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Indirect Extrusion of Tial-Based Intermetallics From Elemental Powders Metallurgy
Proceedings of Sino-Swedish Structural Materials Symposium 2007
Indirect Extrusion of Tial-Based Intermetallics From Elemental Powders Metallurgy WU Y.'32q3, LI X.W. 4,
ZHOU S.X.lY2, HWANG S.K.
( I Central Iron & Steel Research Institute. Beijing 100081, China; 2.Advanced Technology & Materials Co.Ltd.,
Beijing 100081. China; 3.School of Materials Science and Engineering, Inha University, Incheon, 402-75 I , Korea; 4.Collage of Science, Northeastern University, Shenyang 11OOO4, China)
Abstrack Powder metallurgy has been successfully adopted in synthesizing intermetallic compounds based on TiAI. Indirect hot extrusion of elemental powder mixtures resulted in rods of about 2cm in diameter with the compositions based on Ti-46.6A1-1.4Mn-2Mo-0.3C (-0.33Y) (all in at.%). Among the processing parameters, degassing scheme, heating rate, extrusion can, die design, extrusion temperature and extrusion ratio were found to be crucial to ensure the sound mechanical properties at both room temperature and high temperatures. Further improvement in the mechanical properties was obtained by modifying the chemical composition with C and Y,which were effective in refining the microstructure, thus enhancing the tensile strength and plastic elongation at elevated temperature.
1. Introduction Due to the low density, high melting point and high specific strength at elevated temperatures, the intermetallic compounds based on TiAl receive increasing attentions of researchers in the field of high performance automobiles and aircrafts. Among the synthesis methods of the intermetallic compounds, elemental powder method is an attractive route because of the homogeneity in alloy chemical composition and
the fine grain size in the final product. Employment of direct and indirect extrusion of the powder mixtures, resulted in a full density alloys in TiAl-based alloys '1-31. These methods, however, require a careful control of the processing parameters to ensure the soundness in the microstructure that is essential for the mechanical properties. The purpose of this paper is to address some of the crucial technical issues of indirect extrusion in the utilization of the proposed powder methods for synthesizing the TiAl-based intermetallic compounds.
2. Experimental procedure The chemical compositions of the experimental alloys and the characteristics of the powders are summarized in Table 1. Elemental powders used in the present study were of 99.5% or better purity and 4 to 80 pm in the average particle size. For 104
Ti-46.6A1-1.4Mn-2Mo-0.3C(-O.33Y) (all in atomic percent) alloys, powders were mixed in a V-blender for 4h without additive, and compacted into a Cr-Mo steel
(AISI 4140) can of 40 mm and 73 mm in the inner diameter and outer diameter, respectively, and 180 mm in height. The cans were degassed sequentially at room temperature, 250 and 500°C for 1, 2 and 4h, respectively, followed by vacuum sealing at 3x10" tom. Indirect hot extrusion was conducted at 1250°C with extrusion ratios ranging from 8 to 12. Prior to extrusion, cans were pre-heated for 12h. The sketches of fabrication route for indirect extrusion and the can and extrusion die are shown in Fig.1 and Fig.2, respectively. The heat treatment cycles consisted of a solution treatment at 1400°C for l h followed by air-cooling (AC) and an aging treatment at 800°C. Tensile tests were canied out at room temperature and 800°C in an Instron testing machine with a strain rate of l ~ l o - ~ sTensile -'. specimens were 19mm in gauge length and 4mm in gauge diameter. Energy dispersive spectroscopy (EDS) analysis was conducted in transmission electron microscopy (TEM) with the maximum accelerating voltage of 200kV using specimens made by dimple grinding and ion milling.
Proceedings of SineSwedish Structural Materials Symposium 2007
3. Results and Discussion During preparation of powders, the average
reduced porosity in the final material. Depending on temperature, the composition of the extracted gases
powder particle size, mixing conditions, degassing conditions and pre-extrusion heat treatment were
differed. In the range of 25-22oO0C, the major gas component was water vapor that gradually decreased
important parameters affecting the final microstructure and porosity.
with temperature up to 45OoC,which was followed by another increase of the gas release at higher temperatures. Hydrogen gas extraction occurred
Multi-step degassing was an essential step toward
Table 1 Chemical compositionsof experimental TiAI-based alloys and characteristicsof powders. Analysis (at.%)
Nominal Ti
A1
Mn
MQ
c
Ti-46.6A1- 1.4Mn-2Me0.3C
Bal.
46.21
1.32
1.98
0.3 1
Ti-46.6A1- 1.4Mn-2Mo-0.3C-0.33Y
Bal.
45.56
1.29
2.02
0.29
Vender
Micron
CERAC
CERAC
CERAC
CERAC
~
Y
0.28 Micron ~~
Purity, %
99.5
99.5
99.9
99.95
99.5
99.5
Size, rn
44-120
15-70
10
2-4
10-50
7-20
ir-blending:20rpm. 4h
. .. . .. . ... ... ... ... ... ... ...
II
ID:
H
7
U
Cold Compacting: Green Density 60%
Multi-StW DemssinR: 3x1O5tom
. . . . . . . . . . . . .. .. .. .
. . . . . . . . . .
Pre-extrusion Heat Treatment Extrusion: 1250" Cnh, 10:1
Fig. 1 Fabrication route for indirect extrusion of Ti-46.6Al-1.4Mn-2MO-o3c(-O33Y)alloys
Fig. 2 Schematic drawings of (a) can and (b) die used for extruded Ti-46.6Al-1.4Mn-2Mo-O.3C(-O.33Y) alloys. Unit of dimensions: m m
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Proceedines of Sino-Swedish Structural Materials Svmwsium 2007
parameter defined as follows [41:
mainly in the temperature range of 200-400"C whereas COz gas release peaked at 100"C, the former out-weighing the latter over the whole temperature range. Oxygen gas was persistent against degassing treatment, resulting in approximately 1500ppm in the final materials. AS reported in previous paper 14', control of the rate of reactive sintering among powder particles was based on the characteristics of the phase forming at the interfaces of powders. During heating Ti-A1 powder mixtures above 600°C, melting of A1 powder occurred first, which was immediately followed by the formation of Ti-A1 intermetallic compounds. This process involved adiabatic endothermic reaction and exothermic reaction in sequence. The major forms of the compounds were identified as X3Al and Ti2AI,TiAl and TiAl3. Since these phases form shells on pure Ti powder particles, they affect further diffusion of Ti. To maximize the y phase (TiAI), it was necessary to find a processing route to suppress the formation of the undesirable phases, particularly TiA13 since this phase exhausts Al and at the same time retards the diffusion rate of Ti. The heating rate was found to be a crucial parameter for this purpose. The amount of the y phase increased with the heating rate. The reason for this result was thought to be the effect of the heating rate on the ignition temperature and the reaction finish temperature of the major reaction among the powder particles. While the ignition temperature was insensitive to the heating rate, the reaction finish temperature increased sensitively with the heating rate (from 650°C to 1400°C with the heating rate increase from 2 to 300"C/min), resulting in a suppression of TiA13 phase. Hot extrusion was effective in consolidating Ti-46.6A1-I .4Mn-2Mo-0.3C(-0.33Y) alloys. The main cause is considered to be the partial liquid phase sintering. The key processing parameters for hot extrusion were the preheating treatment, the extrusion ratio and the extrusion temperature. To evaluate the effect of the preheating treatment. three different heat treatments were designed over the temperature range of 1200 to 1250°C and the holding time of 1 to 2h. Quantification of the preheating treatment was best described by the annealing
A = Aj = it ; (
2)
where Q is the activation energy for self diffusion of Ti in single phase TiAl, which was assumed to be 300kh1ol 15] and the summation was taken over each heat treatment segment. Porosity was substantially reduced as the value of the parameter A increased from 1 . 0 ~ 1 0 -to' ~1 . 7 ~ 1 0 - 'Further ~. increase of the value up to 5.1x lo-", however, was inconsequential. The extrusion ratio affected the final grain size in the extruded specimens, the grain size decreasing with the extrusion ratio although the capacity of the press, 700ton, limited it to about 1 1:1. From these considerations, the optimum consolidation process for TiAl-based alloys was found to be preheating at 1250°C for more than 2h, followed by extrusion at the same temperature with a ratio of 10:1. The elemental powder metallurgy (EPM) approach brought several advantageous microstructural features as well as the mechanical properties derivable from the microstructures. Among these, the grain size refinement showed the most pronounced effect. Table 2 compares the average grain sizes of the microstructures in the gamma alloys made by various processing methods. The average grain size of Ti-46.6AI- 1.4Mn-2Mo-0.3C-0.33Y alloy produced by the EPM method was approximately 30 pm compared to about 300pm in vacuum arc melted alloys [61 and around 58 pm in arc-melted and subsequently hot extruded alloys 17'. The addition of C and Y to Ti-46.6A1-1.4Mn-2Mo alloy resulted in the refinement of the grain size from 80 pm to 45pm and 30pm, respectively. The fine grain size obtainable in the EPM process of TiAl alloys is a great advantage in comparison to the ingot metallurgy alloys since it is known that the yield strength, room-temperature ductility as well as the fracture resistance can be enhanced by the fine grain size. To realize the fine grain size in the ingot metallurgy alloys thermo-mechanical processing like a two-step forging is required, which will add additional cost to the final product. Furthermore the EPM process can be potentially refined to reduce the final grain size by optimizing the characteristics of the starting powder or
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the consolidation conditions. Alloying additions such as C and Y to the EPM produced y-alloys resulted in further microstructural refinement in the grain size as well as the lath sizes in the fully lamellar microstructures. The effect of C and Y addition on the microstructural refinement is summarized in Table 3. C and Y-addition resulted in a significant reduction of lamella thickness. The thickness of a 2 and y in the lamellar structure were measured by TEM in the specimens cooled from 1400°C at a rate of 200"C/min. C and Y-addition clearly reduced the thickness of the lamellae, from the average of 141nm to 64nm and 50nm for y, respectively, and from the average of 53nm to 28nm and 22nm for a2, respectively. The reason for the lamellar size refinement in the case of C containing alloy was two-fold: reduction in the stacking fault energy of the high temperature a phase"" 12], thus increasing the nucleation rate of the lamellae, and suppression of the growth rate by carbon segregation at grain boundaries and the lamellar interfaces[31. The grain-refining effect of Y was attributed to the oxides of Y2O3 that precipitated prior to
extrusion. During extrusion and subsequent heat treatment, the oxides provided ample sites of heterogeneous nucleation for the product phases (a2 and y from high temperature a phase) while restraining their growth rate. Because it is stable up to 2009°C in TiAl, Y203 is not decomposed during the solution heat treatment at 1400°C. Another important role of C and Y-addition was the formation of carbides and Y203 particles '13, 14]. The carbides along the grain boundaries of y and a2 in 0.3C-containing TiAl alloy and Y2O3 formed in y platelets in 0.33Y-containingTiAl alloy are shown in Fig. 3(a) and (b), respectively. As shown in Fig.3(a), EDS analysis in the E M showed that the plate-like precipitates mainly contained Ti, A1 and a small amount of Mn, Mo, C and Y, with the ratio of Ti: A1 of approximately 3: 1, indicating that the precipitates appeared to be the same as the Ti3AlC phase with the perovskite type crystal structure. As for the Y2O3 precipitates in Fig.3(b), the result of selected area diffraction patterns showed the precipitates having a hexagonal structure and were enriched by yttrium and
Table 2 Grain sizes of y-TiAI alloys produced by various methods. Alloys Process Grain size (pm) TiAl-2.3V-0.9Cr VAR 330 Ti-47A1-3.7(Cr, Nb, Mn, Si)-OSB VAR/Hot Extrusion ( D T J 58 Ti-45Al-2Mn-2Nb 40-200 VAR /HIP +0.8~0l%TiB2(XD45) Ti-47AI-2Mn-2Nb VAR /HIP 50-200 +0.8vol%TiB2 (XD47) Induction skull melting/HIP/2-step Ti-46.5A1-2Cr-3Nb-0.2W 10 forging 80 Ti-46.6A1-1.4Mn-2Mo Indirect Extrusion Ti-46.6A1-1.4Mn-2Mo-0.3C(-0.33Y)
Indirect Extrusion
Ref. 6 7 7 8 9 10
4 Present work
45(30)
Table 3 Average thickness of lamellae in experimental TiAl-based alloys affected by carbon and yttrium additions.
Y (nm)
Alloys Ti-46.6AI-1.4Mn-2Mo
141
a2 (nm) 53
Ti-46.6Al- 1.4Mn-2Mc-0.3C
64
28
Ti-46.6A1-1.4Mn-2Mo-0.3C-0.33Y
50
22
.
107
Proceedings of Sino-Swedish Structural Materials Symposium 2007
Fig. 3 TEM micrographs showing (a) carbides along grain boundaries of 7 and
in 03Ccontaining TIAl alloy and (b)Y203 formed in 7 platelets in 033Ycontaining Ti1 alloy. The samples were heat treated at 1400°C/lh/AC+ 8W0C/12h/AC Table 4 Tensile properties d experimentalTiAl-based alloys at mom temperature and high temperature. Alloys
Ultimate Tensile Strength (MPa)
Elonaation (96) ~~
Fa-
800°C
RT
800°C
526
507
1.8
2.6
Ti46.6A1- 1.4Mn-2Md.3C
610
699
1.7
2.8
Ti-46.6A1- 1.4Mn-2Me0.3C-0.33Y
682
652
3.1
3.7
-T1-46.SAl- 1.4Mn-2Mo
~~
oxygen elements and as expected to be the Y201 precipitates. Therefore, the addition of C and Y brought the microstructural refinement including the grain size and the thickness of lamella of a2and y as well as the formation of the precipitates of carbides and Y203. The microstructural refinement resulted in enhancement of the mechanical properties, particularly the tensile properties at both room temperature and at elevated temperatures' I 3 'I The tensile properties of C-modified and Y -modified Ti-46.6A1-1.4Mn-2Mo alloys are shown in Table 4. Room temperature elongation in the order of 2% together with the yield strength of approximately 526MPa was obtained in the Ti-46.6A1-1.4Mn-2Mo alloys. With the addition of C, the room temperature elongation value was slightly decreased from 1.8% to 1.7% but, in contrast, the tensile strength increased significantly from 526MPa to 610MPa. The variation of the tensile properties of the
samples at 800°C had a similar feature to that at room temperature. Particularly the high temperature tensile strength increased considerably up to 699MPa at temperature of 800°C. With addition of Y, the values of tensile strength and elongation measured at room temperature and 800°C increased further. Ultimate tensile strength increased successfully from 61OMPa to 682MPa in the tensile samples at room temperature. Plastic elongation at room temperature between 1.7% to 3.1% were obtained for specimens in fully lamellar grain microstructure with yttrium contents between 0 to 0.33at.%, whereby the yttrium content of 0.33at.% exhibited the best elongation of 3.7% at 800°C. Therefore, the combination of Y203 and carbide precipitation strengthening mainly contributed to their room and high temperature strength, while the refined grains and reduced lamellar spacing brought the better ductility.
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Proceedings of Sino-Swedish Structural Materials Symposium 2007
4. Conclusion
G Frommeyer ,Z. Metallkd., Vol. 83(1992), pp. 591-595. [6]. J.D. Shi, Z.J. Pu and K.H. Wu, in Gamma Titanium
In this work, some of the key processing parameters of indirect extrusion in synthesizing TiAl-based intermetallic compounds from elemental powders were evaluated. Among others, the powder characteristics, mixing method, degassing procedure, extrusion can and die-design, pre-extrusion heat treatment, extrusion ratio and temperature were the most important variables to control. Control of these processing parameters, combined with proper additions of alloying elements such as C and Y in Ti-46.6A1-1.4Mn-2Mo alloys brought improved mechanical properties at both room temperature and elevated temperature. The present work, therefore, demonstrates that the indirect extrusion from EPM process should be considered as a strong candidate processing technique alternative to the ingot metallurgy.
[lo]. Z. Jin, C. Cady, GT. GrayIII and Y.W. Kim, Metall. Trans. A,
References:
[Ill. K. Kawahara, S. Tsurekawa and H. Nakashima, J. Jpn. Inst.
Aluminides, Y.W. Kim, R. Wagner and M. Yamaguchi, eds., TMS, Las Vegas, Nevada, (1995), pp. 709-716. [7]. M. Oehring, U. Lorentz, R. Niefanger, U. Christoph, F. Appel, R. Wagner, H. Clemens and N. Eberhardt, in Gamma Titanium Aluminides, Y.W. Kim, D.M. Dimiduk and M.H. Loretto, eds., TMS, San Diego, CA, (1999), pp. 439-446. [8]. V. Recina and D. Nilson, in Gamma Titanium Aluminides, Y.W. Kim, D.M. Dimiduk and M.H. Loretto, eds., TMS, San Diego, CA, (1999), pp. 447-454. [9]. D.Y. Seo, T. Everard, P.A. McQuay and T,R, Bieler, in Interstitial and Substitutional Solute Effects in Intermetallics,
I. Baker, R.D. Noebe and E.P. George, eds., TMS, (1998), pp. 227-234. VO~. 31A(2000), pp. 1007-1016.
[ 11. K.J. Park, J.K. Hong and S.K. Hwang, Metall. Trans. A, Vol.
28A(1997), pp. 223-228.
Metals., Vol. 62(1998), pp. 246-254. [12]. D. Blatte, P. Duval, L. Letellier and M. Guttmann, Acta Mater., Vol. 44(1996), pp. 4995-5005.
[2]. K.L. Park and S.K. Hwang, Scr. Mater., VoI. 44(2001), pp. 9-16,
[13]. Y. Wu and S.K. Hwang, Acta Mater., Vol. 50(2002), pp. 1479-1493.
[3]. H.S. Park, S.K. Hwang, C.M. Lee, Y.C. Yoo, S.W. Nam and N.J. Kim, Metall. Trans. A, Vol. 32A(2001), pp. 251-259.
[14]. Y. Wu, S.K. Hwang, S.W. NamandN.J. Kim, Scr. Mater., Vol. 48(2003), pp. 1655-1660.
[4]. T.K. Lee, J.H. Kim and S.K. Hwang, Metall. Trans. A, Vol. 28A(1997), pp. 2723-2729.
[15]. Y. Wu, S.K. Hwang and J.W. Morris, Jr, Metall Mater. Trans. A, Vol. 34A(2003), pp.2077-2087.
[5].S. Kroll, H. Mehrer, N. Stolwijk, C. Herzig, R. Rosenkranz and
109
Indirect Extrusion of Tial-Based Intermetallics From Elemental Powders Metallurgy WU Y.'32q3, LI X.W. 4,
ZHOU S.X.lY2, HWANG S.K.
( I Central Iron & Steel Research Institute. Beijing 100081, China; 2.Advanced Technology & Materials Co.Ltd.,
Beijing 100081. China; 3.School of Materials Science and Engineering, Inha University, Incheon, 402-75 I , Korea; 4.Collage of Science, Northeastern University, Shenyang 11OOO4, China)
Abstrack Powder metallurgy has been successfully adopted in synthesizing intermetallic compounds based on TiAI. Indirect hot extrusion of elemental powder mixtures resulted in rods of about 2cm in diameter with the compositions based on Ti-46.6A1-1.4Mn-2Mo-0.3C (-0.33Y) (all in at.%). Among the processing parameters, degassing scheme, heating rate, extrusion can, die design, extrusion temperature and extrusion ratio were found to be crucial to ensure the sound mechanical properties at both room temperature and high temperatures. Further improvement in the mechanical properties was obtained by modifying the chemical composition with C and Y,which were effective in refining the microstructure, thus enhancing the tensile strength and plastic elongation at elevated temperature.
1. Introduction Due to the low density, high melting point and high specific strength at elevated temperatures, the intermetallic compounds based on TiAl receive increasing attentions of researchers in the field of high performance automobiles and aircrafts. Among the synthesis methods of the intermetallic compounds, elemental powder method is an attractive route because of the homogeneity in alloy chemical composition and
the fine grain size in the final product. Employment of direct and indirect extrusion of the powder mixtures, resulted in a full density alloys in TiAl-based alloys '1-31. These methods, however, require a careful control of the processing parameters to ensure the soundness in the microstructure that is essential for the mechanical properties. The purpose of this paper is to address some of the crucial technical issues of indirect extrusion in the utilization of the proposed powder methods for synthesizing the TiAl-based intermetallic compounds.
2. Experimental procedure The chemical compositions of the experimental alloys and the characteristics of the powders are summarized in Table 1. Elemental powders used in the present study were of 99.5% or better purity and 4 to 80 pm in the average particle size. For 104
Ti-46.6A1-1.4Mn-2Mo-0.3C(-O.33Y) (all in atomic percent) alloys, powders were mixed in a V-blender for 4h without additive, and compacted into a Cr-Mo steel
(AISI 4140) can of 40 mm and 73 mm in the inner diameter and outer diameter, respectively, and 180 mm in height. The cans were degassed sequentially at room temperature, 250 and 500°C for 1, 2 and 4h, respectively, followed by vacuum sealing at 3x10" tom. Indirect hot extrusion was conducted at 1250°C with extrusion ratios ranging from 8 to 12. Prior to extrusion, cans were pre-heated for 12h. The sketches of fabrication route for indirect extrusion and the can and extrusion die are shown in Fig.1 and Fig.2, respectively. The heat treatment cycles consisted of a solution treatment at 1400°C for l h followed by air-cooling (AC) and an aging treatment at 800°C. Tensile tests were canied out at room temperature and 800°C in an Instron testing machine with a strain rate of l ~ l o - ~ sTensile -'. specimens were 19mm in gauge length and 4mm in gauge diameter. Energy dispersive spectroscopy (EDS) analysis was conducted in transmission electron microscopy (TEM) with the maximum accelerating voltage of 200kV using specimens made by dimple grinding and ion milling.
Proceedings of SineSwedish Structural Materials Symposium 2007
3. Results and Discussion During preparation of powders, the average
reduced porosity in the final material. Depending on temperature, the composition of the extracted gases
powder particle size, mixing conditions, degassing conditions and pre-extrusion heat treatment were
differed. In the range of 25-22oO0C, the major gas component was water vapor that gradually decreased
important parameters affecting the final microstructure and porosity.
with temperature up to 45OoC,which was followed by another increase of the gas release at higher temperatures. Hydrogen gas extraction occurred
Multi-step degassing was an essential step toward
Table 1 Chemical compositionsof experimental TiAI-based alloys and characteristicsof powders. Analysis (at.%)
Nominal Ti
A1
Mn
MQ
c
Ti-46.6A1- 1.4Mn-2Me0.3C
Bal.
46.21
1.32
1.98
0.3 1
Ti-46.6A1- 1.4Mn-2Mo-0.3C-0.33Y
Bal.
45.56
1.29
2.02
0.29
Vender
Micron
CERAC
CERAC
CERAC
CERAC
~
Y
0.28 Micron ~~
Purity, %
99.5
99.5
99.9
99.95
99.5
99.5
Size, rn
44-120
15-70
10
2-4
10-50
7-20
ir-blending:20rpm. 4h
. .. . .. . ... ... ... ... ... ... ...
II
ID:
H
7
U
Cold Compacting: Green Density 60%
Multi-StW DemssinR: 3x1O5tom
. . . . . . . . . . . . .. .. .. .
. . . . . . . . . .
Pre-extrusion Heat Treatment Extrusion: 1250" Cnh, 10:1
Fig. 1 Fabrication route for indirect extrusion of Ti-46.6Al-1.4Mn-2MO-o3c(-O33Y)alloys
Fig. 2 Schematic drawings of (a) can and (b) die used for extruded Ti-46.6Al-1.4Mn-2Mo-O.3C(-O.33Y) alloys. Unit of dimensions: m m
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Proceedines of Sino-Swedish Structural Materials Svmwsium 2007
parameter defined as follows [41:
mainly in the temperature range of 200-400"C whereas COz gas release peaked at 100"C, the former out-weighing the latter over the whole temperature range. Oxygen gas was persistent against degassing treatment, resulting in approximately 1500ppm in the final materials. AS reported in previous paper 14', control of the rate of reactive sintering among powder particles was based on the characteristics of the phase forming at the interfaces of powders. During heating Ti-A1 powder mixtures above 600°C, melting of A1 powder occurred first, which was immediately followed by the formation of Ti-A1 intermetallic compounds. This process involved adiabatic endothermic reaction and exothermic reaction in sequence. The major forms of the compounds were identified as X3Al and Ti2AI,TiAl and TiAl3. Since these phases form shells on pure Ti powder particles, they affect further diffusion of Ti. To maximize the y phase (TiAI), it was necessary to find a processing route to suppress the formation of the undesirable phases, particularly TiA13 since this phase exhausts Al and at the same time retards the diffusion rate of Ti. The heating rate was found to be a crucial parameter for this purpose. The amount of the y phase increased with the heating rate. The reason for this result was thought to be the effect of the heating rate on the ignition temperature and the reaction finish temperature of the major reaction among the powder particles. While the ignition temperature was insensitive to the heating rate, the reaction finish temperature increased sensitively with the heating rate (from 650°C to 1400°C with the heating rate increase from 2 to 300"C/min), resulting in a suppression of TiA13 phase. Hot extrusion was effective in consolidating Ti-46.6A1-I .4Mn-2Mo-0.3C(-0.33Y) alloys. The main cause is considered to be the partial liquid phase sintering. The key processing parameters for hot extrusion were the preheating treatment, the extrusion ratio and the extrusion temperature. To evaluate the effect of the preheating treatment. three different heat treatments were designed over the temperature range of 1200 to 1250°C and the holding time of 1 to 2h. Quantification of the preheating treatment was best described by the annealing
A = Aj = it ; (
2)
where Q is the activation energy for self diffusion of Ti in single phase TiAl, which was assumed to be 300kh1ol 15] and the summation was taken over each heat treatment segment. Porosity was substantially reduced as the value of the parameter A increased from 1 . 0 ~ 1 0 -to' ~1 . 7 ~ 1 0 - 'Further ~. increase of the value up to 5.1x lo-", however, was inconsequential. The extrusion ratio affected the final grain size in the extruded specimens, the grain size decreasing with the extrusion ratio although the capacity of the press, 700ton, limited it to about 1 1:1. From these considerations, the optimum consolidation process for TiAl-based alloys was found to be preheating at 1250°C for more than 2h, followed by extrusion at the same temperature with a ratio of 10:1. The elemental powder metallurgy (EPM) approach brought several advantageous microstructural features as well as the mechanical properties derivable from the microstructures. Among these, the grain size refinement showed the most pronounced effect. Table 2 compares the average grain sizes of the microstructures in the gamma alloys made by various processing methods. The average grain size of Ti-46.6AI- 1.4Mn-2Mo-0.3C-0.33Y alloy produced by the EPM method was approximately 30 pm compared to about 300pm in vacuum arc melted alloys [61 and around 58 pm in arc-melted and subsequently hot extruded alloys 17'. The addition of C and Y to Ti-46.6A1-1.4Mn-2Mo alloy resulted in the refinement of the grain size from 80 pm to 45pm and 30pm, respectively. The fine grain size obtainable in the EPM process of TiAl alloys is a great advantage in comparison to the ingot metallurgy alloys since it is known that the yield strength, room-temperature ductility as well as the fracture resistance can be enhanced by the fine grain size. To realize the fine grain size in the ingot metallurgy alloys thermo-mechanical processing like a two-step forging is required, which will add additional cost to the final product. Furthermore the EPM process can be potentially refined to reduce the final grain size by optimizing the characteristics of the starting powder or
106
Proceedines of Sino-Swedish Structural Materials SvmDosium 2007
the consolidation conditions. Alloying additions such as C and Y to the EPM produced y-alloys resulted in further microstructural refinement in the grain size as well as the lath sizes in the fully lamellar microstructures. The effect of C and Y addition on the microstructural refinement is summarized in Table 3. C and Y-addition resulted in a significant reduction of lamella thickness. The thickness of a 2 and y in the lamellar structure were measured by TEM in the specimens cooled from 1400°C at a rate of 200"C/min. C and Y-addition clearly reduced the thickness of the lamellae, from the average of 141nm to 64nm and 50nm for y, respectively, and from the average of 53nm to 28nm and 22nm for a2, respectively. The reason for the lamellar size refinement in the case of C containing alloy was two-fold: reduction in the stacking fault energy of the high temperature a phase"" 12], thus increasing the nucleation rate of the lamellae, and suppression of the growth rate by carbon segregation at grain boundaries and the lamellar interfaces[31. The grain-refining effect of Y was attributed to the oxides of Y2O3 that precipitated prior to
extrusion. During extrusion and subsequent heat treatment, the oxides provided ample sites of heterogeneous nucleation for the product phases (a2 and y from high temperature a phase) while restraining their growth rate. Because it is stable up to 2009°C in TiAl, Y203 is not decomposed during the solution heat treatment at 1400°C. Another important role of C and Y-addition was the formation of carbides and Y203 particles '13, 14]. The carbides along the grain boundaries of y and a2 in 0.3C-containing TiAl alloy and Y2O3 formed in y platelets in 0.33Y-containingTiAl alloy are shown in Fig. 3(a) and (b), respectively. As shown in Fig.3(a), EDS analysis in the E M showed that the plate-like precipitates mainly contained Ti, A1 and a small amount of Mn, Mo, C and Y, with the ratio of Ti: A1 of approximately 3: 1, indicating that the precipitates appeared to be the same as the Ti3AlC phase with the perovskite type crystal structure. As for the Y2O3 precipitates in Fig.3(b), the result of selected area diffraction patterns showed the precipitates having a hexagonal structure and were enriched by yttrium and
Table 2 Grain sizes of y-TiAI alloys produced by various methods. Alloys Process Grain size (pm) TiAl-2.3V-0.9Cr VAR 330 Ti-47A1-3.7(Cr, Nb, Mn, Si)-OSB VAR/Hot Extrusion ( D T J 58 Ti-45Al-2Mn-2Nb 40-200 VAR /HIP +0.8~0l%TiB2(XD45) Ti-47AI-2Mn-2Nb VAR /HIP 50-200 +0.8vol%TiB2 (XD47) Induction skull melting/HIP/2-step Ti-46.5A1-2Cr-3Nb-0.2W 10 forging 80 Ti-46.6A1-1.4Mn-2Mo Indirect Extrusion Ti-46.6A1-1.4Mn-2Mo-0.3C(-0.33Y)
Indirect Extrusion
Ref. 6 7 7 8 9 10
4 Present work
45(30)
Table 3 Average thickness of lamellae in experimental TiAl-based alloys affected by carbon and yttrium additions.
Y (nm)
Alloys Ti-46.6AI-1.4Mn-2Mo
141
a2 (nm) 53
Ti-46.6Al- 1.4Mn-2Mc-0.3C
64
28
Ti-46.6A1-1.4Mn-2Mo-0.3C-0.33Y
50
22
.
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Proceedings of Sino-Swedish Structural Materials Symposium 2007
Fig. 3 TEM micrographs showing (a) carbides along grain boundaries of 7 and
in 03Ccontaining TIAl alloy and (b)Y203 formed in 7 platelets in 033Ycontaining Ti1 alloy. The samples were heat treated at 1400°C/lh/AC+ 8W0C/12h/AC Table 4 Tensile properties d experimentalTiAl-based alloys at mom temperature and high temperature. Alloys
Ultimate Tensile Strength (MPa)
Elonaation (96) ~~
Fa-
800°C
RT
800°C
526
507
1.8
2.6
Ti46.6A1- 1.4Mn-2Md.3C
610
699
1.7
2.8
Ti-46.6A1- 1.4Mn-2Me0.3C-0.33Y
682
652
3.1
3.7
-T1-46.SAl- 1.4Mn-2Mo
~~
oxygen elements and as expected to be the Y201 precipitates. Therefore, the addition of C and Y brought the microstructural refinement including the grain size and the thickness of lamella of a2and y as well as the formation of the precipitates of carbides and Y203. The microstructural refinement resulted in enhancement of the mechanical properties, particularly the tensile properties at both room temperature and at elevated temperatures' I 3 'I The tensile properties of C-modified and Y -modified Ti-46.6A1-1.4Mn-2Mo alloys are shown in Table 4. Room temperature elongation in the order of 2% together with the yield strength of approximately 526MPa was obtained in the Ti-46.6A1-1.4Mn-2Mo alloys. With the addition of C, the room temperature elongation value was slightly decreased from 1.8% to 1.7% but, in contrast, the tensile strength increased significantly from 526MPa to 610MPa. The variation of the tensile properties of the
samples at 800°C had a similar feature to that at room temperature. Particularly the high temperature tensile strength increased considerably up to 699MPa at temperature of 800°C. With addition of Y, the values of tensile strength and elongation measured at room temperature and 800°C increased further. Ultimate tensile strength increased successfully from 61OMPa to 682MPa in the tensile samples at room temperature. Plastic elongation at room temperature between 1.7% to 3.1% were obtained for specimens in fully lamellar grain microstructure with yttrium contents between 0 to 0.33at.%, whereby the yttrium content of 0.33at.% exhibited the best elongation of 3.7% at 800°C. Therefore, the combination of Y203 and carbide precipitation strengthening mainly contributed to their room and high temperature strength, while the refined grains and reduced lamellar spacing brought the better ductility.
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Proceedings of Sino-Swedish Structural Materials Symposium 2007
4. Conclusion
G Frommeyer ,Z. Metallkd., Vol. 83(1992), pp. 591-595. [6]. J.D. Shi, Z.J. Pu and K.H. Wu, in Gamma Titanium
In this work, some of the key processing parameters of indirect extrusion in synthesizing TiAl-based intermetallic compounds from elemental powders were evaluated. Among others, the powder characteristics, mixing method, degassing procedure, extrusion can and die-design, pre-extrusion heat treatment, extrusion ratio and temperature were the most important variables to control. Control of these processing parameters, combined with proper additions of alloying elements such as C and Y in Ti-46.6A1-1.4Mn-2Mo alloys brought improved mechanical properties at both room temperature and elevated temperature. The present work, therefore, demonstrates that the indirect extrusion from EPM process should be considered as a strong candidate processing technique alternative to the ingot metallurgy.
[lo]. Z. Jin, C. Cady, GT. GrayIII and Y.W. Kim, Metall. Trans. A,
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