Friction stress and Hall-Petch relationship in CoCrNi equi-atomic medium entropy alloy processed by severe plastic deformation and subsequent annealing

Ultrastrong Medium‐Entropy Single‐Phase Alloys Designed via Severe Lattice Distortion

“…Room-temperature stress-strain curves reveal that the VCoNi alloy has a higher strength compared to the CrCoNi alloy (389 MPa for the grain size of 11.0 ± 6.7 µm) which is known as the single fcc solid solution alloy currently exhibiting the highest yield strength in a recrystallized state (460 MPa for the grain size of ≈6.5 µm) (Figure 2a). [17,18,21,22,26] Of particular significance is that the fine-grained VCoNi alloy (grain size of 2.0 ± 1.5 µm) reaches a yield strength of nearly 1 GPa despite being in a single fcc solid solution state. In addition, the alloy shows an ultimate tensile strength of 1359 MPa and a large total elongation of 38%, revealing also high strain hardening capability (Table S1, Supporting information).…”

“…This approach is in line with corresponding previous studies on solid solution distortion effects in HEAs and MEAs compared to conventional alloys using friction stress measurements. [20,[22][23][24][25][26][27][28] In addition, ab initio calculations have been recognized as a reliable method to investigate atomic scale solid solution structures and the associated local distortions. [39,40] First, the role of the atomic scale solid solution dependent lattice distortions on the mechanical properties was studied by performing tensile tests after varying the annealing conditions and under consideration of the alloy's average grain sizes (Table S1, Supporting information).…”

“…The calculated lattice friction stress (σ 0 ) of the VCoNi alloy is 383 MPa, which is almost three times higher than that of the previous medium-or high-entropy alloys and of conventional austenitic steels (Table S2, Supporting information). [20,[22][23][24][25][26][27] Therefore, the severe lattice distortion is indeed a most effective alloy design approach for enhancing strength through an adjusted solid solution blend as demonstrated here for the VCoNi alloy. It should also be noted that the Hall-Petch coefficient of 864 MPa µm 1/2 is remarkably large.…”

“…This means that the level of lattice distortion currently exploited in most substitutional solid solution alloys does not contribute significantly to the yield strength. [20,[22][23][24][25][26][27][28] Here, based on a combined theoretical and experimental approach, we show that the degree of lattice distortion is indeed a key parameter in controlling strengthening mechanisms for the design of hitherto unexplored ultrastrong medium-entropy single-phase alloys. For this purpose, we exploit vanadium (V) as a very efficient element in a Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering.…”

Superior strength-ductility combination of a Co-rich CoCrNiAlTi high-entropy alloy at room and cryogenic temperatures

“…Firstly, owing to the addition of 3 at% Al, 3 at% Ti elements, the solid-solution hardening can be estimated to be around 175 MPa [13,14]. Secondly, by accounting for the refined-grain strengthening effect, the yield stress of the alloy with mean grain size of 10 μm can be estimated by σ YS =σ 0 +kd −1/2 [15,16], in which σ 0 is the lattice friction (218 MPa for the CoCrNi alloy [17]); and K is the Hall-Petch constant (568 MPa/μm −1/2 for CoCrNi -AlTi alloy [18]) and d is the mean grain size. Based on the above calculation, the contribution to the yield stress from the refined-grain strengthening is determined to be about 390 MPa.…”