Behavior of Ti-doped CoCrFeMoNi High Entropy Alloy

High entropy alloys are an extremely diverse and interesting class of materials as they are promising materials for a wide range of industrial applications because they are very hard, heat resistant and show great variability in their magnetic behavior [1,2]. High entropy alloys or HEAs consist of five or more different metallic elements; since their macroscopic properties are highly dependent on interatomic interactions, it is of great interest to probe the local structure and structural disorder around each individual element [3,4]. In recent years, the well-known Cantor alloy (CoCrFeMnNi) has attracted considerable attention due to its excellent toughness and fracture strength but its corrosion resistance is very low because Mn interferes with the passivation process and therefore, in our study we have considered an HEA in which Mn has been replaced with Mo (CoCrFeMoNi). To further improve the properties, the alloy obtained has been microalloyed with titanium. The ingots of each alloy are prepared at LAMET laboratory, Romania, by Vacuum Arc Melting (VAR) in a protective argon atmosphere by using the MRF ABJ 900 VAR equipment and eight remelting operations are carried out for complete homogenization. The raw materials used to form these alloys are 99.00 % pure and the obtained alloys have the composition wt % and at % shown in Table 1. One of the samples, CoCrFeMoNi (named HEA) and that with Ti doping was named (HEATi). The samples with and without Ti are compared: their microstructure is analyzed by optical microscopy (Olympus BX5, Japan) , SEM and EDS (HITACHI TM3030); the modulus of elasticity of the alloys was studied by a three-point bending test (Electroforce 3100 BOSE Corporation, USA) and their electrochemical responses, by linear polarization (LP) and electrochemical impedance spectroscopy (EIS) using a SP-150 potentiostat (BioLogic, France). The hardness was measured with Shimadzu HMV2T microhardness apparatus (Tokyo, Japan). The samples preparation and analysis is shown in Figure 1. a) Microstructure The optical examination after electrochemical etching is shown a dendritic structure for both samples (See Figure 2). The microstructures were also analyzed by scanning electron microscopy and the compositions were determined by EDS. b) Elastic modulus and hardness The elastic modulus was determined and the hardness of both samples was calculated and the results are shown in Table 2. It can be observed that the elastic modulus is similar, but the hardness of Ti-doped sample is improved. c) Electrochemical behavior Figure 5 illustrates the potentiodynamic polarization curves of HEA and HEATi in 3.5 wt.% NaCl solution at 25C. The results show that both samples exhibit active dissolution starting at aprox. 0V and almost could not passivate in sodium chloride solution. The graphs obtained from the EIS tests are presented in Figure 6 for HEA and in Figure 7 for HEATi. As can be observed in the Nyquist plots presented in Figure 6, and Figure 7, the radius of the semicircle for HEATi is smaller than for HEA, which indicates a low polarization resistance (a low corrosion resistance) for HEATi. The effects of titanium do ping on the microstructure, elastic modulus, hardness and corrosion properties of CoCrFeNiMo high entropy alloy has been investigated. Simulated corrosive solution of 3.5wt.%NaCl was used to characterize the corrosion behavior of these materials by electrochemical techniques. Both alloys exhibited a compact microstructure, with no cracks, porosity or other defects. The alloy without titanium is more resistant to corrosion, with a very low corrosion rate. The resistance of the passive film decreases with the addition of Ti due to the increasing of the number of defects points in the film. Without Ti the passive layer that forms on the surface of the alloy is more compact and protective [5]. Chemical composition of the experimental samples Elastic modulus and hardness of the samples Schematic preparation and analysis of the samples. a, b) Microstructure of HEA and c,d) HEATi after electrochemical etching. SEM image (a), area of EDS elemental analysis (b), EDS spectra (c) and elemental maps (d-h) of the HEA. SEM image (a), area of EDS elemental analysis (b), EDS spectra (c) and elemental maps (d-h) of the HEATi. Potentiodynamic polarization curves. EIS graphs for the HEA sample. EIS graphs for the HEATi sample. Prof.Victor Geanta is thanked for the preparation of the analyzed samples and his many useful discussion and contributions to this work.