Stacking fault energies in Co and Co-based alloys

Graphic generalized stacking fault energy
Generalized stacking fault energies.

Cobalt and Co-based alloys are important materials that have been subjected to considerable use because of their outstanding mechanical and magnetic properties. In particular, these alloys have found their way into industrial applications where high temperature strength and hardness, excellent wear, galling, corrosion, and erosion resistance are required properties. Unfortunately, serious health concerns related to Co dust and material availability issues may dramatically change the market scenario for Co and hence the search for alternatives has become a need.

Finding a replacement for a material or a part of a material is not easy, specially when a material has been carefully crafted for a specific application. Therefore, in order to find a replacement for a certain material, one needs to identify first a property that is related to the intended functionality of the material. For example, the performance of cutting tools made out of cemented carbides; composite materials that typically use Co as a binder phase, is believed to be related to the low stacking faults energies and the hcp-fcc phase transition in Co.

In "A first principles study of the stacking fault energies for fcc Co-based binary alloys" Acta Materialia 136, 215-223, 2017 the authors investigated the stacking fault energies of CoxM1- x , where M was Cr, Fe, Ni, Mo, Ru, Rh, Pd and W. By analyzing the stacking fault energies, they show that alloying Co with Cr, Ru, and Rh promotes the hcp phase formation while Fe, Ni and Pd favor the fcc phase instead. The effect of Mo and W on the phase transition differs from the other elements, that is, for concentrations below 10% the intrinsic stacking fault energy is lower than that for pure fcc Co and the energy barrier is higher, whereas above 10% the situation reverses. They carried out also thermodynamic calculations using the ThermoCalc software. The trends of the ab initio stacking fault energy were found to agree well with those of the molar Gibbs energy differences and the phase transition temperature in the binary phase diagrams and give a solid support for the phase stability of these alloys. This investigation permits to understand what possible element combination is needed in a material that is intended to replace Co by emulating the stacking faults energies in Cobalt.