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Effect of chlorine on the tensile behavior of reactive-sintered TiAl–Mn alloys
Journal of Materials Processing Technology 130–131 (2002) 235–239
Effect of chlorine on the tensile behavior of reactive-sintered TiAl–Mn alloys Seung-Ho Yang, Mok-Soon Kim* School of Materials Science and Engineering, Inha University, #253 Yonghyun-dong, Nam-gu, Inchon 402-751, South Korea
Abstract Ti–43.5 mol%Al–1.6 mol%Mn–2 vol.%TiB2–(0–600) mass ppm Cl alloys were fabricated by using a reactive sintering process, and tensile behavior of the reactive-sintered TiAl–Mn-based alloys having various chlorine contents was investigated. It was revealed that at room temperature tensile properties are hardly affected by the amount of chlorine. In contrast, at 1073 K, elongation decreased with increasing chlorine content, although yield strength was almost insensitive to the amount of chlorine. # 2002 Elsevier Science B.V. All rights reserved. Keywords: TiAl–Mn; Reactive sintering; Chlorine; Tensile properties
1. Introduction TiAl alloys are considered to be promising as a high temperature structural material, since these alloys have a low density, high melting temperature and high strength at elevated temperature [1]. However, limited formability due to a low ductility below 1100 K, and poor oxidation resistance above 1100 K have been the major obstacles in more widespread development of these alloys. A reactive sintering process is considered to be potential approach to obtain the near-net shaped TiAl alloys [2]. In the reactive sintering process, elemental titanium and aluminum powders, which are less expensive and more ductile than their prealloyed counterparts, are mixed and consolidated to a desired shape using conventional plastic-working procedures. Then, titanium aluminides are formed by a self-sustaining, exothermic reaction between elemental titanium and aluminum powders [2]. Many researches have been studied on improvement in the oxidation resistance of TiAl alloys through various methods, such as ternary element addition, preoxidation and pack cementation. Among them, the beneficial effect of chlorine was dramatic, because only a small amount (200 wt.% ppm) of chlorine enhanced the high temperature oxidation resistance of TiAl-based alloys [3]. However, the role of chlorine on the tensile properties of TiAl-based alloys has not been determined yet. In this study, Ti–43.5 mol%Al–1.6 mol%Mn– 2 vol.%TiB2–(0–600) mass ppm Cl alloys were fabricated * Corresponding author. Tel.: þ82-328607541; fax: þ82-328683607. E-mail address: [email protected] (M.-S. Kim).
by using a reactive sintering process, and tensile behavior of the reactive-sintered TiAl–Mn-based alloys having various chlorine contents was investigated.
2. Experimental The starting materials were elemental titanium powders (<150 mm), argon gas atomized Al–3.6 mol%Mn alloy powders (<150 mm, received from Sumitomo Light Metal Ind., Ltd.) and TiB2 particulates (1.5 mm). The chlorine content was controlled by using three types of elemental titanium powders (see Table 1): MMH-Ti powder: Mg reduced, melted and hydride–dehydride Ti powders. MH-Ti powder: Mg reduced and hydride–dehydried Ti powders. N-Ti powder: Na reduced Ti powders. Marker, impurities and size distribution of each Ti powder are presented in Table 1. As shown in the table, the chlorine content of each Ti powder is near 0, 300 and 900 ppm for MMH-Ti, MH-Ti and N-Ti, respectively. The oxygen content of each Ti powder shows a similar value of 1100– 1200 ppm. Fig. 1 shows the surface morphology of starting powders. On the surface of MH-Ti powder (Fig. 1b) and NTi powder (Fig. 1c), some fine particles are observed, as indicated by an arrow in the figure. According to the microprobe electron analyzer (EPMA) analysis, such particles correspond to MgCl2 and NaCl for MH-Ti powder and N-Ti
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Table 1 Characteristics of elemental titanium powders
Impurities (mass%) O Cl Na Mg Fe C H N Powder size (mm)
MMH-Tia
MH-Tib
0.11 <0.002 – <0.001 0.03 0.01 0.01 0.01
0.11 0.03 – 0.01 0.01 <0.02 0.01 <0.02
<150
Powder size distribution (mass%) þ150 mm 0.1 75–150 mm 66.7 45–75 mm 24.8 45 mm 8.4 Shape of powder
Angular
<150
N-Tic 0.12 0.09 0.07 – <0.01 0.007 0.005 0.001 <150
0.1 60.0 33.5 6.4
0.1 54.6 28.2 17.1
Angular
Irregular
a
Process: magnesium reduction ! melting ! hydride–dehydride and maker: Toho Titanium Co. Ltd. b Process: magnesium reduction ! hydride–dehydride and maker: Toho Titanium Co. Ltd. c Process: sodium reduction and maker: Nippon Sodium Co. Ltd.
powder, respectively. In case of the MMH-Ti powder (Fig. 1a), such chloride particles are not observed. Each elemental titanium powders, Al–Mn alloy powders and TiB2 particulates were mixed in the necessary ratio using a V-type
mixer, and then canned in an aluminum container (AA 6061) to degas at 723 K for 2 h under a vacuum of 101 Pa. The degassed mixtures were consolidated by extrusion. The extrusion was performed at 703 K with the extrusion ratio of 50. The extruded bar was machined to a rod shape with a diameter of 9 mm, and then reactively sintered using hot isostatic pressing (HIP) to produce a dense titanium aluminide. The HIP was performed at 1663 K for 1.5 h under 170 MPa of an argon gas. Tensile specimens having a gage section of 5 mm in diameter and 20 mm in length were machined from the HIP materials. Tensile tests were performed at room temperature and at 1073 K with a strain rate of 8:3 104 s1 under air. Microstructural characterization was done by an optical microscope (OM), a scanning electron microscope (SEM), a X-ray diffraction (XRD) and an EPMA.
3. Results and discussion The chemical composition analysis revealed that the chlorine content of each reactive-sintered (HIP) material is 0, 200 and 600 ppm, if elemental Ti powder used is MMHTi, MH-Ti and N-Ti, respectively. This fact clearly indicates that chlorine content of the titanium aluminide is successively controlled by using the reactive sintering process with the choice of the type of elemental titanium powder.
Fig. 1. Morphology of powders: (a) MMH-Ti powders; (b) MH-Ti powders; (c) N-Ti powders; (d) Al–3.6 mol%Mn alloy powders; (e) TiB2 particulates.
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Fig. 2. Microstructures of HIP specimens having the chlorine content of: (a) 0 ppm; (b) 200 ppm; (c) 600 ppm.
Fig. 2 shows optical micrographs of the reactive-sintered specimens having different chlorine contents. As can be seen in this figure, a fully dense (porosity <1%), lamellar structure is developed for all the specimens. The lamellar structure consists of g (TiAl) and a2 (Ti3Al) lath, verified by XRD and EPMA. The average grain size of the lamellar grains is about 60 mm irrespective of the chlorine content. Fig. 3 shows room temperature tensile properties as a function of chlorine content. It is shown that yield strength, tensile strength and elongation are hardly affected by the amount of chlorine. The result of tensile test at 1073 K is presented in Fig. 4. Note the decrease in elongation with increasing chlorine content, although yield strength does not change markedly with chlorine content. Fracture surface observation by SEM indicates that fracture occurs by a transgranular mode at 1073 K, although the appearance of fracture surface is somewhat different according to the chlorine content as shown in Fig. 5. In the specimen without chlorine (Fig. 5a), which shows a high elongation of 27%, predominant stepwise facets and curved lamellae that reveal an extensive plastic deformation before failure are well visible in the fracture surface. In the speci-
men of high (600 ppm) chlorine content (Fig. 5c), which shows a limited elongation of 2%, fracture surface is covered with a narrow step with smooth lamellae. Relatively predominant stepwise facets are observed on the fracture surface of the specimen with low (200 ppm) chlorine content (Fig. 5b) showing the elongation of 9%. Fig. 6 shows cross-sectional microstructures near the fracture surface of the specimens failured at 1073 K. For the specimens containing 0–200 ppm chlorine (Fig. 6a and b), some microcracks were observed in the lamellar structure. Namely, translamellar microcracks (indicated by an
Fig. 3. The result of tensile test at room temperature.
Fig. 4. The result of tensile test at 1073 K.
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Fig. 5. Fracture surface after tensile test at 1073 K for the specimen having the chlorine content of 0 ppm (a), 200 ppm (b) and 600 ppm (c).
Fig. 6. Cross-sectional microstructures near the fracture surface after tensile test at 1073 K for the specimen having the chlorine content of 0 ppm (a), 200 ppm (b) and 600 ppm (c).
arrow in Fig. 6a) and interlamellar microcracks (indicated by an arrow in Fig. 6b) are exhibited, suggesting that the microcracks are formed in the course of plastic deformation and lead to a transgranular fracture. In contrast, numerous surface microcracks that are perpendicular to the tensile axis are shown in the specimen with 600 ppm chlorine (Fig. 6c). The surface microcracks are also illustrated in Fig. 7. A higher magnification view near a surface microcrack in the specimen in Fig. 6c is presented in Fig. 8 along with the microchemical analysis data on EPMA. As shown from this figure, a very thin (<3 mm) oxide layer, such as A region in Fig. 8, is formed in the surface of specimen. In some cases, a part of oxide scale, such as B and C regions, is lifted to form an envelope, so that an isolated region, such as F region, is made underneath the oxide scale. It should be noted that in the oxide layer (for example, A region) a higher content of carbon is detected compared to the matrix region (D region). Kim et al. [4] proposed that vaporization of TiCl4, CO2, CO can occur at 1073 K in the chlorine containing TiAl–Mn alloys by the following reactions: 1 2 TiO2 ðsÞ
þ Cl2 ðgÞ þ COðgÞ ¼ 12 TiCl4 ðgÞ þ CO2 ðgÞ
(1)
1 2 TiO2 ðsÞ
þ Cl2 ðgÞ þ CðsÞ ¼ 12 TiCl4 ðgÞ þ COðgÞ
(2)
If the volatile TiCl4, CO2, CO vapors are produced at the metal/oxide interface and within the oxide scale, an internal stress is developed at the metal/oxide interface and within the oxide scale. Both the higher content of carbon in the oxide scale and the existence of an isolated region underneath the
Fig. 7. Micrograph near the fracture surface after tensile test at 1073 K for the specimen having the chlorine content of 600 ppm.
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Fig. 8. A higher magnification view (a) and EPMA result of the specimen in Fig. 6c.
oxide scale in Fig. 8 are suggested as the evidences of occurrence of the reactions described by Eqs. (1) and (2). The surface microcracks in Figs. 6c and 7 are also thinkable to be developed by the vaporization reactions during deformation. Namely, the internal stress which is caused by the volatile vapors owing to Eqs. (1) and (2) could contribute to the surface cracking under tensile loading. Therefore, the reduced ductility in the specimen containing 600 ppm chlorine is likely that the surface microcracks that are perpendicular to the tensile stress axis can behave as short notches and propagate unstably in the early stage of deformation to produce a transgranular fracture.
(3) The reduced ductility in the specimen with 600 ppm chlorine is suggested to be related to the surface microcracks caused by vaporization reactions.
Acknowledgements This work was supported by INHA University Research Grant through the Special Research Program in 2001 (INHA-21633).
References 4. Conclusion (1) TiAl–Mn-based alloys with different chlorine contents (0–600 ppm) can be obtained by the reactive sintering process. All the alloys have a fully dense, lamellar structure with the grain size of about 60 mm. (2) Room temperature tensile properties are almost insensitive to chlorine content. At 1073 K, elongation decreases with increasing chlorine content, although yield strength is hardly affected by chlorine content.
[1] Y.W. Kim, D.M. Dimiduk, Progress in the understanding of gamma titanium aluminides, JOM 43 (1991) 40–47. [2] M.S. Kim, Effect of grain size on mechanical properties of fully lamellar TiAl produced by reactive sintering, J. Jpn. Inst. Met. 58 (7) (1994) 819–825. [3] M. Kumagai, K. Shibue, M.S. Kim, M. Yonemitsu, Influence of chlorine on the oxidation behavior of TiAl–Mn intermetallic compound, Intermetallics 4 (7) (1996) 557–566. [4] Y.J. Kim, M.K. Choi, M.S. Kim, Effect of minor elements on oxidation resistance of Ti–45 at.%Al–1.6 at.%Mn intermetallics, J. Kor. Inst. Met. Mater. 36 (1) (1998) 284–292.
Effect of chlorine on the tensile behavior of reactive-sintered TiAl–Mn alloys Seung-Ho Yang, Mok-Soon Kim* School of Materials Science and Engineering, Inha University, #253 Yonghyun-dong, Nam-gu, Inchon 402-751, South Korea
Abstract Ti–43.5 mol%Al–1.6 mol%Mn–2 vol.%TiB2–(0–600) mass ppm Cl alloys were fabricated by using a reactive sintering process, and tensile behavior of the reactive-sintered TiAl–Mn-based alloys having various chlorine contents was investigated. It was revealed that at room temperature tensile properties are hardly affected by the amount of chlorine. In contrast, at 1073 K, elongation decreased with increasing chlorine content, although yield strength was almost insensitive to the amount of chlorine. # 2002 Elsevier Science B.V. All rights reserved. Keywords: TiAl–Mn; Reactive sintering; Chlorine; Tensile properties
1. Introduction TiAl alloys are considered to be promising as a high temperature structural material, since these alloys have a low density, high melting temperature and high strength at elevated temperature [1]. However, limited formability due to a low ductility below 1100 K, and poor oxidation resistance above 1100 K have been the major obstacles in more widespread development of these alloys. A reactive sintering process is considered to be potential approach to obtain the near-net shaped TiAl alloys [2]. In the reactive sintering process, elemental titanium and aluminum powders, which are less expensive and more ductile than their prealloyed counterparts, are mixed and consolidated to a desired shape using conventional plastic-working procedures. Then, titanium aluminides are formed by a self-sustaining, exothermic reaction between elemental titanium and aluminum powders [2]. Many researches have been studied on improvement in the oxidation resistance of TiAl alloys through various methods, such as ternary element addition, preoxidation and pack cementation. Among them, the beneficial effect of chlorine was dramatic, because only a small amount (200 wt.% ppm) of chlorine enhanced the high temperature oxidation resistance of TiAl-based alloys [3]. However, the role of chlorine on the tensile properties of TiAl-based alloys has not been determined yet. In this study, Ti–43.5 mol%Al–1.6 mol%Mn– 2 vol.%TiB2–(0–600) mass ppm Cl alloys were fabricated * Corresponding author. Tel.: þ82-328607541; fax: þ82-328683607. E-mail address: [email protected] (M.-S. Kim).
by using a reactive sintering process, and tensile behavior of the reactive-sintered TiAl–Mn-based alloys having various chlorine contents was investigated.
2. Experimental The starting materials were elemental titanium powders (<150 mm), argon gas atomized Al–3.6 mol%Mn alloy powders (<150 mm, received from Sumitomo Light Metal Ind., Ltd.) and TiB2 particulates (1.5 mm). The chlorine content was controlled by using three types of elemental titanium powders (see Table 1): MMH-Ti powder: Mg reduced, melted and hydride–dehydride Ti powders. MH-Ti powder: Mg reduced and hydride–dehydried Ti powders. N-Ti powder: Na reduced Ti powders. Marker, impurities and size distribution of each Ti powder are presented in Table 1. As shown in the table, the chlorine content of each Ti powder is near 0, 300 and 900 ppm for MMH-Ti, MH-Ti and N-Ti, respectively. The oxygen content of each Ti powder shows a similar value of 1100– 1200 ppm. Fig. 1 shows the surface morphology of starting powders. On the surface of MH-Ti powder (Fig. 1b) and NTi powder (Fig. 1c), some fine particles are observed, as indicated by an arrow in the figure. According to the microprobe electron analyzer (EPMA) analysis, such particles correspond to MgCl2 and NaCl for MH-Ti powder and N-Ti
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 8 0 4 - X
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Table 1 Characteristics of elemental titanium powders
Impurities (mass%) O Cl Na Mg Fe C H N Powder size (mm)
MMH-Tia
MH-Tib
0.11 <0.002 – <0.001 0.03 0.01 0.01 0.01
0.11 0.03 – 0.01 0.01 <0.02 0.01 <0.02
<150
Powder size distribution (mass%) þ150 mm 0.1 75–150 mm 66.7 45–75 mm 24.8 45 mm 8.4 Shape of powder
Angular
<150
N-Tic 0.12 0.09 0.07 – <0.01 0.007 0.005 0.001 <150
0.1 60.0 33.5 6.4
0.1 54.6 28.2 17.1
Angular
Irregular
a
Process: magnesium reduction ! melting ! hydride–dehydride and maker: Toho Titanium Co. Ltd. b Process: magnesium reduction ! hydride–dehydride and maker: Toho Titanium Co. Ltd. c Process: sodium reduction and maker: Nippon Sodium Co. Ltd.
powder, respectively. In case of the MMH-Ti powder (Fig. 1a), such chloride particles are not observed. Each elemental titanium powders, Al–Mn alloy powders and TiB2 particulates were mixed in the necessary ratio using a V-type
mixer, and then canned in an aluminum container (AA 6061) to degas at 723 K for 2 h under a vacuum of 101 Pa. The degassed mixtures were consolidated by extrusion. The extrusion was performed at 703 K with the extrusion ratio of 50. The extruded bar was machined to a rod shape with a diameter of 9 mm, and then reactively sintered using hot isostatic pressing (HIP) to produce a dense titanium aluminide. The HIP was performed at 1663 K for 1.5 h under 170 MPa of an argon gas. Tensile specimens having a gage section of 5 mm in diameter and 20 mm in length were machined from the HIP materials. Tensile tests were performed at room temperature and at 1073 K with a strain rate of 8:3 104 s1 under air. Microstructural characterization was done by an optical microscope (OM), a scanning electron microscope (SEM), a X-ray diffraction (XRD) and an EPMA.
3. Results and discussion The chemical composition analysis revealed that the chlorine content of each reactive-sintered (HIP) material is 0, 200 and 600 ppm, if elemental Ti powder used is MMHTi, MH-Ti and N-Ti, respectively. This fact clearly indicates that chlorine content of the titanium aluminide is successively controlled by using the reactive sintering process with the choice of the type of elemental titanium powder.
Fig. 1. Morphology of powders: (a) MMH-Ti powders; (b) MH-Ti powders; (c) N-Ti powders; (d) Al–3.6 mol%Mn alloy powders; (e) TiB2 particulates.
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Fig. 2. Microstructures of HIP specimens having the chlorine content of: (a) 0 ppm; (b) 200 ppm; (c) 600 ppm.
Fig. 2 shows optical micrographs of the reactive-sintered specimens having different chlorine contents. As can be seen in this figure, a fully dense (porosity <1%), lamellar structure is developed for all the specimens. The lamellar structure consists of g (TiAl) and a2 (Ti3Al) lath, verified by XRD and EPMA. The average grain size of the lamellar grains is about 60 mm irrespective of the chlorine content. Fig. 3 shows room temperature tensile properties as a function of chlorine content. It is shown that yield strength, tensile strength and elongation are hardly affected by the amount of chlorine. The result of tensile test at 1073 K is presented in Fig. 4. Note the decrease in elongation with increasing chlorine content, although yield strength does not change markedly with chlorine content. Fracture surface observation by SEM indicates that fracture occurs by a transgranular mode at 1073 K, although the appearance of fracture surface is somewhat different according to the chlorine content as shown in Fig. 5. In the specimen without chlorine (Fig. 5a), which shows a high elongation of 27%, predominant stepwise facets and curved lamellae that reveal an extensive plastic deformation before failure are well visible in the fracture surface. In the speci-
men of high (600 ppm) chlorine content (Fig. 5c), which shows a limited elongation of 2%, fracture surface is covered with a narrow step with smooth lamellae. Relatively predominant stepwise facets are observed on the fracture surface of the specimen with low (200 ppm) chlorine content (Fig. 5b) showing the elongation of 9%. Fig. 6 shows cross-sectional microstructures near the fracture surface of the specimens failured at 1073 K. For the specimens containing 0–200 ppm chlorine (Fig. 6a and b), some microcracks were observed in the lamellar structure. Namely, translamellar microcracks (indicated by an
Fig. 3. The result of tensile test at room temperature.
Fig. 4. The result of tensile test at 1073 K.
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Fig. 5. Fracture surface after tensile test at 1073 K for the specimen having the chlorine content of 0 ppm (a), 200 ppm (b) and 600 ppm (c).
Fig. 6. Cross-sectional microstructures near the fracture surface after tensile test at 1073 K for the specimen having the chlorine content of 0 ppm (a), 200 ppm (b) and 600 ppm (c).
arrow in Fig. 6a) and interlamellar microcracks (indicated by an arrow in Fig. 6b) are exhibited, suggesting that the microcracks are formed in the course of plastic deformation and lead to a transgranular fracture. In contrast, numerous surface microcracks that are perpendicular to the tensile axis are shown in the specimen with 600 ppm chlorine (Fig. 6c). The surface microcracks are also illustrated in Fig. 7. A higher magnification view near a surface microcrack in the specimen in Fig. 6c is presented in Fig. 8 along with the microchemical analysis data on EPMA. As shown from this figure, a very thin (<3 mm) oxide layer, such as A region in Fig. 8, is formed in the surface of specimen. In some cases, a part of oxide scale, such as B and C regions, is lifted to form an envelope, so that an isolated region, such as F region, is made underneath the oxide scale. It should be noted that in the oxide layer (for example, A region) a higher content of carbon is detected compared to the matrix region (D region). Kim et al. [4] proposed that vaporization of TiCl4, CO2, CO can occur at 1073 K in the chlorine containing TiAl–Mn alloys by the following reactions: 1 2 TiO2 ðsÞ
þ Cl2 ðgÞ þ COðgÞ ¼ 12 TiCl4 ðgÞ þ CO2 ðgÞ
(1)
1 2 TiO2 ðsÞ
þ Cl2 ðgÞ þ CðsÞ ¼ 12 TiCl4 ðgÞ þ COðgÞ
(2)
If the volatile TiCl4, CO2, CO vapors are produced at the metal/oxide interface and within the oxide scale, an internal stress is developed at the metal/oxide interface and within the oxide scale. Both the higher content of carbon in the oxide scale and the existence of an isolated region underneath the
Fig. 7. Micrograph near the fracture surface after tensile test at 1073 K for the specimen having the chlorine content of 600 ppm.
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Fig. 8. A higher magnification view (a) and EPMA result of the specimen in Fig. 6c.
oxide scale in Fig. 8 are suggested as the evidences of occurrence of the reactions described by Eqs. (1) and (2). The surface microcracks in Figs. 6c and 7 are also thinkable to be developed by the vaporization reactions during deformation. Namely, the internal stress which is caused by the volatile vapors owing to Eqs. (1) and (2) could contribute to the surface cracking under tensile loading. Therefore, the reduced ductility in the specimen containing 600 ppm chlorine is likely that the surface microcracks that are perpendicular to the tensile stress axis can behave as short notches and propagate unstably in the early stage of deformation to produce a transgranular fracture.
(3) The reduced ductility in the specimen with 600 ppm chlorine is suggested to be related to the surface microcracks caused by vaporization reactions.
Acknowledgements This work was supported by INHA University Research Grant through the Special Research Program in 2001 (INHA-21633).
References 4. Conclusion (1) TiAl–Mn-based alloys with different chlorine contents (0–600 ppm) can be obtained by the reactive sintering process. All the alloys have a fully dense, lamellar structure with the grain size of about 60 mm. (2) Room temperature tensile properties are almost insensitive to chlorine content. At 1073 K, elongation decreases with increasing chlorine content, although yield strength is hardly affected by chlorine content.
[1] Y.W. Kim, D.M. Dimiduk, Progress in the understanding of gamma titanium aluminides, JOM 43 (1991) 40–47. [2] M.S. Kim, Effect of grain size on mechanical properties of fully lamellar TiAl produced by reactive sintering, J. Jpn. Inst. Met. 58 (7) (1994) 819–825. [3] M. Kumagai, K. Shibue, M.S. Kim, M. Yonemitsu, Influence of chlorine on the oxidation behavior of TiAl–Mn intermetallic compound, Intermetallics 4 (7) (1996) 557–566. [4] Y.J. Kim, M.K. Choi, M.S. Kim, Effect of minor elements on oxidation resistance of Ti–45 at.%Al–1.6 at.%Mn intermetallics, J. Kor. Inst. Met. Mater. 36 (1) (1998) 284–292.