Titanium aluminide alloys are the subject of numerous patents and publications including U.S. Pat. Nos. 4,842,819; 4,917,858; 5,232,661; 5,348,702; 5,350,466; 5,370,839; 5,429,796; 5,503,794; 5,634,992; and 5,746,846, Japanese Patent Publication Nos. 63-171862; 1-259139; and 142539; European Patent Publication No. 365174 and articles by V. R. Ryabov et al entitled "Properties of the Intermetallic Compounds of the System Iron-Alminum" published in Metal Metalloved, 27, No.4, 668673, 1969; S. M. Barinov et al entitled "Deformation and Failure in Titanium Aluminide" published in Izvestiya Akademii Nauk SSSR Metally, No. 3, 164-168, 1984; W. Wunderlich et al entitled "Enhanced Plasticity by Deformation Twinning of Ti-Al-Base Alloys with Cr and S" published in Z. Metallkunde, 802-808, 11/1990; T. Tsujimoto entitled "Research, Development, and Prospects of TiAl Intermetallic Compound Alloys" published in Titanium and Zirconium, Vol. 33, No. 3, 19 pages, 7/1985; N. Maeda entitled "High Temperature Plasticity of Intermetallic Compound TiAl" presented at Material of 53.sup.rd Meeting of Superplasticity, 13 pages, 1/30/1990; N. Maeda et al entitled "Improvement in Ductility of Intermetallic Compound through Grain Super-refinement" presented at Autumn Symposium of the Japan Institute of Metals, 14 pages, 1989; S. Noda et al entiitled "Mechanical Properties of TiAl Intermetallic Compound" presented at Autumn Symposium of the Japan Institute of Metals, 3 pages, 1988; H. A. Lipsitt entitled "Titanium Aluminides--An Overview" published in Mat. Res. Soc. Symp. Proc. Vol. 39, 351-364, 1985; P. L. Martin et al entitled "The Effects of Alloying on the Microstructure and Properties of Ti.sub.3 Al and TiAl" published by ASM in Titanium 80, Vol. 2, 1245-1254, 1980; S. H. Whang et al entitled "Effect of Rapid Solidification in L1.sub.0 TiAl Compound Alloys" ASM Symposium Proceedings on Enhanced Properties in Structural Metals Via Rapid Solidification, Materials Week, 7 pages, 1986; and D. Vujic et al entitled "Effect of Rapid Solidification and Alloying Addition on Lattice Distortion and Atomic Ordering in L1.sub.0 TiAl Alloys and Their Ternary Alloys" published in Metallurgical Transactions A, Vol. 19A, 2445-2455, 10/1988.
Methods by which TiAl aluminides can be processed to achieve desirable properties are disclosed in numerous patents and publications such as those mentioned above. In addition, U.S. Pat. No. 5,489,411 discloses a powder metallurgical technique for preparing titanium aluminide foil by plasma spraying a coilable strip, heat treating the strip to relieve residual stresses, placing the rough sides of two such strips together and squeezing the strips together between pressure bonding rolls, followed by solution annealing, cold rolling and intermediate anneals. U.S. Pat. No. 4,917,858 discloses a powder metallurgical technique for making titanium aluminide foil using elemental titanium, aluminum and other alloying elements. U.S. Pat. No. 5,634,992 discloses a method of processing a gamma titanium aluminide by consolidating a casting and heat treating the consolidated casting above the eutectoid to form gamma grains plus lamellar colonies of alpha and gamma phase, heat treating below the eutectoid to grow gamma grains within the colony structure and heat treating below the alpha tansus to reform any remaining colony structure a structure having % laths within gamma grains.
Still, in view of the extensive efforts to improve properties of titanium aluminides, there is a need for improved alloy compositions and economical processing routes.
According to a first embodiment, the invention provides a two-phase titanium aluminum alloy having a lamellar microstructure controlled by colony size. The alloy can be provided in various forms such as in the as-cast, hot extruded, cold and hot worked, or heat treated condition. As an end product, the alloy can be fabricated into an electrical resistance heating element having a resistivity of 60 to 200 .mu..OMEGA.-cm. The alloy can include additional elements which provide fine particles such as second-phase or boride particles at colony boundaries. The alloy can include grain-boundary equiaxed structures. The additional alloying elements can include, for example, up to 10 at % W, Nb and/or Mo. The alloy can be processed into a thin sheet having a yield strength of more than 80 ksi (560 MPa), an ultimate tensile strength of more than 90 ksi (630 MPa), and/or tensile elongation of at least 1.5%. The aluminum can be present in an amount of 40 to 50 at %, preferably about 46 at %. The titanium can be present in the amount of at least 45 at %, preferably at least 50 at %. As an example, the alloy can include 45 to 55 at % Ti, 40 to 50 at % Al, 1 to 5 at % Nb, 0.5 to 2 at % W, and 0.1 to 0.3 at % B. The alloy is preferably free of Cr, V, Mn and/or Ni.