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Al2O3 syntactic foams
Materials Science & Engineering A 582 (2013) 415–422
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Al–Al2O3 syntactic foams – Part I: Effect of matrix strength and hollow sphere size on the quasi-static properties of Al-A206/Al2O3 syntactic foams J.A. Santa Maria, B.F. Schultz n, J.B. Ferguson, P.K. Rohatgi Materials Science and Engineering Department, University of Wisconsin-Milwaukee, 3200 N. Cramer St., Milwaukee, WI 53201, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2013 Received in revised form 21 May 2013 Accepted 24 May 2013 Available online 14 June 2013
The microstructure and quasi-static compressive mechanical properties of Al-A206/Al2O3 hollow sphere syntactic foams were investigated for foams with three different hollow sphere size ranges (0.106– 0.212 mm, 0.212–0.425 mm, 0.425–0.500 mm) and three different conditions including As cast (F), T4 and T7. The peak strength, plateau strength and toughness of the foams were found to increase with increasing wall thickness to diameter (t/D) ratio. Since the t/D ratio was found to increase with decreasing sphere diameter, the foams produced with the smallest hollow spheres (0.106–0.212 mm) showed superior performance for the peak strength (F: 226 MPa, T4: 312 MPa, T7: 342 MPa), plateau strength (F: 190 MPa, T4: 269 MPa, T7: 269 MPa), and toughness (F: 59 J/cm3, T4: 78 J/cm3, T7: 88 J/cm3). These properties were also found to generally increase with increasing matrix yield strength. The 0.212– 0.425 mm T7 heat treated syntactic foams in this study exhibit the highest specific plateau strength (102 MPa-cm3/g) and the second highest specific energy absorption (41 J/g) of aluminum syntactic foams reported in literature for which comparable data is available. & 2013 Elsevier B.V. All rights reserved.
Keywords: Cellular materials Composites Metal matrix syntactic foam Casting Aluminum alloys Mechanical characterization
1. Introduction Metal matrix syntactic foams (MMSFs), metals containing a high volume percentage (∼50%) of hollow ceramic reinforcements, are desirable materials for blast and energy absorption applications due to their distinctive compressive deformation behavior. Like metallic foams, MMSFs sustain a large strain at a reduced stress level (compared to the monolithic fully dense material) leading to increased energy absorption prior to densification. MMSFs offer the potential to tailor such properties as the peak stress, plateau stress, strain to densification, energy absorption and density; all of which are of interest to engineers designing energy absorbing components [1–3]. Property enhancements may be achieved through careful matrix alloy selection, reinforcement material selection and reinforcement diameter, wall thickness and uniformity, and material processing parameters. Though a variety of reinforcement/matrix combinations have been explored [4–25], few studies have examined the effects of the matrix alloy strength combined with reinforcement size and size distribution on the properties pertinent to the design of MMSFs for energy absorption applications. n
Corresponding author. Tel.: +1 414 229 3956; fax: +1 414 229 6958. E-mail addresses: [email protected] (J.A. Santa Maria). [email protected] (B.F. Schultz), [email protected] (J.B. Ferguson). [email protected] (P.K. Rohatgi). 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.05.081
The objective of many experimental studies involving syntactic foams is to increase energy absorption capabilities. In a previous work [4], the effects of the wall thickness and diameter of hollow alumina sphere reinforced A380 matrix syntactic foams on the quasistatic and dynamic mechanical properties of the resulting MMSFs were explored. The peak strength, plateau strength and toughness of the resulting foams increased with increasing hollow sphere wall thickness to diameter (t/D) ratio. This is in general agreement with previous studies as shown in Fig. 1 [6,7,11,14–16]. It would be expected that increasing the strength of the matrix would also result in a general improvement in the peak and plateau stress. Indeed, as one can see from Fig. 1 for the A201 matrix syntactic foams developed by Kiser et al. [14], the solutionized and artificially aged (T6) foams showed an improvement in peak stress over the annealed (O) foams at equivalent t/D ratios. Likewise, Daoud [17] found that heat treatment of Zn–22Al syntactic foams to improve the matrix strength resulted in an increase in the peak stress, plateau stress, and ductility of the foam. MMSFs are generally synthesized via liquid metal processes including stir mixing [17] and most commonly by pressure infiltration [14]. As in any metal matrix composite system produced by liquid metal processing, the incorporation of reactive reinforcement materials in a metal matrix can result in alterations to the melt chemistry and the resulting mechanical properties of the solidified metal matrix. Balch and Dunand [5] synthesized Al-7075 (T6 and O conditions) and commercially pure (c.p.)
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aluminum/mullite (45 v% crystalline 3Al2O3–2SiO2, 55 v% amorphous SiO2) hollow microsphere syntactic foams by a pressure infiltration technique and measured the quasistatic compressive properties of the resulting foams. Though the foams prepared using the high strength Al-7075 forging alloy showed improved properties compared to the c.p. aluminum matrix foam, careful examination of the microstructure and chemistry of the alloy foam
Fig. 1. Effect of (t/D) ratio on the peak strength of various Aluminum alloy-hollow sphere metal matrix syntactic foams [6,7,11,14–16].
Table 1 Nominal composition of matrix and reinforcements. Material
Component
Nominal content (wt%)
A206
Al Cu Fe Mg Mn Ni Si Sn Al2O3 SiO2 Na2O MgO Fe2O3 CaO
93.3–95.3 4.25–5.0 0.1 0.15–0.35 0.2–0.5 0.05 0.05 0.05 98.8 0.8 0.1 0.05 0.03 0.03
Al2O3 hollow sphere
revealed that the matrix had in fact reacted with the mullite reinforcement to convert the SiO2 to Al2O3, releasing Si into the surrounding metal. As a result, the final composition of the matrix was closer to that of aluminum alloy A390, which resulted in a far reduced strength compared to that expected for 7075 alloy. Orbulov [13] examined the effect of alloy selection on the mechanical properties and final composition of aluminum alloy-E-sphere (36–40 wt% Al2O3, 55–60 wt% SiO2, 1.4–1.6 wt% TiO2, 0.4–0.5 wt% Fe2O3) syntactic foams. The alloys that were explored included 99.5% pure Al, Al–12Si, Al–1.2Mg–1Si–0.3Cu, and Al–4.5Cu. X-ray diffraction studies of the various syntactic foams revealed that in all cases except for the foams having a high starting concentration of Si (Al–12Si), the E-spheres partially converted to α-Al2O3 or γ-Al2O3. This displacement reaction of the Aluminum with the E-spheres to form Al2O3 and Si resulted in a significant increase in the Si content in these alloys. It followed that heat treatments of the resulting foams was not optimal and resulted in only a moderate increase in strength [12,13]. Palmer et al. [16] synthesized un-coated and coated (o 10 nm thick Aluminum) E-sphere reinforced syntactic foams. Three aluminum alloys were examined in this work including 1350, 5083 and 6061. Silicon rich second phases were observed in all cases, and sphere dissolution was observed to be more severe when the processing temperature was increased from 666 1C to 750 1C and in syntactic foams containing 45 mm spheres compared to 270 mm spheres. In the case of the 45 mm spheres, large blocky Silicon phases were observed presumably due to the larger surface area of the E-spheres in contact with the liquid phase aluminum alloy during infiltration processing. Increased dissolution of the reinforcement spheres and the resulting increase in Si content of the alloy resulted in a marked decrease in the mechanical properties of the foams containing 270 mm spheres and a slightly less severe effect for the foams containing 45 mm spheres. It is apparent from the preceding discussion that there is a need for clear and systematic studies on the effect of matrix composition and temper (i.e. strength), and reinforcement size in the absence of deleterious reactions between the reinforcement and the matrix. This work will study the effect of the matrix strength and the size, size ranges, and (t/D) ratios of the hollow spheres on the quasi-static compressive properties of metal matrix syntactic foams where the alloy and reinforcement are selected to avoid such displacement reactions.
2. Experimental procedure Syntactic foams composed of aluminum alloy A206 reinforced with approximately 40–50 v% hollow Al2O3 spheres (ALODUR white bubble alumina) were synthesized via a sub-atmospheric pressure infiltration technique. The nominal compositions of the
Fig. 2. Microstructure of alloy Al-A206 without reinforcement in (a) as cast, (b) T4, and (c) T7 conditions.
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alloy and the hollow spheres are presented in Table 1 (information provided by suppliers). Alumina hollow sphere reinforcements with diameters up to 0.5 mm supplied by C-E Minerals were sorted by size to produce 3 size ranges (0.106–0.212 mm, 0.212–0.425 mm, 0.425– 0.500 mm) for comparison. After sorting, the two larger sizes were floated in chloroform (density of 1.483 g/cm3) causing defective spheres to sink and intact spheres to float, allowing them to be collected. The smallest sphere size could not be processed in this manner due to the bulk density being more than 1.483 g/cm3. The bulk density of the hollow spheres for each size range was determined by measuring the mass of a fixed amount of spheres and the corresponding volume of water displaced by these same spheres. The method of sub-atmospheric pressure infiltration used to synthesize the foams summarized below is described in detail elsewhere [26]. A 13.95 mm ID borosilicate test tube was tappacked with hollow spheres to a height of 70–90 mm. An ingot of aluminum was placed above the hollow spheres, separated by a 2 mm thick layer of zirconia felt. The crucible containing the preform and ingot was heated in a quartz chamber under mechanical vacuum to 750 1C and held until the aluminum had fully melted and uniformly sealed the inner circumference of the crucible. Argon gas was then rapidly introduced into the heated
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quartz chamber, reaching a sub-atmospheric pressure of 0.4 bar, thereby forcing the molten aluminum into the evacuated spaces between the hollow spheres. A pressure of 0.7 bar was used for the smallest size range (0.106–0.212 mm) due to incomplete infiltration at 0.4 bar. The quartz chamber containing the sample was then removed from the tube furnace and the sample was allowed to cool in air under sub-atmospheric pressure for 3 min at which time the sample had solidified. The sample was then removed and quenched in room temperature water. Specimens of the unreinforced alloy were cast using the same procedure for comparison. The specimens, to be heat treated to the T4 and T7 condition were first solutionized at 490–500 1C for 2 h then 525–530 1C for 14–20 h. Following the solution heat treatment the T4 specimens were aged at 21 1C for 120 h and the T7 specimens were aged for 4 h at 200 1C [27].
Table 2 Average hollow sphere bulk density for various sphere size ranges. Size range (mm) 3
Hollow sphere bulk density g(cm ) Composite average density (g/cm3) Area % matrix
0.106–0.212
0.212–0.425
0.425–0.500
2.032 2.29 42.6–48.9%
1.33 1.82 39.6–47.2%
1.29 1.79 34.1–41.2%
Fig. 3. Representative microstructures of Al-A206–Al2O3 syntactic foams having the following heat treatment and hollow sphere diameter range (a) As cast, 0.106–0.212 mm, (b) T4, 0.106–0.212 mm, (c) T7, 0.106–0.212 mm, (d) As cast, 0.212–0.425 mm, (e) T4, 0.212–0.425 mm, (f) T7, 0.212–0.425 mm, (g) As cast, 0.425–0.500 mm, (h) T4, 0.425– 0.500 mm, (i) T7, 0.425–0.500 mm.
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The density of the composites was measured using a Metler Toledo AT261 Delta Range Microbalance equipped with a Density kit (Archimedes method). The specimens were first lightly coated with vacuum grease to prevent infiltration of surface pores during the density measurement. Microstructural analysis was performed on polished and etched (Keller's Reagent) specimens with a Nikon Eclipse TS100 microscope equipped with an automated stage and Clemex Professional Image Analysis Software. Quasi-static compression testing was performed in accordance with ASTM C365-94 on cylindrical specimens (Diameter¼ 14.3 mm Length ¼12.7 mm). Testing was carried out using a SATEC Model 50Ud Universal Testing Machine at constant crosshead speed with an initial strain rate of 10−3 s−1 and a self-leveling platen. Strains were calculated from the crosshead displacement, and were corrected for deflection of the load frame. A minimum of 5 specimens were tested for each syntactic foam composition. The quasi-static compression curves typically exhibited an initial peak followed by a region of lower stress and later densification. Compression was stopped when the densification stress reached the magnitude of the initial peak stress, at approximately 45% strain. The plateau strength reported is the average measured strength from initial peak to densification. Polished cross-sections of the tested specimens were examined via optical microscope.
3. Results and discussion 3.1. Microstructure The properties of the matrix depend on the fineness of the microstructure and the distribution of phases that form as a result of the solidification sequence and inter-relationships between the large number of alloying elements in the A206 alloy. Talamantes-Silva et al. [28] determined that for A206, increasing the cooling rate decreases the dendrite arm spacing, porosity, and grain size of the casting resulting in improved mechanical properties. The solidification sequence that is commonly observed for this alloy can be summarized as follows [29]: (1) Development of an α-Al dendtritic network (2) Precipitation of Al6(MnFeCu) (3) Transformation of liquid and Al6(MnFeCu) to Al and Al20Mn3Cu2 (4) Eutectic reaction to form Al2Cu, Al2Mn3Cu2 and Al7FeCu2 phases and Aluminum. (5) Complex eutectic reaction to form Al, Al2Cu, Al2MgCu, and Mg2Si which is only observed at high cooling rate. The microstructure of the matrix cast under the same conditions as the syntactic foams is shown in Fig. 2 in the as cast, T4, and T7 condition. The as cast microstructure consists of a dendritic structure, whose interdendritic regions are likely made up of Al, Al2Cu, Al2Mn3Cu2 and Al7FeCu2 phases as a result of the eutectic reaction. As shown in Fig. 2b and c, copper-containing eutectic phases have been dissolved by the solutionizing heat treatment in the T4 and T7 specimens. The T4 heat treatment produces a fine dispersion of underaged shearable θ″ throughout the matrix, while the T7 heat treatment possesses a coarse distribution of θ′ and θ overaged precipitates [29]. Representative micrographs of the syntactic foams organized by the size ranges of hollow spheres and heat treatment conditions are shown in Fig. 3. The hollow spheres appear to be uniformly distributed, and fully encapsulated by the metal matrix with little to no visible porosity in the matrix between the hollow spheres. The 0.106–0.212 mm samples contain infiltrated spheres due to the incorporation of defective hollow spheres that could not be separated by floating. The as-cast samples show an Al2Cu network throughout the matrix. Following the T4 and T7 heat treatments, the Al2Cu network present in the as-cast specimens was dissolved and precipitated as a dispersion of underaged shearable or coarse overaged precipitates within the matrix. 3.2. Physical properties
Fig. 4. Dependence of average wall thickness and t/D ratio on reinforcement diameter.
Table 2 presents the densities of the syntactic foam specimens along with the result of image analysis determining the area
Table 3 Summary of quasi-static compression datan for A206/Al2O3 syntactic foams. Heat treatment
Sphere size range (mm)
Peak stress (MPa)
Plateau stress (MPa)
Densification strain (%)
Energy absorption (J/cm3)
As Cast
0.106–0.212 0.212–0.425 0.425–0.500 0.106–0.212 0.212–0.425 0.425–0.500 0.106–0.212 0.212–0.425 0.425–0.500
225.7 7 11.1 155.3 7 6.4 164.67 0.9 312.2 7 3.6 2007 8.7 1827 2.5 341.9 7 12.4 230.3 7 12.3 1967 8.4
189.7 7 7.2 130.5 7 3.3 132.7 7 2.6 269.17 5.4 166.97 4.5 1527 4.6 268.97 7.8 186.5 7 10.7 163.7 7 6
327 3 337 5 427 2 317 357 4 367 3 347 1 417 2 407 1
58.9 7 6.8 41.7 7 6.8 53.4 7 2.8 77.9 7 2 57.2 7 7.6 53.5 7 4.3 87.8 7 5.6 74.4 7 4.7 62.8 7 3.6
T4
T7
n
All the data are reported as average7standard deviation.
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Fig. 5. Typical compressive stress–strain curves for (a) as cast matrix and various sphere sizes, (b) T4 matrix and various sphere sizes, (c) T7 matrix and various sphere sizes, and (d) 0.212–0.425 mm spheres in as cast, T4, and T7 matrix.
Table 4 Yield strength and UTS of Aluminum alloy A206 in the As-cast (estimated), T4 and T7 condition. Matrix condition
sy-matrix (MPa)
sUTS-matrix (MPa)
Al-A206 As-Cast (estimated) Al-A206 T4 [30] Al-A206 T7 [30]
60 262 345
200 428 435
percentage of matrix in the foam. The 0.425–0.500 mm size range has the lowest area fraction of matrix and lowest density due to its spheres being most similar in size, thus allowing for a higher packing efficiency. The 0.212–0.425 mm materials, having a wider range of sizes cannot be packed as densely and result in higher matrix area fraction. The 0.106–0.212 mm materials resulted in a much heavier composite with the highest area fraction of matrix due to the infiltration of defective spheres. The t/D ratio may be determined by considering the sphere geometry and the average density of the sphere, ρsphere, as shown in Eq. (1) where ρAl2 O3 is the density of Al2O3 and d is the internal diameter of the hollow sphere. 3
Fig. 6. Dependence of True Yield Strength and True Maximum Strength on the Inverse Square Root of dendrite arm spacing in Al-A206 T4 and T7 (data from [28]).
ρsphere ¼ ρAl2 O3
ð4=3ÞπðD3 =8Þ−ð4=3Þπðd =8Þ 3
ð4=3ÞπðD =8Þ
3
¼ ρAl2 O3 ð1−Dd 3 Þ
ð1Þ
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Given that d ¼D−2t, the ratio of wall thickness to hollow sphere diameter may be derived as shown in Eq. (2).
3.3. Quasi-static compression properties
" # ρsphere 1=3 t 1 ¼ 1− 1− D 2 ρAl2 O3
Table 3 reports the average and standard deviation of peak strength, plateau strength, densification strain and energy absorption for the syntactic foams synthesized in this study organized by sphere size range and alloy temper. Representative quasi-static engineering stress–strain curves are shown in Fig. 5. It is evident that in most cases, decreasing reinforcement size results in increasing properties. This result is similar to previously reported studies for other alloys [4,6,7,11,14–16], when noting that in the case of the Al2O3 reinforcements used in this study, as sphere size decreases the t/D ratio increases. Therefore, the peak strength, plateau strength, and energy absorption in general increase with
ð2Þ
Fig. 4 shows that though the average hollow sphere wall thickness increases with increasing hollow sphere diameter, the t/D ratio decreases with increasing hollow sphere diameter.
Fig. 7. Plot of the average peak stress of A206-Al2O3 MMSFs vs. the yield strength of the base alloy.
Fig. 9. Plot of the variation of toughness of A206-Al2O3 MMSFs with t/D ratio of the hollow sphere reinforcement.
Fig. 8. Plot of the variation of (a) peak strength and (b) plateau strength of A206-Al2O3 MMSFs with t/D ratio of the hollow sphere reinforcement.
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Fig. 10. Log–Log Plot of specific plateau strength vs. specific energy absorption for different types of aluminum foams [5–9,19,23,24,31–34].
increasing t/D ratio. There is a slight departure from this trend in the case of the As-cast condition, as the peak stress, plateau stress, densification strain and resulting energy absorption are all higher for the largest size range of 0.425–0.5 mm than they are for the middle size range of 0.212–0.425 mm. This may be attributed to a difference in failure mode between the as-cast foam and the heat treated foam. According to data from Talamantes-Silva et al. [28], shown in Fig. 6, the true yield strength of T4 and T7 matrix material should be relatively insensitive to grain size or dendrite arm spacing (DAS) as Orowan strengthening is dominant in these precipitation hardened materials. However, Fig. 6 also shows that the true stress at failure (i.e. UTS converted to true stress) shows a Hall–Petch dependence on grain size or DAS that seems to be independent of the material precipitation condition. Thus the UTS of T4 and T7 materials should be quite similar at equivalent grain sizes or DAS. The as cast material is expected to show a Hall–Petch-type dependence on grain size or DAS for both yield strength and UTS and be significantly weaker than either T4 or T7 due to the absence of the Orowan strengthening effect. Table 4 presents matrix yield and ultimate tensile strengths for As-cast, T4 and T7 conditions. From this data, Fig. 7 suggests that the peak stress in this work is influenced by yield strength, as peak stress increases from as cast to T4 to T7, which is similar to the trend in yield stress in the case of the unreinforced alloy. It is also observed that the peak stress of all of the as-cast syntactic foams, as well as the T4 heat treated foam containing 0.106–0.212 mm spheres exceeded the yield strength of the matrix in the same condition. This suggests that the spheres in these cases have a strengthening effect and therefore failure of the spheres and the matrix will occur concurrently in these foams. As discussed earlier, the MMSFs containing the two largest sphere size ranges (and two smallest t/D ratios) showed a departure from the usual trend of increasing peak stress with increasing t/D ratio. Therefore, the trend lines for these two MMSFs intersect (however the location of this critical intersection point may differ from that shown due to differences in the solidification microstructure between specimens). Comparison of the compressive stress–strain curves for the As-cast MMSF specimens shown in Fig. 5a reveals an apparent change in the failure mode between these specimens. The MMSF containing spheres of size range 0.425–0.500 mm exhibited a peak followed by a relatively sharp drop to a minimum stress. The MMSF containing spheres of size range 0.212–0.425 mm exhibited a more gradual, and less severe drop to an intermediate level, followed by a short plateau, then a further drop to a minimum. Fig. 5d shows the compressive stress strain curves for MMSFs containing spheres of
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size range 0.212–0.425 mm under the three different temper conditions. It can be seen that there is a difference in the shape of the T4 and T7 heat treated curves and the as-cast curve, which also indicates a possible change in the failure mode in this specimen. This suggests that in the case of the As-cast specimens containing 0.212–0.425 mm and 0.425–0.500 mm spheres respectively, a change in failure mode has contributed to the apparent reversal of the otherwise observed trend of increasing properties with increasing t/D ratio. Figs. 8 and 9 show that peak stress, plateau strength, and toughness increase with increasing t/D. The slope of the trend lines for the T4 and T7 heat treated foams is clearly greater for the plots of peak stress and plateau stress vs. t/D ratio. This suggests that in the cases where the peak strength of the foam is less than that of the matrix, there is an increased influence of the matrix strength and reduced influence of the potentially deleterious qualities of the spheres such as wall porosity, size range, volume percentage etc. The properties of the as-cast foam are dependent on the properties of the spheres and therefore the increase in the peak stress and plateau stress with increasing t/D ratio is less dramatic than in the case of the heat treated materials. Fig. 10 shows the Ashby plot of the specific plateau strength vs. the specific energy absorption for aluminum open-celled and syntactic foams available in literature including the data in this study. The A206 T7/0.212–0.425 mm Al2O3 syntactic foams synthesized in this study have the highest specific plateau strength and the second highest specific energy absorption reported in literature to date. This system outperformed the 0.106–0.212 mm syntactic foams in this study despite having a lower t/D ratio due to a lowered density resulting from the absence of infiltrated spheres. If the defective spheres in the 0.106–0.212 mm syntactic foam could be classified, even higher specific energy absorption and specific plateau strength would be expected.
4. Conclusion The microstructure and quasi-static properties of Al A206/ Al2O3 hollow sphere syntactic foams have been determined for foams with 3 different hollow sphere size ranges (0.106–0.212 mm, 0.212–0.425 mm, 0.425–0.500 mm) and tested in the as cast (F) as well as in the T4 and T7 conditions. The following conclusions have been made as a result of this work: 1. The peak strength, plateau strength and toughness of the foams increase with increasing t/D ratio. 2. Because t/D was found to increase with decreasing sphere diameter, the foams produced with the smallest hollow spheres (0.106–0.212 mm) result in superior performance for the peak strength (F: 226 MPa, T4: 312 MPa, T7: 342 MPa), plateau strength (F: 190 MPa, T4: 269 MPa, T7: 269 MPa), and toughness (F: 59 J/cm3, T4: 78 J/cm3, T7: 88 J/cm3). 3. The peak stress of the syntactic foams was shown to follow the yield strength of the base material. 4. The 0.212–0.425 mm T7 heat treated syntactic foams in this study exhibit the highest specific plateau strength (102 MPa-cm3/g) and the second highest specific energy absorption (41 J/g) of any aluminum syntactic foam reported in literature.
Acknowledgments This research was supported by the U.S. Army-TARDEC through TACOM R&D Contract# W56HZV-08-C-0716. The authors would also like to acknowledge and thank C-E Minerals for providing the
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ALODUR alumina hollow sphere material. Disclaimer: Reference herein to any specific commercial company, product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or the Department of the Army (DoA). The opinions of the authors expressed herein do not necessarily state or reflect those of the United States Government or the DoA, and shall not be used for advertising or product endorsement purposes. References [1] A.G. Evans, J.W. Hutchinson, M.F. Ashby, Prog. Mater. Sci. 43 (1999) 171–221. [2] M.F. Ashby, A.G. Evans, J.W. Hutchinson, Cellular Metals, a Design Guide, Cambridge University, Engineering Department, Cambridge, 1998. [3] Y. Chino, D.C. Dunand, Acta Mater. 56 (2008) 105–113. [4] J.A. Santa Maria, M.S. Thesis, University of Wisconsin Milwaukee, 2012. [5] D.K. Balch, J.G. O'Dwyer, G.R. Davis, C.M. Cady, G.T. Gray III, D.C. Dunand, Mater. Sci. Eng. A 391 (2005) 408–417. [6] D.K. Balch, D.C. Dunand, Acta Mater. 54 (2006) 1501–1511. [7] G.H. Wu, Z.Y. Dou, D.L. Sun, L.T. Jiang, B.S. Ding, B.F. He, Scr. Mater. 56 (2007) 221–224. [8] Q. Zhang, P.D. Lee, R. Singh, G. Wu, T.C. Lindley, Acta Mater. 57 (2009) 3003–3011. [9] X.F. Tao, L.P. Zhang, Y.Y. Zhao, Mater. Des. 30 (2009) 2732–2736. [10] X.F. Tao, Y.Y. Zhao, Scr. Mater. 61 (2009) 461–464. [11] Y. Zhao, X. Tao, X. Xue, Proceedings of MS&T 2008 Processing, Properties and Performance of Composite Materials, Materials Science and Technology, Pittsburgh, 2008. [12] I.N. Orbulov, J. Dobránszky, Period. Polytech. Mech. Eng. 52 (2008) 35–42. [13] I.N. Orbulov, J. Ginsztler, Compos. Part A 43 (2012) 553–561. [14] M. Kiser, M.Y. He, F.W. Zok, Acta Mater. 47 (1999) 2685–2694.
[15] W.J. Drury, S.A. Rickles, T.H. Sanders Jr, J.K. Cochran, in: E.W. Lee, E.H. Chia, N. J. Kim (Eds.), Light-Weight Alloys for Aerospace Applications, The Minerals, Metals and Materials Society, Warrendale, 1989. [16] R.A. Palmer, K. Gao, T.M. Doan, L. Green, G. Cavallaro, Mater. Sci. Eng. A 464 (2007) 85–92. [17] A. Daoud, Mater. Sci. Eng. A 488 (2008) 281–295. [18] M. Vesenjak, T. Fiedler, Z. Ren, A. Öchsner, in: A. Öchsner, C. Augustin (Eds.), Multifunctional Metallic Hollow Sphere Structures, Springer, Berlin, 2009, pp. 137–158. [19] Z.Y. Dou, L.T. Jiang, G.H. Wu, Q. Zhang, Z.Y. Xiu, G.Q. Chen, Scr. Mater. 57 (2007) 945–948. [20] P.K. Rohatgi, J.K. Kim, N. Gupta, S. Alaraj, A. Daoud, Compos. Part A 37 (2006) 430–437. [21] L.P. Zhang, Y.Y. Zhao, J. Compos. Mater. 41 (2007) 2105–2117. [22] D.P. Mondal, J.D. Majumder, N. Jha, A. Badkul, S. Das, A. Patel, G. Gupta, Mater. Des. 34 (2012) 82–89. [23] A. Rabiei, L.J. Vendra, Mater. Lett. 63 (2009) 533–536. [24] B.P. Neville, A. Rabiei, Mater. Des. 29 (2008) 388–396. [25] G. Castro, S.R. Nutt, Mater. Sci. Eng. A 535 (2012) 274–280. [26] A. Mortensen, I. Jin, Int. Mater. Rev. 37 (1992) 101–128. [27] H. Chandler (Ed.), Heat Treater's Guide: Practices and Procedures for Nonferrous Alloys, ASM International, Materials Park, 1996. [28] M. Talamantes-Silva, A. Rodríguez, J. Talamantes-Silva, S. Valtierra, R. Colás, Metall. Mater. Trans. B 39 (2008) 911–919. [29] L. Bäckerud, G. Chai, J. Tamminen, Solidification Characteristics of Aluminum Alloys Volume 2 Foundry Alloys, AFS/Skanaluminium Des Plaines, 1990. [30] J.G. Kaufman, E.L. Rooy, Aluminum Alloy Castings: Properties, Processes, and Applications, ASM International, Materials Park, 2004. [31] Y. Feng, N. Tao, Z. Zhu, S. Hu, Y. Pan, Mater. Lett. 57 (2003) 4058–4063. [32] H. Kanahashi, T. Mukai, Y. Yamada, K. Shimojima, M. Mabuchi, T. Aizawa, K. Higashi, Mater. Sci. Eng. A 308 (2001) 283–287. [33] Y. Yamada, K. Shimojima, Y. Sakaguchi, M. Mabuchi, M. Nakamura, T. Asahina, T. Mukai, H. Kanahashi, K. Higashi, J. Mater. Sci. Lett. 18 (1999) 1477–1480. [34] A. Daoud, J. Alloys Compd. 487 (2009) 618–625.
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Al–Al2O3 syntactic foams – Part I: Effect of matrix strength and hollow sphere size on the quasi-static properties of Al-A206/Al2O3 syntactic foams J.A. Santa Maria, B.F. Schultz n, J.B. Ferguson, P.K. Rohatgi Materials Science and Engineering Department, University of Wisconsin-Milwaukee, 3200 N. Cramer St., Milwaukee, WI 53201, USA
art ic l e i nf o
a b s t r a c t
Article history: Received 8 March 2013 Received in revised form 21 May 2013 Accepted 24 May 2013 Available online 14 June 2013
The microstructure and quasi-static compressive mechanical properties of Al-A206/Al2O3 hollow sphere syntactic foams were investigated for foams with three different hollow sphere size ranges (0.106– 0.212 mm, 0.212–0.425 mm, 0.425–0.500 mm) and three different conditions including As cast (F), T4 and T7. The peak strength, plateau strength and toughness of the foams were found to increase with increasing wall thickness to diameter (t/D) ratio. Since the t/D ratio was found to increase with decreasing sphere diameter, the foams produced with the smallest hollow spheres (0.106–0.212 mm) showed superior performance for the peak strength (F: 226 MPa, T4: 312 MPa, T7: 342 MPa), plateau strength (F: 190 MPa, T4: 269 MPa, T7: 269 MPa), and toughness (F: 59 J/cm3, T4: 78 J/cm3, T7: 88 J/cm3). These properties were also found to generally increase with increasing matrix yield strength. The 0.212– 0.425 mm T7 heat treated syntactic foams in this study exhibit the highest specific plateau strength (102 MPa-cm3/g) and the second highest specific energy absorption (41 J/g) of aluminum syntactic foams reported in literature for which comparable data is available. & 2013 Elsevier B.V. All rights reserved.
Keywords: Cellular materials Composites Metal matrix syntactic foam Casting Aluminum alloys Mechanical characterization
1. Introduction Metal matrix syntactic foams (MMSFs), metals containing a high volume percentage (∼50%) of hollow ceramic reinforcements, are desirable materials for blast and energy absorption applications due to their distinctive compressive deformation behavior. Like metallic foams, MMSFs sustain a large strain at a reduced stress level (compared to the monolithic fully dense material) leading to increased energy absorption prior to densification. MMSFs offer the potential to tailor such properties as the peak stress, plateau stress, strain to densification, energy absorption and density; all of which are of interest to engineers designing energy absorbing components [1–3]. Property enhancements may be achieved through careful matrix alloy selection, reinforcement material selection and reinforcement diameter, wall thickness and uniformity, and material processing parameters. Though a variety of reinforcement/matrix combinations have been explored [4–25], few studies have examined the effects of the matrix alloy strength combined with reinforcement size and size distribution on the properties pertinent to the design of MMSFs for energy absorption applications. n
Corresponding author. Tel.: +1 414 229 3956; fax: +1 414 229 6958. E-mail addresses: [email protected] (J.A. Santa Maria). [email protected] (B.F. Schultz), [email protected] (J.B. Ferguson). [email protected] (P.K. Rohatgi). 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.05.081
The objective of many experimental studies involving syntactic foams is to increase energy absorption capabilities. In a previous work [4], the effects of the wall thickness and diameter of hollow alumina sphere reinforced A380 matrix syntactic foams on the quasistatic and dynamic mechanical properties of the resulting MMSFs were explored. The peak strength, plateau strength and toughness of the resulting foams increased with increasing hollow sphere wall thickness to diameter (t/D) ratio. This is in general agreement with previous studies as shown in Fig. 1 [6,7,11,14–16]. It would be expected that increasing the strength of the matrix would also result in a general improvement in the peak and plateau stress. Indeed, as one can see from Fig. 1 for the A201 matrix syntactic foams developed by Kiser et al. [14], the solutionized and artificially aged (T6) foams showed an improvement in peak stress over the annealed (O) foams at equivalent t/D ratios. Likewise, Daoud [17] found that heat treatment of Zn–22Al syntactic foams to improve the matrix strength resulted in an increase in the peak stress, plateau stress, and ductility of the foam. MMSFs are generally synthesized via liquid metal processes including stir mixing [17] and most commonly by pressure infiltration [14]. As in any metal matrix composite system produced by liquid metal processing, the incorporation of reactive reinforcement materials in a metal matrix can result in alterations to the melt chemistry and the resulting mechanical properties of the solidified metal matrix. Balch and Dunand [5] synthesized Al-7075 (T6 and O conditions) and commercially pure (c.p.)
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aluminum/mullite (45 v% crystalline 3Al2O3–2SiO2, 55 v% amorphous SiO2) hollow microsphere syntactic foams by a pressure infiltration technique and measured the quasistatic compressive properties of the resulting foams. Though the foams prepared using the high strength Al-7075 forging alloy showed improved properties compared to the c.p. aluminum matrix foam, careful examination of the microstructure and chemistry of the alloy foam
Fig. 1. Effect of (t/D) ratio on the peak strength of various Aluminum alloy-hollow sphere metal matrix syntactic foams [6,7,11,14–16].
Table 1 Nominal composition of matrix and reinforcements. Material
Component
Nominal content (wt%)
A206
Al Cu Fe Mg Mn Ni Si Sn Al2O3 SiO2 Na2O MgO Fe2O3 CaO
93.3–95.3 4.25–5.0 0.1 0.15–0.35 0.2–0.5 0.05 0.05 0.05 98.8 0.8 0.1 0.05 0.03 0.03
Al2O3 hollow sphere
revealed that the matrix had in fact reacted with the mullite reinforcement to convert the SiO2 to Al2O3, releasing Si into the surrounding metal. As a result, the final composition of the matrix was closer to that of aluminum alloy A390, which resulted in a far reduced strength compared to that expected for 7075 alloy. Orbulov [13] examined the effect of alloy selection on the mechanical properties and final composition of aluminum alloy-E-sphere (36–40 wt% Al2O3, 55–60 wt% SiO2, 1.4–1.6 wt% TiO2, 0.4–0.5 wt% Fe2O3) syntactic foams. The alloys that were explored included 99.5% pure Al, Al–12Si, Al–1.2Mg–1Si–0.3Cu, and Al–4.5Cu. X-ray diffraction studies of the various syntactic foams revealed that in all cases except for the foams having a high starting concentration of Si (Al–12Si), the E-spheres partially converted to α-Al2O3 or γ-Al2O3. This displacement reaction of the Aluminum with the E-spheres to form Al2O3 and Si resulted in a significant increase in the Si content in these alloys. It followed that heat treatments of the resulting foams was not optimal and resulted in only a moderate increase in strength [12,13]. Palmer et al. [16] synthesized un-coated and coated (o 10 nm thick Aluminum) E-sphere reinforced syntactic foams. Three aluminum alloys were examined in this work including 1350, 5083 and 6061. Silicon rich second phases were observed in all cases, and sphere dissolution was observed to be more severe when the processing temperature was increased from 666 1C to 750 1C and in syntactic foams containing 45 mm spheres compared to 270 mm spheres. In the case of the 45 mm spheres, large blocky Silicon phases were observed presumably due to the larger surface area of the E-spheres in contact with the liquid phase aluminum alloy during infiltration processing. Increased dissolution of the reinforcement spheres and the resulting increase in Si content of the alloy resulted in a marked decrease in the mechanical properties of the foams containing 270 mm spheres and a slightly less severe effect for the foams containing 45 mm spheres. It is apparent from the preceding discussion that there is a need for clear and systematic studies on the effect of matrix composition and temper (i.e. strength), and reinforcement size in the absence of deleterious reactions between the reinforcement and the matrix. This work will study the effect of the matrix strength and the size, size ranges, and (t/D) ratios of the hollow spheres on the quasi-static compressive properties of metal matrix syntactic foams where the alloy and reinforcement are selected to avoid such displacement reactions.
2. Experimental procedure Syntactic foams composed of aluminum alloy A206 reinforced with approximately 40–50 v% hollow Al2O3 spheres (ALODUR white bubble alumina) were synthesized via a sub-atmospheric pressure infiltration technique. The nominal compositions of the
Fig. 2. Microstructure of alloy Al-A206 without reinforcement in (a) as cast, (b) T4, and (c) T7 conditions.
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alloy and the hollow spheres are presented in Table 1 (information provided by suppliers). Alumina hollow sphere reinforcements with diameters up to 0.5 mm supplied by C-E Minerals were sorted by size to produce 3 size ranges (0.106–0.212 mm, 0.212–0.425 mm, 0.425– 0.500 mm) for comparison. After sorting, the two larger sizes were floated in chloroform (density of 1.483 g/cm3) causing defective spheres to sink and intact spheres to float, allowing them to be collected. The smallest sphere size could not be processed in this manner due to the bulk density being more than 1.483 g/cm3. The bulk density of the hollow spheres for each size range was determined by measuring the mass of a fixed amount of spheres and the corresponding volume of water displaced by these same spheres. The method of sub-atmospheric pressure infiltration used to synthesize the foams summarized below is described in detail elsewhere [26]. A 13.95 mm ID borosilicate test tube was tappacked with hollow spheres to a height of 70–90 mm. An ingot of aluminum was placed above the hollow spheres, separated by a 2 mm thick layer of zirconia felt. The crucible containing the preform and ingot was heated in a quartz chamber under mechanical vacuum to 750 1C and held until the aluminum had fully melted and uniformly sealed the inner circumference of the crucible. Argon gas was then rapidly introduced into the heated
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quartz chamber, reaching a sub-atmospheric pressure of 0.4 bar, thereby forcing the molten aluminum into the evacuated spaces between the hollow spheres. A pressure of 0.7 bar was used for the smallest size range (0.106–0.212 mm) due to incomplete infiltration at 0.4 bar. The quartz chamber containing the sample was then removed from the tube furnace and the sample was allowed to cool in air under sub-atmospheric pressure for 3 min at which time the sample had solidified. The sample was then removed and quenched in room temperature water. Specimens of the unreinforced alloy were cast using the same procedure for comparison. The specimens, to be heat treated to the T4 and T7 condition were first solutionized at 490–500 1C for 2 h then 525–530 1C for 14–20 h. Following the solution heat treatment the T4 specimens were aged at 21 1C for 120 h and the T7 specimens were aged for 4 h at 200 1C [27].
Table 2 Average hollow sphere bulk density for various sphere size ranges. Size range (mm) 3
Hollow sphere bulk density g(cm ) Composite average density (g/cm3) Area % matrix
0.106–0.212
0.212–0.425
0.425–0.500
2.032 2.29 42.6–48.9%
1.33 1.82 39.6–47.2%
1.29 1.79 34.1–41.2%
Fig. 3. Representative microstructures of Al-A206–Al2O3 syntactic foams having the following heat treatment and hollow sphere diameter range (a) As cast, 0.106–0.212 mm, (b) T4, 0.106–0.212 mm, (c) T7, 0.106–0.212 mm, (d) As cast, 0.212–0.425 mm, (e) T4, 0.212–0.425 mm, (f) T7, 0.212–0.425 mm, (g) As cast, 0.425–0.500 mm, (h) T4, 0.425– 0.500 mm, (i) T7, 0.425–0.500 mm.
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The density of the composites was measured using a Metler Toledo AT261 Delta Range Microbalance equipped with a Density kit (Archimedes method). The specimens were first lightly coated with vacuum grease to prevent infiltration of surface pores during the density measurement. Microstructural analysis was performed on polished and etched (Keller's Reagent) specimens with a Nikon Eclipse TS100 microscope equipped with an automated stage and Clemex Professional Image Analysis Software. Quasi-static compression testing was performed in accordance with ASTM C365-94 on cylindrical specimens (Diameter¼ 14.3 mm Length ¼12.7 mm). Testing was carried out using a SATEC Model 50Ud Universal Testing Machine at constant crosshead speed with an initial strain rate of 10−3 s−1 and a self-leveling platen. Strains were calculated from the crosshead displacement, and were corrected for deflection of the load frame. A minimum of 5 specimens were tested for each syntactic foam composition. The quasi-static compression curves typically exhibited an initial peak followed by a region of lower stress and later densification. Compression was stopped when the densification stress reached the magnitude of the initial peak stress, at approximately 45% strain. The plateau strength reported is the average measured strength from initial peak to densification. Polished cross-sections of the tested specimens were examined via optical microscope.
3. Results and discussion 3.1. Microstructure The properties of the matrix depend on the fineness of the microstructure and the distribution of phases that form as a result of the solidification sequence and inter-relationships between the large number of alloying elements in the A206 alloy. Talamantes-Silva et al. [28] determined that for A206, increasing the cooling rate decreases the dendrite arm spacing, porosity, and grain size of the casting resulting in improved mechanical properties. The solidification sequence that is commonly observed for this alloy can be summarized as follows [29]: (1) Development of an α-Al dendtritic network (2) Precipitation of Al6(MnFeCu) (3) Transformation of liquid and Al6(MnFeCu) to Al and Al20Mn3Cu2 (4) Eutectic reaction to form Al2Cu, Al2Mn3Cu2 and Al7FeCu2 phases and Aluminum. (5) Complex eutectic reaction to form Al, Al2Cu, Al2MgCu, and Mg2Si which is only observed at high cooling rate. The microstructure of the matrix cast under the same conditions as the syntactic foams is shown in Fig. 2 in the as cast, T4, and T7 condition. The as cast microstructure consists of a dendritic structure, whose interdendritic regions are likely made up of Al, Al2Cu, Al2Mn3Cu2 and Al7FeCu2 phases as a result of the eutectic reaction. As shown in Fig. 2b and c, copper-containing eutectic phases have been dissolved by the solutionizing heat treatment in the T4 and T7 specimens. The T4 heat treatment produces a fine dispersion of underaged shearable θ″ throughout the matrix, while the T7 heat treatment possesses a coarse distribution of θ′ and θ overaged precipitates [29]. Representative micrographs of the syntactic foams organized by the size ranges of hollow spheres and heat treatment conditions are shown in Fig. 3. The hollow spheres appear to be uniformly distributed, and fully encapsulated by the metal matrix with little to no visible porosity in the matrix between the hollow spheres. The 0.106–0.212 mm samples contain infiltrated spheres due to the incorporation of defective hollow spheres that could not be separated by floating. The as-cast samples show an Al2Cu network throughout the matrix. Following the T4 and T7 heat treatments, the Al2Cu network present in the as-cast specimens was dissolved and precipitated as a dispersion of underaged shearable or coarse overaged precipitates within the matrix. 3.2. Physical properties
Fig. 4. Dependence of average wall thickness and t/D ratio on reinforcement diameter.
Table 2 presents the densities of the syntactic foam specimens along with the result of image analysis determining the area
Table 3 Summary of quasi-static compression datan for A206/Al2O3 syntactic foams. Heat treatment
Sphere size range (mm)
Peak stress (MPa)
Plateau stress (MPa)
Densification strain (%)
Energy absorption (J/cm3)
As Cast
0.106–0.212 0.212–0.425 0.425–0.500 0.106–0.212 0.212–0.425 0.425–0.500 0.106–0.212 0.212–0.425 0.425–0.500
225.7 7 11.1 155.3 7 6.4 164.67 0.9 312.2 7 3.6 2007 8.7 1827 2.5 341.9 7 12.4 230.3 7 12.3 1967 8.4
189.7 7 7.2 130.5 7 3.3 132.7 7 2.6 269.17 5.4 166.97 4.5 1527 4.6 268.97 7.8 186.5 7 10.7 163.7 7 6
327 3 337 5 427 2 317 357 4 367 3 347 1 417 2 407 1
58.9 7 6.8 41.7 7 6.8 53.4 7 2.8 77.9 7 2 57.2 7 7.6 53.5 7 4.3 87.8 7 5.6 74.4 7 4.7 62.8 7 3.6
T4
T7
n
All the data are reported as average7standard deviation.
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Fig. 5. Typical compressive stress–strain curves for (a) as cast matrix and various sphere sizes, (b) T4 matrix and various sphere sizes, (c) T7 matrix and various sphere sizes, and (d) 0.212–0.425 mm spheres in as cast, T4, and T7 matrix.
Table 4 Yield strength and UTS of Aluminum alloy A206 in the As-cast (estimated), T4 and T7 condition. Matrix condition
sy-matrix (MPa)
sUTS-matrix (MPa)
Al-A206 As-Cast (estimated) Al-A206 T4 [30] Al-A206 T7 [30]
60 262 345
200 428 435
percentage of matrix in the foam. The 0.425–0.500 mm size range has the lowest area fraction of matrix and lowest density due to its spheres being most similar in size, thus allowing for a higher packing efficiency. The 0.212–0.425 mm materials, having a wider range of sizes cannot be packed as densely and result in higher matrix area fraction. The 0.106–0.212 mm materials resulted in a much heavier composite with the highest area fraction of matrix due to the infiltration of defective spheres. The t/D ratio may be determined by considering the sphere geometry and the average density of the sphere, ρsphere, as shown in Eq. (1) where ρAl2 O3 is the density of Al2O3 and d is the internal diameter of the hollow sphere. 3
Fig. 6. Dependence of True Yield Strength and True Maximum Strength on the Inverse Square Root of dendrite arm spacing in Al-A206 T4 and T7 (data from [28]).
ρsphere ¼ ρAl2 O3
ð4=3ÞπðD3 =8Þ−ð4=3Þπðd =8Þ 3
ð4=3ÞπðD =8Þ
3
¼ ρAl2 O3 ð1−Dd 3 Þ
ð1Þ
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Given that d ¼D−2t, the ratio of wall thickness to hollow sphere diameter may be derived as shown in Eq. (2).
3.3. Quasi-static compression properties
" # ρsphere 1=3 t 1 ¼ 1− 1− D 2 ρAl2 O3
Table 3 reports the average and standard deviation of peak strength, plateau strength, densification strain and energy absorption for the syntactic foams synthesized in this study organized by sphere size range and alloy temper. Representative quasi-static engineering stress–strain curves are shown in Fig. 5. It is evident that in most cases, decreasing reinforcement size results in increasing properties. This result is similar to previously reported studies for other alloys [4,6,7,11,14–16], when noting that in the case of the Al2O3 reinforcements used in this study, as sphere size decreases the t/D ratio increases. Therefore, the peak strength, plateau strength, and energy absorption in general increase with
ð2Þ
Fig. 4 shows that though the average hollow sphere wall thickness increases with increasing hollow sphere diameter, the t/D ratio decreases with increasing hollow sphere diameter.
Fig. 7. Plot of the average peak stress of A206-Al2O3 MMSFs vs. the yield strength of the base alloy.
Fig. 9. Plot of the variation of toughness of A206-Al2O3 MMSFs with t/D ratio of the hollow sphere reinforcement.
Fig. 8. Plot of the variation of (a) peak strength and (b) plateau strength of A206-Al2O3 MMSFs with t/D ratio of the hollow sphere reinforcement.
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Fig. 10. Log–Log Plot of specific plateau strength vs. specific energy absorption for different types of aluminum foams [5–9,19,23,24,31–34].
increasing t/D ratio. There is a slight departure from this trend in the case of the As-cast condition, as the peak stress, plateau stress, densification strain and resulting energy absorption are all higher for the largest size range of 0.425–0.5 mm than they are for the middle size range of 0.212–0.425 mm. This may be attributed to a difference in failure mode between the as-cast foam and the heat treated foam. According to data from Talamantes-Silva et al. [28], shown in Fig. 6, the true yield strength of T4 and T7 matrix material should be relatively insensitive to grain size or dendrite arm spacing (DAS) as Orowan strengthening is dominant in these precipitation hardened materials. However, Fig. 6 also shows that the true stress at failure (i.e. UTS converted to true stress) shows a Hall–Petch dependence on grain size or DAS that seems to be independent of the material precipitation condition. Thus the UTS of T4 and T7 materials should be quite similar at equivalent grain sizes or DAS. The as cast material is expected to show a Hall–Petch-type dependence on grain size or DAS for both yield strength and UTS and be significantly weaker than either T4 or T7 due to the absence of the Orowan strengthening effect. Table 4 presents matrix yield and ultimate tensile strengths for As-cast, T4 and T7 conditions. From this data, Fig. 7 suggests that the peak stress in this work is influenced by yield strength, as peak stress increases from as cast to T4 to T7, which is similar to the trend in yield stress in the case of the unreinforced alloy. It is also observed that the peak stress of all of the as-cast syntactic foams, as well as the T4 heat treated foam containing 0.106–0.212 mm spheres exceeded the yield strength of the matrix in the same condition. This suggests that the spheres in these cases have a strengthening effect and therefore failure of the spheres and the matrix will occur concurrently in these foams. As discussed earlier, the MMSFs containing the two largest sphere size ranges (and two smallest t/D ratios) showed a departure from the usual trend of increasing peak stress with increasing t/D ratio. Therefore, the trend lines for these two MMSFs intersect (however the location of this critical intersection point may differ from that shown due to differences in the solidification microstructure between specimens). Comparison of the compressive stress–strain curves for the As-cast MMSF specimens shown in Fig. 5a reveals an apparent change in the failure mode between these specimens. The MMSF containing spheres of size range 0.425–0.500 mm exhibited a peak followed by a relatively sharp drop to a minimum stress. The MMSF containing spheres of size range 0.212–0.425 mm exhibited a more gradual, and less severe drop to an intermediate level, followed by a short plateau, then a further drop to a minimum. Fig. 5d shows the compressive stress strain curves for MMSFs containing spheres of
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size range 0.212–0.425 mm under the three different temper conditions. It can be seen that there is a difference in the shape of the T4 and T7 heat treated curves and the as-cast curve, which also indicates a possible change in the failure mode in this specimen. This suggests that in the case of the As-cast specimens containing 0.212–0.425 mm and 0.425–0.500 mm spheres respectively, a change in failure mode has contributed to the apparent reversal of the otherwise observed trend of increasing properties with increasing t/D ratio. Figs. 8 and 9 show that peak stress, plateau strength, and toughness increase with increasing t/D. The slope of the trend lines for the T4 and T7 heat treated foams is clearly greater for the plots of peak stress and plateau stress vs. t/D ratio. This suggests that in the cases where the peak strength of the foam is less than that of the matrix, there is an increased influence of the matrix strength and reduced influence of the potentially deleterious qualities of the spheres such as wall porosity, size range, volume percentage etc. The properties of the as-cast foam are dependent on the properties of the spheres and therefore the increase in the peak stress and plateau stress with increasing t/D ratio is less dramatic than in the case of the heat treated materials. Fig. 10 shows the Ashby plot of the specific plateau strength vs. the specific energy absorption for aluminum open-celled and syntactic foams available in literature including the data in this study. The A206 T7/0.212–0.425 mm Al2O3 syntactic foams synthesized in this study have the highest specific plateau strength and the second highest specific energy absorption reported in literature to date. This system outperformed the 0.106–0.212 mm syntactic foams in this study despite having a lower t/D ratio due to a lowered density resulting from the absence of infiltrated spheres. If the defective spheres in the 0.106–0.212 mm syntactic foam could be classified, even higher specific energy absorption and specific plateau strength would be expected.
4. Conclusion The microstructure and quasi-static properties of Al A206/ Al2O3 hollow sphere syntactic foams have been determined for foams with 3 different hollow sphere size ranges (0.106–0.212 mm, 0.212–0.425 mm, 0.425–0.500 mm) and tested in the as cast (F) as well as in the T4 and T7 conditions. The following conclusions have been made as a result of this work: 1. The peak strength, plateau strength and toughness of the foams increase with increasing t/D ratio. 2. Because t/D was found to increase with decreasing sphere diameter, the foams produced with the smallest hollow spheres (0.106–0.212 mm) result in superior performance for the peak strength (F: 226 MPa, T4: 312 MPa, T7: 342 MPa), plateau strength (F: 190 MPa, T4: 269 MPa, T7: 269 MPa), and toughness (F: 59 J/cm3, T4: 78 J/cm3, T7: 88 J/cm3). 3. The peak stress of the syntactic foams was shown to follow the yield strength of the base material. 4. The 0.212–0.425 mm T7 heat treated syntactic foams in this study exhibit the highest specific plateau strength (102 MPa-cm3/g) and the second highest specific energy absorption (41 J/g) of any aluminum syntactic foam reported in literature.
Acknowledgments This research was supported by the U.S. Army-TARDEC through TACOM R&D Contract# W56HZV-08-C-0716. The authors would also like to acknowledge and thank C-E Minerals for providing the
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