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Effect of weight percentage of SiC particulates on the ageing behaviour of 6061/SIC metal matrix composites

M. GUPTA Department of Mechanical and Production Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511

M. K. SURAPPA Department of Metallurgy, Indian Institute of Science, Bangalore, India 560012

Metal matrix composites (MMCs), due to their ability to serve a spectrum of diversified applications, have lead to extensive research activities all over the world [1]. The suitability of these composite materials, however, lies in the judicious selection of synthesiz- ing/processing technique, matrix material and ceramic reinforcement. ZUnong the various matrix-reinforce- ment combinations, silicon carbide (SIC) reinforced aluminium matrices have been widely acknowledged as potential candidates for weight-critical automobile and aerospace applications [2]. The addition of SiC particulates has been correlated with an increase in mechanical properties such as yield strength, ultimate tensile strength and elastic modulus of the alumi- nium-based metallic matrices [2]. This improvement can be ascribed, in part, to the particulates-assisted microstructural strengthening. In related studies, for example, investigators have reported changes in microstructural :features such as dislocation density [3], grain size [4], precipitation behaviour [5] and excess solid solubility [6] as a result of the presence of ceramic particulates. The presence of ceramic reinforcement, in addition, has also been correlated with the kinetics of microstructural evolution during the ageing step of the conventional T6 heat treatment [4, 5,7]. The change in ageing kinetics of the metallic matrix as a result of the presence of a fixed weight percentage of ceramic particulates, for example, has been reported by various investigators [4, 5, 7]. However, no systematic studies have been carried out to correlate the effect of weight percent- age of SiC particulates with the ageing behaviour of the metal matrix composites. Accordingly, the present study was undertaken to provide insight into the effect of weight percentage of ceramic reinforce- ment on the ageing respons e of metal matrix composites synthesized using a casting route. Parti- cular emphasis was placed on correlation of the ageing results obtained on the reinforced samples with the microstructural characteristics.

The starting matrix material used in the present study was AA 6061 aluminium alloy containing (in wt%): 0.6Si-l.05Mg-0.46Fe-0.25Cu-0.25 Cr-A1 (bal.). Silicon carbide (o~-SiC) particulates with an average size of 54gin were selected as the reinforcement phase. Three casting experiments were made to synthesize unreinforced alloy, and compo- sites containing 10 and 15 wt % SiC particulates. The

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synthesis of metal matrix composites used in the present study was carried out according to the following procedure. The metal ingots, prior to melting, were cleaned and melted under a cover of nitrogen gas in order to minimize the oxidation of molten metal. SiC particulates, preheated to 900 °C, were then added to the molten metal which was stirred using an impeller. The melt was alloyed with small amounts of Mg and Zr (Mg + Zr < 1 wt %) in order to improve the wettability of the SiC particulates. The composite melt thus obtained was poured into cylindrical cast iron moulds (75 mm diameter and 150 mm height). For the purpose of comparison, the base alloy was cast after adding similar amounts of magnesium and zirconium (Mg + Zr < 1%) into a 6061 A1 alloy melt following similar casting parameters. The as-cast cylindrical bars were homogenized at 540 °C for 3 h and then extruded using an extrusion ratio of 10:1 at 500 °C into 16 mm diameter rods in order to close the residual porosity.

Ageing studies were carried out on extruded unreinforced and reinforced samples (16 mm diame- ter × 10 mm height) in order to find out the time required to attain peak hardness. This esentially involved solutionizing samples for 1 h at 530 °C, quenching in cold water followed by isothermal ageing at 177 °C for various intervals of time. Brinell hardness measurements were made using a 5 mm diameter indentor with a 500 kg load.

Microstructural characterization studies were pri- marily conducted using a JEOL scanning electron microscope (SEM) equipped with EDS (energy dispersive spectroscopy) on the plastic mounted and metallographically polished composite samples in order to examine the precipitation behaviour and segregation of alloying elements in the interfacial region between the A1 alloy matrix and ceramic particulate.

The results of the ageing studies conducted on the monolithic alloy and composites containing 10 and 15 wt% SiC particulates are shown in Fig. 1. The results revealed that ageing time for peak hardness for the monolithic alloy was 8 h, compared with 6 h for 6061/10wt% SiC and 2 h for 6061/15 wt% SiC composite samples.

Scanning electron microscopy caTried out on the composite samples revealed: (a) absence of voids or

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Figure 1 Graphical representation of ageing studies conducted on unreinforced 6061 alloy (D), 6061/10 wt% SiC (O) and 6061/15 wt% SiC (~). All materials were solution-treated at 530 °C for 1 h.

Figure2 Representative SEM micrograph taken from the 6061/ 15 wt% SiC composite sample showing the presence of an Mg-rich intermetallic phase at the 6061/SIC interface (marked A) and in the near vicinity of SiC particulate (marked B).

other discontinuities in the matrix and particulate- matrix interface; (b) good interfacial bonding be- tween dispersed SiC particulates and the matrix; and (c) preferential presence of Mg-rich precipitates at and in the near vicinity of the SiC-matrix interface (Fig. 2). The identity of the Mg-rich precipitates was confirmed using EDS point analyses. In addition, the results of EDS analyses conducted on 6061/15 wt % SiC samples in both as-extruded and peak-aged conditions revealed enrichment of Mg and Si in the near vicinity of SiC particulates when compared to the bulk material composition. The variation in weight percentage silicon observed at various dis- tances froin the SiC-matrix interface, for example, is graphically represented in Fig. 3.

The results of the ageing studies revealed an increase in ageing kinetics with an increase in weight percentage of SiC particulates. The accelerated ageing behaviour observed in the present study can be attributed to the heterogeneous nucleation cap- ability of (a) SiC particulates, and (b) the defect-rich interfacial region formed in the near vicinity of the SiC particulates. The ability of the SiC particulates to act as a heterogeneous nucleation site is consistent with the microstructural characterization studies revealing the presence of Mg-rich phases located at the A1/SiC interface (Fig. 2). The heterogeneous nucleation capability of the interfacial region formed in the near vicinity of SiC particulate s can be attributed, in part, to the high dislocation density present in the composite matrix arising from the mismatch between the coefficients of thermal expan- sion of the metal matrix and the ceramic reinforce- ment [3]. This increase in dislocation density promotes dislocation-assisted diffusion of the alloy- ing elements from the adjacent dislocation-lean areas of the matrix resulting in solute enrichment in the interfacial region, thus making the compositional requirement for precipitation more favourable. The solute enrichment results obtained in the present study are also consistent with transmission electron microscopy results of other investigators conducted on spray-deposited A1-Cu/SiC metal matrix compo- sites [4]. Furthermore, the accelerated ageing kinetics observed in the present study can also be attributed to

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Figure 3 Graphical representation of the segregation pattern of Si at 6061/SiC interfacial region observed in as-extruded (~) and peak-aged (D) 6061/15 wt% SiC samples.

the ability of the disloctaion defect structure formed in the near vicinity of the SiC particulates to nucleate the precipitates. This is also consistent with the microstructural characterization results obtained in the present study indicating the presence of Mg-rich precipitates in the near vicinity of SiC particulates (Fig. 2). In related studies (for example, Song and Baker [5]) the accelerated ageing observed in powder metallurgy processed AA 6061/15 vol% SiC compo- site was attributed to the lower activation energy required for the ageing process arising as a result of the dislocation-assisted nucleation process in the composites. Moreover, the presence of precipitates in the dislocation-rich areas in the near vicinity of SiC particulates has also been reported by other investigators [4]. The results of this study thus clearly indicate that the increase in ageing kinetics of the metallic matrix is associated with the presence of SiC particulates and a dislocation-rich interfacial region formed in the near vicinity of SiC particulates. An increase in ageing kinetics with an increase in weight percentage of SiC particulates can thus be correlated to an increase in the heterogeneous nucleation sites in the bulk matrix in terms of the number of SiC particulates and the volume of the dislocation-rich interfacial region formed around SiC particulates.

In conclusion, the results of the present study illustrate an in increase in ageing kinetics of the metallic matrix with weight percentage of SiC particulates, emphasizing particularly the pivotal role of SiC particulates and the defect-rich interfacial region as heterogeneous nucleation sites for the matrix precipitates.

Acknowledgements The authors wish to acknowledge Mr Thomas Tan, Mr Tung Siew Kong and Mr Boon Heng (National University of Singapore, Singapore) for their valuable assistance in the experimental part of this study and for many useful discussions.

References 1. I. A. IBRAHIM,

2. 3.

4.

5.

6.

F. A. M O H A M E D and E. J. L A V E R N I A , J. Mater. Sci. 26 (1991) 1137. A. L. GEIGER and J. A. W A L K E R , J. Met, 43 (1991) 8. R. J. A R S E N A U L T and N. SHI, Mater. Sci. Eng. (1981) 175. M. GUPTA, T. S. S R I V A T S A N , F. A. M O H A M E D and E. J. L A V E R N I A , J. Mater. Sci. 28 (1993) 2245. Y. SONG and T. N. BAKER, Mater. Sci. Technol. 10 (1994) 406. M. GUPTA, J. J U A R E Z - I S L A S , W. E. FRAZIER, F. A. MOHAMED and E. J. L A V E R N I A , Metall. Trans. 23B (1992) 719. Y. WU and E. J. L A V E R N I A , a~ Met. 43 (1991) 16.

Received 22 December 1994 and accepted 2 May 1995

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