Sigma phase is a type of intermetallic compound that can form in certain alloys, particularly those containing chromium, molybdenum, and/or tungsten, at high temperatures. It forms when the alloy is exposed to temperatures in the range of 1000-1300 °F (540-700 °C) for an extended period of time, typically several hours or more.
Sigma phase is characterized by a tetragonal crystal structure and can have a detrimental effect on the mechanical properties of the alloy. It can cause embrittlement and reduced ductility, which can lead to cracking and failure under stress.
The formation of sigma phase is influenced by several factors, including the chemical composition of the alloy, the temperature and duration of exposure, and the cooling rate after exposure. It is typically more likely to form in alloys with higher levels of chromium, molybdenum, and tungsten, and is more likely to occur at higher temperatures and longer exposure times.
To prevent the formation of sigma phase, it is important to control the temperature and duration of exposure during high-temperature processing and to use alloys with a composition that is less susceptible to sigma phase formation.