Chemical medium-range order in a medium-entropy alloy

Abstract

High-/medium-entropy alloys (H/MEA) have the inherent local chemical order. Yet, as a structural link between the incipient short-range order and mature long-range counterpart, the chemical medium-range order (CMRO) is conjectural and remains open questions as to if, and what kind of, CMRO would be produced and if CMRO is mechanically stable during plastic deformation. Here, we show compelling evidences for CMRO in an Al9.5CrCoNi MEA. Specifically, the electron diffraction under both [$$112$$ 112 ] and [$$013$$ 013 ] zone axis show the definite spots for CMRO of lattice periodicity. CMRO entities are seen directly of medium-ranged in sizes by using dark-field imaging, along with the tendency towards like-pair avoidance and unlike-pair preference based on atomic-resolution EDS mapping. These findings substantiate CMRO with a realistic structural picture in view of crystal periodicity and chemical species occupation, shedding light on understanding the microstructural link at an extended length scale beyond the short-range order.

Mechanically derived short-range order and its impact on the multi-principal-element alloys

“…In contrast with the MSROs the distribution of CSROs is highly irregular throughout the individual grain. This was demonstrated by computational studies 14,15,22,26 and experimental images [16][17][18][19][20][21]33,34 .…”

“…However, to determine which MSRO motif exists in the realdeformation structure, further TEM-energy dispersive spectroscopy characterisation 17,18 is required. CSRO in a few MPEAs can be described in terms of 'preference for unlike species and avoidance of like pairs' 17,18,21,39 . Our MC simulations revealed that lowering the temperature of the Fe 40 Mn 40 Cr 10 Co 10 structure led to a relatively strong tendency for Fe-Co, Mn-Cr, Mn-Co, Fe-Mn, and Fe-Cr, promoting the variability of atomic packing.…”

“…Compared with HEA-L showing the ½{311} diffuse scattering in the EDP, more and larger MSROs were distributed for HEA-M exhibiting the spot-shaped scattering in the EDP. Approximating the clustered MSROs as spheres [16][17][18][19]21,34,35 helps characterise the ordering size distribution (Fig. 5e).…”

“…Unlike traditional metallic alloys, MPEAs are concentrated solid solutions comprising mixtures of elements in which the atoms of principal elements tend to be distributed randomly. However, several studies have indicated that the large-enthalpy interactions among the constituent species can drive a few MPEA systems towards inherent chemical ordering at nanometric scales, as suggested by the results obtained using the computational simulations 14,15 and experimental methods [16][17][18][19][20][21] . When compositionally homogeneous structures have specific low-coordination-number clusters, the preferential local ordering of principal elements generally dominates over the spatial order of a few nearest-neighbour spacings, i.e.…”

“…However, X-ray-based measurements provide relatively low analytical capabilities, restricting the experimental observation of CSROs in the individual grains of the materials, as they are averaged over a comparatively large volume of material 16 . The transmission electron microscopy (TEM) and atom probe tomography techniques were considerably enhanced so as to record the extremely small CSROs within individual fcc grains for metallic alloys [16][17][18][19][20][21] . For instance, TEM dark-field images, formed with CSRO-induced additional diffuse scattering in reciprocal-space selected-area electron diffraction patterns (EDPs) with specific zone axes, enable quantitative measurements of the CSRO degree/extent in NiCrCo and VNiCo MEAs [16][17][18] and Fe 50 Mn 30 Cr 10 Co 10 (at%) HEA 19 with the similar atomic sizes of constituent species.…”