This thesis is focused on three main topics, i.e. the high temperature deformation behavior during hot working process, microstructure control by thermomechanical treatment, and creep deformation behavior of TiAl-base intermetallic compounds.
The high temperature deformation behavior of Ti-47Al-2Cr-4Nb and Ti-(46-48)Al-2W intermetallic compounds have been investigated by isothermal compressive tests at temperatures ranged 1000-1200℃ with strain rates ranged $10^{-3}-10^{-1}s^{-1}$. The stress-strain curve during high temperature deformation exhibited a peak stress, then the flow stress decreased gradually into a steady state stress with increasing the strain. The flow softening behavior after the peak stress is attributed to the dynamic recrystallization. The dependence of flow stress on temperature and strain rate could be formulated well by a hyperbolic sine relationship using the normalized Zener-Hollomon parameter. The dynamic recrystallization rate and recrystallized grain size increased with increasing the temperature and decreasing the strain rate.
The effects of initial microstructures on the high temperature deformation behavior of Ti-48Al-2W intermetallic compounds were investigated by compressive tests at 1100℃ and 1200℃. The initial microstructures were controlled as near γ, duplex and near lamellar structures by heat treatments at 1250℃, 1300℃ and 1350℃. The stress-strain curves showed the flow softening during the high temperature deformation of near γ, duplex and near lamellar intermetallic compounds. The peak flow stress was highest in near lamellar structure, and the flow softening rate was higher in near lamellar structure than near γ structure at a fixed strain. In near γ structure, the dynamic softening was due to the dynamic recrystallization of γ grain. In near lamellar structure, the flow softening was occurred by kinking, rotating and globularization of lamellar. The flow localization and shear bands were observed in near lamellar structure during the high temperature deformation. The flow localization parameter was calculated from the instability condition. It is assumed that the instable deformation occurs when the force required for deformation does not increase with increasing strain. The flow localization in near lamellar structure was due to the higher flow softening rate. In steady state, the dynamically recrystallized grain size decreased with decreasing temperature and with increasing strain rate, i.e. with increasing the Zener-Hollomon parameter. The recrystallized grain sizes were in the range of 1-1㎛ and were maintained constant in the steady state regime. A constitutive equation was suggested to describe the high temperature deformation behavior in steady state regime. The controlling mechanism for the deformation in steady state regime is considered to be the grain boundary sliding accommodated by the lattice diffusion. The flow curves could be simulated by considering the flow softening by dynamic recrystallization. The amount of flow softening was increased with increasing the volume fraction of recrystallized grain.
The microstructure evolutions in Ti-47Al-2Cr-4Nb and Ti-(46-48)Al-2W intermetallic compounds during heat treatment were investigated. The binary phase diagrams were established by microstructure observation after heat treatment at 1200-1370℃, EDS and DTA analysis. The various microstructures could be controlled by heat treatment in α+γ two phase regime for cast and forged intermetallic compounds. The lamellar grain size of Ti-48Al-2W was controlled by heat treatment in α single phase regime and the lamellar volume fraction was controlled by controlling the temperature and time for heat treatment in α+γ two phase regime.
The high temperature creep behavior of Ti-48A-2X(X=Cr, Nb, W) intermetallic compounds has been investigated by constant stress creep tests at temperatures ranged 750-850℃ with applied stresses ranged 170-440MPa. The effects of microstructure and alloying additions of Cr, Nb, W on creep behavior of Ti-48Al-2X intermetallic compounds were analyzed. The alloy addition affected on microstructure such as grain size and lamellar volume fraction. The Ti-48Al-2W exhibited the highest creep resistance, and followed by Ti-48Al-2Nb and Ti-48Al-2Cr intermetallic compound. The reason for higher creep resistance in Ti-48Al-2W is expected due to the solid solution strengthening effect of W. It is suggested that W solute decreased the diffusion kinetics during creep deformation. The effects of lamellar grain size and lamellar volume fraction on creep behavior of Ti-48Al-2W intermetallic compounds were analyzed. The minimum creep rates were almost independent on lamellar grain size in the range of 135-360㎛, but were sensitively dependent on the lamellar volume fraction. The minimum creep rate decreased with increasing lamellar volume fraction from 70% to 100%.
The dependence of minimum creep rate on temperature and stress for each microstructure could be formulated by the power law creep equation. The activation energies were measured as 298kJ/mol for the near γ structure, 340kJ/mol for the near lamellar structure and 345kJ/mol for the fully lamellar structure. The stress exponents were measured as 5.6 in near γ structure and 4.9 in fully lamellar structure. The stress exponents of 5.0-5.5 are obtained in near lamellar structure having the lamellar volume fraction of 70-95%. The similar values of stress exponents and activation energies indicated that the controlling mechanism for creep deformation is identical for different microstructure Ti-48Al-2W. The creep rates of near lamellar Ti-48Al-2W were analyzed by the rule-of-mixture on creep rates of lamellar phase and γ phase.
