheet rolling and oxidation behavior of TiAl containing Nb and Mo elements is still fewer, and both of them are the key issues need to be resolved during the process of industrial trial production, it has great significance in the achievement of commercial production for TiAl alloy. The hot deformation behavior, sheet rolling processing and oxidation behavior of TiAl containing Nb and Mo elements were systematically investigated in the present work. The main work and conclusion can be made as followings:(1) Ti-44.45Al-3.80Nb-1.01Mo-0.29Si-0.14B (such as:TNM, at%) alloy ingot (φ 11 Omm × 190mm) was prepared by vacuum levitation melting with a water cooled copper crucible. The microstructure of as-cast TNM alloy is made up of γ/α2 lamellar structure, y phase and β/B2 phase, the component distribution of HIPed is more uniform, and its tensible strength is 584 MPa and elongation is 1.68% at 800℃. The casting solidification process and phase transition point of TNM alloys are determined.(2) Through the thermal simulation compression experiment show that the β/B2-containing as-casted TNM alloy has fine hot working properties within the range of temperature 1200～1250℃, and strain rate 0.01～0.5s-1. Material constants were solved under the different strain conditions, and the constitutive modeling was established. Verification results show that the modeling can well predict the thermal deformation rheological law of TNM alloy. The low temperature and high temperature deformation mechanism of TNM alloy are based on dislocation slip of y phase and dislocation slip of y, a and β phase, respectively. The existence of P phase plays positive role in high temperature deformation. Dynamic recrystallization of y phase and dynamic recovery of β phase are the softening mechanism.(3) Based on the theory of the dynamic materials model (DMM), the hot processing map of the alloy was established. The hot processing map can be divided into flow instability region and security zone according the power dissipation efficiency in different regions. Then the reasonable process parameters of the hot rolling processing were established. Hot-rolling process was optimized and carried out directly from the ingot without the hot forging. Using conventional rolling mill the TNM alloy sheets with a dimension of 510mm × 105mm × 1.4mm were successfully fabricated by using hot-pack. The rolling condition was nearly isothermal at high temperature (1250℃) within the α+γ+β phase field, and the rolling speed was less than 112 mm/s, the whole reduction of the sheet was up to more than 86.00%. After rolling, the microstructure of as-rolled is near y structure, and the mechanical property was improved by heat treatment, particularly the a single phase zone treatment.(4) First of all, the growth of oxide on y-phase during the stage of initial of oxidation of TNM alloy, and the oxide grows rapidly at 774℃. The experiments of oxidation at 800℃ show that the xide film remained intact after 500h isothermal oxidation, the oxide film is starting to peel off after 710 cycle oxidation and the oxidation rates were aeeelerated by the applied tensile stresses of TNM alloy. Compared with 4822 alloy, the TNM alloy has excellent oxidation resistance. The addition of Nb and Mo in the TNM alloy enhances the oxidation resistance of the alloy. The synergistic effect of Nb and Mo could change the oxide film formation mechanism of alloy, and form Al2O3 layers which were relatively continuous and dense, the indiffusion of oxygen atoms can be prevented and decreases the oxidation rate, compared to 4822 alloy the oxide film is thin. And which contributes to enhanced adhesion between oxide film and substrate, to some extent it can improve high temperature oxidation resistance of TNM alloy. |
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