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A thermomechanical process to make iron aluminide (FeAl) sheet
Materials Science and Engineering A258 Ž1998. 249]257
A thermomechanical process to make iron aluminide Ž FeAl. sheet M.R. Hajaligola,U , S.C. Deevi a , V.K. Sikkab , C.R. Scorey c a
Research and De¨ elopment Center, Philip Morris USA, 4201 Commerce Road, P.O. Box 26581, Richmond, VA 23261, USA b Metals and Ceramics Di¨ ision, Oak Ridge National Laboratory, Oak Ridge, TN 37387, USA c Ametek Specialty Metals Di¨ ision, Wallingford, CT 06492, USA
Abstract An innovative combination of roll compaction, and thermomechanical processing allowed manufacture of FeAl alloy intermetallic sheets with 24 wt.% Al content. Green sheets of FeAl were obtained by roll compaction of water atomized FeAl powder with a polymeric binder. Roll compacted green sheets were de-bindered and partially sintered prior to cold rolling through tungsten carbide rolls. Cold rolling decreased the thickness, reduced the level of porosity and work-hardened the sheets. Several intermediate annealings at or above 11008C were found to be necessary to relieve the work hardening stresses prior to rolling the sheets to a final thickness of 0.20 mm. The annealing temperatures were chosen to be at or above 11008C to allow concurrent sintering of FeAl necessary for the densification of FeAl sheets. Thermomechanical processing of cold rolled sheets allowed commercial manufacture of FeAl intermetallic sheets without the necessity of hot rolling of a cast FeAl ingot. Fully dense sheets possess fine grain microstructure with an average grain size of 20 m m. The electrical resistivities of FeAl sheets can be varied from 140 to 155 m V cmy1, and the high resistivities make them ideally suited for resistive heating applications. Mechanical properties of FeAl sheets are comparable to the properties of hot extruded FeAl alloys. Q 1998 Elsevier Science S.A. All rights reserved. Keywords: Iron alumunide; Intermetallics; Roll-compaction; Powder forming; Cold-rolling; Sintering
1. Introduction Intermetallics based on nickel, iron and titanium aluminides have been the subject of intense research during the last two decades due to their excellent thermal stability at high temperatures coupled with their unique combination of properties such as low densities, good room temperature and high-temperature tensile strengths w1]6x. They have been proposed as structural materials to replace some existing hightemperature alloys. Of the intermetallics, iron aluminides based on FeAl with a B2 structure are of more interest than Fe 3 Al-based alloys. They exhibit excellent oxidation, corrosion and sulfidation resistance at high temperatures, and still possess reason-
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Corresponding author.
able strengths at high temperatures for use as structural materials w7]9x. The room temperature ductility of FeAl alloys are generally in the range of 2]6%, and the elongations are influenced by room temperature embrittlement. The low ductility of FeAl alloys necessitated hot working of cast materials at high temperatures w10]12x, and hot working approaches limited the manufacturability of sheets and rods. Metallurgical processing techniques based on melting and casting, forging and rolling have been successfully used to understand the processability of FeAl alloys. Recently, Mazsiaz et al. w13x have shown that the yield strengths, ultimate tensile strengths and tensile elongations of hot extruded rods of FeAl alloys are superior to the properties of the cast FeAl alloys. This was attributed to the fine grain microstructure of the hot extruded FeAl alloys. Deevi and Sikka w14x and Deevi w15x have also shown by reactive hot extru-
0921-5093r98r$ - see front matter Q 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093Ž98.00941-1
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M.R. Hajaligol et al. r Materials Science and Engineering A258 (1998) 249]257
sion of Fe and Al powders that the properties of fine grained Fe]24 wt.% Al are superior to the properties of cast materials. Therefore, fine grained FeAl alloys are preferable for manufacturing operations such as forming and stamping due to their high tensile elongations. The unique properties of fine grained FeAl alloys prompted Philip Morris USA to initiate an aggressive research and development program on iron aluminides with the goal of commercial production of FeAl sheets with well-defined electrical resistivity, and good room temperature ductility for formability, and reasonable high-temperature creep, fatigue, and tensile strengths. We focused on the manufacturing of 0.20-mm-thick FeAl sheet, and all the processing and manufacturing approaches were directed towards the production of thin sheets of FeAl. As part of this program, we investigated a variety of manufacturing techniques such as hot rolling of cast FeAl alloys; hot rolling of cast sheets obtained by spray forming and rapid solidification processing of FeAl melt; hot extrusion and rolling of pre-alloyed powder in a steel cover; thermal spraying of FeAl sheets; and tape casting of FeAl powder, cold rolling, and thermomechanical processing of FeAl sheets w16]18x. In this paper, we present powder production, manufacture of green sheets, de-bindering followed by partial sintering, and multiple cold rolling and annealing processing steps required to obtain fully dense FeAl sheets. We will also discuss the electrical and mechanical properties of commercial batches of FeAl sheets. 2. Overview of sheet manufacturing Fig. 1 provides a schematic illustration of the process steps employed to obtain FeAl sheets. The
flow diagram shown in Fig. 1 indicates five steps starting from manufacturing of FeAl powder, roll compaction of the powder to obtain green sheets, de-binding followed by partial sintering, cold rolling, and simultaneous annealing and sintering. In step 1, electrolytic grade iron, 99.99% aluminum, and ferroalloys of Zr, Mo and B were inductively melted in air, and then atomized with water to obtain pre-alloyed powders of iron aluminide with specified composition. In step 2, water atomized FeAl alloy powders were compacted into green sheets by roll compaction technique. In step 3, the green sheets were de-bindered in an inert atmosphere, and then partially sintered in a vacuum furnace to provide a reasonable tensile strength for cold rolling. In step 4, sheets were cold rolled in multiple passes through tungsten carbide rolls. In step 5, iron aluminide sheets from step 4 were annealed in a vacuum furnace prior to repeating steps 4 and 5 to obtain a sheet with desired thickness. These steps will be described in detail in the following sections. 2.1. Step 1: manufacturing of FeAl powder Melt atomization techniques were used to obtain powders of either spherical or irregular shape of FeAl alloy with the target composition consisting of: Al, 24; Mo, 0.42; Zr, 0.01; B, 0.01; C, 0.04; and the balance iron Žall in wt.%.. The gas atomization technique was employed to obtain spherical powders, and water atomization technique was used to obtain irregular shape particles using standard atomization techniques w19,20x. The surface morphologies of gas and water powders is shown in Fig. 2. The irregular shape of
Fig. 1. A flow diagram illustrating the manufacturing steps involved in obtaining FeAl sheets by roll compaction of FeAl powders followed by thermomechanical processing.
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Fig. 2. Surface morphologies of Ža. water atomized, and Žb. gas atomized powders.
water atomized particles allowed roll compaction of powders into a green sheet with polymeric binders. Unlike the gas atomization technique, water atomization of FeAl alloy powders required special precautions to reduce the oxygen content of the particles ŽFig. 2., and prevent formation of oxides of iron and aluminum on the surface. Excessive oxidation of water atomized FeAl particles lead to stringers of oxides in the fully densified sheets of FeAl. To avoid formation of oxide stringers in the dense sheet, atomization of the melt was carried out in a well-controlled atmosphere. Atomized powder was dried quickly in a special multi-stage powder dryer tower to minimize oxygen content. The oxygen content of the water atomized powder is close to 0.3 wt.% or higher. On the other hand, the oxygen content of the gas atomized powder is in the range of 0.02]0.04 wt.%, an order of magnitude lower than the water atomized powder. It was extremely difficult to roll compact gas atomized powders due to their highly spherical nature even though FeAl sheets with low oxygen content are de-
sirable. Gas atomized powders were used to obtain green sheets by tape casting technique, and is described elsewhere w17x. 2.2. Step 2: roll compaction of powder into green sheet Green sheets Žcommonly referred to sheets containing polymeric binders. of FeAl of approximately 0.625-mm thickness were obtained by roll compacting the water atomized FeAl powder of y125rq47 m m size distribution mixed with an organic binder such as polyvinyl alcohol. A schematic illustration of roll compaction is shown in Fig. 3. The roll compaction is similar to roll pressing, and involves compaction of solid particles between two counter rotating rolls. A blended mixture of water atomized FeAl powder and polyvinyl alcohol were fed through the hopper into the feed zone of the rolls. The blended powders were drawn into the nip by friction on the roll surface, and were pressed together and compacted. It is important to note that the nature of polymeric binder, powder
Fig. 3. Schematic of a roll compaction unit.
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size, size distribution, and roll pressure determine the densities of roll compacted green sheets. We successfully obtained green sheets with several binders commonly used in the powder metallurgical, and ceramic processing techniques. Uniform densification of the powder during roll compaction is the most critical step in obtaining FeAl sheets of reasonable length with full density. Local fluctuations in the densification of FeAl powder and binders lead to non-uniform sintering during the debindering stage. This ultimately led to cracking of the sheets during the cold rolling stage even though macroscopic on line thickness measurements revealed that the thicknesses were uniform across the sheets. Extensive cracking of the sheets during cold rolling necessitated monitoring of the linear mass density profiles by X-ray method as well as the thickness profiles of the sheets. A feed-back loop control between the hopper, compaction rolls and X-ray detector allow uniform densification of the powders in the green sheets. Control of linear mass density in both rolling and transverse directions was found to be
very critical with very narrow tolerance to obtain green sheets of uniform thickness for further rolling. This is in contrast to the roll compaction of copper and steels which exhibit much higher ductility than FeAl alloys. Fig. 4a shows that the thickness of the sheet is uniform at 0.625 mm across a length of 10 cm, while the mass density profile shown by X-ray varied significantly across the width as shown in Fig. 4a. Green sheet with the mass profile shown in Fig. 4a led to extensive cracking during cold rolling Žcarried out after de-bindering, and sintering. as shown in Fig. 4b. Green sheets with a mass density profile shown in Fig. 4c can successfully be processed to full density with no detectable imperfections. Our experience suggests that the mass density profiles should not deviate by more than 1% from point to point in either direction to ensure uniform sintering of the sheet across its entire length. 2.3. Step 3: de-bindering and initial sintering Green sheets are first de-bindered in an inert atmo-
Fig. 4. Ža. Typical mass profile of a green sheet that results in cracking on cold rolling, Žb. cracks observed during the cold rolling of the de-bindered and partially sintered sheet, and Žc. mass profile of a green sheet that can be cold rolled to full density.
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sphere consisting of either nitrogen Žor argon atmosphere. at approximately 5008C. After the de-bindering step, the sheets were sintered initially in hydrogen and argon atmospheres to a maximum temperature of 13158C. The water vapor and oxygen present in the gases led to the growth of alumina whiskers on the surface and in the interiors of porous FeAl sheets. Growth of alumina whiskers were observed even at very low dew points necessitating sintering of the porous sheets in vacuum. Initial sintering experiments were carried out in vacuum with 10y4 torr pressure in the temperature range of 1050]13158C to ensure full densification. Sintering of highly porous sheets in vacuum close to 13158C led to densification of the sheet in the interior, and evaporation of the particles close to the surface as can be seen in Fig. 5. Increase of sintering time even at a lower temperature led to surface evaporation, thus necessitating a different approach for densification of the porous sheets. Evaporation of metal was observed primarily due to the much higher porosity Ž25]35%. present in the FeAl sheets after de-bindering, and initial sintering of the green sheets. Sintering of FeAl sheets to full density could have been accomplished without evaporation had the porosity been below 15%. Roll compaction technique inherently leads to sheets with much higher porosities after de-bindering step, and the level of porosity led us to alternative approaches. As an alternative approach to full densification in vacuum, we investigated partial sintering of FeAl sheets in vacuum to obtain reasonable strength for densification by roll compaction at room temperature. Fig. 6 shows the microstructures of partially sintered
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Fig. 5. Microstructure of a sheet sintered in vacuum at 12608C.
FeAl sheets at 100 = and 400 = . Partially sintered sheets shown in Fig. 6 have been cold rolled and annealed to obtain dense sheets as described in the next section. 2.4. Step 4: cold rolling and annealing Cold rolling was considered to be the most viable and practical means of obtaining fully dense sheets. The porous nature of the FeAl sheets precluded hot rolling from consideration. Partially sintered FeAl sheets obtained in step 3 were cold rolled to promote densification with each pass through the rolling mill. The thickness of the sheet was measured after every rolling pass to determine the extent of reduction with each pass. Percent reduction calculated from the thickness measurements provided an indication of the cold workability and work hardening characteristics of the porous FeAl sheets. Percent reduction of thick-
Fig. 6. Microstructure of FeAl sheet after de-bindering in nitrogen at 5008C, and partial sintering in vacuum.
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ness was greater during the initial stages of cold rolling, and FeAl sheets work hardened significantly even after several passes. Vickers microhardnesses of FeAl sheets increased substantially with work hardening, and were in the range of 450]500 Hv. The high hardness of the FeAl intermetallic sheets dented the hardened steel rolls used initially, and necessitated the use of carbide rolls for subsequent processing. The intermetallic nature of FeAl sheets as opposed to conventional steels required the design of a special 4-high rolling mill with tungsten carbide work rolls for processing of the FeAl sheets. The rolling mill designed for FeAl processing consisted of two 12.5-cm diameter carbide working rolls and two 50-cm diameter steel backup rolls. Densification of the porous sheets can be achieved by multiple cold rolling of the sheets through the carbide rolls from 0.625 mm to 0.2 mm Žwith a 68% reduction.. Densification in a single stage required a large number of passes through the roll, and the final sheet exhibited extensive cracks. Onset of cracking was followed by monitoring the work hardening characteristics of FeAl sheet by hardness measurements. Sheets cracked invariably when the Vickers microhardnesses reached close to 500 Hv and the onset of cracking was observed to be around 480 Hv. Therefore, densification was achieved in three stages as shown schematically in Fig. 7. In stage I, cold rolling was carried out with approximately 50% reduction in thickness prior to annealing at 11508C for 1 h in vacuum. In stage II, annealing was needed after only approximately 15]20% reduction in thickness. Final sheet thickness was accomplished in stage III. Final annealing resulted in a fully dense sheet.
Fig. 8 shows the evolution of microstructure obtained after annealing at different sheet thickness. The hardness of the FeAl sheets is significantly influenced by the rate of cooling employed in the final annealing step as can be seen in Fig. 9. This is attributed to the vacancy hardening of iron aluminides. Our results are in accordance with the observations reported by several researchers w13x.
3. Mechanical properties
Iron aluminide sheets obtained by roll compaction have been extensively characterized for their electrical properties, specific heat, electrical resistivity, tensile strengths, creep and relaxation behavior and fatigue life times as a function of temperature w21]23x. Yield strength, ultimate tensile strengths and tensile elongation of densified FeAl sheets as a function of temperature at 0.2 mm thickness are shown in Fig. 10. All the tensile specimens were annealed in vacuum at 7008C for 2 h followed by furnace cooling to 4008C before testing. Tensile properties shown in Fig. 10 were obtained on punched specimens in air at 0.02 sy1 strain rate. Yield strength ŽFig. 10a. decreases initially due to environmental embrittlement, and then increases with temperature until 6008C. Yield strength falls down precipitously between 700 and 8008C. Yield strengths of Fe]24 wt.% Al cast in air ŽAIM. and in vacuum ŽVIM. using induction melting techniques are also shown in Fig. 10a. Also shown are the yield strengths of the powder-processed FA-385 wof composition
Fig. 7. Schematic illustration of percent reduction, hardness increase and the annealing sequence as a function of number of times the sheet is passed through the carbide rolls.
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Fig. 8. Microstructures obtained at different sheet thickness illustrating the elimination of porosity with decrease of thickness.
was predominantly transgranular cleavage with a small proportion of intergranular fracture. A mixed fracture mode consisting of transgranular and intergranular was evident between room temperature and 6008C, and the proportion of transgranular fracture mode increased with increase of temperature. The fracture mode was dimple-type rupture with considerable plasticity at or above 7008C w26x. Ductile fracture was observed at 7508C, and Fig. 11b clearly illustrates considerable plasticity and cavitation at the grain boundaries associated with the ductile failure.
4. Conclusions
Fig. 9. Effect of cooling rate on the hardness of fully densified FeAl sheet after the final anneal.
Žat.%.: Al, 35.8; Mo, 0.2; Zr, 0.05; C, 0.13x of Mazsiaz et al. w13x. Temperature dependence of ultimate tensile strength of FeAl is shown in Fig. 10b. Tensile elongation of FeAl sheets is close to 5% at room temperature and increases with temperature. The increase is much more pronounced above 4008C, and the tensile elongation at 8008C is 55%. Tensile properties of FeAl sheets presented in Fig. 10 agree well with the properties of the powder processed iron aluminides w24,25x. The fracture mode at room temperature ŽFig. 11a.
The roll compaction technique allowed densification of water atomized powder into a green sheet. Uniform linear mass profile density was found to be critical to prevent cracking of the sheets, and the on-line detection system based on an X-ray technique allowed manufacture of green sheets of excellent quality. Green sheets were de-bindered in a nitrogen atmosphere, and were partially sintered in vacuum prior to further densification. Sintering of green sheets at high temperatures in hydrogen and in argon formed alumina whiskers in the FeAl sheets due to the presence of trace quantities of water vapor and oxygen. Attempts to sinter porous sheets to full density in vacuum at a pressure of 10y4 torr or less resulted in
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Densification of porous sheets was achieved by a novel combination of cold rolling of FeAl sheets through a carbide roll followed by annealing in three different stages. Annealing was carried out to remove the cold work, and allow further densification of FeAl sheets. Intermediate annealing prevented cracking of the sheets due to the excessive work hardening of the FeAl. Fully dense sheets possess fine grain sizes and excellent yield strengths, ultimate tensile strengths and tensile elongations of 4]6%. Fully dense sheets have been subjected to various metal forming operations such as bending and shaping. Acknowledgements The authors appreciate and acknowledge the discussions and contributions of Mr G.S. Fleischhauer; Mr J. Cunningham, Ms V. Baliga and Dr D. Miser of Philip Morris USA; Dr R. Altomer of Kraft Foods; Mr J. Ricketts, Dr J. Reinshagen, Mr F. Ewing and Mr J. McKernen of Ametek Specialty Products, Inc.; Mr E. Hatfield, Mr D. Harper, Mr K. Blakely, Mr J. Vought and Mr R. Howell of the Oak Ridge National Laboratory. In addition, the authors greatly appreciate the management of Philip Morris USA for their support and encouragement. References
Fig. 10. A comparison of Ža. 0.2% yield strengths, Žb. ultimate tensile strengths, and Žc. tensile elongations of roll compacted FeAl sheets as a function of temperature with powder processed FA-385, and cast Fe]24 wt.% Al alloys.
densification at the center of the sheet, and evaporation of the material at the surface.
w1x C.C. Koch, C.T. Liu, N.S. Stoloff ŽEds.., High Temperature Ordered Intermetallics Alloy, Proceedings of Materials Research Society Symposium, Mater. Res. Soc. Symp. Proc., Pittsburgh, PA, 1985, p. 39. w2x N.S. Stoloff, C.C. Koch, C.T. Liu, O. Izumi ŽEds.., High Temperature Ordered Intermetallic Alloys II, Proceedings of Materials Research Society Symposium, Mater. Res. Soc. Symp. Proc., Pittsburgh, PA, 1987, p. 81. w3x C.T. Liu, A.I. Taub, N.S. Stoloff, C.C. Koch ŽEds.., High Temperature Ordered Intermetallic Alloy III, Proceedings of Materials Research Society Symposium, Mater. Res. Soc. Symp. Proc., Pittsburgh, PA, 1989, p. 133. w4x L.A. Johnson, D.P. Pope, J.O. Stiegler ŽEds.., High Temperature Ordered Intermetallic Alloy IV, Proceedings of Materials Research Society Symposium, Mater. Res. Soc. Symp. Proc., Pittsburgh, PA, 1991, p. 213. w5x S.H. Whang, C.T. Liu, D.P. Pope, J.O. Stiegler ŽEds.., High Temperature Aluminide Intermetallics, Proceedings of TMSrASM Symposium, TMS-AIME, Warrendale, PA, 1990. w6x O. Izumi ŽEd.., Intermetallic Compounds } Structure and Mechanical Properties, Proceedings of JIMIS-6, Japan Institute of Metals, Tokyo, 1991. w7x S.C. Deevi, V.K. Sikka, C.T. Liu, Prog. Mater. Sci. 42 Ž1997. 177. w8x C.T. Liu, E.P. George, V.K. Sikka, S.C. Deevi, in: N.S. Stoloff, R.H. Jones ŽEds.., Processing and Design Issues in High Temperature Materials, TMS, Warrandale, PA, 1996, p. 139.
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Fig. 11. Fracture surface of roll compacted FeAl sheets following straining at Ža. room temperature, and Žb. 7508C. w9x S.C. Deevi, V.K. Sikka, Intermetallics 4 Ž1996. 357. w10x V.K. Sikka, in: J.H. Schneibel, M.A. Crimp ŽEds.., Processing, Properties, and Applications of Iron Aluminides, TMS, Warrandale, PA, 1994, p. 3. w11x D. Pocci, O. Tassa, C. Testani, in: J.H. Schneibel, M.A. Crimp ŽEds.., Processing, Properties, and Applications of Iron Aluminides, Warrandale, PA, 1994, p. 19. w12x C. Testani, A. Di Giafrancesco, O. Tassa, D. Pocci, in: S.C. Deevi, P.J. Maziasz, V.K. Sikka, R.W. Cahn ŽEds.., Nickel and Iron Aluminides: Processing, Properties, and Applications, ASM International, Material Park, OH, 1997, p. 213. w13x P.J. Maziasz, G.M. Goodwin, D.J. Alexander, S. Viswanathan, in: S.C. Deevi, P.J. Maziasz, V.K. Sikka, R.W. Cahn ŽEds.., Nickel and Iron Aluminides: Processing, Properties, and Applications, ASM International, Material Park, OH, 1997, p. 157. w14x S.C. Deevi, V.K. Sikka, in: S.C. Deevi, P.J. Maziasz, V.K. Sikka, R.W. Cahn ŽEds.., Nickel and Iron Aluminides: Processing, Properties, and Applications, ASM International, Material Park, OH, 1997, p. 283. w15x S.C. Deevi, Mater. Sci. Eng. Žaccepted.. w16x M.R. Hajaligol, N. Grant, A. Farah, in: Proceedings of the 1998 International Conference on Powder Metallurgy and Particulate Materials, MPIF, Princeton, NJ, 1998 Žaccepted for publication..
w17x R.E. Mistler, V.K. Sikka, C.R. Scorey, J.E. McKernan, M.R. Hajaligol, Mater. Sci. Eng. A258 Ž1998. 258]1265. w18x M.R. Hajaligol, V.K. Sikka, R. Greenlaw, W. Mickles, in: Proceedings of the 1998 International Conference on Powder Metallurgy and Particulate Materials, MPIF, Princeton, NJ, 1998 Žaccepted for publication.. w19x K.G. Shaw, J.H. Reinshagen, in: S.C. Deevi, V.K. Sikka, P.J. Maziasz, R.W. Cahn ŽEds.., Nickel and Iron Aluminides: Processing, Properties, and Applications, ASM International, Materials Park, OH, 1997. w20x W. Pietsch, Roll Pressing, Monographs in Powder Science and Technology, Heyden and Sons, New York, 1976. w21x A.C. Lilly, S.C. Deevi, Z. Gibbs, Mater. Sci. Eng. A258 Ž1998. 37]41. w22x S.C. Deevi, R.W. Swindeman, Mater. Sci. Eng. A258 Ž1998. 203]210. w23x C.E. Jaske, S.C. Deevi, S.S. Shademan, Mater. Sci. Eng. A258 Ž1998. 211]218. w24x I. Baker, D.J. Gaydosh, Mater. Sci. Eng. 96 Ž1987. 147. w25x M.G. Mendritta, S.K. Shields, D.K. Chatterjee, H.A. Lipsitt, in: R. Mehrbian ŽEd.., Rapid Solidification Processing: Materials and Technologies, NBS, Giathersburg, MD, 1983, p. 220. w26x S.C. Deevi, V.K. Sikka, M.R. Hajaligol Žto be published..
A thermomechanical process to make iron aluminide Ž FeAl. sheet M.R. Hajaligola,U , S.C. Deevi a , V.K. Sikkab , C.R. Scorey c a
Research and De¨ elopment Center, Philip Morris USA, 4201 Commerce Road, P.O. Box 26581, Richmond, VA 23261, USA b Metals and Ceramics Di¨ ision, Oak Ridge National Laboratory, Oak Ridge, TN 37387, USA c Ametek Specialty Metals Di¨ ision, Wallingford, CT 06492, USA
Abstract An innovative combination of roll compaction, and thermomechanical processing allowed manufacture of FeAl alloy intermetallic sheets with 24 wt.% Al content. Green sheets of FeAl were obtained by roll compaction of water atomized FeAl powder with a polymeric binder. Roll compacted green sheets were de-bindered and partially sintered prior to cold rolling through tungsten carbide rolls. Cold rolling decreased the thickness, reduced the level of porosity and work-hardened the sheets. Several intermediate annealings at or above 11008C were found to be necessary to relieve the work hardening stresses prior to rolling the sheets to a final thickness of 0.20 mm. The annealing temperatures were chosen to be at or above 11008C to allow concurrent sintering of FeAl necessary for the densification of FeAl sheets. Thermomechanical processing of cold rolled sheets allowed commercial manufacture of FeAl intermetallic sheets without the necessity of hot rolling of a cast FeAl ingot. Fully dense sheets possess fine grain microstructure with an average grain size of 20 m m. The electrical resistivities of FeAl sheets can be varied from 140 to 155 m V cmy1, and the high resistivities make them ideally suited for resistive heating applications. Mechanical properties of FeAl sheets are comparable to the properties of hot extruded FeAl alloys. Q 1998 Elsevier Science S.A. All rights reserved. Keywords: Iron alumunide; Intermetallics; Roll-compaction; Powder forming; Cold-rolling; Sintering
1. Introduction Intermetallics based on nickel, iron and titanium aluminides have been the subject of intense research during the last two decades due to their excellent thermal stability at high temperatures coupled with their unique combination of properties such as low densities, good room temperature and high-temperature tensile strengths w1]6x. They have been proposed as structural materials to replace some existing hightemperature alloys. Of the intermetallics, iron aluminides based on FeAl with a B2 structure are of more interest than Fe 3 Al-based alloys. They exhibit excellent oxidation, corrosion and sulfidation resistance at high temperatures, and still possess reason-
U
Corresponding author.
able strengths at high temperatures for use as structural materials w7]9x. The room temperature ductility of FeAl alloys are generally in the range of 2]6%, and the elongations are influenced by room temperature embrittlement. The low ductility of FeAl alloys necessitated hot working of cast materials at high temperatures w10]12x, and hot working approaches limited the manufacturability of sheets and rods. Metallurgical processing techniques based on melting and casting, forging and rolling have been successfully used to understand the processability of FeAl alloys. Recently, Mazsiaz et al. w13x have shown that the yield strengths, ultimate tensile strengths and tensile elongations of hot extruded rods of FeAl alloys are superior to the properties of the cast FeAl alloys. This was attributed to the fine grain microstructure of the hot extruded FeAl alloys. Deevi and Sikka w14x and Deevi w15x have also shown by reactive hot extru-
0921-5093r98r$ - see front matter Q 1998 Elsevier Science S.A. All rights reserved. PII S0921-5093Ž98.00941-1
250
M.R. Hajaligol et al. r Materials Science and Engineering A258 (1998) 249]257
sion of Fe and Al powders that the properties of fine grained Fe]24 wt.% Al are superior to the properties of cast materials. Therefore, fine grained FeAl alloys are preferable for manufacturing operations such as forming and stamping due to their high tensile elongations. The unique properties of fine grained FeAl alloys prompted Philip Morris USA to initiate an aggressive research and development program on iron aluminides with the goal of commercial production of FeAl sheets with well-defined electrical resistivity, and good room temperature ductility for formability, and reasonable high-temperature creep, fatigue, and tensile strengths. We focused on the manufacturing of 0.20-mm-thick FeAl sheet, and all the processing and manufacturing approaches were directed towards the production of thin sheets of FeAl. As part of this program, we investigated a variety of manufacturing techniques such as hot rolling of cast FeAl alloys; hot rolling of cast sheets obtained by spray forming and rapid solidification processing of FeAl melt; hot extrusion and rolling of pre-alloyed powder in a steel cover; thermal spraying of FeAl sheets; and tape casting of FeAl powder, cold rolling, and thermomechanical processing of FeAl sheets w16]18x. In this paper, we present powder production, manufacture of green sheets, de-bindering followed by partial sintering, and multiple cold rolling and annealing processing steps required to obtain fully dense FeAl sheets. We will also discuss the electrical and mechanical properties of commercial batches of FeAl sheets. 2. Overview of sheet manufacturing Fig. 1 provides a schematic illustration of the process steps employed to obtain FeAl sheets. The
flow diagram shown in Fig. 1 indicates five steps starting from manufacturing of FeAl powder, roll compaction of the powder to obtain green sheets, de-binding followed by partial sintering, cold rolling, and simultaneous annealing and sintering. In step 1, electrolytic grade iron, 99.99% aluminum, and ferroalloys of Zr, Mo and B were inductively melted in air, and then atomized with water to obtain pre-alloyed powders of iron aluminide with specified composition. In step 2, water atomized FeAl alloy powders were compacted into green sheets by roll compaction technique. In step 3, the green sheets were de-bindered in an inert atmosphere, and then partially sintered in a vacuum furnace to provide a reasonable tensile strength for cold rolling. In step 4, sheets were cold rolled in multiple passes through tungsten carbide rolls. In step 5, iron aluminide sheets from step 4 were annealed in a vacuum furnace prior to repeating steps 4 and 5 to obtain a sheet with desired thickness. These steps will be described in detail in the following sections. 2.1. Step 1: manufacturing of FeAl powder Melt atomization techniques were used to obtain powders of either spherical or irregular shape of FeAl alloy with the target composition consisting of: Al, 24; Mo, 0.42; Zr, 0.01; B, 0.01; C, 0.04; and the balance iron Žall in wt.%.. The gas atomization technique was employed to obtain spherical powders, and water atomization technique was used to obtain irregular shape particles using standard atomization techniques w19,20x. The surface morphologies of gas and water powders is shown in Fig. 2. The irregular shape of
Fig. 1. A flow diagram illustrating the manufacturing steps involved in obtaining FeAl sheets by roll compaction of FeAl powders followed by thermomechanical processing.
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Fig. 2. Surface morphologies of Ža. water atomized, and Žb. gas atomized powders.
water atomized particles allowed roll compaction of powders into a green sheet with polymeric binders. Unlike the gas atomization technique, water atomization of FeAl alloy powders required special precautions to reduce the oxygen content of the particles ŽFig. 2., and prevent formation of oxides of iron and aluminum on the surface. Excessive oxidation of water atomized FeAl particles lead to stringers of oxides in the fully densified sheets of FeAl. To avoid formation of oxide stringers in the dense sheet, atomization of the melt was carried out in a well-controlled atmosphere. Atomized powder was dried quickly in a special multi-stage powder dryer tower to minimize oxygen content. The oxygen content of the water atomized powder is close to 0.3 wt.% or higher. On the other hand, the oxygen content of the gas atomized powder is in the range of 0.02]0.04 wt.%, an order of magnitude lower than the water atomized powder. It was extremely difficult to roll compact gas atomized powders due to their highly spherical nature even though FeAl sheets with low oxygen content are de-
sirable. Gas atomized powders were used to obtain green sheets by tape casting technique, and is described elsewhere w17x. 2.2. Step 2: roll compaction of powder into green sheet Green sheets Žcommonly referred to sheets containing polymeric binders. of FeAl of approximately 0.625-mm thickness were obtained by roll compacting the water atomized FeAl powder of y125rq47 m m size distribution mixed with an organic binder such as polyvinyl alcohol. A schematic illustration of roll compaction is shown in Fig. 3. The roll compaction is similar to roll pressing, and involves compaction of solid particles between two counter rotating rolls. A blended mixture of water atomized FeAl powder and polyvinyl alcohol were fed through the hopper into the feed zone of the rolls. The blended powders were drawn into the nip by friction on the roll surface, and were pressed together and compacted. It is important to note that the nature of polymeric binder, powder
Fig. 3. Schematic of a roll compaction unit.
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size, size distribution, and roll pressure determine the densities of roll compacted green sheets. We successfully obtained green sheets with several binders commonly used in the powder metallurgical, and ceramic processing techniques. Uniform densification of the powder during roll compaction is the most critical step in obtaining FeAl sheets of reasonable length with full density. Local fluctuations in the densification of FeAl powder and binders lead to non-uniform sintering during the debindering stage. This ultimately led to cracking of the sheets during the cold rolling stage even though macroscopic on line thickness measurements revealed that the thicknesses were uniform across the sheets. Extensive cracking of the sheets during cold rolling necessitated monitoring of the linear mass density profiles by X-ray method as well as the thickness profiles of the sheets. A feed-back loop control between the hopper, compaction rolls and X-ray detector allow uniform densification of the powders in the green sheets. Control of linear mass density in both rolling and transverse directions was found to be
very critical with very narrow tolerance to obtain green sheets of uniform thickness for further rolling. This is in contrast to the roll compaction of copper and steels which exhibit much higher ductility than FeAl alloys. Fig. 4a shows that the thickness of the sheet is uniform at 0.625 mm across a length of 10 cm, while the mass density profile shown by X-ray varied significantly across the width as shown in Fig. 4a. Green sheet with the mass profile shown in Fig. 4a led to extensive cracking during cold rolling Žcarried out after de-bindering, and sintering. as shown in Fig. 4b. Green sheets with a mass density profile shown in Fig. 4c can successfully be processed to full density with no detectable imperfections. Our experience suggests that the mass density profiles should not deviate by more than 1% from point to point in either direction to ensure uniform sintering of the sheet across its entire length. 2.3. Step 3: de-bindering and initial sintering Green sheets are first de-bindered in an inert atmo-
Fig. 4. Ža. Typical mass profile of a green sheet that results in cracking on cold rolling, Žb. cracks observed during the cold rolling of the de-bindered and partially sintered sheet, and Žc. mass profile of a green sheet that can be cold rolled to full density.
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sphere consisting of either nitrogen Žor argon atmosphere. at approximately 5008C. After the de-bindering step, the sheets were sintered initially in hydrogen and argon atmospheres to a maximum temperature of 13158C. The water vapor and oxygen present in the gases led to the growth of alumina whiskers on the surface and in the interiors of porous FeAl sheets. Growth of alumina whiskers were observed even at very low dew points necessitating sintering of the porous sheets in vacuum. Initial sintering experiments were carried out in vacuum with 10y4 torr pressure in the temperature range of 1050]13158C to ensure full densification. Sintering of highly porous sheets in vacuum close to 13158C led to densification of the sheet in the interior, and evaporation of the particles close to the surface as can be seen in Fig. 5. Increase of sintering time even at a lower temperature led to surface evaporation, thus necessitating a different approach for densification of the porous sheets. Evaporation of metal was observed primarily due to the much higher porosity Ž25]35%. present in the FeAl sheets after de-bindering, and initial sintering of the green sheets. Sintering of FeAl sheets to full density could have been accomplished without evaporation had the porosity been below 15%. Roll compaction technique inherently leads to sheets with much higher porosities after de-bindering step, and the level of porosity led us to alternative approaches. As an alternative approach to full densification in vacuum, we investigated partial sintering of FeAl sheets in vacuum to obtain reasonable strength for densification by roll compaction at room temperature. Fig. 6 shows the microstructures of partially sintered
253
Fig. 5. Microstructure of a sheet sintered in vacuum at 12608C.
FeAl sheets at 100 = and 400 = . Partially sintered sheets shown in Fig. 6 have been cold rolled and annealed to obtain dense sheets as described in the next section. 2.4. Step 4: cold rolling and annealing Cold rolling was considered to be the most viable and practical means of obtaining fully dense sheets. The porous nature of the FeAl sheets precluded hot rolling from consideration. Partially sintered FeAl sheets obtained in step 3 were cold rolled to promote densification with each pass through the rolling mill. The thickness of the sheet was measured after every rolling pass to determine the extent of reduction with each pass. Percent reduction calculated from the thickness measurements provided an indication of the cold workability and work hardening characteristics of the porous FeAl sheets. Percent reduction of thick-
Fig. 6. Microstructure of FeAl sheet after de-bindering in nitrogen at 5008C, and partial sintering in vacuum.
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ness was greater during the initial stages of cold rolling, and FeAl sheets work hardened significantly even after several passes. Vickers microhardnesses of FeAl sheets increased substantially with work hardening, and were in the range of 450]500 Hv. The high hardness of the FeAl intermetallic sheets dented the hardened steel rolls used initially, and necessitated the use of carbide rolls for subsequent processing. The intermetallic nature of FeAl sheets as opposed to conventional steels required the design of a special 4-high rolling mill with tungsten carbide work rolls for processing of the FeAl sheets. The rolling mill designed for FeAl processing consisted of two 12.5-cm diameter carbide working rolls and two 50-cm diameter steel backup rolls. Densification of the porous sheets can be achieved by multiple cold rolling of the sheets through the carbide rolls from 0.625 mm to 0.2 mm Žwith a 68% reduction.. Densification in a single stage required a large number of passes through the roll, and the final sheet exhibited extensive cracks. Onset of cracking was followed by monitoring the work hardening characteristics of FeAl sheet by hardness measurements. Sheets cracked invariably when the Vickers microhardnesses reached close to 500 Hv and the onset of cracking was observed to be around 480 Hv. Therefore, densification was achieved in three stages as shown schematically in Fig. 7. In stage I, cold rolling was carried out with approximately 50% reduction in thickness prior to annealing at 11508C for 1 h in vacuum. In stage II, annealing was needed after only approximately 15]20% reduction in thickness. Final sheet thickness was accomplished in stage III. Final annealing resulted in a fully dense sheet.
Fig. 8 shows the evolution of microstructure obtained after annealing at different sheet thickness. The hardness of the FeAl sheets is significantly influenced by the rate of cooling employed in the final annealing step as can be seen in Fig. 9. This is attributed to the vacancy hardening of iron aluminides. Our results are in accordance with the observations reported by several researchers w13x.
3. Mechanical properties
Iron aluminide sheets obtained by roll compaction have been extensively characterized for their electrical properties, specific heat, electrical resistivity, tensile strengths, creep and relaxation behavior and fatigue life times as a function of temperature w21]23x. Yield strength, ultimate tensile strengths and tensile elongation of densified FeAl sheets as a function of temperature at 0.2 mm thickness are shown in Fig. 10. All the tensile specimens were annealed in vacuum at 7008C for 2 h followed by furnace cooling to 4008C before testing. Tensile properties shown in Fig. 10 were obtained on punched specimens in air at 0.02 sy1 strain rate. Yield strength ŽFig. 10a. decreases initially due to environmental embrittlement, and then increases with temperature until 6008C. Yield strength falls down precipitously between 700 and 8008C. Yield strengths of Fe]24 wt.% Al cast in air ŽAIM. and in vacuum ŽVIM. using induction melting techniques are also shown in Fig. 10a. Also shown are the yield strengths of the powder-processed FA-385 wof composition
Fig. 7. Schematic illustration of percent reduction, hardness increase and the annealing sequence as a function of number of times the sheet is passed through the carbide rolls.
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Fig. 8. Microstructures obtained at different sheet thickness illustrating the elimination of porosity with decrease of thickness.
was predominantly transgranular cleavage with a small proportion of intergranular fracture. A mixed fracture mode consisting of transgranular and intergranular was evident between room temperature and 6008C, and the proportion of transgranular fracture mode increased with increase of temperature. The fracture mode was dimple-type rupture with considerable plasticity at or above 7008C w26x. Ductile fracture was observed at 7508C, and Fig. 11b clearly illustrates considerable plasticity and cavitation at the grain boundaries associated with the ductile failure.
4. Conclusions
Fig. 9. Effect of cooling rate on the hardness of fully densified FeAl sheet after the final anneal.
Žat.%.: Al, 35.8; Mo, 0.2; Zr, 0.05; C, 0.13x of Mazsiaz et al. w13x. Temperature dependence of ultimate tensile strength of FeAl is shown in Fig. 10b. Tensile elongation of FeAl sheets is close to 5% at room temperature and increases with temperature. The increase is much more pronounced above 4008C, and the tensile elongation at 8008C is 55%. Tensile properties of FeAl sheets presented in Fig. 10 agree well with the properties of the powder processed iron aluminides w24,25x. The fracture mode at room temperature ŽFig. 11a.
The roll compaction technique allowed densification of water atomized powder into a green sheet. Uniform linear mass profile density was found to be critical to prevent cracking of the sheets, and the on-line detection system based on an X-ray technique allowed manufacture of green sheets of excellent quality. Green sheets were de-bindered in a nitrogen atmosphere, and were partially sintered in vacuum prior to further densification. Sintering of green sheets at high temperatures in hydrogen and in argon formed alumina whiskers in the FeAl sheets due to the presence of trace quantities of water vapor and oxygen. Attempts to sinter porous sheets to full density in vacuum at a pressure of 10y4 torr or less resulted in
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Densification of porous sheets was achieved by a novel combination of cold rolling of FeAl sheets through a carbide roll followed by annealing in three different stages. Annealing was carried out to remove the cold work, and allow further densification of FeAl sheets. Intermediate annealing prevented cracking of the sheets due to the excessive work hardening of the FeAl. Fully dense sheets possess fine grain sizes and excellent yield strengths, ultimate tensile strengths and tensile elongations of 4]6%. Fully dense sheets have been subjected to various metal forming operations such as bending and shaping. Acknowledgements The authors appreciate and acknowledge the discussions and contributions of Mr G.S. Fleischhauer; Mr J. Cunningham, Ms V. Baliga and Dr D. Miser of Philip Morris USA; Dr R. Altomer of Kraft Foods; Mr J. Ricketts, Dr J. Reinshagen, Mr F. Ewing and Mr J. McKernen of Ametek Specialty Products, Inc.; Mr E. Hatfield, Mr D. Harper, Mr K. Blakely, Mr J. Vought and Mr R. Howell of the Oak Ridge National Laboratory. In addition, the authors greatly appreciate the management of Philip Morris USA for their support and encouragement. References
Fig. 10. A comparison of Ža. 0.2% yield strengths, Žb. ultimate tensile strengths, and Žc. tensile elongations of roll compacted FeAl sheets as a function of temperature with powder processed FA-385, and cast Fe]24 wt.% Al alloys.
densification at the center of the sheet, and evaporation of the material at the surface.
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Fig. 11. Fracture surface of roll compacted FeAl sheets following straining at Ža. room temperature, and Žb. 7508C. w9x S.C. Deevi, V.K. Sikka, Intermetallics 4 Ž1996. 357. w10x V.K. Sikka, in: J.H. Schneibel, M.A. Crimp ŽEds.., Processing, Properties, and Applications of Iron Aluminides, TMS, Warrandale, PA, 1994, p. 3. w11x D. Pocci, O. Tassa, C. Testani, in: J.H. Schneibel, M.A. Crimp ŽEds.., Processing, Properties, and Applications of Iron Aluminides, Warrandale, PA, 1994, p. 19. w12x C. Testani, A. Di Giafrancesco, O. Tassa, D. Pocci, in: S.C. Deevi, P.J. Maziasz, V.K. Sikka, R.W. Cahn ŽEds.., Nickel and Iron Aluminides: Processing, Properties, and Applications, ASM International, Material Park, OH, 1997, p. 213. w13x P.J. Maziasz, G.M. Goodwin, D.J. Alexander, S. Viswanathan, in: S.C. Deevi, P.J. Maziasz, V.K. Sikka, R.W. Cahn ŽEds.., Nickel and Iron Aluminides: Processing, Properties, and Applications, ASM International, Material Park, OH, 1997, p. 157. w14x S.C. Deevi, V.K. Sikka, in: S.C. Deevi, P.J. Maziasz, V.K. Sikka, R.W. Cahn ŽEds.., Nickel and Iron Aluminides: Processing, Properties, and Applications, ASM International, Material Park, OH, 1997, p. 283. w15x S.C. Deevi, Mater. Sci. Eng. Žaccepted.. w16x M.R. Hajaligol, N. Grant, A. Farah, in: Proceedings of the 1998 International Conference on Powder Metallurgy and Particulate Materials, MPIF, Princeton, NJ, 1998 Žaccepted for publication..
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