Dynamically reinforced heterogeneous grain structure prolongs ductility in a medium-entropy alloy with gigapascal yield strength

Abstract

SignificanceBack stress hardening is usually not obvious in single-phase homogeneous grains, but can be made unusually large and sustained to large tensile strains by creating an unusually heterogeneous grain structure in single-phase alloys with low stacking fault energy (SFE), as demonstrated here for the face-centered cubic CrCoNi medium-entropy alloy. The low SFE facilitates the generation of twinned nanograins and stacking faults during tensile straining, dynamically reinforcing the heterogeneity. Large uniform tensile strain can be achieved after yielding even at gigapascal stress, in the absence of heterogeneities from any second phase.

Tailoring heterogeneities in high-entropy alloys to promote strength–ductility synergy

“…1 (data points A through E (refs. [22][23][24][25] ) and I 26 ). These HEAs 30 offer high σ y and ε u that are clearly superior to others.…”

“…One therefore naturally wonders if concentrated alloys can offer (σ y , ε u ) beyond current benchmark ranges [12][13][14][15][16][17][18][19][20][21] . This proposition brings us to the so-called high-entropy alloys (HEAs), also called multi-principal component alloys or complexconcentrated alloys [22][23][24][25][26] . For a systematic introduction to this new paradigm recently emerged in metallurgy, the readers are referred to several review articles [27][28][29][30][31] .…”

“…Heterogeneities in HEAs. Figure 2 highlights new schemes of nanostructured heterogeneities 22,23,26,33,[36][37][38][39] . Our intent here is to call the readers' attention to the multilevel heterogeneities enabled by HEAs, and explain how they influence dislocation motion to elevate σ y to the gigapascal level, while boosting strain hardening and retaining ε u that rivals simple metals.…”

“…Finally, the grain-size distribution may be intentionally made multimodal or graded (e.g., via partial recrystallization) with length scales from a few nanometers up to the micrometer level. These five levels of heterogeneities, both A,C : [22] B : [23] D : [24] E : [25] I : [26] 1.0 Yield strength versus uniform tensile strain. The yellow-shaded area under the banana-shaped dotted curve covers the strength-ductility data of typical conventional metals.…”

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

“…However, the coarse-grained high-entropy alloys (HEAs) with a face-centered cubic (FCC)-structured single phase usually display good ductility but relative low yield strength at ambient temperatures [2]. Refining the grains to ultrafine scale (UFG) as well as the introduction of high-density nano-precipitates (such as coherent γ′ phase with L1 2 structure) in the FCC-structured matrix phase have proved to be effective methods in enhancing yield strength to a gigapascal level [3][4][5][6][7][8]. However, in such cases, the ductility (UE) is generally scarified severely because phase transformation (transformation-induced plasticity (TRIP)) and twinning (twinning-induced plasticity (TWIP)) all lose their potent in these ultra-fine-grained as well as high-density nano-particles strengthened alloys [3][4][5][6][7][8].…”