Study of high entropy alloys of the Cr-Fe-Mn-Ni system and development of complex concentrated alloys by adding W-C for tribological applications in the nuclear sector

In order to replace Stellite 6, a cobalt-based alloy used as a surface coating for various friction parts in the primary circuit of light water nuclear reactors, the coupling of numerical and experimental methods in the field of HEAs (High Entropy Alloys) was investigated. The Cr-Fe-Mn-Ni system was chosen as HEA basis. Thermodynamic calculations of the Calphad type, via Thermo-Calc and the TCHEA3 database, made it possible to predict the stable phases at 1100 C of an iso-chromium plane at 20% at. and identify the zone of stability of the face-centered cubic (fcc) phase. Two hardening models from the literature, from Varvenne and Walbruhl, were implemented and used in order to identify the hardening tendencies among these compositions. The prediction of the deformation mechanism has been proposed using room temperature data of thermodynamic databases. Then, about ten fcc AHE compositions were developed by arc melting followed by thermomechanical treatments so as to obtain the recrystallization of equiaxed grains. Scanning electron microscopy and X-ray diffraction characterizations made it possible to verify the veracity of the thermodynamic calculations. Tensile tests have made it possible to extrapolate the intrinsic network hardening, a fundamental mechanical property of solid solutions. In a second part, the addition of W and C to HEAs bases was studied, in order to precipitate M7C3 eutectic carbides and form CCAs (Complex Concentrated Alloys). For this, Scheil solidification calculations were carried out, then an experimental verification on 6 ACCs took place. The solidification microstructures were compared to the calculations and demonstrated a very good predictive capacity of the databases. Finally, ball-on-disc friction tests were carried out in order to compare the performance of the CCAs with those of the Stellite 6. The results are encouraging, but it will be important to carry out these tests under same service conditions (temperature, pressure, water and chemistry) as of Stellite 6 to really conclude on the applicability of these new alloys. (author)