| TiAl has been extensively investigated as a promising candidate material for high temperature applications in aviation, automotive and power generation industries. Due to high Nb additions, its low density, relatively high specific strength and excellent corrosion resistance, as well as oxidation and creep resistance at high temperature have been significantly improved compared with low Nb-bearing Ti Al alloys.During the gas atomization process, a liquid stream of molten alloys is broken up into fine spherical droplets of different sizes by the impact of a high-velocity gas flow. The liquid droplet cools rapidly into a supercooled melt by heat dissipation mainly through strong convection. Solidification in the droplet starts with crystal nucleation, mostly at the surface of the droplet, when the temperature is below the liquidus temperature of the alloys. Latent heat released during this process is mostly absorbed by the liquid itself, resulting in recalescence. The initial number of nuclei increases with the droplet size and can be approximated as an exponential function of the droplet diameter. The microstructure pattern transfers from a twinned spherical segment to a concentric liquid/solid interface geometry. The objective of this paper is to formulate a computational Newtonian model in order to predict the progress of droplet solidification as a function of cooling time and provide a theoretical understanding of the microstructure evolution during rapid solidification. Relationships among atomization parameters, thermal history of the droplet and microstructures are obtained.The nucleation temperature TN ranges from 1521 to 1757 K. The temperature of droplet increases sharply to near the liquidus temperature during recalescence. When the recalescence is completed, the droplet solidifies at a relatively slower rate. The cooling rate decreases with increasing droplet size. Finally, the temperature of droplet decreases significantly because of the rapid dissipation of latent heat. The initial cooling rates of the fully liquid droplet are about 105 and 106 K·s-1. After this recalescence, the cooling rates increased and reached to about 105 K·s-1. when solidification was completed. Then the cooling rate of fully solid phase decreased again to about 1×104 K·s-1. The average size of powders is about 75μm range from 0 to 120μm. The microstructural transition from cell to dendrite can be readily represented in a droplet or the droplets of different size. The transition is the result of the decrease of undercooling. The transition undercooling is found to be about 95 K.In the present study, microstructure evolution of undercooled Ti-48 at.% Al droplets was analyzed with the transient nucleation theory in combination with the microstructural observation. It has been demonstrated that the phase transformation and final fraction can be directly related to the undercooling and cooling rate determined by the droplet size. Microstructure observations of the suppressed primary phase are in good agreement with the analysis using transient nucleation theory. The competitive primary phases α and β are dependent on the droplet size with the critical diameter of about 25 μm corresponding to an undercooling of 102 K. With the decrease of undercooling, the volume fraction of α2+γ lamella and net-like structure increases from 20% to 70%. The width of γ lamella increases greatly from about 0.03 μm to 0.15 μm in constant max spacing of α2+γ lamella structure about 0.3 μm. Segregate γ-Ti Al was observed in the final microstructure in all case.The volume fraction of γ phase is easily controlled by the cooling rate, and increased with increasing powder size from 12% to 85%. The hardness increased to the increasing of powder size until 50 μm in diameter(652 HV). As the diameter increased further, the hardness decreased. Variation in hardness is correlated with the phase compositions and morphologies present in the powder.Hot isostatic pressing and spark plasma sintering were carried out for Ti Al alloy powder sintering. The alloy shows a fine microstructure with composition homogeneity. The density of the alloy reached 99.6%. The true stress-true strain cures of the PM Ti-45Al-6Nb-0.3W(at.%) alloy have revealed that the flow stress is substantially sensitive to deformation temperature and strain rate. The active energy of deformation for the alloy was calculated to be 392.6 k J·mol-1. A set of constitutive equations coupling flow stress with strain, strain rate and temperature are presented. The material constants, 1n A, α, n and Q in the constitutive equations, are found to be functions of strain. The predictability of the developed constitutive equation was quantified in terms of the correlation coefficient(R) and average absolute relative error(AARE). R and AARE were found to be 0.99483 and 3.956% respectively, which reflected the good prediction capability on the developed constitutive equation considering the compensation of the strain for Ti-45Al-6Nb-0.3W alloy. The microstructure evolution of the alloys during the hot deformation was studied by TEM and EBSD, and it was found that recrystallization existed during all the thermal deformation. The microstructure of the obtained recrystallization is fine, and some lamellar structure remains in the deformed structure..The processing map of HIPped Ti-45Al-6Nb-0.3W(at.%) alloy was established, and the isothermal forging parameters of the alloy were determined: 1200 °C, 0.01-0.1s-1. Hot deformation behavior of the alloy was analyzed by isothermal forging experiments, and the isothermal forging process of the alloy was completed at the same time in the deformation region of the processing map. The mechanical properties of the alloy were studied. |
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