In fact, for a given volume of precipitate, a sphere has 1.24 less surface area than a cube, and thus is the preferred shape to minimize surface energy. With a coherent particle, however, the interfacial energy can be minimized by forming cubes and allowing the crystalographic planes of the cubic matrix and precipitate to remain continuous. Coherency can be lost by overaging. During heat treatment and service, these begin to decompose and form lower carbides such as M 23 C 6 and M 6 C, which tend to form on the grain boundaries. These common carbides all have an fcc crystal structure.
Results vary on whether carbides are detrimental or advantageous to superalloy properties.
The general opinion is that in superalloys with grain boundaries, carbides are beneficial by increasing rupture strength at high tempeature. These are generally undesirable, brittle phases that can form during heat treatment or service. The cell structure of these phases have close-packed atoms in layers separated by relatively large interatomic distances. Sigma appears to be the most deleterious while strength retention has been observed in some alloys containing mu and Laves. Westinghouse Another listing of manufacturers is available from Gas-Turbines .