Type 52 Alloy, 52% Nickel and 48% Iron. (regular alloy is about 35% iron) Type 52 Alloy is designed for aerospace industry and advanced electronics applications, with a low coefficient of expansion to ensure tuning stability in all environments and is also corrosion resistant. In addition, the Type 52 Alloy has a strong magnetic attraction to the sound surface, so the low to middle stands out.