Synthesis and microstructural analysis of the CoCrFeMnNi high entropy alloy

Details

Supervisors

Jacques, Pascal

Faculty

Ecole polytechnique de Louvain

Degree label

Master [120] : ingénieur civil en chimie et science des matériaux

Abstract

High entropy alloys (HEA), a relatively new class of alloy, manifest a rapidly growing interest because of their attractive properties such as high solid solution hardening capability, good ductility, high thermal stability, excellent corrosion resistance. HEA are composed of at least five major components with atomic percent be- tween 5% and 35%. This thesis explores the single phase (FCC) CoCrFeMnNi HEA, but also the Al-Co-Cu-Fe-Ni system. Three synthesis methods for high entropy alloys have been investigated: (i) arc melting to produce an equi-atomic CoCrFeMnNi alloy; (ii) the construction of a diffusion multiple between Fe-50Mn (at-%), Co-50Ni and Cr, to form a region of quinary mixing and (iii) friction stir welding to mechanically mix 5 elements (Al-Co-Cu-Fe-Ni), followed by a diffusion treatment. For this last method, a region of quinary mixing containing the 5 elements has not been ob- served. A single phase solid solution with a FCC structure has been observed on the arc melted equi-atomic CoCrFeMnNi alloy before and after a homogenization treatment using SEM, EDX and XRD analysis. A HEA phase has also been observed on the diffusion multiple for a larger range of composition. The grain size evolution of a 84% cold rolled CoCrFeMnNi has been analyzed for recrystallization treatments at 800 ◦C between 15 and 45 min. The average grain size has increased with increasing annealing time, from 2.7 μm after 15 min up to 3.4 μm after 45 min. The phase stability of the single phase CoCrFeMnNi has been investigated by annealing a homogenized and a cold rolled alloy at 500 ◦C for respectively 50 and 14 days. A decomposition of the high entropy phase has been observed: 3 new phases along the grain boundaries on the homogenized sample and 2 new phases homogeneously distributed on the cold rolled sample were observed. CoCrFeMnNi tensile specimens with a 3.4 μm grain size have been tested until fracture. A yield strength of 400 MPa, an ultimate tensile strength of 680 MPa and true fracture strain of 0.31 have been measured. The microstructure resulting from a true strain of 0.26 has been examined. Fractographic analysis show a ductile fracture, with two sizes of dimples (1.3 and 10.8 μm). Nucleation of the large dimples seems to occur by decohesion of oxides/sulfides with the matrix. This thesis shows that diffusion multiples are an excellent method to easily and rapidly study a large range of composition of HEA. It can subsequently be associated to arc melting to better characterize the properties of HEA.