TiB2 particulate composites

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Wear 265 (2008) 134–142

Tensile and wear behaviour of in situ Al–7Si/TiB2 particulate composites S. Kumar a , M. Chakraborty b , V. Subramanya Sarma a , B.S. Murty a,∗ a

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India b Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur 721302, India Received 1 September 2006; received in revised form 9 July 2007; accepted 19 September 2007 Available online 26 November 2007

Abstract Al–7Si alloy reinforced with in situ TiB2 particles was synthesized successfully by using salt reaction route. These in situ composites have shown significant improvement in mechanical properties in comparison to the base alloy. The wear resistance of the alloy also significantly improved with the addition of TiB2 particles. The hardness, strength and wear resistance increased with increasing TiB2 content of the composites. TiB2 appears to not only act as a grain refiner for primary ␣-Al but also as a modifier of Si in eutectic mixture. The mechanical properties of the present Al–Si/TiB2 composites are better than those reported earlier with SiC reinforcement. Analysis of the worn surface of Al–Si/TiB2 composites tested under normal loads of 40 and 120 N suggests that adhesion and ploughing are predominant at lower loads and delamination is predominant at higher loads. © 2007 Elsevier B.V. All rights reserved. Keywords: In situ composites; Al–7Si alloy; TiB2 ; Dry sliding wear

1. Introduction Al–Si alloy-based composites are widely used in automotive, aerospace and mineral processing industries because of improved properties such as strength, stiffness, tribological behaviour and a low thermal expansion coefficient. Conventional practice of preparation of Al–Si alloy-based composites (ex situ composites) involves the addition of particles such as SiC to the liquid aluminium by techniques like stir casting which could lead to segregation of reinforcement particles and poor adhesion at the interface, unless the matrix and/or particles are suitably modified [1–4]. Recently, in situ techniques have been developed to fabricate aluminium-based metal matrix composites (MMCs), which can lead to better adhesion at the interface and hence better mechanical properties [5,6]. In the in situ process, ultrafine ceramic particles are formed in situ by the exothermic reaction between the elements or their compounds with molten aluminium alloy. These in situ routes provide advantages such as uniform distribution of reinforcement, finer reinforcement particle size, clear interface and thermodynamically stable reinforcement in comparison the conventional ex situ processes.

∗

Corresponding author. Tel.: +91 44 22574754; fax: +91 44 22574752. E-mail address: [email protected] (B.S. Murty).

0043-1648/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2007.09.007

Eventhough the in situ composites have significant advantages, some synthesis routes may lead to composites with inhomogeneous microstructure with various unstable and/or undesirable phases. These undesirable phases might drastically reduce the mechanical properties. Tee et al. [7] have investigated the wear performance of in situ Al/TiB2 and Al–4Cu/TiB2 composites and found that the presence of Al3 Ti along with TiB2 particles lowers the wear resistance of the MMCs. Tyagi [8] also showed the presence of Al3 Ti phase in the microstructure of Al–Si/TiC composites. Wu et al. [9] varied the volume fraction of the two phases (Al3 Ti and TiB2 ) and showed that higher volume fraction of Al3 Ti leads to higher wear loss. Hence in order to have good mechanical and wear properties it is important to control the Al3 Ti phase formation during the synthesis of in situ Al/TiB2 composites. Among the different routes to synthesise Al–Si/TiB2 in situ MMCs [10–13], the salt route has more advantages in terms of easy control of phases, less contamination and possibility of bulk and continuous casting. In the present work, using salt route, Al3 Ti intermetallic phase formation has been completely eliminated in the Al–7Si matrix to yield only TiB2 particles and similar result has been reported earlier in Al–4Cu system by the present group [14–16]. It had been shown that by eliminating such brittle intermetallic phase, the dry sliding wear resistance [15] and abrasive wear resistance [16] of Al–4Cu alloy can be

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improved significantly. The present report deals with the synthesis of Al–7Si/TiB2 in situ composites and the influence of TiB2 reinforcement on tensile and wear properties of the Al–7Si alloy. 2. Experimental procedure In situ composites of Al–7Si alloy reinforced by 0, 5 and 10 wt.% TiB2 (0, 3.4 and 6.8 vol.%, respectively) were fabricated by salt route. Al–7Si alloy was prepared by melting together appropriate amounts of commercial purity Al (CP Al) and Al–20%Si master alloy. TiB2 is introduced into the alloy by the reaction of the molten alloy with halide salts (K2 TiF6 and KBF4 ) at 800 ◦ C for 60 min. Further details of fabrication processes can be found elsewhere [14,17]. The alloy and composites have been characterized by X-ray diffraction (XRD) using Cu K␣ radiation, scanning electron microscopy (SEM) and energy dispersive X-ray microanalysis (EDS). After conventional metallographic polishing, the samples were electro-polished with 39% orthophosphoric acid, 37% ethanol and 24% water by volume. Image analyser was used to study both secondary dendritic arm spacing (SDAS) and Si particles size distribution from images captured at 200× by optical microscopy. TiB2 particles were extracted by dissolving the Al matrix using 20% NaOH solution (in water) and the extracted particles were characterized by XRD and sizes of extracted TiB2 particles were measured using particle size analyser. The Vickers’s hardness of the composites was determined at 5 kg load. The tensile tests were carried out by Shimadzu AGIS 250 kN unit. Dry sliding wear tests were conducted using pin-on-disc wear testing machine. During the wear test, the specimen pin (alloy and composites) of 8 mm diameter and

Fig. 1. XRD patterns of Al–7Si–xTiB2 composites.

15 mm height slides against AISI 52100 steel disc (hardness ≈61 HRC). A track diameter of 45 mm has been used for all the experiments. All experiments were conducted in air at room temperature (29–32 ◦ C) and at a relative humidity of 50–60%. The wear tests were conducted at constant sliding velocity of 1 ms−1 and sliding distance of 800 m at various loads ranging from 40 to 120 N. Both the pin and disc surface were polished with 600 grit emery paper followed by diamond polishing to get uniform roughness, as surface roughness plays an important role on the wear behaviour. The average surface roughness, Ra , of the machined pin specimen and the disc before wear testing was measured by using Perthometer (M2 Mahr GMBH, Germany), and they lie in the range of 0.32–0.62 ␮m. The wear rate has been calculated by dividing the volume loss by sliding distance, where the volume loss is calculated as the ratio of weight loss of

Fig. 2. SEM micrographs of (a) Al–7Si alloy, (b) Al–7Si/5TiB2 , (c) Al–7Si/10TiB2 and (d) Al–7Si/10TiB2 at higher magnification.

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the sample to the density of the sample. The worn pin surfaces were examined using SEM with EDS. 3. Results and discussion 3.1. Microstructural characterization Fig. 1 shows the XRD pattern of Al–7Si/TiB2 in situ composites. The intensity of TiB2 peaks increases with increase in the amount of TiB2 in the composite and no Al3 Ti peaks were observed. All the peaks in the XRD pattern of the particles extracted from the composite correspond to only TiB2 and no peaks of Al3 Ti have been observed, confirming the absence of the latter in the composite. Fig. 2(a)–(d) shows the SEM micrographs of the composites with different wt.% of TiB2 particles and the base Al–7Si alloy. Fig. 2(a) shows SEM micrograph of as-cast Al–7Si alloy without any reinforcement indicating the presence of dendrites of pro-eutectic ␣-Al and needle/plate like eutectic Si. Fig. 2(b) and (c) shows SEM micrographs of Al–7Si alloy with 5 and 10 wt.% of TiB2 particles. Fig. 2(d) shows the size and shape of the TiB2 particles in the composites. The TiB2 particles are mostly hexagonal in shape and the size distribution of the particles is in the range of 0.6–1.7 ␮m. It is clear from the above figures that the increase in wt.% of TiB2 changes morphology of both proeutectic ␣-Al and the eutectic Si needles. The proeutectic ␣-Al clearly shows the evidence of grain refinement and the dendritic ␣-Al changes to fine equiaxed grain structure in presence of TiB2 . TiB2 is known to be a good grain refiner of Al [18]. The average SDAS of the ␣-Al in the as-cast Al–7Si alloy is 63 ␮m and it decreased to 41–36 ␮m with the addition of 5 and 10 wt.% of TiB2 . It is also interesting to note that TiB2 restricts the growth of Si particles and changes its morphology to more or less spherical particles. Fig. 3 shows the effect of TiB2 addition on Si size distribution. It is evident that with increasing TiB2 addition, the particle size becomes finer and the peak in histogram shifts to lower values. Recently Schaffer et al. [12] had suggested that this modification is a result of interaction between the TiB2 particles and the eutectic Si due to the segregation of these particles to the eutectic Al–Si phase boundary, where they obstruct solute redistribution and refine the eutectic Si. Thus, from the above discussion it is clear that TiB2 particles not only lead to grain refinement but also have a tendency to modify the eutectic Si. 3.2. Hardness and tensile properties Table 1 shows that hardness, 0.2% proof stress and UTS increase with increase in wt.% of TiB2 in composite. It is clear that an increase of 108% in hardness, 123% yield strength, 43% in UTS and 33% in Young’s modulus has been achieved with 10 wt.% of TiB2 in Al–7Si alloy. Tensile properties of the composites in as-cast condition are compared with that of literature in Table 1. The refinement of ␣-Al SDAS and eutectic Si particle size in addition to the presence of fine TiB2 particles accounts for improvement in properties of Al–7Si/TiB2 composites. Si needles in Al–Si alloy act as stress raisers but the addition of TiB2 results in more or less equiaxed Si parti-

Fig. 3. Si particle size distribution with respect to wt.% of TiB2 particles (a) Al–7Si alloy, (b) 5 wt.% and (c) 10 wt.%.

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Table 1 Mechanical properties of Al–7Si alloy-based composites Materiala

E (GPa)

0.2% Proof stress (MPa)

UTS (MPa)

Elongation (%)

Hardness (HV 5)

Ref.

Al–7Si Al–7Si/5TiB2 Al–7Si/10TiB2 Al–7Si–GRMb Al–7Si–0.3Mg/5.5TiB2 c Al–7Si–0.35Mg/30SiCd

69 83 (20) 92 (33) – 86 (15) –

68 126 (85) 152 (124) – 100 (66) –

146 175 (20) 209 (43) 167 (15) 198 (40) 204 (−19)

10.0 7.0 (−30) 4.6 (−54) – 9.0 (−64) 0.2 (−84)

49 73 (49) 102 (108) – – –

PW PW PW [19] [10] [20]

PW—present work. ( )% increment in mechanical properties of composites with respect to matrix alloy. a All compositions are in wt.%. b After addition of both grain refiner and modifier. c As-cast and extruded. d As-cast and heat treated.

cles of 2–3 ␮m which leads to retention of good ductility with increased strength. The mechanical properties of the present ascast Al–7Si/5TiB2 composites are also comparable to as-cast and extruded Al–7Si–0.3Mg/5.5TiB2 composite reported earlier [10]. It is also important to note that the UTS and of elongation of the as-cast Al–Si/10TiB2 composite prepared in the present study are better than heat treated Al–Si–0.3Mg/30SiC composites reported earlier [20]. 3.3. Wear behaviour The variation of coefficient of friction (μ) with sliding distance for the alloy and composites at different loads (40, 80 and

120 N) are shown in Figs. 4(a)–(c). It can be noted that with the addition of TiB2 particle to the Al–Si, the friction coefficient decreases irrespective of applied load. The variation in the average coefficient of friction for the CP Al and Al–Si alloybased composites as a function of the normal load is shown in Fig. 4(d). The average coefficient of friction of the composites is in the range of about 0.25–0.35 and is less than that of matrix Al–Si alloy which lies in the range of 0.37–0.40. The values of μ stated above are mean values of those measured in the given sliding range under steady state condition. The coefficient of friction (μ) of the composites is less than that of the Al–Si and decreases as the amount of TiB2 particles increases. This demonstrates that the friction property of Al–7Si is changed by

Fig. 4. Coefficient of friction of CPAl, Al–7Si alloy and composites as a function of normal load (a) 40 N, (b) 80 N, (c) 120 N and (d) average coefficient of friction.

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ence of secondary phases such as Si and TiB2 which restrict the flow of metal during sliding. This observation was supported by Clarke and Sarkar [23], who reported that the wear resistance of Al improved with addition of Si content up to near-eutectic composition. The wear rate of the Al–7Si/10TiB2 is about 50–60% lower than that of the Al–7Si. The standard deviation for the wear rate is ± 1.71 × 10−5 mm3 /m and the regression coefficient lies in the range of 0.96 and 0.98. Fig. 5(b) shows the wear rate of Al–7Si alloy as the function of wt.% of TiB2 particles. At lower load the effect of TiB2 on the wear rate of Al–7Si is less significant, while at higher load it is quite significant, which could be due to the work hardening tendency of matrix alloy in the presence of TiB2 particles. Fig. 5(b) also shows that the wear rate significantly decreases with increase in amount of TiB2 in the composites. Prasada Rao et al. [24] showed that the wear rate of Al–7Si alloy depends on grain size/dendritic arm spacing and the size of the silicon particles. Thus, the improvement in the wear resistance in the present in situ composites is not only due to the presence of fine TiB2 particles but also due to refinement in grain size and modification and refinement of eutectic Si.

Fig. 5. Wear rate of CP Al and Al–7Si alloy-based composites as a function of (a) normal load and (b) amount of TiB2 particles.

incorporating of TiB2 particles and this could be due to the characteristics of in situ composites such as clear interface (without any reaction product at the interface between the matrix and reinforcement), improved dispersion and smaller particle sizes. Thakur and Dhindaw [21] demonstrated that good dispersion and better interface of the particle in matrix leads to a lower value of coefficient of friction. It has also been proved that the coefficient of friction is reduced by TiB2 more effectively than SiC [10]. Recently, Min et al. [22] compared the coefficient of friction of Al/TiB2 composites with Al/SiC composites and found that it is 0.70 for Al/SiC and 0.16–0.17 for Al/TiB2 . The lower μ values in the present composites in comparison to the base alloy could be attributed to uniformly distributed fine TiB2 particles. The influence of applied normal load on the wear rate of Al–7Si alloy with different wt.% of TiB2 particle is depicted in Figs. 5(a) and (b). Fig. 5 clearly shows that an increase in applied normal load increases the wear rate. However the wear rate increases significantly with load in CP Al while it is much lower in the composites studied. CP Al exhibits very high wear rate and seizes at 100 N. In case of Al–7Si and composites seizure was not observed at 100 N, which is due to the pres-

Fig. 6. (a) Normalized wear rate of Al–7Si alloy and composites as a function of load and amount of TiB2 particles and (b) specific wear rate of Al–7Si alloybased composites as a function of load and hardness.

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Fig. 7. SEM micrographs of worn surface of (a, b) Al–7Si and (c) Al–7Si/10TiB2 at a normal load of 40 N. The arrow mark shows the sliding direction.

Wear resistance is defined as the reciprocal of wear rate. The wear resistance of CP Al is improved by the addition of Si and further significantly improved with increase in the wt.% of TiB2 . It has been suggested that the grain refinement and/or modification improves wear resistance and the load bearing capacity during dry sliding of cast Al–7Si alloy [18]. Similarly, in the present investigation, addition of TiB2 particles resulted in a combined effect of grain refinement, modification and dispersion strengthening leading to an improvement in wear resistance of Al–7Si alloy. The increase in load bearing capacity of Al–7Si alloy by the addition of TiB2 particles can be understood clearly from normalized wear rate, which is the ratio of the wear rate of the Al–7Si/TiB2 composites and that of matrix Al–7Si alloy. Fig. 6(a) shows the relation between the normalized wear rate and the amount of reinforcement as a function of load. The normalized wear rate decreases gradually as the amount of TiB2 particles increases. It is important to note that normalized wear rate decreases as the applied load increases for any given amount of TiB2 particles. This may be attributed to the work hardening of the matrix and possible refinement of TiB2 particles during wear at high loads. The influence of hardness on the specific wear rate (SWR) is depicted in Fig. 6(b). SWR is defined as wear volume per unit load per unit sliding distance. The figure shows the SWR as a function of hardness at 40 and 120 N loads. It can be observed that SWR is strongly dependent on the hardness, and as the hardness increases the SWR decreases. This result is consistent with Archard’s equation [25] (Q = kW/H), in which the wear rate is

inversely proportional to hardness of the soft material (composite pin) in the present case, where Q is the volume removed from the surface per unit sliding distance, W the normal applied load to the surface by the counter body and H is the indentation hardness of the wearing surface and k the wear coefficient. It is also interesting to note that SWR at higher load (120 N) is less than that at lower load (40 N). Fig. 7 shows the SEM micrographs of the worn surfaces of Al–7Si and Al–7Si/10TiB2 composites at a normal load of 40 N, respectively. The worn surface of Al–7Si alloy (Fig. 7(a)) exhibits deep long grooves along the direction of sliding and the average surface roughness (Ra ) of this worn surface is 3.62 ␮m. Ploughing is the processes of displacing material from groove to sideways to form ridges adjacent to the grove produced and thus repeated sliding of hard asperity leads to metal loss. High magnification figure (Fig. 7(b)) shows the evidence of ploughing mechanism predominates in Al–7Si alloy. The worn surface (Fig. 7(c)) of Al–7Si/10TiB2 composites reveals narrow groves and almost flat surface. The average surface roughness (Ra ) of this worn surface is 1.89 ␮m, which is much smaller than that of Al–7Si alloy. This is due to the presence of TiB2 particles which restrict the flow of the material under the applied load. In addition, the worn surface of the composites has higher hardness and hence the ploughing tendency of the hard asperity of the counterface is less when compared to the soft matrix alloy. Fig. 8(a) and (b) shows the SEM micrographs of worn surfaces of the Al–7Si alloy samples tested under a load of 120 N. Fig. 8(a) exhibits deep crater on the worn surface of Al–7Si alloy and a closer examination of the worn surface also shows the

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Fig. 8. SEM micrographs of worn surface of (a, b) Al–7Si alloy and (c, d) Al–7Si/10TiB2 at different magnifications under a normal load of 120 N. The arrow marks show the sliding direction.

evidence of metal flow (Fig. 8(b)). This indicates that Al–7Si alloy on sliding at high load deforms plastically and progressive transfer of material to the counterface and its redepostition leads to seizure of matrix Al–7Si alloy. The morphologies of the worn surface suggest that the wear mechanism is adhesion in case of Al–7Si alloy. SEM micrographs of worn surface of Al–7Si/10TiB2 samples tested under a high load of 120 N are shown in Fig. 8(c) and (d). Fig. 8(c) illustrates granular nature of shallow crater on the worn surface, which had been demonstrated earlier by Clarke and Sarkar [23]. These craters form on the surface due to surface force and grow along the perimeter of the crater and a closer examination at high magnification (Fig. 8(d)) shows the evidence of fracture in transfer layer. The transfer layer on the worn surface of these composites contains a mixture of iron from steel disc along with composite material. Venkataraman and Sundararajan [26] and Animesh et al. [15] have found similar type of layer and it has been called as mechanically mixed layer. The present results also indicate that Al–7Si/10TiB2 composites on sliding at high load exhibit mechanically mixed layer, which was subsequently removed by delamination caused by craze cracks. Under such conditions, as demonstrated by Clarke and Sarkar [23], progressive surface rupture takes place probably as a result of surface force rather than subsurface fracture. Such detached layers get entrapped between the sliding surfaces resulting in three body abrasive wear. In three-body abrasion wear, the loss in metal is due to the abrasive action of the entrapped layer/particle between the two sliding conduct surfaces [27]. In general, three body abra-

sion results in lower wear rate than that of two-body abrasion. Thus in the present case, the entrapped Fe rich layer results in lower wear rate and coefficient of friction for composites when compared to that of Al–7Si alloy. Thus, the wear mechanism of the Al–7Si/10TiB2 at high load is dominated by delamination of mechanical mixed layer. Comparison of the morphologies of the worn surfaces of Al–7Si/10TiB2 samples tested under loads of 40 N (Fig. 7(c)) and 120 N (Fig. 8(c)) suggest that adhesion and ploughing are predominant at lower loads and delamination is predominant at higher loads. In order to understand the changes that occur on the sliding surface of the samples during wear, EDS analysis was carried out. Fig. 9 (a) and (b) shows the EDS spectra on worn surface of Al–7Si and Al–7Si/10TiB2 tested at 120 N and these reveal the presence of Fe on both the alloy and composite specimen. Fig. 9 (c) shows the Fe content variation on the worn surface with TiB2 content and applied load. These results indicate that the worn surfaces of the composites were covered by layers containing Fe from the counterface material confirming the presence of mechanical mixed layer. The presence of Fe rich layers have been reported earlier during wear studies [26–28]. The formation of Fe rich layer on the worn surface may also contribute to lower the coefficient of friction (by acting as a lubricant) and wear rate of the composites in comparison to matrix alloy. It is interesting to note that the maximum Fe content on the surface of the composites observed in the present study is much lower than that observed by Kwok et al. [29] in case of Al/SiC composites (50 wt.% Fe) at 100 N load which suggests that the extent of

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abrasion of TiB2 particles on the steel counter surface is lower than that of SiC particles. 4. Conclusions Al–7Si/TiB2 in situ composites with significant improvement in hardness, yield strength, tensile strength, Young’s modulus and wear resistance have been successfully synthesized by salt reaction route. The mechanical properties of the present Al–7Si/TiB2 composites are better than those reported earlier with SiC reinforcement. TiB2 appears to not only act as a grain refiner for primary ␣-Al but also act as modifier of Si in the eutectic mixture. Analysis of the worn surface of Al–7Si/10TiB2 samples tested under normal loads of 40 and 120 N suggests that adhesion and ploughing are predominant at lower loads and delamination is more predominant at higher loads. Acknowledgements The present work was funded by the Naval Research Board, Government of India. Authors would like to thank Prof. M. Rathinasabapathi, Annamalai University and Dr. M. Kamaraj, IIT Madras for help in SEM and wear studies, respectively. References

Fig. 9. EDS spectrum of worn surface on (a) Al–7Si alloy, (b) Al–7Si/10TiB2 composite when slide under 120 N and (c) average value of Fe content on worn surface measured from EDS as a function of amount of TiB2 and applied load.

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