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SPS synthesis and consolidation of TiAl alloys from elemental powders: Microstructure evolution
Intermetallics 36 (2013) 51e56
Contents lists available at SciVerse ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
SPS synthesis and consolidation of TiAl alloys from elemental powders: Microstructure evolution M.A. Lagos*, I. Agote Tecnalia, Materials for Avanced Systems, Paseo Mikeletegi, 2 e Parque Tecnológico, E-20009 San Sebastián, Gipuzkoa, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2012 Received in revised form 14 December 2012 Accepted 11 January 2013 Available online 7 February 2013
Some works have studied the sintering of TiAl by means of Spark Plasma Sintering (SPS) and it is believed that it is possible to obtain very fine microstructures and very interesting mechanical properties. As raw materials, normally pre-alloyed powders have been used, obtained by atomization or mechanical alloying. However, few works have studied the consolidation of elemental powders. This work presents the synthesis and densification of TiAl-based alloys by means of Spark Plasma Sintering (SPS) using elemental powders as raw materials. The selected composition was Tie48Ale2Cre2Nb. The densification process, the evolution of the microstructure with the temperature and the diffusion of the elements (including heavy metals Cr and Nb) are presented. In addition, SPS products are compared with TiAl alloys from elemental powders obtained by conventional methods. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Titanium aluminides A. Intermetallics C. Reaction synthesis C. Powder metallurgy G. Aerospace constructional uses
1. Introduction
g-TiAl based alloys have been extensively studied as potential replacements for nickel-based superalloys [1e3] for different application fields, such as turbine blades for aircraft engines, stationary turbines and space vehicles. These alloys have relatively high yield strength at elevated temperatures, good creep characteristics, and excellent oxidation/corrosion resistance. The combination of these properties, associated with their low density (3.9e4.1 g/cc), makes them very attractive as high-temperature structural materials for aerospace, automotive and other applications [4,5]. A number of methods have been utilised in the synthesis and processing of g-TiAl based alloys, such as Vacuum Arc Remelting (VAR), conventional melting, casting processes and hot working techniques [1,3,6,7]. Conventionally, the first step is to synthesise the alloy, which is usually done by VAR or other similar technique. The shaping and processing of the material are carried out by casting techniques (typically centrifugal or gravity casting) or hot working (forging, extrusion, etc). In these manufacturing routes, one of the main problems is the scattering in mechanical properties due to the segregation in the composition. Powder metallurgy is an interesting alternative [8] to control the composition. Spark plasma sintering (SPS) is found to compact powders satisfactorily through the simultaneous application of direct current * Corresponding author. Tel.: þ34 667119590. E-mail address: [email protected] (M.A. Lagos). 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.01.006
pulses of high intensity and pressure. The electric current induces a temperature elevation within the sample by Joule’s effect. The whole process lasts just a few minutes. Some works have studied the sintering of TiAl by means of SPS [9e14] and it is believed that it is possible to obtain very fine microstructures. These alloys presented high strength whereas ductility was still limited. As raw materials, normally pre-alloyed powders have been used, obtained by atomization or mechanical alloying. However, few works have studied the consolidation of elemental powders by SPS [15,16]. These works claimed that the sintering of elemental powders is easier and faster than of pre-alloyed powders, but the higher temperature is needed to complete the diffusion of the elements. However, the densification process was not studied in detail and the effect of the load was not studied. This work presents the synthesis and densification of TiAl-based alloys by means of Spark Plasma Sintering (SPS) using elemental powders as raw materials. The densification process and the evolution of the microstructure with the temperature are presented. Particularly interesting is the diffusion of the elements, including heavy metals Cr and Nb. In addition, the SPS densification is compared with other conventional methods like Hot Pressing. 2. Materials and methods The selected composition was Tie48Ale2Cre2Nb. The elemental powders, titanium (<75 mm), chromium (<45 mm) and niobium (<45 mm) were supplied by Alfa Aesar and aluminium
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Density (porosity) was measured by Archimedes method using a Mettler AE 240 weight balance. XRD analysis was carried out using a Siemens D500 apparatus (Cu_Ka radiation, l ¼ 0.154 nm, 2q ¼ 20e100 , 40 kVe30 mA, scan step 0.1 ). Microstructure and semi-quantitative chemical composition were analysed by optical and SEM microscopy (Jeol JSM 5910 LV microscope with an Oxford Inca 300 EDS accessory). The oxygen content of the processed material was determined by chemical analysis. The oxygen was measured as carbon dioxide and carbon monoxide, using a LECO automatic analyser. Carbon was analysed by infrared absorption, nitrogen by thermal conductivity and Al, Cr and Nb by optical emission spectrometry. 3. Results and discussion Fig. 1. Graphite mould in the SPS chamber.
3.1. Densification process Fig. 2 shows the final density of the samples with regards to the temperature of the cycle and two different applied loads, 15 and 50 MPa. It can be observed that the maximum density with a load of 50 MPa was obtained at very low temperature (around 630e 650 C). This temperature is significantly lower than obtained values for pre-alloyed powders in bibliography [9,11]. It is important to note that with a load of 50 MPa and higher temperatures than 700 C, the segregation of the aluminium can occur. In other words, at these temperatures the melting of the aluminium is produced and due to the application of the pressure, the liquid Al can squeeze out of the mould. To perform cycles with more temperature it is necessary to reduce the load to 15 MPa. The maximum densification at this load was obtained around 1300 C.
Fig. 2. Densification curves, two different loads: 15 and 50 MPa.
(<200 mm) by Pometon. Before the sintering, elemental powders were weighed and then dry mixed in a Turbula mixer for 2 h at 69 rpm. The consolidation experiments were performed in a FCT Spark Plasma Sintering furnace (model S8451). This equipment can supply a direct current of 10,000 A of intensity under a maximum tension of 10 V. The DC current was applied with pulse time of 10 ms and a pause time of 5 ms. The powder was filled in a graphite die between two graphite punches (see Fig. 1). The diameter of the samples was 20 mm. During the tests, the chamber was maintained under vacuum (102 Pa). Temperature was controlled by a pyrometer that measures the temperature in the interior part of the graphite punch. Temperatures between 600 and 1300 C were studied with different holding times. The load (15 and 50 MPa) was applied before starting the heating.
3.2. Crystallographic phases (XRD) XRD patterns show that at low temperature (less than 600 C), only the elements titanium and aluminium can be detected (Fig. 3A). Increasing the temperature to 700 C, the intermetallic TiAl3 is present. For temperatures higher than 1100 C (Fig. 4), mainly two phases are present: g-TiAl phase (face-centred tetragonal structure) and a2-Ti3Al (hexagonal structure). Rietveld analysis showed that increasing the temperature from 1100 C to 1300 C reduced the amount of Ti3Al from 20.1% to 15.1%. 3.3. Microstructure evolution In order to study the microstructure evolution, samples were synthesized at 700, 900, 1100 and 1300 C with a holding time of 1 min. Fig. 5A, Fig. 6A and Fig. 7A show SEM images at 700, 900 and
Fig. 3. XRD patterns: A) 650 C, 1 min, B) 700 C 1 min.
M.A. Lagos, I. Agote / Intermetallics 36 (2013) 51e56
53
temperature). It seems that the diffusion of the Al in the Ti particles is very limited at 700 C - 1 min and the reaction between the components occurs only in the surface of the Ti particles. According to previous works [17,18], the exothermic reaction between Ti and Al starts and propagates at 660 C, however, in SPS the propagation of that reaction was very limited. Dark grey particles (points 8 and 10) were composed of aluminium with a significant amount of chromium. Chromium particles (point 9) presented a colour very similar to titanium. There was significant diffusion between aluminium and chromium. Niobium particles were also present in the microstructure like white grains (point 5) and there were not diffusion with other elements. In addition, pores can be observed as black regions.
Fig. 4. XRD patterns: A) 1100 C, 1 min, B) 1300 C 1 min.
1100 C, respectively. In addition, the composition of different points was measured by EDS (points 1 to 10 presented in each figure). The EDS analysis of each point is presented in Figs. 5B, 6B and 7B. The distance between two consecutive measurement points was 20 microns. At 700 C, microstructure was formed by the following parts: The composition of the core of the light grey particles (points 1 and 7) was pure titanium. In the exterior part of these particles (point 2), the composition was similar to the intermetallic Ti3Al. Surrounding the grey particles there were a darker layer (points 3, 4 and 6), the composition of these layer is richer in aluminium, with a composition similar to the intermetallic TiAl3 (this intermetallic was clearly observed in XRD at this
At 900 C microstructure is similar but the diffusion of the elements is more significant. The centre of the light grey particles (points 5 and 8) was still pure titanium and the exterior part of these particles was probably formed by Ti3Al (point 1). The darker layer around the particles corresponded to a composition close to TiAl3. There were not chromium pure particles; however, there are zones richer in this element combined with aluminium. Niobium particles remained without diffusion. At 1100 C (Fig. 7) the composition of the centre of the light grey particles was similar to Ti3Al (point 8), the exterior part of these particles was formed by equi-atomic Al and Ti, most probably TiAl (points 1, 2 and 3). These are the main phases and this results in concordance with XRD analysis. The darkest zone around the particles was rich in Al, corresponding to TiAl3 like compound. In comparison with the sample obtained at 900 C, it is important to remark that there was not free titanium, in other words, the diffusion of the aluminium reached the centre of the titanium particles. There were parts rich in chromium and aluminium. Niobium particles remained un-reacted. Fig. 8 presents SEM images for samples prepared at 1300 C at two different holding times, 1 and 8 min. EDX analysis of these samples is presented in Fig. 9 and Fig. 10. Y-axis presents the atomic percentage of Ti, Al, Cr and Nb at each measurement points (1e10, see Figures A). These different points (1e10) represent the scanning line across the sample and X-axis represents the distance in this scanning line, starting from point 1. The distance between consecutive points was 20 microns. These graphs give an idea about the homogeneity of the composition of the material across the sample.
Fig. 5. SEM images in back scattering mode: 700 C 1 min.
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Fig. 6. SEM images in back scattering mode: 900 C 1 min.
Fig. 7. SEM images in back scattering mode: 1100 C 1 min.
At 1300 C and 1 min the diffusion of all components was clearly higher. Particles were not observed and the composition of the light grey matrix was similar to TiAl. Chromium was present in the whole sample, but the composition was not close to the theoretical 2 at%
(inhomogeneous distribution). Niobium particles were partially diffused (points 1, 8, 9 and 10) in the TiAl matrix. Increasing the holding time at this temperature (see Fig. 8B), it can be observed that the diffusion was almost complete, the composition of the
Fig. 8. SEM images in back scattering mode: A) 1300 C 1 min, B) 1300 C 8 min.
M.A. Lagos, I. Agote / Intermetallics 36 (2013) 51e56 100 Al
90
Cr
Nb
Sample
%C
%O
%N
%Al
% Cr
%Nb
70
SPS-ELEM
<0.020
0.32
0.06
32.5
2.5
4.4
2
4
3
5
6
7
1
50
8
9
10
40 30 20 10 0 0
50
100 Distance (microns)
150
200
Fig. 9. Diffusion of the elements at 1300 C, 1 min.
100 Al
90
Ti
Cr
3.4. Chemical analysis
70 60 50
remain in the sample. Niobium is partially diffused but the percentage of Nb in some regions is near to the 10%. It is believed that 8 min at 1300 C was enough time for the diffusion of Ti, Al and Cr; however, the diffusion of Nb was not completed. Microstructure at 1300 C was also analysed by optical microscopy. Independent of the holding time, microstructure is duplex formed by lamellar colonies and equiaxed grains. Fig. 11 shows images of the optical microscope for samples with 1 and 8 min of holding time. Increasing the holding time the amount of lamellar grains is increased significantly. In addition, grain size is increased from 50 to 70 microns to 150e200 microns.
Nb
80
at %
Table 1 Chemical analysis (wt%).
80
60 at %
Ti
55
1
2
3
4
5
6
7
9
8
10
40 30 20 10 0 0
50
100 Distance (microns)
150
200
Fig. 10. Diffusion of the elements at 1300 C, 8 min.
matrix was near to the 50 at% between Ti and Al and chromium was homogeneously distributed across the sample. There were only few areas with a higher composition in niobium content. Theoretically, the composition of Ti and Al should be 48 at% and for Nb and Cr 2at %. Regarding the diffusion of Ti and Al, at 1300 C and 1 min, there are points richer in aluminium and other in titanium. This fact was also observed in Fig. 9. The difference in composition between Ti and Al is even more that 10 at% in some regions. Chromium is homogeneously distributed across the sample; however, the diffusion of niobium is more limited. Some spots richer in Nb are observed. At 1300 C and 8 min, the diffusion of Ti, Al and Cr is almost complete across the sample. However, some parts richer in Nb
In the chemical analysis, the following elements were determined after SPS sintering: It is interesting to note that carbon and nitrogen impurities were very low. Oxygen content was around 3200 ppm. Oxygen content was measured in raw materials (mixture of powders) and the result was 3800 ppm, similar or even higher than the oxygen content in sintered samples (Table 1). Composition theoretical weight values are 33,3 wt% for Al, 2,7 wt% for Cr and 4,8 wt% for Nb. The obtained values are close to the theoretical ones. 3.5. Comparison with other sintering methods It is interesting to compare the microstructure of specimens produced by SPS using elemental powders with that produced by other sintering methods. For example, one method similar to SPS can be Self propagating High-temperature Synthesis (SHS). In SHS, the material is locally heated until the exothermic reaction between Ti and Al that propagates across the sample. Fig. 12 presents a SEM image from a previous work [17] of a material obtained by SHS by thermal explosion performed in a Hot Press. In this case the same alloy composition was used (Tie48Ale2Cre2Nb). Microstructure obtained by SPS at 900 C (see Fig. 6) and by SHS are very similar. Total duration (heating, holding time and cooling) of the SPS cycle was 15 min and for SHS was 250 min. TiAl elemental powders have been also consolidated by conventional powder metallurgical methods like Hot Press, Hot
Fig. 11. Optical microscope images: A) 1300 C, 1 min, B) 1300 C 8 min.
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a higher temperature (at 1100 C is insignificant) and even at 1300 C and several minutes, areas slightly rich in niobium remained in the sample. Chemical analysis confirmed the theoretical composition. With SPS the diffusion is similar to processes with significantly longer processing time like SHS or Hot Press. Comparison was performed with samples obtained at 900 C by SPS and Hot Pressing and the obtained microstructures are very similar. Acknowledgements This work was supported by the Industry Department of the Basque Government in the frame of the program EMAITEK 2011. References Fig. 12. SEM image, sample obtained by SHS.
Extrusion or Hot Forging [19e21]. The typical hot pressing conditions include holding times of 1 or 2 h. Liu et al. [20] studied microstructure with the increasing of the temperature and it can be observed that at 1300 C and 1 h the diffusion of the elements is limited. The main microstructural inhomogeneity is the presence of island-like a2-Ti3Al. In SPS experiments of this work, it is possible to diffuse these formations around 1300 C. For that reason, it seems that with SPS the diffusion is similar to processes with significantly longer processing time. 4. Conclusions In this work, the densification and synthesis of TiAl elemental powders was studied. Main conclusions are The applied load is a crucial parameter in the densification process. With a load of 50 MPa, the densification occur at around 630e650 C. However, at this low temperature the material did not end up with the full intermetallic range. The plasticity of aluminium at low temperature close the porosity, surrounding the other elemental particles, but there are not diffusion between elements. It is important to note that for high pressure (50 MPa) and temperature more than 700 C, the loss of aluminium can occur. To increase the temperature and improve the diffusion of the elements, the load was reduced to 15 MPa. Increasing the temperature, the diffusion of the elements can be observed. At 900 C 1 min, only the surface of the titanium particles reacted with aluminium to form TiAl3. It means that the reaction between Ti and Al is controlled by diffusion, including the formation of intermediate products like TiAl3. In subsequent heating, Al in TiAl3 diffuses into Ti particles, which makes TiAl3 transform to TiAl phase and Ti particle to Ti3Al phase. At 1300 C, the diffusion of Ti and Al is almost complete. Regarding the diffusion of heavy elements, the diffusion of chromium stared at low temperature (less than 900 C) with aluminium and at 1300 C the diffusion in the intermetallic matrix is complete. Nevertheless, the diffusion of Nb started
[1] Wu X. Review of alloy and process development of TiAl alloys. Intermetallics 2006;14(10e11):1114e22. [2] Leyens C, Peters M. Titanium and titanium alloys. WileyeVCH; 2003. pp: 89e 146. [3] Hu D, Wu X, Loretto MH. Advances in optimisation of mechanical properties in cast TiAl alloys. Intermetallics 2005;13(9):914e9. [4] Loria E. Review Quo vadis gamma titanium aluminide. Intermetallics 2001;9: 997e1001. [5] Lasalmonie A. Intermetallics: why is it so difficult to introduce them in gas turbine engines? Intermetallics 2006;14:1123e9. [6] Clemens H, Schretter P, Kock W, Kneringer G. Rolling of intermetallic gammatitanaluminide based alloys. Metall 1993;47(6):547e50. [7] Jarvis DJ, Voss D. Impress integrated project e an overview paper. Mat Sci Eng 2005;413e414:583e91. [8] LeHolm R, Clemens H, Kestler H. Powder metallurgy (PM) gamma-based titanium aluminide structures for use in various high temperature aerospace applications, gamma titanium aluminides 1999, 2nd international symposium on gamma titanium aluminides held in conjunction with the TMS 1999 annual meeting San Diego, CA, Mar 01e04, pp: 25e33. [9] Couret A, Molénat G, Galy J, Thomas M. Microstructures and mechanical properties of TiAl alloys consolidated by spark plasma sintering. Intermetallics 2008;16:1134e41. [10] Zhang Ch, Zhang K, Wang G. Dependence of heating rate in PCAS on microstructures and high temperature deformation properties of g-TiAl intermetallic alloys. Intermetallics 2010;18:834e40. [11] Jabbar H, Monchoux JP, Houdellier F, Dollé M, Schimansky FP, Pyczak F, et al. Microstructure and mechanical properties of high niobium containing TiAl alloys elaborated by spark plasma sintering. Intermetallics 2010;18(12): 2312e21. [12] Calderon HA, Garibay-Febles V, Umemoto M, Yamaguchi M. Mechanical properties of nanocrystalline TieAleX alloys. Mat Sci Eng A 2002;329e331: 196e205. [13] Wu H, Yue Y, Wu W, Yin H. Fabrication of TiAl intermetallic by spark plasma sintering. Key Eng Mater 2007;336e338:1050e2. [14] Sun Z, Hashimoto H. Fabrication of TiAl alloys by MA-PDS process and the mechanical properties. Intermetallics 2003;11:825e34. [15] Mei B, Miyamoto Y. Preparation of Ti-Al intermetallic compounds by spark plasma sintering. Metallurgical Mater Trans A-Physical Metall Mater Sci 2001; 32(3A):843e7. [16] Um TY, Abe T, Sumi S. Fabrication of intermetallic compounds by spark plasma sintering. J Mater Synth Process 1999;7(5):303e8. [17] Agote I, Coleto J, Gutiérrez M, Sargsyan A, Garcia de Cortazar M, Lagos M, et al. Production of gamma TiAl based alloy by combustion synthesis þ compaction route, characterisation and application. Kovove Mater 2008;46:87e95. [18] Agote I, Coleto J, Gutiérrez M, Sargsyan A, García de Cortazar M, Lagos M, et al. Microstructure and mechanical properties of gamma TiAl based alloys produced by combustion synthesis þ compaction route. Intermetallics 2008;16: 1310e6. [19] Park HS, Park KL, Hwang SK. EPM method of synthesizing intermetallics based on Ti. Mat Sci Eng A 2002;329e331:50e6. [20] Liu Y, Huang B, Yuehui HE, Zhou K. Processing TiAl-based alloy elemental powder metalurgy. J Mater Sci Technol 2000;16(6). [21] Liu B, Liu Y, Zhang W, Huang JS. Hot deformation behavior of TiAl alloys prepared by blended elemental powders. Intermetallics 2010:1e6.
Contents lists available at SciVerse ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
SPS synthesis and consolidation of TiAl alloys from elemental powders: Microstructure evolution M.A. Lagos*, I. Agote Tecnalia, Materials for Avanced Systems, Paseo Mikeletegi, 2 e Parque Tecnológico, E-20009 San Sebastián, Gipuzkoa, Spain
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 October 2012 Received in revised form 14 December 2012 Accepted 11 January 2013 Available online 7 February 2013
Some works have studied the sintering of TiAl by means of Spark Plasma Sintering (SPS) and it is believed that it is possible to obtain very fine microstructures and very interesting mechanical properties. As raw materials, normally pre-alloyed powders have been used, obtained by atomization or mechanical alloying. However, few works have studied the consolidation of elemental powders. This work presents the synthesis and densification of TiAl-based alloys by means of Spark Plasma Sintering (SPS) using elemental powders as raw materials. The selected composition was Tie48Ale2Cre2Nb. The densification process, the evolution of the microstructure with the temperature and the diffusion of the elements (including heavy metals Cr and Nb) are presented. In addition, SPS products are compared with TiAl alloys from elemental powders obtained by conventional methods. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Titanium aluminides A. Intermetallics C. Reaction synthesis C. Powder metallurgy G. Aerospace constructional uses
1. Introduction
g-TiAl based alloys have been extensively studied as potential replacements for nickel-based superalloys [1e3] for different application fields, such as turbine blades for aircraft engines, stationary turbines and space vehicles. These alloys have relatively high yield strength at elevated temperatures, good creep characteristics, and excellent oxidation/corrosion resistance. The combination of these properties, associated with their low density (3.9e4.1 g/cc), makes them very attractive as high-temperature structural materials for aerospace, automotive and other applications [4,5]. A number of methods have been utilised in the synthesis and processing of g-TiAl based alloys, such as Vacuum Arc Remelting (VAR), conventional melting, casting processes and hot working techniques [1,3,6,7]. Conventionally, the first step is to synthesise the alloy, which is usually done by VAR or other similar technique. The shaping and processing of the material are carried out by casting techniques (typically centrifugal or gravity casting) or hot working (forging, extrusion, etc). In these manufacturing routes, one of the main problems is the scattering in mechanical properties due to the segregation in the composition. Powder metallurgy is an interesting alternative [8] to control the composition. Spark plasma sintering (SPS) is found to compact powders satisfactorily through the simultaneous application of direct current * Corresponding author. Tel.: þ34 667119590. E-mail address: [email protected] (M.A. Lagos). 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.01.006
pulses of high intensity and pressure. The electric current induces a temperature elevation within the sample by Joule’s effect. The whole process lasts just a few minutes. Some works have studied the sintering of TiAl by means of SPS [9e14] and it is believed that it is possible to obtain very fine microstructures. These alloys presented high strength whereas ductility was still limited. As raw materials, normally pre-alloyed powders have been used, obtained by atomization or mechanical alloying. However, few works have studied the consolidation of elemental powders by SPS [15,16]. These works claimed that the sintering of elemental powders is easier and faster than of pre-alloyed powders, but the higher temperature is needed to complete the diffusion of the elements. However, the densification process was not studied in detail and the effect of the load was not studied. This work presents the synthesis and densification of TiAl-based alloys by means of Spark Plasma Sintering (SPS) using elemental powders as raw materials. The densification process and the evolution of the microstructure with the temperature are presented. Particularly interesting is the diffusion of the elements, including heavy metals Cr and Nb. In addition, the SPS densification is compared with other conventional methods like Hot Pressing. 2. Materials and methods The selected composition was Tie48Ale2Cre2Nb. The elemental powders, titanium (<75 mm), chromium (<45 mm) and niobium (<45 mm) were supplied by Alfa Aesar and aluminium
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M.A. Lagos, I. Agote / Intermetallics 36 (2013) 51e56
Density (porosity) was measured by Archimedes method using a Mettler AE 240 weight balance. XRD analysis was carried out using a Siemens D500 apparatus (Cu_Ka radiation, l ¼ 0.154 nm, 2q ¼ 20e100 , 40 kVe30 mA, scan step 0.1 ). Microstructure and semi-quantitative chemical composition were analysed by optical and SEM microscopy (Jeol JSM 5910 LV microscope with an Oxford Inca 300 EDS accessory). The oxygen content of the processed material was determined by chemical analysis. The oxygen was measured as carbon dioxide and carbon monoxide, using a LECO automatic analyser. Carbon was analysed by infrared absorption, nitrogen by thermal conductivity and Al, Cr and Nb by optical emission spectrometry. 3. Results and discussion Fig. 1. Graphite mould in the SPS chamber.
3.1. Densification process Fig. 2 shows the final density of the samples with regards to the temperature of the cycle and two different applied loads, 15 and 50 MPa. It can be observed that the maximum density with a load of 50 MPa was obtained at very low temperature (around 630e 650 C). This temperature is significantly lower than obtained values for pre-alloyed powders in bibliography [9,11]. It is important to note that with a load of 50 MPa and higher temperatures than 700 C, the segregation of the aluminium can occur. In other words, at these temperatures the melting of the aluminium is produced and due to the application of the pressure, the liquid Al can squeeze out of the mould. To perform cycles with more temperature it is necessary to reduce the load to 15 MPa. The maximum densification at this load was obtained around 1300 C.
Fig. 2. Densification curves, two different loads: 15 and 50 MPa.
(<200 mm) by Pometon. Before the sintering, elemental powders were weighed and then dry mixed in a Turbula mixer for 2 h at 69 rpm. The consolidation experiments were performed in a FCT Spark Plasma Sintering furnace (model S8451). This equipment can supply a direct current of 10,000 A of intensity under a maximum tension of 10 V. The DC current was applied with pulse time of 10 ms and a pause time of 5 ms. The powder was filled in a graphite die between two graphite punches (see Fig. 1). The diameter of the samples was 20 mm. During the tests, the chamber was maintained under vacuum (102 Pa). Temperature was controlled by a pyrometer that measures the temperature in the interior part of the graphite punch. Temperatures between 600 and 1300 C were studied with different holding times. The load (15 and 50 MPa) was applied before starting the heating.
3.2. Crystallographic phases (XRD) XRD patterns show that at low temperature (less than 600 C), only the elements titanium and aluminium can be detected (Fig. 3A). Increasing the temperature to 700 C, the intermetallic TiAl3 is present. For temperatures higher than 1100 C (Fig. 4), mainly two phases are present: g-TiAl phase (face-centred tetragonal structure) and a2-Ti3Al (hexagonal structure). Rietveld analysis showed that increasing the temperature from 1100 C to 1300 C reduced the amount of Ti3Al from 20.1% to 15.1%. 3.3. Microstructure evolution In order to study the microstructure evolution, samples were synthesized at 700, 900, 1100 and 1300 C with a holding time of 1 min. Fig. 5A, Fig. 6A and Fig. 7A show SEM images at 700, 900 and
Fig. 3. XRD patterns: A) 650 C, 1 min, B) 700 C 1 min.
M.A. Lagos, I. Agote / Intermetallics 36 (2013) 51e56
53
temperature). It seems that the diffusion of the Al in the Ti particles is very limited at 700 C - 1 min and the reaction between the components occurs only in the surface of the Ti particles. According to previous works [17,18], the exothermic reaction between Ti and Al starts and propagates at 660 C, however, in SPS the propagation of that reaction was very limited. Dark grey particles (points 8 and 10) were composed of aluminium with a significant amount of chromium. Chromium particles (point 9) presented a colour very similar to titanium. There was significant diffusion between aluminium and chromium. Niobium particles were also present in the microstructure like white grains (point 5) and there were not diffusion with other elements. In addition, pores can be observed as black regions.
Fig. 4. XRD patterns: A) 1100 C, 1 min, B) 1300 C 1 min.
1100 C, respectively. In addition, the composition of different points was measured by EDS (points 1 to 10 presented in each figure). The EDS analysis of each point is presented in Figs. 5B, 6B and 7B. The distance between two consecutive measurement points was 20 microns. At 700 C, microstructure was formed by the following parts: The composition of the core of the light grey particles (points 1 and 7) was pure titanium. In the exterior part of these particles (point 2), the composition was similar to the intermetallic Ti3Al. Surrounding the grey particles there were a darker layer (points 3, 4 and 6), the composition of these layer is richer in aluminium, with a composition similar to the intermetallic TiAl3 (this intermetallic was clearly observed in XRD at this
At 900 C microstructure is similar but the diffusion of the elements is more significant. The centre of the light grey particles (points 5 and 8) was still pure titanium and the exterior part of these particles was probably formed by Ti3Al (point 1). The darker layer around the particles corresponded to a composition close to TiAl3. There were not chromium pure particles; however, there are zones richer in this element combined with aluminium. Niobium particles remained without diffusion. At 1100 C (Fig. 7) the composition of the centre of the light grey particles was similar to Ti3Al (point 8), the exterior part of these particles was formed by equi-atomic Al and Ti, most probably TiAl (points 1, 2 and 3). These are the main phases and this results in concordance with XRD analysis. The darkest zone around the particles was rich in Al, corresponding to TiAl3 like compound. In comparison with the sample obtained at 900 C, it is important to remark that there was not free titanium, in other words, the diffusion of the aluminium reached the centre of the titanium particles. There were parts rich in chromium and aluminium. Niobium particles remained un-reacted. Fig. 8 presents SEM images for samples prepared at 1300 C at two different holding times, 1 and 8 min. EDX analysis of these samples is presented in Fig. 9 and Fig. 10. Y-axis presents the atomic percentage of Ti, Al, Cr and Nb at each measurement points (1e10, see Figures A). These different points (1e10) represent the scanning line across the sample and X-axis represents the distance in this scanning line, starting from point 1. The distance between consecutive points was 20 microns. These graphs give an idea about the homogeneity of the composition of the material across the sample.
Fig. 5. SEM images in back scattering mode: 700 C 1 min.
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Fig. 6. SEM images in back scattering mode: 900 C 1 min.
Fig. 7. SEM images in back scattering mode: 1100 C 1 min.
At 1300 C and 1 min the diffusion of all components was clearly higher. Particles were not observed and the composition of the light grey matrix was similar to TiAl. Chromium was present in the whole sample, but the composition was not close to the theoretical 2 at%
(inhomogeneous distribution). Niobium particles were partially diffused (points 1, 8, 9 and 10) in the TiAl matrix. Increasing the holding time at this temperature (see Fig. 8B), it can be observed that the diffusion was almost complete, the composition of the
Fig. 8. SEM images in back scattering mode: A) 1300 C 1 min, B) 1300 C 8 min.
M.A. Lagos, I. Agote / Intermetallics 36 (2013) 51e56 100 Al
90
Cr
Nb
Sample
%C
%O
%N
%Al
% Cr
%Nb
70
SPS-ELEM
<0.020
0.32
0.06
32.5
2.5
4.4
2
4
3
5
6
7
1
50
8
9
10
40 30 20 10 0 0
50
100 Distance (microns)
150
200
Fig. 9. Diffusion of the elements at 1300 C, 1 min.
100 Al
90
Ti
Cr
3.4. Chemical analysis
70 60 50
remain in the sample. Niobium is partially diffused but the percentage of Nb in some regions is near to the 10%. It is believed that 8 min at 1300 C was enough time for the diffusion of Ti, Al and Cr; however, the diffusion of Nb was not completed. Microstructure at 1300 C was also analysed by optical microscopy. Independent of the holding time, microstructure is duplex formed by lamellar colonies and equiaxed grains. Fig. 11 shows images of the optical microscope for samples with 1 and 8 min of holding time. Increasing the holding time the amount of lamellar grains is increased significantly. In addition, grain size is increased from 50 to 70 microns to 150e200 microns.
Nb
80
at %
Table 1 Chemical analysis (wt%).
80
60 at %
Ti
55
1
2
3
4
5
6
7
9
8
10
40 30 20 10 0 0
50
100 Distance (microns)
150
200
Fig. 10. Diffusion of the elements at 1300 C, 8 min.
matrix was near to the 50 at% between Ti and Al and chromium was homogeneously distributed across the sample. There were only few areas with a higher composition in niobium content. Theoretically, the composition of Ti and Al should be 48 at% and for Nb and Cr 2at %. Regarding the diffusion of Ti and Al, at 1300 C and 1 min, there are points richer in aluminium and other in titanium. This fact was also observed in Fig. 9. The difference in composition between Ti and Al is even more that 10 at% in some regions. Chromium is homogeneously distributed across the sample; however, the diffusion of niobium is more limited. Some spots richer in Nb are observed. At 1300 C and 8 min, the diffusion of Ti, Al and Cr is almost complete across the sample. However, some parts richer in Nb
In the chemical analysis, the following elements were determined after SPS sintering: It is interesting to note that carbon and nitrogen impurities were very low. Oxygen content was around 3200 ppm. Oxygen content was measured in raw materials (mixture of powders) and the result was 3800 ppm, similar or even higher than the oxygen content in sintered samples (Table 1). Composition theoretical weight values are 33,3 wt% for Al, 2,7 wt% for Cr and 4,8 wt% for Nb. The obtained values are close to the theoretical ones. 3.5. Comparison with other sintering methods It is interesting to compare the microstructure of specimens produced by SPS using elemental powders with that produced by other sintering methods. For example, one method similar to SPS can be Self propagating High-temperature Synthesis (SHS). In SHS, the material is locally heated until the exothermic reaction between Ti and Al that propagates across the sample. Fig. 12 presents a SEM image from a previous work [17] of a material obtained by SHS by thermal explosion performed in a Hot Press. In this case the same alloy composition was used (Tie48Ale2Cre2Nb). Microstructure obtained by SPS at 900 C (see Fig. 6) and by SHS are very similar. Total duration (heating, holding time and cooling) of the SPS cycle was 15 min and for SHS was 250 min. TiAl elemental powders have been also consolidated by conventional powder metallurgical methods like Hot Press, Hot
Fig. 11. Optical microscope images: A) 1300 C, 1 min, B) 1300 C 8 min.
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a higher temperature (at 1100 C is insignificant) and even at 1300 C and several minutes, areas slightly rich in niobium remained in the sample. Chemical analysis confirmed the theoretical composition. With SPS the diffusion is similar to processes with significantly longer processing time like SHS or Hot Press. Comparison was performed with samples obtained at 900 C by SPS and Hot Pressing and the obtained microstructures are very similar. Acknowledgements This work was supported by the Industry Department of the Basque Government in the frame of the program EMAITEK 2011. References Fig. 12. SEM image, sample obtained by SHS.
Extrusion or Hot Forging [19e21]. The typical hot pressing conditions include holding times of 1 or 2 h. Liu et al. [20] studied microstructure with the increasing of the temperature and it can be observed that at 1300 C and 1 h the diffusion of the elements is limited. The main microstructural inhomogeneity is the presence of island-like a2-Ti3Al. In SPS experiments of this work, it is possible to diffuse these formations around 1300 C. For that reason, it seems that with SPS the diffusion is similar to processes with significantly longer processing time. 4. Conclusions In this work, the densification and synthesis of TiAl elemental powders was studied. Main conclusions are The applied load is a crucial parameter in the densification process. With a load of 50 MPa, the densification occur at around 630e650 C. However, at this low temperature the material did not end up with the full intermetallic range. The plasticity of aluminium at low temperature close the porosity, surrounding the other elemental particles, but there are not diffusion between elements. It is important to note that for high pressure (50 MPa) and temperature more than 700 C, the loss of aluminium can occur. To increase the temperature and improve the diffusion of the elements, the load was reduced to 15 MPa. Increasing the temperature, the diffusion of the elements can be observed. At 900 C 1 min, only the surface of the titanium particles reacted with aluminium to form TiAl3. It means that the reaction between Ti and Al is controlled by diffusion, including the formation of intermediate products like TiAl3. In subsequent heating, Al in TiAl3 diffuses into Ti particles, which makes TiAl3 transform to TiAl phase and Ti particle to Ti3Al phase. At 1300 C, the diffusion of Ti and Al is almost complete. Regarding the diffusion of heavy elements, the diffusion of chromium stared at low temperature (less than 900 C) with aluminium and at 1300 C the diffusion in the intermetallic matrix is complete. Nevertheless, the diffusion of Nb started
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