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

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1 Scripta mater. 44 (2001) DEFORMATION AND FRACTURE CHARACTERISTICS OF AA6061-Al 2 O 3 PARTICLE REINFORCED METAL MATRIX COMPOSITES AT ELEVATED TEMPERATURES P. Ganguly 1, W.J. Poole 1 and D.J. Lloyd 2 1 Department of Metals and Materials Engineering, University of British Columbia, Vancouver, B.C., V6T 1Z4, Canada 2 Alcan International Limited, Kingston Research and Development Center, P. O. Box 8400, Kingston, ON, K7L 5L9, Canada (Received July 1, 2000) (Accepted in revised form November 17, 2000) Keywords: Aluminum; Metal matrix composite; High temperature fracture Introduction Aluminum alloy based metal matrix composites reinforced with ceramic particles such as Al 2 O 3 or SiC have a number of advantages over conventional aluminum alloys, primarily enhanced stiffness and improved wear resistance. However, these materials have significantly reduced ductility compared to unreinforced alloys. The improved properties of the composites primarily derive from the transfer of load from the matrix to the hard, stiff reinforcing phase. A consequence of the load transfer process is that the local stresses in the microstructure are raised and this may lead to the intervention of alternative processes during deformation such as particle cracking or interfacial decohesion. The distribution of local stresses in the composite depends on the instrinsic material properties, the geometric characteristics of the composite and the state of loading stresses (e.g. compression, tension etc.). The intrinsic properties of interest include: i) matrix properties such as stiffness, flow stress and rate sensitivity, ii) particle properties such as stiffness and fracture strength, and iii) interfacial properties such as the shear and tensile strengths of the particle-matrix interface. The geometric factors include the volume fraction, shape and distribution of the reinforcing phase. A good knowledge base, from experimental [1 6] and finite element method studies [7 10] exists for the composite properties at low temperature (i.e. room temperature), although fracture behaviour is complicated and is less well understood than elastic or plastic properties. It is of interest to examine situations where the intrinsic properties of the composite are modified by raising the temperature of deformation. Relatively little work has been done on high temperature behaviour [11 15]. The results of Zhao et al. from a cast and extruded AA2014-Al 2 O 3 MMC (10 20 vol. % reinforcement) indicate that there is a significantly lower level of ductility for MMCs (hot extruded) compared to the unreinforced matrix in the temperature range of C [11]. They also observed a shift in the primary mode of damage nucleation from particle cracking to interfacial decohesion at higher temperatures. Syu and Ghosh [14] conducted compression tests with a variety of sample geometries on a AA %Al 2 O 3 to establish the forging limits at temperatures of 300 and 400 C with strain rates of and 0.5 s 1. They found higher forging limits at 400 C and a slight improvement at the lower strain rate /01/$ see front matter Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S (00)

2 1100 DEFORMATION AND FRACTURE CHARACTERISTICS Vol. 44, No. 7 The goal of this work is to measure properties of AA6061/20%Al 2 O 3 composites over a wide range of temperatures and moderately high strain rates and to examine the role of particle distribution by examining as-cast and extruded MMCs. In the present work, the deformation and failure characteristics of both the as-cast and the extruded MMC are investigated at high temperatures ( C) and relatively high strain rates ( s 1 ). This temperature and strain rate regime is industrially significant and corresponds to the hot secondary processing of these materials (hot extrusion or rolling). Experimental The material investigated was a AA %Al 2 O 3 particulate composite fabricated by molten metal mixing and direct chill cast by DURALCAN Inc. Material was obtained in the as-cast condition as well as after hot extrusion. The volume fraction, size and shape of the Al 2 O 3 reinforcing phase were characterized for the as-cast and extruded MMC using the C Imaging Systems image analyzer (version 5.1). The volume fraction of the reinforcements in the MMCs were determined from the area fraction of the particles on the polished sections. For the extruded MMC, the area fraction was estimated over a total area of 8.7 mm 2 (total number of particles 6721), while for the as-cast MMC, an area of 6.5 mm 2 (total number of particles 11362) was used. The extruded and the as-cast MMC samples were tested under uniaxial compression, uniaxial tension and collar compression loading at temperatures between 15 and 550 C with strain rates of 0.1, 1 and 10 s 1. The samples for uniaxial compression were 10 mm in diameter by 14 mm in height while the uniaxial tension samples had diameters and lengths of 3 and 20 mm, respectively. The sample for collar compression tests was similar to the collar upset sample (without lubrication) used by Syu and Ghosh [14] with an outer ring diameter of 14 mm, barrel diameter of 10 mm and collar thickness of 1.5 mm. Prior to testing, all samples were heat treated for 8 hours at 530 C and then water quenched. In addition, to stabilize the matrix at the test temperature, the samples were held for 40 minutes at 300 C prior to the tests at 15 C, 200 C and 300 C, and for 10 minutes and 5 minutes at the test temperatures for the 425 C and 550 C tests respectively. The tests were performed on an MTS/Instron 8500 servohydraulic machine. The heating of the samples to test temperature was achieved by using heated deformation platens. To reduce friction at the sample/platen interface, a Teflon sheet 0.6 mm in thickness coated with molybdenum disulfide was used for the low temperature compression tests (15 C and 200 C). For the high temperature tests, the samples were machined with pocket depressions on both end faces, which were filled with a mixture of Dow Corning grease and colloidal graphite for the 300 C and 425 C tests, and lead borosilicate glass powder (softening point 425 C) for the 550 C tests. The lead borosilicate lubricant was less effective than the graphite/grease mixture, and the samples showed some evidence of barreling at 550 C. For the collar compression tests, the sample was deformed in several steps, each step resulting in increments of 0.1 (for extruded MMC) or 0.05 (for as-cast MMC) in the collar hoop strain. After each step, the sample was visually inspected for any cracks, and the radius of the central barrel (r), and the radius (r c ) and the height of the collar (t) were measured. The radial rr, axial zz and hoop true strains in the collar were calculated from the change in width (r c -r), height (t) and radius (r c ), respectively. Results Table 1 summarizes the key microstructrual characteristics of the reinforcing phase for as-cast and extruded MMCs. It can be observed that the reinforcing particles for the as-cast MMC samples were smaller than that in the extruded composite. The reinforcement volume fraction was also slightly higher for the as-cast MMC. The compressive stress-strain curves for the extruded MMC at a strain rate of 1

3 Vol. 44, No. 7 DEFORMATION AND FRACTURE CHARACTERISTICS 1101 TABLE 1 Mean and Standard Deviation (in brackets) Values for the Reinforcement Characteristics in Extruded and As-Cast MMC Major Axis ( m) Minor Axis ( m) Volume fraction (%) Aspect Ratio Diameter ( m) Extruded MMC 20.8 (11.3) 11.8 (7.3) (0.8) 18.1 As-cast MMC 15.5 (8.2) 8.5 (5.2) (0.8) 12.8 s 1 are plotted in Figure 1a. In general, the strain hardening regime in the materials was limited to imposed strains of less than 0.07 (except for the 15 C tests). For both the MMC and the unreinforced AA6061, the material flow stress increased with decreasing temperature and increasing strain rate (see Figure 1b). The flow stress for the extruded MMC was 10 30% higher than the unreinforced matrix. No significant difference was observed between the flow stress for the extruded and as-cast MMC. Figure 2a compares the tensile failure strains (i.e. reduction in area at failure) between the extruded and as-cast MMCs for a strain rate of 1 s 1. The extruded material showed significantly higher ductility than the as-cast material. In both cases, the ductility was a maximum at 425 C. Microstructural examination of the fracture surfaces revealed that the increased ductility at 425 C is related to the shift in the dominant void nucleation mechanism from particle cracking at 300 C to interfacial decohesion at 550 C. The effect of strain rate on the failure strains was not significant over the range of strain rates examined (i.e s 1 ). In the as-cast material, the cracks followed particle rich regions (see Figure 2b). In order to examine the extent of local damage, the tensile samples were sectioned perpendicular to the fracture surface. Figure 3a and 3b show examples for cast and extruded samples deformed at 550 C and 1 s 1. It can be observed that the damage was localized to a very narrow region near the fracture surface for the as-cast MMC, while it was relatively uniformly distributed in the extruded MMC samples. For lower temperature tests, the damage was predominantly observed near the fracture surface. The role of stress state can be examined by comparing results for failure strains from collar compression and uniaxial tension tests. The failure strains under collar compression were lower than under uniaxial tension for the extruded MMC (see Figure 4a). The failure strains for the as-cast MMC were similar for the two loading paths (see Figure 4b). Figure 1. (a) Stress-strain diagrams for the extruded MMC (strain rate 1 s 1 ), and (b) variation of the steady state flow stress for AA6061 and the extruded MMC with temperature and strain rate.

4 1102 DEFORMATION AND FRACTURE CHARACTERISTICS Vol. 44, No. 7 Figure 2. (a) MMC failure strain variation with temperature (strain rate 1s 1 ). The points indicate the mean, and the error bars the range of the data. (b) crack propagation through a particle cluster. Discussion Fracture of metal matrix composites is thought to occur in a similar manner to traditional engineering alloys. Damage must be nucleated, either by interfacial decohesion or particle cracking, the resulting voids grow and finally coalescence of voids occurs resulting in final fracture. These processes are significantly modified by the non-dilute nature of the second phase. The nucleation of damage is linked to the local stresses in the particles and at the interface. These stresses are significantly modified by the spacing and distribution of nearest neighbour particles. The growth of voids is also modified due to the change in local stress state (void growth is strongly dependent on hydrostatic stress) and due to geometric constraint of the surrounding particles. These complications make the analysis of ductility in these materials challenging. It is useful to consider the effects of changing deformation temperature. When the temperature is raised, the intrinsic properties of the constituent phases are modified. For the matrix, the flow stress is decreased and its rate sensitivity increases while the properties of the alumina reinforcement do not change significantly. The temperature dependence of interfacial properties is difficult to determine, although recent observations by Ziv et al. [16] suggest that the strength of the aluminum/al 2 O 3 interface decreases as temperature increases. Figure 3. Distribution of damage in (a) as-cast and (b) extruded MMC tensile samples, at 550 C and 1s 1.

5 Vol. 44, No. 7 DEFORMATION AND FRACTURE CHARACTERISTICS 1103 Figure 4. Failure strain variation with temperature and strain rate for (a) extruded and (b) as-cast MMC, under uniaxial tensile and collar compressive loadings. The symbols represent the mean, and the error bars the range of the data. The tests were conducted at 300, 425 and 550 C. Some of the points have been horizontally shifted for clarity. The combination of these considerations can be used to understand the observation that the ductility increases with increasing temperature, reaches at maximum near 425 C and then decreases. The lowering of the flow stress as temperature rises directly results in lower stresses in the ceramic reinforcement. This lowers the probability of damage via particle cracking and the ductility initially increases with increasing temperature (in the C temperature range). The lower flow stress in the matrix also lowers the stress at the interface. However, it appears that the interfacial strength decreases at an even greater rate resulting in significant amounts of interfacial decohesion at higher temperatures. Hence, a lowering of ductility for the highest temperature tested. This change in damage mode from particle cracking to interfacial decohesion for these composites has been described in detail elsewhere [17] and are similar to those reported by Syu and Ghosh [13] and Zhao et al. [11] for AA2014/Al 2 O 3 composites. The observations on the role of global stress state are also of interest. Lloyd and Morris have found that at room temperature, these materials show little notch sensitivity suggesting that local stress state was dominated by interparticle interactions [18]. In the collar compression test, the strain path in the collar is initially uniaxial tension but deviates to plane strain tension as the compression is continued [14]. This can be seen in Figure 5 where the hoop strain is plotted vs. axial strain. The strain line for uniaxial tension is seen, deviations above this line indicate a shift towards plane strain conditions. The change in strain path results in a change in the global hydrostatic stress level (tensile in this case). Under uniaxial tension, the hydrostatic stress, H, is simply (1)/ 3 applied where applied is the applied stress. Figure 5. Deformation strain path in the collar in (a) extruded MMC and (b) as-cast MMC samples.

6 1104 DEFORMATION AND FRACTURE CHARACTERISTICS Vol. 44, No. 7 However, under plane strain tension, the hydrostatic stress increases to H 1 / 2 applied. The role of imposed hydrostatic stress field is very important in damage processes as they result in a local volume increase. As a result hydrostatic compression tends to delay fracture [19] while hydrostatic tension promotes fracture. This would explain the lower failure strains for the extruded material in the collar tests. For the as-cast material, the collar test has not proceeded to a point where substantial deviation from uniaxial conditions has occurred (see Figure 5b) and thus, similar failure strains for the collar and tensile tests were observed. Finally, a very significant difference was observed between the cast and extruded MMCs. Although it is difficult to quantify, the spatial distribution of particles was different. In the cast material, microstructure tends to be more clustered. This is consistent with the observations of Hutchison and Palmiere [20] that suggested the particles can be redistributed by deformation and that this was particularly effective for hot deformation conditions. The presence of clustered regions leads to smaller interparticle spacings which result in higher local constraint and a concentration of stress [10]. The higher local stresses promote damage either in the form of particle cracking or by interfacial debonding resulting in failure at lower levels of applied strain. Lloyd [2] has also observed that fracture at room temperature initiates in the particle rich regions and as a result fracture should be very dependent on particle distribution as seems to be the case in the present work. Conclusions The deformation and fracture behaviour of AA6061/20%Al 2 O 3 composites has been examined in the temperature range of C at strain rates of s 1. Ductility was observed to depend strongly on temperature but only weakly on strain rate. At 300 C, fracture was dominated by particle cracking while interfacial debonding was prevalent at 550 C. The highest ductility was found at 425 C where particle cracking and interfacial debonding were both minimized. The magnitude of ductility was observed to depend on macroscopic hydrostatic stress, i.e. ductility was lower for collar compression samples compared to tensile tests. Finally, a significant difference was observed between the ductility of the cast and the extruded MMCs. It is thought that the spatial distribution of the reinforcing phase played a significant role in reducing ductility in the cast materials due to an increase of constraint in particle clusters which promotes the nucleation and growth of damage. Acknowledgments The material and financial support from Alcan International and NSERC (Canada) is gratefully acknowledged. References 1. D. J. Lloyd, Int. Mater. Rev. 39, 1 (1994). 2. D. J. Lloyd, Acta Metall. Mater. 39, 59 (1991). 3. Y. Brechet, J. D. Embury, S. Tao, and L. Luo, Acta Metall. Mater. 39, 1781 (1991). 4. J. J. Lewandowski, C. Liu, and W. H. Hunt, Jr., Mater. Sci. Eng. A107, 241 (1989). 5. M. Manoharan and J. J. Lewandowski, Acta Metall. Mater. 38, 489 (1990). 6. J.-Y. Buffière, E. Maire, P. Cloetens, G. Lormand, and R. Fougères, Acta Mater. 47, 1613 (1999). 7. X. Q. Xu and D. F. Watt, Acta Mater. 44, 801 (1996). 8. J. R. Brockenbrough and F. W. Zok, Acta Metall. Mater. 43, 11 (1995). 9. G. Bao, J. W. Hutchinson, and R. M. McMeeking, Acta Metall. Mater. 39, 1871 (1991). 10. Z. Wang, T.-K. Chen, and D. J. Lloyd, Metall. Trans. 24A, 197 (1993).

7 Vol. 44, No. 7 DEFORMATION AND FRACTURE CHARACTERISTICS D. Zhao, F. R. Tuler, and D. J. Lloyd, Acta Metall. Mater. 42, 2525 (1994). 12. D. J. Lloyd, High Performance Composites: Commonality of Phenomena, ed. K.K. Chawla et al., p. 465, TMS, Warrendale, PA (1994). 13. D.-G. C. Syu and A. K. Ghosh, Mater. Sci. Eng. A184, 27 (1994). 14. D.-G. C. Syu and A. K. Ghosh, Metall. Mater. Trans. 25A, 2027 (1994). 15. E. J. Palmiere and H. Xu, Composites, 30A, 203 (1999). 16. I. Ziv, F. Weinberg, and W. J. Poole, Scripta Mater. 40, 1243 (1999). 17. P. Ganguly and W. J. Poole, in Proceedings of the 8th International Conference on Mechanical Behaviour of Materials, ed. F. Ellyin and J. W. Provan, vol. III.2, p. 818 (1999). 18. D. J. Lloyd and P. L. Morris, in Proceedings of Science and Engineering of Light Metals-RASELM 91, ed. Hirano et al., Japan Institute of Light Metals, 465 (1991). 19. A. K. Vasudevan, O. Richmond, F. Zok, and J. D. Embury, Mater. Sci. Eng. A107, 63 (1989). 20. W. G. Hutchison and E. J. Palmiere, Mater. Trans. JIM. 37, 330, (1996).