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Microstructure and mechanical properties of TiAl-based composites prepared by Stereolithography and gelcasting technologies
Accepted Manuscript Microstructure and mechanical properties of TiAl-based composites prepared by Stereolithography and gelcasting technologies Z.L. Lu, J.W. Cao, S.Z. Bai, M.Y. Wang, D.C. Li PII: DOI: Reference:
S0925-8388(15)00289-3 http://dx.doi.org/10.1016/j.jallcom.2015.01.191 JALCOM 33255
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
1 November 2014 18 December 2014 6 January 2015
Please cite this article as: Z.L. Lu, J.W. Cao, S.Z. Bai, M.Y. Wang, D.C. Li, Microstructure and mechanical properties of TiAl-based composites prepared by Stereolithography and gelcasting technologies, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.01.191
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and mechanical properties of TiAl-based composites prepared by Stereolithography and gelcasting technologies Lu Z.L. 1,2∗, Cao J.W. 1,2, Bai S.Z.1,2, Wang M.Y.1, Li D.C.1 (1, State Key Laboratory of Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an, 710049, China 2, Collaborative Innovation Center for Advanced Aero-Engine, XueYuan Road No.37, HaiDian District, Beijing,100191, China) Abstract: TiAl intermetallic and SiC ceramic are two kinds of important high-temperature structural material, but it is difficult to fabricate complex high-performance composites of them by traditional methods. In the paper, it is a pioneer study to prepare their composites so as to fabricate turbine blade by the hybrid technology of stereolithography and gelcasting. Impregnation and pyrolysis behaviors of polycarbosilane were explored for SiC ceramic preapration, and the microstructure evolution and mechanical properties of TiAl-based composites were mainly analyzed. The results showed that adding trace amount of nickel and aluminum could remarkably promote the sintering of gelcasting TiAl intermetallic samples, the micropores inside TiAl samples were connected after debinding, and the metallurgical structures were all intermetallic. After the porous TiAl samples impregnated with polycarbosilane were then pyrolyzed three times, the final metallurgical structures were TiAl-based composites of TiAl, NiAl, SiC, TiC and Ti3SiC2. Ti3SiC2 was the interface layer formed between TiAl intermetallic and SiC ceramic, and their high-temperature bending strength was between 82MPa and 90MPa at 1100℃. Therefore, it was a promising method for the fabrication of complex high-performance TiAl-based parts such as turbine blade. Keywords: Stereolithography; TiAl-based composites; gelcasting; mechanical properties; Impregnation 1. Introduction TiAl intermetallic is considered as one of the most promising materials for aero-engine applications due to its low density, high specific strength, high specific modulus, good creep resistance and oxidation resistance [1,2]. Therefore, it is of great significance to study the fabrication of complex TiAl parts such as turbine blade [3,4]. However, the application of TiAl intermetallic is limited by its low formability [5]. TiAl intermetallic is mostly as-cast so that its microstructure is usually coarse dendrite, which makes it prone to loose and composition segregation[6,7]. In particular, its plasticity and fracture/creep resistance have the opposite relationship with its tensile strength [8,9]. Some ceramics such as SiC has good chemical stability, ∗Corresponding author, LU ZL, Email: [email protected], Tel:086-29-82665126, Fax: 086-29-82660114.
good oxidation resistance and corrosion resistance, low density and high strength [10-12], however it is hard to fabricate complex parts by traditional methods [13-15]. Therefore, TiAl intermetallic and SiC can play their respective advantages when the method of chemical transformation was developed to form TiAl-based composite materials [16-20]. Turbine blade is the key part of aero-engine and gas turbine, and a few manufacturing methods were put forward only for the fabrication of turbine blade of high-temperature alloy material. At present, with an increase of gas temperature of aero-engine turbine, the existing high-temperature alloy material has almost reached its upper limit bearing temperature [21-24]. In addition, more and more complex cooling structures of the hollow turbine blade is designed to achieve improved cooling efficiency [25], which brings big challenges to manufacturers. With the advancement of material science and manufacturing technology, new composite materials such as TiAl intermetallic and SiC ceramic are expected to be applied in the fabrication of hollow turbine blades [26]. In this paper, it is possible to fabricate complex TiAl-based parts such as tuebine blade using the impregnation and pyrolysis of Polycarbosilane [–SiH(CH3)–CH2–]n(PCS) based on Stereolithography (SL) and gelcasting technologies. The TiAl-based composite will be probably achieved by filling the pores of porous TiAl intermetallic with transformed SiC. The pyrolysis of PCS and transformation into SiC ceramic will be mainly studied, the microstructure of TiAl-based will be analyzed in the process of pyrolysis, and the relationship between microstructure and TiAl-based high-temperature strength will be discussed. 2. Materials and Methods 2.1 Materials TiAl intermetallic powders (D50=40μm and spherical), Al powders (D50=3μm and spherical) and nano-Ni powders (D50=30nm and spherical) were used in this experiment. Acrylamide [C2H3CONH2] (AM) and N,N'-Methylenebisacrylamide [(C2H3CONH)2CH2] (MBAM) were used as the organic monomer and cross-linker, respectively, with a weight ratio of 24 to 1 [27]. 2.2 Methods (1) The fabrication procedures were shown in Fig.1. Firstly, turbine blade mould was fabricated by SL device of SPS600 (Shanxi HengTong Corporation, China, the accuracy of manufacturing was 0.1%) using photosensitive resin (SPR 8981;Zhengbang Ltd, Zhu Hai, China), and the fabrication principle of SL was shown in Fig.2. Then the gelcasting slurry (PH = 10~11) of TiAl/Al/Ni(mass ratios = 94:4:2) was prepared with the rotating speed of ball mill at 360r/min and the milling time for 30min, which was about 15% of organic matter concentration and 60% of solid phase volume concentration. Secondly, it was vacuum freeze dried by the LGJ-30F equipment (freeze temperature of -50℃, drying temperature of 5℃, vacuum degree<10Pa). Thirdly, the heating rate during vacuum debinding (vacuum degree of 6×10-3MPa) was 3℃/min between room temperature and 220℃; 0.5℃/min between 220℃ and 440℃; 1.5℃/min between 440℃ and 600℃; 5℃/min between 600℃ and 1250℃; finally, the samples were kept at 1250℃
for 2h. (2) Precursor Impregnation and Pyrolysis (PIP) processing The samples were vacuum impregnated in the liquid PCS three times at 110℃ for 30min (vacuum vacuity 8×10-2MPa), and then was kept at 150℃ for 10h to completely crosslink PCS. Then the pyrolysis experiment was conducted in tube furnace to fabricate TiAl-SiC. The heating rate with argon shield (flux 50ml/min) was 5℃/min between room-temperature and 200℃; 2℃/min between 200℃~600℃；5℃/min 600℃~1300℃; and then the samples were kept at 1300℃ for 1 hour. 2.3 Measurements The powders were observed by scanning electron microscope (SEM) of S-3000N (Babcock Hitachi in Japan) and transmission electron microscopy (TEM) of JEM-2100F (JEOL in Japan) besides the microstructure of gelcasting and pyrolyzed samples. After the pyrolysis of PCS, the samples’ phase composition and microstructures were analyzed by X-ray diffractometer (XRD) (Product Model: XRD-7000, Shimadzu in Japan). The infrared spectroscopy of PCS was measured every 100℃ from 100℃ to 800℃ using infrared spectrometer of EQUINOX 55 made in Germany（Product Model: SRS-3400, Brooke company）. The high-temperature bending strength were obtained at 1100℃ by three-point bending strength measurement. 3. Results and discussion Fig. 3 shows the SEM morphology of TiAl powders (Fig. 3a) and Al powders (Fig.3c) besides nano-Ni TEM morphology (Fig.3d). All of the powders were spherical, which was beneficial to obtain high solid loading of more than 50% in gelcasting, and Fig.3b shows the energy spectrum analysis of TiAl powders, in which the molar ration between Ti and Al was about 1 to 1. Fig. 4a shows that the Computed Tomography (CT) morphology of gelcasting samples of TiAl, in which the powders of different size were evenly distributed. Fig.4b shows their SEM morphology, in which the large particles were evenly coated by the polymerization reaction of AM. Fig.4c shows the backscattered electron image of gelcasting samples, and the net structure of polyacrylamide was integrated, in which the black area was monomer with network structure, and the white area was mainly TiAl intermetallic powders. TiAl-based prototype was formed and its strength was about 15MPa. Fig.4d shows the monomer’s thermogravimetry curve in gelcasting samples. The thermal decomposition of monomer was very slow from room temperature to 220℃, and the weight loss was about 15wt%. Therefore, the heating rate was set as about 3℃/min. But from 220℃ to 440℃, the thermal decomposition of monomer was most violent, and the weight loss was about 50wt%. Therefore, the heating rate was set as the minimum of 0.5℃/min. Then from 220℃ to 440℃, and the weight loss decreased to about 28wt% so that the heating rate was reduced to 1.5℃/min. After 600℃ to 1250℃, the monomer was totally decomposed, and then the heating rate was increased again, which was set as the maximum of 5℃/min. Fig.5a shows the SEM morphology of TiAl intermetallic samples with connected pores after
debinding, which was beneficial to the following PCS impregnation. Fig.5b shows the energy spectrum analysis of debinded samples, and a little amount of residual carbon appeared, which resulted from the decomposition of AM. After 2wt % nano-Ni was added, according to the Al-Ni phase diagram, liquid phase sintering could be formed when the temperature was above 635℃, which was beneficial to the formation of sintering neck between particles, as shown in Fig.5c and Fig.5d. It prevented the debinded samples from collapsing because of a certain degree of bonding strength between particles. A portion of TiAl was decomposed into Ti and Al in debinding, and then Ti (or Al) was combined with Ni to form AlNi3 or Ni2Ti. But TiAl was still the main phase, as shown in Fig. 5e. To prepare TiAl-based composite, the samples were impregnated with liquid PCS at 110℃. During the impregnation, the pores inside the TiAl preforms were filled by controlling the PCS’s viscosity, and the TiAl-PCS composite material without pores was formed, as shown in Fig. 6a. Fig.6b shows the backscattered electron image of samples impregnated by PCS, in which the black area was PCS, and the white area was mainly TiAl-based particles, and the impregnated samples were dense so as to form easily dense TiAl-based samples after the decomposition of PCS. Fig.6c shows the TG and DSC curve of PCS. Between 0℃ and 200℃, the pyrolysis of PCS was slow, which was 3％; and between 200℃ and 600℃, the decomposition was fast, which was 12％; after 600℃, the decomposition was slow again, which was 2％. The sample was finally transformed into SiC ceramic. At about 80℃, a endothermic peak appeared in the DSC curve, during which period the solvent and absorbed water were removed. At about 300℃, the weight loss peak corresponded to -CH3 groups’ thermal decomposition of hydrogen. Between 450℃ and 550℃, a relatively wide endothermic peak appeared. Due to PCS decomposition of the side chain, which was caused by Si-H or Si-CH3 groups’ thermal decomposition of hydrogen, and the removal of micromolecule PCS, PCS organic functional groups were condensed and fractured, and low molecular hydrogen escaped. This was the main phase of the pyrolytic reaction. Between 600℃ and 900℃, PCS was transformed from organic structure to inorganic structure. At 800℃, the condensation reaction was rapid, and three-dimensional network structure was formed. Above 900℃,Si-H disappeared completely. At 1000℃and 1300℃, the compound formed might mainly include TiAl, NiAl, SiC and Ti3SiC2. Fig.7 shows the infrared spectroscopy of PCS during 100℃ to 800℃. There was a series of absorption peaks between 3500 cm-1 and 4000 cm-1, as shown in Fig.7a (100℃), Fig.7b (300℃) , Fig.7c (500℃) and Fig.7d (800℃), which was probably because of moisture absorption of the samples. Absorption peaks appeared again at about 3250cm-1 because of the stretching vibration of unsaturated C-H bond, which gradually enhanced with an increase of temperature from 100℃ to 800℃. Because of the deformation vibration of Si-CH3 bond and C-H bond etc, there was a series of absorption peaks between 1300 cm-1 and 1800 cm-1, which was nearly invariant during 100℃ to 800℃. Because of the stretching vibration of Si-Si bond after in-plane vibration swing of
Si-CH3, the absorption peaks gradually enhanced and then weakened at about 800 cm-1 during 100℃ to 800℃, which rerached the maximum at 500℃ in Fig. 7c. Fig.8a shows the microstructures of TiAl-based samples after the first pyrolysis of PCS, in which the blocky Ti3SiC2 ceramic easily appeared on TiAl particles. The microstructures became dense gradually, and the SiC ceramic was formed between TiAl particles after the second pyrolysis, as shown in Fig.8b. Then the nearly dense TiAl-SiC composite was prepared in the third pyrolysis, and a very small number of pores were existed, as shown in Fig.8c. Fig.8d shows the XRD analysis of final pyrolyzed TiAl-based samples, and the main metallographic structures were TiAl, SiC, NiAl, TiC and Ti3SiC2. When the crack propagation direction was orthogonal to crystallographic plane, the stress from crack tip to particles was greater than its breaking strength, resulting in transgranular fracture. During crack propagation, the crack was deflected repeatedly inside Ti3SiC2 grains as shown in Fig.8e. TiAl grains inlaid the Ti3SiC2 interlayers further promoted crack deflection, and prevented crystal boundary from sliding, and thus had the strengthening and toughening effect. In brief, the main SiC phase dispersed in the TiAl matrix has energy absorption effect. Stress concentration was reduced by grains’ twisting, pulling out and crack deflection. Material’s resistance to micro-crack propagation was improved, and its toughness was probably enhanced. Fig.9 shows the mechanical properties of TiAl-based samples at 1100℃, in which their bending strength increased from 35MPa to 90MPa after they were impregnated by PCS and pyrolyzed three times, with apparent porosity decreasing from about 31% to 17%. Finally, based on SL and gelcasting technologies, turbine blade of TiAl-based composite was successfully fabricated, as shown in Fig. 10. 4. Conclusions A novel method applying SL and gelcasting technologies was developed and assessed for fabricating complex TiAl-based parts such as turbine blade. Addition of a small amount of nickel and aluminum powders remarkably improved the sintering of TiAl intermetallic. The microstructures of porous TiAl-based preforms were mainly TiAl, AlNi3 and Ni2Ti after debinding. The microstructures of TiAl, NiAl, SiC, TiC and Ti3SiC2 were formed after the pyrolysis of samples impregnated by PCS three times, in which SiC ceramic and TiAl intermetallic combined well. In this pioneer study, the high-temperature strength of samples achieved 90MPa at 1100℃, which demonstrated great promise for the fabrication of complex dense TiAl-base parts. It is advised that further studies shall investigate TiAl intermetallic’s gelcasting with high solid loading, and how to dense TiAl-SiC parts is very important by isostatic pressing or other methods. Acknowledgements The authors gratefully acknowledge for National Natural Science Foundation of China through Grants no.51375372 and 51005177 and ‘‘the Fundamental Research Funds for the Central Universities’’.
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Figure captions Fig.1. The fabrication procedures Fig.2. The fabrication principle of SL Fig.3. SEM morphology of TiAl powders and Al powders besides nano-Ni TEM morphology Fig.4a-4c. CT morphology, SEM morphology and backscattered electron image of gelcasting samples Fig.4d. The monomer’s thermogravimetry curve in gelcasting samples Fig.5a-5b. Fracture morphology and the energy spectrum analysis of debinded samples Fig.5c-5d. Sintering neck between particles of debinded samples Fig. 5e. XRD diagram of debinded samples Fig.6a-6b. Secondary electron image and the backscattered electron image of impregnated samples Fig.6c. TG and DSC curve of PCS Fig.7. Infrared spectroscopy of PCS during 100℃ to 800℃ Fig.8a-8c. SEM morphologies of TiAl-based samples after the first, second and third pyrolysis Fig.8d. XRD analysis of final pyrolyzed TiAl-based samples Fig.8e. Crack morphology inside Ti3SiC2 grains Fig.9. Mechanical properties of TiAl-based samples at 1100℃ Fig. 10. Turbine blade of TiAl-based composite
Highlights Complex and high-performance TiAl-based composite was prepared Trace amount of nickel additive promoted the sintering of the TiAl intermetallic Ti3SiC2 was the interface layer formed between TiAl intermetallic and SiC ceramic
Title: Microstructure and mechanical properties of TiAl-based composites prepared by stereolithography and gelcasting technologies Author names and: Lu Z.L. 1,2, Cao J.W. 1,2, Bai S.Z.1,2, Wang M.Y.1, Li D.C.1 Affiliations： 1, State Key Laboratory of Manufacturing Systems Engineering, School of Mechanical Engineering, Xi’an JiaoTong University, Xi’an, 710049, China; 2, Collaborative Innovation Center for Advanced Aero-Engine, XueYuan Road No.37, HaiDian District, BeiJing,100191, China Corresponding author: ZL Lu, email: [email protected], Tel: 86-29-82665126 State Key Laboratory of Manufacturing Systems Engineering, School of Mechanical Engineering, Xi’an JiaoTong University, Xi’an, 710049, China Present/permanent address: State Key Laboratory of Manufacturing Systems Engineering, School of Mechanical Engineering, Xi’an JiaoTong University, Xi’an, 710049, China
S0925-8388(15)00289-3 http://dx.doi.org/10.1016/j.jallcom.2015.01.191 JALCOM 33255
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
1 November 2014 18 December 2014 6 January 2015
Please cite this article as: Z.L. Lu, J.W. Cao, S.Z. Bai, M.Y. Wang, D.C. Li, Microstructure and mechanical properties of TiAl-based composites prepared by Stereolithography and gelcasting technologies, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.01.191
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Microstructure and mechanical properties of TiAl-based composites prepared by Stereolithography and gelcasting technologies Lu Z.L. 1,2∗, Cao J.W. 1,2, Bai S.Z.1,2, Wang M.Y.1, Li D.C.1 (1, State Key Laboratory of Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an, 710049, China 2, Collaborative Innovation Center for Advanced Aero-Engine, XueYuan Road No.37, HaiDian District, Beijing,100191, China) Abstract: TiAl intermetallic and SiC ceramic are two kinds of important high-temperature structural material, but it is difficult to fabricate complex high-performance composites of them by traditional methods. In the paper, it is a pioneer study to prepare their composites so as to fabricate turbine blade by the hybrid technology of stereolithography and gelcasting. Impregnation and pyrolysis behaviors of polycarbosilane were explored for SiC ceramic preapration, and the microstructure evolution and mechanical properties of TiAl-based composites were mainly analyzed. The results showed that adding trace amount of nickel and aluminum could remarkably promote the sintering of gelcasting TiAl intermetallic samples, the micropores inside TiAl samples were connected after debinding, and the metallurgical structures were all intermetallic. After the porous TiAl samples impregnated with polycarbosilane were then pyrolyzed three times, the final metallurgical structures were TiAl-based composites of TiAl, NiAl, SiC, TiC and Ti3SiC2. Ti3SiC2 was the interface layer formed between TiAl intermetallic and SiC ceramic, and their high-temperature bending strength was between 82MPa and 90MPa at 1100℃. Therefore, it was a promising method for the fabrication of complex high-performance TiAl-based parts such as turbine blade. Keywords: Stereolithography; TiAl-based composites; gelcasting; mechanical properties; Impregnation 1. Introduction TiAl intermetallic is considered as one of the most promising materials for aero-engine applications due to its low density, high specific strength, high specific modulus, good creep resistance and oxidation resistance [1,2]. Therefore, it is of great significance to study the fabrication of complex TiAl parts such as turbine blade [3,4]. However, the application of TiAl intermetallic is limited by its low formability [5]. TiAl intermetallic is mostly as-cast so that its microstructure is usually coarse dendrite, which makes it prone to loose and composition segregation[6,7]. In particular, its plasticity and fracture/creep resistance have the opposite relationship with its tensile strength [8,9]. Some ceramics such as SiC has good chemical stability, ∗Corresponding author, LU ZL, Email: [email protected], Tel:086-29-82665126, Fax: 086-29-82660114.
good oxidation resistance and corrosion resistance, low density and high strength [10-12], however it is hard to fabricate complex parts by traditional methods [13-15]. Therefore, TiAl intermetallic and SiC can play their respective advantages when the method of chemical transformation was developed to form TiAl-based composite materials [16-20]. Turbine blade is the key part of aero-engine and gas turbine, and a few manufacturing methods were put forward only for the fabrication of turbine blade of high-temperature alloy material. At present, with an increase of gas temperature of aero-engine turbine, the existing high-temperature alloy material has almost reached its upper limit bearing temperature [21-24]. In addition, more and more complex cooling structures of the hollow turbine blade is designed to achieve improved cooling efficiency [25], which brings big challenges to manufacturers. With the advancement of material science and manufacturing technology, new composite materials such as TiAl intermetallic and SiC ceramic are expected to be applied in the fabrication of hollow turbine blades [26]. In this paper, it is possible to fabricate complex TiAl-based parts such as tuebine blade using the impregnation and pyrolysis of Polycarbosilane [–SiH(CH3)–CH2–]n(PCS) based on Stereolithography (SL) and gelcasting technologies. The TiAl-based composite will be probably achieved by filling the pores of porous TiAl intermetallic with transformed SiC. The pyrolysis of PCS and transformation into SiC ceramic will be mainly studied, the microstructure of TiAl-based will be analyzed in the process of pyrolysis, and the relationship between microstructure and TiAl-based high-temperature strength will be discussed. 2. Materials and Methods 2.1 Materials TiAl intermetallic powders (D50=40μm and spherical), Al powders (D50=3μm and spherical) and nano-Ni powders (D50=30nm and spherical) were used in this experiment. Acrylamide [C2H3CONH2] (AM) and N,N'-Methylenebisacrylamide [(C2H3CONH)2CH2] (MBAM) were used as the organic monomer and cross-linker, respectively, with a weight ratio of 24 to 1 [27]. 2.2 Methods (1) The fabrication procedures were shown in Fig.1. Firstly, turbine blade mould was fabricated by SL device of SPS600 (Shanxi HengTong Corporation, China, the accuracy of manufacturing was 0.1%) using photosensitive resin (SPR 8981;Zhengbang Ltd, Zhu Hai, China), and the fabrication principle of SL was shown in Fig.2. Then the gelcasting slurry (PH = 10~11) of TiAl/Al/Ni(mass ratios = 94:4:2) was prepared with the rotating speed of ball mill at 360r/min and the milling time for 30min, which was about 15% of organic matter concentration and 60% of solid phase volume concentration. Secondly, it was vacuum freeze dried by the LGJ-30F equipment (freeze temperature of -50℃, drying temperature of 5℃, vacuum degree<10Pa). Thirdly, the heating rate during vacuum debinding (vacuum degree of 6×10-3MPa) was 3℃/min between room temperature and 220℃; 0.5℃/min between 220℃ and 440℃; 1.5℃/min between 440℃ and 600℃; 5℃/min between 600℃ and 1250℃; finally, the samples were kept at 1250℃
for 2h. (2) Precursor Impregnation and Pyrolysis (PIP) processing The samples were vacuum impregnated in the liquid PCS three times at 110℃ for 30min (vacuum vacuity 8×10-2MPa), and then was kept at 150℃ for 10h to completely crosslink PCS. Then the pyrolysis experiment was conducted in tube furnace to fabricate TiAl-SiC. The heating rate with argon shield (flux 50ml/min) was 5℃/min between room-temperature and 200℃; 2℃/min between 200℃~600℃；5℃/min 600℃~1300℃; and then the samples were kept at 1300℃ for 1 hour. 2.3 Measurements The powders were observed by scanning electron microscope (SEM) of S-3000N (Babcock Hitachi in Japan) and transmission electron microscopy (TEM) of JEM-2100F (JEOL in Japan) besides the microstructure of gelcasting and pyrolyzed samples. After the pyrolysis of PCS, the samples’ phase composition and microstructures were analyzed by X-ray diffractometer (XRD) (Product Model: XRD-7000, Shimadzu in Japan). The infrared spectroscopy of PCS was measured every 100℃ from 100℃ to 800℃ using infrared spectrometer of EQUINOX 55 made in Germany（Product Model: SRS-3400, Brooke company）. The high-temperature bending strength were obtained at 1100℃ by three-point bending strength measurement. 3. Results and discussion Fig. 3 shows the SEM morphology of TiAl powders (Fig. 3a) and Al powders (Fig.3c) besides nano-Ni TEM morphology (Fig.3d). All of the powders were spherical, which was beneficial to obtain high solid loading of more than 50% in gelcasting, and Fig.3b shows the energy spectrum analysis of TiAl powders, in which the molar ration between Ti and Al was about 1 to 1. Fig. 4a shows that the Computed Tomography (CT) morphology of gelcasting samples of TiAl, in which the powders of different size were evenly distributed. Fig.4b shows their SEM morphology, in which the large particles were evenly coated by the polymerization reaction of AM. Fig.4c shows the backscattered electron image of gelcasting samples, and the net structure of polyacrylamide was integrated, in which the black area was monomer with network structure, and the white area was mainly TiAl intermetallic powders. TiAl-based prototype was formed and its strength was about 15MPa. Fig.4d shows the monomer’s thermogravimetry curve in gelcasting samples. The thermal decomposition of monomer was very slow from room temperature to 220℃, and the weight loss was about 15wt%. Therefore, the heating rate was set as about 3℃/min. But from 220℃ to 440℃, the thermal decomposition of monomer was most violent, and the weight loss was about 50wt%. Therefore, the heating rate was set as the minimum of 0.5℃/min. Then from 220℃ to 440℃, and the weight loss decreased to about 28wt% so that the heating rate was reduced to 1.5℃/min. After 600℃ to 1250℃, the monomer was totally decomposed, and then the heating rate was increased again, which was set as the maximum of 5℃/min. Fig.5a shows the SEM morphology of TiAl intermetallic samples with connected pores after
debinding, which was beneficial to the following PCS impregnation. Fig.5b shows the energy spectrum analysis of debinded samples, and a little amount of residual carbon appeared, which resulted from the decomposition of AM. After 2wt % nano-Ni was added, according to the Al-Ni phase diagram, liquid phase sintering could be formed when the temperature was above 635℃, which was beneficial to the formation of sintering neck between particles, as shown in Fig.5c and Fig.5d. It prevented the debinded samples from collapsing because of a certain degree of bonding strength between particles. A portion of TiAl was decomposed into Ti and Al in debinding, and then Ti (or Al) was combined with Ni to form AlNi3 or Ni2Ti. But TiAl was still the main phase, as shown in Fig. 5e. To prepare TiAl-based composite, the samples were impregnated with liquid PCS at 110℃. During the impregnation, the pores inside the TiAl preforms were filled by controlling the PCS’s viscosity, and the TiAl-PCS composite material without pores was formed, as shown in Fig. 6a. Fig.6b shows the backscattered electron image of samples impregnated by PCS, in which the black area was PCS, and the white area was mainly TiAl-based particles, and the impregnated samples were dense so as to form easily dense TiAl-based samples after the decomposition of PCS. Fig.6c shows the TG and DSC curve of PCS. Between 0℃ and 200℃, the pyrolysis of PCS was slow, which was 3％; and between 200℃ and 600℃, the decomposition was fast, which was 12％; after 600℃, the decomposition was slow again, which was 2％. The sample was finally transformed into SiC ceramic. At about 80℃, a endothermic peak appeared in the DSC curve, during which period the solvent and absorbed water were removed. At about 300℃, the weight loss peak corresponded to -CH3 groups’ thermal decomposition of hydrogen. Between 450℃ and 550℃, a relatively wide endothermic peak appeared. Due to PCS decomposition of the side chain, which was caused by Si-H or Si-CH3 groups’ thermal decomposition of hydrogen, and the removal of micromolecule PCS, PCS organic functional groups were condensed and fractured, and low molecular hydrogen escaped. This was the main phase of the pyrolytic reaction. Between 600℃ and 900℃, PCS was transformed from organic structure to inorganic structure. At 800℃, the condensation reaction was rapid, and three-dimensional network structure was formed. Above 900℃,Si-H disappeared completely. At 1000℃and 1300℃, the compound formed might mainly include TiAl, NiAl, SiC and Ti3SiC2. Fig.7 shows the infrared spectroscopy of PCS during 100℃ to 800℃. There was a series of absorption peaks between 3500 cm-1 and 4000 cm-1, as shown in Fig.7a (100℃), Fig.7b (300℃) , Fig.7c (500℃) and Fig.7d (800℃), which was probably because of moisture absorption of the samples. Absorption peaks appeared again at about 3250cm-1 because of the stretching vibration of unsaturated C-H bond, which gradually enhanced with an increase of temperature from 100℃ to 800℃. Because of the deformation vibration of Si-CH3 bond and C-H bond etc, there was a series of absorption peaks between 1300 cm-1 and 1800 cm-1, which was nearly invariant during 100℃ to 800℃. Because of the stretching vibration of Si-Si bond after in-plane vibration swing of
Si-CH3, the absorption peaks gradually enhanced and then weakened at about 800 cm-1 during 100℃ to 800℃, which rerached the maximum at 500℃ in Fig. 7c. Fig.8a shows the microstructures of TiAl-based samples after the first pyrolysis of PCS, in which the blocky Ti3SiC2 ceramic easily appeared on TiAl particles. The microstructures became dense gradually, and the SiC ceramic was formed between TiAl particles after the second pyrolysis, as shown in Fig.8b. Then the nearly dense TiAl-SiC composite was prepared in the third pyrolysis, and a very small number of pores were existed, as shown in Fig.8c. Fig.8d shows the XRD analysis of final pyrolyzed TiAl-based samples, and the main metallographic structures were TiAl, SiC, NiAl, TiC and Ti3SiC2. When the crack propagation direction was orthogonal to crystallographic plane, the stress from crack tip to particles was greater than its breaking strength, resulting in transgranular fracture. During crack propagation, the crack was deflected repeatedly inside Ti3SiC2 grains as shown in Fig.8e. TiAl grains inlaid the Ti3SiC2 interlayers further promoted crack deflection, and prevented crystal boundary from sliding, and thus had the strengthening and toughening effect. In brief, the main SiC phase dispersed in the TiAl matrix has energy absorption effect. Stress concentration was reduced by grains’ twisting, pulling out and crack deflection. Material’s resistance to micro-crack propagation was improved, and its toughness was probably enhanced. Fig.9 shows the mechanical properties of TiAl-based samples at 1100℃, in which their bending strength increased from 35MPa to 90MPa after they were impregnated by PCS and pyrolyzed three times, with apparent porosity decreasing from about 31% to 17%. Finally, based on SL and gelcasting technologies, turbine blade of TiAl-based composite was successfully fabricated, as shown in Fig. 10. 4. Conclusions A novel method applying SL and gelcasting technologies was developed and assessed for fabricating complex TiAl-based parts such as turbine blade. Addition of a small amount of nickel and aluminum powders remarkably improved the sintering of TiAl intermetallic. The microstructures of porous TiAl-based preforms were mainly TiAl, AlNi3 and Ni2Ti after debinding. The microstructures of TiAl, NiAl, SiC, TiC and Ti3SiC2 were formed after the pyrolysis of samples impregnated by PCS three times, in which SiC ceramic and TiAl intermetallic combined well. In this pioneer study, the high-temperature strength of samples achieved 90MPa at 1100℃, which demonstrated great promise for the fabrication of complex dense TiAl-base parts. It is advised that further studies shall investigate TiAl intermetallic’s gelcasting with high solid loading, and how to dense TiAl-SiC parts is very important by isostatic pressing or other methods. Acknowledgements The authors gratefully acknowledge for National Natural Science Foundation of China through Grants no.51375372 and 51005177 and ‘‘the Fundamental Research Funds for the Central Universities’’.
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Figure captions Fig.1. The fabrication procedures Fig.2. The fabrication principle of SL Fig.3. SEM morphology of TiAl powders and Al powders besides nano-Ni TEM morphology Fig.4a-4c. CT morphology, SEM morphology and backscattered electron image of gelcasting samples Fig.4d. The monomer’s thermogravimetry curve in gelcasting samples Fig.5a-5b. Fracture morphology and the energy spectrum analysis of debinded samples Fig.5c-5d. Sintering neck between particles of debinded samples Fig. 5e. XRD diagram of debinded samples Fig.6a-6b. Secondary electron image and the backscattered electron image of impregnated samples Fig.6c. TG and DSC curve of PCS Fig.7. Infrared spectroscopy of PCS during 100℃ to 800℃ Fig.8a-8c. SEM morphologies of TiAl-based samples after the first, second and third pyrolysis Fig.8d. XRD analysis of final pyrolyzed TiAl-based samples Fig.8e. Crack morphology inside Ti3SiC2 grains Fig.9. Mechanical properties of TiAl-based samples at 1100℃ Fig. 10. Turbine blade of TiAl-based composite
Highlights Complex and high-performance TiAl-based composite was prepared Trace amount of nickel additive promoted the sintering of the TiAl intermetallic Ti3SiC2 was the interface layer formed between TiAl intermetallic and SiC ceramic
Title: Microstructure and mechanical properties of TiAl-based composites prepared by stereolithography and gelcasting technologies Author names and: Lu Z.L. 1,2, Cao J.W. 1,2, Bai S.Z.1,2, Wang M.Y.1, Li D.C.1 Affiliations： 1, State Key Laboratory of Manufacturing Systems Engineering, School of Mechanical Engineering, Xi’an JiaoTong University, Xi’an, 710049, China; 2, Collaborative Innovation Center for Advanced Aero-Engine, XueYuan Road No.37, HaiDian District, BeiJing,100191, China Corresponding author: ZL Lu, email: [email protected], Tel: 86-29-82665126 State Key Laboratory of Manufacturing Systems Engineering, School of Mechanical Engineering, Xi’an JiaoTong University, Xi’an, 710049, China Present/permanent address: State Key Laboratory of Manufacturing Systems Engineering, School of Mechanical Engineering, Xi’an JiaoTong University, Xi’an, 710049, China