Fabrication and characterization of Mechanically Alloyed NiCoCrFe based High Entropy Alloy and Composites

Abstract

Single phase face-centered cubic (fcc) high entropy alloys (HEAs) are being extensively explored due to their excellent ductility and strength. Reinforcements are generally added to further enhance the mechanical properties of these alloys. This work focuses on the fabrication of NiCoCrFe single phase high entropy alloy followed by addition of Al2O3 and B4C reinforcements. HEA powder prepared by mechanically alloying was subjected to sintering to obtain densified compacts. Mechanical properties of the alloy were investigated based on the microhardness and compression results. X-ray diffraction and scanning electron microscopy was used for phase analysis and microstructure evaluation. All four elements showed complete solubility in nickel based fcc lattice. However, after sintering Cr-rich phase precipitated out to form a thermodynamically stable microstructure. Cr-rich precipitates increased the strength of HEA compared to the reported NiCoCrFe alloy prepared by arc melting. Fully dense compacts showed microhardness of 178 HV and yield point at 335 MPa. Al2O3 and B4C reinforcements were added to the mechanically alloyed HEA powder to form HEA composites. Grain boundary segregation of Al2O3 particles was observed to have led to a detrimental effect on the mechanical properties of the alloy. On the other hand, addition of B4C increased the hardness and yield strength to 425 HV and 890 MPa respectively. In addition, NiCoCrFe was mechanically alloyed in the presence of Ta to form a unique equiaxed microstructure where CoTa3 precipitates were embedded in fcc matrix. This led to enhanced mechanical properties with a hardness of 380 HV.

Single phase face-centered cubic (fcc) high entropy alloys (HEAs) are being extensively explored due to their excellent ductility and strength. Reinforcements are generally added to further enhance the mechanical properties of these alloys. This work focuses on the fabrication of NiCoCrFe single phase high entropy alloy followed by addition of Al2O3 and B4C reinforcements. HEA powder prepared by mechanically alloying was subjected to sintering to obtain densified compacts. Mechanical properties of the alloy were investigated based on the microhardness and compression results. X-ray diffraction and scanning electron microscopy was used for phase analysis and microstructure evaluation. All four elements showed complete solubility in nickel based fcc lattice. However, after sintering Cr-rich phase precipitated out to form a thermodynamically stable microstructure. Cr-rich precipitates increased the strength of HEA compared to the reported NiCoCrFe alloy prepared by arc melting. Fully dense compacts showed microhardness of 178 HV and yield point at 335 MPa. Al2O3 and B4C reinforcements were added to the mechanically alloyed HEA powder to form HEA composites. Grain boundary segregation of Al2O3 particles was observed to have led to a detrimental effect on the mechanical properties of the alloy. On the other hand, addition of B4C increased the hardness and yield strength to 425 HV and 890 MPa respectively. In addition, NiCoCrFe was mechanically alloyed in the presence of Ta to form a unique equiaxed microstructure where CoTa3 precipitates were embedded in fcc matrix. This led to enhanced mechanical properties with a hardness of 380 HV.