Solids: Thermodynamics and Bonding

· Materials may also undergo phase transitions between different solid structures. The diagram illustrates this for pure iron.
· Below 910 C, iron is in the α-phase and has a body centered cubic (bcc) crystal structure.
· Between 910 and 1400 C iron is in the γ-phase which has a face centered cubic (fcc) crystal structure.
· Between 1400 C and its melting point at 1539 C the structure returns to bcc, and is known as δ-iron.
· The phase of lowest Gibbs function is the stable one at any T.

From: Brophy, Rose, & Wulff, "Thermodynamics of Structure," Wiley (1964)