Atomistic Mechanisms of Intergranular Embrittlement

Abstract

Since the composition at grain boundaries generally differs considerably from that in the grain interior, all considerations of intergranular fracture must take this into account. In this survey we consider the historical development of the theories of low temperature intergranular fracture with particular reference to temper brittleness. The theory is followed through Rice and Thompson’ s analysis of the energy balance between ductility and brittle fracture with Rice’s analysis of the effects of segregant on the ideal work of fracture term. As developed by Mason, Hirth and Rice, and Seah the atomistic term involved in this process is the bond energy of the individual atoms across the fracture plane These terms can be calculated simply, to a first approximation, from tabulated thermodynamic data.

Other important forms of low temperature intergranular brittleness, hydrogen embrittlement, liquid metal embrittlement and intergranular stress corrosion cracking are also affected by grain boundary segregation. In hydrogen embrittlement and liquid metal embrittlement, the segregant-environment atom bonding is important whereas in intergranular stress corrosion cracking the emphasis is placed on the electrochemical potential of the s e gre gant atoms.

The high temperature intergranular failures in stress relief annealing and creep embrittlement are similarly affected by segregants but here the terms involve the effects on surface energy and grain boundary diffusivity. Each intergranular fracture phenomenon involves a different atomic parameter and hence a different hierarchy of segregants is observed in each case. Recognition of this is important and should be built into future specification codes.