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Titanium aluminide foils by elemental powder processing
Materials Science and Engineering, A 151 (1992) L27-L29
L27
Letter
Titanium aluminide foils by elemental powder processing M. Dahms GKSS-Forschungszentrurn, Max-Planck-Str., W-2054 Geesthacht (FRG)
S. Schwantes Dornier Luftfahrt Gmb H, Postfach 1303, IV-7990 Friedrichshafen 1 (FRG) (Received November 13, 1991 )
Abstract A procedure is presented for the production of titanium aluminide foils by elemental powder processing which consists of three main parts. (i) Blended elemental powders are cold extruded to produce fully dense rectangular bars. (ii) The bars are rolled to foils of the desired thickness. (iii) The foils are annealed to obtain titanium aluminides. Annealing is carried out under pressure to avoid the formation of Kirkendall pores. It is shown that it is possible to produce fully dense foils using this procedure.
1. Introduction Titanium aluminides have been proposed for application in air frame structures in future aerospace designs [1]. Such an application requires the availability of these alloys as sheets and foils in large quantities. Small-scale foil production has recently been reported [2], but only for Ti3Al-base alloys. For sheet production, it is possible to use low-pressure plasma spray [3]. For TiAl-base alloys, continuous casting is under development [4] to produce TiAl-base sheets with a thickness of approximately 1 mm. For TiAI foil production, isothermal rolling of cast sheets is planned [5]. All of these methods require high temperatures and pressures for the hot extrusion, forging or hot isostatic pressing. It is the aim of this paper to show that it is possible to prepare titanium aluminides using the ductility of the elements which allows cold working.
2. Processing A mixture of titanium and 34 wt.% aluminium, i.e. 48 at.%, was prepared by blending commercially available elemental powders with a maximum grain size of 100/~m, using a conventional tumbling mixer under air. The blended powders were compacted at room temperature in a uniaxial pressing machine (p = 500 MPa) to billets of 50 mm in diameter. The compacts were extruded at room temperature [6] to rectangular bars of 10 mm x 20 mm, i.e. the extrusion ratio was 10. The processing was similar to that described previously [7]. After extrusion, parts of the bars were rolled at temperatures below 300 °C to foils of 100 ~m thickness, i.e. the total rolling strain was 99%. The rolling direction was perpendicular to the extrusion direction, in order to produce foils with a width of more than 20 mm. In this case, the foil width is defined by the cutting length of the bar. Here, a width of 50 mm was chosen because a laboratory rolling mill was used. The length of the foil is defined by the width of the extruded bar. Theoretically, rolling a bar of 10 mm thickness to 100 ~m results in an elongation from 20 mm to 2 m, if no broadening of the foil takes place. In our case, the foil was cut into pieces at different rolling steps for additional investigations. At the final thickness, a piece of 500 mm length was available. After rolling, the foil was reacted in three different ways to form titanium aluminides. (i) Reactive hot isostatic pressing (RHIP) at 1100 °C using a pressure of 125 MPa for 1 h. These conditions have been applied previously to extruded samples [8], and it was known that they would lead to a fully reacted microstructure consisting of TiAI and Ti3AI [8]. The treatment was carried out without encapsulation because no barrier materials are known to prevent diffusion bonding between foil and capsule. (ii) Uniaxial pressing at 540 °C using a pressure of 2 MPa for 1 h at a heating rate of 1 K min- 1. The temperature and pressure are upper limits for aluminium superplastic-forming devices [9]. The successful formation of the aluminides mentioned above would allow processing of large foils with advanced aluminium technology. Elsevier Sequoia
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M. Dahms and S. Schwantes
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Titanium aluminide foils by elemental powder processing
(iii) Uniaxial pressing at 950°C using the same pressure, time and heating rate. The temperature and pressure are again upper limits for superplasticforming devices of titanium alloys [10]. The successful formation of the aluminides would allow processing of large titanium aluminide foils using advanced titanium technology.
3. Microstructurai development Figures 1 and 2 show the as-extruded and as-rolled microstructures respectively. In Fig. 1, it can be seen that, because of the low extrusion ratio, some porosity (dark) is present in the as-extruded sample. Alignment and clustering of titanium particles (grey) parallel to the
horizontal wider side (20 mm) of the extruded bar can be observed, indicating that, due to the higher flow stress of titanium, deformation during extrusion is inhomogeneous. The maximum height of a titanium cluster is about 100/zm. In Fig. 2, it can be seen more clearly that deformation is inhomogeneous in the as-rolled sample. A total rolling strain of 99% will lead to titanium fibres with a maximum thickness of 1 /zm. Nevertheless, the maximum thickness of a titanium cluster after rolling is about 50 /zm being slightly elongated. The rhombohedral shape of the clusters indicates that the major deformation process is shear in the aluminium matrix. Figures 3-5 show the microstructures after the three different treatments of the rolled foil. The RHIP treatment leads to a porous microstructure (see Fig. 3). This correlates with a doubled thickness of the foil. Obviously, the Kirkendall porosity formed during the interdiffusion of titanium and aluminium is open such that
10.'9 ~ m Fig. 1. Microsection of an extruded rod perpendicular to the extrusion direction; the subsequent rolling direction is horizontal. Fig. 3. Microsection of the reactive hot isostatically pressed foil.
Fig. 2. Microsection of a rolled foil; the rolling direction is horizontal. ' Fig.4. Microsection of foil reacted at 540 °C.
M. Dahms and S. Schwantes
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Titanium aluminide foils by elemental powder processing
L29
Fig. 5. Microsection of foil reacted at 950 °C.
Fig. 6. Total view of foil reacted at 950 °C.
argon, which is the pressure medium in the HIP unit, can pass into the pores. In this case, the hydrostatic gas pressure has no effect, and the resulting microstructure is comparable with that obtained without pressure (see ref. 7). At 540 °C (Fig. 4), the applied uniaxial pressure is not sufficient to close the pores, and the reaction is not complete. As determined by energy dispersive X-ray analysis (EDX), the foil consists mainly of a mixture of pure titanium and the intermetallic phase A13Ti. Some pure aluminium still remains as can be seen in Fig. 4. T h e observation of these phases is typical for early stages of interdiffusion between aluminium and titanium [ 11, 12]. At 950 °C (Fig. 5), although the uniaxial pressure is very low, almost no porosity can be seen after the reaction. This may be due to the low heating rate. Pure titanium has a very low creep resistance at elevated temperatures. Finally, the material is fully reacted to give a mixture of Ti3AI and TiAI as determined by EDX. In addition, it was possible to prepare a piece of 70 mm x 50 mm without macroscopic cracks as shown in Fig. 6.
techniques are available on a large scale, it should be possible to produce titanium aluminide foils on a large scale.
4. Conclusions It has been shown that it is possible to produce titanium aluminide foils using a combination of a standard powder metallurgy technique (extrusion), aluminium technology (rolling) and advanced titanium technology (superplastic-forming device). Because all of these
References 1 BMFT-Hypersonic Technology Programme, First Interim Report, IABG, Ottobrunn, February 1990. 2 S. C. Jha and J. A. Forster, in Proc. 1990 Int. Conf. on Titanium Products and Applications, Titanium Development Association, Lake Buena Vista, FL, 1990, pp. 160-169. 3 W. Freeman, Gorham Aluminides Conference, Gorham Institute, Monterey, CA, 1989.
4 M. Matsuo, T. Hanamura, M. Kimura, N. Mashahashi, T. Mizoguchi and T. Miyazawa, Iron and Steel Institute of Japan International 31 (1991 ) 289-297. 5 RD Institute of Metals and Composites for Future Materials, Japan, 1991. 6 W. B6cker and H. J. Bunge, Metall (Berlin), 42 (1988) 466-471. 7 M. Dahms, Mater. Sci. Eng. A, 110 (1989) L5-L8. 8 M. Dahms, T. Pfullmann, J. Seeger and B. Wildhagen, in Y.-W. Kim and R. R. Boyer (eds.), TMS Fall Meeting 1990, Proc. Symp. on Microstructure/Property Relationships in Titanium Alloys and Titanium Aluminides, October 8-11, 1990, Detroit, MI, USA, The Minerals, Metals and Materials
Society, Warrendale, PA, 1991, pp. 337-344. 9 K.A. Padmanahan and G. J. Danes, Superplasticity, Springer, Berlin, 1980. 10 S.P. Agrawal and E. D. Weisert, in Proc. 7th North American Metal-Working Research Conf., Ann Arbor, M1, 1979.
11 F. J. J. van Loo and G. D. Rieck, Acta Metall., 21 (1973) 61-72. 12 M. Dahms, J. Seeger, W. Smarsly and B. Wildhagen, Iron and Steel Institute of Japan International 31 ( 1991 ) 1093.
L27
Letter
Titanium aluminide foils by elemental powder processing M. Dahms GKSS-Forschungszentrurn, Max-Planck-Str., W-2054 Geesthacht (FRG)
S. Schwantes Dornier Luftfahrt Gmb H, Postfach 1303, IV-7990 Friedrichshafen 1 (FRG) (Received November 13, 1991 )
Abstract A procedure is presented for the production of titanium aluminide foils by elemental powder processing which consists of three main parts. (i) Blended elemental powders are cold extruded to produce fully dense rectangular bars. (ii) The bars are rolled to foils of the desired thickness. (iii) The foils are annealed to obtain titanium aluminides. Annealing is carried out under pressure to avoid the formation of Kirkendall pores. It is shown that it is possible to produce fully dense foils using this procedure.
1. Introduction Titanium aluminides have been proposed for application in air frame structures in future aerospace designs [1]. Such an application requires the availability of these alloys as sheets and foils in large quantities. Small-scale foil production has recently been reported [2], but only for Ti3Al-base alloys. For sheet production, it is possible to use low-pressure plasma spray [3]. For TiAl-base alloys, continuous casting is under development [4] to produce TiAl-base sheets with a thickness of approximately 1 mm. For TiAI foil production, isothermal rolling of cast sheets is planned [5]. All of these methods require high temperatures and pressures for the hot extrusion, forging or hot isostatic pressing. It is the aim of this paper to show that it is possible to prepare titanium aluminides using the ductility of the elements which allows cold working.
2. Processing A mixture of titanium and 34 wt.% aluminium, i.e. 48 at.%, was prepared by blending commercially available elemental powders with a maximum grain size of 100/~m, using a conventional tumbling mixer under air. The blended powders were compacted at room temperature in a uniaxial pressing machine (p = 500 MPa) to billets of 50 mm in diameter. The compacts were extruded at room temperature [6] to rectangular bars of 10 mm x 20 mm, i.e. the extrusion ratio was 10. The processing was similar to that described previously [7]. After extrusion, parts of the bars were rolled at temperatures below 300 °C to foils of 100 ~m thickness, i.e. the total rolling strain was 99%. The rolling direction was perpendicular to the extrusion direction, in order to produce foils with a width of more than 20 mm. In this case, the foil width is defined by the cutting length of the bar. Here, a width of 50 mm was chosen because a laboratory rolling mill was used. The length of the foil is defined by the width of the extruded bar. Theoretically, rolling a bar of 10 mm thickness to 100 ~m results in an elongation from 20 mm to 2 m, if no broadening of the foil takes place. In our case, the foil was cut into pieces at different rolling steps for additional investigations. At the final thickness, a piece of 500 mm length was available. After rolling, the foil was reacted in three different ways to form titanium aluminides. (i) Reactive hot isostatic pressing (RHIP) at 1100 °C using a pressure of 125 MPa for 1 h. These conditions have been applied previously to extruded samples [8], and it was known that they would lead to a fully reacted microstructure consisting of TiAI and Ti3AI [8]. The treatment was carried out without encapsulation because no barrier materials are known to prevent diffusion bonding between foil and capsule. (ii) Uniaxial pressing at 540 °C using a pressure of 2 MPa for 1 h at a heating rate of 1 K min- 1. The temperature and pressure are upper limits for aluminium superplastic-forming devices [9]. The successful formation of the aluminides mentioned above would allow processing of large foils with advanced aluminium technology. Elsevier Sequoia
L28
M. Dahms and S. Schwantes
/
Titanium aluminide foils by elemental powder processing
(iii) Uniaxial pressing at 950°C using the same pressure, time and heating rate. The temperature and pressure are again upper limits for superplasticforming devices of titanium alloys [10]. The successful formation of the aluminides would allow processing of large titanium aluminide foils using advanced titanium technology.
3. Microstructurai development Figures 1 and 2 show the as-extruded and as-rolled microstructures respectively. In Fig. 1, it can be seen that, because of the low extrusion ratio, some porosity (dark) is present in the as-extruded sample. Alignment and clustering of titanium particles (grey) parallel to the
horizontal wider side (20 mm) of the extruded bar can be observed, indicating that, due to the higher flow stress of titanium, deformation during extrusion is inhomogeneous. The maximum height of a titanium cluster is about 100/zm. In Fig. 2, it can be seen more clearly that deformation is inhomogeneous in the as-rolled sample. A total rolling strain of 99% will lead to titanium fibres with a maximum thickness of 1 /zm. Nevertheless, the maximum thickness of a titanium cluster after rolling is about 50 /zm being slightly elongated. The rhombohedral shape of the clusters indicates that the major deformation process is shear in the aluminium matrix. Figures 3-5 show the microstructures after the three different treatments of the rolled foil. The RHIP treatment leads to a porous microstructure (see Fig. 3). This correlates with a doubled thickness of the foil. Obviously, the Kirkendall porosity formed during the interdiffusion of titanium and aluminium is open such that
10.'9 ~ m Fig. 1. Microsection of an extruded rod perpendicular to the extrusion direction; the subsequent rolling direction is horizontal. Fig. 3. Microsection of the reactive hot isostatically pressed foil.
Fig. 2. Microsection of a rolled foil; the rolling direction is horizontal. ' Fig.4. Microsection of foil reacted at 540 °C.
M. Dahms and S. Schwantes
/
Titanium aluminide foils by elemental powder processing
L29
Fig. 5. Microsection of foil reacted at 950 °C.
Fig. 6. Total view of foil reacted at 950 °C.
argon, which is the pressure medium in the HIP unit, can pass into the pores. In this case, the hydrostatic gas pressure has no effect, and the resulting microstructure is comparable with that obtained without pressure (see ref. 7). At 540 °C (Fig. 4), the applied uniaxial pressure is not sufficient to close the pores, and the reaction is not complete. As determined by energy dispersive X-ray analysis (EDX), the foil consists mainly of a mixture of pure titanium and the intermetallic phase A13Ti. Some pure aluminium still remains as can be seen in Fig. 4. T h e observation of these phases is typical for early stages of interdiffusion between aluminium and titanium [ 11, 12]. At 950 °C (Fig. 5), although the uniaxial pressure is very low, almost no porosity can be seen after the reaction. This may be due to the low heating rate. Pure titanium has a very low creep resistance at elevated temperatures. Finally, the material is fully reacted to give a mixture of Ti3AI and TiAI as determined by EDX. In addition, it was possible to prepare a piece of 70 mm x 50 mm without macroscopic cracks as shown in Fig. 6.
techniques are available on a large scale, it should be possible to produce titanium aluminide foils on a large scale.
4. Conclusions It has been shown that it is possible to produce titanium aluminide foils using a combination of a standard powder metallurgy technique (extrusion), aluminium technology (rolling) and advanced titanium technology (superplastic-forming device). Because all of these
References 1 BMFT-Hypersonic Technology Programme, First Interim Report, IABG, Ottobrunn, February 1990. 2 S. C. Jha and J. A. Forster, in Proc. 1990 Int. Conf. on Titanium Products and Applications, Titanium Development Association, Lake Buena Vista, FL, 1990, pp. 160-169. 3 W. Freeman, Gorham Aluminides Conference, Gorham Institute, Monterey, CA, 1989.
4 M. Matsuo, T. Hanamura, M. Kimura, N. Mashahashi, T. Mizoguchi and T. Miyazawa, Iron and Steel Institute of Japan International 31 (1991 ) 289-297. 5 RD Institute of Metals and Composites for Future Materials, Japan, 1991. 6 W. B6cker and H. J. Bunge, Metall (Berlin), 42 (1988) 466-471. 7 M. Dahms, Mater. Sci. Eng. A, 110 (1989) L5-L8. 8 M. Dahms, T. Pfullmann, J. Seeger and B. Wildhagen, in Y.-W. Kim and R. R. Boyer (eds.), TMS Fall Meeting 1990, Proc. Symp. on Microstructure/Property Relationships in Titanium Alloys and Titanium Aluminides, October 8-11, 1990, Detroit, MI, USA, The Minerals, Metals and Materials
Society, Warrendale, PA, 1991, pp. 337-344. 9 K.A. Padmanahan and G. J. Danes, Superplasticity, Springer, Berlin, 1980. 10 S.P. Agrawal and E. D. Weisert, in Proc. 7th North American Metal-Working Research Conf., Ann Arbor, M1, 1979.
11 F. J. J. van Loo and G. D. Rieck, Acta Metall., 21 (1973) 61-72. 12 M. Dahms, J. Seeger, W. Smarsly and B. Wildhagen, Iron and Steel Institute of Japan International 31 ( 1991 ) 1093.