The (110) and (320) surfaces of a Cantor alloy

Abstract

The ( 110) and (320) surfaces of the single-phase FeCrMnNiCo solid solution have been studied on two adjacent millimeter size grains using surface science and transmission electron microscopy (TEM) techniques. The structural and chemical evolutions of the high entropy alloy (HEA) surfaces have been determined for various sputtering conditions, annealing temperatures and durations. Up to 873 K, angle-resolved x-ray photoelectron spectroscopy measurements indicate a clear Mn and Ni surface cosegregation. We propose that the surface segregation of Mn is driven by its low surface energy. The attractive interaction between Mn and Ni promotes Ni segregation which accompanied the Mn diffusion to the surface. Regarding the structures investigated by low energy electron diffraction and scanning tunneling microscopy, the (320) surface presents a terraced morphology with an ordered structure consistent with a (1×1) termination. On the contrary, the (110) surface reveals an important degree of structural disorder and local reconstructions. Its highly anisotropic morphology resembles rows propagating along the [001] direction. Above 873 K, Mn desorption occurs while the Ni content keeps increasing linearly with the temperature. TEM analysis show no evidence for HEA decomposition into metallic or intermetallic phases even after repeated annealing and sputtering cycles. The above results set the upper temperature limit above which the surface stoichiometry departs from the quinary HEA concept. It also defines the temperature range for the use of FeCrMnNiCo based coating under high vacuum conditions and for aerospace applications.

Surface Segregation in AgAuCuPdPt High Entropy Alloy: Insights From Molecular Simulations

“…Such segregation dependency on the surface energy of pure elements and pairwise negative mixing enthalpy is also supported by the experimental study in Cantor alloy [25]. It appears that a factor like relative diffusivity of elements in the HEA matrix has no significant role in the thermodynamics driven surface segregations in HEAs and other alloys [25,[32][33][34][35]. The computed atomic strain and the binary mixing enthalpies based on the Miedema model [39][40][41] are presented in table S34 and S35 of the supporting information, respectively.…”

“…Indeed, cluster analysis revealed the presence of at least one Ag (Au) in the neighborhood of almost all the surface Pd (Cu) atoms. Such segregation dependency on the surface energy of pure elements and pairwise negative mixing enthalpy is also supported by the experimental study in Cantor alloy [25]. It appears that a factor like relative diffusivity of elements in the HEA matrix has no significant role in the thermodynamics driven surface segregations in HEAs and other alloys [25,[32][33][34][35].…”

“…While no segregation or very little Ni enrichment was observed at low energy grain boundaries, significant Ni enrichment with Mn co-segregation was observed at general high angle grain boundaries [21]. Mn was the dominant element on the (001), ( 110) and (320) surfaces of the HEA [22,25]. A lower surface energy of Mn and a strongly negative mixing enthalpy of the Ni-Mn pair was attributed to such elemental segregation [22,25].…”

“…Mn was the dominant element on the (001), ( 110) and (320) surfaces of the HEA [22,25]. A lower surface energy of Mn and a strongly negative mixing enthalpy of the Ni-Mn pair was attributed to such elemental segregation [22,25]. While the quantitative (and to some extent qualitative) amounts of the segregating species may differ across these reports, all of them point to HEAs being prone to segregation and that such effects must be taken into account for a more thorough and realistic understanding of surface governed phenomena on HEAs.…”

“…A few studies in the past have addressed segregation phenomena in HEAs, both at surfaces and at grain boundaries [20][21][22][23][24][25]. Much of these studies have been carried out on the cantor alloy (FeMnNiCoCr FCC HEA).…”