Aging evaluation of cast particulate-reinforced SiCAl(2024) composites

Scripta METALLURGICA et MATERIALIA

Vol. 28, pp. 281-286, 1993 Printed in the U.S.A.

Pergamon Press Ltd. All rights reserved

AGING EVALUATION OF CAST PARTICULATE-REINFORCED SIC/A1(2024) COMPOSITES Jun-shan Lin, Peng-xing Li and Ren-jie Wu State Key Lab of MMCs Shanghai Jiao Tong University Shanghai, 200030, P.R. China ( R e c e i v e d J u l y 10, 1992) ( R e v i s e d November 19, 1992) Introduction SiC particulate reinforced aluminum matrix composites have attracted increasing attention because of their superior mechanical properties and lower cost in recent years. Particularly in the aerospace and automotive industries, they are predicted to have extensive application prospects. To date, many aspects of the theories and experiments about discontinuously reinforced metal matrix composites have been studied, but because of a poor model system for study, there is no consensus as to the basic mechanims responsible for their superior properties[l, 2]. This is attributed mainly to the very complicated microstructure in matrices of composites. It has been found that higher density dislocations are generated in the matrix close to the reinforcement-matrix interface during quenching from heat processing temperature due the mismatch of coefficient of thermal expansion(CTE) between the matrix and reinforcement[3, 4], the dislocation density depends on the size and volume fraction, etc., of reinforcements[5], which is believed to be a mainly responsible for strengthening of discontinuously reinforced composites. On the other hand, it has been confirmed[6, 9] that aging kinetics are generally accelerated by the addition of reinforcements to an age-hardenable aluminum alloy because of the enhanced dislocation density can provide a pipe tunnel for atom diffusion. This means that the microstructure in the matrices of composites may be overaged if a traditional optimized age process for the control alloy is used for composites. The experimental data based on the such processes could lead to a misunderstanding on the materials. Unfortunately, previous studies on aging behavior were used to employ composites containing just one or two volume fractions of reinforcements. The essential law of aging acceleration of composites could not be full revealed. So, in present work, the effects of SiC volume fraction on the aging behavior of aluminum-based composites are systematically evaluated by means of hardness measurements and differential scanning calorimetry(DSC) techniques. Exoerimental Procedure Composites for this study were fabricated by ingot casting techniques, where 2024 aluminum alloy was reinforced with 5%, 10%, 15% and 20% volume fractions of SiC particulates of nominal size 14/zm. As-cast ingots of composites were subsequently hot compacted and rolled to plate with thickness of 3.5ram. The control alloy was passed through the same process. The samples used for hardness measurements, which were 10x 10x3.5mm in size and, for DSC heat flow measurements, q56.4xl.lmm in size, were sparkingly cut from as-compacted and rolled composite plates. Solution treatment was carried out in a salt bath furnace at 500"C for 1.Sh. After quenching in ice brine, the samples for DSC were stored in liquid nitrogen, and the sample used for hardness measurements was aged at 190°C for 0-14h. When not in use, all the samples were stored in liquid nitrogen to prevent further aging. In order to find an efficient hardness testing method to evaluate the aging behavior of composites, microhardness and Vicker's hardness measurements were made respectively on the samples of the control alloy and 20 vol% SiC composites. Microhardness measurements were carried out on the samples with a 20Vo1% SiC composite and the control alloy, using a model MHT-1 microhardness tester with a 136" Vicker's diamond 281 0 9 5 6 - 7 1 6 X / 9 3 $ 6 . 0 0 + .00 C o p y r i g h t (c) 1993 Pergamon P r e s s

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pyramid indentor and 10g load. The composite samples were indented in SiC-flee zones in order to obtain the microhardness measurements of the matrix materials only, at least 15 hardness readings were taken for each sample. Vicker's hardness measurements were performed on all the samples of composites and control alloy by using a testing machine with a 136 ° Vicker's diamond pyramid indentor and a 30Kg load. Four hardness readings were taken for each sample. A PERKIN-ELMER DSC-7 thermal analyzer was used to measure the heat flow of precipitation reactions in the control alloy and matrices to examine the aging kinetics of the composites. All of the DSC runs were started at room temperature, ending at 550°C with a constant scanning rate of 20°C/rain. For composites, the contributions of SiC to heat flow were removed by the mass normalizing method. The details of experimental techniques have been described in our previous work[10]. The microstructures of composites were observed by Philips CM12 analytical electron microscopy(AEM). The AEM foils were prepared via mechanical polishing, dimpling and final thinning on a GATAN duo-ion mill equipped with a liquid nitrogen cold stage. Results and Discussion Micros~ructur~ Figure 1 shows the metallographic microstructures of composites. It can be seen that the distribution of SiC particulates is generally uniform except for a 5% SiC volume fraction. In the casting process, SiC particulates are in a low energy state for distribution between the dendrite branches; after cooling they are frequently clustered together, which results in the nonuniform distribution of SiC. This case occurs pronouncedly especially for low reinforcement contents and/or slow solidification rates in the process of composite fabrication. Hardness Figure 2 shows the microhardness variation as a function of aging time at 190*C for both the 20 Vol% SiC composite and control alloy. There is a very large scatter in the microhardness of the composite matrix. In microhardness testing, we could not ensure that the indentor did not contact the subsurface SiC particles though without indenting any SiC particle on the surface. This may be the main cause for the appreciably higher microhardness for the composite in Figure 2. In addition, the inhomogeneous distribution of reinforcements is another cause for the scatter. In general, the microhardness in cluster regions of SiC particles is higher than in regions of reinforcement-free matrix because of the constraint of SiC particles against deformation in the matrix. From Figure 2, it also can be seen that the microhardness vs. aging time curve is higher than that of the control alloy. Christman[7] and Dutta's[9] work on SiC /2124 and A1203 (particulate)/6061 also obtained similar results. Dutta believed that it is the result of the enhanced dislocation density led by the mismatch of CTE between the SiC and aluminum strengthening the matrices of composites. Although the tendency of microhardness to vary with aging time can be observed, the peak on hardness curve is not sharp; on the other hand, taking so many readings to ensure the reliability of data is a large expenditure of time. Thus, it is necessary to find a testing method that is both economical and reliable. Figure 3 shows the Vicker's hardness variation as a function of aging time for all materials. Generally, in age-hardenable aluminum alloy, there is a peak in each hardness vs. age time curve, and the peaks are sharp and evident. Four readings taken from one sample show some scatter. It is inferred that hardness values measured with a large load indentor are affected somewhat by the subsurface particles and inhomogeneous reinforcement distribution because the indentation occupies a large volume which includes a large number of particles. As reported in the literature[6-9], the addition of the SiC reinforcements causes considerable acceleration on the aging kinetics of the matrix alloy. The time to peak hardness decreased linearly with increasing SiC volume fraction for the materials in the present work, as shown in Figure 4. By fitting the curves in Figure 4, a linear relation between the time(q) to peak hardness and SiC volume fraction(Vf ) can be obtained as follows: ti(minute) ~606-1260V f

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From the above function, it can be deduced that whenever the SiC volume fraction was increased by 5%, the time to peak hardness was shortened by about 60 minutes. Differential Seannin2 Calorimetry In general, it is believed that enhanced dislocation density in the matrix of composites provides a pipe tunnel for atom diffusion, which can result in a decreasing incubation period for precipitate formation. Christman and Suresh[7] studied the aging behavior of SiC whisker reinforced 2124 composites and concluded that enhanced dislocation density in the matrix reduced the incubation period for S' precipitation. Knowles and King[Ill, however, believed that the rapid aging is associated with precursor to S', such as GP zones, when they examined the aging response of 8090 aluminum-lithium-based composite. In the present work, the microstructure observations and DSC thermal analyses results proved that a decrease in the incubation period of S' precipitates accelerated the aging process of composites. Figure 5 shows the DSC runs of 5 Vol%, 20 Vol% SiC composites and the control alloy. Two exothermic peaks and one endothermic peak are observed in temperatures ranging from room temperature to 350°C. According to Papazian's publication[8] and our previous work[10], the first exothermic peak, at about 77°C, corresponds to the GP zone formation; another exothermic peak, at about 280*C, corresponds to the S'(A12CuMg) precipitate formation. The endothermic peak represents GP zone dissolution. It can be seen that the addition of SiC reinforcements did not change the sequence of the precipitation reactions in aluminum matrix, but the heat flow vs. temperature curves were horizontally shifted toward a low temperature. Our concern is with temperatures TGp and T S, which correspond to the GP zone and S' precipitation reaction peaks, respectively. Figure 6 shows the variation of the reaction peak temperatures, TGp and T S, as a function of SiC volume fraction. The GP zone formation in the matrix was slightly accelerated by the addition of small volume fraction reinforcements, as shown in Figure 6(a), but the acceleration effect was not enhanced by further increasing the amount of reinforcements. Unlike the case of GP zones, the reaction peak temperature, T S, as shown in Figure 6(b), was lineally decreased by increasing the SiC reinforcements; the relation between the T S, and Vf follows the function as follows: T S,(°C) =288.1-61.2Vf The addition of 20 Vol% SiC decreased the T S, from approximately 289°C in control alloy to about 276 ° C in the composite, thus suggesting that enhanced dislocation density in composite matrices promoted precipitation of the S' phase more violently than that of GP zones; this is also supported by microstructural features. In AEM observation we found, as shown in Figure 7b, that the density of S' precipitates in the matrix close to the SiC-aluminum interface was often higher than in the matrix away from the interface; this is consistent with the fact that the density of dislocations in the matrix close to the interface, as shown in Figure 7a, is higher than in matrix away from the interface. According to the phase transformation theory, the nucleation of precipitates can easily be found at vacancies or dislocations. The addition of SiC enhanced dislocation density and reduced vacancy density as a result of a large number of SiC-aluminum interfaces after quenching. This contrary effect may be the cause leading to the above results, but the details of the mechanism are not clear. Comparing the DSC results with Vicker's hardness measurements and microstructures, it is believed that high dislocation density resulting from the CTE mismatch of SiC and aluminum reduced the incubation period of S' precipitates; the aging responses of composites, thereby, were accelerated by the addition of SiC reinforcements. Conclusions Vicker's hardness testing with a large load is a more economical and reliable method for evaluating the aging behavior of composites than microhardness measurements. The aging kinetics of SIC/2024 composite were found to be significantly accelerated by the addition of SiC. The time to peak hardness was linear decreasing with increasing SiC volume fraction when the materials

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were aged at 190°C. The relation between the time to peak hardness and the SiC volume fraction for SIC/2024 composites follows the following function: ti(minute ) = 606-1200Vf DSC thermal analyses revealed that the addition of SiC in the 2024 aluminum alloy did not change the sequence of precipitation reactions in the matrix alloy, but accelerated the progress of aging. This acceleration effect was mainly caused by a the reduction of the incubation period of S' precipitates. Similar to the results of Vicker's hardness measurements, the peak temperature of S' precipitation reaction decreasing lineally with increasing SiC volume fraction, and followed the function: T S,(°C)=288.1-61.2Vf References 1. J.P. Hirth, Scripta Metail. Mater., 25(1991), 1 2. W.M. Miller and F.J. Humphreys, Scripta Metall. Mater., 25(1991), 2623 3. M. Vogelsang, R.J. Arsenault and R.M. Fisher, Metall. Trans., 17A(1986), 379 4. R.J. Arsenanlt and N. Shi, Mater. Sci. Eng., 81(1986), 175 5. R.J. Arsenault, L. Wang and C.R. Feng, Acta Metall. Mater., 39(1991), 47 6. T.G. Nieh and R.F. Karlak, Scfipta Metall., 18(1984), 25 7. T. Christman and S. Suresh, Acta Metall., 36(1988), 1691 8. J.M. Papazin, Metall. Trans., 19A(1988), 2945 9. I. Durra, S.M. Allen and J.L. Harley, Metall. Trans., 22A(1991), 2553 10. LS. Lin, P.X. Li and R.L Wu, Acta Metall. Sinica(ChinescEdition),in press 11. D.M. Knowles and J.E. King, Acta Metall.Mater., 39(1991),793

Fig. 1. Microstructures of composites a) 20, b) 15, c) i0 ,d) 5 Vol% SiC

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