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Temperature gradient field sintering of Ti–TiB–TiB2 functionally graded material
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 13844–13849 www.elsevier.com/locate/ceramint
Temperature gradient field sintering of Ti–TiB–TiB2 functionally graded material Youfeng Zhangn, Zhongwen Li, Chonggui Li, Zhishui Yu School of Materials Engineering, Shanghai University of Engineering Science, PR China Received 28 February 2015; received in revised form 16 July 2015; accepted 14 August 2015 Available online 28 August 2015
Abstract The six-layered functionally gradient material (FGM) of Ti–TiB–TiB2 was rapidly fabricated in vacuum under 40 MPa for 5 min using the temperature gradient field (TGF) sintering method. A gradient temperature field was formed during the spark plasma sintering (SPS) process. The compositionally graded layers of the FGM resulted in a well-bonded composite structure. The layers consist of only TiB and Ti phase when target volume fraction of TiB increases from 20% to 60%, and TiB2 phase presents in the two layers with higher target volume fraction of TiB. The hardness reaches the maximum value of 12.73 GPa when target volume fraction of TiB is 80%. The needle shape and agglomerated TiB were reaction synthesized in the sintered FGM. The TiB morphologies in layers were investigated and discussed. This TGF sintering method is practicable when sintering any other metal-ceramic FGMs. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Composites; Spark plasma sintering (SPS); X-ray methods; Microstructure
1. Introduction A functionally graded materials (FGMs) is a composite material consisting of two or more phases in which the volume fraction of the constituents changes so that the composition, microstructure and properties vary gradually in one direction. Designing FGMs allows manipulation of many material properties or new functions. TiB and TiB2 exhibit extreme hardness, high melting point, high electrical conductivity, good thermal shock resistance and chemical inertness [1,2], which can be fabricated by in situ reaction between titanium and boron. A FGM with composition changing from Ti alloy on one side to TiB–TiB2 composite on the other side has many potential application fields specifically in defense [3,4]. But the sintering of these FGMs is difficult since the great sintering temperature difference between Ti and TiB, TiB2. So, high n Correspondence to: 1608 Room, Xing Zheng Building, School of Materials Engineering, Shanghai University of Engineering Science, 333 Long Teng Road, Shanghai 201620, China. Tel.: þ 86 21 67791203; fax: þ86 21 67791377. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2015.08.070 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
performance FGMs may be obtained if a temperature gradient field (TGF) with low temperature on Ti side and high temperature on TiB or TiB2 side could be formed in the die during the sintering process. In recent years, TiB–Ti systems FGMs have been fabricated by hot pressing and combustion synthesis method, but crack is the main problem in the FGMs synthesized by these processes [3]. The gradient sintering temperature can be realized by an innovative technology known as spark plasma sintering (SPS). The SPS process is a synthesis and processing technique which makes it possible sintering and sintering bonding at low temperature and short periods by charging at the intervals between powder particles with electrical energy and effectively applying high temperature spark plasma generated momentarily. As a rapid consolidation technique, the SPS process has many advantages on fabrication of advanced materials and FGMs [5–8]. In previous studies, FGMs based on titanium borides by spark plasma sintering (SPS) process were rarely reported, such as the four-layered FGMs of TiB–Ti was rapidly fabricated using Ti powders and TiB2 powders as raw materials [9], but the fabrication of high relative density six-layered
Y. Zhang et al. / Ceramics International 41 (2015) 13844–13849
FGMs of Ti–TiB–TiB2 synthesized by chemical reaction of Ti with B and TiB2 during SPS process has not been reported yet. In this study, the Ti–TiB2–B system was selected to reaction synthesize Ti–TiB–TiB2 FGM. A temperature gradient field (TGF) was formed during the sintering process by spark plasma sintering (SPS). The rapid fabrication of reaction synthesis Ti–TiB–TiB2 FGMs in this TGF formed by SPS technique, and the microstructure and hardness of the FGMs were characterized. 2. Methods and details 2.1. Composition design of FGM Elemental Ti (74 mm, purity 499%), FeMo (74 mm, purity: 4 99%), B (average size 2 mm, purity 4 95%) powder and TiB2 (average size 30 mm, purity 4 99%) powder were used as raw materials. The TiB was synthesized by chemical reaction of Ti with B and TiB2 during the SPS sintering, which is shown as follows: Ti þ B ¼ TiB
ð1Þ
Ti þ TiB2 ¼ 2TiB
ð2Þ
The target TiB volume fraction by different reaction is shown in Table 1 and the layer codes are used in the following discuss. Both B and TiB2 were used in layers of 40, 60, 80 and 100 vol% TiB, as well as only TiB2 was used in layer of 20 vol % TiB. The Ti alloy matrix (Ti–4.0Fe–7.3Mo) was synthesized by blended elementals Ti and FeMo powder. Powder mixtures of target volume fractions of 0, 20, 40, 60, 80 and 100% TiB were blended by planetary ball milling for 600 min, respectively. Then the FGM was stacked layer by layer in a cylinder graphite die with an inner diameter of 20 mm before sintering.
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temperature is higher than that of lower punch, which leads to the forming of temperature gradient field between them during the heating process. Furthermore, the low graphite spacer contacts with the water cooling electrode and the graphite die, which is advantageous to eliminate heat of the die and keeps the difference in temperature between upper and lower punch. The layer of Ti alloy contacted with lower punch and the target 100 vol% TiB contacted with upper punch. The sintering temperature is monitored and regulated by a thermocouple in the die, as shown in Fig. 1, the temperature of upper punch was measured and recorded by means of an optical pyrometer focused on its surface. Then the compact was sintered at 950 1C with a heating rate of 150 1C/min and dwelling 5 min with a pressure of 40 MPa in vacuum. The sintered FGM is cooled to room temperature in furnace. Heating and dwelling of the FGM sintering was completed within 12 min.
2.3. Characterization tests The phase constitution of the composites was identified by X-ray diffraction (XRD) using Cu Kα radiation. The specimens were polished and then etched with a solution of 5 ml HF, 10 ml HNO3 and 85 ml H2O. Microstructure of the sintered FGMs was investigated using a Hitachi S-3500N type scanning electron microscopy (SEM). The Vickers hardness measurement was done on the HX-50 type Vickers machine with a 147 N load for 10 s.
2.2. TGF sintering by SPS The sintering processing was performed using the SPS system (model 1050). Fig. 1 is a schematic illustration of temperature gradient field sintering method. Both lower punch and graphite die contact with the low graphite spacer, while only upper punch contacts with the up graphite spacer. So the resistance of upper electrode is greater than that of lower electrode. When the pulse current passing through the electrodes, the upper punch
Fig. 1. Schematic illustration of temperature gradient field sintering.
Table 1 The designed and calculated TiB volume fraction and calculated data for the layers in the FGM. Layers
Target volume fraction of TiB (%)
Volume fraction of TiB (%)
Thickness (mm)
TiB aspect ratio
Ec (GPa)
Relative density (%)
2.20 1.40 1.63 1.82 1.58 2.00
– 20 10 10 7.5 7.5
115 214 292 366 441 492
98.1 97.1 93.4 90.3 88.5 86.8
Synthesized by reaction (1) Synthesized by reaction (2) Layer Layer Layer Layer Layer Layer
1 2 3 4 5 6
0 20 40 60 80 100
0 0 20 30 40 50
0 20 20 30 40 50
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3. Results and discussion 3.1. Investigation of the temperature gradient field The temperature of upper punch measured by optical pyrometer and that of lower punch measured by thermocouple were recorded per 0.5 min. The change of temperature between upper and lower punch during the heating and dwelling process is shown in Fig. 2. The lower punch was heated with a rate of 150 1C/min, the temperature of upper punch rose to 850 1C in about 1 min and then rises with a heating rate of about 150 1C/min. So the gradient field with a temperature difference of about 650 1C was formed at the beginning of the heating process and maintained during the heating process. The maximum temperature of upper punch was 1569 1C when the lower punch was heated to 900 1C. The sintering pulse current decreased when the temperature was close to dwelling temperature, which leads to the temperature of upper punch decreasing rapidly. With the decreasing of pulse current the temperature difference between upper and lower punch decreased from 650 1C at the beginning to 388 1C at the end of the dwelling process. So, the temperature gradient field can be formed during the sintering process by the method shown in Fig. 1. Formation of the TGF is advantageous to the consolidation of the FGMs and decreasing of the thermal stress during the sintering and cooling process. The interface is an important factor on controlling the properties of FGMs. The sintered FGM was grinded and investigated. As shown in Fig. 3, the compositionally graded layers of the FGM resulted in a well-bonded composite structure. No significant shape change or cracks were detected on the surface of the FGM. The sample was cross-sectioned and no cracks were observed in the center and between the layers. No interlayer cracks are detected in all the cross section of the FGM, which indicates that a well-bonded composite structure can be achieved by the TGF sintering of stacked layers with the designed composition.
Fig. 3. A picture of the sintered Ti–TiB–TiB2 composite.
3.2. X-ray diffraction analysis The graded layers of the FGM were cut through the interface boundaries, and XRD was utilized on each layer for phase Fig. 4. X-ray diffraction spectrum of individual layer of the Ti–TiB–TiB2 FGM.
Fig. 2. Temperature measured by pyrometer and thermocouple during heating and dwelling process.
identification. XRD spectrum from individual layer of the FGM after SPS sintered is shown in Fig. 4. The results of XRD analysis indicate that layer 1 consists of singular β-Ti phase, while the diffraction peaks of α-Ti phase present in the spectrum of layer 2, and peaks of β-Ti phase can hardly be observed in spectrum from layers 3 to 6. Fe and Mo elements were added to the Ti matrix, which solved into titanium during the sintering process and led to the formation of β-Ti phase. Since Fe and Mo elements are β phase stable elements of titanium, the β phase was maintained when cooled to room temperature. The peaks of TiB present and their intensities increase in XRD spectrums of
Y. Zhang et al. / Ceramics International 41 (2015) 13844–13849
layers 2–4. In above three layers, chemical reaction of B and TiB2 with Ti is completed since no peak of TiB2 or B present in these spectrums, and the volume fraction of TiB is increasing. So the target volume fraction of TiB was synthesized as expected in these layers. While in layers 5 and 6, peaks of TiB2, TiB and Ti present in the spectrums. This indicates that the two layers consist of synthesized TiB, TiB2 and small amount of titanium. In these two layers, the chemical reaction of TiB2 with Ti has not been completed. The intensities of TiB2 peaks in spectrum of layer 6 are higher than that of layer 5, which indicates that the volume fraction of TiB2 in layer 6 is higher than that of layer 5. Existence of the residual TiB2 is because it is more stable than TiB phase when there is excess boron existing in the titanium matrix. The volume fraction of reaction synthesized TiB is not as same as expected in these two layers. So there are three kinds of ordinal layers in the SPS sintered FGM, titanium alloy matrix layer (layer 1) and volume fraction increasing from 20% to 60% of synthesized TiB reinforced titanium alloy matrix in next three layers (layers 2–4), and TiBþ TiB2 þ Ti in the last two layers (layers 5 and 6). 3.3. Microstructure Microstructure of the individual layer was observed using SEM, as shown in Fig. 5. These figures illustrate the varied
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microstructure morphologies observed in the FGM. Fig. 5(a) shows the morphology of the Ti alloy matrix, and the grains of β-Ti phase are equiaxial. As designed, the matrix consists of β-Ti phase due to the addition of FeMo powder and solid solution of Fe and Mo elements to the titanium. The needle shape TiB whiskers were observed along the grain boundaries occasionally. A large amount of TiB whiskers were observed along grain boundaries closing to the interface between layers 1 and 2. So the presentation of TiB whiskers in layer1 is related to the diffusion of B element during the sintering process. Fig. 5(b) reveals the deep etched morphology of layer 2, and the needle shape TiB whiskers randomly distributed in the titanium matrix. The diameter of synthesized TiB needles is less than 100 nm. When the target volume fraction of TiB increases to 40%, the agglomerated TiB whiskers present among the needle shape TiB whiskers in layer 3 as shown in Fig. 5(c). The diameter size of needle shape TiB is bigger than that of layer 2. Microstructure of layer 4 is shown in Fig. 5(d), which shows that the two different types of TiB morphologies were observed in layer 4. The randomly distributed needle shape whiskers are similar to that in layer 3. It suggests that the needle shape TiB whiskers morphology underwent a significant change, especially in size, as the volume fraction of TiB increased from 20% to 60%. But the amount of agglomerated TiB increases and their
Fig. 5. SEM photograph of individual layer, and (a)–(f) is morphology of layers 1–6 respectively.
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size is bigger than that in layer 3. These agglomerated TiB are the rod-like whiskers that appear to be interconnected, as shown in Fig. 5(c) and (d), so the synthesis mechanism of the agglomerated TiB is same to that of needle shape TiB. Microstructure of layers 5 and 6 are shown in Fig. 5(e) and (f), the unetched TiB2 presents together with the needle shape and agglomerated TiB. The amount of TiB2 in layer 6 is more than that of layer 5, which confirms the result of XRD analysis. Meanwhile, more small holes were observed between the agglomerated TiB, and the needle shape TiB between them are less than that of layer 5. As TiB has a B27 structure [10], characterized by zigzag chains of boron atoms parallel to the b-direction, where each Β atom lies at the center of a trigonal prism of six Ti atoms, and TiB should exhibit much faster growth along b-direction than a or c direction, which results in the formation of TiB with needle shape or rod-like morphology. Moreover, as shown in Fig. 5, the agglomerated TiB size decreases with an increasing target volume fraction of synthesized TiB whisker from 40% to 80%. With the increasing of content of B and TiB2 particles, mass TiB nucleated and grew simultaneously and the growing up of TiB were restricted each other which led to the formation of agglomerated TiB. The SEM images of the agglomerated TiB shown in Fig. 5(c) and (d), indicate that the agglomerated TiB is composed of small rod-like TiB. With the increasing of B and TiB2 addition and the rising of sintering temperature, the diffusion path increased and speed fasted, which reduced the driving force for TiB grain growth and prevented the sintering between TiB grains to form the agglomerated TiB. It is
Fig. 7. Change of Vickers hardness with the target volume fraction of TiB.
deduced that the grow mechanism of agglomerated TiB is the same as needle shape TiB. While, in layer 5 and 6, the target volume fraction of TiB was not achieved because of the existence of residual TiB2. The amount of agglomerated TiB decreased when more TiB2 remained in layer 6 than that in layer 5. This is the reason why the content of agglomerated TiB decreases with the increasing of target volume fraction of TiB from 80% to 100%. Although TiB2 has not reacted with Ti completely in both layers 5 and 6, the reinforcement fraction (synthesized TiB and residual TiB2) is still higher than that of other layers. The hardness of layer 6 decreased slightly because the fraction of residual Ti between the agglomerated TiB in layer 6 is more than that in layer 5. The ingredient of composite that determines the final product and the morphologies of synthesized TiB. TiB2 is more stable than TiB at high temperature or if there excess boron existing in the titanium matrix. TiB2 phase was also detected in the materials with 100% target volume fraction of synthesized TiB using Ti and B powder. Similar composition of final product was obtained by self-propagating high temperature combustion synthesis method [11,12]. The detailed interfaces between each layer are shown in Fig. 6. It can be seen from Fig. 6 that interfaces between each layer were well bonded except a few pores between layers 3 and 4. 3.4. Hardness The Vickers hardness as a function of layer composition is shown in Fig. 7. This indicates that the hardness of the layers increase dramatically with the increasing of synthesized TiB volume fraction. The hardness is increased with the increasing of target volume fraction of TiB from layers 1 to 5, while the hardness of layer 6 is a little smaller than that of layer 5. The hardness value of layer 1 is 3.52 GPa. The hardness reaches its maximum value in layer 5, which is about 12.73 GPa, and that of layer 6 is 11.98 GPa. 4. Conclusions
Fig. 6. SEM photograph of interfaces between each layer.
The FGM of Ti–TiB–TiB2 was fabricated successfully using the temperature gradient field (TGF) method by spark plasma sintering technique. A gradient field with a temperature
Y. Zhang et al. / Ceramics International 41 (2015) 13844–13849
difference was formed at the beginning of the heating process and maintained during the heating process. The temperature difference between upper and lower punch decreased from 650 1C at the beginning to 388 1C at the end of the dwelling process. The hardness increased with the increasing of target volume fraction of TiB from layers 1 to 5, while the hardness of layer 6 is a little smaller than that of layer 5. The hardness reaches its maximum value in layer 5, which is about 12.73 GPa. There are three kinds of ordinal layers in the SPS sintered FGM, titanium alloy matrix layer (layer 1) and reaction synthesized TiB reinforced titanium alloy matrix in layers 2– 4, and TiB þ TiB2 þ Ti in layers 5 and 6. The layers consist of only TiB and Ti phase when target volume fraction of TiB increases from 20% to 60%, and TiB2 phase presents in the two layers with higher target volume fraction of TiB. TiB phase has two morphologies, i.e. needle shape and agglomerated TiB in the microstructure of the synthesized FGM. The six-layered functionally gradient material (FGM) of Ti–TiB– TiB2 with a well-bonded composite structure can be achieved by the TGF sintering of stacked layers with the designed composition. Acknowledgments This project was supported by the National Natural Science Foundation of China under Grant no. 51402189. References [1] Z. Zhang, X. Shen, C. Zhang, A new rapid route to in-situ synthesize TiB–Ti system functionally graded materials using spark plasma sintering method, Mater. Sci. Eng. A. 565 (2013) 326–332. [2] F.C. Wang, Z. Zhang, J. Luo, A novel rapid route for in situ synthesizing TiB–TiB2 composites, Compos. Sci. Technol. 69 (2009) 2682–2687.
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[3] M. Cirakoglu, S. Bhaduri, S.B. Bhaduri, Combustion synthesis processing of functionally graded materials in the Ti–B binary system, J. Alloys Compd. 347 (2002) 259–265. [4] S.C. Chin Ernest, Army focused research team on functionally graded armor composites, Mater. Sci. Eng. A 259 (1999) 155–161. [5] W. Wang, Y. Zhu, F. Watari, Carbon nanotubes/hydroxyapatite nanocomposites fabricated by spark plasma sintering for bonegraft applications, Appl. Surf. Sci. 262 (2012) 194–199. [6] V. Trombini, E.M.J.A. Pallone, U. Anselmi-Tamburini, Characterization of alumina matrix nanocomposites with ZrO2 inclusions densified by spark plasma sintering, Mater. Sci. Eng. A 501 (2009) 26–29. [7] Z.H. Zhang, F.C. Wang, S.K. Lee, Microstructure characteristic, mechanical properties and sintering mechanism of nanocrystalline copper obtained by SPS process, Mater. Sci. Eng. A 523 (2009) 134–138. [8] Z.M. Xie, R. Liu, Q.F. Fang, Spark plasma sintering and mechanical properties of zirconium micro-alloyed tungsten, J. Nucl. Mater. 444 (2014) 175–180. [9] Z.H. Zhang, X.B. Shen, C. Zhang, A new rapid route to in-situ synthesize TiB–Ti system functionally graded materials using spark plasma sintering method, Mater. Sci. Eng. A 565 (2013) 326–332. [10] Z. Fan, L. Chandrasekeron, C.M. Word-Close, The effect of preconsolidation heat treatment on TiB morphology and mechanicl properties of rapidly solidified Ti–6Al–4V–XB alloys, Scr. Metall. Mater. 32 (1995) 833–838. [11] X.H. Zhang, Q. Xu, J.C. Han, Self-propagating high temperature combustion synthesis of TiB/Ti composites, Mater. Sci. Eng. A 348 (2003) 41–46. [12] M. Cirakoglu, G.L. Watt, S.B. Bhaduri, Controlled combustion synthesis in the Ti–B system with ZrO2 addition, Mater. Sci. Eng. A 282 (2000) 223–231.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 13844–13849 www.elsevier.com/locate/ceramint
Temperature gradient field sintering of Ti–TiB–TiB2 functionally graded material Youfeng Zhangn, Zhongwen Li, Chonggui Li, Zhishui Yu School of Materials Engineering, Shanghai University of Engineering Science, PR China Received 28 February 2015; received in revised form 16 July 2015; accepted 14 August 2015 Available online 28 August 2015
Abstract The six-layered functionally gradient material (FGM) of Ti–TiB–TiB2 was rapidly fabricated in vacuum under 40 MPa for 5 min using the temperature gradient field (TGF) sintering method. A gradient temperature field was formed during the spark plasma sintering (SPS) process. The compositionally graded layers of the FGM resulted in a well-bonded composite structure. The layers consist of only TiB and Ti phase when target volume fraction of TiB increases from 20% to 60%, and TiB2 phase presents in the two layers with higher target volume fraction of TiB. The hardness reaches the maximum value of 12.73 GPa when target volume fraction of TiB is 80%. The needle shape and agglomerated TiB were reaction synthesized in the sintered FGM. The TiB morphologies in layers were investigated and discussed. This TGF sintering method is practicable when sintering any other metal-ceramic FGMs. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Composites; Spark plasma sintering (SPS); X-ray methods; Microstructure
1. Introduction A functionally graded materials (FGMs) is a composite material consisting of two or more phases in which the volume fraction of the constituents changes so that the composition, microstructure and properties vary gradually in one direction. Designing FGMs allows manipulation of many material properties or new functions. TiB and TiB2 exhibit extreme hardness, high melting point, high electrical conductivity, good thermal shock resistance and chemical inertness [1,2], which can be fabricated by in situ reaction between titanium and boron. A FGM with composition changing from Ti alloy on one side to TiB–TiB2 composite on the other side has many potential application fields specifically in defense [3,4]. But the sintering of these FGMs is difficult since the great sintering temperature difference between Ti and TiB, TiB2. So, high n Correspondence to: 1608 Room, Xing Zheng Building, School of Materials Engineering, Shanghai University of Engineering Science, 333 Long Teng Road, Shanghai 201620, China. Tel.: þ 86 21 67791203; fax: þ86 21 67791377. E-mail address: [email protected] (Y. Zhang).
http://dx.doi.org/10.1016/j.ceramint.2015.08.070 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
performance FGMs may be obtained if a temperature gradient field (TGF) with low temperature on Ti side and high temperature on TiB or TiB2 side could be formed in the die during the sintering process. In recent years, TiB–Ti systems FGMs have been fabricated by hot pressing and combustion synthesis method, but crack is the main problem in the FGMs synthesized by these processes [3]. The gradient sintering temperature can be realized by an innovative technology known as spark plasma sintering (SPS). The SPS process is a synthesis and processing technique which makes it possible sintering and sintering bonding at low temperature and short periods by charging at the intervals between powder particles with electrical energy and effectively applying high temperature spark plasma generated momentarily. As a rapid consolidation technique, the SPS process has many advantages on fabrication of advanced materials and FGMs [5–8]. In previous studies, FGMs based on titanium borides by spark plasma sintering (SPS) process were rarely reported, such as the four-layered FGMs of TiB–Ti was rapidly fabricated using Ti powders and TiB2 powders as raw materials [9], but the fabrication of high relative density six-layered
Y. Zhang et al. / Ceramics International 41 (2015) 13844–13849
FGMs of Ti–TiB–TiB2 synthesized by chemical reaction of Ti with B and TiB2 during SPS process has not been reported yet. In this study, the Ti–TiB2–B system was selected to reaction synthesize Ti–TiB–TiB2 FGM. A temperature gradient field (TGF) was formed during the sintering process by spark plasma sintering (SPS). The rapid fabrication of reaction synthesis Ti–TiB–TiB2 FGMs in this TGF formed by SPS technique, and the microstructure and hardness of the FGMs were characterized. 2. Methods and details 2.1. Composition design of FGM Elemental Ti (74 mm, purity 499%), FeMo (74 mm, purity: 4 99%), B (average size 2 mm, purity 4 95%) powder and TiB2 (average size 30 mm, purity 4 99%) powder were used as raw materials. The TiB was synthesized by chemical reaction of Ti with B and TiB2 during the SPS sintering, which is shown as follows: Ti þ B ¼ TiB
ð1Þ
Ti þ TiB2 ¼ 2TiB
ð2Þ
The target TiB volume fraction by different reaction is shown in Table 1 and the layer codes are used in the following discuss. Both B and TiB2 were used in layers of 40, 60, 80 and 100 vol% TiB, as well as only TiB2 was used in layer of 20 vol % TiB. The Ti alloy matrix (Ti–4.0Fe–7.3Mo) was synthesized by blended elementals Ti and FeMo powder. Powder mixtures of target volume fractions of 0, 20, 40, 60, 80 and 100% TiB were blended by planetary ball milling for 600 min, respectively. Then the FGM was stacked layer by layer in a cylinder graphite die with an inner diameter of 20 mm before sintering.
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temperature is higher than that of lower punch, which leads to the forming of temperature gradient field between them during the heating process. Furthermore, the low graphite spacer contacts with the water cooling electrode and the graphite die, which is advantageous to eliminate heat of the die and keeps the difference in temperature between upper and lower punch. The layer of Ti alloy contacted with lower punch and the target 100 vol% TiB contacted with upper punch. The sintering temperature is monitored and regulated by a thermocouple in the die, as shown in Fig. 1, the temperature of upper punch was measured and recorded by means of an optical pyrometer focused on its surface. Then the compact was sintered at 950 1C with a heating rate of 150 1C/min and dwelling 5 min with a pressure of 40 MPa in vacuum. The sintered FGM is cooled to room temperature in furnace. Heating and dwelling of the FGM sintering was completed within 12 min.
2.3. Characterization tests The phase constitution of the composites was identified by X-ray diffraction (XRD) using Cu Kα radiation. The specimens were polished and then etched with a solution of 5 ml HF, 10 ml HNO3 and 85 ml H2O. Microstructure of the sintered FGMs was investigated using a Hitachi S-3500N type scanning electron microscopy (SEM). The Vickers hardness measurement was done on the HX-50 type Vickers machine with a 147 N load for 10 s.
2.2. TGF sintering by SPS The sintering processing was performed using the SPS system (model 1050). Fig. 1 is a schematic illustration of temperature gradient field sintering method. Both lower punch and graphite die contact with the low graphite spacer, while only upper punch contacts with the up graphite spacer. So the resistance of upper electrode is greater than that of lower electrode. When the pulse current passing through the electrodes, the upper punch
Fig. 1. Schematic illustration of temperature gradient field sintering.
Table 1 The designed and calculated TiB volume fraction and calculated data for the layers in the FGM. Layers
Target volume fraction of TiB (%)
Volume fraction of TiB (%)
Thickness (mm)
TiB aspect ratio
Ec (GPa)
Relative density (%)
2.20 1.40 1.63 1.82 1.58 2.00
– 20 10 10 7.5 7.5
115 214 292 366 441 492
98.1 97.1 93.4 90.3 88.5 86.8
Synthesized by reaction (1) Synthesized by reaction (2) Layer Layer Layer Layer Layer Layer
1 2 3 4 5 6
0 20 40 60 80 100
0 0 20 30 40 50
0 20 20 30 40 50
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3. Results and discussion 3.1. Investigation of the temperature gradient field The temperature of upper punch measured by optical pyrometer and that of lower punch measured by thermocouple were recorded per 0.5 min. The change of temperature between upper and lower punch during the heating and dwelling process is shown in Fig. 2. The lower punch was heated with a rate of 150 1C/min, the temperature of upper punch rose to 850 1C in about 1 min and then rises with a heating rate of about 150 1C/min. So the gradient field with a temperature difference of about 650 1C was formed at the beginning of the heating process and maintained during the heating process. The maximum temperature of upper punch was 1569 1C when the lower punch was heated to 900 1C. The sintering pulse current decreased when the temperature was close to dwelling temperature, which leads to the temperature of upper punch decreasing rapidly. With the decreasing of pulse current the temperature difference between upper and lower punch decreased from 650 1C at the beginning to 388 1C at the end of the dwelling process. So, the temperature gradient field can be formed during the sintering process by the method shown in Fig. 1. Formation of the TGF is advantageous to the consolidation of the FGMs and decreasing of the thermal stress during the sintering and cooling process. The interface is an important factor on controlling the properties of FGMs. The sintered FGM was grinded and investigated. As shown in Fig. 3, the compositionally graded layers of the FGM resulted in a well-bonded composite structure. No significant shape change or cracks were detected on the surface of the FGM. The sample was cross-sectioned and no cracks were observed in the center and between the layers. No interlayer cracks are detected in all the cross section of the FGM, which indicates that a well-bonded composite structure can be achieved by the TGF sintering of stacked layers with the designed composition.
Fig. 3. A picture of the sintered Ti–TiB–TiB2 composite.
3.2. X-ray diffraction analysis The graded layers of the FGM were cut through the interface boundaries, and XRD was utilized on each layer for phase Fig. 4. X-ray diffraction spectrum of individual layer of the Ti–TiB–TiB2 FGM.
Fig. 2. Temperature measured by pyrometer and thermocouple during heating and dwelling process.
identification. XRD spectrum from individual layer of the FGM after SPS sintered is shown in Fig. 4. The results of XRD analysis indicate that layer 1 consists of singular β-Ti phase, while the diffraction peaks of α-Ti phase present in the spectrum of layer 2, and peaks of β-Ti phase can hardly be observed in spectrum from layers 3 to 6. Fe and Mo elements were added to the Ti matrix, which solved into titanium during the sintering process and led to the formation of β-Ti phase. Since Fe and Mo elements are β phase stable elements of titanium, the β phase was maintained when cooled to room temperature. The peaks of TiB present and their intensities increase in XRD spectrums of
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layers 2–4. In above three layers, chemical reaction of B and TiB2 with Ti is completed since no peak of TiB2 or B present in these spectrums, and the volume fraction of TiB is increasing. So the target volume fraction of TiB was synthesized as expected in these layers. While in layers 5 and 6, peaks of TiB2, TiB and Ti present in the spectrums. This indicates that the two layers consist of synthesized TiB, TiB2 and small amount of titanium. In these two layers, the chemical reaction of TiB2 with Ti has not been completed. The intensities of TiB2 peaks in spectrum of layer 6 are higher than that of layer 5, which indicates that the volume fraction of TiB2 in layer 6 is higher than that of layer 5. Existence of the residual TiB2 is because it is more stable than TiB phase when there is excess boron existing in the titanium matrix. The volume fraction of reaction synthesized TiB is not as same as expected in these two layers. So there are three kinds of ordinal layers in the SPS sintered FGM, titanium alloy matrix layer (layer 1) and volume fraction increasing from 20% to 60% of synthesized TiB reinforced titanium alloy matrix in next three layers (layers 2–4), and TiBþ TiB2 þ Ti in the last two layers (layers 5 and 6). 3.3. Microstructure Microstructure of the individual layer was observed using SEM, as shown in Fig. 5. These figures illustrate the varied
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microstructure morphologies observed in the FGM. Fig. 5(a) shows the morphology of the Ti alloy matrix, and the grains of β-Ti phase are equiaxial. As designed, the matrix consists of β-Ti phase due to the addition of FeMo powder and solid solution of Fe and Mo elements to the titanium. The needle shape TiB whiskers were observed along the grain boundaries occasionally. A large amount of TiB whiskers were observed along grain boundaries closing to the interface between layers 1 and 2. So the presentation of TiB whiskers in layer1 is related to the diffusion of B element during the sintering process. Fig. 5(b) reveals the deep etched morphology of layer 2, and the needle shape TiB whiskers randomly distributed in the titanium matrix. The diameter of synthesized TiB needles is less than 100 nm. When the target volume fraction of TiB increases to 40%, the agglomerated TiB whiskers present among the needle shape TiB whiskers in layer 3 as shown in Fig. 5(c). The diameter size of needle shape TiB is bigger than that of layer 2. Microstructure of layer 4 is shown in Fig. 5(d), which shows that the two different types of TiB morphologies were observed in layer 4. The randomly distributed needle shape whiskers are similar to that in layer 3. It suggests that the needle shape TiB whiskers morphology underwent a significant change, especially in size, as the volume fraction of TiB increased from 20% to 60%. But the amount of agglomerated TiB increases and their
Fig. 5. SEM photograph of individual layer, and (a)–(f) is morphology of layers 1–6 respectively.
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size is bigger than that in layer 3. These agglomerated TiB are the rod-like whiskers that appear to be interconnected, as shown in Fig. 5(c) and (d), so the synthesis mechanism of the agglomerated TiB is same to that of needle shape TiB. Microstructure of layers 5 and 6 are shown in Fig. 5(e) and (f), the unetched TiB2 presents together with the needle shape and agglomerated TiB. The amount of TiB2 in layer 6 is more than that of layer 5, which confirms the result of XRD analysis. Meanwhile, more small holes were observed between the agglomerated TiB, and the needle shape TiB between them are less than that of layer 5. As TiB has a B27 structure [10], characterized by zigzag chains of boron atoms parallel to the b-direction, where each Β atom lies at the center of a trigonal prism of six Ti atoms, and TiB should exhibit much faster growth along b-direction than a or c direction, which results in the formation of TiB with needle shape or rod-like morphology. Moreover, as shown in Fig. 5, the agglomerated TiB size decreases with an increasing target volume fraction of synthesized TiB whisker from 40% to 80%. With the increasing of content of B and TiB2 particles, mass TiB nucleated and grew simultaneously and the growing up of TiB were restricted each other which led to the formation of agglomerated TiB. The SEM images of the agglomerated TiB shown in Fig. 5(c) and (d), indicate that the agglomerated TiB is composed of small rod-like TiB. With the increasing of B and TiB2 addition and the rising of sintering temperature, the diffusion path increased and speed fasted, which reduced the driving force for TiB grain growth and prevented the sintering between TiB grains to form the agglomerated TiB. It is
Fig. 7. Change of Vickers hardness with the target volume fraction of TiB.
deduced that the grow mechanism of agglomerated TiB is the same as needle shape TiB. While, in layer 5 and 6, the target volume fraction of TiB was not achieved because of the existence of residual TiB2. The amount of agglomerated TiB decreased when more TiB2 remained in layer 6 than that in layer 5. This is the reason why the content of agglomerated TiB decreases with the increasing of target volume fraction of TiB from 80% to 100%. Although TiB2 has not reacted with Ti completely in both layers 5 and 6, the reinforcement fraction (synthesized TiB and residual TiB2) is still higher than that of other layers. The hardness of layer 6 decreased slightly because the fraction of residual Ti between the agglomerated TiB in layer 6 is more than that in layer 5. The ingredient of composite that determines the final product and the morphologies of synthesized TiB. TiB2 is more stable than TiB at high temperature or if there excess boron existing in the titanium matrix. TiB2 phase was also detected in the materials with 100% target volume fraction of synthesized TiB using Ti and B powder. Similar composition of final product was obtained by self-propagating high temperature combustion synthesis method [11,12]. The detailed interfaces between each layer are shown in Fig. 6. It can be seen from Fig. 6 that interfaces between each layer were well bonded except a few pores between layers 3 and 4. 3.4. Hardness The Vickers hardness as a function of layer composition is shown in Fig. 7. This indicates that the hardness of the layers increase dramatically with the increasing of synthesized TiB volume fraction. The hardness is increased with the increasing of target volume fraction of TiB from layers 1 to 5, while the hardness of layer 6 is a little smaller than that of layer 5. The hardness value of layer 1 is 3.52 GPa. The hardness reaches its maximum value in layer 5, which is about 12.73 GPa, and that of layer 6 is 11.98 GPa. 4. Conclusions
Fig. 6. SEM photograph of interfaces between each layer.
The FGM of Ti–TiB–TiB2 was fabricated successfully using the temperature gradient field (TGF) method by spark plasma sintering technique. A gradient field with a temperature
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difference was formed at the beginning of the heating process and maintained during the heating process. The temperature difference between upper and lower punch decreased from 650 1C at the beginning to 388 1C at the end of the dwelling process. The hardness increased with the increasing of target volume fraction of TiB from layers 1 to 5, while the hardness of layer 6 is a little smaller than that of layer 5. The hardness reaches its maximum value in layer 5, which is about 12.73 GPa. There are three kinds of ordinal layers in the SPS sintered FGM, titanium alloy matrix layer (layer 1) and reaction synthesized TiB reinforced titanium alloy matrix in layers 2– 4, and TiB þ TiB2 þ Ti in layers 5 and 6. The layers consist of only TiB and Ti phase when target volume fraction of TiB increases from 20% to 60%, and TiB2 phase presents in the two layers with higher target volume fraction of TiB. TiB phase has two morphologies, i.e. needle shape and agglomerated TiB in the microstructure of the synthesized FGM. The six-layered functionally gradient material (FGM) of Ti–TiB– TiB2 with a well-bonded composite structure can be achieved by the TGF sintering of stacked layers with the designed composition. Acknowledgments This project was supported by the National Natural Science Foundation of China under Grant no. 51402189. References [1] Z. Zhang, X. Shen, C. Zhang, A new rapid route to in-situ synthesize TiB–Ti system functionally graded materials using spark plasma sintering method, Mater. Sci. Eng. A. 565 (2013) 326–332. [2] F.C. Wang, Z. Zhang, J. Luo, A novel rapid route for in situ synthesizing TiB–TiB2 composites, Compos. Sci. Technol. 69 (2009) 2682–2687.
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