Abstract:
A sintering process for producing an aluminide by reacting a first powder with a second powder, the first powder comprising M x Al y  wherein M is Fe, Ni or Ti, x≧1, y≧1 and x&gt;y or y&gt;x and the second powder comprises pure M or M alloy powder. Iron aluminides such as Fe 3 Al, FeAl or alloys thereof can be made by reacting powders of one or more of Fe 3 Al, FeAl 3 , FeAl 2 , Fe 2 Al 5  or alloys thereof with pure iron or an iron alloy. Nickel aluminides such as Ni 3 Al or NiAl or alloys thereof can be made by reacting powders of one or more of NiAl 3 , Ni 2 Al 3 , Ni 3 Al 2 , Ni 5 Al 3  or alloys thereof with pure Ni or a Ni alloy powder. Titanium aluminides such as Ti 3 Al, TiAl or alloys thereof can be made by reacting one or more of TiAl 3 , TiAl 2  or alloys thereof with pure Ti or Ti alloy powder. The process provides a more dense product by solid state reaction of an intermediate intermetallic compound with a component of the final aluminide compact. As a result of the process, the final density can be increased to at least 98% of the theoretical density. Products which can be made by the process include worked products such as rolled sheet, extruded shapes such as tube, drawn products such as wire or bar, or molded/forged products such as fuel injection nozzles.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to improvements in powder processing of aluminides.  
         BACKGROUND OF THE INVENTION  
         [0002]    Iron base alloys containing aluminum can have ordered and disordered body centered crystal structures. For instance, iron aluminide alloys having intermetallic alloy compositions contain iron and aluminum in various atomic proportions such as Fe 3 Al, FeAl, FeAl 2 , FeAl 3 , and Fe 2 Al 5 . Fe 3 Al intermetallic iron aluminides having a body centered cubic ordered crystal structure are disclosed in U.S. Pat. Nos. 5,320,802; 5,158,744; 5,024,109; and 4,961,903. Various articles on iron aluminides are incorporated in a special issue of Materials Science and Engineering A, Vol. A258 (1998) edited by Deevi et al. which includes papers presented at a conference on Iron Aluminides: Alloy Design, Processing, Properties and Applications held Feb. 15-19, 1998 in San Antonio, Tex.  
           [0003]    A 1990 publication in Advances in Powder Metallurgy, Vol. 2, by J. R. Knibloe et al., entitled “Microstructure And Mechanical Properties of P/M Fe 3 Al Alloys”, pp. 219-231, discloses a powder metallurgical process for preparing Fe 3 Al containing 2 and 5% Cr by using an inert gas atomizer. This publication explains that Fe 3 Al alloys have a DO 3  structure at low temperatures and transform to a B2 structure above about 550° C. To make sheet, the powders were canned in mild steel, evacuated and hot extruded at 1000° C. to an area reduction ratio of 9:1.  
           [0004]    A 1991 publication in Mat. Res. Soc. Symp. Proc., Vol. 213, by V. K. Sikka entitled “Powder Processing of Fe 3 Al-Based Iron-Aluminide Alloys,” pp. 901-906, discloses a process of preparing 2 and 5% Cr containing Fe 3 Al-based iron-aluminide powders fabricated into sheet. This publication states that the powders were prepared by nitrogen-gas atomization and argon-gas atomization. The nitrogen-gas atomized powders had low levels of oxygen (130 ppm) and nitrogen (30 ppm). To make sheet, the powders were canned in mild steel and hot extruded at 1000° C. to an area reduction ratio of 9:1.  
           [0005]    A paper by V. K. Sikka et al., entitled “Powder Production, Processing, and Properties of Fe 3 Al”, pp. 1-11, presented at the 1990 Powder Metallurgy Conference Exhibition in Pittsburgh, Pa., discloses a process of preparing Fe 3 Al powder by melting constituent metals under a protective atmosphere, passing the metal through a metering nozzle and disintegrating the melt by impingement of the melt stream with nitrogen atomizing gas. An extruded bar was produced by filling a 76 mm mild steel can with the powder, evacuating the can, heating 1½ hour at 1000° C. and extruding the can through a 25 mm die for a 9:1 reduction. A sheet 0.76 mm thick was produced by removing the can, forging 50% at 1000° C., rolling 50% at 850° C. and finish rolling 50% at 650° C.  
           [0006]    A publication by D. J. Gaydosh et al., entitled “Microstructure and Tensile Properties of Fe-40 At.Pct. Al Alloys with C, Zr, Hf and B Additions” in the September 1989 Met. Trans A, Vol. 20A, pp. 1701-1714, discloses hot extrusion of gas-atomized powder wherein the powder either includes C, Zr and Hf as prealloyed additions or B is added to a previously prepared iron-aluminum powder.  
           [0007]    A publication by C. G. McKamey et al., entitled “A review of recent developments in Fe 3 Al-based Alloys” in the August 1991 J. of Mater. Res., Vol. 6, No. 8, pp. 1779-1805, discloses techniques for obtaining iron-aluminide powders by inert gas atomization and preparing ternary alloy powders based on Fe 3 Al by mixing alloy powders to produce the desired alloy composition and consolidating by hot extrusion, i.e., preparation of Fe 3 Al-based powders by nitrogen- or argon-gas atomization and consolidation to full density by extruding at 1000° C. to an area reduction of &lt;9:1.  
           [0008]    U.S. Pat. Nos. 4,917,858; 5,269,830; and 5,455,001 disclose powder metallurgical techniques for preparation of intermetallic compositions by (1) rolling blended powder into green foil, sintering and pressing the foil to full density, (2) reactive sintering of Fe and Al powders to form iron aluminide or by preparing Ni-B-Al and Ni-B-Ni composite powders by electroless plating, canning the powder in a tube, heat treating the canned powder, cold rolling the tube-canned powder and heat treating the cold rolled powder to obtain an intermetallic compound.  
           [0009]    U.S. Pat. No. 5,484,568 discloses a powder metallurgical technique for preparing heating elements by micropyretic synthesis wherein a combustion wave converts reactants to a desired product. In this process, a filler material, a reactive system and a plasticizer are formed into a slurry and shaped by plastic extrusion, slip casting or coating followed by combusting the shape by ignition.  
           [0010]    U.S. Pat. Nos. 5,098,469 and 5,269,830 disclose techniques for preparing intermetallic alloy compositions by powder metallurgical techniques which include pressureless sintering. The &#39;469 patent discloses a four step pressureless sintering process for producing Ni-Al-Ti intermetallic aluminide alloys wherein a compact of nickel powder and prealloyed aluminide powder is heated without cool down steps and with a heating rate of 10° C. per minute between the processing steps. The &#39;830 patent discloses a pressureless sintering process for producing Fe 3 Al and FeAl compounds wherein elemental powders of iron and aluminum are heated under conditions of temperature and pressure to produce an exothermic reaction and densification is achieved by sintering in vacuum or by pressure assisted densification by heating during compression. According to the &#39;830 patent, pressureless sintering achieves near 75% of full density.  
           [0011]    U.S. Pat. No. 5,768,679 discloses a powder metallurgical technique for making a TiAl intermetallic compound by preparing a mixture of materials selected from Ti, Ti alloys, Al, Al alloys and TiAl compounds, and sintering the mixture.  
           [0012]    U.S. Pat. No. 5,950,063 discloses a method of making powder injection molded parts by sintering a mixture of a powder and binder, wherein the powder can be ceramic, metallic and/or intermetallic and the metallic powder can be a mixture of prealloyed powder and an elemental/semi-elemental powder or a prealloyed powder and an elemental/master-alloy powder.  
           [0013]    U.S. Pat. No. 4,762,558 discloses a reactive sintering process for producing Ni 3 Al by compacting elemental Ni and Al powders and vacuum sintering the powders through an exothermic reaction. U.S. Pat. No. 2,755,184 also discloses a technique for making Ni 3 Al but by using a precursor compound NiAl and elemental Ni powders. Likewise, U.S. Pat. No. 5,905,937 discloses a powder technique for making Ni 3 Al by sintering a mixture of a brittle nickel aluminide powder with elemental Ni.  
           [0014]    A porous aluminide structure is disclosed in U.S. Pat. No. 4,990,181. The &#39;181 patent discloses a porous sintered aluminide made by mixing the aluminide with a metal or aluminide as a prealloyed powder with an organic binder and solvent, burning out carbon in the mixture and sintering to form the porous sintered product.  
           [0015]    Sintered articles made from powder mixtures including ferrous metal compositions including FeAl powder (50 wt. % Al and 50 wt. % Fe) are disclosed in U.S. Pat. Nos. 4,758,272; 4,992,233; and 5,864,071. Of these, the &#39;272 and &#39;233 patents seek to produce a porous body having 5-50 wt. % Al by sintering binder, FeAl powder and Fe powder and the &#39;071 patent seeks to produce a ferrous metal composition having 0.5-5 wt. % Al by sintering FeAl and low alloy or stainless steel compositions.  
           [0016]    Based on the foregoing, there is a need in the art for an economical technique for preparing intermetallic compositions such as iron, titanium or nickel aluminides. For instance, conventional powder metallurgical techniques of preparing iron-aluminides include melting iron and aluminum and inert gas atomizing the melt to form an iron-aluminide powder, canning the powder and working the canned material at elevated temperatures or reaction synthesis can be used to react elemental powders of iron and aluminum. It would be desirable if iron-aluminide could be prepared by a powder metallurgical technique wherein it is not necessary to can the powder and wherein it is not necessary to subject the iron and aluminum to any hot working steps in order to form an iron-aluminide product such as a sheet. It would be further desirable to increase the sintered density of aluminide compacts.  
         SUMMARY OF THE INVENTION  
         [0017]    The invention provides a method of manufacturing a sintered aluminide compact by a powder metallurgical technique, comprising steps of forming a powder mixture comprising a first powder of Me x Al y  or alloy thereof wherein M is Fe, Ti or Ni, x≧1, y≧1, x&gt;y or y&gt;x and a second powder comprising M or alloy thereof, and heating the powder mixture so as to react the first powder with the second powder to form a sintered aluminide compact.  
           [0018]    In preparing an FeAl or Fe 3 Al iron aluminide compact, the first powder can comprise one or more materials selected from Fe 2 Al 5 , FeAl 3 , FeAl 2 , Fe 3 Al or alloys thereof and the second powder can comprise one or more materials selected from FeAl, Fe 2 Al 5 , FeAl 3 , FeAl 2 , Fe 3 Al or alloys thereof and/or Fe or an iron base alloy powder. For example, during the heating step, Fe 2 Al 5  can be reacted with Fe to form FeAl or Fe 3 Al. By adjusting the contents of aluminum, iron and optional alloying additions in the powder mixture, it is possible to form a sintered compact consisting of FeAl or Fe 3 Al or alloys thereof. The heating step is preferably carried out in a vacuum or inert gas (e.g., Ar, He, N 2 , etc.) environment such that expansion of the sintered compact due to volume change during formation of the FeAl or Fe 3 Al is less than 10% and/or the FeAl or Fe 3 Al initially forms as a layer between the iron or iron base alloy powder and the Fe 2 Al 5 . In a preferred process, the powder mixture is heated at a heating rate of less than 15° C./min and/or the sintered compact is heated sufficiently to increase the density of the sintered compact to at least 98% of the theoretical density. The process can include a step of pressing the powder mixture into a shaped article such as a molded part or a worked article such as a sheet. According to the process, reactions which can sequentially occur during the heating steps include the initial formation of FeAl or Fe 3 Al by an interfacial reaction between pure Fe or an iron base alloy powder and Fe x Al y , and the balance of the FeAl or Fe 3 Al is formed by solid state diffusion.  
           [0019]    In preparing an NiAl or Ni 3 Al iron aluminide compact, the first powder can comprise one or more materials selected from Ni 2 Al 3 , Ni 3 Al 2 , Ni 5 Al 3 , NiAl 3  or alloys thereof and the second powder can comprise one or more materials selected from Ni 2 Al 3 , Ni 3 Al 2 , Ni 5 Al 3 , NiAl 3  or alloys thereof and/or Ni or Ni base alloy powder. For example, during the heating step, NiAl 3  can be reacted with Ni to form NiAl or Ni 3 Al. By adjusting the contents of aluminum, Ni and optional alloying additions in the powder mixture, it is possible to form a sintered compact consisting of NiAl or Ni 3 Al or alloys thereof. The heating step is preferably carried out in a vacuum or inert gas (e.g., Ar, He, N 2 , etc.) environment such that expansion of the sintered compact due to volume change during formation of the NiAl or Ni 3 Al is less than 10% and/or the NiAl or Ni 3 Al initially forms as a layer between the Ni or Ni base alloy powder and the Ni x Al y . In a preferred process, the powder mixture is heated at a heating rate of less than 15° C./min and/or the sintered compact is heated sufficiently to increase the density of the sintered compact to at least 98% of the theoretical density. The process can include a step of pressing the powder mixture into a shaped article such as a molded part or a worked article such as a sheet.  
           [0020]    In preparing an TiAl or Ti 3 Al iron aluminide compact, the first powder can comprise one or more materials selected from TiAl 2 , TiAl 3  or alloys thereof and the second powder can comprise one or more materials selected from TiAl 2 , TiAl 3  or alloys thereof and/or Ti or Ti base alloy powder. For example, during the heating step, TiAl 3  can be reacted with Ti to form TiAl or Ti 3 Al. By adjusting the contents of aluminum, Ti and optional alloying additions in the powder mixture, it is possible to form a sintered compact consisting of TiAl or Ti 3 Al or alloys thereof. The heating step is preferably carried out in a vacuum or inert gas (e.g., Ar, He, N 2 , etc.) environment such that expansion of the sintered compact due to volume change during formation of the TiAl or Ti 3 Al is less than 10% and/or the TiAl or Ti 3 Al initially forms as a layer between the Ti or Ti base alloy powder and the Ti x Al y . In a preferred process, the powder mixture is heated at a heating rate of less than 15° C./min and/or the sintered compact is heated sufficiently to increase the density of the sintered compact to at least 98% of the theoretical density. The process can include a step of pressing the powder mixture into a shaped article such as a molded part or a worked article such as a sheet. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    The invention provides a powder metallurgical technique for making high strength, high density powder products of aluminides which can be designed to have excellent strength and ductility at room temperature as well as high tensile and creep strengths at elevated temperatures. The sintering process according to the invention is an improvement over conventional processes wherein water or gas atomized prealloyed powders are combined with a binder and the sintered product includes porosity due to binder burnout with the result that the product attains a maximum density of around 90% of theoretical density. According to the invention it is possible to obtain densities of at least 95%, preferably at least 98% or higher.  
         [0022]    According to the invention, powder products are manufactured by a powder metallurgical technique which provides a highly dense aluminide. In the process, a powder mixture of the aluminide can be formed into continuous shapes such as sheet, tube, rod, wire or extruded shapes. For instance, the powder mixture can be formed into continuous sheet by tape casting or roll compaction. Alternatively, the powder mixture can be formed into desired shapes by die compaction or injection molding. The powder mixture preferably includes powder in the form of an intermediate aluminum-containing intermetallic compound to avoid thermal expansion associated with the formation of intermediate aluminum-containing intermetallic compounds during sintering of the powder mixture. Such thermal expansion can produce Kirkendall porosity which results in lowered density during the sintering process. Thus, because the powder mixture starts off with an intermediate aluminum-containing intermetallic compound, the powder mixture can be sintered to higher density than in the case of using elemental Fe, Ti or Ni and aluminum powders.  
         [0023]    According to one embodiment of the invention wherein an iron aluminide is produced, it is possible to include alloying additions such as pure iron powder and/or prealloyed iron or aluminum base powders to achieve a desired iron aluminide composition such as Fe 3 Al or FeAl (36 to 50 at % Al) or alloys thereof. The powders can optionally include conventional binder materials to aid in molding of the powder mixture. Alternatively, the powder mixture can be binder-free to reduce the porosity otherwise associated with porosity formed during binder burnout. Also, the powder mixture can be free of elemental aluminum to avoid the volume expansion which otherwise may occur during formation of intermediate iron-aluminum intermetallic compounds during the sintering step. In the case of a binder-free composition, it is advantageous to include elemental iron powder to improve compaction of the powder mixture.  
         [0024]    Of the iron aluminides, FeAl has a B2 structure and exists over a wide range of Al concentrations at room temperature (36 to˜50 atomic %). Iron aluminides based on FeAl exhibit better oxidation, carburization and sulfidation resistance than Fe 3 Al alloys and have lower densities compared to the steels and commercial iron based alloys, offering better strength-to weight ratio. In addition, FeAl exhibits high electrical resistivity in the range of 130 to 170 μΩ-cm as compared to many of the commercial metallic heating elements. These properties allow them to be considered as high temperature structural materials, gas filters, heating elements, and as fasteners.  
         [0025]    According to one embodiment of the present invention, M x Al y  or alloy thereof is prepared by a sintering process. Sintering is useful for forming precision, high-performance products operating in demanding applications such as automotive engines, aerospace hardware, manufacturing tools and electronic components. Sintering delivers net shape processing, uses limited material, and eliminates deformation processing and machining of the components. It also allows microstructural control of the product. After shaping the powder into compacts, the compacts are heated to elevated temperatures (approximately one-half of the absolute melting temperature) to bond the particles and increase the strength.  
         [0026]    In forming binary iron aluminide or iron aluminide alloy compositions, various iron containing powders can be mixed together and reacted to form the iron aluminide. For example, Fe or its alloy can be reacted to form FeAl, Fe 3 Al can be reacted with FeAl 2  to form FeAl, FeAl 2  can be reacted with Fe to form FeAl or Fe 3 Al, Fe 2 Al 5  can be reacted with Fe to form FeAl or Fe 3 Al, and FeAl 3  can be reacted with Fe to form FeAl or Fe 3 Al. These are just a few examples of using high aluminum intermetallics with low melting points to form a valuable high melting point intermetallic.  
         [0027]    In forming binary Ni aluminide or Ni aluminide alloy compositions, various Ni containing powders can be mixed together and reacted to form the nickel aluminide. For example, Ni 2 Al 3  can be reacted with Ni 3 Al 2 , Ni 5 Al 3  or NiAl 3  to form NiAl or Ni 3 Al, Ni 3 Al 2  can be reacted with Ni 5 Al 3  or NiAl 3  to form NiAl, Ni 5 Al 3  can be reacted with NiAl 3  to form NiAl or Ni 3 Al, etc.  
         [0028]    In forming binary Ti aluminide or Ti aluminide alloy compositions, various Ti containing powders can be mixed together and reacted to form the Ti aluminide. For example, Ti can be reacted with TiAl 2  or TiAl 3  to form TiAl or Ti 3 Al, TiAl 2  can be reacted with TiAl 3  to form TiAl, etc.  
         [0029]    The bonds between the particles grow by various mechanisms, which occur at the atomic level. Common mechanisms for metal bonding are solid-state diffusion and liquid state sintering (with liquid phase present during the process). Classical sintering processes include several stages: contact formation, neck growth, pore rounding and pore closure, and final densification of the product.  
         [0030]    Many powder metallurgical processes use prealloyed powders as a starting mixture for dense intermetallic production, which can be obtained by atomization or mechanical alloying. Further consolidation (sintering) involves the use of complex and costly processes based on hot isostatic pressing or hot extrusion. Therefore, it would be desirable to develop low cost processing methods for intermetallic products. The invention provides a novel processing technique which involves use of an intermediate iron-aluminum intermetallic compound such as Fe 2 Al 5 , an aluminum-containing powder which undergoes reaction with iron to produce FeAl during sintering. By varying the iron/aluminum ratio in the composition to be sintered, the same technique can be used to form Fe 3 Al compacts.  
         [0031]    The process according to one embodiment of the invention is an improvement over the conventional reaction synthesis processes wherein heating of elemental iron and aluminum powders is accompanied by heat generation due to an exothermic reaction between the mixed powders. It has been reported that the main disadvantage of this process is the large porosity of the final products. To eliminate porosity, some researchers have reported the use of the application of pressure during the combustion or sintering, which increases the complexity of the process. Such large porosity can be avoided according to the process of the invention wherein an intermediate intermetallic composition is used to form value added FeAl or Fe 3 Al intermetallic alloys.  
         [0032]    According to one aspect of the present invention, pressureless sintering can be used to from the sintered aluminide products such as FeAl, Fe 3 Al, NiAl, Ni 3 Al, TiAl, Ti 3 Al or alloys thereof. Pressureless sintering is based on the thermal bonding of the particles into the solid structure without the assistance of the pressure and is widely used by automotive industry.  
         [0033]    According to the invention it is possible to prepare aluminide composites with minimum or no melting of aluminum. In particular, by providing the aluminum content of the final compact in the form of an intermediate compound M x Al y  such as Fe 2 Al 5  or alloy thereof it is possible to form a sintered aluminide compact by a solid state process wherein the components of the aluminide undergo an interfacial reaction, e.g., an interfacial reaction between Fe and Fe 2 Al 5  or alloys thereof to form FeAl or Fe 3 Al. This reaction may be accompanied by the formation of voids which provide an escape path for volatile impurities present in the original powder. After formation of a small amount of the FeAl or Fe 3 Al phase, the dominant process is solid state diffusion. At 1000° C., formation of the desired FeAl or Fe 3 Al phase can be completed. However, a controlled thermal expansion or swelling of the compact may occur up to 1150° C. followed by the sintering.  
         [0034]    After synthesizing of 100% FeAl, Fe 3 Al or alloy thereof, the next step is the densification of the compacts involving diffusion, which is driven by a reduction of the surface area. Final densification of the samples depends on the expansion, which occurs prior to the complete formation of FeAl or Fe 3 Al. Densification can be expected to start at 1150° C. and rapidly increase from 1200° C.  
         [0035]    The sintering limit can be achieved faster at higher (e.g., 1350° C.) temperatures. The same densification can be obtained at the lower temperatures (e.g., 1200° C.), but with longer heating times. Accordingly, high temperatures on the order of 1200-1350° C. can be used to achieve densities of ˜98% and above.  
         [0036]    One of the challenges in Fe-Al sintering is to reduce the large pores left by diffusion of aluminum. According to sintering theory, early in the sintering process the pores remain attached to the grain boundaries and as the temperature increases the rate of grain boundary motion increases. After isolation of the pores from each other and further shrinkage, the grain boundaries break away from the pores leaving them trapped in the interior of the grains. Generally, pores at the grain interior shrink much slower than pores on the grain boundary. Separation of the pores from the boundaries thus limits the final density. As such, it is desirable to avoid the formation of pores inside the grains. According to the present invention, pore formation and location can be minimized by using an intermediate intermetallic compound as a precursor powder in the sintering process.  
         [0037]    In preparing powders for use in the process according to the present invention, fine powders on the order of 0.1 to 40 μm, preferably 2 to 20 μm can be prepared by an atomization technique wherein the nozzle configuration and/or pressure of atomization is modified to achieve the fine powder size. Also, nanosized powders can be incorporated in the powder mixture to improve compaction of the powder mixture. Conventional powder metallurgical techniques use powder mixtures wherein the powders are 40 to 150 μm in size. By using finer sized powders in the process according to the invention, it is possible to achieve a smoother surface in the final sintered product and thereby reduce production cost by obviating the need for additional machining of the sintered part.  
         [0038]    According to a second embodiment of the invention, an intermetallic alloy composition is formed into a worked product such as a sheet by consolidating a mixture of M x Al y  or alloy thereof with M or M alloy powder, rolling and heat treating the rolled sheet. The invention overcomes porosity problems associated with working intermetallic alloys made by tape casting or roll compacting prealloyed powders.  
         [0039]    According to this embodiment, a sheet having an intermetallic alloy composition is prepared by a powder metallurgical technique wherein a non-densified metal sheet is formed by consolidating a powder mixture having elements in amounts which form an intermetallic alloy composition upon sintering thereof. The non-densified sheet can be debindered (if the powder mixture contains binders), react the M x Al with the M or M alloy powder to form the desired aluminide or aluminide alloy and sinter the powders and thus increase the density of the sheet prior to or after rolling the non-densified sheet. A rolled sheet is formed by hot and/or cold rolling the densified or non-densified metal sheet so as to reduce the thickness thereof, and the rolled sheet is heat treated to further sinter, anneal and stress relieve the sheet. For example, a sheet or strip can be formed by hot rolling or cold rolling the sintered powders in one or more passes to a final desired thickness with at least one heat treating step such as a sintering, annealing or stress relief heat treatment. This technique can be employed to manufacture a sheet of Fe 3 Al or an FeAl alloy with 6-32 atomic % Al, preferably 6-26 at. % Al. Nickel aluminide and titanium aluminide intermetallic alloys can be made in like manner. If desired, the heat treatment can be carried out by passing the sheet or strip through a furnace arrangement which heats the strip to a desired temperature such as up to the melting point of one or more constituents of the aluminide, e.g., flash annealing as described in commonly owned U.S. Pat. No. 6,143,241, the disclosure of which is hereby incorporated by reference.  
         [0040]    The foregoing process provides a simple and economic manufacturing technique for preparing intermetallic alloy materials such as iron, nickel or titanium aluminides which are known to have poor ductility and high work hardening potential at room temperature.  
         [0041]    In roll compacting the powder mixture, water or polymer atomized powder is preferred over gas atomized powder for subsequent roll compaction since the irregularly shaped surfaces of the water atomized powder provide better mechanical interlocking than the spherical powder obtained from gas atomization. Details of the roll compaction process can be found in commonly owned U.S. Pat. No. 6,030,472, the disclosure of which is hereby incorporated by reference.  
         [0042]    In the roll compaction process, the powder is sieved to a desired particle size range, blended with an organic binder, mixed with an optional solvent and blended together to form a blended powder. The sieving step preferably provides a powder having a particle size within the range of −100 to +325 mesh which corresponds to a particle size of 43 to 150 μm. In order to improve the flow properties of the powder, less than 5%, preferably 3-5% of the powder has a particle size of less than 43 μm. The binder is preferably an organic binder which will decompose in a controlled manner such as polyvinyl alcohol and methyl cellulose, etc. and is blended with the powder in an amount such as up to about 5 wt %. The cellulose based binder can be methylcellulose (MS), carboxymethylcellulose (CMS) or any other suitable binder such as polyvinylalcohol (PVA). The surface of the powder is preferably contacted with enough binder to cause mechanical bonding of the powder (i.e., the powder particles stick to each other when pressed together). The solvent can be a liquid such as purified water in any suitable amount such as up to about 5 wt %. The mixture of the binder-adhered powder and solvent provides a “dry” blend which is free flowing while providing mechanical interlocking of the powders when roll compacted together.  
         [0043]    Green strips are prepared by roll compaction wherein the blended powder is fed from a hopper through a slot into a space between two compaction rolls. In a preferred embodiment, the roll compaction produces a green strip of aluminide having a thickness of about 0.026 inch and the green strip can be cut into strips having dimensions such as 36 inches by 4 inches. The green strips are subjected to a heat treatment step to remove volatile components such as the binder and any organic solvents. The binder burn out can be carried out in a furnace at atmospheric or reduced pressure in a continuous or batch manner. For instance, a batch of iron aluminide strips can be heated in a furnace set at a suitable temperature such as 700-900° F. (371-482° C.) for a suitable amount of time such as 6-8 hours or for shorter times at a higher temperature such as 950° F. (510° C.). During this step, the furnace can be at 1 atmosphere pressure with nitrogen gas flowing therethrough so as to remove most of the binder, e.g., at least 99% binder removal. This binder removal step results in very fragile green strips which are then subjected to primary sintering in a vacuum furnace.  
         [0044]    In the primary sintering step, the porous brittle de-bindened strips are preferably heated under conditions suitable for effecting reaction of the Fe x Al y  powders with the Fe and/or Fe alloy powders so as to form FeAl or Fe 3 Al as well as sintering and densification of the powder. This sintering step can be carried out in a furnace at reduced pressure in a continuous or batch manner. For instance, a batch of the de-bindened iron aluminide strips can be heated in a vacuum furnace at a suitable temperature such as 1000 to 1260° C. for a suitable time such as one hour. The vacuum furnace can be maintained at any suitable vacuum pressure such as 10 −4  to 10 −5  Torr. In order to prevent loss of aluminum from the strips during sintering, it is preferable to maintain the sintering temperature low enough to avoid vaporizing aluminum yet provide enough metallurgical bonding to allow subsequent rolling. Further, vacuum sintering is preferred to avoid oxidation of the non-densified strips. However, protective atmospheres such as hydrogen, argon and/or nitrogen with proper dew points such as −50° F. or less thereof could be used in place of the vacuum.  
         [0045]    In the next step, the presintered strips are preferably subjected to hot or cold rolling in air to a final or intermediate thickness. Due to the hardness of the intermetallic alloy, it is advantageous to use a 4-high rolling mill wherein the rollers in contact with the intermetallic alloy strip preferably have carbide rolling surfaces. However, any suitable roller construction can be used such as stainless steel rolls. If steel rollers are used, the amount of reduction is preferably limited such that the rolled material does not deform the rollers as a result of work hardening of the intermetallic alloy. The hot/cold rolling step is preferably carried out to reduce the strip thickness by at least 30%, preferably at least about 50%. For instance, the 0.026 inch thick presintered iron aluminide strips can be cold rolled to 0.013 inch thickness in a single cold rolling step with single or multiple passes.  
         [0046]    After the hot/cold rolling, the hot/cold rolled strips are subjected to heat treating to anneal the strips. This primary annealing step can be carried out in a vacuum furnace in a batch manner or in a furnace with gases like H 2 , N 2  and/or Ar in a continuous manner and at a suitable temperature to relieve stress and/or effect further densification of the powder. In the case of iron aluminide, the primary annealing can be carried at any suitable temperature such as 1652-2372° F. (900 to 1300° C.), preferably 1742-2102° F. (950 to 1150° C.) for one or more hours in a vacuum furnace. For example, the hot/cold rolled iron aluminide strip can be annealed for one hour at 2012° F. (11000° C.) but surface quality of the sheet can be improved in the same or different heating step by annealing at higher temperatures such as 2300° F. (1260° C.) for one hour.  
         [0047]    After the primary annealing step, the strips can be optionally trimmed to desirable sizes. For instance, the strip can be cut in half and subjected to further rolling and heat treating steps.  
         [0048]    In the next step, the rolled strips can be further rolled to reduce the thickness thereof. For instance, the iron aluminide strips can be rolled in a 4-high rolling mill so as to reduce the thickness thereof from 0.013 inch to 0.010 inch. This step achieves a reduction of at least 15%, preferably about 25%. However, if desired, one or more annealing steps can be eliminated, e.g., a 0.024 inch strip can be rolled directly to 0.010 inch. Subsequently, the rolled strips are subjected to secondary sintering and annealing. In the secondary sintering and annealing step, the strips can be heated in a vacuum furnace in a batch manner or in a furnace with gases like H 2 , N 2  and/or Ar in a continuous manner to achieve full density. For example, a batch of the iron aluminide strips can be heated in a vacuum furnace to a temperature of 2300° F. (1260° C.) for one hour.  
         [0049]    After the secondary sintering and annealing step, the strips can optionally be subjected to secondary trimming to shear off ends and edges as needed such as in the case of edge cracking. Then, the strips can optionally be subjected to further rolling wherein the thickness of the strips is further reduced such as by 15% or more. For example, the strips can be cold rolled to a final desired thickness such as from 0.010 inch to 0.008 inch.  
         [0050]    After the final rolling step, the strips can be subjected to a final annealing step in a continuous or batch manner at a temperature above the recrystallization temperature. For instance, in the final annealing step, a batch of the iron aluminide strips can be heated in a vacuum furnace to a suitable temperature such as 2012° F. (1100° C.) for about one hour. During the final annealing the rolled sheet is preferably recrystallized to a desired average grain size such as about 10 to 30 μm, preferably around 20 μm. Then, the strips can optionally be subjected to a final trimming step wherein the ends and edges are trimmed and the strip is slit into narrow strips having the desired dimensions for further processing such as into tubular heating elements or other desired product.  
         [0051]    The trimmed strips can be subjected to a stress relieving heat treatment to remove thermal vacancies created during the previous processing steps. The stress relief treatment increases ductility of the strip material (e.g., the room temperature ductility can be raised from around 1% to around 3-4%). In the stress relief heat treatment, a batch of the strips can be heated in a furnace at atmospheric pressure or in a vacuum furnace. For instance, the iron aluminide strips can be heated to around 1292° F. (700° C.) for two hours and cooled by slow cooling in the furnace (e.g., at ≦2-5° F./min) to a suitable temperature such as around 662° F. (350° C.) followed by quenching. During stress relief annealing it is preferable to maintain the iron aluminide strip material in a temperature range wherein the iron aluminide is in the B2 ordered phase.  
         [0052]    The stress relieved strips can be processed as tubular heating elements, honeycomb structures, thermal protection structures or other products by any suitable technique. Iron aluminide, nickel aluminide and titanium aluminide strips can be made by any combination of the foregoing steps or modification of such steps to achieve desired dimensions and/or properties. For heater elements, the strips can be laser cut, mechanically stamped or chemical photoetched to provide a desired pattern of individual heating blades. For instance, the cut pattern can provide a series of hairpin shaped blades extending from a rectangular base portion which when rolled into a tubular shape and joined provides a tubular heating element with a cylindrical base and a series of axially extending and circumferentially spaced apart heating blades. Alternatively, an uncut strip could be formed into a tubular shape and the desired pattern cut into the tubular shape to provide a heating element having the desired configuration.  
         [0053]    In general, rolled FeAl material having a composition, in weight %, of 23% Al, 0.005% B, 0.42% Mo, 0.1% Zr, 0.2% Y, 0.03% C, balance Fe can exhibit room temperature yield strength of 55-70 ksi, ultimate tensile strength of 65-75 ksi, total elongation of 1-6%, reduction of area of 7-12% and electrical resistivity of about 150-160 μΩ·cm whereas the elevated temperature strength properties at 750° C. include yield strength of 36-43 ksi, ultimate tensile strength of 42-49 ksi, total elongation of 22-48% and reduction of area of 26-41% elongation values.  
         [0054]    Iron aluminide, nickel aluminide and titanium aluminide sheet products according to the invention can be made by roll compaction such as by the various steps described above or other technique such as tape casting. Such techniques, however, can be modified to form other shaped products as will be apparent to those skilled in the art.  
         [0055]    In the tape casting process, a powder mixture is processed by substituting tape casting for the roll compaction step in the foregoing roll compaction embodiment. However, whereas an irregularly shaped powder is preferred for the roll compaction process, gas atomized powder is preferred for tape casting due to its spherical shape and low oxide contents. The gas atomized powder is sieved as in the roll compaction process and the sieved powder is blended with organic binder and solvent so as to produce a slip, the slip is tape cast into a thin sheet and the tape cast sheet is hot/cold rolled and heat treated as set forth in the roll compaction embodiment. Details of the tape casting process can be found in commonly owned U.S. Pat. No. 6,030,472, the disclosure of which is hereby incorporated by reference  
         [0056]    The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those -embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.