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Characterisation of titanium–titanium boride composites processed by powder metallurgy techniques
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
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Characterisation of titanium–titanium boride composites processed by powder metallurgy techniques M. Selva Kumara,⁎, P. Chandrasekarb , P. Chandramohanb , M. Mohanrajc a
Department of Mechanical Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi-642003, India School of Engineering, Professional Group of Institutions, Coimbatore-641662, India c Department of Mechanical Engineering, Info Institute of Engineering, Coimbatore-641107, India b
AR TIC LE D ATA
ABSTR ACT
Article history:
In this work, a detailed characterisation of titanium–titanium boride composites processed
Received 8 November 2011
by three powder metallurgy techniques, namely, hot isostatic pressing, spark plasma
Received in revised form 5 July 2012
sintering and vacuum sintering, was conducted. Two composites with different volume
Accepted 20 July 2012
percents of titanium boride reinforcement were used for the investigation. One was titanium with 20% titanium boride, and the other was titanium with 40% titanium boride
Keywords:
(by volume). Characterisation was performed using X-ray diffraction, electron probe micro
Titanium–titanium
analysis — energy dispersive spectroscopy and wavelength dispersive spectroscopy, image
boride composites
analysis and scanning electron microscopy. The characterisation results confirm the
Hot isostatic pressing
completion of the titanium boride reaction. The results reveal the presence of titanium
Spark plasma sintering
boride reinforcement in different morphologies such as needle-shaped whiskers, short
Vacuum sintering
agglomerated whiskers and fine plates. The paper also discusses how mechanical properties such as microhardness, elastic modulus and Poisson's ratio are influenced by the processing techniques as well as the volume fraction of the titanium boride reinforcement. © 2012 Elsevier Inc. All rights reserved.
1.
Introduction
Components used in many engineering applications, especially in the aerospace and automobile industries and defence, are required to meet certain property requirements, such as possess a certain specific strength, elastic modulus, toughness, etc. [1]. Conventional metallic materials rarely meet such requirements. However, recent advancements in the field of metal matrix composites have overcome this difficulty, and many metal matrix composites developed in recent years have the characteristics to meet various combinations of property requirements. Titanium-based composites constitute an important group of such metal matrix composites. Though titanium-based composites are mostly used for aerospace applications, they are finding increasing use in other applications such as in the fabrication of automotive components and consumer utilities ⁎ Corresponding author. Tel.: +91 9865930803. E-mail address: [email protected] (M. Selva Kumar). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2012.07.014
[2,3]. Titanium-based composites possess several advantageous properties such as high specific modulus, high specific strength, good corrosion resistance and good wear resistance [4–7]. Various reinforcements are used in titanium-based composites. The incorporation of reinforcements such as SiC, Al2O3 and B4C leads to the formation of undesirable reaction products, which act as major barriers to the development of viable titanium composites. However, titanium boride (TiB) is well suited for use as reinforcement due to the absence of any intermediate phase between Ti and TiB [8–10]. Moreover, TiB reinforcement has certain desirable characteristics such as high elastic modulus, good thermal stability at high temperature and a density nearly equal to that of titanium [11]. Many research studies on the processing of Ti–TiB composites using many techniques such as solidification, rapid solidification [12–14], combustion synthesis [15] and laser cladding [16]
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MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
have been reported. In all of these techniques, TiB reinforcement is developed either by an in situ method or by the addition of ceramic particles of TiB. Fibre-reinforced composites involve the addition of costly fibres and intricate processing steps [9]. However, in situ composites are created using simpler fabrication steps and hence are more cost effective than fibrereinforced composites. Additionally, in situ titanium matrix composites overcome the shortcomings associated with the casting process, such as the pollution of reinforcements and the wettability between ceramic particles and a matrix [17]. Considerable research studies have also been carried out to understand the mechanical properties of Ti–TiB composites [18–23]. With respect to mechanical properties, hardness and the elastic properties of Ti–TiB composites are of great interest due to the higher modulus of TiB relative to that of Ti. A review of the available literature reveals that no significant work has been reported on Ti–TiB composites processed by powder metallurgical techniques such as Hot Isostatic Pressing (HIP), Spark Plasma Sintering (SPS) and vacuum sintering. Hence, the main objective of this work was to process a high-density Ti– TiB composites (aiming 20 and 40 volume percentage (vol.%) TiB) using the above-mentioned powder metallurgical techniques. The processed composites were characterised by X-ray diffraction (XRD), Electron Probe Micro Analysis (EPMA)-Energy Dispersing Spectroscopy (EDS) and Wavelength Dispersing Spectroscopy (WDS), optical microscopy (with an image analyser) and scanning electron microscopy (SEM) techniques. In addition, the mechanical properties of the composites, including microhardness, elastic moduli, shear moduli and Poisson's ratio were measured.
2.
Materials
powder was 10:1 during the mixing process. The as-milled powder was preserved in the vacuum atmosphere for the processing of the Ti–TiB composites.
3.
The main drawback associated with the liquid route is the high cost of material. This difficulty was overcome by processing the composites through powder metallurgy techniques (via in situ preparation). In earlier research studies, rapid solidification and combustion synthesis were employed to process Ti–TiB composites [9,23]. In this work, in situ Ti composites (aiming 20 and 40 vol.% TiB) were processed by HIP, SPS and vacuum sintering.
3.1.
Material Processing Techniques
3.1.1.
Hot Isostatic Pressing (HIP)
In the HIP process, a powder mixture is subjected to an elevated temperature and pressure to eliminate internal micro shrinkage. During the manufacturing process carried out in this study, a powder mixture was placed in a container (typically steel can) and maintained at a temperature of 1200 °C and at a pressure of 120 MPa under a high-vacuum atmosphere (approximately 10− 6 m bar) for approximately 5 h to remove air and moisture from the mixture. Then, the container was sealed and pressed using HIP. The circulation of inert gas at high pressure and temperature resulted in the removal of internal voids and created a strong metallurgical bond throughout the material. This produced a clean and homogeneous material with nearly 100% density.
3.1.2.
The elemental powders used in this work were Ti (325 mesh, 99.5%) and TiB2 (325 mesh) and Ferro Molybdenum FeMo (β stabilisers). The mixture was used to produce a composite containing a 0.2 and 0.4 volume fraction of TiB according to the following reaction: Ti þ TiB2 →2TiB
ð1Þ
Table 1 illustrates the target volume percentage and the composition of the mixture in weight percentage. The first step of the process consisted of the mechanical mixing of elemental powders (Ti, TiB2 and FeMo). β-stabilising elements (FeMo) were added to increase the fraction of the β phase in the matrix, which is more ductile than the α phase. Powders were prepared under vacuum to reduce the oxidation process. Mechanical alloying was performed in a dry ball mill for approximately 20 h. The weight ratio of stainless steel balls to
Target vol pct of TiB
20 40
Composition of mixture (wt. pct) Ti
TiB2
FeMo
78 70
6 15
16 15
Spark Plasma Sintering (SPS)
Spark plasma sintering is a newly developed consolidation technique that enables a composite powder to be fully densified at a relatively low temperature and short time (10–15 min) [24–27]. During the SPS process carried out in this study (at the International Research Center for Advanced Newer Materials — ARCI, Hyderabad, India), the as-milled powders were placed in a graphite die and then pressed uniaxially at 20 MPa, and a direct current pulse voltage was applied. The powders were heated by spark discharge between the particles. The graphite die was heated from both inside and outside. Hence, rapid heating occurred and the sintering time was reduced. The sintering temperature was maintained at approximately 1100 °C, with a holding time of 5 min. Therefore, consolidation occurred rapidly at a relatively low temperature, resulting in control over grain structure and microstructure.
3.1.3. Table 1 – Compositions of Ti–TiB composites.
Approach
Vacuum Sintering
The milled composite powder was compacted in a Universal Testing Machine (UTM-100 t) under a load of 350 kN. Then, cylindrical green composite powder billets were consolidated by sintering in a vacuum furnace (processed at Non-Ferrous Technology Development Center — NFTDC, Hyderabad, India) maintained at approximately 1200 °C for approximately 5 h. TiB2 reacted with the Ti powder and transformed into TiB during sintering.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
3.2.
Characterisation
4.
Small cylindrical samples were cut from the sintered Ti–TiB composites by electrical discharge machining. The densities of the Ti–TiB composites were measured by the Archimedes method (water immersion method). These densities were compared to the theoretical density values calculated using the volumetric rule of mixtures (shown in Table 2). The samples were etched with Kroll's reagent. The microstructures of the composites were observed using an optical microscope with an image analyser (Envision 3.0 software) attachment and a Scanning Electron Microscope (HITACHI S-3000 H). The identification of Ti and TiB phases present in the composites after sintering were confirmed by XRD (Rigaku Ultima, model-IIIXRay Diffractometer, CuKα radiation, λ = 1.540598 A°) and EPMA (EDS and WDS) techniques (JOEL JXA-8530F Electron Probe Micro Analyzer). Moreover, the Vickers hardness of the specimen was measured using a Mitutoya microhardness tester at a load of 300 g for a dwell time of 15 s. The Young's modulus, shear modulus and Poisson's ratio were determined from the longitudinal and transverse velocity of ultrasonic waves propagating through the material.
3.3. Estimation of Elastic Properties Through Ultrasonic Technique The velocity of ultrasonic waves in a solid medium is directly related to the elastic properties and density of the material. The velocity of ultrasonic waves in materials is generally obtained from the time of flight of ultrasonic waves through a known thickness of the sample. Hence, the elastic modulus of a material measured by the ultrasonic technique is a good representative bulk material property [28,29]. The relationships between ultrasonic velocity and the elastic properties of materials are presented below: E ¼ ρV2L
3V2L −4V2T
ð2Þ
V2L −V2T
G ¼ ρV2T
ð3Þ
VL 2 −2 V ν ¼ " T 2 # VL −1 2 VT
ð4Þ
V L −longitudinal velocity; V T −transverse velocity; ρ−density of the samples; ν−Poisson0 s ratio; G−shear modulus; E−Young0 s modulus: Table 2 – Density of Ti–TiB composites. Technique
SPS HIP Vacuum Sintering
Composite
Ti-24% TiB Ti-38.5% TiB Ti-20.6% TiB Ti-38.3% TiB Ti-17.6% TiB Ti-37.9% TiB
Theoretical density g/cm3 4.963 4.933 4.963 4.933 4.963 4.933
Measured Density density % g/cm3 4.90 4.75 4.92 4.9 4.2 4.685
98.73 96.29 99.12 99.33 84.62 94.97
Results and Discussion
The microstructural characterisation and the mechanical properties of the Ti–TiB composites (processed by HIP, SPS and vacuum sintering) are discussed in this section.
4.1. Estimation of TiB Volume Fraction Through Image Analyser Technique The volume fraction of TiB phases measured by an image analyser with acceptable accuracy [12] is shown in Table 3. Through this method, the TiB phases were identified and their volume percentages were estimated using an image analyser attached to an optical microscope. All dark boride phases in the matrix were identified, and their volumes in all of the composites were estimated after a few iterations. TiB was determined to be the predominant boride phase. The average value of the resulting volume fraction of TiB is presented in Table 3. As shown in Table 3, the estimated TiB volume percentages of all of the composites processed using the three techniques (HIP, SPS and vacuum sintering) agreed reasonably well with another, revealing the target volume percentage with acceptable deviations. The HIP and SPS processes yielded highly dense sintered composites whose densities are closer to the theoretical densities and also closer to the target volume percentage of TiB (as shown in Table 2). However, the composites fabricated by vacuum sintering show some marginal differences in density and TiB volume fraction from the theoretical values as shown in Table 2.
4.2.
Microstructure
The composites were investigated by X-ray diffractometry, SEM (SEI mode) and EPMA to reveal the morphology of the TiB fabricated in situ in a Ti matrix.
4.2.1.
X-ray Diffraction Technique
XRD analysis was carried out on metallographically polished composite specimens. The XRD patterns of the composites processed using the three different powder metallurgical techniques (HIP, SPS and vacuum sintering) are shown in Fig. 1a and b. The diffractogram confirms the completion of the chemical reaction. The patterns of all of the composites show the presence of β Ti and α Ti peaks, suggesting that the matrix contains both β and α phases. It is ascertained that the addition of β-stabilising agents (FeMo) resulted in a β Ti matrix. The patterns also reveal that there is no existence of Ti3B4, which is a possible intermediate phase in the matrix [9,27]. The observation also confirms the transformation of TiB2 particles into TiB in the resultant matrix.
Table 3 – Actual Vol pct of TiB. Target vol pct of TiB
20 40
Composition of mixture (wt. pct)
Estimated vol pct of TiB
Ti
TiB2
FeMo
SPS
HIP
Vacuum sintering
78 70
6 15
16 15
24 38.5
20.6 38.3
17.6 37.9
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MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
in the composites (17.6 and 37.9 vol.% TiB) are observed. This indicates the slow diffusion of boron atoms in TiB, which is similar to the result reported in an earlier reported study [12]. In the composites (both 17.6 and 37.9 vol.% TiB), the dominant TiB (112), (111) and (201) peaks at 2θ = 52.44°, 37.62° and 34.7°, respectively, are observed. It is evident from the pattern (shown in Fig. 1a and b) that the peak corresponding to TiB increases with the volume fraction of TiB. Additionally, the intensity of the TiB peaks in the 37.9 vol.% TiB composite is quite higher than that of the peaks in the composite containing 17.6 vol.% TiB. The pattern also reveals the presence of Ti (101) and β Ti (211) at 2θ = 39.74° and 70.16°, respectively, in the composites containing 17.6 and 37.9 vol.% TiB. Moreover, there is evidence of FeMo, with a small (310) peak at 2θ = 35.94°, in the composite containing 17.6 vol.% TiB. Fig. 1b shows the XRD pattern of the composite (38.3 vol.% TiB) processed through HIP. The existence of β Ti and α Ti is evidenced from the peaks. However, the composite with 20.6 vol.% TiB contains few TiB2 peaks, indicating a minimal probability of an incomplete reaction. The overlapping of α Ti (101) and β Ti (110) peaks at 2θ = 39° to 40° is observed in Fig. 1. Accordingly, more TiB (101), (201), (112), and (210) peaks exist between 2θ = 35° and 50° in the 38.3 vol.% TiB composite. The intensities of the Ti and TiB peaks in both composites are considerably lower than those of the composites processed through vacuum sintering, as depicted in the diffractogram (Fig. 1). The spectra of the composites sintered through SPS are shown in Fig. 1a and b. The sintered composite matrix mainly consists of β Ti and few α Ti phases. Unlike the composites prepared through vacuum sintering and hot isostatic pressing, it is reasonable to conclude that the full transformation of TiB2 into TiB has occurred. The spectra of the composites reveal the existence of dominant TiB (101), (112) and (201) peaks at 2θ values of 24.3°, 52.3° and 35.3°, respectively, with high intensity. The intensity of the TiB (101) peak is found to be marginally higher in the composite with 38.5 vol.% TiB than in that with 24 vol.% TiB. Moreover, the spectra clearly indicate that the composites sintered through the SPS process feature more dominant TiB and Ti phases than the composites processed through the other two techniques (HIP and vacuum sintering). It is noted that the β Ti (112) peaks are common to the spectra of all of the composites, regardless of the processing route. It is known that the β phase is relatively more ductile than the α phase in Ti alloys [23,27].
4.2.2.
Fig. 1 – XRD Pattern of Ti–TiB composite. (a) Vacuum sintering — Ti-17.6 vol.% TiB, HIP — Ti-20.6 vol.% TiB, SPS — Ti-24 vol.% TiB. (b) Vacuum sintering — Ti-37.9 vol.% TiB, HIP — Ti-38.3 vol.% TiB, SPS — Ti-38.5 vol.% TiB.
The XRD pattern of the composites processed through vacuum sintering (Fig. 1a) shows a few TiB2 peaks. The presence of TiB2 (001) and (002) peaks at 2θ =27.64° and 57.36°, respectively,
Electron Probe Micro Analysis (EPMA)
The EPMA (EDS) results obtain in normal mode are depicted in Fig. 2a and b for the composites processed through SPS and vacuum sintering. The results (as shown in Table 4 and 5) confirm the distribution of Ti and boride phases in the composites, which have not been reported in earlier studies [17]. Fig. 3a and b (observed using WDS) show three distinct regions: one is a very dark boride-rich region, which is predominantly composed of TiB, and the other regions are lighter and grey, which confirm the presence of titanium. However, the presence of a few TiB whiskers in the titanium outer matrix may be due to the diffusion of boron atoms to the Ti-rich regions during sintering.
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MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
Fig. 2 – EPMA (EDS) of Ti–TiB composites processed through SPS and vacuum sintering. (a) EPMA (EDS) of Ti‐38.5 vol.% TiB composites processed through SPS. (b) EPMA (EDS) of Ti‐37.9 vol.% TiB composites processed through vacuum sintering.
4.2.3.
Scanning Electron Microscopy (SEM)
High-magnification micrographs were taken in SEI mode using SEM after deep etching the samples to reveal the distribution of TiB whiskers. The morphologies of the TiB whiskers formed in the composites (38.3 and 20.6 vol.% TiB) processed by HIP are considerably different from those in the composites synthesised using the other techniques (SPS and vacuum sintering) and hot pressing [12]. The composite with 38.3 vol.% TiB (Fig. 4a) shows fine irregular reinforcement, with a diameter of approximately 1 μm and an aspect ratio close to unity. Additionally, hard spherical TiB particles are embedded in the matrix. Fig. 4b illustrates the microstructure of the composite with 20.6 vol.% TiB. The image shows that α platelets nucleated within the β matrix. Few TiB whiskers are also observed in the composite. FeMo particles were not
detected in the composites, which confirm the notion that all of the Fe and Mo dissolved in the Ti matrix, a result similar to that reported in a previous work by Feng et al. [24]. The microstructure of the Ti–TiB composites (with 38.5 and 24 vol.% TiB) consolidated using SPS is shown in Fig. 5a and b. The figures show both β Ti and α phases, which are consistent with the XRD results. It is ascertained that the addition of a β-stabilising agent (FeMo) results in a β Ti matrix. The different types of TiB whisker morphologies observed are as follows. (i) A small amount of fine TiB needles on the order of 1 μm in length and 0.6 μm in width is present along the β Ti grain boundaries in the composites with 38.5 vol.% TiB; (ii) some TiB whiskers are grouped into colonies of monolithic form, which is not so in the case of the 24 vol.% TiB composite; (iii) spherical TiB particles nucleated in the α and β Ti regions in the composite with
Table 4 – EDS analysis of Ti‐38.5 vol.% TiB composites processed through SPS.
Table 5 – EDS analysis of Ti‐37.9 vol.% TiB composites processed through vacuum sintering. Composition of element (pct)
Composition of element (pct) Ti
B
Fe
Mo
Ti
B
Fe
Mo
67.67
17.22
6.18
8.93
72.17
14.95
4.21
8.66
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MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
Fig. 3 – EPMA (WDS) of Ti–TiB composites processed through SPS and vacuum sintering. (a) SEM-WDS picture of Ti-37.9 vol.% TiB composite processed by vacuum sintering. (b) SEM-WDS picture of Ti-38.5 vol.% TiB composite processed by SPS.
24 vol.% TiB; (iv) it is interesting to note that the TiB whiskers(1.4 μm in length and 0.7 μm in width, with an aspect ratio of 2) are short and agglomerated and appear to be interconnected and randomly oriented in the composite. The distribution of TiB whiskers throughout the composites with 37.9 and 17.6 vol.% TiB and processed using vacuum sintering are illustrated in Fig. 6. Three types of whiskers are observed: (i) short agglomerates or clusters of short whiskers, (ii) coarse needle-shaped whiskers and (iii) fine plates of TiB. The short agglomerated TiB whisker clusters appear to be interconnected and uniformly distributed in the composite with 37.9 vol.% TiB, as shown in Fig. 6a. The short whiskers are approximately 1.5 μm long and 0.6 μm wide, with an aspect ratio of 2. Such clusters of short TiB whiskers have been reported in previous studies [9]. A few coarse, elongated TiB whiskers growing from the homogeneous layer into the β matrix of the composite with 17.6 vol.% TiB are shown in Fig. 6b. The long needle-shaped structures measure approximately
Fig. 4 – SEM images of Ti–TiB composites processed through HIP. (a) SEM pictures of Ti–TiB composites processed through HIP (Ti-38.3 vol.% TiB). (b) SEM pictures of Ti–TiB composites processed through HIP (Ti-20.6 vol.% TiB).
30 μm in length and 4.5 μm in width, with an aspect ratio of 7. Additionally, fine plates of TiB on the order of 20 μm in length, 5 μm in width and 1 μm in thickness are shown in Fig. 6b. Fine plates are not observed in the composite processed through hot pressing, as reported by Sahay et al. [9] and Panda and Ravichandran [30].
4.3.
Comparison of Processing Techniques
The materials characterisation confirms that the composites processed through HIP and SPS yield highly dense sintered composites, as their densities and volume percentages are closer to the theoretical values. Concerning the microstructure, the hot isostatic pressed composites possess very hard TiB particles with embedded α platelets, which are not observed in the composites processed by SPS and vacuum sintering. These α platelets contribute to the high elastic modulus of the composite [28]. Short agglomerated TiB whiskers are detected in both the SPS and vacuum sintered composites, though with densely packed short fibres in the latter. Very fine TiB needles with an aspect ratio of 2 are distributed in the matrix of the SPS-processed
49
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
Fig. 5 – SEM images of Ti–TiB composites processed by SPS. (a) SEM pictures of Ti–TiB composites processed by SPS (Ti-38.5 vol.% TiB). (b) SEM pictures of Ti–TiB composites processed by SPS (Ti-24 vol.% TiB).
Fig. 6 – SEM images of vacuum-sintered Ti–TiB composites. (a) SEM pictures of vacuum sintered Ti–TiB composites (Ti-37.9 vol.% TiB). (b) SEM pictures of vacuum sintered Ti–TiB composites (Ti-17.6 vol.% TiB).
composite (38.5 vol.% TiB). The same is observed in the vacuumsintered composites, which show long needle-shaped structures with an aspect ratio of 7. It is noted that the SPS-sintered composite is significantly different from the composites fabricated using the other processing techniques with respect to their monolithic TiB whiskers (with an aspect ratio of 10), which are randomly distributed in the composites with 38.5 vol.% TiB. These colonies of fibres do not exist in the HIP- or vacuumsintered composites. However, fine plates of TiB whiskers are found in the vacuum-sintered composite. These are not evident in the composites synthesised through SPS or vacuum sintering. Though SPS, HIP and vacuum sintering yield highly dense composites, as shown in Table 2, there is a considerable difference in the density (84.6 g/cm3) of the vacuum-sintered composite (17.6 vol.% TiB) compared to the composites processed using HIP and SPS.
values. Details regarding these properties are presented in this subsection.
4.4.
Mechanical Properties
The Ti–TiB composites exhibit good mechanical properties such as high hardness and high Young's modulus and shear modulus
4.4.1.
Microhardness
The microhardness of the composites processed through spark plasma sintering, hot isostatic pressing and vacuum sintering techniques is illustrated in Table 6. The average microhardness values of the spark-plasma-sintered Ti–TiB composite with 24% TiB and 38.5% TiB (by volume) are 710 HV and 890 HV, respectively.
Table 6 – Vickers hardness of Ti–TiB composite. Technique SPS HIP Vacuum Sintering
Composite
Hardness (HV)
Ti-24% TiB Ti-38.5% TiB Ti-20.6% TiB Ti-38.3% TiB Ti-17.6% TiB Ti-37.9% TiB
710 890 658 823 424 618
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The microhardness values of the composites (Ti-20.6 vol.% TiB and 38.3 vol.% TiB) processed by hot isostatic pressing, are 658 HV and 823 HV, respectively. The microhardness of the composites (Ti-17.6 vol.% TiB and 37.9 vol.% TiB) processed through vacuum sintering was found to be 424 HV and 618 HV, respectively. It is interesting to note that the maximum hardness value (890 HV) was observed for an SPS-processed composite, in which the microstructure was controlled. This may be attributed to the short time required for consolidation at a high heating rate, which leads to the formation of fine TiB needles and short agglomerated interconnected fibres in the matrix. This resulted in the high hardness of the composites. Similarly, for the composites processed through HIP, hard TiB particles embedded in the matrix and the fine irregular TiB whiskers in the composites led to an increase in hardness, though to marginally lower value than that obtained by SPS processing. However, the results indicate that the vacuum-sintered composites contain coarse interconnected and long needle-shaped whiskers in the matrix, which contributed to lower hardness values than those of the composites fabricated using the other two processing techniques. The results confirm that the microhardness increases with the volume fraction of TiB. The effect of TiB on the Ti matrix results in high composite hardness [13,23].
4.4.2.
and HIP. It is also ascertained that the Young's moduli of the Ti– TiB composites are higher than the Young's modulus of the Ti matrix (110 GPa). The synthesis of TiB reinforcement leads to an increase in shear modulus with an increase in the TiB volume fraction, as shown in Table 7. The shear modulus values range between 48.80 GPa and 72.54 GPa in the composites processed through vacuum sintering and SPS, respectively. It is evident that there is a marginal difference in the shear modulus values of the composites processed by HIP. This may be due to the existence of α platelets, which exhibit higher elastic modulus values. The average shear modulus values of the composites (Ti-20.6 vol.% TiB and 38.3 vol.% TiB) processed by this technique are 61.90 GPa and 67.88 GPa, respectively. The results show that the presence of the TiB phase in the form of whiskers in all of the composites may contribute to the increase in the modulus of TiB, aside from the increase in volume fraction [18]. As expected, the Poisson's ratio was reduced to a value lying between 0.27 and 0.31 with an increase in the amount of TiB [18]. The composites with both volume fractions processed through SPS and vacuum sintering exhibited the same range of Poisson's ratio (0.31–0.27), with an average deviation of 0.3. However, the Poisson's ratio of the composites (Ti-17.6 vol.% TiB and 37.9 vol.% TiB) processed by HIP are almost identical (0.29), as shown in Table 7.
Elastic Properties
The elastic modulus and Poisson's ratio values of the composites determined through ultrasonic wave propagation [22] are presented in Table 7. The Young's modulus and shear modulus values increase remarkably, while the Poisson's ratio decreases with an increase in the volume fraction of TiB [20]. The maximum Young's modulus was 185 GPa, observed in the composite (with 38.5 vol.% TiB) processed by SPS, which is marginally higher than the Young's modulus of the composites processed through HIP and vacuum sintering. This resulted from randomly oriented interconnected TiB whiskers and fine TiB needles in the composites. It is interesting to note that the Young's modulus of the composite (with 37.9 vol.% TiB) processed by vacuum sintering is comparable with that of the composite processed by HIP. This may be due to the lower amount of TiB2 in the processed composite, as evidenced by the XRD studies and SEM micrographs. This TiB2 content can substantially increase the modulus beyond that of the Ti– TiB composites due to the high elastic modulus (540 GPa) of TiB2 [18]. However, for the composite (with 17.6 vol.% TiB) processed by vacuum sintering, the elastic modulus (127.86 GPa) is relatively lower than that of the composites processed by SPS
5.
Conclusion
Ti–TiB composites with two different volume fractions (aiming 20 and 40 vol.% TiB) were processed through HIP, SPS and vacuum sintering. The following conclusions can be drawn: • The EPMA results reveal the distribution of Ti and boride phases in the composites. TiB is identified as the predominant boride phase in the Ti matrix. The same is confirmed by the XRD results (Fig. 2a and b). • The processed composites also reveal the presence of an unreacted TiB2 phase. • There is no evidence of a Ti3B4 phase in any of the composites. • The volume fractions of the TiB phase in the composites were determined through image analysis, which confirms a reasonable agreement with the target volume percentages of 20 and 40 of TiB as reinforcement. • The microstructure of TiB reinforcement reveals three morphologies, namely needle-shaped whiskers, short agglomerates or clusters of whiskers and very fine plates.
Table 7 – Experimental values of Young's modulus, shear modulus and Poisson's ratio. Technique SPS HIP Vacuum sintering
Composite
Measured density (g/cm3)
Longitudinal velocity (m/s)
Transverse velocity (m/s)
Young's modulus (GPa)
Shear modulus (GPa)
Poisson's ratio
Ti-24% TiB Ti-38.5% TiB Ti-20.6% TiB Ti-38.3% TiB Ti-17.6% TiB Ti-37.9% TiB
4.90 4.75 4.92 4.90 4.20 4.685
6484 7016 6418 6788 6488 6572
3429 3908 3547 3700 3409 3661
150.94 185.00 159.70 172.88 127.86 160.12
57.61 72.54 61.90 67.88 48.80 62.77
0.31 0.28 0.29 0.28 0.31 0.27
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
• The HIP and SPS processes yield highly dense sintered composites. • The elastic modulus was measured by the ultrasonic method. The elastic modulus, shear modulus and microhardness were found to increase with the increasing volume fraction of TiB. • The composites processed using SPS and HIP exhibit better elastic properties and higher hardness compared to the composites processed by vacuum sintering. • The Poisson's ratio of the composites is reduced with an increase in the volume fraction of TiB.
Acknowledgements The authors acknowledge the All India Council for Technical Education (AICTE), New Delhi for providing a grant to carry out this work at the Dr. Mahalingam College of Engineering and Technology, Pollachi, India. The authors also acknowledge Dr. G. Apparao, DMRL, Hyderabad, Dr. Anish Kumar, IGCAR, Kalpakkam, Mr. Govindaraj, NFTDC, Hyderabad and Dibyendu Chackravarthy, ARCI, Hyderabad for their support in processing the composites. The guidance received from Dr. B. Ravisankar, NIT-Trichy, Ms. Kalavathy, IGCAR-Kalpakkam, Dr. V. Balusamy, and Dr. R. Subramanian, PSG Tech, Coimbatore is also gratefully acknowledged.
REFERENCES [1] Radhakrishnabhat BV, Subramanyam J, Bhanuprasad VV. Preparation of Ti–TiB–TiC & Ti–TiB composites by in-situ reaction hot pressing. Mater Sci Eng, A 2002;325:126. [2] Chandrasekar P, Balusamy V, Ravi Chandran KS, Harish kumar. Laser surface hardening of titanium–titanium boride (Ti–TiB) metal matrix composites. Scr Mater 2007;56:641-4. [3] Alman DE, Hawk JA. The abrasive wear of sintered titanium matrix — ceramic particle reinforced composites. Wear 1999;225–229:629-39. [4] Guo C, Zhou J, Zhao J, Guo B, Yu Y, Zhou H, et al. Microstructure and friction and wear behaviour of laser boronizing composite coatings on titanium substrate. Appl Surf Sci 2011;257:4398-405. [5] Dong H, Bell T. Enhanced wear resistance of titanium surfaces by a new thermal oxidation treatment. Wear 2000;238:131-7. [6] Lee C, Sanders A, Tikekar N, Ravichandran KS. Tribology of titanium boride‐coated titanium balls against alumina ceramic: wear, friction, and micromechanisms. Wear 2008;265:375-86. [7] Morsi K, Patel VV, Naraghi S, Garay JE. Processing of titanium–titanium boride dual matrix composites. J Mater Process Technol 2008;196:236-42. [8] Courant B, Hantzpergue JJ, Avril L, Benayoun S. Structure and hardness of titanium surfaces carburized by pulsed laser melting with graphite addition. J Mater Process Technol 2005;160:374-81. [9] Sahay SS, Ravi Chandran KS, Atri R. Evolution of microstructure and phases in situ processed Ti–TiB composites containing high volume fractions of TiB whiskers. J Mater Res 1999;14:4214-23. [10] Ravi Chandran KS, Panda KB. Discontinuosly reinforced titanium with titanium boride whiskers on the horizon. Adv Mater Process 2002;160:59-62.
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[11] Geng K, Lu W, Yang Z, Zhang D. In situ preparation of titanium matrix composites reinforced by TiB and Nd2O3. Mater Lett 2003;57:4054-7. [12] Kim M-G, Sung Si -Y, Kim Y-J. Synthesis of in situ Titanium carbide particle reinforced titanium composites. Mater Sci Forum 2005;475–479:963-6. [13] Gorsse S, Chaminade JP, Le Petitcorps Y. In situ preparation of titanium base composites reinforced by TiB single crystals using a powder metallurgy technique. Compos A 1998;29: 1229-34. [14] Pettersson A, Magnusson P, Lundberg P, Nygren M. Titanium–titanium diboride composites as part of a gradient armour material. Int J Impact Eng 2005;32:387-99. [15] Ranganath S, Vijaykumar M, Subramanyam. Combustion-assisted synthesis of Ti–TiB–TiC composite via the casting route. J Mater Sci Eng A 1992;149:253-7. [16] DuttaMajumdar J, Li L. Development of titanium boride (TiB) dispersed titanium (Ti) matrix composite by direct laser cladding. Mater Lett 2010;64:1010-2. [17] Min-Min W, Wei-jie L, Ji-ning Q, Di Z, Bo J, Feng Z. The effect of reinforcements on superplasticity of in situ synthesized (TiB + TiC) / Ti matrix composite. Scr Mater 2006;54:1955-9. [18] Atri RR, Ravichandran KS, Jha SK. Elastic properties of in-situ processed Ti–TiB composites measured by impulse excitation of vibration. Mater Sci Eng, A 1999;271:150-9. [19] Gorsse S, Petitcorps YL, Matar S, Rebillat F. Investigation of the young's modulus of TiB needles in situ produced in titanium matrix composite. Mater Sci Eng, A 2003;340:80-7. [20] Feng HB, Zhou Y, Jia DC, Meng QC. Microstructure and mechanical properties of in situ TiB reinforced titanium matrix composites based on Ti-FeMo-B prepared by spark plasma sintering. Compos Sci Technol 2004;64:2495-500. [21] Shen XB, Zhang ZH, Wei S, Wang FC, Lee SK. Microstructure and mechanical properties of the in situ TiB–Ti metal — matrix composites synthesized by spark plasma sintering process. J Alloys Compd 2011;509:7692-6. [22] Lia LH, Kim HE, Kang ES. Sintering and mechanical properties of titanium diboride with aluminium nitride as a sintering aid. J Eur Ceram Soc 2002;22:973-7. [23] Panda KB, Ravichandran KS. Titanium–titanium boride (TiB) functionally graded materials through reaction sintering: synthesis. microstructure and properties. Metall Mater Trans A 2003;34A:1993-2003. [24] Feng HB, Zhou Y, Jia DC, Meng QC. Rapid synthesis of Ti alloy with B addition by spark plasma sintering. Mater Sci Eng, A 2005;390:344-9. [25] Zhang Zh, Shen XB, Wen S, Luo J, Lee SK, Wang FC. In situ reaction synthesis of Ti–TiB composites containing high volume fraction of TiB by spark plasma sintering process. J Alloys Compd 2010;503:145-50. [26] Wang FC, Zhang Zh, Luo J, Huang CC, Lee SK. A novel rapid route for in situ synthesizing TiB–TiB2 composites. Compos Sci Technol 2009;69:2682-7. [27] Spear KE, McDowell P, McMahon F. Experimental evidence for the existence of the Ti3B4 phase. J Am Ceram Soc 1986;69: C4-5. [28] Majumdar P, Singh SB, Chakraborty M. Elastic modulus of biomedical titanium alloys by nanoindentation and ultrasonic techniques — A comparative study. Mater Sci Eng, A 2008;489:419-25. [29] Doghmane M, Hadjoub F, Doghmane A, Hadjoub Z. Approaches for evaluating Young's and shear moduli in terms of a single SAW velocity via the SAM technique. Mater Lett 2007;61:813-6. [30] Panda KB, Ravichandran KS. Synthesis of ductile titanium–titanium boride (Ti–TiB) composites with a beta-titanium matrix: the nature of TiB formation and composite properties. Metall Mater Trans A 2003;34:1371-85.
Available online at www.sciencedirect.com
www.elsevier.com/locate/matchar
Characterisation of titanium–titanium boride composites processed by powder metallurgy techniques M. Selva Kumara,⁎, P. Chandrasekarb , P. Chandramohanb , M. Mohanrajc a
Department of Mechanical Engineering, Dr. Mahalingam College of Engineering and Technology, Pollachi-642003, India School of Engineering, Professional Group of Institutions, Coimbatore-641662, India c Department of Mechanical Engineering, Info Institute of Engineering, Coimbatore-641107, India b
AR TIC LE D ATA
ABSTR ACT
Article history:
In this work, a detailed characterisation of titanium–titanium boride composites processed
Received 8 November 2011
by three powder metallurgy techniques, namely, hot isostatic pressing, spark plasma
Received in revised form 5 July 2012
sintering and vacuum sintering, was conducted. Two composites with different volume
Accepted 20 July 2012
percents of titanium boride reinforcement were used for the investigation. One was titanium with 20% titanium boride, and the other was titanium with 40% titanium boride
Keywords:
(by volume). Characterisation was performed using X-ray diffraction, electron probe micro
Titanium–titanium
analysis — energy dispersive spectroscopy and wavelength dispersive spectroscopy, image
boride composites
analysis and scanning electron microscopy. The characterisation results confirm the
Hot isostatic pressing
completion of the titanium boride reaction. The results reveal the presence of titanium
Spark plasma sintering
boride reinforcement in different morphologies such as needle-shaped whiskers, short
Vacuum sintering
agglomerated whiskers and fine plates. The paper also discusses how mechanical properties such as microhardness, elastic modulus and Poisson's ratio are influenced by the processing techniques as well as the volume fraction of the titanium boride reinforcement. © 2012 Elsevier Inc. All rights reserved.
1.
Introduction
Components used in many engineering applications, especially in the aerospace and automobile industries and defence, are required to meet certain property requirements, such as possess a certain specific strength, elastic modulus, toughness, etc. [1]. Conventional metallic materials rarely meet such requirements. However, recent advancements in the field of metal matrix composites have overcome this difficulty, and many metal matrix composites developed in recent years have the characteristics to meet various combinations of property requirements. Titanium-based composites constitute an important group of such metal matrix composites. Though titanium-based composites are mostly used for aerospace applications, they are finding increasing use in other applications such as in the fabrication of automotive components and consumer utilities ⁎ Corresponding author. Tel.: +91 9865930803. E-mail address: [email protected] (M. Selva Kumar). 1044-5803/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.matchar.2012.07.014
[2,3]. Titanium-based composites possess several advantageous properties such as high specific modulus, high specific strength, good corrosion resistance and good wear resistance [4–7]. Various reinforcements are used in titanium-based composites. The incorporation of reinforcements such as SiC, Al2O3 and B4C leads to the formation of undesirable reaction products, which act as major barriers to the development of viable titanium composites. However, titanium boride (TiB) is well suited for use as reinforcement due to the absence of any intermediate phase between Ti and TiB [8–10]. Moreover, TiB reinforcement has certain desirable characteristics such as high elastic modulus, good thermal stability at high temperature and a density nearly equal to that of titanium [11]. Many research studies on the processing of Ti–TiB composites using many techniques such as solidification, rapid solidification [12–14], combustion synthesis [15] and laser cladding [16]
44
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
have been reported. In all of these techniques, TiB reinforcement is developed either by an in situ method or by the addition of ceramic particles of TiB. Fibre-reinforced composites involve the addition of costly fibres and intricate processing steps [9]. However, in situ composites are created using simpler fabrication steps and hence are more cost effective than fibrereinforced composites. Additionally, in situ titanium matrix composites overcome the shortcomings associated with the casting process, such as the pollution of reinforcements and the wettability between ceramic particles and a matrix [17]. Considerable research studies have also been carried out to understand the mechanical properties of Ti–TiB composites [18–23]. With respect to mechanical properties, hardness and the elastic properties of Ti–TiB composites are of great interest due to the higher modulus of TiB relative to that of Ti. A review of the available literature reveals that no significant work has been reported on Ti–TiB composites processed by powder metallurgical techniques such as Hot Isostatic Pressing (HIP), Spark Plasma Sintering (SPS) and vacuum sintering. Hence, the main objective of this work was to process a high-density Ti– TiB composites (aiming 20 and 40 volume percentage (vol.%) TiB) using the above-mentioned powder metallurgical techniques. The processed composites were characterised by X-ray diffraction (XRD), Electron Probe Micro Analysis (EPMA)-Energy Dispersing Spectroscopy (EDS) and Wavelength Dispersing Spectroscopy (WDS), optical microscopy (with an image analyser) and scanning electron microscopy (SEM) techniques. In addition, the mechanical properties of the composites, including microhardness, elastic moduli, shear moduli and Poisson's ratio were measured.
2.
Materials
powder was 10:1 during the mixing process. The as-milled powder was preserved in the vacuum atmosphere for the processing of the Ti–TiB composites.
3.
The main drawback associated with the liquid route is the high cost of material. This difficulty was overcome by processing the composites through powder metallurgy techniques (via in situ preparation). In earlier research studies, rapid solidification and combustion synthesis were employed to process Ti–TiB composites [9,23]. In this work, in situ Ti composites (aiming 20 and 40 vol.% TiB) were processed by HIP, SPS and vacuum sintering.
3.1.
Material Processing Techniques
3.1.1.
Hot Isostatic Pressing (HIP)
In the HIP process, a powder mixture is subjected to an elevated temperature and pressure to eliminate internal micro shrinkage. During the manufacturing process carried out in this study, a powder mixture was placed in a container (typically steel can) and maintained at a temperature of 1200 °C and at a pressure of 120 MPa under a high-vacuum atmosphere (approximately 10− 6 m bar) for approximately 5 h to remove air and moisture from the mixture. Then, the container was sealed and pressed using HIP. The circulation of inert gas at high pressure and temperature resulted in the removal of internal voids and created a strong metallurgical bond throughout the material. This produced a clean and homogeneous material with nearly 100% density.
3.1.2.
The elemental powders used in this work were Ti (325 mesh, 99.5%) and TiB2 (325 mesh) and Ferro Molybdenum FeMo (β stabilisers). The mixture was used to produce a composite containing a 0.2 and 0.4 volume fraction of TiB according to the following reaction: Ti þ TiB2 →2TiB
ð1Þ
Table 1 illustrates the target volume percentage and the composition of the mixture in weight percentage. The first step of the process consisted of the mechanical mixing of elemental powders (Ti, TiB2 and FeMo). β-stabilising elements (FeMo) were added to increase the fraction of the β phase in the matrix, which is more ductile than the α phase. Powders were prepared under vacuum to reduce the oxidation process. Mechanical alloying was performed in a dry ball mill for approximately 20 h. The weight ratio of stainless steel balls to
Target vol pct of TiB
20 40
Composition of mixture (wt. pct) Ti
TiB2
FeMo
78 70
6 15
16 15
Spark Plasma Sintering (SPS)
Spark plasma sintering is a newly developed consolidation technique that enables a composite powder to be fully densified at a relatively low temperature and short time (10–15 min) [24–27]. During the SPS process carried out in this study (at the International Research Center for Advanced Newer Materials — ARCI, Hyderabad, India), the as-milled powders were placed in a graphite die and then pressed uniaxially at 20 MPa, and a direct current pulse voltage was applied. The powders were heated by spark discharge between the particles. The graphite die was heated from both inside and outside. Hence, rapid heating occurred and the sintering time was reduced. The sintering temperature was maintained at approximately 1100 °C, with a holding time of 5 min. Therefore, consolidation occurred rapidly at a relatively low temperature, resulting in control over grain structure and microstructure.
3.1.3. Table 1 – Compositions of Ti–TiB composites.
Approach
Vacuum Sintering
The milled composite powder was compacted in a Universal Testing Machine (UTM-100 t) under a load of 350 kN. Then, cylindrical green composite powder billets were consolidated by sintering in a vacuum furnace (processed at Non-Ferrous Technology Development Center — NFTDC, Hyderabad, India) maintained at approximately 1200 °C for approximately 5 h. TiB2 reacted with the Ti powder and transformed into TiB during sintering.
45
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
3.2.
Characterisation
4.
Small cylindrical samples were cut from the sintered Ti–TiB composites by electrical discharge machining. The densities of the Ti–TiB composites were measured by the Archimedes method (water immersion method). These densities were compared to the theoretical density values calculated using the volumetric rule of mixtures (shown in Table 2). The samples were etched with Kroll's reagent. The microstructures of the composites were observed using an optical microscope with an image analyser (Envision 3.0 software) attachment and a Scanning Electron Microscope (HITACHI S-3000 H). The identification of Ti and TiB phases present in the composites after sintering were confirmed by XRD (Rigaku Ultima, model-IIIXRay Diffractometer, CuKα radiation, λ = 1.540598 A°) and EPMA (EDS and WDS) techniques (JOEL JXA-8530F Electron Probe Micro Analyzer). Moreover, the Vickers hardness of the specimen was measured using a Mitutoya microhardness tester at a load of 300 g for a dwell time of 15 s. The Young's modulus, shear modulus and Poisson's ratio were determined from the longitudinal and transverse velocity of ultrasonic waves propagating through the material.
3.3. Estimation of Elastic Properties Through Ultrasonic Technique The velocity of ultrasonic waves in a solid medium is directly related to the elastic properties and density of the material. The velocity of ultrasonic waves in materials is generally obtained from the time of flight of ultrasonic waves through a known thickness of the sample. Hence, the elastic modulus of a material measured by the ultrasonic technique is a good representative bulk material property [28,29]. The relationships between ultrasonic velocity and the elastic properties of materials are presented below: E ¼ ρV2L
3V2L −4V2T
ð2Þ
V2L −V2T
G ¼ ρV2T
ð3Þ
VL 2 −2 V ν ¼ " T 2 # VL −1 2 VT
ð4Þ
V L −longitudinal velocity; V T −transverse velocity; ρ−density of the samples; ν−Poisson0 s ratio; G−shear modulus; E−Young0 s modulus: Table 2 – Density of Ti–TiB composites. Technique
SPS HIP Vacuum Sintering
Composite
Ti-24% TiB Ti-38.5% TiB Ti-20.6% TiB Ti-38.3% TiB Ti-17.6% TiB Ti-37.9% TiB
Theoretical density g/cm3 4.963 4.933 4.963 4.933 4.963 4.933
Measured Density density % g/cm3 4.90 4.75 4.92 4.9 4.2 4.685
98.73 96.29 99.12 99.33 84.62 94.97
Results and Discussion
The microstructural characterisation and the mechanical properties of the Ti–TiB composites (processed by HIP, SPS and vacuum sintering) are discussed in this section.
4.1. Estimation of TiB Volume Fraction Through Image Analyser Technique The volume fraction of TiB phases measured by an image analyser with acceptable accuracy [12] is shown in Table 3. Through this method, the TiB phases were identified and their volume percentages were estimated using an image analyser attached to an optical microscope. All dark boride phases in the matrix were identified, and their volumes in all of the composites were estimated after a few iterations. TiB was determined to be the predominant boride phase. The average value of the resulting volume fraction of TiB is presented in Table 3. As shown in Table 3, the estimated TiB volume percentages of all of the composites processed using the three techniques (HIP, SPS and vacuum sintering) agreed reasonably well with another, revealing the target volume percentage with acceptable deviations. The HIP and SPS processes yielded highly dense sintered composites whose densities are closer to the theoretical densities and also closer to the target volume percentage of TiB (as shown in Table 2). However, the composites fabricated by vacuum sintering show some marginal differences in density and TiB volume fraction from the theoretical values as shown in Table 2.
4.2.
Microstructure
The composites were investigated by X-ray diffractometry, SEM (SEI mode) and EPMA to reveal the morphology of the TiB fabricated in situ in a Ti matrix.
4.2.1.
X-ray Diffraction Technique
XRD analysis was carried out on metallographically polished composite specimens. The XRD patterns of the composites processed using the three different powder metallurgical techniques (HIP, SPS and vacuum sintering) are shown in Fig. 1a and b. The diffractogram confirms the completion of the chemical reaction. The patterns of all of the composites show the presence of β Ti and α Ti peaks, suggesting that the matrix contains both β and α phases. It is ascertained that the addition of β-stabilising agents (FeMo) resulted in a β Ti matrix. The patterns also reveal that there is no existence of Ti3B4, which is a possible intermediate phase in the matrix [9,27]. The observation also confirms the transformation of TiB2 particles into TiB in the resultant matrix.
Table 3 – Actual Vol pct of TiB. Target vol pct of TiB
20 40
Composition of mixture (wt. pct)
Estimated vol pct of TiB
Ti
TiB2
FeMo
SPS
HIP
Vacuum sintering
78 70
6 15
16 15
24 38.5
20.6 38.3
17.6 37.9
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MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
in the composites (17.6 and 37.9 vol.% TiB) are observed. This indicates the slow diffusion of boron atoms in TiB, which is similar to the result reported in an earlier reported study [12]. In the composites (both 17.6 and 37.9 vol.% TiB), the dominant TiB (112), (111) and (201) peaks at 2θ = 52.44°, 37.62° and 34.7°, respectively, are observed. It is evident from the pattern (shown in Fig. 1a and b) that the peak corresponding to TiB increases with the volume fraction of TiB. Additionally, the intensity of the TiB peaks in the 37.9 vol.% TiB composite is quite higher than that of the peaks in the composite containing 17.6 vol.% TiB. The pattern also reveals the presence of Ti (101) and β Ti (211) at 2θ = 39.74° and 70.16°, respectively, in the composites containing 17.6 and 37.9 vol.% TiB. Moreover, there is evidence of FeMo, with a small (310) peak at 2θ = 35.94°, in the composite containing 17.6 vol.% TiB. Fig. 1b shows the XRD pattern of the composite (38.3 vol.% TiB) processed through HIP. The existence of β Ti and α Ti is evidenced from the peaks. However, the composite with 20.6 vol.% TiB contains few TiB2 peaks, indicating a minimal probability of an incomplete reaction. The overlapping of α Ti (101) and β Ti (110) peaks at 2θ = 39° to 40° is observed in Fig. 1. Accordingly, more TiB (101), (201), (112), and (210) peaks exist between 2θ = 35° and 50° in the 38.3 vol.% TiB composite. The intensities of the Ti and TiB peaks in both composites are considerably lower than those of the composites processed through vacuum sintering, as depicted in the diffractogram (Fig. 1). The spectra of the composites sintered through SPS are shown in Fig. 1a and b. The sintered composite matrix mainly consists of β Ti and few α Ti phases. Unlike the composites prepared through vacuum sintering and hot isostatic pressing, it is reasonable to conclude that the full transformation of TiB2 into TiB has occurred. The spectra of the composites reveal the existence of dominant TiB (101), (112) and (201) peaks at 2θ values of 24.3°, 52.3° and 35.3°, respectively, with high intensity. The intensity of the TiB (101) peak is found to be marginally higher in the composite with 38.5 vol.% TiB than in that with 24 vol.% TiB. Moreover, the spectra clearly indicate that the composites sintered through the SPS process feature more dominant TiB and Ti phases than the composites processed through the other two techniques (HIP and vacuum sintering). It is noted that the β Ti (112) peaks are common to the spectra of all of the composites, regardless of the processing route. It is known that the β phase is relatively more ductile than the α phase in Ti alloys [23,27].
4.2.2.
Fig. 1 – XRD Pattern of Ti–TiB composite. (a) Vacuum sintering — Ti-17.6 vol.% TiB, HIP — Ti-20.6 vol.% TiB, SPS — Ti-24 vol.% TiB. (b) Vacuum sintering — Ti-37.9 vol.% TiB, HIP — Ti-38.3 vol.% TiB, SPS — Ti-38.5 vol.% TiB.
The XRD pattern of the composites processed through vacuum sintering (Fig. 1a) shows a few TiB2 peaks. The presence of TiB2 (001) and (002) peaks at 2θ =27.64° and 57.36°, respectively,
Electron Probe Micro Analysis (EPMA)
The EPMA (EDS) results obtain in normal mode are depicted in Fig. 2a and b for the composites processed through SPS and vacuum sintering. The results (as shown in Table 4 and 5) confirm the distribution of Ti and boride phases in the composites, which have not been reported in earlier studies [17]. Fig. 3a and b (observed using WDS) show three distinct regions: one is a very dark boride-rich region, which is predominantly composed of TiB, and the other regions are lighter and grey, which confirm the presence of titanium. However, the presence of a few TiB whiskers in the titanium outer matrix may be due to the diffusion of boron atoms to the Ti-rich regions during sintering.
47
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
Fig. 2 – EPMA (EDS) of Ti–TiB composites processed through SPS and vacuum sintering. (a) EPMA (EDS) of Ti‐38.5 vol.% TiB composites processed through SPS. (b) EPMA (EDS) of Ti‐37.9 vol.% TiB composites processed through vacuum sintering.
4.2.3.
Scanning Electron Microscopy (SEM)
High-magnification micrographs were taken in SEI mode using SEM after deep etching the samples to reveal the distribution of TiB whiskers. The morphologies of the TiB whiskers formed in the composites (38.3 and 20.6 vol.% TiB) processed by HIP are considerably different from those in the composites synthesised using the other techniques (SPS and vacuum sintering) and hot pressing [12]. The composite with 38.3 vol.% TiB (Fig. 4a) shows fine irregular reinforcement, with a diameter of approximately 1 μm and an aspect ratio close to unity. Additionally, hard spherical TiB particles are embedded in the matrix. Fig. 4b illustrates the microstructure of the composite with 20.6 vol.% TiB. The image shows that α platelets nucleated within the β matrix. Few TiB whiskers are also observed in the composite. FeMo particles were not
detected in the composites, which confirm the notion that all of the Fe and Mo dissolved in the Ti matrix, a result similar to that reported in a previous work by Feng et al. [24]. The microstructure of the Ti–TiB composites (with 38.5 and 24 vol.% TiB) consolidated using SPS is shown in Fig. 5a and b. The figures show both β Ti and α phases, which are consistent with the XRD results. It is ascertained that the addition of a β-stabilising agent (FeMo) results in a β Ti matrix. The different types of TiB whisker morphologies observed are as follows. (i) A small amount of fine TiB needles on the order of 1 μm in length and 0.6 μm in width is present along the β Ti grain boundaries in the composites with 38.5 vol.% TiB; (ii) some TiB whiskers are grouped into colonies of monolithic form, which is not so in the case of the 24 vol.% TiB composite; (iii) spherical TiB particles nucleated in the α and β Ti regions in the composite with
Table 4 – EDS analysis of Ti‐38.5 vol.% TiB composites processed through SPS.
Table 5 – EDS analysis of Ti‐37.9 vol.% TiB composites processed through vacuum sintering. Composition of element (pct)
Composition of element (pct) Ti
B
Fe
Mo
Ti
B
Fe
Mo
67.67
17.22
6.18
8.93
72.17
14.95
4.21
8.66
48
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
Fig. 3 – EPMA (WDS) of Ti–TiB composites processed through SPS and vacuum sintering. (a) SEM-WDS picture of Ti-37.9 vol.% TiB composite processed by vacuum sintering. (b) SEM-WDS picture of Ti-38.5 vol.% TiB composite processed by SPS.
24 vol.% TiB; (iv) it is interesting to note that the TiB whiskers(1.4 μm in length and 0.7 μm in width, with an aspect ratio of 2) are short and agglomerated and appear to be interconnected and randomly oriented in the composite. The distribution of TiB whiskers throughout the composites with 37.9 and 17.6 vol.% TiB and processed using vacuum sintering are illustrated in Fig. 6. Three types of whiskers are observed: (i) short agglomerates or clusters of short whiskers, (ii) coarse needle-shaped whiskers and (iii) fine plates of TiB. The short agglomerated TiB whisker clusters appear to be interconnected and uniformly distributed in the composite with 37.9 vol.% TiB, as shown in Fig. 6a. The short whiskers are approximately 1.5 μm long and 0.6 μm wide, with an aspect ratio of 2. Such clusters of short TiB whiskers have been reported in previous studies [9]. A few coarse, elongated TiB whiskers growing from the homogeneous layer into the β matrix of the composite with 17.6 vol.% TiB are shown in Fig. 6b. The long needle-shaped structures measure approximately
Fig. 4 – SEM images of Ti–TiB composites processed through HIP. (a) SEM pictures of Ti–TiB composites processed through HIP (Ti-38.3 vol.% TiB). (b) SEM pictures of Ti–TiB composites processed through HIP (Ti-20.6 vol.% TiB).
30 μm in length and 4.5 μm in width, with an aspect ratio of 7. Additionally, fine plates of TiB on the order of 20 μm in length, 5 μm in width and 1 μm in thickness are shown in Fig. 6b. Fine plates are not observed in the composite processed through hot pressing, as reported by Sahay et al. [9] and Panda and Ravichandran [30].
4.3.
Comparison of Processing Techniques
The materials characterisation confirms that the composites processed through HIP and SPS yield highly dense sintered composites, as their densities and volume percentages are closer to the theoretical values. Concerning the microstructure, the hot isostatic pressed composites possess very hard TiB particles with embedded α platelets, which are not observed in the composites processed by SPS and vacuum sintering. These α platelets contribute to the high elastic modulus of the composite [28]. Short agglomerated TiB whiskers are detected in both the SPS and vacuum sintered composites, though with densely packed short fibres in the latter. Very fine TiB needles with an aspect ratio of 2 are distributed in the matrix of the SPS-processed
49
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
Fig. 5 – SEM images of Ti–TiB composites processed by SPS. (a) SEM pictures of Ti–TiB composites processed by SPS (Ti-38.5 vol.% TiB). (b) SEM pictures of Ti–TiB composites processed by SPS (Ti-24 vol.% TiB).
Fig. 6 – SEM images of vacuum-sintered Ti–TiB composites. (a) SEM pictures of vacuum sintered Ti–TiB composites (Ti-37.9 vol.% TiB). (b) SEM pictures of vacuum sintered Ti–TiB composites (Ti-17.6 vol.% TiB).
composite (38.5 vol.% TiB). The same is observed in the vacuumsintered composites, which show long needle-shaped structures with an aspect ratio of 7. It is noted that the SPS-sintered composite is significantly different from the composites fabricated using the other processing techniques with respect to their monolithic TiB whiskers (with an aspect ratio of 10), which are randomly distributed in the composites with 38.5 vol.% TiB. These colonies of fibres do not exist in the HIP- or vacuumsintered composites. However, fine plates of TiB whiskers are found in the vacuum-sintered composite. These are not evident in the composites synthesised through SPS or vacuum sintering. Though SPS, HIP and vacuum sintering yield highly dense composites, as shown in Table 2, there is a considerable difference in the density (84.6 g/cm3) of the vacuum-sintered composite (17.6 vol.% TiB) compared to the composites processed using HIP and SPS.
values. Details regarding these properties are presented in this subsection.
4.4.
Mechanical Properties
The Ti–TiB composites exhibit good mechanical properties such as high hardness and high Young's modulus and shear modulus
4.4.1.
Microhardness
The microhardness of the composites processed through spark plasma sintering, hot isostatic pressing and vacuum sintering techniques is illustrated in Table 6. The average microhardness values of the spark-plasma-sintered Ti–TiB composite with 24% TiB and 38.5% TiB (by volume) are 710 HV and 890 HV, respectively.
Table 6 – Vickers hardness of Ti–TiB composite. Technique SPS HIP Vacuum Sintering
Composite
Hardness (HV)
Ti-24% TiB Ti-38.5% TiB Ti-20.6% TiB Ti-38.3% TiB Ti-17.6% TiB Ti-37.9% TiB
710 890 658 823 424 618
50
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
The microhardness values of the composites (Ti-20.6 vol.% TiB and 38.3 vol.% TiB) processed by hot isostatic pressing, are 658 HV and 823 HV, respectively. The microhardness of the composites (Ti-17.6 vol.% TiB and 37.9 vol.% TiB) processed through vacuum sintering was found to be 424 HV and 618 HV, respectively. It is interesting to note that the maximum hardness value (890 HV) was observed for an SPS-processed composite, in which the microstructure was controlled. This may be attributed to the short time required for consolidation at a high heating rate, which leads to the formation of fine TiB needles and short agglomerated interconnected fibres in the matrix. This resulted in the high hardness of the composites. Similarly, for the composites processed through HIP, hard TiB particles embedded in the matrix and the fine irregular TiB whiskers in the composites led to an increase in hardness, though to marginally lower value than that obtained by SPS processing. However, the results indicate that the vacuum-sintered composites contain coarse interconnected and long needle-shaped whiskers in the matrix, which contributed to lower hardness values than those of the composites fabricated using the other two processing techniques. The results confirm that the microhardness increases with the volume fraction of TiB. The effect of TiB on the Ti matrix results in high composite hardness [13,23].
4.4.2.
and HIP. It is also ascertained that the Young's moduli of the Ti– TiB composites are higher than the Young's modulus of the Ti matrix (110 GPa). The synthesis of TiB reinforcement leads to an increase in shear modulus with an increase in the TiB volume fraction, as shown in Table 7. The shear modulus values range between 48.80 GPa and 72.54 GPa in the composites processed through vacuum sintering and SPS, respectively. It is evident that there is a marginal difference in the shear modulus values of the composites processed by HIP. This may be due to the existence of α platelets, which exhibit higher elastic modulus values. The average shear modulus values of the composites (Ti-20.6 vol.% TiB and 38.3 vol.% TiB) processed by this technique are 61.90 GPa and 67.88 GPa, respectively. The results show that the presence of the TiB phase in the form of whiskers in all of the composites may contribute to the increase in the modulus of TiB, aside from the increase in volume fraction [18]. As expected, the Poisson's ratio was reduced to a value lying between 0.27 and 0.31 with an increase in the amount of TiB [18]. The composites with both volume fractions processed through SPS and vacuum sintering exhibited the same range of Poisson's ratio (0.31–0.27), with an average deviation of 0.3. However, the Poisson's ratio of the composites (Ti-17.6 vol.% TiB and 37.9 vol.% TiB) processed by HIP are almost identical (0.29), as shown in Table 7.
Elastic Properties
The elastic modulus and Poisson's ratio values of the composites determined through ultrasonic wave propagation [22] are presented in Table 7. The Young's modulus and shear modulus values increase remarkably, while the Poisson's ratio decreases with an increase in the volume fraction of TiB [20]. The maximum Young's modulus was 185 GPa, observed in the composite (with 38.5 vol.% TiB) processed by SPS, which is marginally higher than the Young's modulus of the composites processed through HIP and vacuum sintering. This resulted from randomly oriented interconnected TiB whiskers and fine TiB needles in the composites. It is interesting to note that the Young's modulus of the composite (with 37.9 vol.% TiB) processed by vacuum sintering is comparable with that of the composite processed by HIP. This may be due to the lower amount of TiB2 in the processed composite, as evidenced by the XRD studies and SEM micrographs. This TiB2 content can substantially increase the modulus beyond that of the Ti– TiB composites due to the high elastic modulus (540 GPa) of TiB2 [18]. However, for the composite (with 17.6 vol.% TiB) processed by vacuum sintering, the elastic modulus (127.86 GPa) is relatively lower than that of the composites processed by SPS
5.
Conclusion
Ti–TiB composites with two different volume fractions (aiming 20 and 40 vol.% TiB) were processed through HIP, SPS and vacuum sintering. The following conclusions can be drawn: • The EPMA results reveal the distribution of Ti and boride phases in the composites. TiB is identified as the predominant boride phase in the Ti matrix. The same is confirmed by the XRD results (Fig. 2a and b). • The processed composites also reveal the presence of an unreacted TiB2 phase. • There is no evidence of a Ti3B4 phase in any of the composites. • The volume fractions of the TiB phase in the composites were determined through image analysis, which confirms a reasonable agreement with the target volume percentages of 20 and 40 of TiB as reinforcement. • The microstructure of TiB reinforcement reveals three morphologies, namely needle-shaped whiskers, short agglomerates or clusters of whiskers and very fine plates.
Table 7 – Experimental values of Young's modulus, shear modulus and Poisson's ratio. Technique SPS HIP Vacuum sintering
Composite
Measured density (g/cm3)
Longitudinal velocity (m/s)
Transverse velocity (m/s)
Young's modulus (GPa)
Shear modulus (GPa)
Poisson's ratio
Ti-24% TiB Ti-38.5% TiB Ti-20.6% TiB Ti-38.3% TiB Ti-17.6% TiB Ti-37.9% TiB
4.90 4.75 4.92 4.90 4.20 4.685
6484 7016 6418 6788 6488 6572
3429 3908 3547 3700 3409 3661
150.94 185.00 159.70 172.88 127.86 160.12
57.61 72.54 61.90 67.88 48.80 62.77
0.31 0.28 0.29 0.28 0.31 0.27
MA TE RI A L S CH A R A CT ER IZ A TI O N 7 3 (2 0 1 2) 4 3–5 1
• The HIP and SPS processes yield highly dense sintered composites. • The elastic modulus was measured by the ultrasonic method. The elastic modulus, shear modulus and microhardness were found to increase with the increasing volume fraction of TiB. • The composites processed using SPS and HIP exhibit better elastic properties and higher hardness compared to the composites processed by vacuum sintering. • The Poisson's ratio of the composites is reduced with an increase in the volume fraction of TiB.
Acknowledgements The authors acknowledge the All India Council for Technical Education (AICTE), New Delhi for providing a grant to carry out this work at the Dr. Mahalingam College of Engineering and Technology, Pollachi, India. The authors also acknowledge Dr. G. Apparao, DMRL, Hyderabad, Dr. Anish Kumar, IGCAR, Kalpakkam, Mr. Govindaraj, NFTDC, Hyderabad and Dibyendu Chackravarthy, ARCI, Hyderabad for their support in processing the composites. The guidance received from Dr. B. Ravisankar, NIT-Trichy, Ms. Kalavathy, IGCAR-Kalpakkam, Dr. V. Balusamy, and Dr. R. Subramanian, PSG Tech, Coimbatore is also gratefully acknowledged.
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