MICROSTRUCTURE AND HIGH-TEMPERATURE TENSILE DEFORMATION OF TIAL(SI) ALLOYS MADE FROM ELEMENTAL POWDERS

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at . pet Al-1.4 at. pet Si (Si poor) and Ti-45 at. pet Al-2.7 at. pet Si (Si rich), which were prepared by reaction powder processing, have bee n investigated. Both alloys consist of the intermetallic compounds gam ma-TiAl, alpha(2)-Ti3Al, and zeta-Ti-5(Si, Al)(3). The microstructure can be described as a duplex structure (i.e., lamellar gamma/alpha(2) regions distributed in gamma matrix) containing zeta precipitates. The higher Si content leads to a larger amount of zeta precipitates and a finer gamma grain size in the Si-rich alloy. The tensile properties o f both alloys depend on test temperature. At room temperature and 700 degrees C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 degrees C, the opposite is true. E xaminations of tensile deformed specimens reveal zeta-Ti-5(Si, Al)(3) particle debonding and particle cracking at lower test temperatures. A t 900 degrees C, nucleation of voids and microcracks along lamellar gr ain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate zeta-T i-5(Si, Al)(3) particles at high temperature, where the positive effec t of grain refinement on both strength and ductility can be utilized.