SiC composites

Scripta Materialia, Vol. 35, No. 2, pp. 193-198,1996 Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights resewed 1359-6462/96 $12.00 + .OO

PI1 S1359-6462(96)00128-5

THE ESSENTIAL WORK OF FRACTURE AS A MEANS FOR CHARACTERIZING THE INFLUENCE OF PARTICLE SIZE AND VOLUME FRACTION ON THE FRACTURE TOUGHNESS OF PLATES OF Al/Sic COMPOSITES Y. Marchal’, F. Delannay’ and L. Froyen2 ‘Universite catholique de Louvain, Departement des sciences des materiaux et des procedes, PCIM, Reaumur, Place Sainte Barbe 2, B- 1348 Louvain-la-Neuve, Belgium :!Katholieke Universiteit Leuven, Departement Metaalkunde en Toegepaste Materiaalkunde, de Croylaan 2, B-300 1 Leuven, Belgium (Received January 19, 1996) Introduction Over the past .20 years, particulate-reinforced aluminium-matrix composites have been increasingly considered for weight-saving applications in the automotive and aerospace industries. The addition of ceramic particles can lead to higher modulus, yield strength and tensile strength. However, the low ductility and fracture toughness of MMCs remain a major obstacle for applications. Many investigators have shown that the low fracture toughness of particulate-reinforced MMCs is due to the role of particles acting as damage initiation sites (l-6). When the particle volume fraction is high and when plane strain conditions prevail, the inelastic zone surrounding the crack tip is generally small enough as to allow the testing of fracture toughness on laboratory size specimens using conventional linear elastic fracture mechanics (LEFM) methods (1). However, for composites containing low volume fractions of particles and/or for samples in the shape of thin plates, extensive yielding invalidates LEFM methods. In such cases, fracture toughness may be measured using the essential work of fracture (EWF) approach. This method, which can be applied to any material, consists in measuring the work spent for fracturing plates with Deep Double Edge Notched Tensile (DENT]1geometry. The total work of fracture can be considered as the sum of an essential work W, spent in the process zone ahead of the crack tip and a nonessential work W,, dissipated in an outer plastic zone. Cotterell and Reddel(7) have suggested that, if the initial ligament length 1 of the specimen is not larger than the plastic zone size, W, is proportional to the initial ligament area It (where t is the thickness of the plate) and that W, is proportional to the volume of the plastic zone. This writes w = w,

+ w,

= Itw, +

or

193

pl*twp

HI

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W -_=w=w It

e + w,

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where w, and wPare respectively the essential and non-essential specific works of fracture and p is a shape factor which depends on the shape of the plastic zone. As sketched in Figure 1, a plot of the total specific work of fracture w as a function of the initial ligament length 1gives thus a straight line whose ordinate intercept at origin is the specific essential work of fracture w,. Cotterell and Mai (8) have demonstrated that, for an elastic-perfectly plastic material, w, is a measure of J,, the critical J integral at the initiation of cracking. It has also been suggested by Cotterell and Reddel that the values of the final displacement df at completion of fracture of the DENT specimens increase linearly with increasing ligament length and that the displacement 6, obtained by extrapolation of Ml) to zero ligament length can be interpreted as the critical crack tip opening displacement (CTOD) of the plate (7). This work aims at investigating the application of the EWF method for measuring the fracture toughness of Al/Sic composite plates with different reinforcement sizes and volume fractions. It will be shown that this method allows to highlight the respective contributions of reinforcement size and matrix strain hardening to the toughness of the plates. Materials and Exuerimental Procedure The materials were produced via the powder metallurgy technique. 99.5% pure Al powder was mixed with 2,5 or 8 weight percents of Sic with particle sizes either ranging between 5 and 20 urn or above 120 urn. The powder mixtures were cold isostatically compacted to billets and, after degassing, the billets were extruded at 490°C into 15mm diameter rods. These rods were rolled at 500°C to form 1.5mm thick plates which were finally annealed for 30 minutes at 400°C. For comparison purposes, a sample of unreinforced matrix was processed in the same way as the composites. Uniaxial tensile tests were performed to determine the whole stress-strain behaviour of the plates until tensile fracture. DENT specimens were machined from the plates. The initial notch tips were made sharp using fresh razor blades. The notch lengths were measured using a travelling microscope. The gauge length was 45 mm and the displacement rate during the test was 0.25 mm/mm Thanks to the absence of instability, the load/displacement curves could be recorded until completion of cracking along the ligament. For each

Figure 1. Sketch of the essential work of fracture method.

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specimen, the total work of fracture was obtained by integrating the whole load/displacement cufves. For each sample, the values of the essential work of fracture w, and crack tip opening displacement i&were determined by linear extrapolation on the values of w and df measured on at least 8 DENT specimens with ligaments varying from 6 to 20 mm. Fracture surfaces were also studied by scanning electron microscopy (SEM). The extent of necking along the craclced ligament was measured on the SEM images. Results

The tensile true stress - true strain curve of the samples was fitted to a Ramberg-Osgood relationship e _= e0

-

a

QO

+

a

-

t

m

a 00

i

Table 1 shows,, for the materials investigated in this work, the corresponding values of yield strength o, and strain-hardening exponent n = l/m. It can be seen that the yield strength (I~increases slightly with increasing Sic weight fYactionbut seems not to depend very much on the reinforcement size. The strainhardening exponent n is lower for composites reinforced with particles with sizes above 120pm than for composites reinforced with particles with sizes ranging between 5 and 20pm. The presence of Sic particles has a much larger effect on the fracture behaviour. Figure 2 shows the values of the essential work of fracture w, and crack tip opening displacement i&for the different samples. Both w, and 8, are observed to decrease with increasing volume fraction of reinforcement. However, the trends differ depending on the particle size and weight fraction. For weight fractions of 2% and 5%, the decrease of w, is much larger in the case of large particle sizes than in the case of small particle sizes. This indicates that, for composites with low reinforcement volume fractions, large particle sizes are more detrimental to fracture toughness than small particle sizes. However, fracture toughness would have been considered identical if it had been expressed only in terms of & values. In contrast, an increase of the weight fraction from 5 to 8% causes a significant further reduction of both w, and hc in the case of small particle sizes whereas the same increase causes hardly any change of these parameters in the case of large particle sizes. Due to the plane stress conditions prevailing in the plates, DENT specimens exhibit very extensive necking along the cracked ligament. The average extent of necking along the ligament was measured by observation of -thefracture surface by scanning electron microscopy. This measurement being fairly time consuming, it was made only on the specimens with ligament sizes of (or close to) 15 mm. (In contrast, 6, is an average for the whole range of ligament lengths and can thus be considered more representative). As shown in Figure 3, quite unexpectedly, the trend of the variation of the extent of necking, for a given particle weight fraction but different reinforcement sizes (between 5 and 20 pm or above 120pm), is somewhat contrary to the trend of the variation of w, (Fig. 2): the extent of necking is larger when the

Yield Strength (ooo & and Strain-Hardening

TABLE 1 Exponent (n) of the Different Investigated

Composites

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essential work of fracture w, is lower. Similar phenomena have been observed already for several other materials such as for example zinc plates (9). SEM investigations of the fracture surfaces suggested the presence of two kinds of voids: large voids around Sic particles and smaller voids in the aluminium matrix, most probably at the location of small oxide particles resulting from the oxide skin on the Al powder particles. When the reinforcement consists of particles smaller than 20 urn, the larger voids appear to have nucleated most often by decohesion at the interface between the matrix and the Sic particle. In contrast, for particle sizes larger than 120um, both particle fracture and interface decohesion are observed. In composites containing 8 weight percent of particles, significant particle clustering was observed when using low particle sizes whereas no clustering was observed in the case of large particle sizes. Discussion The limited difference between the plastic parameters listed in Table 1 is not unexpected as it is well known that the plastic behaviour of composites made by powder metallurgy is often governed by the hardening effect due to the tiny oxide particles originating from the oxide skin on the Al powder particles. The role assigned to Sic particles in these composites is to increase stiffness. The larger strain hardening exponent of composites reinforced with particles ranging fi-om 5 to 20 pm can presumably be ascribed to a higher dislocation density generated by the finest fragments of these particles. The EWF method can be applied subject to the condition that the whole ligament has yielded before crack propagation, which means that the plastic zones at the two crack tips must have met before the initiation of cracking. In plane stress, the plastic zone size rp can be estimated using the relation

Using E = 70 GPa and the values of u,, and w, given in Table 1 and Figure 2, one calculates values of rp ranging from 0.4 to 0.8 m. This indicates that the EWF method fully applies to the samples investigated in this study. In thin plates, the fracture process zone (in which the essential work of fracture is spent) includes the zone undergoing necking. The plastic work spent for necking is thus the major contribution to the essential work of fracture. Also, the crack tip opening displacement i& is primarily due to the elongation of the neck. The damage work spent for void nucleation, growth, and coalescence in the neck and the displacement corresponding to this damage process constitute only minor contributions to w, and &. w, and hc depend thus very much on the parameters characterizing the plastic behaviour, namely the yield strength u0 and strain hardening exponent n. As shown in Table 1, composites containing small particles display larger strain-hardening exponents than composites containing large particles.

(a~~~~;

;.ii~‘““‘I

0 SIC weigh!%

2

4 6 SIC wcighld

8

10

Figure 2. Variation of (a) the essential work of fracture w, and (b) crack tip opening displacement 6, as a fimction of the Sic weight fraction.

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When the scale of the fmite strain zone (process zone) is sufficiently small, crack initiation is governed by the value of the J integral. In such cases, the influence of strain hardening reflects on the value of the so-called Shih’a;parameter d,,which relates the crack tip opening displacement, the J-integral and the yield strength a,. At the initiation of crack extension, J = J, and this relationship writes: CTOD

= d,

’ 5 00

d, varies as a function of the strain-hardening coefficient n and of the elastic strain ae, = au& in the Ramberg-Osgood equation [3]. This dependence has been tabulated by Shih (10). The dependence on aeO is much weaker than the dependence on n. Using our measurements of w, and &, we are thus tempted to define the experimental parameter

(6) The values of d, and di have been determined for the materials investigated in this work. As shown in figure 4, these values are very similar. This gives support to the approximation, confirmed experimentally by several authors (8, 11,12,13,14,15) , that w, = J,, and to the proposal of Cotterell and Reddel(7) that 6, can be identified with the CTOD. The correspondence between the values of d, and di indicates also that the differences (shown in figure 2) between the trends of w, and b, for the two sizes of particles should be related to the influence (shown in Table 1) of particle size on strain hardening. For weight fractions of 2% and 5%, the fact that smaller particle sizes bring about larger values of w, in spite of identical values of hc is due to the larger plastic dissipation during necking as a result of the larger strain hardening exponent. A high hardening exponent thus gives the plate a higher overall toughness. If significant, the observation of different extents of necking (Figure 3) in spite of identical values of he should be related to the influence of the strain exponent on the shape of the neck. It is important to notice that the present work demonstrates again the fact that the extent of necking of a plate before cracking does not always scale with the fracture toughness of this plate (expressed by w,) (9). Little change of cracking behaviour is observed in the case of particle sizes larger than 120 pm when the particle wieight fraction increases from 5% to 8%. In contrast, this increase causes a very drastic decrease of both w, and 6, in the case of small particle sizes. The reason of this difference appears to be

0 0 Is!

1

,s

0.5 0.4

%

0.3

0 0

Pure Numinium Al + x%SiC (5_2Opm)

2

0.2

0

N

5 ,c

0.1

l-4

0

0 0

Y g

F4 )

.

0.6

0

0,8

0

5! + x”&iC

l= -0

.

.

.

Pure Aluminium Al + Z%siC(5..Z@tm) Al +S?LSiC (5..ZWm)

’

*

0.6

(12Ottm)

m Al+ Z%SiC(120pm) + Al +SxSiC (120pm)

.

& Al + 8%SiC (129rm) 0

0

2

8 Si:

10

weight%

Figure 3. Average extent of necking as a function of the volume fraction of SIC for DENT specimens with ligament size of (or close to) 15mm.

0

0.2

0.4 Shih

factor

0.6 dn

0.8

1

Figure 4. Comparison of the theoretical (dJ and experimental (0 values of the Shih’s factor.

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due to the large amount of clustering observed by SEM in the latter sample. This clustering phenomenon is difficult to avoid in composites prepared by powder metallurgy when the particle volume fraction exceeds a critical value which depends on the ratio between the sizes of the Al and Sic particles. It appears that the cracking behaviour becomes then governed by the size of the clusters rather than by the size of the individual particles. These clusters bring about high stress concentrations, making void nucleation easier. Conclusion The essential work of fracture approach has been used to characterize the fracture toughness of plates of Al/Sic composites. It has been possible to correlate fracture toughness to the fracture behaviour. Pure aluminium and composites reinforced with low particle volume fractions display a high toughness. Increasing the volume fraction of reinforcement yields a pronounced drop of fracture toughness. Reinforcement particles size has an obvious influence on the strain hardening behaviour of the materials and therefore on their fracture toughness. In the case of composites containing 2 or 5 weight percent of reinforcement, both strain-hardening coefficient and essential work of fracture are higher for small particles than for larger particles. When the volume fraction of the reinforcement increases and the size of the particles decreases, the probability for a crack to meet particle clusters increases. Composites containing 8 weight percent of 5..20um particles present such clusters and display therefore a drastic loss of fracture toughness.

AcknowledPments

This work has been supported by SSTC - DWTC (Belgium) in the frame of PAI-UIAP 4 1. References 1. 2. 3. 4. 5. 6. 7. 8. 9. IO. 11. 12. 13. 14. 15.

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