Communication A Novel Approach for Extracting and Characterizing Interfacial Reaction Products in Al-SiC p Composites K.R. RAVI, R.M. PILLAI, B.C. PAI, and M. CHAKRABORTY The reclamation process by salt flux addition has been employed to extract the interfacial reaction products as well as SiC p from Al-SiC p composites. It has been observed that this process enables the use of X-ray diffraction (XRD) to identify the interfacial reaction product Al 4 C 3 in Al-SiC p composites even in the early stages of reaction. Further, it allows an easy and more accurate quantification of the interfacial reaction in Al-SiC p composites using spectroscopic techniques such as optical emission spectroscopy (OES). DOI: 10.1007/s11661-007-9254-y Ó The Minerals, Metals & Materials Society and ASM International 2007 In Al-SiC composites, a direct reaction between Al and SiC can occur during the fabrication stage forming Al 4 C 3 and Si according to the reaction 4Al þ 3SiC ! Al 4 C 3 þ 3Si ½1 This reaction is well known to have undesirable effects on the overall Al-SiC composite properties such as (1) degradation of mechanical properties due to the forma- tion of Al 4 C 3 ; [1] (2) reduction in corrosion resistance in water, methanol, HCl, etc. because of the unstable nature of the reaction product, Al 4 C 3 , in such environ- ments; [2,3] and (3) changing the matrix alloy composition by Si released during interfacial reaction. As a result, composite interface plays an important role in deter- mining the resultant composite properties. Most studies carried out to identify the structures and morphologies of the interfacial reaction products of composites have used transmission electron microscopy, although the same goals can be achieved by X-ray diffraction (XRD) crystallography and scanning elec- tron microscopy (SEM). Only a few researchers [4,5,6] have used XRD despite its accuracy and reliability in analyzing crystal structures. This is due to experimental difficulties involved in determining the exact peak positions required for phase identification when volume fractions of the interfacial reaction products within the composite materials become small. For example, Lloyd et al. [4] could observe sufficiently strong Al 4 C 3 peak only after the reaction releases 6.0 wt pct of Si (conversion of 40 pct of SiC into Al 4 C 3 ) in 6061–20 vol pct SiC p composites. In order to overcome this problem, Lee et al. [7] developed a electrochemical dissolution tech- nique to extract interfacial reaction products as well as SiC p from Al-SiC p composites using an electrolyte consisting of either 33 vol pct HNO 3 + 67 vol pct CH 3 OH [7] or 33 vol pct HNO 3 + 67 vol pct water. [8] Because methanol and water are known to dissolve Al 4 C 3 with the evolution of methane, special care has to be taken to make sure that the interfacial reaction products are not dissolved into the electrolyte during the extraction process. Further, it has been observed that, when the extent of the reaction is weak, the XRD peak corresponding to Al 4 C 3 is very faint or absent due to possible dissolution of Al 4 C 3 during the electrochemical extraction process. [7,8,9] As a result, identification of the reaction is possible only when the extent of interfacial reaction is strong so that the metal matrix composites (MMCs) become practically unsuitable because of their deleterious effect on the properties. This article commu- nicates an alternate approach to extract the interfacial reaction products as well as SiC p from Al-SiC p com- posites, which enables identification and quantification of the interfacial reaction product even in the early stages of reaction. The material used in the present study is A356 Al alloy (Table I) reinforced with 15 vol pct SiC p (A356 Al-SiC p composites), fabricated by the stir casting technique. The entire composite fabrication process (particle addition, stirring, and pouring) is carried out at 700 °C. The time of contact of the aluminum melt with SiC at this temperature does not exceed 30 minutes. Silicon carbide particles introduced into the melt are of 14 lm average size. The composite is heat treated at different elevated temperatures (held at 710 °C, 740 °C, and 800 °C for 2 hours) to have a sufficient amount of interfacial reaction. The reclamation process by salt flux addition em- ployed in this investigation to extract interfacial reaction products as well as SiC p from A356 Al-SiC p composite is shown in Figure 1. In this process, after heat treating the composites (including as processed), 1 wt pct of salt flux (equimolar NaCl-KCl + 5 pct NaF) added to the composite melt through the vortex, which was created by mechanical stirring. After mixing of 30 to 60 seconds, separation of reinforcement particles along with the reaction products starts leading to their floating on the melt surface and skimming off. The powders extracted by this process are named as reclaimed powder. The leftover matrix melt in the crucible is poured into a cast iron mold and is named as reclaimed alloy. For comparison, the electrochemical dissolution method is also employed to extract interfacial reaction products as well as SiC p using electrolyte consisting of 33 vol pct HNO 3 + 67 vol pct CH 3 OH. The extraction process is conducted using 11 V (d.c.) and 6 A, and the K.R. RAVI, Senior Research Fellow, and R.M. PILLAI, Senior Deputy Director and Head, Materials and Minerals Division, and B.C. PAI, Senior Deputy Director and Head, Research Planning and Business Development, are with the National Institute for Interdisciplinary Science and Technology (CSIR), Thiruvananthapu- ram, Kerala 695 019, India. Contact e-mail: rmpillai@rediffmail.com M. CHAKRABORTY, Professor, is with the Metallurgical and Materials Engineering Department, Indian Institute of Technology, Kharagpur 721 302, India. Manuscript submitted March 27, 2007. Article published online July 3, 2007. 1666—VOLUME 38A, JULY 2007 METALLURGICAL AND MATERIALS TRANSACTIONS A