The BCC/B2 Morphologies in AlxNiCoFeCr High-Entropy Alloys

Abstract

Abstract:The present work investigates primarily the morphology evolution of the body-centeredcubic (BCC)/B2 phases in AlxNiCoFeCr high-entropy alloys (HEAs) with increasing Al content, which has been neglected so far. There exist two types of microscopic morphologies of BCC and B2 phases in this HEA series: one is the weave-like morphology induced by the spinodal decomposition, and the other is the microstructure of a spherical disordered BCC precipitation on the ordered B2 matrix that appears in HEAs with a much higher Al content. The shape of coherent precipitates is found to be closely related to the lattice misfit between BCC and B2 phases, which is sensitive to Al. The mechanical properties, including the compressive yielding strength and microhardness of the AlxNiCoFeCr HEAs, are also discussed in light of the concept of the valence electron concentration (VEC).

Structure and mechanical properties of B2 ordered refractory AlNbTiVZrx (x = 0–1.5) high-entropy alloys

“…The B2 ordering is often observed in Al-containing high-entropy alloys [29][30][31][32][33][34]. Al was found to be a bcc/B2 stabilizer in those HEAs which are based on late transition metals (TMs) such as Fe, Ni, Co and Cu [29][30][31][32][33][34][35]. Having a high electron density (the outermost shell holds three electrons) and a high Fermi level (small work-function and high ionization tendency), Al prefers to transfer electrons to TMs, such as Ni, Co, Fe, Cr and Mn, to form intermetallic compounds with covalent bonds between the elements [36].…”

Effect of Ti substitution for Al on the cuboidal nanoprecipitates in Al0.7NiCoFeCr2 high-entropy alloys

“…On the other side, the atomic size of Ti is relatively larger than those of Al and Ni, which can result in a large lattice distortion and, thus, induces an ordered phase transformation from B2 to L2 1 phase. More importantly, in our recent work, 23,24,38 the formation of a special coherent microstructure with cuboidal B2 nanoprecipitates in BCC HEAs was found to be related closely to the lattice misfit, e, between B2 and BCC phases, which is expressed with the formula of e ¼ 2 Â a B2 À a BCC ð Þ = a B2 þ a BCC ð Þ . Generally, a small e corresponds to the formation of spherical particles and it is difficult to control the shape of coherent particles in the case with a large e. [44][45][46] Only a moderate e can produce such a coherent microstructure with cuboidal particles that contributes to the improvement of hightemperature mechanical properties of alloys, especially the creep-resistant property.…”

“…23 It is difficult to achieve cuboidal or spherical shapes of coherent B2 precipitates in BCC-based HEAs due to the large composition difference between BCC and B2 phases that can induce a large lattice misfit. 24 The weave-like microstructure caused by spinodal decomposition between these two phases is always existed in BCC/B2 Al-contained HEAs, like AlNiCoFeCr, 25,26 resulting in a serious brittleness. Further addition of a small amount of Ti into AlNiCoFeCrTi x (x , 0.5, Ti , 9.1 at.%) HEAs does not change the morphology of alloys, still keeping the spinodal decomposition of BCC and B2 phases.…”

Effect of Cold Rolling on the Phase Transformation Kinetics of an Al0.5CoCrFeNi High-Entropy Alloy

“…There is not much difference for the microstructure ( Figure 1 ) and phases ( Figure 2 ) between the as-cast and deformed sample; however, according to the f – T curve, activation energy and kinetic exponent, there is a large difference. The microstructure of Al x CoCrFeNi (0 ≤ x ≤ 2) HEAs has been intensively investigated in different heat treatment conditions [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ]. Different from the thermo-mechanical processing that can affect the transformation pathways in Al 0.3 CoCrFeNi [ 34 ], 20% cold rolling cannot alter the phase transition product [ 12 ].…”

“…First, this alloy system owns very good mechanical [ 12 , 13 , 14 , 15 ] and physical properties [ 16 , 17 , 18 ]. Second, with the increasing Al content, the main phase of the alloy moves from pure face-centered-cubic (FCC) to FCC + pure body-centered-cubic (BCC), and pure BCC phase, making the mechanical and physical properties adjustable [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 ]. Thus, the microstructure, phases and properties of this alloy system have been intensively investigated.…”