Nonlinear Oxidation Behavior in Pure Ni and Ni-Containing Entropic Alloys

Abstract

We performed a combined experimental and theoretical investigation of the oxidation behavior of pure Ni and of the following multi-component Ni-containing alloys with nearly equiatomic compositions: FeNi, CoFeNi, CoCrFeNi, and CoCrFeMnNi. The materials were exposed to air at ambient pressure and at a temperature of 800 • C for 150 min, their weight-gain due to oxidation was continuously monitored and the products of oxidation were subsequently characterized by XRD. The most common oxides formed have spinel or halite structure and the materials resistance to oxidation increases as: FeNi < CoFeNi < Ni < CoCrFeMnNi < CoCrFeNi. We found further that the oxidation-resistance of the materials does not correlate linearly with the number of elements present, instead the type of elements impacts significantly the materials susceptibility to oxidative damage. Cr is the element that imparted higher resistance to oxidation while Mn and Fe worsened the materials performance. In order to better understand the mechanisms of oxidation we employed thermodynamic equilibrium calculations and predicted the phase stability of oxides of the elements that are present in the materials, in different ranges of temperature, composition and oxygen activity. Additionally, we determined the phase compositions for the thermodynamically stable oxides at 800 • C. The results from the thermodynamic modeling are in good agreement with the experimental finds. The alloys with low resistance to oxidation such as CoFeNi and FeNi, form the Fe 3 O 4 spinel phase which tends to dominate the phase diagram for these materials. The presence of Cr increases the resistance to atomic rearrangement due to slow diffusion in the complex structure of Cr containing spinel phases. This causes the extremely high resistance to oxidation of the CoCrFeNi alloy. The presence of Mn in CoCrFeNi stabilizes the Mn 3 O 4 spinel, which reduces the oxidation-resistance of the alloys due to the high mobility of Mn.

High-entropy alloy superconductors: Status, opportunities, and challenges

“…Most HEAs studied have involved primarily constituents from the 3d series of metals, but in 2014, the first HEA superconductor, consisting primarily of 4d and 5d series elements, was found [15], revealing a new facet of the capabilities of these materials. A variety of studies have been performed on HEA superconductors since that time [7,[16][17][18][19][20][21][22][23][24][25][26], in part because the combined electronic and mechanical properties found in HEAs suggest their potential for practical applications. Being in the relatively early days in the study of HEA superconductors, it is possible to consider all of the known ones and present a reasonably complete snapshot of HEA superconductors at this point in time.…”

An Experimental and Theoretical Study of Duplex fcc+hcp Cobalt Based Entropic Alloys

“…In this work the possibility to partially substitute Cobalt with high-entropy alloys [11,29,30] or, using the more appropriate term in the present context, so-called entropic alloys [31,32] has been investigated both theoretically and experimentally with a focus on the relative stabilities of the fcc and hcp structures. Moreover, for a long time high entropy alloys have been believed to form primarily bcc and/or fcc phases.…”

Reaction with hot air of high entropy alloys strengthened by monocarbides formed from heavy metals: assessment of the oxidation kinetics from the analysis of the chemically changed subsurfaces

“…A series of investigations were already carried about the behavior of HEAs in high temperature oxidation. Among the studied alloys which are the closest to the Cantor composition but which are slightly different, there are Mn-free Al-containing ones (AlCoCrFeNi) [19,20], or some Ni-free ones [21,22], for instance. The oxidation behavior of the equimolar CoNiFeMnCr composition was specifically investigated in laboratory air at temperatures from 500°C to 900°C [23,24], and in synthetic air between 900 and 1100°C [25].…”