Influence of grain boundary chemistry and crystallography on creep cavity formation
In the present project, the influence of small amounts of Bi and Sb on the intergranular creep damage accumulation in Cu is investigated. Pure Cu model materials as well as Cu-0.008 wt.% Bi and Cu-0.92 wt.% Sb alloys are produced via an ingot metallurgy route. Comparable microstructures and grain boundary networks are established by thermomechanical processing. Therefore, changes in the damage behavior can be predominantly related to effects of Bi and Sb, which segregate strongly to grain boundaries. Creep tests are conducted with the three model materials and the cavitation damage is determined as a function of creep strain.
Results show that creep cavity nucleation is accelerated in materials containing grain boundary segregation of Bi and Sb. Rupture is therefore cavity nucleation controlled. In pure Cu rupture is cavity growth controlled.
The crystallographic grain boundary character also has a significant influence on the susceptibility of a grain boundary to suffer creep cavitation. High-angle grain boundaries are most prone to grain boundary cavitation while low-angle grain boundaries and special coincidence site lattice boundaries are more damage resistant.
High resolution scanning transmission electron microscopy image showing segregated Bi atoms as bright dots at a random high-angle grain boundary in Cu.
Fracture surface of plain Cu: ductile.
Fracture surface of Cu-Bi: brittle.