Fig. 1. MMC 30, Unetched (50x)


Fig. 2. MMC 30, Unetched (100x)


Fig. 3. MMC 30, Unetched (500x)


Fig. 4. MMC 30 Etched (50x)


Fig. 5. MMC30, Etched (100x)


Fig. 6. MMC 30, Etched (500x)


Figures 1-6 show the metallography of a 28% F500 MMC at magnifications of 50x/100x/500x in the etched and unetched conditions. (Twenty-eight percent is the concentration by volume and F500 refers to the size of the silicon carbide particulates.) The micrographs show the uniformly distributed silicon carbide particulates (brown) in the aluminum (tan) matrix. Also shown are intermittent interden- dritic voids (black indicates shrinkage cavities).


(Young’s modulus). Using the univer- sally accepted linear and inverse rule of mixtures, this potential increase can be estimated whereby the well-known border cases apply only to certain geometric alignments of the compo- nents in the composite materials. As the percentage of SiCp increases, the modulus moves linearly in the direc- tion of the SiCp modulus and away from the aluminum alloy. Tis reaction is similar in other properties such as thermal conductivity and expansion. Aluminum alloy/silicon carbide


MMC castings can be produced in 20%, 30% and 40% concentrations of silicon carbide by volume and heat treatable, commonly being furnished to a T77P heat treatment.


This enables engineers to design


components to their desired operat- ing environment. For instance, the property of wear resistance can be understood easily because the alumi- num alloys are relatively soft com-


Fig. 7. Shown is an A356 micrograph (unetched, 100x), without silicon carbide particulates, to compare to the MMC microstructure.


Nov/Dec 2014 | METAL CASTING DESIGN & PURCHASING | 33


pared to the hard carbide particles, enhancing the wear properties with increased silicon carbide particulate


(SiCp). Bonding between the SiCp particles and the aluminum matrix is an important consideration. Pur-


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