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Consolidation of Ti6Al4V alloy and refractory nitride nanoparticles by spark plasma sintering method: Microstructure, mechanical, corrosion and oxidation characteristics
Materials Science & Engineering A 774 (2020) 138920
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Consolidation of Ti6Al4V alloy and refractory nitride nanoparticles by spark plasma sintering method: Microstructure, mechanical, corrosion and oxidation characteristics J.O. Abe a, b, *, A.P.I. Popoola a, O.M. Popoola b a b
Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, P.M.B. X680, Pretoria, South Africa Centre for Energy and Electric Power, Tshwane University of Technology, Pretoria, South Africa
A R T I C L E I N F O
A B S T R A C T
Keywords: Spark plasma sintering Refractory nitrides Densification Microindentation Linear polarization Thermal gravimetric analysis
Ti6Al4V alloy combines excellent mechanical and chemical characteristics attractive in some important appli cations in the automotive and aerospace industries but its insufficient hardness and high-temperature oxidation subdue its extensive use. This work was undertaken to modify the microstructure and improve the current limitation of Ti6Al4V alloy with the intention of not affecting its desirable properties. Therefore, the effects of different refractory nitrides: aluminium nitride (AlN), titanium nitride (TiN) and hexagonal boron nitride (h-BN) reinforcements on the microstructure, mechanical, chemical and oxidation properties of spark plasma sintered Ti6Al4V-based binary composites were investigated. Spark plasma sintering technique was effectively utilized to consolidate the Ti6Al4V powder and 3 wt% nanoparticles of AlN, TiN and h-BN respectively. The microstructure and phase composition of the sintered composites were examined by scanning electron microscopy, optical microscopy, and X-ray diffractometry; densification was evaluated according to Archimedes’ principle and microhardness was measured by Vickers’ microhardness test. The corrosion and oxidation behaviour of the samples were studied by linear polarization experiment and thermal gravimetric analysis respectively. It was found that the binary composites produced attained almost full theoretical relative densification (98.23–99.54%) due to adequate diffusional mass transport in solidly bonded particles at the matrix-reinforcement interfaces. Ti6Al4V-3h-BN composite gave the optimal combination of relative densification (99.54%) and microhardness (7030 MPa) which exceeds 200% of the monolithic alloy and about 48% superior to both composites with AlN and TiN reinforcements. The yield and ultimate tensile strengths of the matrix were improved by approximately 47% via the AlN and TiN nanoparticle additions and significantly by 116% through the nano-h-BN addition. Ti6Al4V–3AlN demonstrated most superior electrochemical corrosion resistance with a current density of 3.66 μA/cm2 and a polarization resistance of 8760.2 Ω in the sulphuric acid medium while Ti6Al4V–3TiN with the least normalized weight gain of 0.85 mg/cm2 showed the greatest resistance against oxidation in the hightemperature oxidizing environment.
1. Introduction Titanium and its alloys have gained considerable prominence in automotive and aerospace applications including airframe structures, landing gear and jet engine components owing to their exceptional combination of properties: low density, high structural properties (e.g. specific strength, stiffness, toughness and fatigue resistance), creep strength, corrosion resistance and high-temperature properties [1–3]. These alloys also can retain their mechanical properties at high
temperature (up to 500–600 � C), which exceed the limit of the operating temperature of lightweight aerospace materials such as aluminium al loys, magnesium alloys and fiber–polymer composites. Although heat-resistant steels and nickel alloys were originally the chosen mate rials in the early development of jet engines, these alloys are too heavy, and replacing them with titanium alloys, especially in discs and fan blades makes it possible for substantial weight savings [3,4]. Among the different compositions of titanium alloys in use today, the most versatile is Ti6Al4V. The alloy is comprised of both α-Ti and β-Ti, and therefore,
* Corresponding author. Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, P.M.B. X680, Pretoria, South Africa. E-mail address: [email protected] (J.O. Abe). https://doi.org/10.1016/j.msea.2020.138920 Received 28 October 2019; Received in revised form 28 December 2019; Accepted 3 January 2020 Available online 7 January 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
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regarded as (α þ β)-Ti alloy. The α-Ti is responsible for the high toughness and creep resistance while the β-Ti is responsible for the high strength and fatigue resistance of the alloy. Ti6Al4V alloy accounts for over 50% the sales of titanium in the United States, Europe and South Africa, and is the most used titanium alloy in aircraft. It makes up about 60% of the titanium used in jet engines and up to 80–90% for airframes [4,5]. Despite the obvious benefits, extensive applications of Ti6Al4V alloy in the automotive and aerospace industries have been curtailed because of low hardness leading to severe adhesive wear and insufficient high-temperature oxidation resistance [1,6–8]. Over the last few de cades, the researchers in the field have done a great deal of work in the development of composites in enhancing the identified limitations. One outstanding achievement so far is the consolidation of Titanium Matrix Composites (TMCs) with different compositions of ceramic re inforcements [9–13]. These TMCs have good wear resistance, high hardness and high strength stability through the added reinforcements, exhibit advanced mechanical and chemical properties. Ceramic rein forced TMCs have been in use for quite some time now, which may characteristically involve continuous (array of filaments or fibers) or discontinuous (particles or chopped fibers) reinforcement types. How ever, the particulate ceramic reinforced TMCs are of keen interest due to their ease of fabrication, lower cost and more isotropic properties. An optimum set of mechanical properties tends to be obtained when fine and thermally stable ceramic particulates are dispersed in a TMC [7,9, 13,14]. The commonly used ceramic reinforcements include the car bides, oxides, borides and nitrides [11,13,15,16]. Recent studies have shown tremendous advantages of applications of advanced refractory nitride reinforcements in the development of materials targeted for use in the aerospace industries. For example, Ray et al. [10] fabricated Al-based composites with 10 and 30 wt% titanium nitride (TiN) by using pressureless sintering and hot-pressing techniques. The presence of ti tanium nitride (TiN) particles at grain boundaries played an important role in improving the densification and thus, resulted in improved me chanical properties. Xiang et al. [13] also studied the characteristics of Ti6Al4V/TiB composite prepared via direct laser fabrication by injection of pre-mixed powders. The authors concluded that the modulus of elasticity, yield strength, ultimate strength and wear resistance increased by the presence of TiB particles in the monolithic alloy. Re fractory nitrides show unusual combination of mechanical and chemical properties which are very attractive in applications involving high temperature and/or corrosive environments where most metals are no longer adequate. They are characterized by high hardness, wear resis tance, high melting points (>1800 � C) and good chemical resistance. These nitrides include the nitrides of Ti, V, Zr, Nb, Hf, Ta, B, Al and Si [10,17,18]. Moreover, the researchers have also been able to improve the properties of titanium and its alloys via numerous processing routes adopted in the synthesis of TMCs [11]. The powder metallurgy route has been reported frequently used involving various sintering techniques such as hot isostatic pressing (HIP), hot extrusion and hot-pressing methods to accomplish improved mechanical and physical properties [10]. Yu et al. [19] fabricated Ti6Al4V-3.5 vol %TiB composite via pre-sintering and hot extrusion process and stated that sintering tem peratures and time played significant roles on the densification and mechanical properties. Choe et al. [20] likewise produced Ti–Al–4V reinforced with 7.5 mass% TiC and 7.5 mass% W hybrid composites via combined actions of cold and hot isostatic pressing. The authors iden tified superior mechanical properties including hardness, tensile strength and yield strength in the fabricated composite but with a reduced ductility compared to its monolithic matrix and other com posites. However, Cao et al. [21] and Matizamhuka [22] reported the frequent challenges manufacturers encounter in the synthesis of nano sized materials by conventional sintering methods. More recently, an advanced sintering technique known as spark plasma sintering (SPS) has been developed which has been proven very effective [20]. In addition, SPS offers rapid densification, with minimal grain growth, enhanced
microstructure combined with improved mechanical properties of a wide range of powders, which include ceramics, polymers, composites, metallic-based and more in lesser sintering times when compared to the conventional hot-pressing technology [22]. For example, Al and BN are two dissimilar materials reported by Du et al. [23] that cannot be pro duced in the form of a composite by the conventional powder metal lurgical technique were successfully manufactured Firestein et al. [24] where Al-based composite with nano-BN particles reinforcement was synthesized through SPS method and the microstructure in conjunction with the mechanical properties were characterized. Kgoete et al. [15] studied the effect of SPS on the microstructure and corrosion behaviour of Si3N4 additions on Ti6Al4V alloy and Kgoete et al. [16] presented the data on the influence of TiN particles on wear and corrosion properties of Ti6Al4V alloy produced by SPS as well. In all these studies, remark able improvement in the mechanical and chemical properties of the metal matrix composites (MMCs) was achieved. However, the compar ative influence of three different refractory nitride nanoparticles comprising AlN, TiN and h-BN on the properties of Ti6Al4V matrix composites via SPS method has not yet been considered. Therefore, in the present study, spark plasma sintering technique was employed to fabricate the Ti6Al4V-based composites reinforced with nanoparticles of AlN, TiN and h-BN. The influence of the refractory nitride reinforcements on the microstructure, mechanical, chemical and oxidation characteristics of the composites were systematically investigated. 2. Experimental procedure In this work, four different powders were utilized. The commercially pure Ti6Al4V alloy powder (APS 60 μm and purity >99% supplied by TLS Technik, Germany) was used as the matrix and AlN, TiN and h-BN (APS 125 nm and purity >99% supplied by Hongwu International Group Ltd., China) were used as the reinforcements respectively with the SEM and XRD images shown in Figs. 1 and 2 respectively. Three sets of Ti6Al4V-based powders of the same weight with each consisting 3 wt% of each reinforcement and a control of pure Ti6Al4V were measured with a balance of high precision accuracy of 0.001 g and mixed in a tubular mixer rotating at a speed of 100 rpm for a duration of 10 h in order to achieve standardized mixture. During the mixing some steel balls of approximately φ8mm were incorporated into the powder container at the ball-to-powder weight ratio of 2:5 for better distribution of the reinforcement in the matrix [25]. The blended powders were weighed and poured into a φ30mm graphite die in order to produce composite samples of 10 mm thickness. Then the prepared powders were consolidated by using spark plasma sintering machine (SPS FCT Systeme GmbH, Germany) under argon atmosphere with an optimized process parameters: temperature of 1000 � C, pressure of 30 MPa, heating rate of 100 � C/min and dwell time of 10 min obtained from a Taguchi design and optimization experiment per formed separately prior to this work [26]. Afterward, the density measurements of the sintered samples were performed with a densitometer based on Archimedes’ principle where the arithmetic mean of six successive measurements was taken for each sample as the effective density and the theoretical density was calcu lated according to the rule of mixture. Subsequently, the relative densification of each sample was calculated as the ratio of the effective density and theoretical density multiplied by 100% [15,25,26]. The samples were prepared for metallographic examination by using P120 to P4000 grit papers for grinding and diamax powder of 1 μm in a solution for polishing until mirrorlike shine was achieved. Next, samples were etched with Kroll’s reagent and a scanning electron microscope (JEOL JSM 7600 F) equipped with energy dispersive spectroscopy (SEM/EDS), an optical microscope (Olympus BX 51 TRF, Japan) and Xray diffractometer (Philips PW1710) were used to examine the sample microstructure and phase characteristics. Microindentation hardness testing of the as-SPSed samples was 2
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Fig. 1. SEM micrographs of as-received powders: a. Ti6Al4V b. AlN c. TiN d. h-BN.
The electrochemical corrosion performance of the samples was studied in 0.5 M of H2SO4 by using linear polarization experiment with the aid of an Autolab Potentiostat (PGSTAT302 N) equipped with NOVA software version (1.10). The polarization curves were recorded at a start potential of 1.5 V to stop the potential of 1.5 V with a scan rate of 1 mV/s. The electrodes included the reference electrode of Ag/AgCl in saturated KCl solution, the counter electrode of platinum, and the working electrode. The exposed surface area of specimens was 1 cm2. Tafel extrapolation was employed to analyze the electrochemical corrosion results [27,28]. Finally, the oxidation behaviour of the sintered samples was assessed by using thermal gravimetric analysis (TGA). The samples were studied between the temperatures of 40 and 900 � C, at the heating rate of 10 � C/ min and feeding rate of 20 mL/min of air [29]. 3. Results and discussion 3.1. Microstructure and XRD analysis The as-received powders of Ti6Al4V alloy, AlN, TiN and h-BN were characterized by using SEM and XRD as shown in Fig. 1(a–d) and 2 (a-d) respectively. The SEM micrograph in Fig. 1a reveals non-porous spher ical particles of Ti6Al4V alloy, Fig. 1b shows wurzite-shaped particles of AlN, Fig. 1c clearly reveals a lumpy spherical particles of TiN and Fig. 1d presents the platy-hexagonal morphology of h-BN particles [30]. On the other hand, the XRD diffractogram of Ti6Al4V powder is depicted in Fig. 2a with a notable pristine α-Ti phase peaks at the corresponding diffraction angles, 2θ ¼ 35.22� , 38.49� , 40.38� , 53.31� , 63.58� , 71.03� and 77.41� [31]. The XRD diffractograms in Fig. 2(b–c) show clear and distinct peaks of crystalline AlN phase at 2θ ¼ 33.11� , 36.05� , 37.96� , 50.03� , 59.16� , 66.23� and 71.38� and crystalline TiN phases at 2θ ¼ 36.63� , 42.57� , 61.82� , 74.12� and 78.04� respectively. Likewise, Fig. 2d reveals a well-defined high intensity peaks depicting highly crystalline hexagonal BN phase at 2θ ¼ 26.78� , 41.64� , 43.56� , 50.18� , 55.19� , 75.94� and 82.12� exhibiting size range between 100 and 150 nm formed from the transformation of a highly disordered amorphous
Fig. 2. XRD plots of the as-received powders: a. Ti6Al4V b. AlN c. TiN d. h-BN.
performed by using dura scan diamond indenter Vickers microhardness tester (FUTURE-TECH FM-800, Japan) and 300gf indenting load for a duration of 15 s with 0.5 mm intervals. The arithmetic mean of five successive indentations was taken for each sample as the effective microhardness value of the samples. The yield strength (YS) and ulti mate tensile strength (UTS) of the composites were calculated from the results obtained for the Vickers’ hardness values and the strain hard ening coefficients of the composites by using Eqs. (1) and (2) respec tively [1]. � � H YS ¼ ð0:1Þn (1) 3 � UTS ¼
� H � n �n 2:9 0:217
(2) 3
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structure via isothermal annealing and presence of no other phases [32]. The SEM and EDS micrographs showing the morphologies and in tensity peaks of the samples: Ti6Al4V alloy, Ti6Al4V–3AlN, Ti6Al4V–3TiN and Ti6Al4V-3h-BN sintered at 1000 � C temperature, 30 MPa pressure, 100 � C/min heating rate and for dwelling time 10 min are designated in Fig. 3(a–d). A typical microstructure of Ti6Al4V alloy produced via spark plasma sintering is presented in Fig. 3(a1) with a clear display of two distinct phases comprising both the α-Ti and β-Ti phases. The stability of these dual phases is believed to be due to the presence of Al and V respectively as substantiated by the EDS peaks in Fig. 3(a2) at about the expected proportions. On the other hand, the microstructures of other composites as shown in Fig. 3 (b1-d1) are distinguished from the one observed in Fig. 3a by the different re inforcements. However, the peaks of the elements of the reinforcements could not be resolutely affirmed by EDS which may be attributed to its very low content added making them hard to detect [33]. Fig. 3(b1) reveals the presence of coarsened AlN within an (α þ β) Ti-rich matrix while Fig. 3(c1) shows evidence of a bimodal distribution of TiN ceramic delineated by a distinct pattern in the matrix and a bimodal micro structure. Fig. 3(d1) presented uniform distribution of h-BN in the ma trix and makes the microstructure of the sintered composite homogeneous. Nonetheless, it can be seen from Fig. 3 (a1-d1) that all the samples produced are free from porosities, cracks or other defects which tend to negatively affect their desirable properties. The observation shows that Ti6Al4V alloy and Ti6Al4V matrix composites with 3 wt % refractory nitride nanoparticles of AlN, TiN and h-BN could be fabricated effectively by using spark plasma sintering technique. The XRD plots of the sintered samples are stacked as shown in Fig. 4 (a–d). The XRD diffractogram in Fig. 4a confirms the presence of both α-Ti and β-Ti phases only [30,34]. The diffraction peaks of α-Ti phase can also be identified at the same positions in all the sintered samples at the diffraction angles, 2θ ¼ 35.80� , 40.18� , 52.73� , 63.00� , and 70.68� ) as depicted in Fig. 4 (b,c and d). This phenomenon may be due to the fact that all the samples have a common Ti6Al4V matrix. It can also be noticed that the high magnitude of the intensities of the as-received reinforcement nitride powder peaks shown in Fig. 2 (b, c and d) were drastically reduced after thorough dispersion in the Ti6Al4V matrix and sintering as evidenced in Fig. 4 (b, c and d). Moreover, it can be equally observed in Fig. 4 (b and c) that almost identical XRD peak positions and intensities of α-Ti phase (2θ ¼ 35.80� , 38.11� , 40.18� , 52.73� , 63.00� , and 70.68� ), VN phase (2θ ¼ 37.28� and 43.38� ) and TiN phase (2θ ¼ 42.48� ) are present in both Ti6Al4V–3AlN and Ti6Al4V–3TiN compos ites with an exception of the presence of AlN phase at 2θ ¼ 76.63� in Fig. 4b and another TiN phase at 2θ ¼ 78.08� in Fig. 4c which is an indication that both composites share some similarities in their com positions and physical and chemical properties [35]. However, Fig. 4d displays different XRD patterns comprising the phases of VN (2θ ¼ 36.40� , 42.18� , 36.40� and 42.18� ), AlN (2θ ¼ 37.99� ) and the charac teristic peaks of hexagonal BN phase at 2θ ¼ 26.37� and 41.59� which gives the reason for the superior structural properties (hardness and strength) of the Ti6Al4V-3h-BN composite. On the whole, the X-ray diffraction analysis has revealed that none of the diffractogram patterns in Fig. 4(a–d) shows formation of detrimental intermetallic peaks which indicates that no unfavourable interfacial reactions occurred between the Ti6Al4V matrix and the nitride reinforcements during the SPS operation [33].
The relative densification of the monolithic sintered alloy calculated from an effective density of 4.42 g/cm3 and theoretical density of 4.50 g/cm3 and Vickers microhardness were 98.22% and 3250 MPa respec tively. The micrograph in Fig. 5a shows a dominance of α-Ti lamellar (which is responsible for toughness and creep resistance) and little ev idence of prior β-Ti grain boundaries (which is responsible for high strength and fatigue resistance) in the (αþ β)Ti-rich matrix can be said to be the reason for the relatively low microhardness value of the alloy [4,5] as evidenced by the large size of the diamond indentation. The relative densification results (98.23–99.54%) obtained as presented in Table 1 indicate that the SPS consolidation technique applied to syn thesize the Ti6Al4V-based composites was effective and produced almost full theoretical densification (99.54%) with h-BN reinforcement. Fig. 5b presents the micrograph of a decent distribution of coarsened nanoparticles of AlN in the (α þ β) Ti-rich matrix with relative densifi cation value of 99.10% and increased microhardness value of 4790 MPa than the matrix as represented by a relatively moderate diamond indentation. The increased hardness can be traced to the presence of hard AlN phase which imparted more hardness [30]. The micrograph of Ti6Al4V–3TiN as shown in Fig. 5c with 98.23% relative densification and moderate microhardness value of 4750 MPa as depicted by the relatively smaller diamond indentation in the figure can be attributed to the formation of dendritic structure indicating a significantly coarsened α-Ti þ TiN grains and a relatively refined homogeneous dispersion at the grain boundaries of the (α þ β) Ti-rich matrix. Meanwhile, Ti6Al4V-3h-BN binary composite gave the optimal combination of relative densification and microhardness values of 99.54% and 7030 MPa which is more than 200% that of the monolithic alloy (3250 MPa) and about 48% superior to both composites with TiN (4750 MPa) and AlN (4790 MPa) reinforcements as well represented by the smallest size of the diamond indentation in Fig. 5d. This phenomenon can be attrib uted to adequate mixing of the powders where the nanoparticles of h-BN filled up pore spaces within the titanium alloy matrix which are the primary causes of poor densification and the effectiveness of the consolidation method which enables the development of dense products [36]. In addition, there is an indication of refined grains and better filling of the pores as observed in the microstructure (Fig. 3d1) which explain the reason for the complete densification [25]. Generally, it is found that the hardness property is proportional to the relative densi fication and facilitated by homogeneous distribution of the re inforcements and adequate diffusional mass transport resulting in solidly bonded particles at the interface between the matrix and the refractory nitride reinforcements [37]. Moreover, the significant improvement in the hardness property of the matrix through the various additions suggests that SPS facilitates the synthesis of refractory nitride nanoparticles of AlN, TiN and h-BN reinforced titanium matrix com posites with superior mechanical characteristics useful in the automo tive and aerospace industries. In accordance with the equation involving the yield strength (YS) and ultimate tensile strength (UTS) of a material to its hardness (H) and strain hardening coefficient (n) as proposed by Cahoon et al. [38] and applied by Ujah et al. [25] and Adebiyi and Popoola [1] in Eqs. (1) and (2) respectively, the yield and ultimate tensile strengths of the com posites were calculated from the results obtained for the Vickers’ hardness value for Ti6Al4V alloy (3250 MPa), Ti6Al4V–3AlN (4790 MPa), Ti6Al4V–3TiN (4750 MPa) and Ti6Al4V-3h-BN (7030 MPa) and the strain hardening coefficient was assumed to be 0.15 for all the ma terial since they are related in properties as ceramic reinforcements and do behave in almost the same manner under mechanical loading [38]. Thus, by using Eqs. (1) and (2) the yield and ultimate tensile strengths obtained are as follows: 770 MPa and 1060 MPa for Ti6Al4V alloy, 1130 MPa and 1563 MPa for Ti6Al4V–3AlN, 1121 MPa and 1550 MPa for Ti6Al4V–3TiN, 1659 MPa and 2293 MPa for Ti6Al4V-3h-BN respec tively. Fig. 7 shows the comparison of the yield and ultimate tensile strength results for the composite samples.
3.2. Densification and mechanical properties In order to reveal the dispersion of the reinforcement nanoparticles of AlN, TiN and h-BN and their effects on the densification and hardness property of the matrix, the optical micrographs of the sintered samples and their representative microindentations on the surface are presented in Fig. 5(a–d) respectively while the details of the results of the exper iment conducted for relative densification and microhardness mea surements are presented in Table 1 and Fig. 6. 4
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Fig. 3. SEM and EDS micrographs showing the morphologies and peaks of as-sintered samples: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN.
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Table 1 The results of the densification and microhardness measurements. Sample
Effective density (g/cm3)
Theoretical density (g/ cm3)
Relative densification (%)
Microhardness (MPa)
Ti6Al4V Ti6Al4V–3AlN Ti6Al4V–3TiN Ti6Al4V-3hBN
4.42 4.41 4.44 4.36
4.50 4.45 4.52 4.38
98.22 99.10 98.23 99.54
3250 4790 4750 7030
Fig. 4. XRD plots of the as-sintered samples: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN.
3.3. Electrochemical corrosion behaviour The electrochemical corrosion behaviour of the as-sintered samples was investigated in 0.5 M H2SO4. The potentiodynamic polarization curves of the samples are plotted in Fig. 8 and the extrapolated Tafel data for the experiment are presented in Table 2. The results obtained show that the different nanoparticle additions affect the chemical behaviour of the matrix in one way or the other. From the polarization results, the Ti6Al4V matrix itself showed good corrosion performance in the 0.5 M H2SO4 solution where the anodic and cathodic curves were characterized by moderately positive corrosion potential ( 0.53192 V) and low current density (7.10 μA/cm2) compared to other reinforced composites. This is an indication that a passivating film was formed
Fig. 6. Comparison of the relative densification and microhardness values of Ti6Al4V alloy, Ti6Al4V–3AlN, Ti6Al4V–3TiN and Ti6Al4V-3h-BN.
Fig. 5. Optical micrographs depicting the microstructure and representative diamond-shaped microindentation on the surface of: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN. 6
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respect to the unreinforced alloy. This resulted in a commensurate in crease in the polarization resistance from 3855.7 Ω to 8760.2 Ω and better corrosion resistance [39]. Similarly, Ti6Al4V–3TiN demonstrated an improved corrosion resistance than the matrix (but less when compared to Ti6Al4V–3AlN) by increased polarization resistance of 4338.8 Ω characterized by a slightly less positive corrosion potential ( 0.53986 V) and reduced current density (6.12 μA/cm2) [16]. On the other hand, Ti6Al4V-3h-BN displayed the least corrosion resistance with the least polarization resistance of 2714.4 Ω in the same corrosion me dium. The lower corrosion potential ( 0.53986 V) and increased cur rent density (9.76 μA/cm2) values of the samples which resulted in the outcome may be caused by debilitating chemistry or changes in the surface condition of the TMC, even though all the samples were pre pared by the same procedure because the reinforcement is naturally expected to be highly resistance to corrosion in normal acids [27]. Thus, Ti6Al4V–3AlN in this work presents better electrochemical corrosion resistance properties in the 0.5 M H2SO4 solution compared to the pure Ti6Al4V and other TMCs with 3 wt % reinforcements of TiN and h-BN.
Fig. 7. Comparison of the yield and ultimate tensile strength values of Ti6Al4V alloy, Ti6Al4V–3AlN, Ti6Al4V–3TiN and Ti6Al4V-3h-BN.
3.4. Oxidation behaviour The oxidation behaviour of the sintered samples was examined using thermal gravimetric analysis (TGA). The samples were heated in the air under a controlled atmosphere from 40 to 900 � C for a duration of 86 min. The heating rate of 10 � C/min was used while air was being constantly fed at 20 mL/min [27,29]. The samples were oxidized, and the weight gains of the composites were compared with that of the unreinforced alloy [40]. The weight gain was calculated as the change in weight (ΔW) of the sample tested per unit total surface area of the sample exposed (A), that is, ΔW/A also regarded as normalized or spe cific weight gain measured in the unit of mg/cm2 [41–44]. The oxidation behaviour of the samples between 45 and 900 � C as depicted by the plots of the oxidation temperature and time against the weight gains of the samples is presented in Fig. 9 and the comparison of the weight gains of the oxidized samples is shown in Fig. 10. The high affinity that Ti and Al have for oxygen at temperatures beyond 450 � C cause them to form stable thin oxide layers of titania, TiO2 and alumina, Al2O3. The prop erties of the oxidation products formed are of major consideration in determining the oxidation behaviour of the samples [43,44]. Thus, the predominant oxide layers increase in thickness in turns protect the surface of the samples against further destructive interaction with the oxidizing environment leading to improvement in the oxidation resis tance until about 600 � C where oxide diffusion layer develops [29,41, 42]. This phenomenon has been seen to be true for all the sintered samples in Fig. 9(a–d) following a parabolic kinetic pattern with time which suggests diffusion control process except for Ti6Al4V–3TiN in Fig. 9c which displays complex oxidation kinetics until at temperatures around 800 � C. At temperatures above 800 � C, paralinear oxidation occurs where higher temperatures diffusion by means of micro-cracks becomes significant resulting in weight gains for all the samples. The varying weight gains of all the samples (0.85–1.08 mg/cm2) is an indi cation that all the samples were someway affected by the oxidation process in varying degrees and low weight gain means that the sample demonstrates very good resistance to oxidation in the operating tem peratures [27,42]. The results obtained as seen in Fig. 10 show that there is a trend in the improvement of the oxidation behaviour from the TMC with the addition of 3 wt % of TiN (0.85 mg/cm2), followed by h-BN reinforcement (0.9 mg/cm2), unreinforced Ti6Al4V alloy (1.05 mg/cm2) and then to the AlN reinforcement (1.08 mg/cm2). This suggests that the AlN addition did not provide desirable protection against oxidation of Ti6Al4V alloy in the high-temperature environment, especially at tem peratures above 800 � C whereas h-BN addition showed better oxidation resistance [17,18]. Meanwhile, the sintered Ti6Al4V–3TiN with the least weight gain of 0.85 mg/cm2 indicated that the TiN reinforcement of the Ti6Al4V matrix offered the greatest high-temperature oxidation resistance attributable to the crucial role that higher concentration of Ti
Fig. 8. Potentiodynamic polarization curves of Ti6Al4V alloy, Ti6Al4V–3AlN, Ti6Al4V–3TiN and Ti6Al4V-3h-BN in 0.5 M H2SO4. Table 2 Potentiodynamic polarization extrapolated Tafel data for the experiment. Sample
Ti6Al4V alloy Ti6Al4V–3AlN Ti6Al4V–3TiN Ti6Al4V-3hBN
Ecorr (V)
0.53192 0.47333 0.53986 0.53355
jcorr (μA/ cm2)
Corrosion rate (mm/year)
Polarization resistance (Ω)
7.10 3.66 6.12 9.76
0.23451 0.11791 0.19584 0.3189
3855.7 8760.2 4338.8 2714.4
during the interaction between the corrosive environment and the ele ments that constituted the alloy. Aluminium and titanium have been reported to predominantly form stable passivating films of alumina, Al2O3 and titania, TiO2; and their presence in the alloy can be credited for its good corrosion resistance [27,29]. The film forms a boundary between the corrosive medium and the alloy which protects the surface from further corrosion. Furthermore, it was observed that there is a slight shift in potential from 0.53192 V to a more positive potential of 0.47333 V and current density from 7.10 μA/cm2 to a lower value of 3.66 μA/cm2 in the sample with 3 wt % reinforcement of AlN with 7
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Fig. 9. Plots depicting the oxidation behaviour of: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN with respect to temperature and time.
play in the formation of an adherent and protective TiO2 layer against insidious oxidation [40–42]. 4. Conclusion The effects of nanoparticle reinforcements of AlN, TiN and h-BN on the microstructure, mechanical, chemical and oxidation properties of spark plasma sintered Ti6Al4V-based composites have been investi gated. The following conclusions have been made: 1. The SEM images clearly revealed that the sintered composites are free from cracks, porosities and other defects; and the XRD results showed no traces of detrimental intermetallic phases which are potentially capable to impair the desirable properties of the com posites. This is a good indication that no unfavourable interfacial reactions occurred between the Ti6Al4V matrix and the nitride re inforcements during the SPS sintering and also suggests that Ti6Al4V matrix composites with 3 wt % refractory nitride nanoparticles of AlN, TiN and h-BN could be fabricated effectively by using spark plasma sintering technique. 2. The consolidation of Ti6Al4V powder and nanoparticles of 3 wt% of AlN, TiN and h-BN by SPS produced nearly full theoretical densifi cation (98.23–99.54%) of the sintered composites which supports the effectiveness of the consolidation technique enabled by homo geneous distribution of the reinforcements and adequate diffusional
Fig. 10. Comparison of the weight gains after oxidation of: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN.
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mass transport resulting in solidly bonded particles at the interface between the matrix and the refractory nitride reinforcements. 3. Ti6Al4V-3h-BN binary composite gave the optimal combination of relative densification and microhardness values of 99.54% and 7030 MPa which indicated an improvement of more than 200% of the microhardness of the monolithic alloy (3250 MPa) and about 48% superior to both composites reinforced with AlN (4790 MPa) and TiN (4750 MPa). Similarly, the yield and ultimate tensile strengths of the matrix were improved by approximately 47% via the AlN and TiN nanoparticle additions and significantly by 116% through the nanoh-BN addition. 4. The corrosion results revealed that Ti6Al4V–3AlN demonstrated most superior electrochemical corrosion resistance properties with current density of 3.66 μA/cm2 and polarization resistance of 8760.2 Ω to the unreinforced Ti6Al4V, and TMCs with 3 wt % re inforcements of TiN and h-BN respectively. 5. Among the sintered samples, Ti6Al4V–3TiN with the least normal ized weight gain of 0.85 mg/cm2 indicated that the TiN reinforce ment offered the greatest improvement in the Ti6Al4V alloy resistance against oxidation in the high-temperature environment.
[9] [10] [11] [12]
[13] [14] [15] [16] [17]
Data availability
[18]
The raw and processed data are not available at the time of sub mission because they are part of our ongoing research work.
[19]
Declaration of competing interest
[20]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[21] [22]
CRediT authorship contribution statement
[23]
J.O. Abe: Conceptualization, Investigation, Methodology, Data curation, Writing - original draft, Visualization. A.P.I. Popoola: Su pervision, Project administration. O.M. Popoola: Supervision, Writing review & editing.
[24]
[25]
Acknowledgment
[26]
The authors wish to thank the Department of Chemical, Metallur gical and Materials Engineering and Centre for Energy and Electric Power (CEEP), Tshwane University of Technology (TUT), Pretoria, South Africa for the support given all through this work. National Research Fund (NRF), South Africa is also appreciated.
[27] [28]
References
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Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Consolidation of Ti6Al4V alloy and refractory nitride nanoparticles by spark plasma sintering method: Microstructure, mechanical, corrosion and oxidation characteristics J.O. Abe a, b, *, A.P.I. Popoola a, O.M. Popoola b a b
Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, P.M.B. X680, Pretoria, South Africa Centre for Energy and Electric Power, Tshwane University of Technology, Pretoria, South Africa
A R T I C L E I N F O
A B S T R A C T
Keywords: Spark plasma sintering Refractory nitrides Densification Microindentation Linear polarization Thermal gravimetric analysis
Ti6Al4V alloy combines excellent mechanical and chemical characteristics attractive in some important appli cations in the automotive and aerospace industries but its insufficient hardness and high-temperature oxidation subdue its extensive use. This work was undertaken to modify the microstructure and improve the current limitation of Ti6Al4V alloy with the intention of not affecting its desirable properties. Therefore, the effects of different refractory nitrides: aluminium nitride (AlN), titanium nitride (TiN) and hexagonal boron nitride (h-BN) reinforcements on the microstructure, mechanical, chemical and oxidation properties of spark plasma sintered Ti6Al4V-based binary composites were investigated. Spark plasma sintering technique was effectively utilized to consolidate the Ti6Al4V powder and 3 wt% nanoparticles of AlN, TiN and h-BN respectively. The microstructure and phase composition of the sintered composites were examined by scanning electron microscopy, optical microscopy, and X-ray diffractometry; densification was evaluated according to Archimedes’ principle and microhardness was measured by Vickers’ microhardness test. The corrosion and oxidation behaviour of the samples were studied by linear polarization experiment and thermal gravimetric analysis respectively. It was found that the binary composites produced attained almost full theoretical relative densification (98.23–99.54%) due to adequate diffusional mass transport in solidly bonded particles at the matrix-reinforcement interfaces. Ti6Al4V-3h-BN composite gave the optimal combination of relative densification (99.54%) and microhardness (7030 MPa) which exceeds 200% of the monolithic alloy and about 48% superior to both composites with AlN and TiN reinforcements. The yield and ultimate tensile strengths of the matrix were improved by approximately 47% via the AlN and TiN nanoparticle additions and significantly by 116% through the nano-h-BN addition. Ti6Al4V–3AlN demonstrated most superior electrochemical corrosion resistance with a current density of 3.66 μA/cm2 and a polarization resistance of 8760.2 Ω in the sulphuric acid medium while Ti6Al4V–3TiN with the least normalized weight gain of 0.85 mg/cm2 showed the greatest resistance against oxidation in the hightemperature oxidizing environment.
1. Introduction Titanium and its alloys have gained considerable prominence in automotive and aerospace applications including airframe structures, landing gear and jet engine components owing to their exceptional combination of properties: low density, high structural properties (e.g. specific strength, stiffness, toughness and fatigue resistance), creep strength, corrosion resistance and high-temperature properties [1–3]. These alloys also can retain their mechanical properties at high
temperature (up to 500–600 � C), which exceed the limit of the operating temperature of lightweight aerospace materials such as aluminium al loys, magnesium alloys and fiber–polymer composites. Although heat-resistant steels and nickel alloys were originally the chosen mate rials in the early development of jet engines, these alloys are too heavy, and replacing them with titanium alloys, especially in discs and fan blades makes it possible for substantial weight savings [3,4]. Among the different compositions of titanium alloys in use today, the most versatile is Ti6Al4V. The alloy is comprised of both α-Ti and β-Ti, and therefore,
* Corresponding author. Department of Chemical, Metallurgical and Materials Engineering, Tshwane University of Technology, P.M.B. X680, Pretoria, South Africa. E-mail address: [email protected] (J.O. Abe). https://doi.org/10.1016/j.msea.2020.138920 Received 28 October 2019; Received in revised form 28 December 2019; Accepted 3 January 2020 Available online 7 January 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
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regarded as (α þ β)-Ti alloy. The α-Ti is responsible for the high toughness and creep resistance while the β-Ti is responsible for the high strength and fatigue resistance of the alloy. Ti6Al4V alloy accounts for over 50% the sales of titanium in the United States, Europe and South Africa, and is the most used titanium alloy in aircraft. It makes up about 60% of the titanium used in jet engines and up to 80–90% for airframes [4,5]. Despite the obvious benefits, extensive applications of Ti6Al4V alloy in the automotive and aerospace industries have been curtailed because of low hardness leading to severe adhesive wear and insufficient high-temperature oxidation resistance [1,6–8]. Over the last few de cades, the researchers in the field have done a great deal of work in the development of composites in enhancing the identified limitations. One outstanding achievement so far is the consolidation of Titanium Matrix Composites (TMCs) with different compositions of ceramic re inforcements [9–13]. These TMCs have good wear resistance, high hardness and high strength stability through the added reinforcements, exhibit advanced mechanical and chemical properties. Ceramic rein forced TMCs have been in use for quite some time now, which may characteristically involve continuous (array of filaments or fibers) or discontinuous (particles or chopped fibers) reinforcement types. How ever, the particulate ceramic reinforced TMCs are of keen interest due to their ease of fabrication, lower cost and more isotropic properties. An optimum set of mechanical properties tends to be obtained when fine and thermally stable ceramic particulates are dispersed in a TMC [7,9, 13,14]. The commonly used ceramic reinforcements include the car bides, oxides, borides and nitrides [11,13,15,16]. Recent studies have shown tremendous advantages of applications of advanced refractory nitride reinforcements in the development of materials targeted for use in the aerospace industries. For example, Ray et al. [10] fabricated Al-based composites with 10 and 30 wt% titanium nitride (TiN) by using pressureless sintering and hot-pressing techniques. The presence of ti tanium nitride (TiN) particles at grain boundaries played an important role in improving the densification and thus, resulted in improved me chanical properties. Xiang et al. [13] also studied the characteristics of Ti6Al4V/TiB composite prepared via direct laser fabrication by injection of pre-mixed powders. The authors concluded that the modulus of elasticity, yield strength, ultimate strength and wear resistance increased by the presence of TiB particles in the monolithic alloy. Re fractory nitrides show unusual combination of mechanical and chemical properties which are very attractive in applications involving high temperature and/or corrosive environments where most metals are no longer adequate. They are characterized by high hardness, wear resis tance, high melting points (>1800 � C) and good chemical resistance. These nitrides include the nitrides of Ti, V, Zr, Nb, Hf, Ta, B, Al and Si [10,17,18]. Moreover, the researchers have also been able to improve the properties of titanium and its alloys via numerous processing routes adopted in the synthesis of TMCs [11]. The powder metallurgy route has been reported frequently used involving various sintering techniques such as hot isostatic pressing (HIP), hot extrusion and hot-pressing methods to accomplish improved mechanical and physical properties [10]. Yu et al. [19] fabricated Ti6Al4V-3.5 vol %TiB composite via pre-sintering and hot extrusion process and stated that sintering tem peratures and time played significant roles on the densification and mechanical properties. Choe et al. [20] likewise produced Ti–Al–4V reinforced with 7.5 mass% TiC and 7.5 mass% W hybrid composites via combined actions of cold and hot isostatic pressing. The authors iden tified superior mechanical properties including hardness, tensile strength and yield strength in the fabricated composite but with a reduced ductility compared to its monolithic matrix and other com posites. However, Cao et al. [21] and Matizamhuka [22] reported the frequent challenges manufacturers encounter in the synthesis of nano sized materials by conventional sintering methods. More recently, an advanced sintering technique known as spark plasma sintering (SPS) has been developed which has been proven very effective [20]. In addition, SPS offers rapid densification, with minimal grain growth, enhanced
microstructure combined with improved mechanical properties of a wide range of powders, which include ceramics, polymers, composites, metallic-based and more in lesser sintering times when compared to the conventional hot-pressing technology [22]. For example, Al and BN are two dissimilar materials reported by Du et al. [23] that cannot be pro duced in the form of a composite by the conventional powder metal lurgical technique were successfully manufactured Firestein et al. [24] where Al-based composite with nano-BN particles reinforcement was synthesized through SPS method and the microstructure in conjunction with the mechanical properties were characterized. Kgoete et al. [15] studied the effect of SPS on the microstructure and corrosion behaviour of Si3N4 additions on Ti6Al4V alloy and Kgoete et al. [16] presented the data on the influence of TiN particles on wear and corrosion properties of Ti6Al4V alloy produced by SPS as well. In all these studies, remark able improvement in the mechanical and chemical properties of the metal matrix composites (MMCs) was achieved. However, the compar ative influence of three different refractory nitride nanoparticles comprising AlN, TiN and h-BN on the properties of Ti6Al4V matrix composites via SPS method has not yet been considered. Therefore, in the present study, spark plasma sintering technique was employed to fabricate the Ti6Al4V-based composites reinforced with nanoparticles of AlN, TiN and h-BN. The influence of the refractory nitride reinforcements on the microstructure, mechanical, chemical and oxidation characteristics of the composites were systematically investigated. 2. Experimental procedure In this work, four different powders were utilized. The commercially pure Ti6Al4V alloy powder (APS 60 μm and purity >99% supplied by TLS Technik, Germany) was used as the matrix and AlN, TiN and h-BN (APS 125 nm and purity >99% supplied by Hongwu International Group Ltd., China) were used as the reinforcements respectively with the SEM and XRD images shown in Figs. 1 and 2 respectively. Three sets of Ti6Al4V-based powders of the same weight with each consisting 3 wt% of each reinforcement and a control of pure Ti6Al4V were measured with a balance of high precision accuracy of 0.001 g and mixed in a tubular mixer rotating at a speed of 100 rpm for a duration of 10 h in order to achieve standardized mixture. During the mixing some steel balls of approximately φ8mm were incorporated into the powder container at the ball-to-powder weight ratio of 2:5 for better distribution of the reinforcement in the matrix [25]. The blended powders were weighed and poured into a φ30mm graphite die in order to produce composite samples of 10 mm thickness. Then the prepared powders were consolidated by using spark plasma sintering machine (SPS FCT Systeme GmbH, Germany) under argon atmosphere with an optimized process parameters: temperature of 1000 � C, pressure of 30 MPa, heating rate of 100 � C/min and dwell time of 10 min obtained from a Taguchi design and optimization experiment per formed separately prior to this work [26]. Afterward, the density measurements of the sintered samples were performed with a densitometer based on Archimedes’ principle where the arithmetic mean of six successive measurements was taken for each sample as the effective density and the theoretical density was calcu lated according to the rule of mixture. Subsequently, the relative densification of each sample was calculated as the ratio of the effective density and theoretical density multiplied by 100% [15,25,26]. The samples were prepared for metallographic examination by using P120 to P4000 grit papers for grinding and diamax powder of 1 μm in a solution for polishing until mirrorlike shine was achieved. Next, samples were etched with Kroll’s reagent and a scanning electron microscope (JEOL JSM 7600 F) equipped with energy dispersive spectroscopy (SEM/EDS), an optical microscope (Olympus BX 51 TRF, Japan) and Xray diffractometer (Philips PW1710) were used to examine the sample microstructure and phase characteristics. Microindentation hardness testing of the as-SPSed samples was 2
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Fig. 1. SEM micrographs of as-received powders: a. Ti6Al4V b. AlN c. TiN d. h-BN.
The electrochemical corrosion performance of the samples was studied in 0.5 M of H2SO4 by using linear polarization experiment with the aid of an Autolab Potentiostat (PGSTAT302 N) equipped with NOVA software version (1.10). The polarization curves were recorded at a start potential of 1.5 V to stop the potential of 1.5 V with a scan rate of 1 mV/s. The electrodes included the reference electrode of Ag/AgCl in saturated KCl solution, the counter electrode of platinum, and the working electrode. The exposed surface area of specimens was 1 cm2. Tafel extrapolation was employed to analyze the electrochemical corrosion results [27,28]. Finally, the oxidation behaviour of the sintered samples was assessed by using thermal gravimetric analysis (TGA). The samples were studied between the temperatures of 40 and 900 � C, at the heating rate of 10 � C/ min and feeding rate of 20 mL/min of air [29]. 3. Results and discussion 3.1. Microstructure and XRD analysis The as-received powders of Ti6Al4V alloy, AlN, TiN and h-BN were characterized by using SEM and XRD as shown in Fig. 1(a–d) and 2 (a-d) respectively. The SEM micrograph in Fig. 1a reveals non-porous spher ical particles of Ti6Al4V alloy, Fig. 1b shows wurzite-shaped particles of AlN, Fig. 1c clearly reveals a lumpy spherical particles of TiN and Fig. 1d presents the platy-hexagonal morphology of h-BN particles [30]. On the other hand, the XRD diffractogram of Ti6Al4V powder is depicted in Fig. 2a with a notable pristine α-Ti phase peaks at the corresponding diffraction angles, 2θ ¼ 35.22� , 38.49� , 40.38� , 53.31� , 63.58� , 71.03� and 77.41� [31]. The XRD diffractograms in Fig. 2(b–c) show clear and distinct peaks of crystalline AlN phase at 2θ ¼ 33.11� , 36.05� , 37.96� , 50.03� , 59.16� , 66.23� and 71.38� and crystalline TiN phases at 2θ ¼ 36.63� , 42.57� , 61.82� , 74.12� and 78.04� respectively. Likewise, Fig. 2d reveals a well-defined high intensity peaks depicting highly crystalline hexagonal BN phase at 2θ ¼ 26.78� , 41.64� , 43.56� , 50.18� , 55.19� , 75.94� and 82.12� exhibiting size range between 100 and 150 nm formed from the transformation of a highly disordered amorphous
Fig. 2. XRD plots of the as-received powders: a. Ti6Al4V b. AlN c. TiN d. h-BN.
performed by using dura scan diamond indenter Vickers microhardness tester (FUTURE-TECH FM-800, Japan) and 300gf indenting load for a duration of 15 s with 0.5 mm intervals. The arithmetic mean of five successive indentations was taken for each sample as the effective microhardness value of the samples. The yield strength (YS) and ulti mate tensile strength (UTS) of the composites were calculated from the results obtained for the Vickers’ hardness values and the strain hard ening coefficients of the composites by using Eqs. (1) and (2) respec tively [1]. � � H YS ¼ ð0:1Þn (1) 3 � UTS ¼
� H � n �n 2:9 0:217
(2) 3
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structure via isothermal annealing and presence of no other phases [32]. The SEM and EDS micrographs showing the morphologies and in tensity peaks of the samples: Ti6Al4V alloy, Ti6Al4V–3AlN, Ti6Al4V–3TiN and Ti6Al4V-3h-BN sintered at 1000 � C temperature, 30 MPa pressure, 100 � C/min heating rate and for dwelling time 10 min are designated in Fig. 3(a–d). A typical microstructure of Ti6Al4V alloy produced via spark plasma sintering is presented in Fig. 3(a1) with a clear display of two distinct phases comprising both the α-Ti and β-Ti phases. The stability of these dual phases is believed to be due to the presence of Al and V respectively as substantiated by the EDS peaks in Fig. 3(a2) at about the expected proportions. On the other hand, the microstructures of other composites as shown in Fig. 3 (b1-d1) are distinguished from the one observed in Fig. 3a by the different re inforcements. However, the peaks of the elements of the reinforcements could not be resolutely affirmed by EDS which may be attributed to its very low content added making them hard to detect [33]. Fig. 3(b1) reveals the presence of coarsened AlN within an (α þ β) Ti-rich matrix while Fig. 3(c1) shows evidence of a bimodal distribution of TiN ceramic delineated by a distinct pattern in the matrix and a bimodal micro structure. Fig. 3(d1) presented uniform distribution of h-BN in the ma trix and makes the microstructure of the sintered composite homogeneous. Nonetheless, it can be seen from Fig. 3 (a1-d1) that all the samples produced are free from porosities, cracks or other defects which tend to negatively affect their desirable properties. The observation shows that Ti6Al4V alloy and Ti6Al4V matrix composites with 3 wt % refractory nitride nanoparticles of AlN, TiN and h-BN could be fabricated effectively by using spark plasma sintering technique. The XRD plots of the sintered samples are stacked as shown in Fig. 4 (a–d). The XRD diffractogram in Fig. 4a confirms the presence of both α-Ti and β-Ti phases only [30,34]. The diffraction peaks of α-Ti phase can also be identified at the same positions in all the sintered samples at the diffraction angles, 2θ ¼ 35.80� , 40.18� , 52.73� , 63.00� , and 70.68� ) as depicted in Fig. 4 (b,c and d). This phenomenon may be due to the fact that all the samples have a common Ti6Al4V matrix. It can also be noticed that the high magnitude of the intensities of the as-received reinforcement nitride powder peaks shown in Fig. 2 (b, c and d) were drastically reduced after thorough dispersion in the Ti6Al4V matrix and sintering as evidenced in Fig. 4 (b, c and d). Moreover, it can be equally observed in Fig. 4 (b and c) that almost identical XRD peak positions and intensities of α-Ti phase (2θ ¼ 35.80� , 38.11� , 40.18� , 52.73� , 63.00� , and 70.68� ), VN phase (2θ ¼ 37.28� and 43.38� ) and TiN phase (2θ ¼ 42.48� ) are present in both Ti6Al4V–3AlN and Ti6Al4V–3TiN compos ites with an exception of the presence of AlN phase at 2θ ¼ 76.63� in Fig. 4b and another TiN phase at 2θ ¼ 78.08� in Fig. 4c which is an indication that both composites share some similarities in their com positions and physical and chemical properties [35]. However, Fig. 4d displays different XRD patterns comprising the phases of VN (2θ ¼ 36.40� , 42.18� , 36.40� and 42.18� ), AlN (2θ ¼ 37.99� ) and the charac teristic peaks of hexagonal BN phase at 2θ ¼ 26.37� and 41.59� which gives the reason for the superior structural properties (hardness and strength) of the Ti6Al4V-3h-BN composite. On the whole, the X-ray diffraction analysis has revealed that none of the diffractogram patterns in Fig. 4(a–d) shows formation of detrimental intermetallic peaks which indicates that no unfavourable interfacial reactions occurred between the Ti6Al4V matrix and the nitride reinforcements during the SPS operation [33].
The relative densification of the monolithic sintered alloy calculated from an effective density of 4.42 g/cm3 and theoretical density of 4.50 g/cm3 and Vickers microhardness were 98.22% and 3250 MPa respec tively. The micrograph in Fig. 5a shows a dominance of α-Ti lamellar (which is responsible for toughness and creep resistance) and little ev idence of prior β-Ti grain boundaries (which is responsible for high strength and fatigue resistance) in the (αþ β)Ti-rich matrix can be said to be the reason for the relatively low microhardness value of the alloy [4,5] as evidenced by the large size of the diamond indentation. The relative densification results (98.23–99.54%) obtained as presented in Table 1 indicate that the SPS consolidation technique applied to syn thesize the Ti6Al4V-based composites was effective and produced almost full theoretical densification (99.54%) with h-BN reinforcement. Fig. 5b presents the micrograph of a decent distribution of coarsened nanoparticles of AlN in the (α þ β) Ti-rich matrix with relative densifi cation value of 99.10% and increased microhardness value of 4790 MPa than the matrix as represented by a relatively moderate diamond indentation. The increased hardness can be traced to the presence of hard AlN phase which imparted more hardness [30]. The micrograph of Ti6Al4V–3TiN as shown in Fig. 5c with 98.23% relative densification and moderate microhardness value of 4750 MPa as depicted by the relatively smaller diamond indentation in the figure can be attributed to the formation of dendritic structure indicating a significantly coarsened α-Ti þ TiN grains and a relatively refined homogeneous dispersion at the grain boundaries of the (α þ β) Ti-rich matrix. Meanwhile, Ti6Al4V-3h-BN binary composite gave the optimal combination of relative densification and microhardness values of 99.54% and 7030 MPa which is more than 200% that of the monolithic alloy (3250 MPa) and about 48% superior to both composites with TiN (4750 MPa) and AlN (4790 MPa) reinforcements as well represented by the smallest size of the diamond indentation in Fig. 5d. This phenomenon can be attrib uted to adequate mixing of the powders where the nanoparticles of h-BN filled up pore spaces within the titanium alloy matrix which are the primary causes of poor densification and the effectiveness of the consolidation method which enables the development of dense products [36]. In addition, there is an indication of refined grains and better filling of the pores as observed in the microstructure (Fig. 3d1) which explain the reason for the complete densification [25]. Generally, it is found that the hardness property is proportional to the relative densi fication and facilitated by homogeneous distribution of the re inforcements and adequate diffusional mass transport resulting in solidly bonded particles at the interface between the matrix and the refractory nitride reinforcements [37]. Moreover, the significant improvement in the hardness property of the matrix through the various additions suggests that SPS facilitates the synthesis of refractory nitride nanoparticles of AlN, TiN and h-BN reinforced titanium matrix com posites with superior mechanical characteristics useful in the automo tive and aerospace industries. In accordance with the equation involving the yield strength (YS) and ultimate tensile strength (UTS) of a material to its hardness (H) and strain hardening coefficient (n) as proposed by Cahoon et al. [38] and applied by Ujah et al. [25] and Adebiyi and Popoola [1] in Eqs. (1) and (2) respectively, the yield and ultimate tensile strengths of the com posites were calculated from the results obtained for the Vickers’ hardness value for Ti6Al4V alloy (3250 MPa), Ti6Al4V–3AlN (4790 MPa), Ti6Al4V–3TiN (4750 MPa) and Ti6Al4V-3h-BN (7030 MPa) and the strain hardening coefficient was assumed to be 0.15 for all the ma terial since they are related in properties as ceramic reinforcements and do behave in almost the same manner under mechanical loading [38]. Thus, by using Eqs. (1) and (2) the yield and ultimate tensile strengths obtained are as follows: 770 MPa and 1060 MPa for Ti6Al4V alloy, 1130 MPa and 1563 MPa for Ti6Al4V–3AlN, 1121 MPa and 1550 MPa for Ti6Al4V–3TiN, 1659 MPa and 2293 MPa for Ti6Al4V-3h-BN respec tively. Fig. 7 shows the comparison of the yield and ultimate tensile strength results for the composite samples.
3.2. Densification and mechanical properties In order to reveal the dispersion of the reinforcement nanoparticles of AlN, TiN and h-BN and their effects on the densification and hardness property of the matrix, the optical micrographs of the sintered samples and their representative microindentations on the surface are presented in Fig. 5(a–d) respectively while the details of the results of the exper iment conducted for relative densification and microhardness mea surements are presented in Table 1 and Fig. 6. 4
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Fig. 3. SEM and EDS micrographs showing the morphologies and peaks of as-sintered samples: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN.
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Table 1 The results of the densification and microhardness measurements. Sample
Effective density (g/cm3)
Theoretical density (g/ cm3)
Relative densification (%)
Microhardness (MPa)
Ti6Al4V Ti6Al4V–3AlN Ti6Al4V–3TiN Ti6Al4V-3hBN
4.42 4.41 4.44 4.36
4.50 4.45 4.52 4.38
98.22 99.10 98.23 99.54
3250 4790 4750 7030
Fig. 4. XRD plots of the as-sintered samples: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN.
3.3. Electrochemical corrosion behaviour The electrochemical corrosion behaviour of the as-sintered samples was investigated in 0.5 M H2SO4. The potentiodynamic polarization curves of the samples are plotted in Fig. 8 and the extrapolated Tafel data for the experiment are presented in Table 2. The results obtained show that the different nanoparticle additions affect the chemical behaviour of the matrix in one way or the other. From the polarization results, the Ti6Al4V matrix itself showed good corrosion performance in the 0.5 M H2SO4 solution where the anodic and cathodic curves were characterized by moderately positive corrosion potential ( 0.53192 V) and low current density (7.10 μA/cm2) compared to other reinforced composites. This is an indication that a passivating film was formed
Fig. 6. Comparison of the relative densification and microhardness values of Ti6Al4V alloy, Ti6Al4V–3AlN, Ti6Al4V–3TiN and Ti6Al4V-3h-BN.
Fig. 5. Optical micrographs depicting the microstructure and representative diamond-shaped microindentation on the surface of: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN. 6
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respect to the unreinforced alloy. This resulted in a commensurate in crease in the polarization resistance from 3855.7 Ω to 8760.2 Ω and better corrosion resistance [39]. Similarly, Ti6Al4V–3TiN demonstrated an improved corrosion resistance than the matrix (but less when compared to Ti6Al4V–3AlN) by increased polarization resistance of 4338.8 Ω characterized by a slightly less positive corrosion potential ( 0.53986 V) and reduced current density (6.12 μA/cm2) [16]. On the other hand, Ti6Al4V-3h-BN displayed the least corrosion resistance with the least polarization resistance of 2714.4 Ω in the same corrosion me dium. The lower corrosion potential ( 0.53986 V) and increased cur rent density (9.76 μA/cm2) values of the samples which resulted in the outcome may be caused by debilitating chemistry or changes in the surface condition of the TMC, even though all the samples were pre pared by the same procedure because the reinforcement is naturally expected to be highly resistance to corrosion in normal acids [27]. Thus, Ti6Al4V–3AlN in this work presents better electrochemical corrosion resistance properties in the 0.5 M H2SO4 solution compared to the pure Ti6Al4V and other TMCs with 3 wt % reinforcements of TiN and h-BN.
Fig. 7. Comparison of the yield and ultimate tensile strength values of Ti6Al4V alloy, Ti6Al4V–3AlN, Ti6Al4V–3TiN and Ti6Al4V-3h-BN.
3.4. Oxidation behaviour The oxidation behaviour of the sintered samples was examined using thermal gravimetric analysis (TGA). The samples were heated in the air under a controlled atmosphere from 40 to 900 � C for a duration of 86 min. The heating rate of 10 � C/min was used while air was being constantly fed at 20 mL/min [27,29]. The samples were oxidized, and the weight gains of the composites were compared with that of the unreinforced alloy [40]. The weight gain was calculated as the change in weight (ΔW) of the sample tested per unit total surface area of the sample exposed (A), that is, ΔW/A also regarded as normalized or spe cific weight gain measured in the unit of mg/cm2 [41–44]. The oxidation behaviour of the samples between 45 and 900 � C as depicted by the plots of the oxidation temperature and time against the weight gains of the samples is presented in Fig. 9 and the comparison of the weight gains of the oxidized samples is shown in Fig. 10. The high affinity that Ti and Al have for oxygen at temperatures beyond 450 � C cause them to form stable thin oxide layers of titania, TiO2 and alumina, Al2O3. The prop erties of the oxidation products formed are of major consideration in determining the oxidation behaviour of the samples [43,44]. Thus, the predominant oxide layers increase in thickness in turns protect the surface of the samples against further destructive interaction with the oxidizing environment leading to improvement in the oxidation resis tance until about 600 � C where oxide diffusion layer develops [29,41, 42]. This phenomenon has been seen to be true for all the sintered samples in Fig. 9(a–d) following a parabolic kinetic pattern with time which suggests diffusion control process except for Ti6Al4V–3TiN in Fig. 9c which displays complex oxidation kinetics until at temperatures around 800 � C. At temperatures above 800 � C, paralinear oxidation occurs where higher temperatures diffusion by means of micro-cracks becomes significant resulting in weight gains for all the samples. The varying weight gains of all the samples (0.85–1.08 mg/cm2) is an indi cation that all the samples were someway affected by the oxidation process in varying degrees and low weight gain means that the sample demonstrates very good resistance to oxidation in the operating tem peratures [27,42]. The results obtained as seen in Fig. 10 show that there is a trend in the improvement of the oxidation behaviour from the TMC with the addition of 3 wt % of TiN (0.85 mg/cm2), followed by h-BN reinforcement (0.9 mg/cm2), unreinforced Ti6Al4V alloy (1.05 mg/cm2) and then to the AlN reinforcement (1.08 mg/cm2). This suggests that the AlN addition did not provide desirable protection against oxidation of Ti6Al4V alloy in the high-temperature environment, especially at tem peratures above 800 � C whereas h-BN addition showed better oxidation resistance [17,18]. Meanwhile, the sintered Ti6Al4V–3TiN with the least weight gain of 0.85 mg/cm2 indicated that the TiN reinforcement of the Ti6Al4V matrix offered the greatest high-temperature oxidation resistance attributable to the crucial role that higher concentration of Ti
Fig. 8. Potentiodynamic polarization curves of Ti6Al4V alloy, Ti6Al4V–3AlN, Ti6Al4V–3TiN and Ti6Al4V-3h-BN in 0.5 M H2SO4. Table 2 Potentiodynamic polarization extrapolated Tafel data for the experiment. Sample
Ti6Al4V alloy Ti6Al4V–3AlN Ti6Al4V–3TiN Ti6Al4V-3hBN
Ecorr (V)
0.53192 0.47333 0.53986 0.53355
jcorr (μA/ cm2)
Corrosion rate (mm/year)
Polarization resistance (Ω)
7.10 3.66 6.12 9.76
0.23451 0.11791 0.19584 0.3189
3855.7 8760.2 4338.8 2714.4
during the interaction between the corrosive environment and the ele ments that constituted the alloy. Aluminium and titanium have been reported to predominantly form stable passivating films of alumina, Al2O3 and titania, TiO2; and their presence in the alloy can be credited for its good corrosion resistance [27,29]. The film forms a boundary between the corrosive medium and the alloy which protects the surface from further corrosion. Furthermore, it was observed that there is a slight shift in potential from 0.53192 V to a more positive potential of 0.47333 V and current density from 7.10 μA/cm2 to a lower value of 3.66 μA/cm2 in the sample with 3 wt % reinforcement of AlN with 7
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Fig. 9. Plots depicting the oxidation behaviour of: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN with respect to temperature and time.
play in the formation of an adherent and protective TiO2 layer against insidious oxidation [40–42]. 4. Conclusion The effects of nanoparticle reinforcements of AlN, TiN and h-BN on the microstructure, mechanical, chemical and oxidation properties of spark plasma sintered Ti6Al4V-based composites have been investi gated. The following conclusions have been made: 1. The SEM images clearly revealed that the sintered composites are free from cracks, porosities and other defects; and the XRD results showed no traces of detrimental intermetallic phases which are potentially capable to impair the desirable properties of the com posites. This is a good indication that no unfavourable interfacial reactions occurred between the Ti6Al4V matrix and the nitride re inforcements during the SPS sintering and also suggests that Ti6Al4V matrix composites with 3 wt % refractory nitride nanoparticles of AlN, TiN and h-BN could be fabricated effectively by using spark plasma sintering technique. 2. The consolidation of Ti6Al4V powder and nanoparticles of 3 wt% of AlN, TiN and h-BN by SPS produced nearly full theoretical densifi cation (98.23–99.54%) of the sintered composites which supports the effectiveness of the consolidation technique enabled by homo geneous distribution of the reinforcements and adequate diffusional
Fig. 10. Comparison of the weight gains after oxidation of: a. Ti6Al4V alloy b. Ti6Al4V–3AlN c. Ti6Al4V–3TiN d. Ti6Al4V-3h-BN.
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mass transport resulting in solidly bonded particles at the interface between the matrix and the refractory nitride reinforcements. 3. Ti6Al4V-3h-BN binary composite gave the optimal combination of relative densification and microhardness values of 99.54% and 7030 MPa which indicated an improvement of more than 200% of the microhardness of the monolithic alloy (3250 MPa) and about 48% superior to both composites reinforced with AlN (4790 MPa) and TiN (4750 MPa). Similarly, the yield and ultimate tensile strengths of the matrix were improved by approximately 47% via the AlN and TiN nanoparticle additions and significantly by 116% through the nanoh-BN addition. 4. The corrosion results revealed that Ti6Al4V–3AlN demonstrated most superior electrochemical corrosion resistance properties with current density of 3.66 μA/cm2 and polarization resistance of 8760.2 Ω to the unreinforced Ti6Al4V, and TMCs with 3 wt % re inforcements of TiN and h-BN respectively. 5. Among the sintered samples, Ti6Al4V–3TiN with the least normal ized weight gain of 0.85 mg/cm2 indicated that the TiN reinforce ment offered the greatest improvement in the Ti6Al4V alloy resistance against oxidation in the high-temperature environment.
[9] [10] [11] [12]
[13] [14] [15] [16] [17]
Data availability
[18]
The raw and processed data are not available at the time of sub mission because they are part of our ongoing research work.
[19]
Declaration of competing interest
[20]
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[21] [22]
CRediT authorship contribution statement
[23]
J.O. Abe: Conceptualization, Investigation, Methodology, Data curation, Writing - original draft, Visualization. A.P.I. Popoola: Su pervision, Project administration. O.M. Popoola: Supervision, Writing review & editing.
[24]
[25]
Acknowledgment
[26]
The authors wish to thank the Department of Chemical, Metallur gical and Materials Engineering and Centre for Energy and Electric Power (CEEP), Tshwane University of Technology (TUT), Pretoria, South Africa for the support given all through this work. National Research Fund (NRF), South Africa is also appreciated.
[27] [28]
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