The strength-ductility trade-off has been a long-standing dilemma in materials science. This has limited the potential of many structural materials, steels in particular. Here we report a way of enhancing the strength of twinning-induced plasticity steel at no ductility trade-off. After applying torsion to cylindrical twinning-induced plasticity steel samples to generate a gradient nanotwinned structure along the radial direction, we find that the yielding strength of the material can be doubled at no reduction in ductility. It is shown that this evasion of strength-ductility trade-off is due to the formation of a gradient hierarchical nanotwinned structure during pre-torsion and subsequent tensile deformation. A series of finite element simulations based on crystal plasticity are performed to understand why the gradient twin structure can cause strengthening and ductility retention, and how sequential torsion and tension lead to the observed hierarchical nanotwinned structure through activation of different twinning systems.

In this study, we report the effect of elemental combinations on the friction stress and Hall-Petch relationship in medium entropy alloys (MEAs) and high entropy alloys (HEAs) which are defined as the alloys composed of four or less and five or more principal elements, respectively, with (near-) equiatomic concentrations. The MEAs (CoCrFeNi, CoCrNi, etc.), which are subsystems of equi-atomic CoCr-FeMnNi HEA, were highly deformed by high-pressure torsion (HPT) and subsequently annealed at different temperatures. The specimens with fully-recrystallized microstructures of FCC single-phase with various mean grain sizes down to sub-micrometer scale were obtained. Subsequently, tensile tests were performed at room temperature to obtain precise Hall-Petch relationships and friction stresses of the materials. Co-20(CrNi)(80) was successfully predicted as the alloy showing the highest strength among the MEAs by the modified Labusch model (so-called mean field Labusch model) for solution hardening. Experimental values of the friction stresses were found to fit with the model very well, indicating that the strength of the alloys was closely related to entirely distorted crystal lattice acting as high-density obstacles for dislocation motion in the alloys. At the same time, values of the average lattice distortion in the alloys were found comparable to those in some dilute alloys, although "severe"" lattice distortion had been believed as a reason for the higher strength than dilute systems. Finally, a strengthening mechanism by element-element interaction was proposed as an additional mechanism to enhance the strength in FCC HEAs and MEAs. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd.

Bulk ultrafine-grained (UFG) CoCrFeMnNi high-entropy alloy (HEA) with fully recrystallized microstructure was processed by cold rolling and annealing treatment. The high-cycle fatigue behaviors of the UFG HEA and a coarse-grained (CG) counterpart were investigated under fully reversed cyclic deformation. The fatigue strength of the UFG HEA can be significantly enhanced by refining the grain size. However, no grain coarsening was observed in the UFG HEA during fatigue tests. Mechanisms for the superior mechanical properties of the UFG HEA were explored.

High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m(1/2). Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening. Copyright © 2014, American Association for the Advancement of Science.

C-N co-doped interstitial high entropy alloy (iHEA) was reported to have high strength and ductility. However, iHEA with fully recrystallized ultrafine grains (UFGs) and underlying thermally activated processes associated with dislocation slip, twinning, and solute drag have not been reported yet. In this work, a C-N co-doped iHEA with nominal composition Fe_48.5Mn₃₀Co₁₀Cr₁₀C_0.5N_1.0 (at.%) was prepared, and the microstructures were tuned by cold-rolling and annealing treatments to improve mechanical properties. Upon cold-rolling with a strain of 1.74, the main microstructures in the iHEA are composed of nano-grains, nano-twins, HCP laminates, and high density of dislocations, leading to ultrahigh hardness of 466.7 HV and tensile strength of 1730 MPa at the expense of ductility (2.44%). Both the nanostructures and the high hardness of the iHEA can be maintained up to an annealing temperature of 600 &#x000b0;C (462.5 HV). After annealing at 650 &#x000b0;C for 1 h, the UFG microstructures are obtained in the iHEA, containing recrystallized grains with an average grain size of 0.91 &#x000b5;m and nanoprecipitates with an average diameter of 90.8 nm. The combined strengthening and hardening effects of UFGs, nanoprecipitates, twinning, and solutes contribute to high strain hardening (<em>n</em> = 0.81), gigapascal yield strength (984 MPa), and good ductility (20%). The C-N co-doping leads to a strong drag effect on dislocation slip, resulting in a nano-scale mean free path of dislocation slip &#x003bb;&#x000af; (1.44 nm) and much small apparent activation volume V&#x0002A; (15.8<em>b³</em>) of the UFG iHEA.

High-entropy alloys (HEAs) consisting of multiple principle elements provide an avenue for realizing exceptional mechanical, physical and chemical properties. We report a novel strategy for designing a new class of HEAs incorporating the additional interstitial element carbon. This results in joint activation of twinning-and transformation-induced plasticity (TWIP and TRIP) by tuning the matrix phase's instability in a metastable TRIP-assisted dual-phase HEA. Besides TWIP and TRIP, such alloys benefit from massive substitutional and interstitial solid solution strengthening as well as from the composite effect associated with its dual-phase structure. Nanosize particle formation and grain size reduction are also utilized. The new interstitial TWIP-TRIP-HEA thus unifies all metallic strengthening mechanisms in one material, leading to twice the tensile strength compared to a single-phase HEA with similar composition, yet, at identical ductility.

High-entropy alloys offer a promising alternative in several high-technology applications concerning functional, safety and health aspects. Many of these new alloys compete with traditional structural materials in terms of mechanical characteristics. Understanding and controlling their properties are of the outmost importance in order to find the best single-or multiphase solutions for specific uses. Here, we employ first-principles alloy theory to address the micro-mechanical properties of five polymorphic high-entropy alloys in their face-centered cubic (fcc) and hexagonal close-packed (hcp) phases. Using the calculated elastic parameters, we analyze the mechanical stability, elastic anisotropy, and reveal a strong correlation between the polycrystalline moduli and the average valence electron concentration. We investigate the ideal shear strength of two selected alloys under shear loading and show that the hcp phase possesses more than two times larger intrinsic strength than that of the fcc phase. The derived half-width of the dislocation core predicts a smaller Peierls barrier in the fcc phase confirming its increased ductility compared to the hcp one. The present theoretical findings explain a series of important observations made on dual-phase alloys and provide an atomic-level knowledge for an intelligent design of further high-entropy materials.

<p>Ultra-high strength alloys with good ductility are ideal materials for lightweight structural application in various industries. However, improving the strength of alloys frequently results in a reduction in ductility, which is known as the strength-ductility trade-off in metallic materials. Current alloy design strategies for improving the ductility of ultra-high strength alloys mainly focus on the selection of alloy composition (atomic length scale) or manipulating ultra-fine and nano-grained microstructure (grain length scale). The intermediate length scale between atomic and grain scales is the dislocation length scale. A new alloy design concept based on such dislocation length scale, namely dislocation engineering, is illustrated in the present work. This dislocation engineering concept has been successfully substantiated by the design and fabrication of a deformed and partitioned (D&P) steel with a yield strength of 2.2 GPa and an uniform elongation of 16%. In this D&P steel, high dislocation density can not only increase strength but also improve ductility. High dislocation density is mainly responsible for the improved yield strength through dislocation forest hardening, whilst the improved ductility is achieved by the glide of intensive mobile dislocations and well-controlled transformation-induced plasticity (TRIP) effect, both of which are governed by the high dislocation density resulting from warm rolling and martensitic transformation during cold rolling. In addition, the present work proposes for the first time to apply such dislocation engineering concept to the quenching and partitioning (Q&P) steel by incorporating a warm rolling process prior to the quenching step, with an aim to improve simultaneously the strength and ductility of the Q&P steel. It is believed that dislocation engineering provides a new promising alloy design strategy for producing novel strong and ductile alloys.</p>

A wide variety of industrial applications require materials with high strength and ductility. Unfortunately, the strategies for increasing material strength, such as processing to create line defects (dislocations), tend to decrease ductility. We developed a strategy to circumvent this in inexpensive, medium manganese steel. Cold rolling followed by low-temperature tempering developed steel with metastable austenite grains embedded in a highly dislocated martensite matrix. This deformed and partitioned (D and P) process produced dislocation hardening but retained high ductility, both through the glide of intensive mobile dislocations and by allowing us to control martensitic transformation. The D and P strategy should apply to any other alloy with deformation-induced martensitic transformation and provides a pathway for the development of high-strength, high-ductility materials.Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

High-entropy alloys (HEAs) have attracted considerable research attention in the material field because of their outstanding mechanical properties. For metallic materials, grain boundary plays a crucial role in the mechanical behavior and plastic deformation mechanisms. To show the effect of grain boundary on deformation mechanisms in HEAs, the mechanical behavior and evolution of deformation systems in the equiatomic FeMnCoCrNi HEA bicrystals with various orientation combinations during uniaxial tension are investigated using molecular dynamic simulations, and the effect of the orientation relationship between the grain boundary and tensile direction on mechanical behavior is demonstrated. The findings reveal that for all models studied, dislocations nucleate preferentially at the grain boundary and slip into the grains on both sides. Grain boundaries are widened and curved during deformation. Necking tends to occur at the grain boundary when the grain boundary is perpendicular to the tensile direction, which decreases flow stress with increasing loading. For the model with a grain boundary parallel to the deformation direction, the model's flow stress remains at a level above 1 GPa during the whole plastic deformation. The bicrystal with a combination of [111] and [110] orientations shows the most significant fluctuation of flow stress and the highest work hardening ability compared with other models. The decrease in stress with deformation is due to the slip of numerous dislocations, while the high strain hardening ability is caused by the formation of ε-martensite, stacking faults, and twins. Furthermore, the deformation behavior of FeMnCoCrNi, FeCuCoCrNi HEAs, and pure Cu are compared. Compared with Cu, the larger lattice distortion in FeMnCoCrNi and FeCuCoCrNi HEAs makes the grain boundaries coarser, which makes dislocations easy to nucleate under loading, and the formation of ε-martensite is the most outstanding in FeMnCoCrNi HEA with a lower stacking fault energy. The results of this study can guide the design of microstructures and orientations in high-performance HEAs with micron- and nanoscaled grains.

为了揭示高熵合金中晶界对塑性变形机制的影响，利用分子动力学模拟方法研究了具有不同初始取向组合的等主元FeMnCoCrNi高熵合金双晶在单轴拉伸变形中的力学性能与变形系统演化，并揭示了晶界与拉伸方向的位向关系对高熵合金力学行为的影响。结果表明，对研究的所有双晶模型而言，位错优先在晶界处形核并向两侧的晶粒内滑移。在变形过程中，晶界发生了不同程度的宽化和弯曲。当晶界与拉伸方向垂直时，颈缩易于在晶界处发生，这导致双晶的流变应力随外加载荷增大而降低。而当晶界平行于拉伸方向时，在整个塑性变形过程中模型保持1 GPa以上的流变应力。相对于其他双晶而言，[111]与[110]取向组合的双晶流变应力波动幅度最大，同时呈现出最强的加工硬化能力。其中应力的下降归因于大量的位错发生了滑移，而高的硬化能力则是由较多的ε-马氏体、层错以及孪晶形成所致。此外，还对比了FeMnCoCrNi、FeCuCoCrNi和纯Cu 3种材料的变形行为。与Cu相比，FeMnCoCrNi和FeCuCoCrNi高熵合金中的晶格畸变使晶界更加粗糙，这使得外加载荷作用下位错易于形核，且层错能较低的FeMnCoCrNi中形成的ε-马氏体最多。