Mechanical and Impact Behaviour of (Al 2 O 3 ) p /2014 and (Al 2 O 3 ) p /6061 Al Metal Matrix Composites in the C Range

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1 Applied Composite Materials 4: , c 1997 Kluwer Academic Publishers. Printed in the Netherlands. Mechanical and Impact Behaviour of (Al 2 O 3 ) p /2014 and (Al 2 O 3 ) p /6061 Al Metal Matrix Composites in the C Range F. BONOLLO DIMEG, University of Padova, via Marzolo, Padova, Italy L. CESCHINI and G. L. GARAGNANI Institute of Metallurgy, University of Bologna, Via Risorgimento, Bologna, Italy (Received 2 October 1996; accepted 17 October 1996) Abstract. The present work is aimed at studying the impact behaviour of commercially available Aluminium matrix composites, in a temperature interval ranging from 25 C to 200 C. The results of instrumented impact tests and of microstructural and fractographic observations are correlated with the tensile properties of these materials. A description of the phenomena involved (particles cracking, interfacial failure associated to matrix-reinforcement reaction layers, ductile behaviour of the matrix) is given. The effect of testing temperature as well as that of the matrix characteristics are presented and discussed. Key words: aluminum matrix composites, impact strength, tensile strength, high temperature testing, fractography. 1. Introduction The mechanical behaviour of Aluminium matrix composites has been investigated thoroughly in various research works [1 8]. The increased mechanical properties, with respect to unreinforced Aluminium, constitute in fact the main driving force for the applications of these materials in different engineering fields. The competitive price levels achieved in some cases by discontinuously reinforced Aluminium composites make them very attractive. A key-point, for the commercial success of these materials, is related to their good mechanical behaviour at high temperatures. The toughness and the impact strength, as functions of the different materials and processing variables, have been deeply analysed at room temperatures, and are reviewed by several researchers [9 14]. At higher temperatures, on the other hand, there is less information about these characteristics, which, however, may play a fundamental role in the in-service duration of Aluminium matrix composites. The goal of this work is to investigate the impact behaviour of some discontinuously reinforced Aluminum matrix composites from room temperature up to 200 C, correlating the results of the test with the microstructural features VTEX(P) PIPS No.: MATHKAP ACMA148.tex; 25/02/1997; 17:19; v.5; p.1

2 174 F. BONOLLO ET AL. and with the fractographic characteristics of the specimens, as well as with their tensile behaviour. 2. Materials and Methods The experimental work has been carried out on two kinds of composites: an AA2014 alloy reinforced by 17%vol of Al 2 O 3 particles (average size = 15 µm), extruded and heat treated (T6), called in the followings as 2014MMC, an AA6061 alloy reinforced by 20%vol of Al 2 O 3 particles (average size = 15 µm), extruded, forged and heat treated (T6), called in the followings as 6061MMC, in both cases manufactured by Duralcan and hot worked and treated by Simbi SpA (Turin). Typically, each composite was extruded from the as-received conditions (7 inches extruded bars) to 50 mm diameter and subsequently heat treated; specimens were drawn by electro-discharge machining (EDM) both for tensile and for instrumented impact tests (available energy = 300 J). In this case, the tests were carried out on a Ceast MK3 R testing machine at 25 C, 100 C, 150 C and 200 C, using Charpy-V notch specimens, according to ASTM standard E23. For each specimen, the load vs. time curve was recorded, allowing the evaluation of both the energy required for the fracture of the specimen (E f ), and the contributions associated with the nucleation (E n ) and propagation stages (E p ) of the fracture itself. The microstructural features of the composites have been analysed by light, scanning and transmission electron microscopy (LM, SEM and TEM, respectively). The specimens for the TEM observations were prepared following a procedure involving mechanical thinning (up to a thickness of 80 90µm), punching (to achieve 3 mm diameter disks) and, finally, electrolytical thinning (a 25% perchloric acid solution in ethanol at 30 C was employed in a Struers Tenupol R electrolytic jet machine). The examinations were carried out on a JEOL 200CX R transmission electron microscope with an acceleration voltage of 160 kv. On the broken specimens, a complete fractographic analysis has been carried out, in order to better define the mechanical behaviour of the materials investigated. Quantitative metallography procedures have been also employed for evaluating the characteristics of fracture paths as function of the composite conditions as well as of the testing temperature. In detail, micrographs were taken of the profiles of the broken surfaces (as schematically shown in Figure 1), and a PC computer code in BASIC language was developed to digitize the images and to count on them the number of particles along a random path (RPPC) and along the fracture path (FPPC). The same code allowed the evaluation of the linear roughness (RL) of the fracture surface. In Figure 1 the meanings of RPPC, FPPC and RL are also graphically explained. ACMA148.tex; 25/02/1997; 17:19; v.5; p.2

MECHANICAL AND IMPACT BEHAVIOUR 175 Figure 1. Scheme of the procedure for the evaluation of RPPC, FPPC and RL. a Figure 2.

Results and Discussion The typical microstructure of the composites is shown in Figure 2, in which the good homogeneity in the reinforcement distribution

In the as-extruded condition, a typical high dislocation density can be seen around an Al 2 O 3 particle (Figure 3); the heat treatment leads to the

3 MECHANICAL AND IMPACT BEHAVIOUR 175 Figure 1. Scheme of the procedure for the evaluation of RPPC, FPPC and RL. a Figure 2. Typical microstructure of the 2014MMC (a) and of the 6061MMC (b). b 3. Results and Discussion The typical microstructure of the composites is shown in Figure 2, in which the good homogeneity in the reinforcement distribution is clearly evident. The TEM investigations were used to study the microstructural evolution of the 6061MMC composite. In the as-extruded condition, a typical high dislocation density can be seen around an Al 2 O 3 particle (Figure 3); the heat treatment leads to the formation of reinforceing precipitates, constituting sites for dislocation piling-up phenomena (Figures 4 and 5). The tensile characteristics of the composites under investigation are collected in Table I: analyses on these data are reported in previous works [15, 16]. ACMA148.tex; 25/02/1997; 17:19; v.5; p.3

$The fractographic observations constitute a good tool for a better understanding of the mechanical behaviour of these materials.$ $The broken surfaces of the tensile tested specimens revealed a dimpled morphology in the matrix, typical of a ductile fracture,$

4 176 F. BONOLLO ET AL. Figure 3. Dislocations in the as-extruded 6061MMC material. Figure 4. Piling-up of dislocations in the 6061MMC material extruded and T6 heat treated. Figure 5. Piling-up of dislocations in the 6061MMC material extruded and T6 heat treated. The fractographic observations constitute a good tool for a better understanding of the mechanical behaviour of these materials. The broken surfaces of the tensile tested specimens revealed a dimpled morphology in the matrix, typical of a ductile fracture, independently of the processes (extruded or extruded and T6 heat treated) and of the test temperatures. As shown in Figures 6 and 7 the situation is similar for the two kinds of matrices: the reinforcement particles are mainly ACMA148.tex; 25/02/1997; 17:19; v.5; p.4

5 MECHANICAL AND IMPACT BEHAVIOUR 177 Table I. Results of the tensile tests (E =extruded, T6 = heat treated, artificial ageing). Material Testing UTS YS e temperature ( C) (MPa) (MPa) (%) 6061MMC [E] MMC [E,T6] MMC [E,T6] localized on the bottom of the dimples, in agreement with the void growth mechanism generally associated with discontinuously reinforced materials [3, 4, 11]. In both composites, however, no preferential sites for cracking (such as clusters of particles) have been found on the fractured surfaces, due to the substantial homogeneity of reinforcement distribution into the matrix. At higher magnifications, in the 6061MMC material, it is possible to observe (Figure 8) that: on the particles surface, a layer of precipitates is evident: such a layer, constituted by Mg-rich spinels, is probably due to the manufacturing process and is related with interfacial debonding and failure phenomena [17]; in the interparticle regions, the matrix reveals its ductility, showing microdimples. Further information about the microstructural and mechanical behaviour of the material under investigation are supplied by means of quantitative fractography, results of which are collected in Table II. In all cases, the number of particles along the fracture path is lower than that measured randomly in the composite. It means that, independently of the matrix composition and of the processing conditions, the reinforcement particles do not constitute weakening points for the composites. This is obviously correlated with the matrix characteristics: an overaged matrix (this is in fact the case of the composites tested at 200 or 220 C, after 1 h soaking time) becomes the preferential path for fracture; such consideration is also supported by the RL value. For what concerns the 6061MMC, at 220 C, with respect to 140 C, a higher dimple density on the fracture surface has been observed, as well as a more pronounced roughness (i.e. a higher RL value). From the data on broken particles on the fracture surface, the 2014MMC seems to have a slightly better particle-matrix interfacial bonding (the fracture of particles takes place when the interface is stronger than the particles themselves); ACMA148.tex; 25/02/1997; 17:19; v.5; p.5

6 178 F. BONOLLO ET AL. Figure 6. Fractured surface of the 6061MMC extruded and T6 treated, tensile tested at 140 C. Figure 7. Fractured surface of the 2014MMC extruded and T6 treated, tested at 150 C. Figure 8. Fractured surface of the 6061MMC extruded and T6 treated, tensile tested at 100 C. ACMA148.tex; 25/02/1997; 17:19; v.5; p.6

7 MECHANICAL AND IMPACT BEHAVIOUR 179 Table II. Results of quantitative fractography measurements. Material Testing RPPC FPPC FPPC/ RL Broken particles temperature RPPC on the fracture ( C) (part./mm) (part./mm) surface (%) 6061MMC [E] MMC [E,T6] MMC [E,T6] MMC [E,T6] Table III. Results of the impact tests. Material Testing Impact Nuclation Propagation temperature energy energy energy ( C) (J) (J) (J) β AA6061 [T6] MMC [T6] AA2014 [T6] MMC [T6] this is due to the presence, as discussed above, of an interfacial reaction layer in the 6061MMC materials. However, the measured percentages of broken particles on the fracture surface are generally in agreement with other literature data [5]. The results of the instrumented impact tests are collected in Table III, in terms of overall impact energy, nucleation energy and propagation energy; the parameter β, defined as the ratio of the nucleation to the overall impact energy (E n /E f ), is also introduced. For comparison, the data concerning the unreinforced matrices are also presented. In Figure 9 an example of the load vs time and energy vs time diagrams obtained is given, while in Figures 10 and 11 the data are summarized, allowing the comparison among the various experimental variables and showing, for the materials under investigation, the contribution of ACMA148.tex; 25/02/1997; 17:19; v.5; p.7

8 180 F. BONOLLO ET AL. a b Figure MMC-T6, impact tested at 200 C: load vs time (a) and energy vs time (b) diagrams. the nucleation and of the propagation steps to the overall fracture energies, at various temperatures. A first consideration of these data concerns the values of β, which are substantially higher for the AA6061 alloy (always β>0.5) with respect to the AA2014 alloy (β 0.5). In other words, the energy required for the crack nucleation is higher than that for the propagation in the case of AA6061, while the contrary is true for AA2014. The introduction of the reinforceing particles produces, for both matrices, a decrease in β, whose values are in the ranges for 6061MMC and for 2014MMC, i.e. a decrease in the nucleation energy contribution to the failure. As a general trend, however, it must be observed that ACMA148.tex; 25/02/1997; 17:19; v.5; p.8

9 MECHANICAL AND IMPACT BEHAVIOUR 181 Figure 10. Impact energies as functions of material and testing temperature. a b c Figure 11. Contribution of the nucleation and propagation stages to the facture energies for AA6061-T6 (a), AA2014-T6 (b), 6061MMC-T6 (c) and 2014MMC-T6 (d). d ACMA148.tex; 25/02/1997; 17:19; v.5; p.9

182 F. BONOLLO ET AL. Figure 12. Fractured surface of the 6061MMC impact tested at RT. Figure 13. Fractured surface of the 2014MMC impact tested at RT.

Ranking the various materials in terms of impact energy, we have: AA6061 AA2014 > 6061MMC > 2014MMC, with a significant effect due to the matrix impact strength.

10 182 F. BONOLLO ET AL. Figure 12. Fractured surface of the 6061MMC impact tested at RT. Figure 13. Fractured surface of the 2014MMC impact tested at RT. the temperature does not significantly affect the values of β. On the other hand, an increase in temperature is usually associated with an increase in the impact energy. Ranking the various materials in terms of impact energy, we have: AA6061 AA2014 > 6061MMC > 2014MMC, with a significant effect due to the matrix impact strength. As a further remark, values higher than that reported in literature have been achieved at 25 C for the 6061MMC: an impact energy of 2.5 J was measured, while 1.4 J are mentioned for an AA %SiCp composite [14]. From the SEM examinations, a good correspondance can be observed between the tensile and the impact fractured surfaces, even if the testing velocities are substantially different (typically, the traverse speed is 0.2 mm/min in the tensile test, while the hammer speed is 5 m/s in the impact test). The presence of reinforcement particles into the dimples has been detected for both composites, as can be seen in Figures 12 and 13. In particular, for the 6061MMC material, the ACMA148.tex; 25/02/1997; 17:19; v.5; p.10

11 MECHANICAL AND IMPACT BEHAVIOUR 183 reaction layer around particles is very evident, as well as the debonding phenomenon induced during the test (Figure 12). It is worthnoting, however, that, together with the interfacial debonding, cracking of particles is present. Fractured reinforcement particles can be detected also in Figure 12, with, in one case, a crack oriented at 45 with respect to the main particle axis. Again, as in the fractographs after tensile test, some dimples are detectable in the matrix. Another proof of the correlation between tensile and impact test comes by the examination of the profile roughness parameter, which was proposed in the past as a gauge of fracture toughness [12]: the 6061MMC material shows similar trends for RL ( ) and for impact strength (3J 5J)on changing the testing temperature (from 140 C to 220 C and from 150 C to 200 C, respectively). In the case of the 2014MMC material, the fractographic examination allows to observe clean particle surfaces, i.e. the absence of precipitates: this results in failure mechanism involving particle breaking (Figure 13). From Figure 13 is also evident the lower ductility of the AA2014 matrix, with respect to the dimpled morphology showed by the AA6061 matrix in Figure 12. An overview of the experimental data suggests some considerations about the behaviour of the composites and of their matrices. The highest impact strengths were shown by the AA6061-T6 alloy, for which the more part of the fracture energy is associated with the crack nucleation stage. This is in agreement with the better ductility at room temperature of AA6061-T6 (elongation = 17%) compared to AA2014-T6 (elongation = 13%) [18]. On the other side, the AA2014-T6 alloy displayed a significant improvement in fracture energy at 200 C, which can be put in correlation with the highly increased ductility at this temperature (elongations of 38% and of 28% are reported at 204 C, respectively for AA2014-T6 and for AA6061-T6 [18]). The presence of the reinforceing particles causes, for both alloys, a significant decrease in impact strengths. This is certainly obvious (the microstructural effect of hard particles in a ductile matrix is well known), but the almost similar impact strengths of the two composites result from different reasons. In fact, for the 6061MMC material the positive effect on impact strength theoretically given by a ductile matrix is lost because of the interfacial reaction leading to embrittling, Mg-rich, spinels. No interfacial reactions, on the other hand, were detected in the 2014MMC, for which it was evident from fractographs the negative effect due to the lower matrix deformation, compared to that of 6061MMC. Finally, it must be remarked that the temperature promotes improvements in fracture energies for all the materials under investigation, but it seems not significantly affect the energy partition between the nucleation and the propagation stages, i.e. it does not really affect the failure mechanism. 4. Conclusions From the instrumented impact test carried out in the 25 C 200 C temperature range, the behaviour of AA6061- and AA2014-based composites has been eval- ACMA148.tex; 25/02/1997; 17:19; v.5; p.11

12 184 F. BONOLLO ET AL. uated, as well as that of the unreinforced matrices. The impact strength ranking is AA6061 AA2014 > 6061MMC > 2014MMC. For both composites and independently of the testing temperature, the presence of the reinforcement lowers the crack nucleation energies, with respect to the unreinforced matrices. The testing temperature seems not to play a significant effect on the failure mechanisms, while it is generally associated with increased impact energies. The fractographic examinations carried out on the broken surfaces of tensile and impact tested specimens suggest that also the loading speed has a limited influence on the failure mechanism. Such a mechanism consisted mainly in matrix damage and reinforcement particle breaking for the AA2014-based composite. For the 6061MMC, in addition, significant interfacial damage phenomena have been observed, associated with a reaction layer on the reinforcement particles surface. Acknowledgements The authors would like to think the Italian Ministry of University and Technological Research (MURST) for the finanical support. Many thanks are also due to Prof. F. Zucchi (University of Ferrara) for his cooperation and for helpful discussions, to Dr. M. Castelli (Simbi SpA) for the supply of composites, and to ing. S. Luciani (University of Padova) and ing. F. Fornaciari (University of Ferrara) for their help in the experimental work. References 1. Clyne, T. W. and Withers, P. J., An Introduction to Metal Matrix Composites, Cambridge University Press, Cambridge, Lloyd, D. J., Particle Reinforced Aluminium and Magnesium Matrix Composites, International Materials Review 39(1), 1994, Nutt, S. R. and Duva, J. M., A Failure Mechanism in Al-SiC Composites, Scripta Metallurgica 20, 1986, You, C. P., Thompson, A. W. and Bernstein, I. M., Proposed Failure Mechanism in a Discontinuously Reinforced Aluminum Alloy, Scripta Metallurgica 21, 1987, Flom, Y. and Arsenault, R. J., Fracture of SiC/Al Composites, in Proceedings 6th Int. Conf. on Composite Materials, Elsevier, Amsterdam, 1987, pp Lloyd, D. J., Aspects of Fracture in Particulate Reinforced Metal Matrix Composites, Acta Metallurgica et Materialia 39(1), 1991, Lewandowski, J. J., Liu, C. and Hunt, W. H., Effects of Matrix Microstructure and Particle Distribution on Fracture of an Aluminum Metal Matrix Composite, Materials Science and Engineering A107, 1989, Hunt, W. H., Osman, T. M. and Lewandowski, J. J., Micro- and Macrostructural Factors in DRA Fracture Resistance, Journal of Metals 45(1), 1993, Mortensen, A., A Review of the Fracture Toughness of Particle reinforced Aluminum Alloys, in Proceedings Int. Conf. on Fabrication of particulates reinforced metal composites, Montreal, 1990, pp ACMA148.tex; 25/02/1997; 17:19; v.5; p.12

13 MECHANICAL AND IMPACT BEHAVIOUR Friend, C. M., Toughness in Metal Matrix Composites, Materials Science and Technology 5, 1989, Hasson, D. F. and Crowe, C. R., Fracture Toughness of SiC/Al Metal Matrix Composites, in Proceedings Conf. Strength of metals and alloys, Pergamon Press, Montreal, 1985, pp Davidson, D. L., Fracture Surface Roughness as a Gauge of Fracture Toughness: Aluminium- Particulate SiC Composites, Journal of Materials Science 24, 1989, Hamouda, A. M. S. and Hashmi, M. S. J., Mechanical Properties of Aluminium Metal Matrix Composites Under Impact loading, in Proceedings Int. Conf. on Advances in Materials and Processing Technologies, Dublin, 1993, pp Nardone, V. C., Strife, J. R. and Prewo, K. M., Microstructurally Toughened Particulate- Reinforced Aluminum Matrix Composites, Metallurgical Transactions 22A, 1991, Bonollo, F., Brondolin, V., Castelli M. and Garagnani, G. L., Mechanical Behaviour of Forged Al Composites, in Proceedings 4th Int. Conf. ATA-MAT 94, Turin, 1994, pp Ceschini, L., Garagnani, G. L., Poli, G. and Bonollo, F., Tensile Tests of Al2O3 Particles Reinforced AA2014 Aluminium Alloy at Temperatures Ranging from 25 to 200 C, in Proceedings 4th Eur. Conf. on Advanced Materials and Processes EUROMAT 95, Padova, vol. 1, 1995, pp McLeod, A. D. and Gabriel, C. M., Kinetics of the Growth of Spinel, MgAl 2O 4,onAlumina Particulate in Aluminum Alloys Containing Magnesium, Metallurgical Transactions 23A, 1992, ASM Metals Handbook, Vol. 2: Properties and Selection: Non-Ferrous Alloys and Pure metals, ASM, Metals Park, Ohio, ACMA148.tex; 25/02/1997; 17:19; v.5; p.13

DEFORMATION AND FRACTURE CHARACTERISTICS OF AA6061-Al 2 O 3 PARTICLE REINFORCED METAL MATRIX COMPOSITES AT ELEVATED TEMPERATURES

Scripta mater. 44 (2001) 1099 1105 www.elsevier.com/locate/scriptamat DEFORMATION AND FRACTURE CHARACTERISTICS OF AA6061-Al 2 O 3 PARTICLE REINFORCED METAL MATRIX COMPOSITES AT ELEVATED TEMPERATURES P.