DEVELOPMENT OF ULTRAFINE LAMELLAR STRUCTURES IN 2-PHASE GAMMA-TIAL ALLOYS

Processing of two-phase gamma-TiAl alloys (Ti-47Al-2Cr-2Nb, or minor m odifications thereof) above the alpha-transus temperature (T-alpha) pr oduced unique refined-colony/ultrafine lamellar structures in both pow der-and ingot-metallurgy (PM and IM, respectively) alloys. These ultra fine lamellar structures consist of fine laths of the gamma and alpha( 2) phases, with average interlamellar spacings (lambda(L)) of 100 to 2 00 nm and alpha(2)-alpha(2) spacings (lambda(alpha)) of 200 to 500 nm, and are dominated by gamma/alpha(2) interfaces. This characteristic m icrostructure forms by extruding PM Ti-47Al-2Cr-2Nb alloys at 1400 deg rees C and also forms with finer colony size but slightly coarser, ful ly lamellar structures by hot-extruding similar IM alloys. Alloying ad ditions of B and W refine lambda(L) and lambda(alpha) in both IM Ti-47 Al (cast and heat treated at 1400 degrees C) and IM Ti-47Al-2Cr-2Nb al loys (extruded at 1400 degrees C). The ultrafine lamellar structure in the PM alloy remains stable during heat treatment at 900 degrees C fo r 2 hours but becomes unstable after 4 hours at 982 degrees C; the ult rafine lamellar structure remains relatively stable after aging for >5 000 hours at 800 degrees C. Additions of B + W dramatically improve th e coarsening resistance of lambda(L) and lambda(alpha) in the IM Ti-47 Al alloys aged for 168 hours at 1000 degrees C. In both the PM and IM Ti-47Al-2Cr-2Nb alloys, these refined-colony/ultrafine lamellar struct ures correlate with high strength and good ductility at room temperatu re, and very good strength at high temperatures. While refining the co lony size improves the room-temperature ductility, alloys with finer l ambda(L) are stronger at both room and high temperatures. Additions of B + W produce finer as-processed lambda(L) and lambda(alpha) in IM Ti Al alloys and stabilize such structures during heat treatment or aging .