DE69902245T2 - TITANALUMINID ALLOY WITH TWO PHASES - Google Patents

Abstract

A two phase titanium aluminide alloy having a lamellar microstructure with little intercolony structures. The alloy can include fine particles such as boride particles at colony boundaries and/or grain boundary equiaxed structures. The alloy can include alloying additions such as </= 10 at % W, Nb and/or Mo. The alloy can be free of Cr, V, Mn, Cu and/or Ni and can include, in atomic %, 45 to 55 % Ti, 40 to 50 % Al, 1 to 5 % Nb, 0.3 to 2 % W, up to 1 % Mo and 0.1 to 0.3 % B. In weight %, the alloy can include 57 to 60 % Ti, 30 to 32 % Al, 4 to 9 % Nb, up to 2 % Mo, 2 to 8 % W and 0.02 to 0.08 % B.

Description

Scope of the invention

The invention relates generally to two-phase titanium aluminide alloy compositions useful for ohmic heating and other applications such as structural applications.

Background of the invention

Titanium aluminide alloys are covered in numerous patents and publications, such as U.S. Patent 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 Publications Nos. 63-171862; 1-259139 and 1-42539; in European Patent Publication No. 365174 and in articles by VR Ryabov et al. entitled "Properties of the Intermetallic Compounds of the System Iron-Aluminium", published in Metal Metalloved, 27, No. 4, 668-673, 1969; SM 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 Si", 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 the Material of 53rd Meeting of Superplasticity, 13 pages, 30.01.1990; N. Maeda et al entitled "Improvement in Ductility of Intermetallic Compound through Grain Super-refinement", presented at the Autumn Symposium of the Japan Institute of Metals, 14 pages, 1989; S. Noda et al entitled "Mechanical Properties of TiAl Intermetallic Compound", presented at the Autumn Symposium of the Japan Institute of Metals, 3 pages, 1988; HA Lipsitt entitled "Titanium Aluminides - An Overview", published in Mat. Res. Soc. Symp. Proc., Vol. 39, 351-364, 1985; PL Martin et al entitled "The Effects of Alloying on the Microstructure and Properties of Ti3 Al and TiAl", published by ASM in Titanium 80, Vol. 2, 1245-1254, 1980; SH Whang et al entitled "Effect of Rapid Solidification in L10 TiAl Compound Alloys" ASM Symposium Proceedings an 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 an Lattice Distortion and Atomic Ordering in L10 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 desired properties are disclosed in numerous patents and publications such as those mentioned above. In addition, U.S. Patent No. 5,489,411 discloses a powder metallurgy technique for producing titanium aluminide foil by plasma spraying a spoolable strip, heat treating the strip to relieve residual stresses, bringing the rough sides of two such strips together, and pressing the strips together between compression bonding rolls, followed by solution annealing, cold rolling, and intermediate annealing. U.S. Patent No. 4,917,858 discloses a powder metallurgy technique for producing titanium aluminide foil using elemental titanium, aluminum, and other alloying elements. U.S. Patent No. 5,634,992 discloses a method of processing a gamma titanium aluminide by solidifying a casting and heat treating the solidified casting above the eutectic point to form gamma grains and lamellar colonies of alpha and gamma phases, heat treating below the eutectic point to grow gamma grains within the colony structure, and heat treating below the alpha transus to reform any remaining colony structure to a structure having α₂. Streaks within gamma grains. US-A-5 417 781 discloses titanium aluminide articles containing elements such as chromium and/or manganese and a relatively small amount of tungsten.

Given the extensive efforts to improve the properties of titanium aluminides, there is still a need for improved alloy compositions and economical processing methods.

Summary of the invention

The invention defined by the alloy composition of claim 1 provides a two-phase titanium-aluminum alloy with lamellar microstructure controlled by colony size. The alloy may be provided in various forms such as in the cast, hot extruded, cold and hot worked or heat treated condition. As a final product, an electrical resistance heating element having a resistivity of 60 to 200 uΩ-cm may be made from the alloy. The alloy may contain additional elements which provide fine particles such as second phase or boride particles at colony boundaries. The alloy may contain equiaxed grain boundary structures. The alloying elements described within the definition of claim 1 may contain, for example, up to 10 atomic percent W, Nb and/or Mo. The alloy can be processed into a thin plate having a tensile strength of greater than 80 ksi (560 MPa), a ultimate tensile strength of greater than 90 ksi (630 MPa), and/or a tensile elongation of at least 1.5%. The aluminum can be present in an amount of 40 to 50 atomic percent, preferably about 46 atomic percent. The titanium can be present in an amount of at least 45 atomic percent, preferably at least 50 atomic percent. For example, the alloy can contain 45 to 55 atomic percent Ti, 40 to 50 atomic percent Al, 1 to 5 atomic percent Nb, 0.5 to 2 atomic percent W, and 0.1 to 0.3 atomic percent B. The alloy is free of Cr, V, Mn and/or Ni.

Short description of the drawings

Figures 1a-d show 200x magnification optical micrographs of PMTA-TiAl alloys hot extruded at 1400°C and annealed at 1000°C for 2 hours. Figure 1a shows the microstructure of PMTA-1, Figure 1b shows the microstructure of PMTA-2, Figure 1c shows the microstructure of PMTA-3 and Figure 1d shows the microstructure of PMTA-4;

Figures 2a-d show 500x magnification optical micrographs of PMTA alloys hot extruded at 1400°C and annealed at 1000°C for 2 hours. Figure 2a shows the microstructure of PMTA-1, Figure 2b shows the microstructure of PMAT-2, Figure 2c shows the microstructure of PMAT-3 and Figure 2d shows the microstructure of PMTA-4;

Fig. 3 represents phantom pattern bands observed in a backscattered image of PMTA-2 hot-extruded at 1400°C and annealed at 1000°C for 2 hours, showing the non-uniform distribution of W;

Fig. 4 shows a backscattered image of PMTA-2 hot extruded at 1400ºC and annealed at 1000ºC for 2 hours;

Fig. 5a is a 200x micrograph of PMTA-3 hot extruded at 1400°C and annealed at 1000°C for one day, and Fig. 5b shows the same microstructure at 500x magnification;

Fig. 6a shows the microstructure of PMTA-2 hot-extruded at 1400°C and annealed at 1000°C for 3 days at 200x magnification, and Fig. 6b shows the same microstructure at 500x magnification;

Fig. 7a is an optical micrograph of an as-received TiAl plate (Ti-45Al-5Cr, atomic %) and Fig. 7b shows the same microstructure after annealing at 1000ºC for 3 days; both micrographs are magnified 500x;

Fig. 8a represents a micrograph of PMTA-6 and Fig. 8b represents a micrograph of PMTA-7, both hot-extruded at 1380°C (200x magnification);

Fig. 9a is a micrograph of PMTA-6 and Fig. 9b is a micrograph of PMTA-7, both hot extruded at 1365ºC (200x magnification);

Fig. 10 is a micrograph showing abnormal grain growth in PMTA hot extruded at 1380ºC;

Figures 11a-d are micrographs of PMTA-8 heat treated under various conditions after hot extrusion at 1335°C, with the heat treatments including two hours at 1000°C (Fig. 11a), 30 minutes at 1340°C (Fig. 11b), 30 minutes at 1320°C (Fig. 11c), and 30 minutes at 1315°C (Fig. 11d) (200x magnification);

Figure 12 is a plot of resistivity in microohms versus temperature for Samples 1 and 2 cut from a block of nominal PMTA-4 composition;

Fig. 13 is a plot of hemisphere total emissivity versus temperature for Samples 1 and 2;

Fig. 14 is a plot of diffusion coefficient versus temperature for samples 80259-1, 80259-2 and 80259-3 cut from the same block as samples 1 and 2;

Fig. 15 is a graph of specific heat versus temperature for titanium aluminide according to the invention; and

Fig. 16 is a plot of thermal expansion versus temperature for samples 80259-1H, 80259-1C, 80259-2H, 80259-3H and 80259-3C cut from the same block as samples 1 and 2.

Detailed description of the preferred embodiments

The invention provides two-phase TiAl alloys with thermophysical and mechanical properties, which are useful for a variety of applications such as resistance heating elements. The alloys exhibit useful mechanical properties and corrosion resistance at elevated temperatures up to 1000ºC and above. The TiAl alloys have extremely low material density (about 4.0 g/cm³), a desirable combination of tensile ductility and strength at room temperature and elevated temperatures, high electrical resistivity and/or can be manufactured into sheet material with a thickness of < 10 mils. This sheet material can be used for resistance heating elements in devices such as cigarette lighters, among others. For example, the plate may be formed into a tubular heating element having a series of heating strips operated to light portions of a cigarette individually in an electric smoking device of the type disclosed in U.S. Patent Nos. 5,591,368 and 5,530,225, the disclosures of which are hereby incorporated by reference. In addition, the alloys may be free of 1 elements such as Cr, V, Mn and/or Ni.

Compared to TiAl alloys containing 1 to 4 atomic % Cr, V and/or Mn to improve tensile ductility at ambient temperatures, the tensile ductility of dual-phase TiAl alloys with lamellar structures according to the present invention can be controlled mainly by the colony size rather than such alloying elements. The invention thus provides high-strength TiAl alloys that may be free of Cr, V, Mn and/or Ni.

Table 1 lists the nominal compositions of alloys investigated, with the base alloy containing 46.5 atomic % Al and the balance Ti. Small amounts of alloying additives were added to investigate the effects on mechanical and metallurgical properties of the two-phase TiAl alloys. Nb was added in Amounts up to 4% were investigated for possible effects on oxidation resistance, W was investigated in amounts up to 1.0% for effects on microstructural stability and creep resistance, and Mo was investigated in amounts up to 0.5% for effects on hot fabrication. Boron was added in amounts up to 0.18% to refine lamellar structures in the dual-phase TiAl alloys.

Eight alloys, designated PMTA-1 through 9, having the compositions listed in Table 1, were prepared by arc melting and drip casting into a 1" diameter by 5" long copper mold using commercially pure metals. All alloys were successfully cast with no casting defects. Seven alloy ingots (PMTA-1 through 4 and 6-9) were then placed in Mo cans and hot extruded at 1335 to 1400ºC with a strain ratio of 5:1 to 6:1. The extrusion conditions are listed in Table 2. The cooling rate after extrusion was controlled by air cooling and quenching the extruded rods in water for a short period of time. The alloy rods extruded at 1365 to 1400ºC had an irregular shape, whereas the PMTA-8 hot-extruded at 1335ºC had a much smoother surface with no surface irregularities. However, no cracks were detected in any of the hot-extruded alloy rods.

The microstructures of the alloys were examined by optical metallography and electron superposition analysis in the as-cast and heat-treated conditions (listed in Table 2). In the as-cast condition, all alloys exhibited a lamellar structure with some degree of segregation and funneling. Figs. 1 and 2 show optical micrographs at 200 and 500x magnification, respectively, for the hot-extruded PMTA-1 to 4 alloys, which were stress-relaxed at 1000°C for 2 hours. All Alloys exhibited complete lamellar structures with a small amount of equiaxed grain structures at colony boundaries. Some fine particles were observed at colony boundaries, which were identified as borides by electron microprobe analysis. There is also no obvious difference in the microstructural features among these four PMTA alloys.

Electron microprobe analyses reveal that tungsten is not evenly distributed even in the hot-extruded alloys. As can be seen in Fig. 3, the darker contrasting phantom pattern bands are depleted by about 0.33 atomic % W. Fig. 4 is a backscattered image of PMTA-2 showing the formation of second-phase particles (borides) in brighter contrast at colony boundaries. The composition of the borides was determined and listed in Table 3 along with that of the lamellar matrix. The second-phase particles are essentially (Ti, W, Nb) borides that are decorated and pinned lamellar colony boundaries.

Figures 5 and 6 show optical microstructures of hot-extruded PMTA-3 and 2 annealed at 1000ºC for 1 day and 3 days, respectively. Equiaxed grain boundary structures are clearly visible in these long-time annealed samples, and the amount increases with annealing time at 1000ºC. A significant amount of equiaxed grain structures is present in the sample annealed at 1000ºC for 3 days.

For comparison, a 9 mil thick TiAl plate (Ti-45Al-5Cr, atomic %) was evaluated. Fig. 7 shows the optical microstructures of the TiAlCr plate both in the as-received and annealed (3 days at 1000°C) condition. In contrast to the dual-phase lamellar structure of the alloys of the invention, the TiAlCr plate has a duplex structure and its grain structure does not exhibit any significant Coarsening occurs at 1000ºC.

Tensile plate specimens 9-20 mils thick and 0.5 inch long were sectioned from the hot extruded alloy bars after annealing at 1000ºC for 2 hours using an EDM machine. Some of the specimens were reannealed at 1000ºC for up to 3 days prior to tensile testing. Tensile tests were conducted on an Instron testing machine at a load rate of 0.1 inch/second at room temperature. Table 4 lists the tensile test results.

All alloys relaxed at 1000ºC for 2 hours had a tensile elongation of 1% or more at room temperature in air. The tensile elongation was not affected when the specimen thickness was changed from 9 to 20 mils. As can be seen in Table 4, the PMTA-4 alloy appears to have the best tensile ductility of the 4 alloys. Note that a tensile elongation of 1.6% obtained in a 20 mil thick plate specimen corresponds to an elongation of 4% in a 0.12 inch diameter bar specimen. The tensile elongation appears to increase slightly with annealing time at 1000ºC, and the maximum ductility is obtained in the specimen annealed at 1000ºC for 1 day.

All alloys are extremely strong, with yield strengths in excess of 100 ksi (700 MPa) and ultimate tensile strengths in excess of 115 ksi (800 MPa) at room temperature. The high strength is due to the refined, fully lamellar structures created in these TiAl alloys. In comparison, the TiAlCr sheet material has a yield strength of only 61 ksi (420 MPa) at room temperature. The PMTA alloys are thus up to 67% stronger than the TiAlCr sheet. The PMTA alloys with 0.5% Mo exhibited significantly higher strengths, but slightly lower tensile elongation. at room temperature.

Figures 8a-b and 9a-b present optical micrographs of PMTA-6 and 7 hot extruded at 1380ºC and 1365ºC, respectively. Both alloys exhibited lamellar grain structures with few intercolony structures. Large colony grains (see Figure 10) were observed in both alloys hot extruded at 1380ºC and 1365ºC, which may have been the result of abnormal grain growth in the alloys containing low amounts of boron after hot extrusion. There are no significant differences in microstructural features between these two PMTA alloys.

Figures 11a-d show the effects of heat treatment on microstructures of PMTA-8 hot extruded at 1335ºC. The alloy extruded at 1335ºC had a much finer colony size and many more intercolony structures compared to those hot extruded at 1380ºC and 1365ºC. Heat treatment at 1000ºC for 2 hours did not result in any significant change in the extruded structure (Fig. 11a). However, heat treatment at 1340ºC for 30 minutes resulted in a much larger colony structure (Fig. 11b). Lowering the heat treatment temperature from 1340ºC to 1320-1325ºC (a difference of 20-25ºC) resulted in a large decrease in colony size, as can be seen in Figs. 11c and 11d. Annealing at 1320-1315ºC also appears to induce more intercolony structures in PMTA-8. The abnormal grain growth is almost completely eliminated by hot extrusion at 1335ºC.

Tensile plate specimens of PMTA-6 to 8 with a thickness between 8 and 22 mils and a length of 0.5 inch were cut from the hot extruded alloy bars after a final heat treatment of 2 hours at 1000ºC or 20 minutes at 1320-1315ºC using an EDM machine. Tensile tests were performed on an Instron testing machine with at a loading rate of 0.1 inch at temperatures up to 800ºC in air. All tensile test results are shown in Tables 5 through 8. Alloys PMTA-4, -6 and -7, heat treated at 1000ºC for 2 hours, exhibited excellent strength at all temperatures, regardless of hot extrusion temperature. Hot extrusion at 1400-1365ºC produces low tensile ductility (< 4%) at room temperature and at elevated temperatures. A significant increase in tensile ductility is obtained at all temperatures when hot extrusion takes place at 1335ºC. PMTA-8, hot extruded at 1335ºC, exhibited the highest strength and tensile ductility at all test temperatures. There appeared to be no systematic change in tensile ductility as the specimen thickness changed from 8 to 22 mils.

Tables 7 and 8 also show the tensile properties of PMTA-6 and 7 heat treated at 1320ºC and 1315ºC for 20 minutes, respectively. Compared to the results from the heat treatment at 1000ºC, the heat treatment at 1320-1315ºC resulted in higher tensile elongation but lower strength at the test temperatures. Among all the alloys and heat treatments, PMTA-8 hot extruded at 1335ºC and annealed at 1315ºC for 20 minutes had the best tensile ductility at room temperature and elevated temperatures. This alloy had tensile ductility of 3.3% and 11.7% at room temperature and 800ºC, respectively. PMTA-8, heat treated at 1315ºC, appears to be significantly stronger than known TiAl alloys.

In an attempt to demonstrate the bending ductility of TiAl sheet material, several pieces of 11 to 20 mil PMTA-7 and PMTA-8 alloy sheet, produced by hot extrusion and heat treating at 1320ºC, were bent at room temperature. None of the Alloy pieces broke after bending at 42º. These results clearly demonstrate that PMTA alloys with a controlled microstructure are bendable at room temperature.

The oxidation behavior of PMTA-2, -5 and -7 was investigated by exposing plate samples (9-20 mils thick) to 800ºC in air. The samples were periodically removed from the furnaces for weight measurements and surface examinations. The samples showed very little weight gain without any signs of spalling. It appears that the alloying additions W and Nb affect the oxidation rate of the alloys at 800ºC, with W being more effective in increasing the oxidation resistance of TiAl alloys. Among the alloys, PMTA-7 shows the least weight gain and the best oxidation resistance at 800ºC. The oxidation of PMTA-7 showed that oxide deposits completely adhere without any signs of microcracks and spalling. These observations clearly indicate that the oxide deposits formed at 800ºC adhere well to the base material and provide a high level of protection.

Figure 12 is a plot of resistivity in microohms versus temperature for Samples 1 and 2 cut from a block of nominal PMTA-4 composition, i.e., 30.8 wt% Al, 7.1 wt% Nb, 2.4 wt% W, and 0.045 wt% B. Figure 13 is a plot of hemisphere total emissivity versus temperature for Samples 1 and 2; Figure 14 is a plot of diffusion coefficient versus temperature for Samples 80259-1, 80259-2, and 80259-3 cut from the same block as Samples 1 and 2. Fig. 15 is a graph of specific heat versus temperature for titanium aluminide according to the invention and Fig. 16 is a graph of thermal expansion versus temperature for samples 80259-1H, 80259-1C, 80259-2H, 80259-3H and 80259-3C, cut from the same block as samples 1 and 2.

In summary, the hot PMTA alloys extruded at 1365-1400ºC exhibited mainly lamellar structures with few intercolony structures, whereas PMTA-8 extruded at 1335ºC had much finer colony structures and more intercolony structures. Heat treatment of PMTA-8 at 1315-1320ºC for 20 min resulted in fine lamellar structures. The alloys may contain (Ti, W, Nb) borides formed at colony boundaries. Furthermore, tungsten is not evenly distributed in the hot extruded alloys, suggesting the possibility of high electrical resistivity of TiAl alloys with W additions. The inclusion of 0.5 at.% Mo significantly increases the elongation strength and ultimate tensile strength of the TiAl alloys, but reduces the tensile elongation to some extent at room temperature. Of the four hot-extruded PMTA 1-4 alloys, PMTA-4 with alloy composition Ti-46.5 Al-3 Nb-0.5 W-0.2 B (atomic %) has the best combination of tensile ductility and strength at room temperature. Compared to TiAlCr sheet material (Ti-45 Al-5Cr), PMTA-4 is 67% stronger than TiAlCr sheet. In addition, TiAlCr sheet showed no bending ductility at room temperature, while PMTA-4 showed 1.4% elongation. The tensile elongation of TiAl alloys is independent of sheet thickness in the range of 9 to 20 mils. PMTA 4, 6 and 7 alloys, heat treated at 1000ºC for 2 hours, showed excellent strength at all temperatures up to 800ºC, regardless of hot extrusion temperature. However, hot extrusion temperatures of 1400-1365ºC provide lower tensile ductilities (< 4%) at room temperature and elevated temperatures. A significant increase in tensile ductility is achieved at all temperatures when the extrusion temperature is 1335ºC. At 1335ºC PMTA-8 (Ti-46.5 Al-3 Nb-1W-0.5B) hot extruded and annealed at 1315ºC for 20 minutes showed the best tensile ductility at room temperature and elevated temperatures (3.3% at room temperature and 11.7% at 800ºC). Table 1. Nominal alloy compositions Table 2. Manufacturing and heat treatment conditions for PMTA alloys Table 3. Phase compositions in PMTA-2 alloy determined by electron microprobe analysis

* only metal elements Table 4. Tensile properties of PMTA alloys hot extruded at 1400ºC and tested at room temperature Table 5. Tensile properties of PMTA-4 hot extruded at 1400ºC and annealed at 1000ºC for 2 hours Table 6. Tensile properties of PMTA-6 hot extruded at 1365ºC and annealed at 1000ºC for 2 hours Table 7. Tensile properties of PMTA-7 hot extruded at 1365ºC Table 8. Tensile properties of PMTA-8 hot extruded at 1335ºC

The titanium aluminide described above can be made into various shapes or products such as electrical resistance heating elements. However, the compositions disclosed herein can also be used for other purposes such as thermal spray applications where they could be used as coatings with oxidation and corrosion resistance. In addition, the compositions could be used for oxidation and corrosion resistant electrodes, furnace components, chemical reactors, sulfidation resistant materials, corrosion resistant materials for use in the chemical industry, piping for conveying coal slurry or coal tar, substrate materials for catalysts, exhaust walls and turbocharger rotors for automotive and diesel engines, porous filters, etc.

For resistance heating elements, the geometry of the heating element blades can be varied to optimize the heating resistance according to the formula R = ρ (L/W · T); where R = resistance of the heater, ρ = specific resistance of the heater material, L = length of heater, W = width of heater and T = thickness of heater. The resistivity of the heater material can be varied by compositional changes such as adjusting the aluminum content of the heater material, treating or incorporating alloying additives. The resistivity can be increased substantially by, for example, incorporating aluminum oxide particles into the heater material. The heater material may contain ceramic particles if required to improve creep resistance and/or thermal conductivity. The heater material may contain, for example, particles or fibers of electrically conductive material such as nitrides of transition metals (Zr, Ti, Hf), carbides of transition metals, borides of transition metals and MoSi₂ to provide good high temperature creep resistance up to 1200ºC and excellent oxidation resistance. The heating material may also contain particles of electrically insulating material such as Al₂O₃, Y₂O₃, Si₃N₄, ZrO₂ to make the heating material creep resistant at high temperature and also to increase the thermal conductivity and/or reduce the thermal expansion coefficient of the heating material. The electrically insulating/conductive particles/fibers may be added to a powder mixture of Fe, Al, Ti or iron aluminide, or such particles/fibers may be obtained by reaction synthesis of elemental powders which react exothermically during the manufacture of the heating element.

The principles, preferred embodiments and modes of operation of the present invention have been described above. However, the invention should not be construed as limited to the specific embodiments discussed. The embodiments described above are to be considered illustrative rather than restrictive, and it is to be understood that those skilled in the art may modify these embodiments without departing from the scope of the present invention as defined in the following claims.

Claims (19)

1. A two-phase titanium aluminide alloy comprising, in, wt%, 50 to 65% Ti, 25 to 35% Al, 2 to 15% Nb, less than 5% Mo, 1 to 10% W and 0.01 to 0.2% B. 2. Titanium aluminide alloy according to claim 1, in the cast, hot extruded, cold worked or heat treated state. 3. A titanium aluminide alloy according to claim 1, wherein the alloy has a two-phase lamellar microstructure, with fine particles located at colony boundaries. 4. Titanium aluminide alloy according to claim 3, wherein fine boride particles are located at colony boundaries. 5. Titanium aluminide alloy according to claim 3, wherein fine second phase particles are located at colony boundaries. 6. Titanium aluminide alloy according to claim 1, wherein the alloy has a two-phase microstructure with equiaxed grain boundary structures. 7. Titanium aluminide alloy according to claim 1, wherein the Ti content is between 57 and 60%, the Al content is between 30 and 32%, the Nb content is between 4 and 9%, the Mo content is at most 2%, the W content is between 2 and 8% and the B content is between 0.02 and 0.08%. 8. The titanium aluminide alloy of claim 1 having a yield strength of greater than 80 ksi (560 MPa), a tensile strength of greater than 90 ksi (680 MPa), and/or a tensile elongation of at least 1%. 9. The titanium aluminide alloy of claim 1, wherein the alloy has a microstructure in which W is non-uniformly distributed. 10. Titanium aluminide alloy according to claim 1, wherein aluminum is present in a lenge of about 46 to 47 atomic %. 11. A titanium aluminide alloy according to claim 1, wherein the alloy has a lamellar microstructure which is substantially free of equiaxed structures at colony boundaries. 12. Titanium aluminide alloy according to claim 1, wherein the alloy does not contain Mo or Cr. 13. Titanium aluminide alloy according to claim 1, wherein the Ti content is 57 to 60%, the Al content is 30 to 32%, the Nb content is 4 to 9%, the W content is 2 to 8% and the β content is 0.02 to 0.08%. 14. Titanium aluminide alloy according to claim 1, including 45 to 55 atomic % Ti, 40 to 50 atomic % Al, 1 to 5 atomic % Nb, 0.3 to 1.5 atomic % W and 0.1 to 0.3 atomic % B. 15. The titanium aluminide alloy of claim 1 comprising a layer having a thickness of 8 to 30 mils. 16. Titanium aluminide alloy according to claim 1, free from Cr, V, Mn, Co, Cu and Ni. 17. Titanium aluminide alloy according to claim 1, comprising TiAl with 2 to 4 atomic % Nb, ≤ 1 atomic % Mo and 0.5 to 2 atomic % W, 0.1 to 0.3 atomic % B. 18. Titanium aluminide alloy according to claim 1, including 1 to 4 atomic % Nb, ≤ 1 atomic % Mo and 0.25 to 2 atomic % W. 19. A titanium aluminide alloy according to claim 1, wherein the alloy is formed into an electrical resistance heating element capable of heating to 900°C in less than 1 second when a voltage of up to 10 volts and up to 6 amps is passed through the heating element.