EVOLUTION OF MICROSTRUCTURES IN THE NICKEL MODIFIED TITANIUM TRIALUMINIDES NEAR THE L1(2) PHASE FIELD

This study focuses upon the evolution of microstructures during solidi fication processing of several intermetallic alloys around the Ll(2) p hase in the Al-rich corner of the Al-Ti-Ni ternary system. The alloys were produced by double induction melting and subsequent homogenizatio n followed by furnace cooling. The microstructure was characterized by means of optical and scanning electron microscopy with energy-dispers ive spectroscopy (EDS) analysis and X-ray diffraction. The microstruct ural evolution in homogenized alloys was dependent on both nickel and titanium content. Very fine precipitates of Al2Ti were observed within the Ll(2) phase in alloys containing 62 to 65 at. pet Al and at least 25 at. pct Ti. The Al2Ti precipitates are stable at least up to 1000 degrees C and undergo complete dissolution at 1200 degrees C. In alloy s containing around 66 at. pct Al and 25 to 31 at. pct Ti, phases such as Al3Ti, Al5Ti2, and Al11Ti5 were observed. A modified room temperat ure isotherm in the Al-Ti-Ni ternary system is proposed, taking into a ccount the existence of Al2Ti, Al11Ti5, Al5Ti2, and Al3Ti in equilibri um with the Ll(2) phase. It seems that at room temperature, the Ll(2) phase field for homogenized alloys is extremely small. It will be prac tically impossible to obtain a single-phase microstructure at room tem perature in the Al-Ti-Ni ternary alloys after homogenization at 1000 d egrees C followed by furnace cooling.