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Dispersion characteristics, microstructural evolution and sintering behaviour of Al2O3-Ti6Al4V composites fabricated by spark plasma sintering
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 3791–3797
www.materialstoday.com/proceedings
ICMPC-2019
Dispersion characteristics, microstructural evolution and sintering behaviour of Al2O3-Ti6Al4V composites fabricated by spark plasma sintering Okoro Avwerosuoghene Mosesa*, Cosssa Themba Edmonda, Thaba Thapelo Preciousa, Lephuthing Senzeni Siphoa, Oke Samuel Rantia, Olubambi Peter Apataa a
Centre for Nanoengineering and Tribocorrosion, Department of Metallurgy, School of Mining, Metallurgy and Chemical Engineering, University of Johannesburg, 2028, Johannesburg, Republic of South Africa
Abstract Experimental investigations were undertaken to understand the dispersion characteristics and sintering behaviour of alumina reinforced Ti6Al4V composites. Alumina of varying volume percent (5, 10 and 15) was dispersed in Ti6Al4V via low energy ball mill (PM 100) using alumina balls of 5 mm in an alumina vial for 6 hours, at 150 rpm with a ball to powder ratio of 10:1. The admixed powders were sintered at 1000 and 1100 0C, 100 0C/min, 50 MPa and held for 10 minutes using spark plasma sintering. The effect of the addition of alumina to Ti6Al4V were evaluated by analyzing the dispersion characteristics, sintering behaviour and the relative displacement of the punches during sintering. The results indicated that the alumina particles were homogeneous dispersed and adhered to the Ti6Al4V particles after low energy ball milling. Additionally, the incorporation of alumina into the Ti6Al4V promotes the sinterability and the rapid displacement of the punches during the sintering process. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Alumina; Titanium alloy; Spark plasma sintering; dispersion characteristics; sintering behaviours
1. Introduction Metal matrix composites are highly desirable in various advanced engineering applications because of their excellent mechanical, thermal and electrical properties. They are usually synthesized by the integration of ceramic or other metallic materials as reinforcements in the form of fibers or particulates with exclusive properties to augment the metallic matrix material. These reinforcing materials helps to modify the microstructures of metallic matrix and improve the mechanical properties by various strengthening mechanisms. * Corresponding author. E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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Over the years, the exclusive properties of metal matrix composites have been linked to the excellent properties of the reinforcing phases. Where various ceramics materials (SiC, WC, TiC, SiO2 and Al2O3) [1, 2] with outstanding mechanical and tribological properties have been adopted to improve the physical, mechanical and tribological properties of various metal matrix materials to form advanced metal matrix composites. Among the enlisted ceramic materials used as reinforcing phase to improve metal matrices, Alumina (Al2O3) has attracted the attention of researchers in recent times. They are desirable due to their exclusive wear resistance, elevated temperature mechanical properties (stiffness and strength), cost friendliness, electrical conductivity and thermal stability [3]. The combination of these unique properties of alumina has motivated researchers to apply them as reinforcing materials to improve the properties of various metal matrices namely, copper [3, 4], aluminium [5, 6], and magnesium [7, 8] for advanced engineering applications. The quest to improve the properties of titanium alloy (Ti6Al4V) for various industrial, structural and biomedical applications have encouraged the integration of ceramic particulates such as alumina into it structure. Titanium metals and its alloys are considered for diverse engineering applications because of their light weight, excellent corrosion resistances and good specific strength [9, 10]. However, titanium alloys tend to oxidize easily, experience loss of its mechanical properties at elevated temperature especially above 400 0C and undergo creep at temperature above 300 0C. These drawbacks of the alloy have justified the reason for incorporating alumina into its structures to augment its properties by the transfer of load from the metallic based matrix to the alumina particulate and inhibiting dislocation motion by pinning effects of the alumina particles. In order to achieve the effective transfer of load from the matrix materials to the reinforcement phase, there must be homogeneous dispersion of the particulate phase within the matrix phase and good consolidation technique with appropriate processing parameters to form a composite structure with improved properties than the matrix material must be adopted. Recently, spark plasma sintering has attracted the interest of researchers because of its flexibility, ease of operation and ability to consolidate powder materials within limited time. This consolidation technique proffers numerous benefits than conventional sintering techniques because it allows the regulation of sintering energy with limited grain growth and support hot compaction up to full densification. In a bid to synthesize alumina reinforced Ti6Al4V composites, spark plasma sintering technique is adopted in this study. The dispersion characteristics of the alumina in Ti6Al4V and sintering behaviour of the sintered composites were evaluated to understand the sintering phenomena associated with the fabrication of the composites. 2. Materials and Method 2.1 Starting Materials The alumina powders used for this study was supplied by supplied by Glass blown & Volumetric Glassware & Chemicals in Johannesburg, South Africa with percentage purity of (~97.7), it has particle sizes varying from 12 to 45µm. An argon atomized, prealloyed Ti6Al4V powders with particle size ~25 μm supplied by TLS Technik GmbH & Co. Germany was used as the matrix material. 2.2 Dispersion of Al2O3 in Ti6Al4V powders The alumina powders of different volume fraction (5, 10 and 15 vol.%) were weighed and dispersed into Ti6Al4V powders using a low energy ball mill (Retsch 100 PM, Germany) with the following milling parameters, milling speed of 150 revolution per minute (rpm), time of 6 hours and ball to powder ration (BPR) of 10:1. The mixing operation was carried out using alumina balls of 5 mm diameter to enhance collisions of the balls with the powders . This mixing operation was done at an interval of 10 minutes for every 10 minutes of mixing to prevent the powders from overheating and prevent unwanted reactions between the powders during mixing operation. 2.3 Sintering of the Admixed Powders The mixed composite powders were measured and poured into a Ø 20 mm graphite die with a gauged height of 5 mm and cold compacted with a load of 10 KN to promote good electrical conductivity between the powders and the die. The pre-compacted composites are then transferred into a spark plasma sintering (model HHPD-25, FCT Germany) chamber where sintering operation was carried out. The composite mixture was sintered at a sintering temperature of 1000 oC and 1100 oC respectively. Other sintering parameters adopted are pressure of 50 MPa, heating rate of 100 oC/min and holding time of 10 minutes. The sintered samples were collected from the sintering chamber after cooling and sand blasted to remove graphite foils from the sintered samples. The sintering data of the samples were acquired and evaluated.
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2.4 Characterization of starting, admixed powders and sintered composites The morphology of both the starting, admixed powders and sintered composites were examined under scanning electron microscopy (SEM), model TESCAN furnished with energy dispersive X-ray spectrometry (EDX). The phase identification and crystalline phases of starting and mixed powders and sintered composites were characterized by X-ray diffractometer (XRD, Rigaku D/max-rB). This was carried out by a scan using Cu-Kα (λ= 0.154 nm) radiation at a scanning rate of 1°/min over the angular range of 10–90°. 3. Results and Discussion 3.1 Dispersion characteristics and microstructural evolution of the composites The effectiveness of the low energy ball milling adopted during the dispersion was adjudged by the morphology and structures of the starting, admixed and sintered composites acquired from the SEM analysis. Figure 1 (a-d) depict SEM images and EDS of the starting powders. From figure 1(a &c), it was observed that the morphology of the Ti6Al4V are spherical with even shapes and they have an average particle size of (~26 µm) alongside the EDS which confirm the elemental composition of the Ti6Al4V. Similarly, figure 1(b &d) shows the morphology of the Al2O3 which depict irregular particles with coarse surface and the EDS confirmed the elemental composition of the alumina.
Fig.1. Illustrates the SEM images of the starting powders, Ti6Al4V(a), Al2O3 (b) and their accompanying EDS (c & d).
Additionally, figure 2 (a-c) shows the SEM images of the admixed powder. From the SEM images in figure 2, it was observed that the alumina particles are adhered to the Ti6Al4V particles which gave the metal matrix particles a coarse morphology. The SEM images of the admixed powders also show that there was increase in the amount of the alumina particles adhered to the Ti6Al4V particles when the volume fraction of the alumina was increased from 5 vol.% to 15 vol.%. The adhesion of the alumina particles to the Ti6Al4V particles was ascribed to the adequate impact energy exerted from the alumina balls on the powders during mixing which resulted in good bonding between the powders.
Fig.2. Illustrates the SEM images of the Admixed composite powders (Al2O3-Ti6Al4V) containing 5 vol.% (a), 10 vol. % (b) and 15 vol. % (c)
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Fig.3. Illustrates the SEM images of the sintered Ti6Al4V at 1000 oC (a), 1100 oC (b), 5 vol.% Al2O3- Ti6Al4V at 1000 oC (c), 1100 oC (d), 10 vol.% Al2O3- Ti6Al4V at 1000 oC (e), 1100 oC (f), and 15 vol.% Al2O3- Ti6Al4V Composites at 1000 oC (g) and1100 oC (h).
In a bid to consolidate the composites powders, the admixed powders were sintered at 1000 oC and 1100 oC and the SEM images in figure 3 shows the morphology of the sintered titanium alloy and the composites. From the SEM images shown in figure 3 (a-h), the SEM images in figure 3 (a & b) represent the micrograph of Ti6Al4V sintered at 1000 oC and 1100 oC respectively. It was observed that the microstructures were made up of the different phases[11]. The presence of aluminium helps to stabilize the alpha phase and vanadium stabilizes the beta phase [12, 13]. In addition, there are more lamellar structures on the sample sintered at 1000 oC however, when the sintering temperature was increased to 1100 oC, it was observed that the lamellar structures were gradually transformed to flakelike structure this transformation is ascribed to the higher sintering temperature which resulted in the change of the microstructure. Similarly, the SEM images in figure 3 (c & d) depict the composites containing 5 vol.% of alumina that was sintered at 1000 oC and 1100 oC respectively. From the SEM micrographs, it was observed that the alumina particles settled at the grain boundaries of the Ti6Al4V grains. However, when the volume fraction was increased from 5 to 15 vol.%, it was observed that the alumina particles diffused into the alpha phase of the titanium alloy. From all indications in the SEM images of the sintered composites, it was observed that
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the alumina particles were uniformly dispersed in the titanium alloy with good interfacial bonding which will assist in inhibiting dislocation motion by particle strengthening mechanism during service application of the synthesized composites. The microstructural evolution of the sintered composites also indicated that there were interfacial interaction between the alumina and the Ti6Al4V structure during sintering. 3.2 Phase analysis of the composites The phase analysis of the starting, admixed powders and sintered composites at varying sintering temperatures was conducted to understand the transformation of the titanium alloy by the addition of alumina. Figure 4 below shows the XRD pattern of the powders and sintered composites.
Fig.4. Illustrates the XRD pattern of the starting powders Al2O3 (a), Ti6Al4V(b), admixed composite powders with varying amount of alumina (b), sintered composites at 1000 oC (c) and at 1100 oC (d)
From the XRD pattern in figure 4 (a), which indicates the XRD spectra of pristine alumina, various Corundum (crystalline) alumina phases were observed at 2θ = 25.55o ,35.11 o, 37.74 o, 43.31 o, 52.50 o, 57.44 o, 59.68 o, 61.06 o , 66.45 o, 68.14 o,77.8 o, 80.33 o, 84.26 o, 86.25 o and 88.90 o which corresponds to the following planes (012), (104), (110), (006), (024), (116), (211), (122), (116), (300), (119), (217), (223), (312) and (102) respectively. From the XRD pattern of the alumina reinforced Ti6Al4V admixed powders in figure 4(b), it was glaring that the crystalline alumina became more pronounced with the increased in alumina volume fraction. However, only alpha titanium phases were observed from the XRD pattern of the Ti6Al4V patterns. Similarly, figure 4(c &d) shows the XRD pattern of the composites sintered at 1000 oC and 1100 oC respectively. From the XRD patterns of the sintered composites, it was observed that the crystalline alumina peaks were more pronounced on the titanium alloy. This may be as a result of increase in temperature during sintering which favors homogeneity between the composites components. However, it was observed that the prominent peaks in the sintered composites tend to decrease with the increase in volume fraction of the alumina. Additionally, the presence of alumina phases in the composites were pronounced with increase in sintering temperature. 3.3 Sintering Behaviours of Al2O3-Ti6Al4V Composites In a bid to understand the sintering behaviours of the titanium alloy and the sintered composites, the sintering data were collected and analyzed after spark plasma sintering. Figure 5 (a-d) shows the sintering behavours of the titanium alloy and the synthesized composites at distinct sintering temperature ( 1000 oC & 1100 oC) and the
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displacement of the punches during sintering process which depicts the deformation of the samples under the influence of pressure MPa and the varying sintering temperature. From figure 5 (a-d) which showed the sintering behavior of the titanium alloy and the synthesized composites sintered at 1000 oC, it was observed that the temperature was maintained at 250 oC for 450 seconds (s) before it increased to 1000 oC for the synthesized composites. However, the temperature was maintained at 250 oC for 600s for the titanium alloy before it increased to 1000 oC. This behavior may be ascribed to the high thermal conductivity of the alumina particles which promotes the rapid heating during the synthesis of the composites. Additionally, when the samples were sintered at higher sintering temperature as shown in figure 5(b), both the sintered alloy and composites started the sintering cycle at almost the same time, however, the samples sintered at higher temperature completed the sintering cycle at lesser time and this may be ascribed to the higher sintering temperature which activated the sintering rate that resulted in the speedy consolidation process.
Fig.5. Illustrates the sintering behaviour of titanium alloy and the synthesized composites (a &b) and the displacement curves of the punches (c &d) during sintering at 1000 oC and 1100 oC respectively.
Similarly, figure 5(c &d) depict the displacement of the graphite punches during sintering of the titanium alloy and the synthesized composites. From all indication, it was observed that the samples sintered at 1100 oC exhibits fast sintering process. However, both samples sintered at different sintering temperature shows that the displacement of the punches increased from 0 to 3.5 mm because of the exerted pressure on the materials. The short displacement observed during sintering was due to the cold compaction that was done on the powders before sintering process. From the displacement curves in figure 5 (c), it was observed that the applied pressure was not effective in aiding the displacement of the punches at 0 to 14 mins for the titanium alloy and this is due to low temperature at that time, however, for the synthesized composites which has alumina content with good thermal conductivity, the displacement of the punches was observed at lesser time (10 min) even at low temperature. This fast displacement of the punches was ascribed to the composites powders with improved thermal conductivity that aided the sintering rate of the composites. When the temperature was increased, the effect of the applied pressure was dominant that resulted in the huge displacements of the punches thereby resulting in the rearrangements of the particles [14]. The effectiveness of the exerted pressure during sintering promotes the compaction of the samples by aiding the escape of trapped gases present in the sample during cold compaction. It was observed from the curves that the displacement was continuous after the escape of the trapped gas from the samples, thereby showing the three stages of the sintering process (heating, holding and cooling). Apart from the rearrangement of the powder particles, the sintering process also aid grain growth and necking of the particles as a result of bulk mass transport mechanisms thereby causing grain boundary diffusion, plastic flow and dislocation climb and overall deformation of the material [15, 16].
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4. Conclusion The adoption of low energy ball milling technique is an effective method of dispersing alumina in titanium alloy since it resulted in homogeneous dispersion and good interfacial bonding of alumina in Ti6Al4V. Additionally, the incorporation of alumina into titanium alloy promotes the sinterability of the composites. The compressive pressure during sintering is ineffective to result in the movement of the punches at lower temperature. However, at higher sintering temperature, there were continuous displacement of the punches which result in rapid compaction of the samples. Acknowledgement The authors would like to appreciate the National research foundation of South Africa in association with the world academic of science (NRF-TWAS) and Global Excellence and Stature for funding this research. References [1] S. Moustafa, Z. Abdel-Hamid, A. Abd-Elhay, Copper matrix SiC and Al2O3 particulate composites by powder metallurgy technique, Materials Letters, 53 (2002) 244-249. [2] H. Simchi, A. Simchi, Tensile and fatigue fracture of nanometric alumina reinforced copper with bimodal grain size distribution, Materials Science and Engineering: A, 507 (2009) 200-206. [3] V. Rajkovic, D. Bozic, J. Stasic, H. Wang, M.T. Jovanovic, Processing, characterization and properties of copper-based composites strengthened by low amount of alumina particles, Powder Technology, 268 (2014) 392-400. [4] D. Ying, D. Zhang, Processing of Cu–Al2O3 metal matrix nanocomposite materials by using high energy ball milling, Materials Science and Engineering: A, 286 (2000) 152-156. [5] A. Kurşun, E. Bayraktar, H.M. Enginsoy, Experimental and numerical study of alumina reinforced aluminum matrix composites: Processing, microstructural aspects and properties, Composites Part B: Engineering, 90 (2016) 302-314. [6] Y. Iwai, T. Honda, T. Miyajima, Y. Iwasaki, M. Surappa, J. Xu, Dry sliding wear behavior of Al2O3 fiber reinforced aluminum composites, Composites science and technology, 60 (2000) 1781-1789. [7] E. Ghasali, M. Alizadeh, K. Shirvanimoghaddam, R. Mirzajany, M. Niazmand, A. Faeghi-Nia, T. Ebadzadeh, Porous and non-porous alumina reinforced magnesium matrix composite through microwave and spark plasma sintering processes, Materials Chemistry and Physics, 212 (2018) 252-259. [8] H. Li, L. Cheng, X. Sun, Y. Li, B. Li, C. Liang, H. Wang, J. Fan, Fabrication and properties of magnesium matrix composite reinforced by urchin-like carbon nanotube-alumina in situ composite structure, Journal of Alloys and Compounds, 746 (2018) 320-327. [9] A. Okoro, R. Machaka, S. Lephuthing, M. Awotunde, P. Olubambi, Structural integrity and dispersion characteristics of carbon nanotubes in titanium-based alloy, IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2018, pp. 012004. [10] A. Okoro, M. Awotunde, O. Ajiteru, S. Lephuthing, P. Olubambi, R. Machaka, Effects of carbon nanotubes on the mechanical properties of spark plasma sintered titanium matrix composites—A review, Mechanical and Intelligent Manufacturing Technologies (ICMIMT), 2018 IEEE 9th International Conference on, IEEE, 2018, pp. 54-59. [11] D.M. Brunette, P. Tengvall, M. Textor, P. Thomsen, Titanium in medicine: material science, surface science, engineering, biological responses and medical applications, Springer Science & Business Media, 2012. [12] L. Da Silva, M. Ueda, M. Silva, E. Codaro, Corrosion behavior of Ti–6Al–4V alloy treated by plasma immersion ion implantation process, Surface and Coatings Technology, 201 (2007) 8136-8139. [13] H.-q. Duan, Y.-f. Han, W.-j. LÜ, J.-w. Mao, L.-q. Wang, D. Zhang, Effect of solid carburization on surface microstructure and hardness of Ti-6Al-4V alloy and (TiB+ La2O3)/Ti-6Al-4V composite, Transactions of Nonferrous Metals Society of China, 26 (2016) 1871-1877. [14] O.E. Falodun, B.A. Obadele, S.R. Oke, M.E. Maja, P.A. Olubambi, Effect of sintering parameters on densification and microstructural evolution of nano-sized titanium nitride reinforced titanium alloys, Journal of Alloys and Compounds, 736 (2018) 202-210. [15] A.M. Okoro, S.S. Lephuthing, S.R. Oke, O.E. Falodun, M.A. Awotunde, P.A. Olubambi, A Review of Spark Plasma Sintering of Carbon Nanotubes Reinforced Titanium-Based Nanocomposites: Fabrication, Densification, and Mechanical Properties, JOM, 1-18. [16] Z. Zhang, X. Shen, F. Wang, S. Lee, L. Wang, Densification behavior and mechanical properties of the spark plasma sintered monolithic TiB2 ceramics, Materials Science and Engineering: A, 527 (2010) 5947-5951.
ScienceDirect Materials Today: Proceedings 18 (2019) 3791–3797
www.materialstoday.com/proceedings
ICMPC-2019
Dispersion characteristics, microstructural evolution and sintering behaviour of Al2O3-Ti6Al4V composites fabricated by spark plasma sintering Okoro Avwerosuoghene Mosesa*, Cosssa Themba Edmonda, Thaba Thapelo Preciousa, Lephuthing Senzeni Siphoa, Oke Samuel Rantia, Olubambi Peter Apataa a
Centre for Nanoengineering and Tribocorrosion, Department of Metallurgy, School of Mining, Metallurgy and Chemical Engineering, University of Johannesburg, 2028, Johannesburg, Republic of South Africa
Abstract Experimental investigations were undertaken to understand the dispersion characteristics and sintering behaviour of alumina reinforced Ti6Al4V composites. Alumina of varying volume percent (5, 10 and 15) was dispersed in Ti6Al4V via low energy ball mill (PM 100) using alumina balls of 5 mm in an alumina vial for 6 hours, at 150 rpm with a ball to powder ratio of 10:1. The admixed powders were sintered at 1000 and 1100 0C, 100 0C/min, 50 MPa and held for 10 minutes using spark plasma sintering. The effect of the addition of alumina to Ti6Al4V were evaluated by analyzing the dispersion characteristics, sintering behaviour and the relative displacement of the punches during sintering. The results indicated that the alumina particles were homogeneous dispersed and adhered to the Ti6Al4V particles after low energy ball milling. Additionally, the incorporation of alumina into the Ti6Al4V promotes the sinterability and the rapid displacement of the punches during the sintering process. © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Alumina; Titanium alloy; Spark plasma sintering; dispersion characteristics; sintering behaviours
1. Introduction Metal matrix composites are highly desirable in various advanced engineering applications because of their excellent mechanical, thermal and electrical properties. They are usually synthesized by the integration of ceramic or other metallic materials as reinforcements in the form of fibers or particulates with exclusive properties to augment the metallic matrix material. These reinforcing materials helps to modify the microstructures of metallic matrix and improve the mechanical properties by various strengthening mechanisms. * Corresponding author. E-mail address: [email protected] 2214-7853© 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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Over the years, the exclusive properties of metal matrix composites have been linked to the excellent properties of the reinforcing phases. Where various ceramics materials (SiC, WC, TiC, SiO2 and Al2O3) [1, 2] with outstanding mechanical and tribological properties have been adopted to improve the physical, mechanical and tribological properties of various metal matrix materials to form advanced metal matrix composites. Among the enlisted ceramic materials used as reinforcing phase to improve metal matrices, Alumina (Al2O3) has attracted the attention of researchers in recent times. They are desirable due to their exclusive wear resistance, elevated temperature mechanical properties (stiffness and strength), cost friendliness, electrical conductivity and thermal stability [3]. The combination of these unique properties of alumina has motivated researchers to apply them as reinforcing materials to improve the properties of various metal matrices namely, copper [3, 4], aluminium [5, 6], and magnesium [7, 8] for advanced engineering applications. The quest to improve the properties of titanium alloy (Ti6Al4V) for various industrial, structural and biomedical applications have encouraged the integration of ceramic particulates such as alumina into it structure. Titanium metals and its alloys are considered for diverse engineering applications because of their light weight, excellent corrosion resistances and good specific strength [9, 10]. However, titanium alloys tend to oxidize easily, experience loss of its mechanical properties at elevated temperature especially above 400 0C and undergo creep at temperature above 300 0C. These drawbacks of the alloy have justified the reason for incorporating alumina into its structures to augment its properties by the transfer of load from the metallic based matrix to the alumina particulate and inhibiting dislocation motion by pinning effects of the alumina particles. In order to achieve the effective transfer of load from the matrix materials to the reinforcement phase, there must be homogeneous dispersion of the particulate phase within the matrix phase and good consolidation technique with appropriate processing parameters to form a composite structure with improved properties than the matrix material must be adopted. Recently, spark plasma sintering has attracted the interest of researchers because of its flexibility, ease of operation and ability to consolidate powder materials within limited time. This consolidation technique proffers numerous benefits than conventional sintering techniques because it allows the regulation of sintering energy with limited grain growth and support hot compaction up to full densification. In a bid to synthesize alumina reinforced Ti6Al4V composites, spark plasma sintering technique is adopted in this study. The dispersion characteristics of the alumina in Ti6Al4V and sintering behaviour of the sintered composites were evaluated to understand the sintering phenomena associated with the fabrication of the composites. 2. Materials and Method 2.1 Starting Materials The alumina powders used for this study was supplied by supplied by Glass blown & Volumetric Glassware & Chemicals in Johannesburg, South Africa with percentage purity of (~97.7), it has particle sizes varying from 12 to 45µm. An argon atomized, prealloyed Ti6Al4V powders with particle size ~25 μm supplied by TLS Technik GmbH & Co. Germany was used as the matrix material. 2.2 Dispersion of Al2O3 in Ti6Al4V powders The alumina powders of different volume fraction (5, 10 and 15 vol.%) were weighed and dispersed into Ti6Al4V powders using a low energy ball mill (Retsch 100 PM, Germany) with the following milling parameters, milling speed of 150 revolution per minute (rpm), time of 6 hours and ball to powder ration (BPR) of 10:1. The mixing operation was carried out using alumina balls of 5 mm diameter to enhance collisions of the balls with the powders . This mixing operation was done at an interval of 10 minutes for every 10 minutes of mixing to prevent the powders from overheating and prevent unwanted reactions between the powders during mixing operation. 2.3 Sintering of the Admixed Powders The mixed composite powders were measured and poured into a Ø 20 mm graphite die with a gauged height of 5 mm and cold compacted with a load of 10 KN to promote good electrical conductivity between the powders and the die. The pre-compacted composites are then transferred into a spark plasma sintering (model HHPD-25, FCT Germany) chamber where sintering operation was carried out. The composite mixture was sintered at a sintering temperature of 1000 oC and 1100 oC respectively. Other sintering parameters adopted are pressure of 50 MPa, heating rate of 100 oC/min and holding time of 10 minutes. The sintered samples were collected from the sintering chamber after cooling and sand blasted to remove graphite foils from the sintered samples. The sintering data of the samples were acquired and evaluated.
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2.4 Characterization of starting, admixed powders and sintered composites The morphology of both the starting, admixed powders and sintered composites were examined under scanning electron microscopy (SEM), model TESCAN furnished with energy dispersive X-ray spectrometry (EDX). The phase identification and crystalline phases of starting and mixed powders and sintered composites were characterized by X-ray diffractometer (XRD, Rigaku D/max-rB). This was carried out by a scan using Cu-Kα (λ= 0.154 nm) radiation at a scanning rate of 1°/min over the angular range of 10–90°. 3. Results and Discussion 3.1 Dispersion characteristics and microstructural evolution of the composites The effectiveness of the low energy ball milling adopted during the dispersion was adjudged by the morphology and structures of the starting, admixed and sintered composites acquired from the SEM analysis. Figure 1 (a-d) depict SEM images and EDS of the starting powders. From figure 1(a &c), it was observed that the morphology of the Ti6Al4V are spherical with even shapes and they have an average particle size of (~26 µm) alongside the EDS which confirm the elemental composition of the Ti6Al4V. Similarly, figure 1(b &d) shows the morphology of the Al2O3 which depict irregular particles with coarse surface and the EDS confirmed the elemental composition of the alumina.
Fig.1. Illustrates the SEM images of the starting powders, Ti6Al4V(a), Al2O3 (b) and their accompanying EDS (c & d).
Additionally, figure 2 (a-c) shows the SEM images of the admixed powder. From the SEM images in figure 2, it was observed that the alumina particles are adhered to the Ti6Al4V particles which gave the metal matrix particles a coarse morphology. The SEM images of the admixed powders also show that there was increase in the amount of the alumina particles adhered to the Ti6Al4V particles when the volume fraction of the alumina was increased from 5 vol.% to 15 vol.%. The adhesion of the alumina particles to the Ti6Al4V particles was ascribed to the adequate impact energy exerted from the alumina balls on the powders during mixing which resulted in good bonding between the powders.
Fig.2. Illustrates the SEM images of the Admixed composite powders (Al2O3-Ti6Al4V) containing 5 vol.% (a), 10 vol. % (b) and 15 vol. % (c)
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Fig.3. Illustrates the SEM images of the sintered Ti6Al4V at 1000 oC (a), 1100 oC (b), 5 vol.% Al2O3- Ti6Al4V at 1000 oC (c), 1100 oC (d), 10 vol.% Al2O3- Ti6Al4V at 1000 oC (e), 1100 oC (f), and 15 vol.% Al2O3- Ti6Al4V Composites at 1000 oC (g) and1100 oC (h).
In a bid to consolidate the composites powders, the admixed powders were sintered at 1000 oC and 1100 oC and the SEM images in figure 3 shows the morphology of the sintered titanium alloy and the composites. From the SEM images shown in figure 3 (a-h), the SEM images in figure 3 (a & b) represent the micrograph of Ti6Al4V sintered at 1000 oC and 1100 oC respectively. It was observed that the microstructures were made up of the different phases[11]. The presence of aluminium helps to stabilize the alpha phase and vanadium stabilizes the beta phase [12, 13]. In addition, there are more lamellar structures on the sample sintered at 1000 oC however, when the sintering temperature was increased to 1100 oC, it was observed that the lamellar structures were gradually transformed to flakelike structure this transformation is ascribed to the higher sintering temperature which resulted in the change of the microstructure. Similarly, the SEM images in figure 3 (c & d) depict the composites containing 5 vol.% of alumina that was sintered at 1000 oC and 1100 oC respectively. From the SEM micrographs, it was observed that the alumina particles settled at the grain boundaries of the Ti6Al4V grains. However, when the volume fraction was increased from 5 to 15 vol.%, it was observed that the alumina particles diffused into the alpha phase of the titanium alloy. From all indications in the SEM images of the sintered composites, it was observed that
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the alumina particles were uniformly dispersed in the titanium alloy with good interfacial bonding which will assist in inhibiting dislocation motion by particle strengthening mechanism during service application of the synthesized composites. The microstructural evolution of the sintered composites also indicated that there were interfacial interaction between the alumina and the Ti6Al4V structure during sintering. 3.2 Phase analysis of the composites The phase analysis of the starting, admixed powders and sintered composites at varying sintering temperatures was conducted to understand the transformation of the titanium alloy by the addition of alumina. Figure 4 below shows the XRD pattern of the powders and sintered composites.
Fig.4. Illustrates the XRD pattern of the starting powders Al2O3 (a), Ti6Al4V(b), admixed composite powders with varying amount of alumina (b), sintered composites at 1000 oC (c) and at 1100 oC (d)
From the XRD pattern in figure 4 (a), which indicates the XRD spectra of pristine alumina, various Corundum (crystalline) alumina phases were observed at 2θ = 25.55o ,35.11 o, 37.74 o, 43.31 o, 52.50 o, 57.44 o, 59.68 o, 61.06 o , 66.45 o, 68.14 o,77.8 o, 80.33 o, 84.26 o, 86.25 o and 88.90 o which corresponds to the following planes (012), (104), (110), (006), (024), (116), (211), (122), (116), (300), (119), (217), (223), (312) and (102) respectively. From the XRD pattern of the alumina reinforced Ti6Al4V admixed powders in figure 4(b), it was glaring that the crystalline alumina became more pronounced with the increased in alumina volume fraction. However, only alpha titanium phases were observed from the XRD pattern of the Ti6Al4V patterns. Similarly, figure 4(c &d) shows the XRD pattern of the composites sintered at 1000 oC and 1100 oC respectively. From the XRD patterns of the sintered composites, it was observed that the crystalline alumina peaks were more pronounced on the titanium alloy. This may be as a result of increase in temperature during sintering which favors homogeneity between the composites components. However, it was observed that the prominent peaks in the sintered composites tend to decrease with the increase in volume fraction of the alumina. Additionally, the presence of alumina phases in the composites were pronounced with increase in sintering temperature. 3.3 Sintering Behaviours of Al2O3-Ti6Al4V Composites In a bid to understand the sintering behaviours of the titanium alloy and the sintered composites, the sintering data were collected and analyzed after spark plasma sintering. Figure 5 (a-d) shows the sintering behavours of the titanium alloy and the synthesized composites at distinct sintering temperature ( 1000 oC & 1100 oC) and the
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displacement of the punches during sintering process which depicts the deformation of the samples under the influence of pressure MPa and the varying sintering temperature. From figure 5 (a-d) which showed the sintering behavior of the titanium alloy and the synthesized composites sintered at 1000 oC, it was observed that the temperature was maintained at 250 oC for 450 seconds (s) before it increased to 1000 oC for the synthesized composites. However, the temperature was maintained at 250 oC for 600s for the titanium alloy before it increased to 1000 oC. This behavior may be ascribed to the high thermal conductivity of the alumina particles which promotes the rapid heating during the synthesis of the composites. Additionally, when the samples were sintered at higher sintering temperature as shown in figure 5(b), both the sintered alloy and composites started the sintering cycle at almost the same time, however, the samples sintered at higher temperature completed the sintering cycle at lesser time and this may be ascribed to the higher sintering temperature which activated the sintering rate that resulted in the speedy consolidation process.
Fig.5. Illustrates the sintering behaviour of titanium alloy and the synthesized composites (a &b) and the displacement curves of the punches (c &d) during sintering at 1000 oC and 1100 oC respectively.
Similarly, figure 5(c &d) depict the displacement of the graphite punches during sintering of the titanium alloy and the synthesized composites. From all indication, it was observed that the samples sintered at 1100 oC exhibits fast sintering process. However, both samples sintered at different sintering temperature shows that the displacement of the punches increased from 0 to 3.5 mm because of the exerted pressure on the materials. The short displacement observed during sintering was due to the cold compaction that was done on the powders before sintering process. From the displacement curves in figure 5 (c), it was observed that the applied pressure was not effective in aiding the displacement of the punches at 0 to 14 mins for the titanium alloy and this is due to low temperature at that time, however, for the synthesized composites which has alumina content with good thermal conductivity, the displacement of the punches was observed at lesser time (10 min) even at low temperature. This fast displacement of the punches was ascribed to the composites powders with improved thermal conductivity that aided the sintering rate of the composites. When the temperature was increased, the effect of the applied pressure was dominant that resulted in the huge displacements of the punches thereby resulting in the rearrangements of the particles [14]. The effectiveness of the exerted pressure during sintering promotes the compaction of the samples by aiding the escape of trapped gases present in the sample during cold compaction. It was observed from the curves that the displacement was continuous after the escape of the trapped gas from the samples, thereby showing the three stages of the sintering process (heating, holding and cooling). Apart from the rearrangement of the powder particles, the sintering process also aid grain growth and necking of the particles as a result of bulk mass transport mechanisms thereby causing grain boundary diffusion, plastic flow and dislocation climb and overall deformation of the material [15, 16].
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4. Conclusion The adoption of low energy ball milling technique is an effective method of dispersing alumina in titanium alloy since it resulted in homogeneous dispersion and good interfacial bonding of alumina in Ti6Al4V. Additionally, the incorporation of alumina into titanium alloy promotes the sinterability of the composites. The compressive pressure during sintering is ineffective to result in the movement of the punches at lower temperature. However, at higher sintering temperature, there were continuous displacement of the punches which result in rapid compaction of the samples. Acknowledgement The authors would like to appreciate the National research foundation of South Africa in association with the world academic of science (NRF-TWAS) and Global Excellence and Stature for funding this research. References [1] S. Moustafa, Z. Abdel-Hamid, A. Abd-Elhay, Copper matrix SiC and Al2O3 particulate composites by powder metallurgy technique, Materials Letters, 53 (2002) 244-249. [2] H. Simchi, A. Simchi, Tensile and fatigue fracture of nanometric alumina reinforced copper with bimodal grain size distribution, Materials Science and Engineering: A, 507 (2009) 200-206. [3] V. Rajkovic, D. Bozic, J. Stasic, H. Wang, M.T. Jovanovic, Processing, characterization and properties of copper-based composites strengthened by low amount of alumina particles, Powder Technology, 268 (2014) 392-400. [4] D. Ying, D. Zhang, Processing of Cu–Al2O3 metal matrix nanocomposite materials by using high energy ball milling, Materials Science and Engineering: A, 286 (2000) 152-156. [5] A. Kurşun, E. Bayraktar, H.M. Enginsoy, Experimental and numerical study of alumina reinforced aluminum matrix composites: Processing, microstructural aspects and properties, Composites Part B: Engineering, 90 (2016) 302-314. [6] Y. Iwai, T. Honda, T. Miyajima, Y. Iwasaki, M. Surappa, J. Xu, Dry sliding wear behavior of Al2O3 fiber reinforced aluminum composites, Composites science and technology, 60 (2000) 1781-1789. [7] E. Ghasali, M. Alizadeh, K. Shirvanimoghaddam, R. Mirzajany, M. Niazmand, A. Faeghi-Nia, T. Ebadzadeh, Porous and non-porous alumina reinforced magnesium matrix composite through microwave and spark plasma sintering processes, Materials Chemistry and Physics, 212 (2018) 252-259. [8] H. Li, L. Cheng, X. Sun, Y. Li, B. Li, C. Liang, H. Wang, J. Fan, Fabrication and properties of magnesium matrix composite reinforced by urchin-like carbon nanotube-alumina in situ composite structure, Journal of Alloys and Compounds, 746 (2018) 320-327. [9] A. Okoro, R. Machaka, S. Lephuthing, M. Awotunde, P. Olubambi, Structural integrity and dispersion characteristics of carbon nanotubes in titanium-based alloy, IOP Conference Series: Materials Science and Engineering, IOP Publishing, 2018, pp. 012004. [10] A. Okoro, M. Awotunde, O. Ajiteru, S. Lephuthing, P. Olubambi, R. Machaka, Effects of carbon nanotubes on the mechanical properties of spark plasma sintered titanium matrix composites—A review, Mechanical and Intelligent Manufacturing Technologies (ICMIMT), 2018 IEEE 9th International Conference on, IEEE, 2018, pp. 54-59. [11] D.M. Brunette, P. Tengvall, M. Textor, P. Thomsen, Titanium in medicine: material science, surface science, engineering, biological responses and medical applications, Springer Science & Business Media, 2012. [12] L. Da Silva, M. Ueda, M. Silva, E. Codaro, Corrosion behavior of Ti–6Al–4V alloy treated by plasma immersion ion implantation process, Surface and Coatings Technology, 201 (2007) 8136-8139. [13] H.-q. Duan, Y.-f. Han, W.-j. LÜ, J.-w. Mao, L.-q. Wang, D. Zhang, Effect of solid carburization on surface microstructure and hardness of Ti-6Al-4V alloy and (TiB+ La2O3)/Ti-6Al-4V composite, Transactions of Nonferrous Metals Society of China, 26 (2016) 1871-1877. [14] O.E. Falodun, B.A. Obadele, S.R. Oke, M.E. Maja, P.A. Olubambi, Effect of sintering parameters on densification and microstructural evolution of nano-sized titanium nitride reinforced titanium alloys, Journal of Alloys and Compounds, 736 (2018) 202-210. [15] A.M. Okoro, S.S. Lephuthing, S.R. Oke, O.E. Falodun, M.A. Awotunde, P.A. Olubambi, A Review of Spark Plasma Sintering of Carbon Nanotubes Reinforced Titanium-Based Nanocomposites: Fabrication, Densification, and Mechanical Properties, JOM, 1-18. [16] Z. Zhang, X. Shen, F. Wang, S. Lee, L. Wang, Densification behavior and mechanical properties of the spark plasma sintered monolithic TiB2 ceramics, Materials Science and Engineering: A, 527 (2010) 5947-5951.