Development of Ultrafine, Lamellar Structures in Two-Phase (gamma)-TiAl Alloys

摘要

Processing of two-phase gamma-TiAl alloys (Ti-47Al-2Cr-2Nb or minor modifications thereof) above the alpha-transus temperature (T (sub alpha)) produced unique refined-colony/ultrafine lamellar structures in both powder- and ingot-metallurgy (P/M and I/M, respectively) alloys. These ultrafine lamellar structures consist of fine laths of the gamma and alpha (sub 2) phases, with average interlamellar spacings (lambda (sub Lambda)) of 100-200 nm and alpha (sub 2)-alpha (sub 2) spacings (lambda (sub alpha)) of 200-500 nm, and are dominated by gamma/alpha (sub 2) interfaces. This characteristic microstructure forms by extruding P/M Ti-47Al-2Cr-2Nb alloys above T (sub alpha), and also forms with finer colony size but slightly coarser fully-lamellar structures by hot-extruding similar I/M alloys. Alloying additions of B and W refine lambda (sub L) and lambda (sub alpha) in both I/M Ti-47Al (cast and heat-treated above T (sub alpha)) or in extruded Ti-47Al-2Cr-2Nb alloys. The ultrafine lamellar structure in the P/M alloy remains stable during heat-treatment at 900 (degrees)C for 2h, but becomes unstable after 4h at 982 (degrees)C; the ultrafine lamellar structure remains relatively stable after aging for (gt)5000 h at 800 (degrees)C. Additions of B+W dramatically improve the coarsening resistance of lambda L and lambda alpha in the I/M Ti-47Al alloys aged for 168 h at 1000(degrees)C. In both the P/M and I/M Ti-47Al-2Cr-2Nb alloys, these refined-colony/ultrafine-lamellar structures correlate with high strength and good ductility at room temperature, and very good strength at high temperatures. While refining the colony size improves the room-temperature ductility, alloys with finer lambda (sub L) are stronger at both room- and high-temperatures. Additions of B+W produce finer as-processed lambda (sub L) and lambda (sub alpha) in I/M TiAl alloys, and stabilize such structures during heat-treatment or aging.