α₂+γ型Ti-Al系金属間化合物のα域加工によるラメラ粒の微細化

抄録

Hot working in the high temperature single α phase region have been employed to achieve a fine equiaxed lamellar grain structure in an α₂+γ type Ti-Al intermetallic compound, which is expected to be the optimum microstructure for balancing the mechanical properties, such as room temperature ductility, fracture toughness and high temperature strength. Effects of hot working parameters, such as temperature, reduction and strain rates, and the subsequent cooling rate on the microstructures were systematically examined in the Ti-40 mol%Al alloy. The microstructures were characterized by the hardness measurement, X-ray diffractometry, optical microscopy and TEM.
It was clearly demonstrated that the refinement of fully lamellar grain structure is possible with the above method. It was revealed that the critical cooling rate for the formation of γ phase exists at 0.2 K/s when the alloy was deformed by 60% with 10⁻²s⁻¹ at 1473 K. Parameters of hot working also strongly influence the lamellar grain size. Lower temperature, higher reduction and higher strain rates respectively promotes the refinement of lamellar grain size and an optimum combination of the working parameters produced the grain size of 100 μm. Furthermore, an isothermal process for the precipitation of γ phase after quenching into the α₂+γ region achieved the grain size of 70 μm. The formation of γ phase in the α phase during continuous cooling and the role of hot working in the α phase on the eventual lamellar grain structures were discussed.