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Fabrication of unidirectional porous TiAl–Mn intermetallic compounds by reactive sintering using extruded powder mixtures
Intermetallics 11 (2003) 849–855 www.elsevier.com/locate/intermet
Fabrication of unidirectional porous TiAl–Mn intermetallic compounds by reactive sintering using extruded powder mixtures S.H. Yanga, W.Y. Kimb, M.S. Kima,* a
School of Materials Science and Engineering, Inha University, #253 Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea b Korea Institute of Industrial Technology, Incheon 404-251, South Korea Received 8 January 2003; received in revised form 1 April 2003; accepted 23 April 2003
Abstract The unidirectional porous Ti–45at.%Al–1.6at.%Mn intermetallic compounds having fully lamellar structure were fabricated by reactive sintering method using the extruded mixtures consisting of elemental titanium and aluminum-manganese powders. The average porosity of the specimens is measured to have a distribution in the range from 25% to 35% in volume fraction within the experimental conditions investigated. The porosity of the specimen produced decreased with increasing heating rate in the reactive sintering process. The maximum porosity of 35% can be obtained when the specimen was reactively sintered under the heating rate of 0.17 K/s. The porosity of the reactively sintered specimen can be controlled by a variation of heating rate. From the microstructural observation, we found a directionality of pores oriented parallel to the extrusion direction and suggested that an alignedstructure of pores of the present alloy is closely associated with the extrusion microstructure consisting of elongated feature of constituent mixtures. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; C. Extrusion; C. Reaction synthesis; C. Sintering
1. Introduction Porous metals are of great interest as a potential engineering material in the field of various industry because of their unique properties such as impact energy absorption capacity, air and water permeability, thermal conductivity, electrical insulating properties and so on [1–5]. During the last decades, a number of investigations for the formation of gas hole during solidification have been carried out on mainly metals and alloys [4]. However, few studies are conducted on porous intermetallic compounds [5]. Intermetallic alloys have been studied for high temperature structural and functional applications in the next generation because they have excellent mechanical properties at high temperature, good corrosion resistance and oxidation resistance. Most extensive studies have been performed on nickel- and titanium-aluminides such as Ni3Al, NiAl, Ti3Al and TiAl. Among * Corresponding author. Tel.: +82-32-860-7541; fax: +82-32-8683607. E-mail address: [email protected] (M.S. Kim).
those intermetallic alloys, TiAl-based alloys are of interest. Gamma (g) TiAl-based intermetallic compounds have a strong potential for high temperature structural applications because of their high melting point (1800 K), high specific strength, low density (3.8 g/cm3), and therefore can be a good candidate to use the material at temperature up to 1173 K in the replacement of Ni-base superalloys those density is more than 2 times compared to that of the present alloy [6]. Since high temperature strength properties have been found to be similar to both alloys, an improved energy efficiency would be expected due to lightweight of the present TiAl-based alloys. With incorporation of desired pores, porous structure, in the microstructure of TiAl-based alloys, an application field of TiAl-based intermetallic compounds would be extensively broadened as not only structural materials but also functional ones. To the best of author’s knowledge, there is no report on porous TiAlbased intermetallic alloys. As one of applications of the present alloys based on gamma (g) TiAl, various kinds of artificial composites could be attained by infiltration
0966-9795/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0966-9795(03)00082-7
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methods. Depending on desired properties and applications, the most appropriate composites could be chosen in terms of a variation of filling materials. Moreover, porous TiAl-based intermetallic alloy itself may be highly recommended as a high temperature filter material since it has excellent thermal stability even at very high temperature. Recently, Nakajima et al. have produced porous metals with unidirectional pore structure in terms of unidirectional solidification method [3]. The porous metals with unidirectional pore structure have mechanical property superior to that of conventional porous metals with randomly distributed pore structure [4]. It is therefore recognized that unidirectional structure of porous materials has highly potential in the structural and functional fields of materials. A reactive sintering has been considered to be an attractive process in order to produce TiAl-based intermetallic compounds [7]. In conventional procedure for reactive sintering, it contains three kinds of processing stages in the following order; mixing of elemental powders, compaction for desired shape and finally sintering. In the present study, a severe plastic working was applied using warm extrusion in order to produce a rodshaped mixture consisting of elemental titanium and aluminum–manganese, which has an alined microstructure. The mixture consolidated was then processed by reactive sintering, which proceeds by exothermic reaction between elemental titanium and aluminum– manganese powders. In our previous study on the fabrication of TiAl–Mn intermetallic compounds by reactive sintering it was revealed that heating rate plays an important role in order to control the pores developed during the processing [8]. In this study, unidirectional porous Ti– 45at.%Al–1.6at.%Mn intermetallic compounds are fabricated by reactive sintering using extruded mixtures of elemental titanium and aluminum–manganese powders. We aim at understanding the effect of heating rate in the reactive sintering process on the pore formation for the present alloy.
2. Experimental procedure The starting raw materials were a commercially pure titanium powder (< 150 mm, supplied by Toho Titanium Co., Ltd.) and an Al–3.6at.%Mn alloy powder produced by argon gas atomization (< 150 mm, supplied by Sumitomo Light Metal Ind., Ltd.). Mn was added as an agent in the reactive sintering processing in order to expect improved mechanical properties. These powders were mixed in a desired molecular fraction using a V-type mixer for 2 h, and then canned in an aluminum container for degassing at 723 K for 2 h. The degassed powder mixtures were consolidated by warm extrusion.
Warm extrusion was performed at 700 K with the extrusion ratio of 20 to 1. The aluminum can sealed was removed by mechanical machining after warm extrusion. Then, the extruded specimen was reactively sintered up to 1073 K with various heating rates in the range from 0.17 K/s to 1.7 K/s using specially-designed rapid heating/cooling system in air. If necessary, argon or vacuum atmosphere were also used. The holding time at target temperature was 1s and the cooling rate was controlled to 10 K/s. Then, the reactively sintered specimen was heat treated at 1623 K for 2 h in vacuum in an attempt to obtain a homogeneous, fully lamellar structure. Microstructural observation was carried out using an optical microscope (OM), an electron probe micro analyzer (EPMA), and image analyzer. X-ray diffraction (XRD) analysis was conducted on the reactively sintered specimens to identify the constituent phases. And, thermal analysis of the extruded specimens was carried out using differential thermal analysis (DTA) equipment.
3. Results 3.1. Microstructure Back scattering electron microscope (BEM) was used in distinguishing the constituent phases in the as-extruded microstructure. Fig. 1(a) and (b) show the typical microstructures corresponding to the transverse and longitudinal sections of extrusion direction for the powder mixtures extruded, respectively. The lightly and darkly contrasted regions in the figures correspond to aluminum-manganese powders and titanium powders, respectively, as indicated by arrows in Fig. 1(a) and (b). Well-defined microstructure with elongated feature of powder mixtures to the extrusion direction is observed through the whole specimen, as seen in Fig. 1(b). This result may indicate that the powder mixtures were severely strained during the warm extrusion. The distribution of powder mixtures is observed to be homogeneous without any aggregates that may result in insufficient mixing of elemental powders. Apparent feature of the microstructures shown in Fig. 1(a) and (b) exhibits that there are no detectable defects such as micro pores or cracks and additional phases which can be a reaction product if the warm extrusion applied was controlled by a diffusion process. Typical OM micrographs of some specimens that were prepared by reactive sintering after warm extrusion are presented in Fig. 2. The lightly and darkly contrasted regions in the microstructure are confirmed to be reaction product and pore, respectively. Fig. 2(a) and (c) are microstructures corresponding transverse and longitudinal sections of the specimen which was produced by reactive sintering up to 1073 K with the
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using warm extrusion and subsequent reactive sintering process used in this study. Measurement of porosity was conducted on the transverse section of specimens using an image analyzer equipped with a computer system. Fig. 3 shows the average porosity of the specimens plotted as a function of heating rate in the reactive sintering process. It is very evident that porosity decreases with increasing heating rate. The maximum porosity of 35% can be obtained for the specimen reactively sintered with the heating rate of 0.17 K/s. 3.2. Phase analysis
Fig. 1. OM micrographs of the (a) transverse and (b) longitudinal sections of extrusion direction for the extruded specimen consisting of titanium and aluminum–3.6 at.% manganese powders. Note that lightly and darkly contrasted regions are aluminum-manganese and titanium powders, respectively.
heating rate of 0.17 K/s. Fig. 2(b) and (d) are microstructures corresponding transverse and longitudinal sections of the specimen which was produced by reactive sintering up to 1073 K with the heating rate of 1.7 K/s. A noticeable difference in volume fraction of pore between specimens prepared with different heating rate can be observed through the microstructure. Clearly, the volume fraction of the specimen prepared with the heating rate of 0.17 K/s is higher than that of the specimen processed with the heating rate of 1.7 K/s. This result evidently indicates that heating rate plays a significant role to control porosity in the present process. An aligned-pore structure parallel to the extrusion direction is observed in the microstructures of longitudinal sections, as seen in Fig. 2(c) and (d). This may be closely associated with the warm-extruded microstructure before reactive sintering. It is therefore suggested that a unidirectional pore structure can be successfully formed in the present TiAl-based alloys
Phase analysis for three different reactively sintered specimens was done by XRD, and the results are presented in Fig. 4. The data points shown as filled circles and filled squares in the figure indicate peaks due to the DO19 structured-Ti3Al and L10 structured-TiAl, respectively. No distinguishable difference in relative intensity for the three alloys is found, indicating that heating rate may not be important factor to verify the reaction products within the range studied. Those phases diffracted in XRD patterns have been known to exist at the given experimental condition of the Ti–45at.%Al in binary Ti–Al phase diagram. Besides the Ti3Al and TiAl intermetallic phases, intermediate TiAl3 and Ti3Al5 phases are observed, as seen in Fig. 4. The occurrence of TiAl3 and Ti3Al5 phases may be explained by insufficient diffusion activity during reactive sintering process since the chemical composition of the present alloy is within the two-phase region consisting of TiAl and Ti3Al. This result directly implies that an appropriate heat treatment after reactive sintering should be provided in order to obtain an equilibrium microstructure with a fully lamellar structure consisting of TiAl and Ti3Al. Fig. 5 shows an OM micrograph for the specimen heat treated at 1623 K for 2 h in vacuum after reactive sintering with the heating rate of 0.17 K/s. As expected, a fully lamellar microstructure with elongated feature of pore parallel to extrusion direction is seen over the whole specimen. Basically, it is observed that no discernable change for the pore structure, volume fraction of pore and its morphology even after equilibrium heat treatment at 1623 K for 2 h is attained, indicating that the porous TiAl–Mn intermetallic alloys are thermally stable and can be produced by means of reactive sintering and subsequent heat treatment after warm extrusion of elemental powder mixtures.
4. Discussion In the present study, we have focused on pore formation of the TiAl–Mn intermetallic alloys having a fully lamellar microstructure through the reactive sintering and subsequent heat treatment after warm extrusion
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Fig. 2. OM micrographs of the transverse (a, b) and longitudinal sections (c, d) for the specimens prepared by reactive sintering with the heating rate of 0.17-K/s (a, c) and 1.7-K/s (b, d).
Fig. 3. The average porosity of reactively sintered specimen plotted as a function of heating rate.
Fig. 4. The XRD profiles of the reactively sintered specimens produced as a function of heating rate: (a) 0.17 K/s, (b) 0.8 K/s, (c) 1.7 K/s.
using elemental powder mixtures. The average porosity of reactively sintered specimens decreased with increasing heating rate in the reactive sintering process, as shown in Fig. 3. In order to investigate the effect of heating rate on formation behavior of the pore during reactive sintering process, DTA for the extruded mixtures was carried out under argon atmosphere with various heating rates, and the results are presented in Fig. 6. At a low heating rate of 0.17 K/s, DTA result shows that the peak corresponding to the exothermic
reaction is only detected at around 930 K which temperature is slightly higher than Al melting point. In contrasts, when the heating rate is at and above 0.5 K/s, peaks showing endothermic reaction appear in front of the exothermic reaction peak. The peaks exhibiting endothermic reaction are detected at around Al melting point. Therefore, the endothermic peak can be correlated to the partial melting of Al due to the higher heating rate, which may be insufficient to cause an intermetallic phase as a reaction product between aluminum-manganese and
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Fig. 5. OM micrograph of the specimen produced by reactive sintering with the heating rate of 0.17 K/s using extruded mixtures consisting of titanium and aluminum–3.6 at.% manganese powders and subsequent heat treatment at 1623 K for 2 h.
Fig. 6. The DTA profiles of the specimens produced by reactive sintering with various heating rates: (a) 1.7 K/s, (b) 1.4 K/s, (c) 0.8 K/s, (d) 0.5 K/s, (e) 0.17 K/s.
titanium powders. Similar result has been reported by Wang et al. [9]. With increasing heating rate the integrated area of endothermic peak that can be related to the heat developed from the melting of Al is found to be larger. The increase in integrated area means the increase in the amount of molten Al. Thus, a melting of Al can be accelerated with increasing heating rate. The present result is well consistent with the previous report by Wang et al. [10] that endothermic reaction is activated with increasing heating rate and Al content in Ti–Al alloy system. From the DTA result shown in Fig. 6, we suggest that reactive sintering process for the present alloy can be divided into two stages. The first stage is for the solid-state
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diffusion regime in front of the occurrence of endothermic reaction. The second stage is for the reaction regime of constituent elements involving both endothermic and exothermic reaction at temperatures higher than Al melting point. In the solid-state diffusion regime, the Kirkendall void could be caused by large difference in diffusion coefficient between constituent elements [9]. When the extruded specimen was heat treated at 903 K for 1 s with the heating rate of 0.17 K/s which temperature is lower than the reaction temperatures showing endothermic and exothermic reactions, no detectable difference in the microstructure was observed between the heat treated and the extruded samples, indicating that porous structure in the microstructure seems hardly formed in the solid-state diffusion stage. This would be most likely due to the insufficient keeping time of 1 s at 903 K. It is therefore considered that porous structure of the present extruded alloy can be developed during the second stage involving exothermic and endothermic reactions. It was reported that pore formation during an exothermic reaction is due to vaporization of gases in NiAl and FeAl system [11]. Such explanation, however, may not be useful for the present result since the present extruded specimen has very low gas content since a sufficient degassing treatment was performed on the powder mixtures before warm extrusion. Also, concerning the atmospheric influence on pore formation, it is found that the average porosities of the reactively sintered specimens in air, Ar and vacuum using the same heating rate of 1.7 K/s are 25.6, 24.6 and 26.4% in volume fraction, respectively, indicating that volume fraction of pore is insensitive to atmosphere of reactive sintering process. The foregoing experimental results imply that reactive sintering of the present extruded-mixtures consisting of titanium and aluminum-manganese powders will be proceeded in the following manner; Al melting which appears to an endothermic reaction peak at about 930 K, and then exothermic reaction to produce various Ti–Al based intermetallic compounds will occur in the temperature range from 930 K to 1010 K, which the reaction temperature is found to be a strong function of heating rate, as seen in Fig. 6. Thus, the formation mechanism of porous structure of the present alloy can be correlated with the combination of endothermic and exothermic reaction during reactive sintering at the given temperature with a heating rate at and above 0.5 K/s. At first Al would be melted in a prior reaction stage and then Al diffusion to Ti powders takes place to produce various intermetallic phases such as Ti3Al, TiAl, TiAl3, Ti3Al5 resulting in porous structure at the place of molten Al region since no applied pressure is attained during the processing. If we ignore the other factors to control the porosity, much porous structure should be attained with increasing heating rate during reactive
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sintering. However, the porosity decreased with increasing heating rate. This is completely reversal against our expectation. The plausible reason for this phenomenon seems related to a role of molten Al. With increasing heating rate we may consider two competitive phenomenons in relation with volume fraction of pore. One is an increase in the amount of molten Al which can decrease in volume fraction of pore. The other one is an increase in volume fraction of pore due .to activated Al diffusion to Ti powders. Therefore, the former effect may be predominant to decrease the volume fraction of pore compared to the latter effect. It has been well known that densification of a specimen prepared by reactive sintering is promoted with increasing amount of liquid phase [11]. The porosity of the present alloy decreased with increasing amount of molten Al that is characterized by an endothermic reaction in DTA result shown in Fig. 6. This observation is therefore in good agreement with the result reported previously. Fig. 7(a) and (b) show that both the relative areal fractions of endothermic and exothermic reaction increased with increasing heating rate in
reactive sintering process. As mentioned above, the former can be related with an increase in the amount of molten Al. While, the latter can be explained in relation with an increase of reaction heat for the synthesis of various intermetallic compounds by reactive sintering. From the result obtained, an amount of molten Al in a reactive sintering process may have a positive role in order to produce a dense specimen. Therefore, higher heating rate leads to dense structure of the present alloy in the reactive sintering process. In the present study, we have successfully produced unidirectional porous TiAl–Mn intermetallic alloy having fully lamellar structure by means of warm extrusion of elemental powder mixtures and then subsequent reactive sintering up to 1073 K for 1s and finally an appropriate heat treatment at 1623 K for 2 h, as shown in Figs. 2 and 5. This unidirectional pore structure is likely correlated to the extruded microstructure consisting of elongated feature of constituent powder mixtures. This suggests that warm extrusion of powder mixtures is effective to produce a unidirectional porous TiAl–Mn intermetallic alloy with fully lamellar microstructure using reactive sintering and an appropriate heat treatment. Much more works such as mechanical and physical properties are required as a next subject.
5. Conclusions The pore characteristics in TiAl–Mn intermetallic compounds were investigated as a function of heating rate using reactive sintering of extruded mixtures consisting of titanium and aluminum-manganese powders. The following results were obtained from the present study. 1. The unidirectional porous TiAl–Mn intermetallic compounds having fully lamellar structure can be successfully produced in terms of warm extrusion of powder mixtures consisting of titanium and aluminum-manganese powders and subsequent reactive sintering and finally heat treatment at 1623 K for 2 h. It is suggested that warm extrusion of powder mixtures plays a key role to produce unidirectional porous TiAl–Mn intermetallic alloy. 2. The average porosity decreases with increasing heating rate in the reactive sintering process, and the maximum porosity of 35% is obtained for the reactively sintered specimen with the heating rate of 0.17 K/s. 3. It is revealed that the decrease in porosity with increasing heating rate is closely related to the increase of molten aluminum.
References
Fig. 7. Relationship between normalized areal fraction and porosity for both the endothermic reaction (a) and exothermic reaction (b).
[1] Wen CE, Mabuchi M, Yamada Y, Shimojima K, Chino Y, Asahina T. Scripta Mater 2001;45:1147. [2] Simone AE, Gibson LJ. Mater Sci and Eng 1997;A229:55.
S.H. Yang et al. / Intermetallics 11 (2003) 849–855 [3] Yamanura S, Shiota H, Murakami K, Nakajima H. Mater Sci and Eng 2001;A318:137. [4] Hyun SK, Murakami K, Nakajima H. Mater Sci and Eng 2001; A299:241. [5] Li BY, Rong LJ, Li YY, Gjunter VE. Acta Mater 2000;48:3895. [6] Clemens H, Kestler H. Advanced Engineering Materials 2000; 9:551.
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[7] Kim MS. J Japan Inst Metals 1994;58:819. [8] Yang SH, Hyun SK, Kim MS. J Kor Inst Met & Mater 1999; 37:1246. [9] Wang GX, Dahms M. Powder Metallurgy 1992;24:219. [10] Wang X, Sohn HY, Schlesinger ME. Mat Sci Eng 1994; A186:151. [11] Munir ZA. Ceramic Bulletin 1998;67:342.
Fabrication of unidirectional porous TiAl–Mn intermetallic compounds by reactive sintering using extruded powder mixtures S.H. Yanga, W.Y. Kimb, M.S. Kima,* a
School of Materials Science and Engineering, Inha University, #253 Yonghyun-dong, Nam-gu, Incheon 402-751, South Korea b Korea Institute of Industrial Technology, Incheon 404-251, South Korea Received 8 January 2003; received in revised form 1 April 2003; accepted 23 April 2003
Abstract The unidirectional porous Ti–45at.%Al–1.6at.%Mn intermetallic compounds having fully lamellar structure were fabricated by reactive sintering method using the extruded mixtures consisting of elemental titanium and aluminum-manganese powders. The average porosity of the specimens is measured to have a distribution in the range from 25% to 35% in volume fraction within the experimental conditions investigated. The porosity of the specimen produced decreased with increasing heating rate in the reactive sintering process. The maximum porosity of 35% can be obtained when the specimen was reactively sintered under the heating rate of 0.17 K/s. The porosity of the reactively sintered specimen can be controlled by a variation of heating rate. From the microstructural observation, we found a directionality of pores oriented parallel to the extrusion direction and suggested that an alignedstructure of pores of the present alloy is closely associated with the extrusion microstructure consisting of elongated feature of constituent mixtures. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; C. Extrusion; C. Reaction synthesis; C. Sintering
1. Introduction Porous metals are of great interest as a potential engineering material in the field of various industry because of their unique properties such as impact energy absorption capacity, air and water permeability, thermal conductivity, electrical insulating properties and so on [1–5]. During the last decades, a number of investigations for the formation of gas hole during solidification have been carried out on mainly metals and alloys [4]. However, few studies are conducted on porous intermetallic compounds [5]. Intermetallic alloys have been studied for high temperature structural and functional applications in the next generation because they have excellent mechanical properties at high temperature, good corrosion resistance and oxidation resistance. Most extensive studies have been performed on nickel- and titanium-aluminides such as Ni3Al, NiAl, Ti3Al and TiAl. Among * Corresponding author. Tel.: +82-32-860-7541; fax: +82-32-8683607. E-mail address: [email protected] (M.S. Kim).
those intermetallic alloys, TiAl-based alloys are of interest. Gamma (g) TiAl-based intermetallic compounds have a strong potential for high temperature structural applications because of their high melting point (1800 K), high specific strength, low density (3.8 g/cm3), and therefore can be a good candidate to use the material at temperature up to 1173 K in the replacement of Ni-base superalloys those density is more than 2 times compared to that of the present alloy [6]. Since high temperature strength properties have been found to be similar to both alloys, an improved energy efficiency would be expected due to lightweight of the present TiAl-based alloys. With incorporation of desired pores, porous structure, in the microstructure of TiAl-based alloys, an application field of TiAl-based intermetallic compounds would be extensively broadened as not only structural materials but also functional ones. To the best of author’s knowledge, there is no report on porous TiAlbased intermetallic alloys. As one of applications of the present alloys based on gamma (g) TiAl, various kinds of artificial composites could be attained by infiltration
0966-9795/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0966-9795(03)00082-7
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methods. Depending on desired properties and applications, the most appropriate composites could be chosen in terms of a variation of filling materials. Moreover, porous TiAl-based intermetallic alloy itself may be highly recommended as a high temperature filter material since it has excellent thermal stability even at very high temperature. Recently, Nakajima et al. have produced porous metals with unidirectional pore structure in terms of unidirectional solidification method [3]. The porous metals with unidirectional pore structure have mechanical property superior to that of conventional porous metals with randomly distributed pore structure [4]. It is therefore recognized that unidirectional structure of porous materials has highly potential in the structural and functional fields of materials. A reactive sintering has been considered to be an attractive process in order to produce TiAl-based intermetallic compounds [7]. In conventional procedure for reactive sintering, it contains three kinds of processing stages in the following order; mixing of elemental powders, compaction for desired shape and finally sintering. In the present study, a severe plastic working was applied using warm extrusion in order to produce a rodshaped mixture consisting of elemental titanium and aluminum–manganese, which has an alined microstructure. The mixture consolidated was then processed by reactive sintering, which proceeds by exothermic reaction between elemental titanium and aluminum– manganese powders. In our previous study on the fabrication of TiAl–Mn intermetallic compounds by reactive sintering it was revealed that heating rate plays an important role in order to control the pores developed during the processing [8]. In this study, unidirectional porous Ti– 45at.%Al–1.6at.%Mn intermetallic compounds are fabricated by reactive sintering using extruded mixtures of elemental titanium and aluminum–manganese powders. We aim at understanding the effect of heating rate in the reactive sintering process on the pore formation for the present alloy.
2. Experimental procedure The starting raw materials were a commercially pure titanium powder (< 150 mm, supplied by Toho Titanium Co., Ltd.) and an Al–3.6at.%Mn alloy powder produced by argon gas atomization (< 150 mm, supplied by Sumitomo Light Metal Ind., Ltd.). Mn was added as an agent in the reactive sintering processing in order to expect improved mechanical properties. These powders were mixed in a desired molecular fraction using a V-type mixer for 2 h, and then canned in an aluminum container for degassing at 723 K for 2 h. The degassed powder mixtures were consolidated by warm extrusion.
Warm extrusion was performed at 700 K with the extrusion ratio of 20 to 1. The aluminum can sealed was removed by mechanical machining after warm extrusion. Then, the extruded specimen was reactively sintered up to 1073 K with various heating rates in the range from 0.17 K/s to 1.7 K/s using specially-designed rapid heating/cooling system in air. If necessary, argon or vacuum atmosphere were also used. The holding time at target temperature was 1s and the cooling rate was controlled to 10 K/s. Then, the reactively sintered specimen was heat treated at 1623 K for 2 h in vacuum in an attempt to obtain a homogeneous, fully lamellar structure. Microstructural observation was carried out using an optical microscope (OM), an electron probe micro analyzer (EPMA), and image analyzer. X-ray diffraction (XRD) analysis was conducted on the reactively sintered specimens to identify the constituent phases. And, thermal analysis of the extruded specimens was carried out using differential thermal analysis (DTA) equipment.
3. Results 3.1. Microstructure Back scattering electron microscope (BEM) was used in distinguishing the constituent phases in the as-extruded microstructure. Fig. 1(a) and (b) show the typical microstructures corresponding to the transverse and longitudinal sections of extrusion direction for the powder mixtures extruded, respectively. The lightly and darkly contrasted regions in the figures correspond to aluminum-manganese powders and titanium powders, respectively, as indicated by arrows in Fig. 1(a) and (b). Well-defined microstructure with elongated feature of powder mixtures to the extrusion direction is observed through the whole specimen, as seen in Fig. 1(b). This result may indicate that the powder mixtures were severely strained during the warm extrusion. The distribution of powder mixtures is observed to be homogeneous without any aggregates that may result in insufficient mixing of elemental powders. Apparent feature of the microstructures shown in Fig. 1(a) and (b) exhibits that there are no detectable defects such as micro pores or cracks and additional phases which can be a reaction product if the warm extrusion applied was controlled by a diffusion process. Typical OM micrographs of some specimens that were prepared by reactive sintering after warm extrusion are presented in Fig. 2. The lightly and darkly contrasted regions in the microstructure are confirmed to be reaction product and pore, respectively. Fig. 2(a) and (c) are microstructures corresponding transverse and longitudinal sections of the specimen which was produced by reactive sintering up to 1073 K with the
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using warm extrusion and subsequent reactive sintering process used in this study. Measurement of porosity was conducted on the transverse section of specimens using an image analyzer equipped with a computer system. Fig. 3 shows the average porosity of the specimens plotted as a function of heating rate in the reactive sintering process. It is very evident that porosity decreases with increasing heating rate. The maximum porosity of 35% can be obtained for the specimen reactively sintered with the heating rate of 0.17 K/s. 3.2. Phase analysis
Fig. 1. OM micrographs of the (a) transverse and (b) longitudinal sections of extrusion direction for the extruded specimen consisting of titanium and aluminum–3.6 at.% manganese powders. Note that lightly and darkly contrasted regions are aluminum-manganese and titanium powders, respectively.
heating rate of 0.17 K/s. Fig. 2(b) and (d) are microstructures corresponding transverse and longitudinal sections of the specimen which was produced by reactive sintering up to 1073 K with the heating rate of 1.7 K/s. A noticeable difference in volume fraction of pore between specimens prepared with different heating rate can be observed through the microstructure. Clearly, the volume fraction of the specimen prepared with the heating rate of 0.17 K/s is higher than that of the specimen processed with the heating rate of 1.7 K/s. This result evidently indicates that heating rate plays a significant role to control porosity in the present process. An aligned-pore structure parallel to the extrusion direction is observed in the microstructures of longitudinal sections, as seen in Fig. 2(c) and (d). This may be closely associated with the warm-extruded microstructure before reactive sintering. It is therefore suggested that a unidirectional pore structure can be successfully formed in the present TiAl-based alloys
Phase analysis for three different reactively sintered specimens was done by XRD, and the results are presented in Fig. 4. The data points shown as filled circles and filled squares in the figure indicate peaks due to the DO19 structured-Ti3Al and L10 structured-TiAl, respectively. No distinguishable difference in relative intensity for the three alloys is found, indicating that heating rate may not be important factor to verify the reaction products within the range studied. Those phases diffracted in XRD patterns have been known to exist at the given experimental condition of the Ti–45at.%Al in binary Ti–Al phase diagram. Besides the Ti3Al and TiAl intermetallic phases, intermediate TiAl3 and Ti3Al5 phases are observed, as seen in Fig. 4. The occurrence of TiAl3 and Ti3Al5 phases may be explained by insufficient diffusion activity during reactive sintering process since the chemical composition of the present alloy is within the two-phase region consisting of TiAl and Ti3Al. This result directly implies that an appropriate heat treatment after reactive sintering should be provided in order to obtain an equilibrium microstructure with a fully lamellar structure consisting of TiAl and Ti3Al. Fig. 5 shows an OM micrograph for the specimen heat treated at 1623 K for 2 h in vacuum after reactive sintering with the heating rate of 0.17 K/s. As expected, a fully lamellar microstructure with elongated feature of pore parallel to extrusion direction is seen over the whole specimen. Basically, it is observed that no discernable change for the pore structure, volume fraction of pore and its morphology even after equilibrium heat treatment at 1623 K for 2 h is attained, indicating that the porous TiAl–Mn intermetallic alloys are thermally stable and can be produced by means of reactive sintering and subsequent heat treatment after warm extrusion of elemental powder mixtures.
4. Discussion In the present study, we have focused on pore formation of the TiAl–Mn intermetallic alloys having a fully lamellar microstructure through the reactive sintering and subsequent heat treatment after warm extrusion
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Fig. 2. OM micrographs of the transverse (a, b) and longitudinal sections (c, d) for the specimens prepared by reactive sintering with the heating rate of 0.17-K/s (a, c) and 1.7-K/s (b, d).
Fig. 3. The average porosity of reactively sintered specimen plotted as a function of heating rate.
Fig. 4. The XRD profiles of the reactively sintered specimens produced as a function of heating rate: (a) 0.17 K/s, (b) 0.8 K/s, (c) 1.7 K/s.
using elemental powder mixtures. The average porosity of reactively sintered specimens decreased with increasing heating rate in the reactive sintering process, as shown in Fig. 3. In order to investigate the effect of heating rate on formation behavior of the pore during reactive sintering process, DTA for the extruded mixtures was carried out under argon atmosphere with various heating rates, and the results are presented in Fig. 6. At a low heating rate of 0.17 K/s, DTA result shows that the peak corresponding to the exothermic
reaction is only detected at around 930 K which temperature is slightly higher than Al melting point. In contrasts, when the heating rate is at and above 0.5 K/s, peaks showing endothermic reaction appear in front of the exothermic reaction peak. The peaks exhibiting endothermic reaction are detected at around Al melting point. Therefore, the endothermic peak can be correlated to the partial melting of Al due to the higher heating rate, which may be insufficient to cause an intermetallic phase as a reaction product between aluminum-manganese and
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Fig. 5. OM micrograph of the specimen produced by reactive sintering with the heating rate of 0.17 K/s using extruded mixtures consisting of titanium and aluminum–3.6 at.% manganese powders and subsequent heat treatment at 1623 K for 2 h.
Fig. 6. The DTA profiles of the specimens produced by reactive sintering with various heating rates: (a) 1.7 K/s, (b) 1.4 K/s, (c) 0.8 K/s, (d) 0.5 K/s, (e) 0.17 K/s.
titanium powders. Similar result has been reported by Wang et al. [9]. With increasing heating rate the integrated area of endothermic peak that can be related to the heat developed from the melting of Al is found to be larger. The increase in integrated area means the increase in the amount of molten Al. Thus, a melting of Al can be accelerated with increasing heating rate. The present result is well consistent with the previous report by Wang et al. [10] that endothermic reaction is activated with increasing heating rate and Al content in Ti–Al alloy system. From the DTA result shown in Fig. 6, we suggest that reactive sintering process for the present alloy can be divided into two stages. The first stage is for the solid-state
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diffusion regime in front of the occurrence of endothermic reaction. The second stage is for the reaction regime of constituent elements involving both endothermic and exothermic reaction at temperatures higher than Al melting point. In the solid-state diffusion regime, the Kirkendall void could be caused by large difference in diffusion coefficient between constituent elements [9]. When the extruded specimen was heat treated at 903 K for 1 s with the heating rate of 0.17 K/s which temperature is lower than the reaction temperatures showing endothermic and exothermic reactions, no detectable difference in the microstructure was observed between the heat treated and the extruded samples, indicating that porous structure in the microstructure seems hardly formed in the solid-state diffusion stage. This would be most likely due to the insufficient keeping time of 1 s at 903 K. It is therefore considered that porous structure of the present extruded alloy can be developed during the second stage involving exothermic and endothermic reactions. It was reported that pore formation during an exothermic reaction is due to vaporization of gases in NiAl and FeAl system [11]. Such explanation, however, may not be useful for the present result since the present extruded specimen has very low gas content since a sufficient degassing treatment was performed on the powder mixtures before warm extrusion. Also, concerning the atmospheric influence on pore formation, it is found that the average porosities of the reactively sintered specimens in air, Ar and vacuum using the same heating rate of 1.7 K/s are 25.6, 24.6 and 26.4% in volume fraction, respectively, indicating that volume fraction of pore is insensitive to atmosphere of reactive sintering process. The foregoing experimental results imply that reactive sintering of the present extruded-mixtures consisting of titanium and aluminum-manganese powders will be proceeded in the following manner; Al melting which appears to an endothermic reaction peak at about 930 K, and then exothermic reaction to produce various Ti–Al based intermetallic compounds will occur in the temperature range from 930 K to 1010 K, which the reaction temperature is found to be a strong function of heating rate, as seen in Fig. 6. Thus, the formation mechanism of porous structure of the present alloy can be correlated with the combination of endothermic and exothermic reaction during reactive sintering at the given temperature with a heating rate at and above 0.5 K/s. At first Al would be melted in a prior reaction stage and then Al diffusion to Ti powders takes place to produce various intermetallic phases such as Ti3Al, TiAl, TiAl3, Ti3Al5 resulting in porous structure at the place of molten Al region since no applied pressure is attained during the processing. If we ignore the other factors to control the porosity, much porous structure should be attained with increasing heating rate during reactive
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sintering. However, the porosity decreased with increasing heating rate. This is completely reversal against our expectation. The plausible reason for this phenomenon seems related to a role of molten Al. With increasing heating rate we may consider two competitive phenomenons in relation with volume fraction of pore. One is an increase in the amount of molten Al which can decrease in volume fraction of pore. The other one is an increase in volume fraction of pore due .to activated Al diffusion to Ti powders. Therefore, the former effect may be predominant to decrease the volume fraction of pore compared to the latter effect. It has been well known that densification of a specimen prepared by reactive sintering is promoted with increasing amount of liquid phase [11]. The porosity of the present alloy decreased with increasing amount of molten Al that is characterized by an endothermic reaction in DTA result shown in Fig. 6. This observation is therefore in good agreement with the result reported previously. Fig. 7(a) and (b) show that both the relative areal fractions of endothermic and exothermic reaction increased with increasing heating rate in
reactive sintering process. As mentioned above, the former can be related with an increase in the amount of molten Al. While, the latter can be explained in relation with an increase of reaction heat for the synthesis of various intermetallic compounds by reactive sintering. From the result obtained, an amount of molten Al in a reactive sintering process may have a positive role in order to produce a dense specimen. Therefore, higher heating rate leads to dense structure of the present alloy in the reactive sintering process. In the present study, we have successfully produced unidirectional porous TiAl–Mn intermetallic alloy having fully lamellar structure by means of warm extrusion of elemental powder mixtures and then subsequent reactive sintering up to 1073 K for 1s and finally an appropriate heat treatment at 1623 K for 2 h, as shown in Figs. 2 and 5. This unidirectional pore structure is likely correlated to the extruded microstructure consisting of elongated feature of constituent powder mixtures. This suggests that warm extrusion of powder mixtures is effective to produce a unidirectional porous TiAl–Mn intermetallic alloy with fully lamellar microstructure using reactive sintering and an appropriate heat treatment. Much more works such as mechanical and physical properties are required as a next subject.
5. Conclusions The pore characteristics in TiAl–Mn intermetallic compounds were investigated as a function of heating rate using reactive sintering of extruded mixtures consisting of titanium and aluminum-manganese powders. The following results were obtained from the present study. 1. The unidirectional porous TiAl–Mn intermetallic compounds having fully lamellar structure can be successfully produced in terms of warm extrusion of powder mixtures consisting of titanium and aluminum-manganese powders and subsequent reactive sintering and finally heat treatment at 1623 K for 2 h. It is suggested that warm extrusion of powder mixtures plays a key role to produce unidirectional porous TiAl–Mn intermetallic alloy. 2. The average porosity decreases with increasing heating rate in the reactive sintering process, and the maximum porosity of 35% is obtained for the reactively sintered specimen with the heating rate of 0.17 K/s. 3. It is revealed that the decrease in porosity with increasing heating rate is closely related to the increase of molten aluminum.
References
Fig. 7. Relationship between normalized areal fraction and porosity for both the endothermic reaction (a) and exothermic reaction (b).
[1] Wen CE, Mabuchi M, Yamada Y, Shimojima K, Chino Y, Asahina T. Scripta Mater 2001;45:1147. [2] Simone AE, Gibson LJ. Mater Sci and Eng 1997;A229:55.
S.H. Yang et al. / Intermetallics 11 (2003) 849–855 [3] Yamanura S, Shiota H, Murakami K, Nakajima H. Mater Sci and Eng 2001;A318:137. [4] Hyun SK, Murakami K, Nakajima H. Mater Sci and Eng 2001; A299:241. [5] Li BY, Rong LJ, Li YY, Gjunter VE. Acta Mater 2000;48:3895. [6] Clemens H, Kestler H. Advanced Engineering Materials 2000; 9:551.
855
[7] Kim MS. J Japan Inst Metals 1994;58:819. [8] Yang SH, Hyun SK, Kim MS. J Kor Inst Met & Mater 1999; 37:1246. [9] Wang GX, Dahms M. Powder Metallurgy 1992;24:219. [10] Wang X, Sohn HY, Schlesinger ME. Mat Sci Eng 1994; A186:151. [11] Munir ZA. Ceramic Bulletin 1998;67:342.