Trying to make heads or tails of the word and the subject of "hypereutectic" is no easy matter. Our article on piston alloys covers the subject of how Primary Silicon is Formed, and our article on Heat Treat,relates how the alloy strength is increased with the T6 Heat Treatment. This page is intended to give you a visual idea of what the 390 alloy looks like under a microscope.
The dark particles distributed throughout this photo (fig. 1), are primary silicon. These particles form when the silicon in the metal exceeds 12%. Primary silicon particles represent 4 - 6% of the total piston.The key to the successful use of the 390 alloy is to keep the particles small and well distributed. This is accomplished with modern foundry practice and mold design. The largest particles in the photographs, (fig. 1 and fig. 3), are about four ten thousandths of an inch in length and are so hard diamond tooling is required to machine the alloy.
The gray mottled area in the photo, (fig. 1), is base metal alloy that makes up the balance of the piston. The silicon in the base metal is dissolved with aluminum to form a 12% silicon/aluminum solution. This base metal is eutectic, (u-tec-tic), aluminum, (fig. 2)..
The bottom line is that the structure of 390 alloy gives superior wear resistance, 15% less thermal expansion and increased thermal barrier properties. KB Performance Pistons are designed around our hypereutectic alloy. The characteristics of the 390 alloy, combined with modern permanent mold process, gives design freedom for a superior product, the KB Performance Pistons from United Engine & Machine Company.