F. Tancret, E. Bertrand, L. Couturier
The definition of Complex Concentrated Alloys, or Compositionally Complex Alloys (CCA), is not very precise. One could define them as “baseless”, “multi-concentrated” or “multi-principal” alloys (MPEA), similarly to HEAs but that, contrarily to the latter, would develop polyphaser microstructures, may it be in the as-solidified state, following a precipitation heat treatment or during deformation (“TRIP” effect). It would then be an extension of the concept of HEA towards more classical metallurgies like those of steels or superalloys. As in the case of HEAs, original combinations of properties are looked for, that could potentially be exploited for certain applications. Here are presented various projects in which we are working on the computational design of CCAs.
Keywords: Hgh entropy Alloys, multi-element alloys, high entropy superalloys
The computational design of CCAs is notably performed in upstream phases of projects on the development of cobalt-free alloys for the nuclear industry, as in the PhD studentship of Dinesh Ram (2020-2023) with the ANR project Heria (structural materials) as a collaboration with Saint-Étienne school of Mines, Framatome, EDF and Aperam, or in the PhD studentship of Lisa Rateau (2019-2022) as a collaboration with Framatome (wear-resistant hardfacing alloys).
Examples deal with the design of alloys strengthened by the precipitation of γ’ intermetallics, that could also be called “high entropy superalloys” (HESA), in which we want to maximise simultaneously the volume fraction of the hardening phase (Vf), the high temperature solid solution hardening brought by heavy elements like Mo, Nb or W, and the chromium content to ensure a good corrosion / oxidation resistance. The example below gives the result of a multi-objective optimisation of such alloys within the system Al-Co-Cr-Fe-Mn-Mo-Nb-Ni-Ti-V-W. The Pareto-optimal set evidences the necessary trade-off to be found between high temperature strength (for which we want to maximise Vf and the sum Mo+Nb+W, in other words to move towards the right in the chart) and the resistance to corrosion / oxidation (for which we want to increase the content in Cr, in other words to move upwards in the chart).
Another example is the design of alloys that are single phase after fabrication (hence being “HEAs”), and undergo a martensitic transformation under strain (TRIP effect), potentially allowing significant gains in strain hardenability, strength and ductility. This aspect has notably been dealt with in the PhD thesis of Madeleine Bignon (2020, funded by DGA, collaboration with Lancaster University). The analysis, by calculation, of actual HEAs, shows that the TRIP behaviour can be obtained within a certain range of the driving force for transformation, ΔG.
This concept has then been exploited to design “TRIP HEAs” by multi-objective optimisation, also exhibiting a high solid solution hardening.
. Laboratoire des Sciences du Numérique de Nantes (LS2N) – Université de Nantes
. Laboratoire Georges Friedel (LGF) – Mines Saint-Étienne
. Framatome, EDF, Aperam