Evaluation of Equiatomic CrMnFeCoNiCu System and Subsequent Derivation of a Non-Equiatomic MnFeCoNiCu Alloy

Abstract

Investigation into non-equiatomic high-entropy alloys has grown in recent years due to questions about the role of entropy stabilization in forming single-phase solid solutions. Non-equiatomic alloys have been shown to retain the outstanding mechanical properties exhibited by their equiatomic counterparts and even improve electrical, thermal, and magnetic properties, albeit with relaxed composition bounds. However, much remains to understand the processing-structure-property relationships in all classes of so-called high-entropy alloys (HEAs). Here, we are motivated by the natural phenomena of crystal growth and equilibrium conditions to introduce a method of HEA development where controlled processing conditions determine the most probable and stable composition. This is demonstrated by cooling an equiatomic CrMnFeCoNiCu alloy from the melt steadily over 3 days (cooling rate ~4 °C/h). The result is an alloy containing large Cr-rich precipitates and an almost Cr-free matrix exhibiting compositions within the MnFeCoNiCu system (with trace amounts of Cr). From this juncture, it is argued that the most stable composition is within the CrMnFeCoNiCu system rather than the CrMnFeCoNi system. With further optimization and evaluation, a unique non-equiatomic alloy, Mn₁₇Fe₂₁Co₂₄Ni₂₄Cu₁₄, is derived. The alloy solidifies and recrystallizes into a single-phase face-centered cubic (FCC) polycrystal. In addition to possible applications where Invar is currently utilized, this alloy can be used in fundamental studies that contrast its behavior with its equiatomic counterpart and shed light on the development of HEAs.

Keywords: high-entropy alloys; microstructure; multi-principal element alloys; solidification; transition metal alloys and compounds.

Figure 1

Illustration of the tube furnace setup for slow cooling. A double-crucible method was used, which incorporates the alloy in the inner crucible and a sacrificial element (Cu) in the outer crucible. Temperature is controlled by the thermocouple touching the crucible.

Figure 2

Microstructure of as-cast CrMnFeCoNiCu alloy. (a) SEM micrograph showing dendritic as-cast structure and (b) layered SEM+EDS elemental map along with elemental maps of the constituent metals.

Figure 3

XRD pattern of equiatomic CrMnFeCoNiCu, exhibiting peaks for two FCC phases and minor amounts of MnO identified on the basis of d-spacing.

Figure 4

XRD pattern of equiatomic CrMnFeCoNiCu after 30% rolling reduction and annealing at 1200 °C for 48 h.

Figure 5

Morphology and elemental maps of the CrMnFeCoNiCu alloy after rolling and isothermal heat treatment. (a) Layered EDS map, (b) SEM micrograph, and constituent elemental maps of Cr, Mn, Fe, Co, Ni, and Cu, respectively. Arrows in (a,b) mark the presence of Cr-rich oxide inclusions.

Figure 6

Morphology and elemental maps of a heat-treated CrMnFeCoNiCu alloy, focusing on a Cu-rich zone. (a) SEM micrograph and (b) corresponding layered EDS map and constituent elemental maps of Cr, Mn, Fe, Co, Ni, and Cu, respectively. Arrows indicate a region exhibiting compositional and morphological variations.

Figure 7

Section views of the as-cast ingot. (a) Cross-section optical micrograph and (b) corresponding SEM micrograph showing phase morphology. (c) Plan-view image of as-cast ingot showing acicular precipitates on the surface and (d) corresponding SEM micrograph.

Figure 8

Image-stitched layered EDS elemental map of a 1 mm wide cross-section from bottom to top, with constituent elemental maps of Cr, Mn, Fe, Co, Ni, and Cu, respectively.

Figure 9

Layered EDS maps taken from regions of interest from the (a) top, (b) middle, and (c) bottom of the ingot. Bar charts quantifying the composition of microstructural features C1, C2, M1, and M2 are shown for regions of interest at the (a1) top, (b1) middle, and (c1) bottom of the sample.

Figure 10

XRD pattern of CrMnFeCoNiCu alloy after slow cooling and exhibiting multiple phases. For unindexed peaks, the interplanar spacing d is given in angstroms.

Figure 11

XRD diffractogram of as-cast Mn₁₇Fe₂₁Co₂₄Ni₂₄Cu₁₄ alloy showing single phase FCC diffraction peaks.

Figure 12

Layered EDS map and constituent elemental maps from an arc-melted, Mn₁₇Fe₂₁Co₂₄Ni₂₄Cu₁₄ alloy cross-section.

Figure 13

(a) XRD diffractogram of homogenized Mn₁₇Fe₂₁Co₂₄Ni₂₄Cu₁₄ alloy showing single-phase FCC diffraction peaks, and (b) optical micrograph showing a nearly equiaxed grain structure.

Figure 14

Layered EDS map and constituent elemental maps of homogenized Mn₁₇Fe₂₁Co₂₄Ni₂₄Cu₁₄ alloy. A small number of Cu-enriched oxide inclusions are observed.

Figure 15

Tensile stress–strain curves for both the equiatomic and non-equiatomic alloys.