Adding elements is usually helpful because of solid solution strengthening, but can result in unwanted precipitation. Precipitates can be classified as geometrically close-packed (GCP), topologically close-packed (TCP), or carbides. GCP phases usually benefit mechanical properties, but TCP phases are often deleterious. Because TCP phases are not truly close packed, they have few slip systems and are brittle. Also they "scavenge" elements from GCP phases. Many elements that are good for forming γ' or have great solid solution strengthening may precipitate TCPs. The proper balance promotes GCPs while avoiding TCPs.
TCP phase formation areas are weak because they:
* have inherently poor mechanical properties
* are incoherent with the γ matrix
* are surrounded by a "depletion zone" where there is no γ'
* usually form sharp plate or needle-like morphologies which nucleate cracks
The main GCP phase is γ. Almost all superalloys are Ni-based because of this phase. γ is an ordered L1 (pronounced L-one-two), which means it has a certain atom on the face of the unit cell, and a certain atom on the corners of the unit cell. Ni-based superalloys usually present Ni on the faces and Ti or Al on the corners.
Another "good" GCP phase is γ. It is also coherent with γ, but it dissolves at high temperatures.