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Assessment of particle–matrix debonding in particulate metal matrix composites using ultrasonic velocity measurements
Materials Science and Engineering A323 (2002) 42 – 51 www.elsevier.com/locate/msea
Assessment of particle–matrix debonding in particulate metal matrix composites using ultrasonic velocity measurements P.N. Bindumadhavan, Heng Keng Wah, O. Prabhakar * School of Materials Engineering, Nanyang Technological Uni6ersity, Nanyang A6enue, Singapore 639798, Singapore Received 6 October 2000; received in revised form 8 February 2001
Abstract Particulate metal matrix composites are a class of materials that have evoked keen interest, largely due to the promise of improved properties over conventional metals and alloys. However, the effective realization of the improved properties of these composites is contingent upon the presence of good bonding between the reinforcement particles and the metal matrix. Therefore, a quantitative measure of the particle–matrix bonding efficiency would be particularly useful in helping to fully realize the potential of these materials. It is known that particle– matrix debonding degrades the elastic modulus of the composite considerably while having practically no effect on its density. Based on this, a model has been developed and a quantitative measure of debonding has been proposed. In conjunction with measurement of ultrasonic velocity, this model is capable of giving quantitative estimates of the debonding factor of the composite material. In the present study, aluminum alloy A356–SiCp composites containing between 3 and 15 vol.% SiC particles have been synthesized by the melt stirring method. Debonding factors as high as 0.29 have been estimated in low SiC volume fraction composites. In the case of the high SiC volume fraction composites, the estimated debonding factor was found to be as low as 0.10, corresponding to the lesser particle–porosity interaction observed in these composites. Elastic modulus measurements on composite specimens also confirm the observation that particle–porosity association is seen to a greater extent in the low SiC volume fraction composites. Elastic modulus deviation of up to 14% was observed in the case of the low SiC volume fraction composites while the high SiC volume fraction composites showed a maximum modulus deviation of less than 10%. It was also seen that measurement of debonding factor before and after hot rolling of the composites provides a method of evaluating the effectiveness of the rolling operation in improving the integrity of the composites. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Particulate composites; Melt stirring; Aluminum alloy; Ultrasonic velocity; Debonding factor; Particle– porosity association
1. Introduction Metal matrix particulate composites are a class of tailor-made materials with the capability to suit such diverse combinations of properties as high modulus, strength, hardness, wear resistance and low thermal expansion coefficients. These materials have emerged as viable alternatives for use in automotive and aerospace industries, mainly because they are considerably cheaper and easier to manufacture and process as compared to fiber reinforced composites. In recent years, a significant number of fabrication routes have been developed for the manufacture of particulate composites. * Corresponding author. Tel.: +65-790-4599; fax: + 65-790-9081. E-mail address: [email protected] (O. Prabhakar).
Though these methods are capable of generating material of high microstructural quality, the widespread use of these composites is currently limited by the economic non-viability of many of these fabrication methods and the paucity of reliable material qualification techniques. Melt stirring, which belongs to a class of fabrication techniques called liquid metallurgy techniques, is currently one of the most economically viable and popular fabrication routes for the manufacture of particulate composites. However, a large number of process variables must be controlled in the melt stirring technique to create a quality product. Non-optimized values of these variables generally adversely affect the final mechanical properties of the composite due to poor wetting and subsequent rejection of the reinforcement particles and non-uniform distribution (clustering) of
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the particles. The introduction of porosity into the composite during stirring also has a deleterious effect on the composite properties. If the particles are found in association with porosity, the properties of the composite are further degraded as these particles are effectively debonded from the matrix and do not contribute to strengthening of the composite. Nondestructive evaluation (NDE) techniques are particularly useful for the inspection of materials because the output from such a test depend, to a large extent, on the microstructure of the material. In addition, these tests are capable of providing rapid results without the need to destroy test samples, thus being well suited for a manufacturing environment. Ultrasonic test techniques and methods have been studied and developed by various researchers for the nondestructive characterization of composites. Roth et al. [1] have developed a semi-empirical model relating the velocity of longitudinal ultrasonic waves to the elastic properties and density of the specimens. Using this model they have shown that the longitudinal velocity of the material decreases almost linearly with an increase in the volume fraction of porosity in the material. Tests were conducted on silicon nitride and steel samples and these researchers observed a high degree of agreement of measured values with predicted values of velocity. Rokhlin et al. [2] have conducted a detailed study on the ultrasonic phase velocity behavior of fiber reinforced composites. They found that the velocity is significantly affected by the degradation of the elastic properties of the interphasial layer. Degradation in the interphasial layer could occur during manufacture (debonding of the fiber from the matrix due to insufficient wetting or thermal incompatibility) or during service (oxidation damage or fatigue related damage). They observed a reduction in phase velocity for composites which were manufactured using fibers that were
not appropriately pre-coated (thereby inducing debond at the fiber –matrix interface), as well as in composites subjected to fatigue or oxidation cycles. They have explained this behavior by considering the degradation of elastic modulus of the interphase when fiber debond takes place as a result of fatigue or oxidation damage. A review of the literature on ultrasonic evaluation of composites shows that the extent of work done on particulate composites is considerably lesser than that on fiber reinforced composites. Also, majority of the work on particulate composites has been focused on powder metallurgy (P/M) processed composites. However, as already explained, melt stirring followed by casting remains one of the most economically viable fabrication techniques for particulate composites [3–8]. It is also well known that the improvement in mechanical properties of the composite are based on the presence of good particle– matrix bonding [9,10]. Thus, simple techniques to evaluate the bonding efficiency of the particles should be developed for materials design to translate effectively to engineering products. So, it was felt that investigation of the debonding of particles due to particle–porosity association and clustering would be very useful. Hence, the present paper involves a study of the ultrasonic velocity behavior of an aluminum alloy (A356) based SiC particulate composite, processed by the melt stirring technique. An attempt has been made to quantify the extent of particle debonding in the composite as indicated by the association between particles and porosity. Elastic moduli of the composites have been determined by tensile testing and discussed with reference to the measured ultrasonic velocity as well as the extent of debonding. The composites were also hot rolled in order to improve their microstructural integrity, and the extent of such improvement was again checked using ultrasonic velocity measurements.
Table 1 Elemental composition of matrix alloy used in this study (wt.%)
2. Details of experimental work
Si
Mg
Cu
Zn
Ni
Fe
Ti
Al
6.37
0.20
0.04
0.015
0.037
0.15
0.19
Balance
Table 2 Properties of matrix alloy [6] and reinforcement used in this study Property
Matrix (A356)
Reinforcement (SiC)
Elastic modulus (E), GPa Bulk modulus (K), Gpa Shear modulus (G), GPa Poisson’s ratio, w Standard density (z), kg m−3
– 70.0 23.0 0.33 2680
430 – – – 3200
2.1. Materials used The aluminum alloy matrix used in the present work is a commercial casting-grade alloy (A356) whose elemental composition is given in Table 1. The reinforcement particle used is silicon carbide of average diameter 47 mm. The properties of the matrix and reinforcement are summarized in Table 2.
2.2. Casting of composites Composite specimens were processed by the melt stirring technique, which has been described in detail by Rohatgi [11]. Composites with approximately 3, 7, 11 and 15 vol.% of SiC reinforcement were synthesized.
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All specimens were top-poured into a permanent mold for solidification.
2.3. Characterization of microstructure The porosity in the castings was measured by three methods. These were the standard metallographic technique (using light microscopic observation of a polished cross-section), density measurement using the water displacement principle, and volume measurement using a helium gas-based pycnometer. The SiC particle volume fraction was measured by two techniques. These were the standard metallographic technique and the acid digestion technique (using hydrochloric acid).
2.4. Measurement of ultrasonic 6elocity and elastic modulus Ultrasonic velocity in the specimens was measured using a Krautkramer USD 15SX model instrument using the pulse-echo method. For longitudinal velocity measurements, a standard probe of 2 MHz frequency was used. The accuracy of velocity measurements was within 91 m s − 1. Elastic modulus of the composite specimens was measured using an Instron model 4206 tensile tester, using a constant crosshead speed of 50 mm min − 1. For longitudinal velocity, five measurements were made and the average values have been reported. For elastic modulus, three samples were tested and the average values have been reported.
2.5. Hot rolling of composite samples Composite samples were heated to temperatures between 500 and 530°C in an electric resistance furnace prior to rolling. All samples were given a thickness reduction of 55– 60% by rolling. The thickness reduction by rolling was done in small steps in order to minimize sample cracking.
3. Model for prediction of ultrasonic velocity Particulate composites can generally be treated as statistically homogeneous and isotropic [12]. Thus, a model for the prediction of ultrasonic velocity in particulate composites containing porosity can be developed as follows. An unreinforced matrix containing porosity of volume fraction 6p is considered. The elastic modulus of such a material (assuming that the size and shape of the pores do not affect the elastic modulus) is given by the well-known Mackenzie’s [13] equation: E6 p =
9K6 pG6 p 3K6 p + G6 p
(1)
where K6 p and G6 p are the bulk modulus and shear modulus respectively of the material containing porosity of volume fraction 6p. These can be readily calculated from the equations: K6 p = and
4KG(1− 6p) 4G+ 36pK
G6 p = G 1− 5
n
(2)
n
3K + 4G 6 9K + 8G p
(3)
where K and G are the respective moduli for the matrix material that is pore free. These values can be obtained from literature [14], and are summarized in Table 2 for the matrix alloy used in this study. In the above porous matrix material, it is assumed that reinforcement particles of volume fraction 6r are uniformly dispersed. It is further assumed that the effects due porosity and particles are linearly superposed. This allows the use of the rule-of-mixtures law to be used to estimate the effective properties of the composite. It is also assumed that out of the total reinforcement volume fraction of 6r, a fraction ‘f’ is debonded from the matrix due to association with porosity or clustering with porosity. Thus the volume fraction of reinforcement particles that are effectively bonded to the matrix is given by 6r (1−f ). It is well known that only effectively bonded particles contribute to the improvement of elastic modulus of the composite. Therefore, the corresponding fraction of the reinforcement particles, 6rf, which are debonded from the matrix, do not contribute to the increase in elastic modulus of the composite. Thus, the effective elastic modulus, Ec, of the composite containing porosity of volume fraction 6p and effectively bonded reinforcement particles of volume fraction 6r (1−f ), is given by the rule-of-mixtures law to be: Ec = Er[6r(1−f)]+ E6 p(1−6r)
(4)
where Er is the elastic modulus of the reinforcement particle. At this juncture the two limiting cases of f=0 (all particles are fully bonded and have no interaction with porosity) and f= 1 (all particles are fully debonded and do not contribute in any way to the increase in elastic modulus) need to be discussed. In the case where f=0, it is seen that the value of elastic modulus obtained from Eq. (4) would be the theoretical upper bound value. In the case where f= 1, it is realized that the fully debonded particles should be considered as porosity as far as their contribution to the elastic modulus of the composite is concerned. Thus, the value of elastic modulus for the case of f= 1 gives the theoretical lower bound value. Actual values of elastic modulus would lie in between these bounds, corresponding to values of f that satisfy 0B fB 1.
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Substituting the appropriate expressions from Eqs. (1)– (3) into Eq. (4) gives the following equation for the effective elastic modulus of the composite:
n ! !
Ec =Er[6r(1− f)]+
"n "
Æ 9 4KG(1− 6p) G 1 −5 3K +4G 6 Ã p 4G + 36pK 9K +8G Ã 12KG(1−6p) 3K + 4G Ã +G 1 −5 6 È 4G+36pK 9K + 8G p
(5a)
Upon substitution of the values of parameters from Table 2, the above expression reduces to: Ec =4306r(1− f)+
0.54 −1.546p +6 2p 0.0086 − 0.00746p −0.00366 2p
(1 − 6r)
n
(5b)
giving the elastic modulus of the composite in GPa. As far as the density of the composite is concerned, if the standard pore free density of the matrix material is taken to be zstd, then the density of the matrix with porosity of volume fraction 6p is given by the equation: zspm =zstd(1−6p)
(6)
With respect to density of the composite, it is realized that the relative bonding efficiency of the particles with the matrix does not affect the density values, in contrast to the case of mechanical properties like elastic modulus. Thus, if reinforcement particle of volume fraction 6r is now added to this material, the effective density of the composite, zc, is given by the rule-of-mixtures law to be: zc = zr6r +zstd(1−6p)(1 −6r)
(7)
where zr is the density of the reinforcement particle. It is well known that the longitudinal velocity, VL, in a homogeneous, isotropic medium is given by the expression: VL =
E(1− w) z(1+w)(1− 2w)
n
1/2
(8)
where E, w, and z are the effective elastic modulus, Poisson’s ratio and density, respectively, of the medium under consideration. Thus, substituting equations Eqs. (5a), (5b) and (7) into Eq. (8) gives an expression for the longitudinal velocity in the composite containing porosity of volume fraction 6p and effectively bonded reinforcement particles of volume fraction 6r (1 −f ) as:
Again, substitution of various parameters from Table 2 results in a simplified relation for the longitudinal velocity (in ms − 1) as:
<
VL =
Ç Ã Ã(1−6r) Ã É
n ! !
n=
0.54− 1.546p + 6 2p (1−6r) 0.013−0.0116p − 0.0056 2p 14476r + 1212(1−6p)(1 −6r)
109 2886r(1− f) +
1/2
(9b) It is clear that when all the reinforcement particles exist in the composite in complete isolation from the porosity in the composite and are well bonded to the matrix, the factor f will be equal to zero. In that case, the longitudinal velocity calculated from Eq. (9a) or Eq. (9b) would give the theoretical upper bound velocity for the composite assuming no particle–porosity interaction and perfect bonding. Likewise, in the case when all particles are debonded due to complete association between particles and porosity or due to particle– porosity clustering, the factor f would be equal to 1 and the velocity calculated would give the theoretical lower bound velocity for the composite. The velocity actually measured on the composite would lie in between these two limits, corresponding to a value of the debonding factor that satisfies 0BfB1. Therefore, the debonding factor, calculated using longitudinal (L) velocities, can now be defined by the ratio: f=
VL(max) − VLm VL(max) − VL(min)
(10)
where the subscript (max) refers to the theoretical upper bound, (min) refers to the theoretical lower bound, and m refers to the actually measured velocities, respectively. In the above formulation, Poisson’s ratio, w, has been assumed to be unaffected by the presence of both porosity and particles. This has been shown [15] to be a fairly valid assumption. Thus, a value of 0.33 for w, corresponding to the value generally assumed for aluminum and its alloys [14], has been used throughout this study.
4. Results and discussion
4.1. Microstructure Average values and S.D. of porosity and reinforcement content in the cast specimens, before rolling,
"n "
1/2 Æ 9 4KG(1 − 6p) G 1− 5 3K+4G 6 Ç Ç ÆÆ Ç p à à à à 4G+ 36pK 9K+ 8G ÃÃE 6 (1− f) + à Ã(1− 6r)Ã(1−w)à r r Ãà 12KG(1 − 6 3K + 4G à p) Ã Ã È 4G +36 K +G 1− 5 9K + 8G 6p Ã É É ÃÈ Ã p VL = È É [zr6r +zstd(1 − 6p)(1 −6r)](1+ w)(1− 2w)
45
(9a)
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Table 3 Average values and standard deviation of reinforcement and porosity content and corresponding factor k (before rolling) Casting identification
SiC mean 9| (%)
Porosity mean 9 | (%)
k= 6r/(1−6p)
A356-0 A356-1 A356-2 A356-3 A356-4
0 3 9 0.125 7 9 0.138 11 9 0.143 15 9 0.119
2.29 0.148 3.2 90.157 3.9 90.172 4.490.163 6.1 90.140
0.0000 0.0309 0.0728 0.1150 0.1597
measured by various methods are summarized in Table 3. A356-0 refers to the unreinforced matrix alloy while A356-1 to A356-4 refer to specimens with increasing amounts of SiC reinforcement. The particle distribution was found to be fairly uniform; no large-scale particle segregation was observed. However, the uniformity of particle distribution in high SiC volume fraction composites was much better than that observed in low SiC volume fraction composites. The higher particle concentration gives rise to multiple solidification fronts, and geometric trapping of the particles between these fronts leads to reduced particle movement within the melt. In contrast, in composites with lower particle concentrations, particle pushing occurs to a greater extent and leads to particle segregation in the interdendritic regions. The above mechanism is considered to be the prime reason for the better uniformity of particle distribution in the high SiC volume fraction composites [16]. Porosity content of the specimen was observed to increase with an increase in the SiC content. There are two possible reasons for this observation. Firstly, since the time of stirring for higher SiC content composites is slightly longer, there is a higher likelihood for more air
bubbles to be sucked through the vortex into the stirred metal. Secondly, at high SiC content, the melt becomes less fluid, creating conditions that are unfavorable for already entrapped air bubbles to escape from the melt during solidification.
4.2. Obser6ations on ultrasonic 6elocity The comparison between the upper and lower bounds on the predicted velocity and the velocity values actually measured for longitudinal waves in the specimens are plotted in Fig. 1 against a factor ‘k’ defined as the ratio (6r/(1− 6p)). 6r And 6p represent the volume fraction of reinforcement and porosity, respectively. The measured velocity value shown in Fig. 1 is the average of five measurements made on each sample. Two general trends are apparent from Fig. 1. The first is that the measured velocities show a fairly linear increasing trend. Using values of the measured velocity, it is, therefore, possible to predict the factor k. In the present study, the following equation has been derived: VL = 11902 (6r/(1− 6p))+5692.7
(11)
where VL is the measured longitudinal velocity in ms − 1. The volume fraction porosity, 6p, can be fairly accurately measured by determining the density of the specimen using water displacement methods and comparing this density with the maximum (theoretical) density obtained by hot pressing or rolling. Thus, the above Eq. (11) can be used to determine the volume fraction of SiC in the composite, 6r, in a relatively straightforward manner. The second trend that is clear is that the measured longitudinal velocities are smaller than the predicted longitudinal velocity values. It is to be noted that the velocity value predicted by Eq. (9a) or Eq. (9b) takes into
Fig. 1. Plot of predicted and measured longitudinal velocities against k (k = 6r/1 −6p).
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Fig. 2. Plot of debonding factor against k (k = 6r/1 −6p).
account the presence of both reinforcement particles and porosity. Therefore, the observed deviation of measured velocity from that predicted by equation Eq. (9a) or Eq. (9b) can be attributed to the presence of some degree of particle– matrix debonding (corresponding to a value of debonding factor f such that 0B f B 1). As explained earlier this debonding would be caused by the association of particles with porosity or the formation of particle–porosity clusters, resulting in the reduction of elastic modulus, which has been shown to reduce velocity values greatly. The debonding factor, f, has been estimated for the various composites from the measured velocities and the predicted velocities using Eq. (10), and has been plotted against the factor k= 6r/ (1− 6p), calculated using values of 6r and 6p and shown in Table 3. This plot is shown in Fig. 2. It can be seen from Fig. 2 that the estimated debonding factor values follow a generally decreasing trend with an increase in factor k. It is seen that the value of f is a maximum for the composite with k =0.0309 (casting A356-1, Table 3). If the association between particles and porosity is assumed to be completely random, an increasing interaction (or an increasing value of debonding factor f ) is expected between pores and particles when the volume fraction of both increases. However, in the present study, it is observed that although the particle and porosity volume fractions increase for the composites A356-1 to A356-4, the debonding factor f decreases. This seems to indicate that although both porosity and particle concentration increases in these composites, this does not necessarily lead to an increased interaction or association between the porosity and particles. It also suggests that the possible mechanism of particle– porosity interaction or association would depend on the particular conditions under which these composites
were manufactured. A similar observation has been made by Rohatgi et al. [17] in their work on an aluminum alloy–zircon particle reinforced composite. They have reported that they did not observe any significant increasing trend in particle–porosity interaction although both porosity and particle content increased, in the composites tested by them. In the present case, the composite with k =0.0309 (composite A356-1, Table 3) showed the maximum value of the estimated debond factor. This implies that the particle–porosity association or interaction is present to a greater extent in this composite than in the other composites. Optical microscopic observation of carefully polished sections of the composites show that this is the case. It was observed that the tendency for particle–porosity association or clustering is seen to a much greater extent in the composite with k =0.0309 (composite A356-1, Table 3). A typical optical mi-
Fig. 3. Optical micrograph of a particle – porosity cluster, observed in the composite with k = 0.0309 (unrolled specimen).
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Fig. 4. Plot of comparison between predicted and measured elastic modulus against (k = 6r/1 −6p).
crograph of a particle– porosity cluster, observed in the composite with k = 0.0309 (composite A356-1, Table 3), is shown in Fig. 3. The particles that are clustered around the edge of the pore are effectively debonded from the matrix and would not contribute to any increase in mechanical properties of the composite. Thus, the composite with k =0.0309 (composite A3561, Table 3), which showed a larger extent of such clusters, also exhibited the largest value of the estimated debond factor, which in turn caused the largest deviation between predicted and measured velocity. Microstructural examination of composites with k = 0.0728, 0.1150, and 0.1597 (composites A356-2, 3 and 4, respectively, Table 3) showed that although there is an increase in the overall particle and porosity content in these composites, the tendency for particle– porosity association or clustering is reduced. Samuel and Samuel [18] have shown that the porosity size and shape were affected by the presence of SiC reinforcement particles through the tendency these particles display to block or restrict the growth of pores. In composites with low volume fraction of reinforcement particles, they found that porosity growth was less restricted leading to larger pores in the interdendritic regions, these large pores being generally found surrounded by reinforcement particles. On the other hand, high volume fraction composites were generally found to have smaller pores, which were associated to a lesser extent with the reinforcement particles. Thus, it appears that low volume fraction composites generally have bigger pores, which are found surrounded by and in association with reinforcement particles. Therefore, the observed trend of estimated debonding factors in the present study is understandable.
4.3. Obser6ations on elastic modulus Theoretical upper and lower bounds of the elastic moduli of the composites were calculated using equation Eq. (5b), using values of 0 and 1 for the factor f. The elastic moduli of the specimens were also determined from tensile tests, carried out on an Instron tensile testing machine. These results are shown in Fig. 4. The measured elastic modulus data reported in Fig. 4, is the average of three tests performed on composite samples with each value of k. The trend of the variation in the moduli values is seen to be qualitatively similar to that of longitudinal velocity values. As already mentioned, microstructural observations show that the composite with k= 0.0309 (A356-1) exhibited the largest extent of particle–porosity association or clustering, which is thus expected to show the greatest difference between predicted (with f= 0) and measured elastic moduli values. An SEM micrograph of the frac-
Fig. 5. SEM micrograph of the fractured surface of a composite with k = 0.0309 (unrolled specimen), showing particle – porosity association.
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Table 4 Average values and S.D. of porosity and corresponding factor k, before rolling (BR) and after rolling (AR) Casting identification
Porosity BR mean 9 | (%)
k =6r/(1−6p) BR
Porosity AR mean 9 |
k =6r/(1−6p) AR
A356-0 A356-1 A356-2 A356-3 A356-4
2.2 90.148 3.2 90.157 3.9 90.172 4.4 90.163 6.1 90.140
0.0000 0.0309 0.0728 0.1150 0.1597
0.8 90.135 1.4 90.144 1.9 90.128 2.6 90.115 3.5 90.106
0.0000 0.0304 0.0713 0.1129 0.1554
tured surface of the composite with k = 0.0309 (composite A356-1, Table 3) is shown in Fig. 5. Again, the association of SiC particles with porosity can be clearly seen.
4.4. Obser6ations on hot rolled samples As already mentioned, during manufacture of composites by the melt-stirring method, unless expensive vacuum atmosphere is maintained, there is a high likelihood that air or gas bubbles are sucked into the melt, leading to porosity or particle– porosity clusters. In order to improve the properties of such composites, manufacturers routinely perform secondary processing operations like rolling or forging on the as-cast composites. These operations reduce or eliminate gas porosity (either isolated or associated with reinforcement particles) inside the composite, thereby improving the properties of the composite. At the same time, non-optimized rolling or forging parameters could lead to internal defects like reinforcement particle cracking or particle–matrix interface cracking, which would degrade material properties. This is especially true of composites with higher volume fraction of reinforcement. Therefore, it is necessary to be able to measure the extent of such improvement (or degradation) so that the secondary processing operation can be optimized and material wastage can be minimized. In the present study, composite samples were rolled as explained in Section 2. The longitudinal ultrasonic velocity was measured after rolling. It was observed that the velocity measured on the rolled samples was higher than that measured on the unrolled samples. This is understandable because the rolling operation reduces porosity in the sample, thereby improving its mechanical integrity. This leads to a corresponding increase in the measured ultrasonic velocity. In order to check the effectiveness of the rolling operation in reducing or eliminating the associated porosity, porosity of the rolled samples was estimated by measuring the density after rolling, using the water displacement method as well as the Pycnometer method, and comparing this density with the theoretical density. Values of average porosity measured in the samples, before and after rolling are summarized in Table 4. This
porosity value was then used to calculate the bounds on the predicted value of the longitudinal velocity. Comparison of these bounds on predicted velocity with the actual measured velocity, using Eq. (10), enables estimation of the debonding factor after the rolling operation. A comparison of the estimated debonding factors before and after the rolling operation would be very useful in determining the effectiveness of the rolling operation in reducing or eliminating associated porosity. This comparison is shown in Fig. 6. Here too, the reported value of longitudinal velocity is the average of five measurements taken on each sample. It is seen from Fig. 6 that for the low SiC volume fraction composites (or composites with low k), the reduction in debonding factor is comparatively higher. In the case of the high SiC volume fraction composites, reduction in debonding factor is lower or even negative, as in the case of the composite with k= 0.1554 (composite A3564 after rolling, Table 4). This indicates that the positive effect of rolling is manifested to a much greater extent in the low SiC volume fraction composites than in the high SiC volume fraction composites. This observation could be attributed to the presence of a larger number of brittle SiC particles in the high SiC volume fraction composites, which could lead to particle cracking as well as particle–matrix interface microcracking or debonding during the rolling process. As already discussed, such cracked or debonded particles do not fully (effectively) contribute to the elastic modulus of the composite, thus causing an increase in the debonding factor. Fig. 7 shows a typical optical micrograph of the composite with 15 vol.% SiC, after rolling. In Fig. 7, reinforcement particle cracking can be clearly seen, as indicated by the arrows. Such cracked particles would not fully contribute to the increase in elastic modulus of the composite, thus causing a reduction in the measured velocity and contributing to an increase in the debonding factor. In a similar manner, particle–matrix interface microcracking would lead to an increase in the debonding factor. Rolling parameters that are not an optimum would cause reinforcement particle cracking or particle–matrix interface microcracking. Thus, it can be seen that estimation of the debonding factor would be very useful in improving the effectiveness of the rolling operation.
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P.N. Bindumadha6an et al. / Materials Science and Engineering A323 (2002) 42–51
Fig. 6. Comparison of debonding factors before and after rolling.
Fig. 7. Optical micrograph of a 15 vol.% composite, showing reinforcement particle cracking.
tween measured and predicted values of up to 14% was observed in the case of poorly bonded composites while relatively well bonded composites showed modulus deviation of less than 10%. It was also found that the effect of hot rolling in improving the integrity of the composite could be conveniently estimated by using measured values of the velocity in conjunction with the model developed. This can potentially lead to cost and material savings by helping to optimize the parameters of the secondary processing operation. The results obtained would be of considerable importance in industrial situations to obtain a first estimate of the effect of changes in process parameters on the extent of particle–matrix debonding in composite materials.
5. Conclusions The present study shows that measurement of ultrasonic velocity in the particulate composite can give a good indication of the extent of particle– porosity association in the composite. In composites that showed a lower value of the factor k (or a higher degree of particle–porosity association), the debonding factor f was found to be higher. Debonding factors estimated from measured velocities were found to be as high as 0.29 in the case of poorly bonded composites (A356-1 with k = 0.0309). Debonding factors measured in relatively well bonded composites (A356-4 with k = 0.1597) were as low as 0.10. These results also agreed well with microstructural observations on particle– porosity association or clustering in the composites. Results from measurement of elastic modulus of the composites were also consistent with the results of velocity measurements. Elastic modulus deviation be-
Acknowledgements The authors would like to thank Professor Fong Hock Sun (Dean, School of Materials Engineering, Nanyang Technological University) for financial support and permission to use various resources of the School. One of the authors (P.N. Bindumadhavan) was supported by a research scholarship granted by the school during the course of this work.
References [1] D.J. Roth, D.B. Stang, S.M. Swickard, M.R. DeGuire, L.E. Dolhert, Mater. Eval. July (1991) 883. [2] S.I. Rokhlin, W. Huang, Y.C. Chu, Ultrasonics 33 (5) (1995) 351. [3] M. Skibo, P.L. Morris, D.J. Lloyd, Proc. Int. Symp. on Advances in Cast Reinforced Metal Composites, September 1998, Chicago IL, p. 257.
P.N. Bindumadha6an et al. / Materials Science and Engineering A323 (2002) 42–51 [4] A.M. Samuel, F.H. Samuel, Key Eng. Mater. 104 – 107 (1995) 65. [5] S. Ray, J. Mater. Sci. 128 (1993) 5397. [6] A.M. Samuel, A. Gotmare, F.H. Samuel, Compos. Sci. Technol. 53 (1995) 301. [7] J.E. Allison, G.S. Cole, JOM, January (1993) 19. [8] W. Hoover, in: N. Hansen, et al. (Eds.), Proceedings of the 12th International Symposium, Riso National Laboratories, Roskilde, Denmark, 19912, p. 387. [9] R.J. Arsenault, Composites 25 (7) (1994) 540. [10] Y.H. Teng, J.D. Boyd, Composites 25 (10) (1994) 960. [11] P. Rohatgi, in: D.M. Stetanescu, J.R. Davis (Eds.), Metals Handbook, 9th edn., Vol. 15, Casting, 1991, p. 842.
.
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[12] R.M. Fulrath, J.A. Pask (Eds.), Ceramic Microstructures, Robert Krieger, Huntington, NY, 1976, p. 313. [13] J.K. Mackenzie, Proc. Phys. Soc. B63 (1950) 2. [14] F. King, Aluminum and its Alloys, Ellis Harwood, London, 1987, p. 95. [15] R.W. Rice, in: R.A. MacCrone (Ed.), Microstructure Dependence of Mechanical Behavior of Ceramics, Academic Press, New York, 1977, p. 199. [16] D.J. Lloyd, Compos. Sci. Technol. 35 (1989) 159. [17] P.K. Rohatgi, S. Raman, B.S. Majumdar, A. Banerjee, Mater. Sci. Eng. A123 (1990) 89. [18] A.M. Samuel, F.H. Samuel, Metall. Trans. A24 (1993) 1857.
Assessment of particle–matrix debonding in particulate metal matrix composites using ultrasonic velocity measurements P.N. Bindumadhavan, Heng Keng Wah, O. Prabhakar * School of Materials Engineering, Nanyang Technological Uni6ersity, Nanyang A6enue, Singapore 639798, Singapore Received 6 October 2000; received in revised form 8 February 2001
Abstract Particulate metal matrix composites are a class of materials that have evoked keen interest, largely due to the promise of improved properties over conventional metals and alloys. However, the effective realization of the improved properties of these composites is contingent upon the presence of good bonding between the reinforcement particles and the metal matrix. Therefore, a quantitative measure of the particle–matrix bonding efficiency would be particularly useful in helping to fully realize the potential of these materials. It is known that particle– matrix debonding degrades the elastic modulus of the composite considerably while having practically no effect on its density. Based on this, a model has been developed and a quantitative measure of debonding has been proposed. In conjunction with measurement of ultrasonic velocity, this model is capable of giving quantitative estimates of the debonding factor of the composite material. In the present study, aluminum alloy A356–SiCp composites containing between 3 and 15 vol.% SiC particles have been synthesized by the melt stirring method. Debonding factors as high as 0.29 have been estimated in low SiC volume fraction composites. In the case of the high SiC volume fraction composites, the estimated debonding factor was found to be as low as 0.10, corresponding to the lesser particle–porosity interaction observed in these composites. Elastic modulus measurements on composite specimens also confirm the observation that particle–porosity association is seen to a greater extent in the low SiC volume fraction composites. Elastic modulus deviation of up to 14% was observed in the case of the low SiC volume fraction composites while the high SiC volume fraction composites showed a maximum modulus deviation of less than 10%. It was also seen that measurement of debonding factor before and after hot rolling of the composites provides a method of evaluating the effectiveness of the rolling operation in improving the integrity of the composites. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Particulate composites; Melt stirring; Aluminum alloy; Ultrasonic velocity; Debonding factor; Particle– porosity association
1. Introduction Metal matrix particulate composites are a class of tailor-made materials with the capability to suit such diverse combinations of properties as high modulus, strength, hardness, wear resistance and low thermal expansion coefficients. These materials have emerged as viable alternatives for use in automotive and aerospace industries, mainly because they are considerably cheaper and easier to manufacture and process as compared to fiber reinforced composites. In recent years, a significant number of fabrication routes have been developed for the manufacture of particulate composites. * Corresponding author. Tel.: +65-790-4599; fax: + 65-790-9081. E-mail address: [email protected] (O. Prabhakar).
Though these methods are capable of generating material of high microstructural quality, the widespread use of these composites is currently limited by the economic non-viability of many of these fabrication methods and the paucity of reliable material qualification techniques. Melt stirring, which belongs to a class of fabrication techniques called liquid metallurgy techniques, is currently one of the most economically viable and popular fabrication routes for the manufacture of particulate composites. However, a large number of process variables must be controlled in the melt stirring technique to create a quality product. Non-optimized values of these variables generally adversely affect the final mechanical properties of the composite due to poor wetting and subsequent rejection of the reinforcement particles and non-uniform distribution (clustering) of
0921-5093/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 1 ) 0 1 3 9 9 - 5
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the particles. The introduction of porosity into the composite during stirring also has a deleterious effect on the composite properties. If the particles are found in association with porosity, the properties of the composite are further degraded as these particles are effectively debonded from the matrix and do not contribute to strengthening of the composite. Nondestructive evaluation (NDE) techniques are particularly useful for the inspection of materials because the output from such a test depend, to a large extent, on the microstructure of the material. In addition, these tests are capable of providing rapid results without the need to destroy test samples, thus being well suited for a manufacturing environment. Ultrasonic test techniques and methods have been studied and developed by various researchers for the nondestructive characterization of composites. Roth et al. [1] have developed a semi-empirical model relating the velocity of longitudinal ultrasonic waves to the elastic properties and density of the specimens. Using this model they have shown that the longitudinal velocity of the material decreases almost linearly with an increase in the volume fraction of porosity in the material. Tests were conducted on silicon nitride and steel samples and these researchers observed a high degree of agreement of measured values with predicted values of velocity. Rokhlin et al. [2] have conducted a detailed study on the ultrasonic phase velocity behavior of fiber reinforced composites. They found that the velocity is significantly affected by the degradation of the elastic properties of the interphasial layer. Degradation in the interphasial layer could occur during manufacture (debonding of the fiber from the matrix due to insufficient wetting or thermal incompatibility) or during service (oxidation damage or fatigue related damage). They observed a reduction in phase velocity for composites which were manufactured using fibers that were
not appropriately pre-coated (thereby inducing debond at the fiber –matrix interface), as well as in composites subjected to fatigue or oxidation cycles. They have explained this behavior by considering the degradation of elastic modulus of the interphase when fiber debond takes place as a result of fatigue or oxidation damage. A review of the literature on ultrasonic evaluation of composites shows that the extent of work done on particulate composites is considerably lesser than that on fiber reinforced composites. Also, majority of the work on particulate composites has been focused on powder metallurgy (P/M) processed composites. However, as already explained, melt stirring followed by casting remains one of the most economically viable fabrication techniques for particulate composites [3–8]. It is also well known that the improvement in mechanical properties of the composite are based on the presence of good particle– matrix bonding [9,10]. Thus, simple techniques to evaluate the bonding efficiency of the particles should be developed for materials design to translate effectively to engineering products. So, it was felt that investigation of the debonding of particles due to particle–porosity association and clustering would be very useful. Hence, the present paper involves a study of the ultrasonic velocity behavior of an aluminum alloy (A356) based SiC particulate composite, processed by the melt stirring technique. An attempt has been made to quantify the extent of particle debonding in the composite as indicated by the association between particles and porosity. Elastic moduli of the composites have been determined by tensile testing and discussed with reference to the measured ultrasonic velocity as well as the extent of debonding. The composites were also hot rolled in order to improve their microstructural integrity, and the extent of such improvement was again checked using ultrasonic velocity measurements.
Table 1 Elemental composition of matrix alloy used in this study (wt.%)
2. Details of experimental work
Si
Mg
Cu
Zn
Ni
Fe
Ti
Al
6.37
0.20
0.04
0.015
0.037
0.15
0.19
Balance
Table 2 Properties of matrix alloy [6] and reinforcement used in this study Property
Matrix (A356)
Reinforcement (SiC)
Elastic modulus (E), GPa Bulk modulus (K), Gpa Shear modulus (G), GPa Poisson’s ratio, w Standard density (z), kg m−3
– 70.0 23.0 0.33 2680
430 – – – 3200
2.1. Materials used The aluminum alloy matrix used in the present work is a commercial casting-grade alloy (A356) whose elemental composition is given in Table 1. The reinforcement particle used is silicon carbide of average diameter 47 mm. The properties of the matrix and reinforcement are summarized in Table 2.
2.2. Casting of composites Composite specimens were processed by the melt stirring technique, which has been described in detail by Rohatgi [11]. Composites with approximately 3, 7, 11 and 15 vol.% of SiC reinforcement were synthesized.
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All specimens were top-poured into a permanent mold for solidification.
2.3. Characterization of microstructure The porosity in the castings was measured by three methods. These were the standard metallographic technique (using light microscopic observation of a polished cross-section), density measurement using the water displacement principle, and volume measurement using a helium gas-based pycnometer. The SiC particle volume fraction was measured by two techniques. These were the standard metallographic technique and the acid digestion technique (using hydrochloric acid).
2.4. Measurement of ultrasonic 6elocity and elastic modulus Ultrasonic velocity in the specimens was measured using a Krautkramer USD 15SX model instrument using the pulse-echo method. For longitudinal velocity measurements, a standard probe of 2 MHz frequency was used. The accuracy of velocity measurements was within 91 m s − 1. Elastic modulus of the composite specimens was measured using an Instron model 4206 tensile tester, using a constant crosshead speed of 50 mm min − 1. For longitudinal velocity, five measurements were made and the average values have been reported. For elastic modulus, three samples were tested and the average values have been reported.
2.5. Hot rolling of composite samples Composite samples were heated to temperatures between 500 and 530°C in an electric resistance furnace prior to rolling. All samples were given a thickness reduction of 55– 60% by rolling. The thickness reduction by rolling was done in small steps in order to minimize sample cracking.
3. Model for prediction of ultrasonic velocity Particulate composites can generally be treated as statistically homogeneous and isotropic [12]. Thus, a model for the prediction of ultrasonic velocity in particulate composites containing porosity can be developed as follows. An unreinforced matrix containing porosity of volume fraction 6p is considered. The elastic modulus of such a material (assuming that the size and shape of the pores do not affect the elastic modulus) is given by the well-known Mackenzie’s [13] equation: E6 p =
9K6 pG6 p 3K6 p + G6 p
(1)
where K6 p and G6 p are the bulk modulus and shear modulus respectively of the material containing porosity of volume fraction 6p. These can be readily calculated from the equations: K6 p = and
4KG(1− 6p) 4G+ 36pK
G6 p = G 1− 5
n
(2)
n
3K + 4G 6 9K + 8G p
(3)
where K and G are the respective moduli for the matrix material that is pore free. These values can be obtained from literature [14], and are summarized in Table 2 for the matrix alloy used in this study. In the above porous matrix material, it is assumed that reinforcement particles of volume fraction 6r are uniformly dispersed. It is further assumed that the effects due porosity and particles are linearly superposed. This allows the use of the rule-of-mixtures law to be used to estimate the effective properties of the composite. It is also assumed that out of the total reinforcement volume fraction of 6r, a fraction ‘f’ is debonded from the matrix due to association with porosity or clustering with porosity. Thus the volume fraction of reinforcement particles that are effectively bonded to the matrix is given by 6r (1−f ). It is well known that only effectively bonded particles contribute to the improvement of elastic modulus of the composite. Therefore, the corresponding fraction of the reinforcement particles, 6rf, which are debonded from the matrix, do not contribute to the increase in elastic modulus of the composite. Thus, the effective elastic modulus, Ec, of the composite containing porosity of volume fraction 6p and effectively bonded reinforcement particles of volume fraction 6r (1−f ), is given by the rule-of-mixtures law to be: Ec = Er[6r(1−f)]+ E6 p(1−6r)
(4)
where Er is the elastic modulus of the reinforcement particle. At this juncture the two limiting cases of f=0 (all particles are fully bonded and have no interaction with porosity) and f= 1 (all particles are fully debonded and do not contribute in any way to the increase in elastic modulus) need to be discussed. In the case where f=0, it is seen that the value of elastic modulus obtained from Eq. (4) would be the theoretical upper bound value. In the case where f= 1, it is realized that the fully debonded particles should be considered as porosity as far as their contribution to the elastic modulus of the composite is concerned. Thus, the value of elastic modulus for the case of f= 1 gives the theoretical lower bound value. Actual values of elastic modulus would lie in between these bounds, corresponding to values of f that satisfy 0B fB 1.
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Substituting the appropriate expressions from Eqs. (1)– (3) into Eq. (4) gives the following equation for the effective elastic modulus of the composite:
n ! !
Ec =Er[6r(1− f)]+
"n "
Æ 9 4KG(1− 6p) G 1 −5 3K +4G 6 Ã p 4G + 36pK 9K +8G Ã 12KG(1−6p) 3K + 4G Ã +G 1 −5 6 È 4G+36pK 9K + 8G p
(5a)
Upon substitution of the values of parameters from Table 2, the above expression reduces to: Ec =4306r(1− f)+
0.54 −1.546p +6 2p 0.0086 − 0.00746p −0.00366 2p
(1 − 6r)
n
(5b)
giving the elastic modulus of the composite in GPa. As far as the density of the composite is concerned, if the standard pore free density of the matrix material is taken to be zstd, then the density of the matrix with porosity of volume fraction 6p is given by the equation: zspm =zstd(1−6p)
(6)
With respect to density of the composite, it is realized that the relative bonding efficiency of the particles with the matrix does not affect the density values, in contrast to the case of mechanical properties like elastic modulus. Thus, if reinforcement particle of volume fraction 6r is now added to this material, the effective density of the composite, zc, is given by the rule-of-mixtures law to be: zc = zr6r +zstd(1−6p)(1 −6r)
(7)
where zr is the density of the reinforcement particle. It is well known that the longitudinal velocity, VL, in a homogeneous, isotropic medium is given by the expression: VL =
E(1− w) z(1+w)(1− 2w)
n
1/2
(8)
where E, w, and z are the effective elastic modulus, Poisson’s ratio and density, respectively, of the medium under consideration. Thus, substituting equations Eqs. (5a), (5b) and (7) into Eq. (8) gives an expression for the longitudinal velocity in the composite containing porosity of volume fraction 6p and effectively bonded reinforcement particles of volume fraction 6r (1 −f ) as:
Again, substitution of various parameters from Table 2 results in a simplified relation for the longitudinal velocity (in ms − 1) as:
<
VL =
Ç Ã Ã(1−6r) Ã É
n ! !
n=
0.54− 1.546p + 6 2p (1−6r) 0.013−0.0116p − 0.0056 2p 14476r + 1212(1−6p)(1 −6r)
109 2886r(1− f) +
1/2
(9b) It is clear that when all the reinforcement particles exist in the composite in complete isolation from the porosity in the composite and are well bonded to the matrix, the factor f will be equal to zero. In that case, the longitudinal velocity calculated from Eq. (9a) or Eq. (9b) would give the theoretical upper bound velocity for the composite assuming no particle–porosity interaction and perfect bonding. Likewise, in the case when all particles are debonded due to complete association between particles and porosity or due to particle– porosity clustering, the factor f would be equal to 1 and the velocity calculated would give the theoretical lower bound velocity for the composite. The velocity actually measured on the composite would lie in between these two limits, corresponding to a value of the debonding factor that satisfies 0BfB1. Therefore, the debonding factor, calculated using longitudinal (L) velocities, can now be defined by the ratio: f=
VL(max) − VLm VL(max) − VL(min)
(10)
where the subscript (max) refers to the theoretical upper bound, (min) refers to the theoretical lower bound, and m refers to the actually measured velocities, respectively. In the above formulation, Poisson’s ratio, w, has been assumed to be unaffected by the presence of both porosity and particles. This has been shown [15] to be a fairly valid assumption. Thus, a value of 0.33 for w, corresponding to the value generally assumed for aluminum and its alloys [14], has been used throughout this study.
4. Results and discussion
4.1. Microstructure Average values and S.D. of porosity and reinforcement content in the cast specimens, before rolling,
"n "
1/2 Æ 9 4KG(1 − 6p) G 1− 5 3K+4G 6 Ç Ç ÆÆ Ç p à à à à 4G+ 36pK 9K+ 8G ÃÃE 6 (1− f) + à Ã(1− 6r)Ã(1−w)à r r Ãà 12KG(1 − 6 3K + 4G à p) Ã Ã È 4G +36 K +G 1− 5 9K + 8G 6p Ã É É ÃÈ Ã p VL = È É [zr6r +zstd(1 − 6p)(1 −6r)](1+ w)(1− 2w)
45
(9a)
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Table 3 Average values and standard deviation of reinforcement and porosity content and corresponding factor k (before rolling) Casting identification
SiC mean 9| (%)
Porosity mean 9 | (%)
k= 6r/(1−6p)
A356-0 A356-1 A356-2 A356-3 A356-4
0 3 9 0.125 7 9 0.138 11 9 0.143 15 9 0.119
2.29 0.148 3.2 90.157 3.9 90.172 4.490.163 6.1 90.140
0.0000 0.0309 0.0728 0.1150 0.1597
measured by various methods are summarized in Table 3. A356-0 refers to the unreinforced matrix alloy while A356-1 to A356-4 refer to specimens with increasing amounts of SiC reinforcement. The particle distribution was found to be fairly uniform; no large-scale particle segregation was observed. However, the uniformity of particle distribution in high SiC volume fraction composites was much better than that observed in low SiC volume fraction composites. The higher particle concentration gives rise to multiple solidification fronts, and geometric trapping of the particles between these fronts leads to reduced particle movement within the melt. In contrast, in composites with lower particle concentrations, particle pushing occurs to a greater extent and leads to particle segregation in the interdendritic regions. The above mechanism is considered to be the prime reason for the better uniformity of particle distribution in the high SiC volume fraction composites [16]. Porosity content of the specimen was observed to increase with an increase in the SiC content. There are two possible reasons for this observation. Firstly, since the time of stirring for higher SiC content composites is slightly longer, there is a higher likelihood for more air
bubbles to be sucked through the vortex into the stirred metal. Secondly, at high SiC content, the melt becomes less fluid, creating conditions that are unfavorable for already entrapped air bubbles to escape from the melt during solidification.
4.2. Obser6ations on ultrasonic 6elocity The comparison between the upper and lower bounds on the predicted velocity and the velocity values actually measured for longitudinal waves in the specimens are plotted in Fig. 1 against a factor ‘k’ defined as the ratio (6r/(1− 6p)). 6r And 6p represent the volume fraction of reinforcement and porosity, respectively. The measured velocity value shown in Fig. 1 is the average of five measurements made on each sample. Two general trends are apparent from Fig. 1. The first is that the measured velocities show a fairly linear increasing trend. Using values of the measured velocity, it is, therefore, possible to predict the factor k. In the present study, the following equation has been derived: VL = 11902 (6r/(1− 6p))+5692.7
(11)
where VL is the measured longitudinal velocity in ms − 1. The volume fraction porosity, 6p, can be fairly accurately measured by determining the density of the specimen using water displacement methods and comparing this density with the maximum (theoretical) density obtained by hot pressing or rolling. Thus, the above Eq. (11) can be used to determine the volume fraction of SiC in the composite, 6r, in a relatively straightforward manner. The second trend that is clear is that the measured longitudinal velocities are smaller than the predicted longitudinal velocity values. It is to be noted that the velocity value predicted by Eq. (9a) or Eq. (9b) takes into
Fig. 1. Plot of predicted and measured longitudinal velocities against k (k = 6r/1 −6p).
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Fig. 2. Plot of debonding factor against k (k = 6r/1 −6p).
account the presence of both reinforcement particles and porosity. Therefore, the observed deviation of measured velocity from that predicted by equation Eq. (9a) or Eq. (9b) can be attributed to the presence of some degree of particle– matrix debonding (corresponding to a value of debonding factor f such that 0B f B 1). As explained earlier this debonding would be caused by the association of particles with porosity or the formation of particle–porosity clusters, resulting in the reduction of elastic modulus, which has been shown to reduce velocity values greatly. The debonding factor, f, has been estimated for the various composites from the measured velocities and the predicted velocities using Eq. (10), and has been plotted against the factor k= 6r/ (1− 6p), calculated using values of 6r and 6p and shown in Table 3. This plot is shown in Fig. 2. It can be seen from Fig. 2 that the estimated debonding factor values follow a generally decreasing trend with an increase in factor k. It is seen that the value of f is a maximum for the composite with k =0.0309 (casting A356-1, Table 3). If the association between particles and porosity is assumed to be completely random, an increasing interaction (or an increasing value of debonding factor f ) is expected between pores and particles when the volume fraction of both increases. However, in the present study, it is observed that although the particle and porosity volume fractions increase for the composites A356-1 to A356-4, the debonding factor f decreases. This seems to indicate that although both porosity and particle concentration increases in these composites, this does not necessarily lead to an increased interaction or association between the porosity and particles. It also suggests that the possible mechanism of particle– porosity interaction or association would depend on the particular conditions under which these composites
were manufactured. A similar observation has been made by Rohatgi et al. [17] in their work on an aluminum alloy–zircon particle reinforced composite. They have reported that they did not observe any significant increasing trend in particle–porosity interaction although both porosity and particle content increased, in the composites tested by them. In the present case, the composite with k =0.0309 (composite A356-1, Table 3) showed the maximum value of the estimated debond factor. This implies that the particle–porosity association or interaction is present to a greater extent in this composite than in the other composites. Optical microscopic observation of carefully polished sections of the composites show that this is the case. It was observed that the tendency for particle–porosity association or clustering is seen to a much greater extent in the composite with k =0.0309 (composite A356-1, Table 3). A typical optical mi-
Fig. 3. Optical micrograph of a particle – porosity cluster, observed in the composite with k = 0.0309 (unrolled specimen).
48
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Fig. 4. Plot of comparison between predicted and measured elastic modulus against (k = 6r/1 −6p).
crograph of a particle– porosity cluster, observed in the composite with k = 0.0309 (composite A356-1, Table 3), is shown in Fig. 3. The particles that are clustered around the edge of the pore are effectively debonded from the matrix and would not contribute to any increase in mechanical properties of the composite. Thus, the composite with k =0.0309 (composite A3561, Table 3), which showed a larger extent of such clusters, also exhibited the largest value of the estimated debond factor, which in turn caused the largest deviation between predicted and measured velocity. Microstructural examination of composites with k = 0.0728, 0.1150, and 0.1597 (composites A356-2, 3 and 4, respectively, Table 3) showed that although there is an increase in the overall particle and porosity content in these composites, the tendency for particle– porosity association or clustering is reduced. Samuel and Samuel [18] have shown that the porosity size and shape were affected by the presence of SiC reinforcement particles through the tendency these particles display to block or restrict the growth of pores. In composites with low volume fraction of reinforcement particles, they found that porosity growth was less restricted leading to larger pores in the interdendritic regions, these large pores being generally found surrounded by reinforcement particles. On the other hand, high volume fraction composites were generally found to have smaller pores, which were associated to a lesser extent with the reinforcement particles. Thus, it appears that low volume fraction composites generally have bigger pores, which are found surrounded by and in association with reinforcement particles. Therefore, the observed trend of estimated debonding factors in the present study is understandable.
4.3. Obser6ations on elastic modulus Theoretical upper and lower bounds of the elastic moduli of the composites were calculated using equation Eq. (5b), using values of 0 and 1 for the factor f. The elastic moduli of the specimens were also determined from tensile tests, carried out on an Instron tensile testing machine. These results are shown in Fig. 4. The measured elastic modulus data reported in Fig. 4, is the average of three tests performed on composite samples with each value of k. The trend of the variation in the moduli values is seen to be qualitatively similar to that of longitudinal velocity values. As already mentioned, microstructural observations show that the composite with k= 0.0309 (A356-1) exhibited the largest extent of particle–porosity association or clustering, which is thus expected to show the greatest difference between predicted (with f= 0) and measured elastic moduli values. An SEM micrograph of the frac-
Fig. 5. SEM micrograph of the fractured surface of a composite with k = 0.0309 (unrolled specimen), showing particle – porosity association.
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Table 4 Average values and S.D. of porosity and corresponding factor k, before rolling (BR) and after rolling (AR) Casting identification
Porosity BR mean 9 | (%)
k =6r/(1−6p) BR
Porosity AR mean 9 |
k =6r/(1−6p) AR
A356-0 A356-1 A356-2 A356-3 A356-4
2.2 90.148 3.2 90.157 3.9 90.172 4.4 90.163 6.1 90.140
0.0000 0.0309 0.0728 0.1150 0.1597
0.8 90.135 1.4 90.144 1.9 90.128 2.6 90.115 3.5 90.106
0.0000 0.0304 0.0713 0.1129 0.1554
tured surface of the composite with k = 0.0309 (composite A356-1, Table 3) is shown in Fig. 5. Again, the association of SiC particles with porosity can be clearly seen.
4.4. Obser6ations on hot rolled samples As already mentioned, during manufacture of composites by the melt-stirring method, unless expensive vacuum atmosphere is maintained, there is a high likelihood that air or gas bubbles are sucked into the melt, leading to porosity or particle– porosity clusters. In order to improve the properties of such composites, manufacturers routinely perform secondary processing operations like rolling or forging on the as-cast composites. These operations reduce or eliminate gas porosity (either isolated or associated with reinforcement particles) inside the composite, thereby improving the properties of the composite. At the same time, non-optimized rolling or forging parameters could lead to internal defects like reinforcement particle cracking or particle–matrix interface cracking, which would degrade material properties. This is especially true of composites with higher volume fraction of reinforcement. Therefore, it is necessary to be able to measure the extent of such improvement (or degradation) so that the secondary processing operation can be optimized and material wastage can be minimized. In the present study, composite samples were rolled as explained in Section 2. The longitudinal ultrasonic velocity was measured after rolling. It was observed that the velocity measured on the rolled samples was higher than that measured on the unrolled samples. This is understandable because the rolling operation reduces porosity in the sample, thereby improving its mechanical integrity. This leads to a corresponding increase in the measured ultrasonic velocity. In order to check the effectiveness of the rolling operation in reducing or eliminating the associated porosity, porosity of the rolled samples was estimated by measuring the density after rolling, using the water displacement method as well as the Pycnometer method, and comparing this density with the theoretical density. Values of average porosity measured in the samples, before and after rolling are summarized in Table 4. This
porosity value was then used to calculate the bounds on the predicted value of the longitudinal velocity. Comparison of these bounds on predicted velocity with the actual measured velocity, using Eq. (10), enables estimation of the debonding factor after the rolling operation. A comparison of the estimated debonding factors before and after the rolling operation would be very useful in determining the effectiveness of the rolling operation in reducing or eliminating associated porosity. This comparison is shown in Fig. 6. Here too, the reported value of longitudinal velocity is the average of five measurements taken on each sample. It is seen from Fig. 6 that for the low SiC volume fraction composites (or composites with low k), the reduction in debonding factor is comparatively higher. In the case of the high SiC volume fraction composites, reduction in debonding factor is lower or even negative, as in the case of the composite with k= 0.1554 (composite A3564 after rolling, Table 4). This indicates that the positive effect of rolling is manifested to a much greater extent in the low SiC volume fraction composites than in the high SiC volume fraction composites. This observation could be attributed to the presence of a larger number of brittle SiC particles in the high SiC volume fraction composites, which could lead to particle cracking as well as particle–matrix interface microcracking or debonding during the rolling process. As already discussed, such cracked or debonded particles do not fully (effectively) contribute to the elastic modulus of the composite, thus causing an increase in the debonding factor. Fig. 7 shows a typical optical micrograph of the composite with 15 vol.% SiC, after rolling. In Fig. 7, reinforcement particle cracking can be clearly seen, as indicated by the arrows. Such cracked particles would not fully contribute to the increase in elastic modulus of the composite, thus causing a reduction in the measured velocity and contributing to an increase in the debonding factor. In a similar manner, particle–matrix interface microcracking would lead to an increase in the debonding factor. Rolling parameters that are not an optimum would cause reinforcement particle cracking or particle–matrix interface microcracking. Thus, it can be seen that estimation of the debonding factor would be very useful in improving the effectiveness of the rolling operation.
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Fig. 6. Comparison of debonding factors before and after rolling.
Fig. 7. Optical micrograph of a 15 vol.% composite, showing reinforcement particle cracking.
tween measured and predicted values of up to 14% was observed in the case of poorly bonded composites while relatively well bonded composites showed modulus deviation of less than 10%. It was also found that the effect of hot rolling in improving the integrity of the composite could be conveniently estimated by using measured values of the velocity in conjunction with the model developed. This can potentially lead to cost and material savings by helping to optimize the parameters of the secondary processing operation. The results obtained would be of considerable importance in industrial situations to obtain a first estimate of the effect of changes in process parameters on the extent of particle–matrix debonding in composite materials.
5. Conclusions The present study shows that measurement of ultrasonic velocity in the particulate composite can give a good indication of the extent of particle– porosity association in the composite. In composites that showed a lower value of the factor k (or a higher degree of particle–porosity association), the debonding factor f was found to be higher. Debonding factors estimated from measured velocities were found to be as high as 0.29 in the case of poorly bonded composites (A356-1 with k = 0.0309). Debonding factors measured in relatively well bonded composites (A356-4 with k = 0.1597) were as low as 0.10. These results also agreed well with microstructural observations on particle– porosity association or clustering in the composites. Results from measurement of elastic modulus of the composites were also consistent with the results of velocity measurements. Elastic modulus deviation be-
Acknowledgements The authors would like to thank Professor Fong Hock Sun (Dean, School of Materials Engineering, Nanyang Technological University) for financial support and permission to use various resources of the School. One of the authors (P.N. Bindumadhavan) was supported by a research scholarship granted by the school during the course of this work.
References [1] D.J. Roth, D.B. Stang, S.M. Swickard, M.R. DeGuire, L.E. Dolhert, Mater. Eval. July (1991) 883. [2] S.I. Rokhlin, W. Huang, Y.C. Chu, Ultrasonics 33 (5) (1995) 351. [3] M. Skibo, P.L. Morris, D.J. Lloyd, Proc. Int. Symp. on Advances in Cast Reinforced Metal Composites, September 1998, Chicago IL, p. 257.
P.N. Bindumadha6an et al. / Materials Science and Engineering A323 (2002) 42–51 [4] A.M. Samuel, F.H. Samuel, Key Eng. Mater. 104 – 107 (1995) 65. [5] S. Ray, J. Mater. Sci. 128 (1993) 5397. [6] A.M. Samuel, A. Gotmare, F.H. Samuel, Compos. Sci. Technol. 53 (1995) 301. [7] J.E. Allison, G.S. Cole, JOM, January (1993) 19. [8] W. Hoover, in: N. Hansen, et al. (Eds.), Proceedings of the 12th International Symposium, Riso National Laboratories, Roskilde, Denmark, 19912, p. 387. [9] R.J. Arsenault, Composites 25 (7) (1994) 540. [10] Y.H. Teng, J.D. Boyd, Composites 25 (10) (1994) 960. [11] P. Rohatgi, in: D.M. Stetanescu, J.R. Davis (Eds.), Metals Handbook, 9th edn., Vol. 15, Casting, 1991, p. 842.
.
51
[12] R.M. Fulrath, J.A. Pask (Eds.), Ceramic Microstructures, Robert Krieger, Huntington, NY, 1976, p. 313. [13] J.K. Mackenzie, Proc. Phys. Soc. B63 (1950) 2. [14] F. King, Aluminum and its Alloys, Ellis Harwood, London, 1987, p. 95. [15] R.W. Rice, in: R.A. MacCrone (Ed.), Microstructure Dependence of Mechanical Behavior of Ceramics, Academic Press, New York, 1977, p. 199. [16] D.J. Lloyd, Compos. Sci. Technol. 35 (1989) 159. [17] P.K. Rohatgi, S. Raman, B.S. Majumdar, A. Banerjee, Mater. Sci. Eng. A123 (1990) 89. [18] A.M. Samuel, F.H. Samuel, Metall. Trans. A24 (1993) 1857.