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Ageing behaviour of A356 alloy reinforced with in-situ formed TiB2 particles
Materials Science and Engineering A 489 (2008) 220–226
Ageing behaviour of A356 alloy reinforced with in-situ formed TiB2 particles A. Mandal a,∗ , M. Chakraborty a , B.S. Murty b a
Department of Metallurgical & Materials Engineering, Indian Institute of Technology, Kharagpur 721302, West Bengal, India b Department of Metallurgical & Materials Engineering, Indian Institute of Technology, Madras, Chennai 600036, Tamil Nadu, India Received 1 July 2007; received in revised form 8 December 2007; accepted 16 January 2008
Abstract The alloy A356 was reinforced with in-situ TiB2 particles via salt-metal reaction, i.e., the reaction of K2 TiF6 and KBF4 salts with the molten alloy. The study was undertaken to investigate the ageing response of the alloy when dispersed with TiB2 particles. The results indicated that the ageing time comes down from 12 h in the base alloy to 4 h in the composite with 10 wt.% TiB2 . It was also observed that different amounts of TiB2 significantly alter the particle size distribution of Si which consequently affect the mechanical properties. Further, it was also noted that thermal modification plays an important role in retaining the ductility of composites. © 2008 Elsevier B.V. All rights reserved. Keywords: In-situ composites; Al-TiB2 ; Age hardening; Mechanical properties; Particle size distribution
1. Introduction It has been well established that addition of ceramic particles to age hardenable aluminium matrices results in composites exhibiting different ageing kinetics compared to the unreinforced matrix. Such age hardening behaviour was demonstrated with different combinations of matrices and reinforcements. While some studies show that the addition of reinforcement leads to an acceleration of the ageing kinetics [1–4], some others conclude a decrease or very little alteration in ageing kinetics of the base alloy with reinforcement addition [5–8]. In one of the studies on A356/SiC composites, the decrease in ageing kinetics was attributed to the depletion of Mg in the form of a spinel, MgAlO4 [9]. Since then several authors have investigated the effect of different reinforcement particles on A356 alloy [9–11]. Few others have also studied the effect of TiB2 particles reinforced in A356 alloy by different routes [8,12,13]. Earlier, Wood et al. [13] had synthesized A356/TiB2 composites via a salt route technique involving K2 TiF6 and KBF4 salts, but they did not investigate ∗ Corresponding author at: Metal Processing Institute, Worcester Polytechnic Institute, Massachusetts 01609, USA. Tel.: +1 508 831 6503; fax: +1 508 831 5993. E-mail address: [email protected] (A. Mandal).
0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.01.042
the age hardening behaviour of the composites. Thereafter no systematic ageing study on such composites was carried out. The present study is an attempt to bring out the effect of in-situ TiB2 particles on the age hardening behaviour of A356 alloy. Hence different weight fractions of TiB2 were reinforced in A356 alloy via a salt route technique involving K2 TiF6 and KBF4 salts. A systematic study of the base alloy and composites was done using the Vickers hardness measurement and the corresponding age hardening curves were obtained. 2. Experimental details Commercial A356 alloy reinforced with TiB2 particles were used for the present study. The composition of the base alloy is indicated in Table 1. In the present study, A356/xTiB2 (x = 2.5, 5, 7.5, 10 wt.%) in-situ composites were prepared by melting together appropriate amounts of A356 alloy and allowing the melt to react with K2 TiF6 and KBF4 salt mixture at 800 ◦ C for 60 min. The melting was carried out in an electrical resistance furnace. The melt was mechanically stirred intermittently with a graphite rod coated with zirconia paste at regular intervals of 10 min so as to allow complete reaction of the salts with the alloy. A schematic diagram of the set up is shown in Fig. 1. After the completion of reaction time of 60 min the dross which
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Fig. 1. Schematic diagram of synthesis of A356/TiB2 composites. Table 1 Composition of A356 alloy Si
Mg
Fe
Cu
Mn
Al
6.641
0.486
0.263
0.029
0.023
balance
floats on the top of the melt was decanted off and the composites were cast in horizontal metal moulds. Thereafter the composites were characterised by X-ray diffraction (XRD) using Cu K␣ radiation, scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) microanalysis and the Leica image analyser. Prior to the ageing studies, the alloy and composites were solutionised at 540 ◦ C for 8 h followed by quenching in cold water. This is referred to as solutionised condition. The ageing temperature was maintained at 155 ◦ C for different time intervals. The above heat treatment sequence is termed as T6 treatment. The composites subjected to ageing were tested for their hardness using Vickers hardness tester at 5 kg load (hence the hardness is represented as HV5 ). Each hardness value presented is an average of at least ten symmetrical indentations. The tensile properties were evaluated using an electronic Hounsfeld Tensometer. The samples were machined as per ASTM E8 specifications. The tensile properties reported are an average of three tests at each condition. 3. Results and discussion 3.1. Synthesis and characterization of A356/TiB2 composites The age-hardenable A356 alloy was reinforced with various amounts of in-situ TiB2 particles. The XRD patterns show that TiB2 peaks are prominent only at higher wt.% of the particles (Fig. 2a). The complete absence of brittle Al3 Ti particles is also evident from the XRD patterns. The presence of TiB2 particles was further confirmed by XRD of the extracted particles obtained by chemical dissolution technique (Fig. 2b). The technique involves the dissolution of the matrix in NaOH solution which leaves behind TiB2 particles only.
Fig. 2. XRD patterns of (a) A356/TiB2 MMCs (b) extracted TiB2 particles from A356/10 wt.% TiB2 composite.
The optical photomicrographs of A356 alloy and composites with different wt.% of TiB2 particles in their as cast condition were observed. The typical as cast microstructure of A356 alloy consists of a coarse eutectic phase located in between the Al dendrites. The micrographs suggest that as the amount of TiB2 particles in the matrix increases, the dendritic regions become concentrated with TiB2 particles. As a result the size of the Si particles in the dendritic region decreases progressively with TiB2 addition. This is because as the melt solidifies fine TiB2 particles along with Si particles are pushed away by the solidification front towards the interdendritic region thereby restricting the growth of Si particles. A comparison of A356/5TiB2 (Fig. 3a) and A356/10TiB2 composites (Fig. 3b) suggest that with the increasing number of TiB2 particles, the tendency for the agglomeration increases. The continuous network structure comprising of “TiB2 and Si particles” becomes more prominent in case of A356/10TiB2 composite (Fig. 3b). The decrease in the size of Si particles is more evident at higher magnification. The decrease is clearly evident in the composites with 2.5 and 5 wt.% TiB2 particles. Earlier, such observations were not reported by Wood et al. [13].
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Fig. 3. Optical micrographs (at 500×) of as cast A356 based composites reinforced with (a) 5 wt.% (b) 10 wt.% TiB2 particles.
In peak-aged condition, the Si particles in the alloy as well as composites are completely spheroidized due to a phenomenon termed as thermal modification. This is quite evident from SEM photomicrographs in Fig. 4 which shows that the size of the modified Si particles in the alloy (Fig. 4a) are significantly reduced on addition of 2.5 wt.% TiB2 particles (Fig. 4b). The microstructures also suggest that the size of already reduced Si particles due to increment of TiB2 particles is further spheroidized due to T6 treatment. The decrease in the size of the Si needles, which is an attribute of growth restriction imposed by the increasing amount of TiB2 particles, is prominent in composites with higher wt.% of TiB2 particles. Such reduction in particle size of silicon has been reported earlier in other Al–Si alloy system [14]. The thermal modification of reduced Si particles may have an important implication on the tensile properties of the composites. Fig. 5a is the histogram showing the size distribution of Si particles in the alloy as well as composites with 2.5 and 10 wt.% TiB2 particles under T6 condition. In case of the base alloy, majority of the Si particles fall in the size range of 3–12 m, while a small percentage lie on the either extreme. Addition of 2.5 wt.% TiB2 to the base alloy changes the microstructure as well as the size distribution of Si particles with most of the particles in the size lying in range of 2–6 m. Further incorporation of TiB2 particles in the composite leads to a drastic change in the distribution of Si particles. In the composite with 10 wt.% TiB2 particles the size of Si particles drops down in the range
Fig. 4. SEM photomicrographs of peak aged A356 based composites reinforced with (a) 0 wt.%, base alloy (b) 2.5 wt.%.
of 1–4 m. The change in the size distribution of Si particles is clearly reflected in the SEM photomicrographs shown in Fig. 4. It was calculated that the percentage of Si particles in the size range of 1–3 m increases from a mere 4% in the base alloy to as high as 46% and 96% in the composite with 2.5 and 10 wt.% TiB2 particles. A higher amount of fine Si particles at higher weight fractions of TiB2 could have a beneficial effect on the properties of the composites, especially the ductility. Fig. 5b shows the mean size of Si particles as a function of wt.% TiB2 . The plot shows that the decrease in size of Si particles is very sharp at lower TiB2 content (2.5 and 5 wt.%) while at higher TiB2 content (7.5 and 10 wt.%) the decrease is very gradual. 3.2. Ageing studies of A356/TiB2 composites Vickers hardness measurements were used to ascertain the age hardening behaviour of the composites in the present study. Fig. 6a shows the bulk hardness of the alloy and composites as a function of ageing time. In absence of TiB2 particles, i.e., A356 alloy, the age hardening curve is very sharp and a peak hardness value of 116 VHN is attained after 12 h of ageing, while in case of the composites the curve is quite smooth, i.e., the drop in hardness with time is gradual. The time required to attain the peak hardness in the composites with 2.5 and 5 wt.% TiB2 is 6 h, and for the composites with 7.5 and 10 wt.% TiB2 the time further reduces to 4 h. So the time to attain the peak hardness is reduced by 2–3 times in case of composites. This is in agreement with
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Fig. 5. (a) Particle size distribution of Si in A356/TiB2 MMCs and (b) mean size of Si particles as a function of TiB2 in A356/TiB2 MMCs.
the previous studies on Al–4Cu–TiB2 composites which showed similar accelerated ageing behaviour [4]. It is worth mentioning that the peak hardness of the composite with even 10 wt.% TiB2 (HV 89) is lower than that of the base alloy (HV 116). The plot also shows that with the increase in amount of TiB2 , the peak hardness attained by the composite decreases and then increases slightly after 5 wt.% TiB2 (Fig. 6b). This is unlike the previous observations involving TiB2 particles by the same authors [4]. The marginal increase in bulk hardness of the composites at higher weight fractions of TiB2 particles (in T6 condition) can be attributed to the compensation of loss in hardness (due to loss of Mg2 Si) by increasing weight fraction of the hard TiB2 particles. Such an anomaly in the age hardening behaviour of A356 alloy reinforced with TiB2 particles has not been reported earlier, though only few studies have been carried out in such system [13]. Thus it is strongly suggested that such a behaviour is due to the loss of Mg, which otherwise would have led to a still higher
hardness of the composites. However the decrease in the peak ageing duration can be attributed to the development of strain field around the matrix-reinforcement region due to differential coefficients of thermal expansion between Al (23.5 × 10−6 /K) and TiB2 (7 × 10−6 /K) [16]. Thus the resulting dislocations act as nucleating sites for the precipitation of Mg2 Si, though to a lesser extent under the present circumstances. A comparison of the hardness of the base alloy and composites under different conditions is shown in Figure 6b. In the as cast condition, there is an overall increase of hardness by about 21 VHN in A356/10TiB2 composite as compared to the base alloy. Thus, it can be said that the increase in hardness is not quite significant even after addition of 10 wt.% of TiB2 particles. Also it can be seen that addition of 2.5 wt.% TiB2 particles into the alloy causes a slight drop in the hardness, which then increases on further addition of TiB2 particles. The alloy and the composites show a higher bulk hardness in the as cast condition
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Fig. 6. (a) Hardness of the composites as a function of ageing time (b) bulk hardness of the A356/TiB2 composites under different conditions.
than in the solutionised condition. This is due to the phenomena known as “delayed ageing” exhibited in such alloys [15]. It is interesting to note that the drop in hardness for the above two conditions is approximately by the same amount (HV5 = 15) in all the cases. However, there is a marked increase in the peak hardness of the base alloy as compared to composites. In case of the composite reinforced with 2.5 wt.% TiB2 particles the increase in peak hardness is maximum (as compared to solutionised condition), while in other composites the increase is to a much lesser extent. This may be due to the depletion of age hardening precipitates, Mg2 Si, from the composites. It is suggested that there is a possible loss of Mg as a result of the reaction of molten alloy with K2 TiF6 and KBF4 salts leading to the formation of complex compounds. One of the difficulties encountered in the present system is the detection of Mg content in the melts. The lower Mg content in the base alloy and a further lower amount in the composites makes Mg analysis a difficult proposition by the present characterization techniques. So it was attempted to analyze the dross (decanted after the completion of the reaction) for its Mg content. The EDX microanalysis of the dross (Fig. 7a) clearly showed the presence of Mg peaks thus providing a clue to the drop in hardness of the composites in their as cast condition. The reaction of the molten alloy with
Fig. 7. (a) EDX microanalysis of the dross (b) difference in hardness between the peak aged and solution treated composites.
the fluoride salts leads to the formation of some complex compounds that is lost along with the dross. So the final composite is deprived of Mg and consequently the age hardening precipitates of Mg2 Si. Such a possible loss of Mg was not reported earlier by Wood et al. [13]. It can be said that the overall hardness of the composite is a function of the amount of in-situ TiB2 particles and Mg2 Si precipitates. At lower weight fraction of TiB2 , the contribution of Mg2 Si precipitates is significant, while at higher weight fraction the hard TiB2 particles themselves dictate the hardness of the composite. Fig. 7b shows the difference between the peak hardness of different composites and the corresponding solution treated composites (lower curve) and the solution treated base alloy (upper curve). The figure clearly distinguishes the increment in the hardness due to precipitates and TiB2 particles. It is evident that without the addition of TiB2 particles the increment in the hardening is solely by Mg2 Si precipitates. However in presence of 2.5 wt.% TiB2 , the contribution from TiB2 particles diminishes till 5 wt.% TiB2 and then increases on further addition of particles. The closeness of the hardness of the composite with 5 wt.% TiB2 and the base alloy in the solutionised condition
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(also see Fig. 6b) results in very little difference in hardness as indicated in Fig. 7b. However it may not be deduced that the contribution of TiB2 particles is negligible but rather lower than expected. Also it can be said the composite with 5 wt.% TiB2 , the hardness contribution from Mg2 Si and TiB2 particles are very close. Thus, it may be stated that Fig. 7b is an effective way of delineating the hardness contribution form different phases in such composites. 3.3. Tensile properties of A356/TiB2 composites The tensile properties of composites in their as cast and peak aged conditions are compared with that of the base alloy in Table 2. The solutionised condition refers to the state of the alloy/composite after quenching in water after holding at 540 ◦ C for 8 h, whereas peak aged condition refers to the condition at which maximum hardness is attained. The tensile properties show a considerable increase in the peak-aged condition as compared to as cast condition, as expected. The increase in tensile properties is not linear with respect to TiB2 content. This phenomenon can be ascribed to the loss of Mg from the base alloy as discussed earlier in Section 3.2. Wood et al. [13] has also shown similar trends in tensile strength but did not give any reasons for the same. The tensile strength increases from 184 MPa in case of base alloy to 256 MPa in A356/10TiB2 composite in the as cast condition, while the corresponding values are 276 and 328 MPa in the peak aged condition. So the percentage increment in the values of UTS is lower in case of the composite. This may be attributed to the lower amount of precipitation hardenable Mg2 Si precipitates in the composites owing to the loss of Mg. It is quite interesting to note that even though the peak hardness of the composite is lower than that of the base alloy, the UTS values are significantly higher. Thus, it may be said that the strength of the composites is not directly related to its hardness. The tensile properties are a strong function of the amount of precipitates and reinforcement. The shape and size of second phase particles also dictates the properties of the composites. In the present system, globular morphology of Si particles due to T6 treatment along with equiaxed TiB2 particles play a significant role in enhancing the properties of composites by minimizing the stress concentration in the matrix. The as cast composites exhibit sufficient ductility which is further increased in their peak aged condition (Table 2). The
Fig. 8. SEM fractographs of A356/TiB2 composites in peak aged (T6) condition (a) 2.5 wt.% (b) 10 wt.%.
improvement in ductility of composites can be attributed to couple effect of numerous small Si particles due to growth restriction and thermal modification during the T6 heat treatment. Both the factors help in lowering the stress concentration in the matrix to a great extent. Hence, the composites with maximum wt.% of TiB2 exhibit sufficient ductility. It is reported that the fracture occurs by preferential debonding of larger Si particles rather than the smaller ones [17]. This could be another reason for good ductility and strength of the composites. Fig. 8 shows the SEM fractograph of A356/TiB2 composites in their peak aged condition (T6 treatment). The base alloy shows a typical ductile failure but the fracture behaviour is substantially
Table 2 Mechanical properties of A356/TiB2 composites Alloy/compositea
A356 A356/2.5TiB2 A356/5TiB2 A356/7.5TiB2 A356/10TiB2 A356/4TiB2 A356/5.5TiB2 a
As cast
Peak aged
Reference
YS (0.2%) (MPa)
UTS (MPa)
El. (%)
Hardness HV5
YS (0.2%) (MPa)
UTS (MPa)
El. (%)
Hardness HV5
110 135 151 196 200 – –
184 206 215 231 256 – –
9 7 6 5 4 – –
69 61 66 75 85 – –
195 241 251 256 267 237 240
276 290 302 317 328 300 303
13 10 9 8 6 10 7
116 92 69 78 89 – –
All compositions are in wt.%.
Present work Present work Present work Present work Present work [93Woo] [93Woo]
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altered by the heat treatment and the amount of reinforcement in the composite. The fracture surface of composites with 2.5 and 10 wt.% TiB2 revealed the coexistence of ductile and cleavage fracture. The SEM photomicrograph of A356/10TiB2 composite shows shallow dimples as well as ductile shear bands indicating sufficient amount of ductility retained in the composite. It was observed that with the increase in the amount of TiB2 particles, the size and depth of the dimples decreases. The composite with 10 wt.% TiB2 shows shallower dimples than the one containing 2.5 wt.% TiB2 particles. The numerous small dimples are nucleated by large number of closely spaced particles in the “TiB2 and Si” network in A356/10TiB2 composite. 4. Conclusions The salt route technique was adopted to disperse TiB2 particles in A356 alloy successfully. The presence of in-situ TiB2 particles accelerates the ageing kinetics significantly. Also, the size distribution of Si particles is narrowed down at higher weight fraction of TiB2 particles due to their growth restriction. The finer size of Si leads to easier spheroidisation of Si particles during subsequent heat treatment. These thermally modified Si particles have beneficial affect on the tensile properties, especially the ductility.
References [1] T. Christman, S. Suresh, Acta Metall. 36 (1988) 1691–1704. [2] S. Suresh, T. Christman, Y. Sugimura, Scripta Metall. 23 (1989) 1599–1602. [3] L. Lu, M.O. Lai, Y. Su, H.L. Teo, C.F. Feng, Scripta Mater. 45 (2001) 1017–1023. [4] A. Mandal, R. Maiti, M. Chakraborty, B.S. Murty, Mater. Sci. Eng. A 386 (2004) 296–300. [5] A.N. Abdel-Azim, Y. Shash, S.F. Mostafa, A. Younan, J. Mater. Proc. Tech. 55 (1995) 140–145. [6] K.C. Chen, C.G. Chao, Metall. Trans. 26A (1995) 1035–1043. [7] W. Li, J.P. Long, S. Jing, B.L. Shen, S.J. Gao, M.J. Tu, J. Mater. Eng. Perform. 12 (2003) 19–22. [8] A. Daoud, W. Reif, J. Mater. Proc. Tech. 123 (2002) 313–318. [9] N. Wang, Z. Wang, G.C. Weatherly, Metall. Trans. 23A (1992) 1423–1430. [10] Y.T. Zhao, Q.X. Dai, X.N. Cheng, S.C. Sun, Int. J. Mod. Phy. B 17 (2003) 1292–1296. [11] H. Akbulut, Y. Yalcin, F. Yilmaz, Proc. I MECH E Part L J. Mater.-Des. Appl. 216 (2002) 1–7. [12] M.D. Salvador, V. Amigo, N. Martinez, C. Ferrer, J. Mater. Proc. Tech. 143/144 (2003) 598–604. [13] J.V. Wood, P. Davies, J.L.F. Kellie, Mater. Sci. Tech. 9 (1993) 833–840. [14] A. Mandal, M. Chakraborty, B.S. Murty, International Symposium of Research Students on Material Science and Engineering Conference Proceedings (CDROM), IIT Madras, India, 2004, pp. 1–6. [15] S. Murali, K.T. Kashwap, K.S. Raman, K.S.S. Murthy, Scripta Metall. Mater. 29 (1993) 1421–1426. [16] S.K. Varma, J. Ponce, M. Solis, S. Andrews, D. Salas, Metall. Trans. 27A (1996) 2023–2034. [17] M.D. Dighe, A.M. Gokhale, Scripta Mater. 37 (1997) 1435–1440.
Ageing behaviour of A356 alloy reinforced with in-situ formed TiB2 particles A. Mandal a,∗ , M. Chakraborty a , B.S. Murty b a
Department of Metallurgical & Materials Engineering, Indian Institute of Technology, Kharagpur 721302, West Bengal, India b Department of Metallurgical & Materials Engineering, Indian Institute of Technology, Madras, Chennai 600036, Tamil Nadu, India Received 1 July 2007; received in revised form 8 December 2007; accepted 16 January 2008
Abstract The alloy A356 was reinforced with in-situ TiB2 particles via salt-metal reaction, i.e., the reaction of K2 TiF6 and KBF4 salts with the molten alloy. The study was undertaken to investigate the ageing response of the alloy when dispersed with TiB2 particles. The results indicated that the ageing time comes down from 12 h in the base alloy to 4 h in the composite with 10 wt.% TiB2 . It was also observed that different amounts of TiB2 significantly alter the particle size distribution of Si which consequently affect the mechanical properties. Further, it was also noted that thermal modification plays an important role in retaining the ductility of composites. © 2008 Elsevier B.V. All rights reserved. Keywords: In-situ composites; Al-TiB2 ; Age hardening; Mechanical properties; Particle size distribution
1. Introduction It has been well established that addition of ceramic particles to age hardenable aluminium matrices results in composites exhibiting different ageing kinetics compared to the unreinforced matrix. Such age hardening behaviour was demonstrated with different combinations of matrices and reinforcements. While some studies show that the addition of reinforcement leads to an acceleration of the ageing kinetics [1–4], some others conclude a decrease or very little alteration in ageing kinetics of the base alloy with reinforcement addition [5–8]. In one of the studies on A356/SiC composites, the decrease in ageing kinetics was attributed to the depletion of Mg in the form of a spinel, MgAlO4 [9]. Since then several authors have investigated the effect of different reinforcement particles on A356 alloy [9–11]. Few others have also studied the effect of TiB2 particles reinforced in A356 alloy by different routes [8,12,13]. Earlier, Wood et al. [13] had synthesized A356/TiB2 composites via a salt route technique involving K2 TiF6 and KBF4 salts, but they did not investigate ∗ Corresponding author at: Metal Processing Institute, Worcester Polytechnic Institute, Massachusetts 01609, USA. Tel.: +1 508 831 6503; fax: +1 508 831 5993. E-mail address: [email protected] (A. Mandal).
0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.01.042
the age hardening behaviour of the composites. Thereafter no systematic ageing study on such composites was carried out. The present study is an attempt to bring out the effect of in-situ TiB2 particles on the age hardening behaviour of A356 alloy. Hence different weight fractions of TiB2 were reinforced in A356 alloy via a salt route technique involving K2 TiF6 and KBF4 salts. A systematic study of the base alloy and composites was done using the Vickers hardness measurement and the corresponding age hardening curves were obtained. 2. Experimental details Commercial A356 alloy reinforced with TiB2 particles were used for the present study. The composition of the base alloy is indicated in Table 1. In the present study, A356/xTiB2 (x = 2.5, 5, 7.5, 10 wt.%) in-situ composites were prepared by melting together appropriate amounts of A356 alloy and allowing the melt to react with K2 TiF6 and KBF4 salt mixture at 800 ◦ C for 60 min. The melting was carried out in an electrical resistance furnace. The melt was mechanically stirred intermittently with a graphite rod coated with zirconia paste at regular intervals of 10 min so as to allow complete reaction of the salts with the alloy. A schematic diagram of the set up is shown in Fig. 1. After the completion of reaction time of 60 min the dross which
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221
Fig. 1. Schematic diagram of synthesis of A356/TiB2 composites. Table 1 Composition of A356 alloy Si
Mg
Fe
Cu
Mn
Al
6.641
0.486
0.263
0.029
0.023
balance
floats on the top of the melt was decanted off and the composites were cast in horizontal metal moulds. Thereafter the composites were characterised by X-ray diffraction (XRD) using Cu K␣ radiation, scanning electron microscopy (SEM) equipped with energy dispersive X-ray (EDX) microanalysis and the Leica image analyser. Prior to the ageing studies, the alloy and composites were solutionised at 540 ◦ C for 8 h followed by quenching in cold water. This is referred to as solutionised condition. The ageing temperature was maintained at 155 ◦ C for different time intervals. The above heat treatment sequence is termed as T6 treatment. The composites subjected to ageing were tested for their hardness using Vickers hardness tester at 5 kg load (hence the hardness is represented as HV5 ). Each hardness value presented is an average of at least ten symmetrical indentations. The tensile properties were evaluated using an electronic Hounsfeld Tensometer. The samples were machined as per ASTM E8 specifications. The tensile properties reported are an average of three tests at each condition. 3. Results and discussion 3.1. Synthesis and characterization of A356/TiB2 composites The age-hardenable A356 alloy was reinforced with various amounts of in-situ TiB2 particles. The XRD patterns show that TiB2 peaks are prominent only at higher wt.% of the particles (Fig. 2a). The complete absence of brittle Al3 Ti particles is also evident from the XRD patterns. The presence of TiB2 particles was further confirmed by XRD of the extracted particles obtained by chemical dissolution technique (Fig. 2b). The technique involves the dissolution of the matrix in NaOH solution which leaves behind TiB2 particles only.
Fig. 2. XRD patterns of (a) A356/TiB2 MMCs (b) extracted TiB2 particles from A356/10 wt.% TiB2 composite.
The optical photomicrographs of A356 alloy and composites with different wt.% of TiB2 particles in their as cast condition were observed. The typical as cast microstructure of A356 alloy consists of a coarse eutectic phase located in between the Al dendrites. The micrographs suggest that as the amount of TiB2 particles in the matrix increases, the dendritic regions become concentrated with TiB2 particles. As a result the size of the Si particles in the dendritic region decreases progressively with TiB2 addition. This is because as the melt solidifies fine TiB2 particles along with Si particles are pushed away by the solidification front towards the interdendritic region thereby restricting the growth of Si particles. A comparison of A356/5TiB2 (Fig. 3a) and A356/10TiB2 composites (Fig. 3b) suggest that with the increasing number of TiB2 particles, the tendency for the agglomeration increases. The continuous network structure comprising of “TiB2 and Si particles” becomes more prominent in case of A356/10TiB2 composite (Fig. 3b). The decrease in the size of Si particles is more evident at higher magnification. The decrease is clearly evident in the composites with 2.5 and 5 wt.% TiB2 particles. Earlier, such observations were not reported by Wood et al. [13].
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Fig. 3. Optical micrographs (at 500×) of as cast A356 based composites reinforced with (a) 5 wt.% (b) 10 wt.% TiB2 particles.
In peak-aged condition, the Si particles in the alloy as well as composites are completely spheroidized due to a phenomenon termed as thermal modification. This is quite evident from SEM photomicrographs in Fig. 4 which shows that the size of the modified Si particles in the alloy (Fig. 4a) are significantly reduced on addition of 2.5 wt.% TiB2 particles (Fig. 4b). The microstructures also suggest that the size of already reduced Si particles due to increment of TiB2 particles is further spheroidized due to T6 treatment. The decrease in the size of the Si needles, which is an attribute of growth restriction imposed by the increasing amount of TiB2 particles, is prominent in composites with higher wt.% of TiB2 particles. Such reduction in particle size of silicon has been reported earlier in other Al–Si alloy system [14]. The thermal modification of reduced Si particles may have an important implication on the tensile properties of the composites. Fig. 5a is the histogram showing the size distribution of Si particles in the alloy as well as composites with 2.5 and 10 wt.% TiB2 particles under T6 condition. In case of the base alloy, majority of the Si particles fall in the size range of 3–12 m, while a small percentage lie on the either extreme. Addition of 2.5 wt.% TiB2 to the base alloy changes the microstructure as well as the size distribution of Si particles with most of the particles in the size lying in range of 2–6 m. Further incorporation of TiB2 particles in the composite leads to a drastic change in the distribution of Si particles. In the composite with 10 wt.% TiB2 particles the size of Si particles drops down in the range
Fig. 4. SEM photomicrographs of peak aged A356 based composites reinforced with (a) 0 wt.%, base alloy (b) 2.5 wt.%.
of 1–4 m. The change in the size distribution of Si particles is clearly reflected in the SEM photomicrographs shown in Fig. 4. It was calculated that the percentage of Si particles in the size range of 1–3 m increases from a mere 4% in the base alloy to as high as 46% and 96% in the composite with 2.5 and 10 wt.% TiB2 particles. A higher amount of fine Si particles at higher weight fractions of TiB2 could have a beneficial effect on the properties of the composites, especially the ductility. Fig. 5b shows the mean size of Si particles as a function of wt.% TiB2 . The plot shows that the decrease in size of Si particles is very sharp at lower TiB2 content (2.5 and 5 wt.%) while at higher TiB2 content (7.5 and 10 wt.%) the decrease is very gradual. 3.2. Ageing studies of A356/TiB2 composites Vickers hardness measurements were used to ascertain the age hardening behaviour of the composites in the present study. Fig. 6a shows the bulk hardness of the alloy and composites as a function of ageing time. In absence of TiB2 particles, i.e., A356 alloy, the age hardening curve is very sharp and a peak hardness value of 116 VHN is attained after 12 h of ageing, while in case of the composites the curve is quite smooth, i.e., the drop in hardness with time is gradual. The time required to attain the peak hardness in the composites with 2.5 and 5 wt.% TiB2 is 6 h, and for the composites with 7.5 and 10 wt.% TiB2 the time further reduces to 4 h. So the time to attain the peak hardness is reduced by 2–3 times in case of composites. This is in agreement with
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Fig. 5. (a) Particle size distribution of Si in A356/TiB2 MMCs and (b) mean size of Si particles as a function of TiB2 in A356/TiB2 MMCs.
the previous studies on Al–4Cu–TiB2 composites which showed similar accelerated ageing behaviour [4]. It is worth mentioning that the peak hardness of the composite with even 10 wt.% TiB2 (HV 89) is lower than that of the base alloy (HV 116). The plot also shows that with the increase in amount of TiB2 , the peak hardness attained by the composite decreases and then increases slightly after 5 wt.% TiB2 (Fig. 6b). This is unlike the previous observations involving TiB2 particles by the same authors [4]. The marginal increase in bulk hardness of the composites at higher weight fractions of TiB2 particles (in T6 condition) can be attributed to the compensation of loss in hardness (due to loss of Mg2 Si) by increasing weight fraction of the hard TiB2 particles. Such an anomaly in the age hardening behaviour of A356 alloy reinforced with TiB2 particles has not been reported earlier, though only few studies have been carried out in such system [13]. Thus it is strongly suggested that such a behaviour is due to the loss of Mg, which otherwise would have led to a still higher
hardness of the composites. However the decrease in the peak ageing duration can be attributed to the development of strain field around the matrix-reinforcement region due to differential coefficients of thermal expansion between Al (23.5 × 10−6 /K) and TiB2 (7 × 10−6 /K) [16]. Thus the resulting dislocations act as nucleating sites for the precipitation of Mg2 Si, though to a lesser extent under the present circumstances. A comparison of the hardness of the base alloy and composites under different conditions is shown in Figure 6b. In the as cast condition, there is an overall increase of hardness by about 21 VHN in A356/10TiB2 composite as compared to the base alloy. Thus, it can be said that the increase in hardness is not quite significant even after addition of 10 wt.% of TiB2 particles. Also it can be seen that addition of 2.5 wt.% TiB2 particles into the alloy causes a slight drop in the hardness, which then increases on further addition of TiB2 particles. The alloy and the composites show a higher bulk hardness in the as cast condition
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Fig. 6. (a) Hardness of the composites as a function of ageing time (b) bulk hardness of the A356/TiB2 composites under different conditions.
than in the solutionised condition. This is due to the phenomena known as “delayed ageing” exhibited in such alloys [15]. It is interesting to note that the drop in hardness for the above two conditions is approximately by the same amount (HV5 = 15) in all the cases. However, there is a marked increase in the peak hardness of the base alloy as compared to composites. In case of the composite reinforced with 2.5 wt.% TiB2 particles the increase in peak hardness is maximum (as compared to solutionised condition), while in other composites the increase is to a much lesser extent. This may be due to the depletion of age hardening precipitates, Mg2 Si, from the composites. It is suggested that there is a possible loss of Mg as a result of the reaction of molten alloy with K2 TiF6 and KBF4 salts leading to the formation of complex compounds. One of the difficulties encountered in the present system is the detection of Mg content in the melts. The lower Mg content in the base alloy and a further lower amount in the composites makes Mg analysis a difficult proposition by the present characterization techniques. So it was attempted to analyze the dross (decanted after the completion of the reaction) for its Mg content. The EDX microanalysis of the dross (Fig. 7a) clearly showed the presence of Mg peaks thus providing a clue to the drop in hardness of the composites in their as cast condition. The reaction of the molten alloy with
Fig. 7. (a) EDX microanalysis of the dross (b) difference in hardness between the peak aged and solution treated composites.
the fluoride salts leads to the formation of some complex compounds that is lost along with the dross. So the final composite is deprived of Mg and consequently the age hardening precipitates of Mg2 Si. Such a possible loss of Mg was not reported earlier by Wood et al. [13]. It can be said that the overall hardness of the composite is a function of the amount of in-situ TiB2 particles and Mg2 Si precipitates. At lower weight fraction of TiB2 , the contribution of Mg2 Si precipitates is significant, while at higher weight fraction the hard TiB2 particles themselves dictate the hardness of the composite. Fig. 7b shows the difference between the peak hardness of different composites and the corresponding solution treated composites (lower curve) and the solution treated base alloy (upper curve). The figure clearly distinguishes the increment in the hardness due to precipitates and TiB2 particles. It is evident that without the addition of TiB2 particles the increment in the hardening is solely by Mg2 Si precipitates. However in presence of 2.5 wt.% TiB2 , the contribution from TiB2 particles diminishes till 5 wt.% TiB2 and then increases on further addition of particles. The closeness of the hardness of the composite with 5 wt.% TiB2 and the base alloy in the solutionised condition
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(also see Fig. 6b) results in very little difference in hardness as indicated in Fig. 7b. However it may not be deduced that the contribution of TiB2 particles is negligible but rather lower than expected. Also it can be said the composite with 5 wt.% TiB2 , the hardness contribution from Mg2 Si and TiB2 particles are very close. Thus, it may be stated that Fig. 7b is an effective way of delineating the hardness contribution form different phases in such composites. 3.3. Tensile properties of A356/TiB2 composites The tensile properties of composites in their as cast and peak aged conditions are compared with that of the base alloy in Table 2. The solutionised condition refers to the state of the alloy/composite after quenching in water after holding at 540 ◦ C for 8 h, whereas peak aged condition refers to the condition at which maximum hardness is attained. The tensile properties show a considerable increase in the peak-aged condition as compared to as cast condition, as expected. The increase in tensile properties is not linear with respect to TiB2 content. This phenomenon can be ascribed to the loss of Mg from the base alloy as discussed earlier in Section 3.2. Wood et al. [13] has also shown similar trends in tensile strength but did not give any reasons for the same. The tensile strength increases from 184 MPa in case of base alloy to 256 MPa in A356/10TiB2 composite in the as cast condition, while the corresponding values are 276 and 328 MPa in the peak aged condition. So the percentage increment in the values of UTS is lower in case of the composite. This may be attributed to the lower amount of precipitation hardenable Mg2 Si precipitates in the composites owing to the loss of Mg. It is quite interesting to note that even though the peak hardness of the composite is lower than that of the base alloy, the UTS values are significantly higher. Thus, it may be said that the strength of the composites is not directly related to its hardness. The tensile properties are a strong function of the amount of precipitates and reinforcement. The shape and size of second phase particles also dictates the properties of the composites. In the present system, globular morphology of Si particles due to T6 treatment along with equiaxed TiB2 particles play a significant role in enhancing the properties of composites by minimizing the stress concentration in the matrix. The as cast composites exhibit sufficient ductility which is further increased in their peak aged condition (Table 2). The
Fig. 8. SEM fractographs of A356/TiB2 composites in peak aged (T6) condition (a) 2.5 wt.% (b) 10 wt.%.
improvement in ductility of composites can be attributed to couple effect of numerous small Si particles due to growth restriction and thermal modification during the T6 heat treatment. Both the factors help in lowering the stress concentration in the matrix to a great extent. Hence, the composites with maximum wt.% of TiB2 exhibit sufficient ductility. It is reported that the fracture occurs by preferential debonding of larger Si particles rather than the smaller ones [17]. This could be another reason for good ductility and strength of the composites. Fig. 8 shows the SEM fractograph of A356/TiB2 composites in their peak aged condition (T6 treatment). The base alloy shows a typical ductile failure but the fracture behaviour is substantially
Table 2 Mechanical properties of A356/TiB2 composites Alloy/compositea
A356 A356/2.5TiB2 A356/5TiB2 A356/7.5TiB2 A356/10TiB2 A356/4TiB2 A356/5.5TiB2 a
As cast
Peak aged
Reference
YS (0.2%) (MPa)
UTS (MPa)
El. (%)
Hardness HV5
YS (0.2%) (MPa)
UTS (MPa)
El. (%)
Hardness HV5
110 135 151 196 200 – –
184 206 215 231 256 – –
9 7 6 5 4 – –
69 61 66 75 85 – –
195 241 251 256 267 237 240
276 290 302 317 328 300 303
13 10 9 8 6 10 7
116 92 69 78 89 – –
All compositions are in wt.%.
Present work Present work Present work Present work Present work [93Woo] [93Woo]
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altered by the heat treatment and the amount of reinforcement in the composite. The fracture surface of composites with 2.5 and 10 wt.% TiB2 revealed the coexistence of ductile and cleavage fracture. The SEM photomicrograph of A356/10TiB2 composite shows shallow dimples as well as ductile shear bands indicating sufficient amount of ductility retained in the composite. It was observed that with the increase in the amount of TiB2 particles, the size and depth of the dimples decreases. The composite with 10 wt.% TiB2 shows shallower dimples than the one containing 2.5 wt.% TiB2 particles. The numerous small dimples are nucleated by large number of closely spaced particles in the “TiB2 and Si” network in A356/10TiB2 composite. 4. Conclusions The salt route technique was adopted to disperse TiB2 particles in A356 alloy successfully. The presence of in-situ TiB2 particles accelerates the ageing kinetics significantly. Also, the size distribution of Si particles is narrowed down at higher weight fraction of TiB2 particles due to their growth restriction. The finer size of Si leads to easier spheroidisation of Si particles during subsequent heat treatment. These thermally modified Si particles have beneficial affect on the tensile properties, especially the ductility.
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