Meaning
Thermodynamic model describing the grain boundary segregation of solute atoms as a function of temperature and bulk concentration. Understanding this behavior allows metallurgists to predict how solute atoms gather at crystalline boundaries and alter the structural integrity of alloys. This model assumes a fixed number of identical boundary sites where solute atoms can reside, creating an equilibrium between the grain interior and the boundaries.
Segregation Thermodynamics
Temperature changes modify the grain boundary concentration of solute atoms, with lower temperatures typically promoting greater segregation. The McLean isotherm mathematically connects this equilibrium concentration to the bulk concentration and the free energy of segregation. When the material is held at elevated temperatures during manufacturing, solute segregation can lead to temper embrittlement or susceptibility to stress corrosion cracking.
By calculating these equilibrium states, engineers can specify the heat treatment profiles that avoid critical ranges where harmful elements concentrate at the boundaries.
Model Verification
Electron microscopy and atom probe tomography provide the analytical data to validate the theoretical predictions. These measurements confirm the local chemistry of the grain boundaries after specific cooling cycles.
Industrial Impact
Controlling grain boundary segregation is essential for the production of high-strength steels and nickel-based superalloys. Proper alloying strategies based on these calculations prevent premature structural failures in demanding environments.
