| Multi-principal-element solid solution alloy, commonly termed as high entropy alloy, is typically composed of at least five principal elements in equal or near equal atomic amount. Its superior entropy, compared with that of traditional alloy, contributes to forming solid solution phases with simple structures. The alloy was identified by the name of multi-principal-element solid solution so as to differentiate with traditional solid solution alloys. Multi-principal-element solid solution alloy is the focus of present materials studies and is expected to be materials with outstanding performance in extreme environments, due to its excellent mechanical strength and hardness, stability at high temperature, impressive wear behavior, good resistance to corrosion and oxidation. Composition design of multi-principal-element alloy has been simplified by equi-molar strategy, however, this method is not a prerequisite for new composition developing. Previous researches show that short range interaction between elements is decisive to alloys’structures and performances, but lacking of short range structure model impedes the further optimization of composition and properties.Recently, a "cluster plus glue atom" model has been developed to describe short range order in solid solution alloy, revealing that prominent solid solution alloy usually have stable local structure unit----first nearest neighbour cluster and second nearest neighbour glue atoms. Then cluster formula was given as [cluster] (glue atom)x, while x means the number of glue atoms; close-packed cluster of BCC structure is polyhedron with coordination number of 14. Thus, this paper target composition design of Al-Ni-Co-Fe-Cr five-principal-element alloys using "cluster plus glue atom" model. By varying Al content, composition series of Alx[Ni1/4Co1/4Fe1/4Cr1/4]16-x (1.5≤x≤4) was studied and basic cluster formula [Al-(Ni1/4Co1/4Fe1/4Cr1/4)14]Al1 was settled. [Al-(Ni1/(1+x+y+z)Cox/(1+x+y+z)Fey/(1+x+y+z)Crz/(1+x+y+Z))14]Al1 (Series R2, 1≤x≤2, 1≤y≤2, 1≤z≤2) and [Al-(Nix/(1+x)Co1/(1+x))8(Fey/(1+y)Cr1(1+y))6]Al1 (Series R3, 1/7≤x≤7, 1/5≤y≤5) were developed from the basic cluster formula by adjusting the proportion of Ni, Co, Fe, Cr. Then systematical experiments were conducted to verify the alloys’performances and the feasibility of cluster model in multi-principal-element solid solution alloy system.Alloy rods of (?)6mm were prepared using arc-melting furnace and copper-mould suction-casting method in argon atmosphere. Structural identification, micro structure and elemental distributions were investigated by means of XRD, OM, SEM and EPMA respectively. Mechanical property results were carried out on HV micro-hardness facility and MTS universal test machine. The main conclusions are:(1) Results of the alloy series Rl-Alx[Ni1/4Co1/4Fe1/4Cr1/4]16-x show that structure of as-cast alloys change with the increasing Al content:alloys maintain FCC structure whenx≤1.5; alloys stay BCC structure when x≥2.5; alloys form (FCC+BCC) in range of 1.5<x<2.5. Cluster formula[Al-(Ni1/4Co1/4Fe1/4Cr1/4)14]Al1(Al12.5Ni21.875Co21.875Fe21.875Cr21.875 at.%) in series R1 exhibits the micro structure of FCC dendrites and BCC inter-dendrites, whose Al content is in accordance with the critical content in FCC/BCC structure. It suggests the rationality to make further optimization based on cluster formula [A1-M14]Al1,where M=Ni1/4Co1/4Fe1/4Cr1/4.(2) Structure of alloys also vary with proportion of transition metal elements M (M=Ni, Co, Fe, Cr), as it shows from results of the series R2-[Al-M14]Al1:while valence electron concentration VEC increasing, structures evolve as (BCC/B2)â†'(BCC+FCC) binary phasesâ†' FCC phase solid solution. BCC/B2 structure alloy [Al-(Ni1/5Co1/5Fe1/5Cr2/5)14]Al (Al12.5Ni17.5Co17.5Fe17.5Cr35 at.%) performs highest HV and yielding strength of all, HV=626,σy=1791MPa; FCC structure alloy[Al-(Ni4Co4Fe3Cr3)14]Al (Al12.5Ni25Co25Fe18.75Cr18.75 at.%) owns better tensile strength and ductility, with ultimate tensile strengthσb=774MPa and final elongation δ=35.6%.(3) In the series R3-[Al-(Nix/(1+x)Co1/(1+x))8(Fey/(1+y)Cr1/(1+y))6]Al1, the 8 (Ni+Co) atoms and 6 (Fe+Cr) atomsat shell position of CN14 cluster were fixed, but each proportion of Ni/Co and Fe/Cr has been adjusted. R3 alloys exhibit higher yielding strength than series R1, R2 alloys in the same VEC range. Results suggest that transition metal elements Ni, Co, Fe, Cr have distinguishing effects towards structure and properties of multi-principal-element alloys, which means transition metal elements should be considered specifically.(4) It reveals that structure stability of multi-principal-element solid solution alloy is closely linked with valence electron concentration VEC, from the results of alloy series R1-Alx[Ni1/4Co1/4Fe1/4Cr1/4]16-x, R2-[Al-(Ni1/(1+x+y+z)Cox/(1+x+y+z)Fey/(1+x+y+z)Crz/(1+x+y+z))14]Al1, R3-[Al-(Nix/(1+X)Co1/(1+x))8(Fey/(1+y)Cr1/(1+y))6]Al1, furthermore, mechanical properties also vary as structure changes. Alloy forms FCC structure when VEC≥7.61, yielding strength varies from 318MPa to 520MPa; Alloy forms both FCC and BCC structure when 7.46<VEC<7.61, proportion of BCC phase increasesas VEC decreases, and yielding strength also increases from 607MPa to 1100MPa; Alloy forms BCC structure when VEC≤7.46, yielding strengthvaries from 1340MPa to 1791MPa. |
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