Strength and toughness of L1_0-type intermetallic TiAl with duplex structure.

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Experimental studies were made on microstructure-controlled L1_0-TiAl mainly by means of transmission electron microscopy. Results are as follows.1. Microstructural variations of L1_0-ordered TiAl with Al contents.TiAl compounds ranged from 47 to 52 at% AI at intervals of 0.25 at% AI were solution heat treated at 1373 K. The structure was nearly single-phase gamma by optical microscope(OM)examination. During aging at 973 K, transmission electron mocroscope(TEM)revealed that no precipitates were found in the matrix gamma. TiAl compounds ranged from 56 to 62 at% Al at intervals of 2.0 at% AI were solution heat treated at 1473 K. The structure was the single-phase gamma by OM. TEM observation that TiAl_2 precipitate, which was not so fine, was formed during aging at temperatures in the range of 973 to 1173 K.2. Microstructural variations of stoichiometric L1_0-ordered TiAl containing 0.1 to 2.0 at% C or N.Fine needle-shaped Ti_3AlC and Ti_3AlN of perovskite type were formed only [001] direction of the L1_0 matrix during aging. The cross-section of the needle was not circular but square. In the carbon containing alloys, precipitates with the similar morphology and the same crystal structure was formed. The selected area electron diffraction analyses suggested that the precipitates had a perovskite-type crystal structure and had the orientation relationship(100)-Ti_3AlC/Ti_3AlN parallel to(100)TiAl and[001]-Ti_3AlC/Ti_3AlN parallel to [001]-TiAl. At higher temperatures, Ti_3AlC/Ti_3AlN precipitates were replaced by Ti_2AlC and Ti_2AlN with hexagonal lattice structure.3. Room temperature compression strength.The strength increase might be not so anticipatated by the precipitation of perovskite phase. The alloy containing 0.5 at% N indicated higher strength than that containing 0.5 at% C.