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Fabrication and SPS microstructures of Ti–45Al–8.5Nb–(W,B,Y) alloying powders
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Intermetallics 16 (2008) 215e224 www.elsevier.com/locate/intermet
Fabrication and SPS microstructures of Tie45Ale8.5Nbe(W,B,Y) alloying powders Y.H. Wang a, J.P. Lin a,*, Y.H. He b, Y.L. Wang a, G.L. Chen a a
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, PR China b State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, PR China Received 21 June 2007; received in revised form 22 September 2007; accepted 26 September 2007 Available online 26 November 2007
Abstract Tie45Ale8.5Nbe(W,B,Y) alloying powders were fabricated by Ar gas atomization process, and then were consolidated by spark plasma sintering (SPS). Results showed that the phases in the powders are related with the particle size, but both exterior and interior structures of the powders are dendritic. There are two types of microsegregations in the alloying powders, designated as S-segregation and NT-segregation, and they become finer with decreasing particle size. A typical fully lamellar (FL) microstructure along with two types of microsegregations, such as b-segregation and a-segregation, is successfully formed at 1200 C. However, FL microstructure becomes coarser at 1300 C. Three kinds of alloys sintered with different particle sizes have FL microstructure after the treatment at 1200 C/40 MPa for 10 min, but there are differences in their lamellar colony sizes and microsegregations. It is favorable to refining lamellar colony and homogenizing microstructure if holding time is extended. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; B. Phase transformations; C. Powder metallurgy
1. Introduction High Nb containing TiAl alloys are considered as the potential high temperature structural materials due to their low densities, high melting points, good elevated temperature strengths and environmental resistances [1e4]. High Nb addition significantly improves high temperature properties for TiAl based alloys, but it also enhances the difficulties of their fabrication [5]. These alloys reported so far are mostly fabricated by ingot metallurgy (IM) [6e8]. However, IM process of high Nb containing TiAl alloys is usually difficult due to their relatively high melting point and the extreme reactivity of Ti. Recently, powder metallurgy (PM) technique appears to be more attractive since high degrees of chemical homogeneities can be obtained and macrosegregations are avoided.
* Corresponding author. Tel.: þ86 10 62332192; fax: þ86 10 62332508. E-mail address: [email protected] (J.P. Lin). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.09.010
PM process comprises elemental powder metallurgy (EPM) and alloying powder metallurgy (APM). EPM TiAl based alloys have drawn intensive attention because of its low cost and convenient addition of alloy elements [9e11]. However, the concentrations of impurities such as oxygen and carbon are relatively high, reducing the mechanical properties. Therefore, numerous works have been conducted on the microstructures and properties of APM TiAl based alloy [12e14]. In this study, high Nb containing TiAl alloying powders were fabricated by Ar gas atomization process, and then were consolidated in SPS1050 furnace. The effect of SPS process on microstructures of high Nb containing TiAl alloys was investigated in detail. 2. Experimental Tie45Ale8.5Nbe(W,B,Y) (at.%) ingot with large scale was fabricated using plasma arc furnace twice and annealed at 1200 C for 50 h in order to reduce the composition
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inhomogeneity. The homogeneous ingot was machined to the size of F 18 220 mm and then used to produce alloying powders by Ar gas atomization process. The alloying powders were classified by Thaler sieves. SPS process was employed to sinter and concurrently consolidate the alloying powders under the different sintering parameters. Fig. 1 shows a sintering curve of process parameters at 1200 C/40 MPa for 10 min. The alloying powders were filled into a graphite mold with two graphite punches pressed at both ends. Dense compacts of F 30 15 mm were obtained. The concentrations of gas elements such as oxygen and nitrogen in the alloying powders and as-SPS alloys were analyzed with the inert gas melting-IR absorption spectrometry. The concentration of carbon was measured by high frequency-IR absorption. X-ray diffraction (XRD) analysis was conducted using Cu Ka radiation to examine the phase transformation. Microstructural observation was carried out by scanning electron microscopy using back scattering electron (BSE) imaging and energy dispersive spectroscopy (EDS). Lamellar spacing was determined by transmission electron microscopy (TEM). The lamellar spacing given in this paper was arithmetic average value measured without taking account of types of lamellar boundaries. TEM foils were prepared by twin jet electropolished in a solution of 60% (vol.%) methanol, 35% butyl alcohol and 5% perchloric acid at 15 V and 30 C. The lamellar colony size was determined by the intersection linear method. 3. Results and discussion 3.1. Characteristics of high Nb containing TiAl alloying powder Fig. 2 shows the particle size distribution taining TiAl alloying powders fabricated by tion process. The particle size of high Nb alloying powders ranges mostly from 100
of high Nb conAr gas atomizacontaining TiAl to 200 mm, and
Fig. 1. Sintering curve of the process parameters at 1200 C/40 MPa for 10 min.
Fig. 2. Particle size distribution of Tie45Ale8.5Nbe(W,B,Y) alloying powder.
the average particle diameter d50 is approximately 138 mm. The powder particles with the different sizes are predominantly spherical and satellite formation is hardly observed, as seen in Fig. 3a and b, indicating that the atomized powders have excellent flowability. However, a small amount of Ar gas is trapped in the alloy droplets during the atomization, resulting in a certain porosity of the alloying powder. Therefore, a little amount of powders with closed or opening pores are found (Fig. 3c and d). Further examination shows that the proportion of powders with opening or closed pores is less than 8%. It has been reported that the existence of the pores in powder, especially closed pores, is not beneficial to further hot isostatic pressing (HIP) process [12]. During HIP-compaction these pores shrink until their inner gas pressure equals the applied HIP-pressure. Therefore, the compression of closed pores is of practical importance to obtain dense alloys. The oxygen concentration shows an increase with decreasing particle size from 900 ppm (wt.%) at 40 þ 80 mesh to 1150 ppm at 200 mesh. Finer particles are easier to be oxygenated due to their larger specific surface area. In contrast, the nitrogen level is about constant for all volume fractions: w140 ppm. In addition, a little amount of carbon, about 25 ppm, is detected because the ingot was contaminated in the machining process. XRD analysis, of the four series of powder size fractions, showed that the structure of the finer powder (200 mesh) consists of single a2 (Fig. 4). When the particle size becomes coarser, the proportion of a2 phase decreases and that of g phase increases. The powders with 150 þ 200 mesh contain predominantly a2 phase and a small amount of g phase. However, the powders with 40 þ 80 mesh are composed mainly of g phase and a little amount of a2 phase. The increase in a2 phase content in the finest powder particles is attributed to the faster cooling rate. This similar changes in the relative fractions of the g/a2 phases have been observed in TiAl powders (<0.03 wt.% C) produced by comparable atomization
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Fig. 3. Macrostructures of Tie45Ale8.5Nbe(W,B,Y) alloying powders: (a) 40 þ 80 mesh; (b) 200 mesh; (c) closed pore; (d) opening pore.
conditions, although some g phase was found in all of the powder size fractions [15]. Therefore, the alloying powders are presumed to be not in equilibrium state, which can be related to the rapid solidification of the powder particles.
Fig. 4. XRD patterns of Tie45Ale8.5Nbe(W,B,Y) alloying powders: (a) 40 þ 80 mesh; (b) 80 þ 150 mesh; (c) 150 þ 200 mesh; (d) 200 mesh.
Fig. 5a and b show the surface morphology of the alloying powders with different particle sizes. It indicates that their surfaces look like dendritic, which is related to the higher cooling rate. Moreover, the dendritic morphology becomes finer with decreasing the particle sizes because the cooling rate is higher for the finer powders. However, Masaharu Tokizane found that most of the particles have dendritic surface, and a little amount of the particles look like martensitic for Tie47Al alloying powders fabricated by plasma rotating electrode (PRE) process [16]. This is different from our observation owing to the different cooling rates. The cooling rate is as high as 104 K/s for the PRE process. However, the cooling rate of the atomized alloying powder is not clearly known, but it can be estimated rather high as to be the order of 103 K/s. The interior morphology of high Nb containing TiAl alloying powders is also dendritic, as shown in Fig. 5c and e. From the microstructure and the composition, microsegregation in alloying powders is of two types: S-segregation and NTsegregation. Table 1 shows the EDS analysis results of the microsegregations in Fig. 5c. It reveals that S-segregation located in the interdendritic area contains the higher Al content. However, NT-segregation located in the dendritic area has the higher Ti, Nb and W than the nominal composition of the
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Fig. 5. Microstructures of Tie45Ale8.5Nbe(W,B,Y) alloying powders: (a) the exterior (40 þ 80 mesh); (b) the exterior (200 mesh); (c) and (d) the interior (40 þ 80 mesh); (e) and (f) the interior (200 mesh).
alloy. Therefore, it can be deduced that NT-segregation is b (B2) phase from its chemical composition. It should be noted that b phase in the alloying powders is difficult to designate as b-segregation or a-segregation reported in high Nb containing TiAl ingots because the alloying powders are in the non-equilibrium state [17]. Many boride and yttrium oxide particles with bright contrast in SEM-BSE images are distributed mainly in the interdendritic regions due to B- and Y-segregations (Fig. 5c). The compositions of boride and yttrium
oxide are as also shown in Table 1. Closer examination of borides in deeply etched sample reveals that they are mainly in the form of particle and rod for the coarser powders (Fig. 5d), but their sizes in alloying powders are smaller than those in the ingot. It is interesting that the rod-shaped borides are hardly found in the finer powders (Fig. 5f). Y2O3 particles came from the prior ingot. In general, the microsegregations become finer with decreasing the particle sizes, which is in accordance with the previous results [14,18].
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Table 1 EDS analysis in Fig. 5c Region
Ti
Al
Nb
W
B
Y
O
S-segregation NT-segregation Borides Y2O3
34.48 59.27 17.19 13.33
63.09 28.58 12.05 4.06
2.43 12.02 5.33 e
e 0.13 0.11 e
e e 65.32 e
e e e 37.47
e e e 45.14
In fact, Ar gas atomization process is a second solidification of Tie45Ale8.5Nbe(W,B,Y) alloy, similarly to the first that of the ingot. According to TieAl quasi-binary phase diagram (Fig. 6), Tie45Ale8.5Nbe(W,B,Y) alloy evolves the following path solidification: L / L þ b / b / b þ a / a / a þ g / lamella (a2 þ g) [19]. It indicates that b phase forms firstly, and subsequently b transforms to a, following the orientation relationship of f110gb//(0001)a and <111>b// < 1120 > a [20]. However, the transformation is incomplete owing to the low diffusivity of these elements and quick solidification rate. During b solidification, Al is rejected to the interdendritic region, while those elements stabilizing the b phase, such as Nb and W, are concentrated in the dendritic arm. The primary b grain boundaries are rich in Al, and liquid phase in the boundaries of primary b grains forms g phase directly. As a result, S-segregation forms in b solidification stage. 3.2. Microstructure of Tie45Ale8.5Nbe(W,B,Y) alloy by SPS process 3.2.1. Effect of sintering temperature on microstructure Relative densities of >99.5% were achieved in SPS consolidated samples when sintering temperature is higher than 1100 C. Table 2 shows the effect of SPS process on the impurities concentrations in as-sintered alloy with 200 mesh
powders. It indicates that the oxygen and nitrogen concentrations slightly increase, but carbon concentration rapidly increases. This is correlated with the characteristics of SPS process, such as the high heating rate, the rapid cooling rate and the adoption of graphite mold. The microstructures formed during SPS process were found to change dramatically with sintering temperature, as shown in Fig. 7. Alloying powders are deformed and bonded, and the interfaces of these particles are very evident in the sintered alloy at 1000 C. The microstructure of the alloy is similar to that of the alloying powder. It was also found that a small amount of pores exist at prior particle boundaries. When sintering temperature is up to 1100 C, the interfaces of these particles are still discernible, but a near g microstructure with equiaxed grain size of 6 mm appears. It contains small amounts of a2 and b phases, the latter appearing light gray and white, respectively, in BSE-SEM imaging (Fig. 7b). A typical FL microstructure containing fine and homogeneous lamellar colonies a2/g is successfully formed at 1200 C. The average lamellar colony size is about 27 mm and the width of g lamellae and that of a lamellae is about 210 nm and 270 nm, respectively (Fig. 7c). In addition, it can be seen that there are composition segregations in the FL microstructure from BSE micrographs. The EDS analysis was performed to identify the microsegregations, as shown in Table 3. Microsegregations will be discussed in detail as follows. FL microstructure becomes coarser at 1300 C. Therefore, it can be deduced that the sintering temperature has significant effect on microstructure, and 1200 C is an appropriate sintering temperature to fabricate high Nb containing TiAl alloys with FL microstructure consisting of fine and homogeneous a2/g lamellar colonies. During SPS process, the dendritic structures of alloying powders were removed with increasing the sintering temperature. At 1100 C, a small amount of a2 phase formed, which is related with two transformation mechanisms: (i) a phase nucleates and grows at the g grain boundaries during the phase transformation of a / g þ a / g þ a2; and (ii) a particles, formed at g grain boundaries in the course of prior powder consolidation, grow into a2 during SPS treatment. When sintering temperature increases up to 1200 C, FL microstructure similar to that of ingot was formed in terms of a strict orientation relationship: {111}g//(0001)a2 and <110>g//[1120]a2 Table 2 Effect of SPS process on the impurities contents
Fig. 6. 8Nb(at.%) containing TieAl quasi-phase diagram.
Impurities
O
N
C
Alloying powders (ppm) SPS alloy (ppm)
1150 1170
140 145
24 35
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Fig. 7. Microstructures at the different sintering temperatures: (a) 1000 C; (b) 1100 C; (c) 1200 C; (d) 1300 C.
[20], but the lamellar colony size is smaller. The lamellar microstructure are probably defined by a / a þ g / a2 / g / L(a2/g) transformations [21]. However, the effect of SPS temperature on the lamellar characteristics is not yet clear. It has been proposed that the eutectoid transformation of a / a2 / g is diffusion-controlled by the interface reaction that involves uniform atomic detachment or a screw dislocation mechanism [21]. FL microstructure coarsening happens at 1300 C. This is not favorable to control microstructure and leads to the difficulty in the grain boundary gliding and dislocation movement. 3.2.2. Effect of particle sizes for the alloying powders on microstructure Fig. 8 shows the effect of particle size on microstructures for Tie45Ale8.5Nbe(W,B,Y) alloy. It reveals that three alloys sintered with different particle sizes have FL microstructure after Table 3 EDS analysis in Fig. 7c Phase
Ti
Al
Nb
W
B
Y
O
b phase Borides Y2O3
58.66 18.79 13.50
29.43 11.22 4.54
11.79 6.01 e
0.12 0.10 e
e 63.88 e
e e 35.24
e e 46.72
the treatment at 1200 C/40 MPa for 10 min, but there are differences in their lamellar colony sizes and microsegregations. Fig. 9 is the relationship between lamellar colony size and particle sizes. When the particle sizes change from 40 þ 80 mesh to 80 þ 150 mesh, the lamellar colony sizes decrease quickly. However, the lamellar colony sizes slightly decrease even if the alloying powders become finer. There are differences in the size and form of microsegregations in SPS alloys fabricated with the different particle sizes. For the alloy sintered with 40 þ 80 mesh powders, the size of b phase is larger, and its distribution in FL microstructure is inhomogeneous. Most of b phase exist in the form of strip, and only a small amount of b phase was jointed to ring. However, b phase in SPS alloy sintered by 150 þ 200 mesh powders are similar to that in the ingot and distributes homogeneously in the form of ring. When alloying powders with 200 mesh are adopted, the microsegregations become finer. The effect of particle size on SPS microstructures for Tie 45Ale8.5Nbe(W,B,Y) alloy is related with the microstructures of alloying powders. 3.2.3. Effect of holding time on microstructure Fig. 10 shows the effect of holding time at the highest sintering temperature (1200 C) on microstructures. It can be seen that SPS alloys have FL microstructures for both the
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Fig. 8. Effect of particle size on microstructures: (a) 40 þ 80mesh; (b) 150 þ 200 mesh; (c) 200 mesh.
holding times, but the lamellar colony size is coarser and the distribution of b phase appears inhomogeneous for 4 min, indicating that it is favorable to refine lamellar colony and to homogenize microstructure if holding time is extended. It is
Fig. 9. Relationship between lamellar colony size and particle size.
related with the formation of FL microstructure. The formation of FL microstructures in SPS alloys is different from that in ingot. Generally, it is a solidification process for the ingot. However, the microstructural evolution in SPS alloys is a diffusioncontrolled phase transformations, i.e. a / a þ g / a2 þ g / L(a2/g). In other words, the eutectoid transformation a / a2 þ g followed by the b precipitation from a2 phase takes place, which leads to the formation of a2/g lamellar structure. It must be noted that the transformation a / a2 þ g will proceed if the sintering temperature is higher than 1100 C. It can be deduced that the ordering transformation of aea2 is incomplete when the sintering temperature is up to 1200 C because of the rapid heating rate of SPS process. Thus, the transformation will be more complete if holding time at the highest sintering temperature is extended. The decomposition of a2 / a2 þ g occurs, then lamellar structure forms during cooling from sintering temperature. During subsequent cooling, the growths of lamellar colonies are restrained each other owing to their excessive number. As a result, the lamellar colony size is finer when the holding time is 10 min. Moreover, the ordering transformation of a also improves the homogenous precipitation and distribution of b phase. It must be pointed out that the suitable holding time should be adopted for different fabrication process. In this study,
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Fig. 10. Effect of holding time on microstructures: (a) and (b) 4 min (200 mesh); (c) 10 min (200 mesh).
10 min are favorable holding time to fabricate high Nb containing TiAl alloys because SPS process is a method of short time fabrication, and finer and more homogenous lamellar structure can be obtained. 3.2.4. Microsegregations in as-SPS alloys It is worth noting that the presence of b phase in TiAl based alloy with alloying additions of b stabilizers such as Nb, W, Cr and Mo has been reported previously [6,22]. The main microsegregation is b phase except for B-segregation (borides) and Y-segregation (Y2O3) in SPS microstructures, as shown in Fig. 11a. The results of EDS analysis suggests that the light reticulate phases are b; the light particulate or bar phases are borides and the lightest particles are Y2O3. High magnification SEM-BSE image reveals that b phase can be classified two different types of segregations in fact (Fig. 11a). The one is the b-segregation at the boundary and triple junctions among a grain due to the phase transformation of b / a; the other is the a-segregation that forms local lamellar structure composed of b, g and a plates due to phase transformation of a / a2 þ b þ g. For SPS process, the lamellar colony forms when sintering temperature increases to 1200 C. At the same time, S-segregation was melted because of the higher Al addition and SPS
characteristics. As well known that SPS process has an activating sintering by discharging effect between particles. The interior temperature is approximately 200 C higher than the programmed, as the temperature of the mold was controlled [21,23]. During solidification, b phase (b-segregation) as the first solidified phase becomes richer in Nb and W. However, the volume fraction of b phase is small due to the lower concentrations of Nb and W in S-segregation region. Subsequently, b phase transforms to a. However, the transformation of bea, which later transformed to lamellar colony, is incomplete because the cooling rate is often too rapid to completely eliminate differences in composition by diffusion. During further cooling, very fine b phase (a-segregation) precipitates from a2 and locates in the lamellar colonies. Lamellar colonies and b phase simultaneously grow up, but the growth rate of lamellar colonies is faster than that of b phase due to the low diffusivities of Nb and W. Finally, lamellar colonies except for a-segregation and b-segregation are formed. In general, B addition is beneficial to refining grain for TiAl based alloys [24,25]. It can be seen that the effect of holding time on the sizes of borides is evident from Fig. 11b and c. The length of borides is less than 15 mm if the holding time is 10 min. However, their sizes are about 40 mm when the
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Fig. 11. Microsegregations in the as-SPS alloys: (a) b-segregations and a-segregation; (b) borides (10 min); (c) borides (4 min); (d) borides (section image).
holding time is 4 min. Therefore, extending holding time is beneficial to reducing boride size. It has been reported by other investigators, that borides exist in different morphologies in TiAl based alloys, and their crystallographic structures mainly depend on the boron content [26]. For the studied alloy, the crystal structure of the flakes is not known yet, but from the quasi-hexagonal sections of the borides it is concluded that the boride phase is TiB2 (Fig. 11d) [26]. In addition, Y2O3 particles cannot be eliminated through SPS process. 4. Conclusions Tie45Ale8.5Nbe(W,B,Y) alloying powders were fabricated by Ar gas atomization process. The particle size of the alloying powder ranges mostly from 100 to 200 mm. The oxygen concentration shows an increase with decreasing particle size and the nitrogen level is about constant. In addition, a little amount of carbon is observed. The phases of the alloying powders are related with the particle size, but both the exterior and the interior structures of the alloying powders are dendritic. There are two types of microsegregations in the powder, i.e. S-segregation and NT-segregation, and they become finer with decreasing particle size. The sintering temperature has
a significant effect on the microstructures. When the sintering temperature is 1000 C, the dendritic microstructure of SPS alloy is similar to that of the alloying powder. At 1100 C, the interfaces of these particles are still discernible, but near g microstructure appears in every particle. A typical FL microstructure followed by two types of microsegregations, i.e. b-segregation and a-segregation, is successfully formed at 1200 C. However, FL microstructure becomes coarser at 1300 C. Both particle size and holding time have a significant effect on the microstructural homogeneity. Three alloys sintered with different particle sizes have FL microstructure after the treatment at 1200 C/40 MPa for 10 min, but there are differences in their lamellar colony sizes and microsegregations. It is favorable to refining lamellar colony and homogenizing microstructure if holding time is extended.
Acknowledgements This research was performed under the auspices of the HiTech Research and Development Program of China (No. 2006AA03Z511), the Key Grant Project of Chinese Ministry of Education (No. 704008) and the program from New Century Excellent Talents in University (No. NCETe04e01017).
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[14] Gerling R, Bartels A, Clemens H, Kestler H, Schimansky FP. Intermetallics 2004;12:275. [15] Gouma PI. M.Phil (Eng) Qualifying Thesis, 1994, The University of Birmingham, Birmingham. [16] Masaharu T, Toshisuke F, Teruhiko I. ISIJ Int. 1991;31:1088. [17] Chen G, Xu XJ, Teng ZK, Wang YL, Lin JP. Intermetallics 2007;15: 625. [18] Gouma PI, Davey SJ, Loretto MH. Mater. Sci. Eng. A 1998;241:151. [19] Wang YH, Lin JP, He YH, Wang YL, Chen GL. Mater. Sci. Eng. A 2007;471:82. [20] Chen G, Zhang W, Liu Z, Li S, Kim YW. In: Kim YW, Dimiduk DM, Loretto MH, editors. Gamma Titanium Aluminides 1999. Warrendale, USA: TMS; 1999. p. 371. [21] Zhao L, Beddoes J, Morphy D, Wallace W. Mater. Manuf. Process. 1994;9:695. [22] Hsiung LM, Nieh TG. Mater. Sci. Eng. A 2004;364:1. [23] Sun ZM, Hashimoto H. Intermetallics 2003;11:825. [24] Hu D. Intermetallics 2002;10:851. [25] Huang L, Liu CT, Liaw PK, Huang BY. JOM 2004;56:361. [26] Abdel-Hamid AA, Hamar Thibault S, Hamar RJ. Cryst. Growth 1985;71: 744.
Intermetallics 16 (2008) 215e224 www.elsevier.com/locate/intermet
Fabrication and SPS microstructures of Tie45Ale8.5Nbe(W,B,Y) alloying powders Y.H. Wang a, J.P. Lin a,*, Y.H. He b, Y.L. Wang a, G.L. Chen a a
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, PR China b State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, PR China Received 21 June 2007; received in revised form 22 September 2007; accepted 26 September 2007 Available online 26 November 2007
Abstract Tie45Ale8.5Nbe(W,B,Y) alloying powders were fabricated by Ar gas atomization process, and then were consolidated by spark plasma sintering (SPS). Results showed that the phases in the powders are related with the particle size, but both exterior and interior structures of the powders are dendritic. There are two types of microsegregations in the alloying powders, designated as S-segregation and NT-segregation, and they become finer with decreasing particle size. A typical fully lamellar (FL) microstructure along with two types of microsegregations, such as b-segregation and a-segregation, is successfully formed at 1200 C. However, FL microstructure becomes coarser at 1300 C. Three kinds of alloys sintered with different particle sizes have FL microstructure after the treatment at 1200 C/40 MPa for 10 min, but there are differences in their lamellar colony sizes and microsegregations. It is favorable to refining lamellar colony and homogenizing microstructure if holding time is extended. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: A. Titanium aluminides, based on TiAl; B. Phase transformations; C. Powder metallurgy
1. Introduction High Nb containing TiAl alloys are considered as the potential high temperature structural materials due to their low densities, high melting points, good elevated temperature strengths and environmental resistances [1e4]. High Nb addition significantly improves high temperature properties for TiAl based alloys, but it also enhances the difficulties of their fabrication [5]. These alloys reported so far are mostly fabricated by ingot metallurgy (IM) [6e8]. However, IM process of high Nb containing TiAl alloys is usually difficult due to their relatively high melting point and the extreme reactivity of Ti. Recently, powder metallurgy (PM) technique appears to be more attractive since high degrees of chemical homogeneities can be obtained and macrosegregations are avoided.
* Corresponding author. Tel.: þ86 10 62332192; fax: þ86 10 62332508. E-mail address: [email protected] (J.P. Lin). 0966-9795/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2007.09.010
PM process comprises elemental powder metallurgy (EPM) and alloying powder metallurgy (APM). EPM TiAl based alloys have drawn intensive attention because of its low cost and convenient addition of alloy elements [9e11]. However, the concentrations of impurities such as oxygen and carbon are relatively high, reducing the mechanical properties. Therefore, numerous works have been conducted on the microstructures and properties of APM TiAl based alloy [12e14]. In this study, high Nb containing TiAl alloying powders were fabricated by Ar gas atomization process, and then were consolidated in SPS1050 furnace. The effect of SPS process on microstructures of high Nb containing TiAl alloys was investigated in detail. 2. Experimental Tie45Ale8.5Nbe(W,B,Y) (at.%) ingot with large scale was fabricated using plasma arc furnace twice and annealed at 1200 C for 50 h in order to reduce the composition
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inhomogeneity. The homogeneous ingot was machined to the size of F 18 220 mm and then used to produce alloying powders by Ar gas atomization process. The alloying powders were classified by Thaler sieves. SPS process was employed to sinter and concurrently consolidate the alloying powders under the different sintering parameters. Fig. 1 shows a sintering curve of process parameters at 1200 C/40 MPa for 10 min. The alloying powders were filled into a graphite mold with two graphite punches pressed at both ends. Dense compacts of F 30 15 mm were obtained. The concentrations of gas elements such as oxygen and nitrogen in the alloying powders and as-SPS alloys were analyzed with the inert gas melting-IR absorption spectrometry. The concentration of carbon was measured by high frequency-IR absorption. X-ray diffraction (XRD) analysis was conducted using Cu Ka radiation to examine the phase transformation. Microstructural observation was carried out by scanning electron microscopy using back scattering electron (BSE) imaging and energy dispersive spectroscopy (EDS). Lamellar spacing was determined by transmission electron microscopy (TEM). The lamellar spacing given in this paper was arithmetic average value measured without taking account of types of lamellar boundaries. TEM foils were prepared by twin jet electropolished in a solution of 60% (vol.%) methanol, 35% butyl alcohol and 5% perchloric acid at 15 V and 30 C. The lamellar colony size was determined by the intersection linear method. 3. Results and discussion 3.1. Characteristics of high Nb containing TiAl alloying powder Fig. 2 shows the particle size distribution taining TiAl alloying powders fabricated by tion process. The particle size of high Nb alloying powders ranges mostly from 100
of high Nb conAr gas atomizacontaining TiAl to 200 mm, and
Fig. 1. Sintering curve of the process parameters at 1200 C/40 MPa for 10 min.
Fig. 2. Particle size distribution of Tie45Ale8.5Nbe(W,B,Y) alloying powder.
the average particle diameter d50 is approximately 138 mm. The powder particles with the different sizes are predominantly spherical and satellite formation is hardly observed, as seen in Fig. 3a and b, indicating that the atomized powders have excellent flowability. However, a small amount of Ar gas is trapped in the alloy droplets during the atomization, resulting in a certain porosity of the alloying powder. Therefore, a little amount of powders with closed or opening pores are found (Fig. 3c and d). Further examination shows that the proportion of powders with opening or closed pores is less than 8%. It has been reported that the existence of the pores in powder, especially closed pores, is not beneficial to further hot isostatic pressing (HIP) process [12]. During HIP-compaction these pores shrink until their inner gas pressure equals the applied HIP-pressure. Therefore, the compression of closed pores is of practical importance to obtain dense alloys. The oxygen concentration shows an increase with decreasing particle size from 900 ppm (wt.%) at 40 þ 80 mesh to 1150 ppm at 200 mesh. Finer particles are easier to be oxygenated due to their larger specific surface area. In contrast, the nitrogen level is about constant for all volume fractions: w140 ppm. In addition, a little amount of carbon, about 25 ppm, is detected because the ingot was contaminated in the machining process. XRD analysis, of the four series of powder size fractions, showed that the structure of the finer powder (200 mesh) consists of single a2 (Fig. 4). When the particle size becomes coarser, the proportion of a2 phase decreases and that of g phase increases. The powders with 150 þ 200 mesh contain predominantly a2 phase and a small amount of g phase. However, the powders with 40 þ 80 mesh are composed mainly of g phase and a little amount of a2 phase. The increase in a2 phase content in the finest powder particles is attributed to the faster cooling rate. This similar changes in the relative fractions of the g/a2 phases have been observed in TiAl powders (<0.03 wt.% C) produced by comparable atomization
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Fig. 3. Macrostructures of Tie45Ale8.5Nbe(W,B,Y) alloying powders: (a) 40 þ 80 mesh; (b) 200 mesh; (c) closed pore; (d) opening pore.
conditions, although some g phase was found in all of the powder size fractions [15]. Therefore, the alloying powders are presumed to be not in equilibrium state, which can be related to the rapid solidification of the powder particles.
Fig. 4. XRD patterns of Tie45Ale8.5Nbe(W,B,Y) alloying powders: (a) 40 þ 80 mesh; (b) 80 þ 150 mesh; (c) 150 þ 200 mesh; (d) 200 mesh.
Fig. 5a and b show the surface morphology of the alloying powders with different particle sizes. It indicates that their surfaces look like dendritic, which is related to the higher cooling rate. Moreover, the dendritic morphology becomes finer with decreasing the particle sizes because the cooling rate is higher for the finer powders. However, Masaharu Tokizane found that most of the particles have dendritic surface, and a little amount of the particles look like martensitic for Tie47Al alloying powders fabricated by plasma rotating electrode (PRE) process [16]. This is different from our observation owing to the different cooling rates. The cooling rate is as high as 104 K/s for the PRE process. However, the cooling rate of the atomized alloying powder is not clearly known, but it can be estimated rather high as to be the order of 103 K/s. The interior morphology of high Nb containing TiAl alloying powders is also dendritic, as shown in Fig. 5c and e. From the microstructure and the composition, microsegregation in alloying powders is of two types: S-segregation and NTsegregation. Table 1 shows the EDS analysis results of the microsegregations in Fig. 5c. It reveals that S-segregation located in the interdendritic area contains the higher Al content. However, NT-segregation located in the dendritic area has the higher Ti, Nb and W than the nominal composition of the
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Fig. 5. Microstructures of Tie45Ale8.5Nbe(W,B,Y) alloying powders: (a) the exterior (40 þ 80 mesh); (b) the exterior (200 mesh); (c) and (d) the interior (40 þ 80 mesh); (e) and (f) the interior (200 mesh).
alloy. Therefore, it can be deduced that NT-segregation is b (B2) phase from its chemical composition. It should be noted that b phase in the alloying powders is difficult to designate as b-segregation or a-segregation reported in high Nb containing TiAl ingots because the alloying powders are in the non-equilibrium state [17]. Many boride and yttrium oxide particles with bright contrast in SEM-BSE images are distributed mainly in the interdendritic regions due to B- and Y-segregations (Fig. 5c). The compositions of boride and yttrium
oxide are as also shown in Table 1. Closer examination of borides in deeply etched sample reveals that they are mainly in the form of particle and rod for the coarser powders (Fig. 5d), but their sizes in alloying powders are smaller than those in the ingot. It is interesting that the rod-shaped borides are hardly found in the finer powders (Fig. 5f). Y2O3 particles came from the prior ingot. In general, the microsegregations become finer with decreasing the particle sizes, which is in accordance with the previous results [14,18].
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Table 1 EDS analysis in Fig. 5c Region
Ti
Al
Nb
W
B
Y
O
S-segregation NT-segregation Borides Y2O3
34.48 59.27 17.19 13.33
63.09 28.58 12.05 4.06
2.43 12.02 5.33 e
e 0.13 0.11 e
e e 65.32 e
e e e 37.47
e e e 45.14
In fact, Ar gas atomization process is a second solidification of Tie45Ale8.5Nbe(W,B,Y) alloy, similarly to the first that of the ingot. According to TieAl quasi-binary phase diagram (Fig. 6), Tie45Ale8.5Nbe(W,B,Y) alloy evolves the following path solidification: L / L þ b / b / b þ a / a / a þ g / lamella (a2 þ g) [19]. It indicates that b phase forms firstly, and subsequently b transforms to a, following the orientation relationship of f110gb//(0001)a and <111>b// < 1120 > a [20]. However, the transformation is incomplete owing to the low diffusivity of these elements and quick solidification rate. During b solidification, Al is rejected to the interdendritic region, while those elements stabilizing the b phase, such as Nb and W, are concentrated in the dendritic arm. The primary b grain boundaries are rich in Al, and liquid phase in the boundaries of primary b grains forms g phase directly. As a result, S-segregation forms in b solidification stage. 3.2. Microstructure of Tie45Ale8.5Nbe(W,B,Y) alloy by SPS process 3.2.1. Effect of sintering temperature on microstructure Relative densities of >99.5% were achieved in SPS consolidated samples when sintering temperature is higher than 1100 C. Table 2 shows the effect of SPS process on the impurities concentrations in as-sintered alloy with 200 mesh
powders. It indicates that the oxygen and nitrogen concentrations slightly increase, but carbon concentration rapidly increases. This is correlated with the characteristics of SPS process, such as the high heating rate, the rapid cooling rate and the adoption of graphite mold. The microstructures formed during SPS process were found to change dramatically with sintering temperature, as shown in Fig. 7. Alloying powders are deformed and bonded, and the interfaces of these particles are very evident in the sintered alloy at 1000 C. The microstructure of the alloy is similar to that of the alloying powder. It was also found that a small amount of pores exist at prior particle boundaries. When sintering temperature is up to 1100 C, the interfaces of these particles are still discernible, but a near g microstructure with equiaxed grain size of 6 mm appears. It contains small amounts of a2 and b phases, the latter appearing light gray and white, respectively, in BSE-SEM imaging (Fig. 7b). A typical FL microstructure containing fine and homogeneous lamellar colonies a2/g is successfully formed at 1200 C. The average lamellar colony size is about 27 mm and the width of g lamellae and that of a lamellae is about 210 nm and 270 nm, respectively (Fig. 7c). In addition, it can be seen that there are composition segregations in the FL microstructure from BSE micrographs. The EDS analysis was performed to identify the microsegregations, as shown in Table 3. Microsegregations will be discussed in detail as follows. FL microstructure becomes coarser at 1300 C. Therefore, it can be deduced that the sintering temperature has significant effect on microstructure, and 1200 C is an appropriate sintering temperature to fabricate high Nb containing TiAl alloys with FL microstructure consisting of fine and homogeneous a2/g lamellar colonies. During SPS process, the dendritic structures of alloying powders were removed with increasing the sintering temperature. At 1100 C, a small amount of a2 phase formed, which is related with two transformation mechanisms: (i) a phase nucleates and grows at the g grain boundaries during the phase transformation of a / g þ a / g þ a2; and (ii) a particles, formed at g grain boundaries in the course of prior powder consolidation, grow into a2 during SPS treatment. When sintering temperature increases up to 1200 C, FL microstructure similar to that of ingot was formed in terms of a strict orientation relationship: {111}g//(0001)a2 and <110>g//[1120]a2 Table 2 Effect of SPS process on the impurities contents
Fig. 6. 8Nb(at.%) containing TieAl quasi-phase diagram.
Impurities
O
N
C
Alloying powders (ppm) SPS alloy (ppm)
1150 1170
140 145
24 35
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Fig. 7. Microstructures at the different sintering temperatures: (a) 1000 C; (b) 1100 C; (c) 1200 C; (d) 1300 C.
[20], but the lamellar colony size is smaller. The lamellar microstructure are probably defined by a / a þ g / a2 / g / L(a2/g) transformations [21]. However, the effect of SPS temperature on the lamellar characteristics is not yet clear. It has been proposed that the eutectoid transformation of a / a2 / g is diffusion-controlled by the interface reaction that involves uniform atomic detachment or a screw dislocation mechanism [21]. FL microstructure coarsening happens at 1300 C. This is not favorable to control microstructure and leads to the difficulty in the grain boundary gliding and dislocation movement. 3.2.2. Effect of particle sizes for the alloying powders on microstructure Fig. 8 shows the effect of particle size on microstructures for Tie45Ale8.5Nbe(W,B,Y) alloy. It reveals that three alloys sintered with different particle sizes have FL microstructure after Table 3 EDS analysis in Fig. 7c Phase
Ti
Al
Nb
W
B
Y
O
b phase Borides Y2O3
58.66 18.79 13.50
29.43 11.22 4.54
11.79 6.01 e
0.12 0.10 e
e 63.88 e
e e 35.24
e e 46.72
the treatment at 1200 C/40 MPa for 10 min, but there are differences in their lamellar colony sizes and microsegregations. Fig. 9 is the relationship between lamellar colony size and particle sizes. When the particle sizes change from 40 þ 80 mesh to 80 þ 150 mesh, the lamellar colony sizes decrease quickly. However, the lamellar colony sizes slightly decrease even if the alloying powders become finer. There are differences in the size and form of microsegregations in SPS alloys fabricated with the different particle sizes. For the alloy sintered with 40 þ 80 mesh powders, the size of b phase is larger, and its distribution in FL microstructure is inhomogeneous. Most of b phase exist in the form of strip, and only a small amount of b phase was jointed to ring. However, b phase in SPS alloy sintered by 150 þ 200 mesh powders are similar to that in the ingot and distributes homogeneously in the form of ring. When alloying powders with 200 mesh are adopted, the microsegregations become finer. The effect of particle size on SPS microstructures for Tie 45Ale8.5Nbe(W,B,Y) alloy is related with the microstructures of alloying powders. 3.2.3. Effect of holding time on microstructure Fig. 10 shows the effect of holding time at the highest sintering temperature (1200 C) on microstructures. It can be seen that SPS alloys have FL microstructures for both the
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Fig. 8. Effect of particle size on microstructures: (a) 40 þ 80mesh; (b) 150 þ 200 mesh; (c) 200 mesh.
holding times, but the lamellar colony size is coarser and the distribution of b phase appears inhomogeneous for 4 min, indicating that it is favorable to refine lamellar colony and to homogenize microstructure if holding time is extended. It is
Fig. 9. Relationship between lamellar colony size and particle size.
related with the formation of FL microstructure. The formation of FL microstructures in SPS alloys is different from that in ingot. Generally, it is a solidification process for the ingot. However, the microstructural evolution in SPS alloys is a diffusioncontrolled phase transformations, i.e. a / a þ g / a2 þ g / L(a2/g). In other words, the eutectoid transformation a / a2 þ g followed by the b precipitation from a2 phase takes place, which leads to the formation of a2/g lamellar structure. It must be noted that the transformation a / a2 þ g will proceed if the sintering temperature is higher than 1100 C. It can be deduced that the ordering transformation of aea2 is incomplete when the sintering temperature is up to 1200 C because of the rapid heating rate of SPS process. Thus, the transformation will be more complete if holding time at the highest sintering temperature is extended. The decomposition of a2 / a2 þ g occurs, then lamellar structure forms during cooling from sintering temperature. During subsequent cooling, the growths of lamellar colonies are restrained each other owing to their excessive number. As a result, the lamellar colony size is finer when the holding time is 10 min. Moreover, the ordering transformation of a also improves the homogenous precipitation and distribution of b phase. It must be pointed out that the suitable holding time should be adopted for different fabrication process. In this study,
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Fig. 10. Effect of holding time on microstructures: (a) and (b) 4 min (200 mesh); (c) 10 min (200 mesh).
10 min are favorable holding time to fabricate high Nb containing TiAl alloys because SPS process is a method of short time fabrication, and finer and more homogenous lamellar structure can be obtained. 3.2.4. Microsegregations in as-SPS alloys It is worth noting that the presence of b phase in TiAl based alloy with alloying additions of b stabilizers such as Nb, W, Cr and Mo has been reported previously [6,22]. The main microsegregation is b phase except for B-segregation (borides) and Y-segregation (Y2O3) in SPS microstructures, as shown in Fig. 11a. The results of EDS analysis suggests that the light reticulate phases are b; the light particulate or bar phases are borides and the lightest particles are Y2O3. High magnification SEM-BSE image reveals that b phase can be classified two different types of segregations in fact (Fig. 11a). The one is the b-segregation at the boundary and triple junctions among a grain due to the phase transformation of b / a; the other is the a-segregation that forms local lamellar structure composed of b, g and a plates due to phase transformation of a / a2 þ b þ g. For SPS process, the lamellar colony forms when sintering temperature increases to 1200 C. At the same time, S-segregation was melted because of the higher Al addition and SPS
characteristics. As well known that SPS process has an activating sintering by discharging effect between particles. The interior temperature is approximately 200 C higher than the programmed, as the temperature of the mold was controlled [21,23]. During solidification, b phase (b-segregation) as the first solidified phase becomes richer in Nb and W. However, the volume fraction of b phase is small due to the lower concentrations of Nb and W in S-segregation region. Subsequently, b phase transforms to a. However, the transformation of bea, which later transformed to lamellar colony, is incomplete because the cooling rate is often too rapid to completely eliminate differences in composition by diffusion. During further cooling, very fine b phase (a-segregation) precipitates from a2 and locates in the lamellar colonies. Lamellar colonies and b phase simultaneously grow up, but the growth rate of lamellar colonies is faster than that of b phase due to the low diffusivities of Nb and W. Finally, lamellar colonies except for a-segregation and b-segregation are formed. In general, B addition is beneficial to refining grain for TiAl based alloys [24,25]. It can be seen that the effect of holding time on the sizes of borides is evident from Fig. 11b and c. The length of borides is less than 15 mm if the holding time is 10 min. However, their sizes are about 40 mm when the
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Fig. 11. Microsegregations in the as-SPS alloys: (a) b-segregations and a-segregation; (b) borides (10 min); (c) borides (4 min); (d) borides (section image).
holding time is 4 min. Therefore, extending holding time is beneficial to reducing boride size. It has been reported by other investigators, that borides exist in different morphologies in TiAl based alloys, and their crystallographic structures mainly depend on the boron content [26]. For the studied alloy, the crystal structure of the flakes is not known yet, but from the quasi-hexagonal sections of the borides it is concluded that the boride phase is TiB2 (Fig. 11d) [26]. In addition, Y2O3 particles cannot be eliminated through SPS process. 4. Conclusions Tie45Ale8.5Nbe(W,B,Y) alloying powders were fabricated by Ar gas atomization process. The particle size of the alloying powder ranges mostly from 100 to 200 mm. The oxygen concentration shows an increase with decreasing particle size and the nitrogen level is about constant. In addition, a little amount of carbon is observed. The phases of the alloying powders are related with the particle size, but both the exterior and the interior structures of the alloying powders are dendritic. There are two types of microsegregations in the powder, i.e. S-segregation and NT-segregation, and they become finer with decreasing particle size. The sintering temperature has
a significant effect on the microstructures. When the sintering temperature is 1000 C, the dendritic microstructure of SPS alloy is similar to that of the alloying powder. At 1100 C, the interfaces of these particles are still discernible, but near g microstructure appears in every particle. A typical FL microstructure followed by two types of microsegregations, i.e. b-segregation and a-segregation, is successfully formed at 1200 C. However, FL microstructure becomes coarser at 1300 C. Both particle size and holding time have a significant effect on the microstructural homogeneity. Three alloys sintered with different particle sizes have FL microstructure after the treatment at 1200 C/40 MPa for 10 min, but there are differences in their lamellar colony sizes and microsegregations. It is favorable to refining lamellar colony and homogenizing microstructure if holding time is extended.
Acknowledgements This research was performed under the auspices of the HiTech Research and Development Program of China (No. 2006AA03Z511), the Key Grant Project of Chinese Ministry of Education (No. 704008) and the program from New Century Excellent Talents in University (No. NCETe04e01017).
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