Fabrication and characterization of CrFeMoV based high entropy alloy with modified design by the addition of Mn and Al

Abstract

High-entropy alloys (HEAs) are emerging materials where five or more elements usually contribute in equiatomic-ratio. These alloys have anticipated attraction from the researchers around the world due to their superior mechanical, oxidation, and corrosion behavior in comparison to conventional alloys. The designing of HEAs required special consideration as it would greatly influence the obtained feature of the alloys by the formation of the single-phase, or multiphase with intermetallic inclusions. After rigorous screening, a CrFeMoV based quaternary system has been designed and the effect of Mn and Al addition on the microstructure and mechanical behavior have been analyzed. The fabrication of both systems was conducted through powder metallurgy by employing high-energy ball milling followed by spark plasma sintering. The milling of CrFeMoVMnx based system was performed in the dry environment without using any PCA. However, due to the higher ductility of Al and the possibility of cold-welding, while milling, the AlxCrFeMoV based system was fabricated by using Methanol as process control agent. The optimized milling time for the Mn-based system was 6 hours and for Al-based was 7 hours.

The comprehensive microstructural analysis of CrFeMoVMnx exhibited that the reduction of Mn decreased the formation of sigma phase (intermetallic), and the content of sigma varied directly with the concentration of Mn. The decrement of intermetallic phase in single-phase solid solution substantially influenced the mechanical features of the alloys. The systems with lower Mn-content showed improved ductility with a slight decrease in ductility. The Mn1 system showed the highest compressive yield strength of 3180MPa. On the other hand, the Al addition in CrFeMoV system influenced the characteristics by the formation of an additional solid solution phase. The contentof the secondary solid solution phase varied directly with the Al concentration. The appearance of Al significantly enhanced the compressive yield strength of the alloy

however, the effect on the ductility was not as detrimental as it was in case of Mn-containing systems. The Al0.6 and Al1 system showed highest specific yield strength of 490 MPa.cm3/g and 510 MPa.cm3/g as compared to any previously reported high entropy alloy. In addition, the strengthening mechanism in CrFeMoV alloy was assess by fabricating the alloy through arc melting and comparing it with powder metallurgy samples. Furthermore, the electrochemical behavior of AlxCrFeMoV system was also assessed in this thesis study. The corrosion behavior was analyzed in 3.5 wt.% NaCl solution by using a potentiodynamic test and electrochemical impedance spectroscopy. Due to high pitting resistant equivalent in the alloys, the alloys showed exceptionally good pitting resistant. The XPS depth profiling showed the formation of a double oxide layer, firstly contributed by all constituents, and secondary dominated by Cr based oxides.