Effect of Oxygen on the Interaction of a High-Entropy Alloy (NiFeCoCrTiAl) with Cubic Boron Nitride

Some thermodynamic aspects of interaction between Ni–Fe–Co–Cr–Ti–Al high-entropy alloys (HEAs) and cubic boron nitride (cBN) at high temperatures in a medium with and without oxygen are studied. CALPHAD modeling with literature data are used to estimate the temperature dependence of change in the Gibbs energy for the formation of nitrides, borides, and oxides. It is shown that the presence of oxygen essentially changes thermodynamic conditions for the reactions, in particularly, decreases the probability of direct interaction between Ti, Al, and cBN due to the formation of oxide phases. Comparative analysis with reactions of pure metals indicates that Ti and Al in the composition of HEAs have a low activity to agree with CALPHAD calculation results. Experimental high-temperature studied performed by using the method of synchronous thermal analysis fully confirm thermodynamic calculations. The obtained results can be used to develop the compositions of high-entropy alloys for application in cBN based composite materials.