In human-made malleable materials, microdamage such as cracking usually limits material lifetime. Some biological composites, such as bone, have hierarchical microstructures that tolerate cracks but cannot withstand high elongation. We demonstrate a directionally solidified eutectic high-entropy alloy (EHEA) that successfully reconciles crack tolerance and high elongation. The solidified alloy has a hierarchically organized herringbone structure that enables bionic-inspired hierarchical crack buffering. This effect guides stable, persistent crystallographic nucleation and growth of multiple microcracks in abundant poor-deformability microstructures. Hierarchical buffering by adjacent dynamic strain-hardened features helps the cracks to avoid catastrophic growth and percolation. Our self-buffering herringbone material yields an ultrahigh uniform tensile elongation (~50%), three times that of conventional nonbuffering EHEAs, without sacrificing strength.Copyright © 2021 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Deformation twinning is rarely found in bulk face-centered cubic (FCC) alloys with very high stacking fault energy (SFE) under standard loading conditions. Here, based on results from bulk quasi-static tensile experiments, we report deformation twinning in a micrometer grain-sized compositionally complex steel (CCS) with a very high SFE of ~79 mJ/m, far above the SFE regime for twinning (<~50 mJ/m) reported for FCC steels. The dual-nanoprecipitation, enabled by the compositional degrees of freedom, contributes to an ultrahigh true tensile stress up to 1.9 GPa in our CCS. The strengthening effect enhances the flow stress to reach the high critical value for the onset of mechanical twinning. The formation of nanotwins in turn enables further strain hardening and toughening mechanisms that enhance the mechanical performance. The high stress twinning effect introduces a so far untapped strengthening and toughening mechanism, for enabling the design of high SFEs alloys with improved mechanical properties.© 2022. The Author(s).

The elastic constants, ideal tensile strength (ITS), stacking fault energy (SFE), lattice constant and magnetic moment of FeMnCoCrNi high entropy alloys with varying Co and Cr contents at 0 and 300 K were systematically investigated by first-principle calculations. For the alloys with Co substitution for Ni, at both temperatures the elastic stability of the face-centered cubic (fcc) phase, bulk elastic modulus (B), Young's modulus (E), shear modulus (G) and ITS increase monotonically with increasing Co content. However, the Cauchy pressure (CP), Pugh ratio (B/G), Poisson ratio (v), Zener anisotropy ratio (AZ) and elastic anisotropy ratio (AVR) decrease monotonically. The SFE also decreases with the increase of Co, resulting in the change of plastic deformation mechanism from dislocation slip to mechanical twinning, and then to hcp-martensitic transformation. This elucidates the underlying mechanism of the effect of Co addition on the strength and micromechanical behavior of FeMnCoCrNi alloys. Compared with Co, the Cr substitution for Ni leads to the more complex change of elastic constants and ITS. The increase of Cr shows the similar effect on SFE and deformation mechanism as that of Co. The variation of valence electron concentration and magnetism affect the SFE. The increase of either Co or Cr leads to the reduced magnetic moments of Fe and Mn. This could be responsible for the monotonic decrease of both lattice constant and SFE as the Co content increases. However, for the Cr addition case, multiple factors may affect the evolution of lattice constant and SFE. These findings shed light on the deformation mechanism of the alloys with different compositions.

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。

High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centred cubic solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature, the alloy shows tensile strengths of almost 1 GPa, failure strains of similar to 70% and K-JIc fracture-toughness values above 200 MPa m(1/2); at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and K-JIc values of 275 MPa m(1/2). Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning.

Multi-principal element alloys usually exhibit outstanding strength and toughness at cryogenic temperatures, especially in CrMnFeCoNi and CrCoNi alloys. These remarkable cryogenic properties are attributed to the occurrence of deformation twins, and it is envisaged that a reduced stacking fault energy (SFE) transforms the deformation mechanisms into advantageous properties at cryogenic temperatures. A recently reported high-strength VCoNi alloy is expected to exhibit further notable cryogenic properties. However, no attempt has been made to investigate the cryogenic properties in detail as well as the underlying deformation mechanisms. Here, the effects of cryogenic temperature on the tensile and impact properties are investigated, and the underlying mechanisms determining those properties are revealed in terms of the temperature dependence of the yield strength and deformation mechanism. Both the strength and ductility were enhanced at 77 K compared to 298 K, while the Charpy impact toughness gradually decreased with temperature. The planar dislocation glides remained unchanged at 77 K in contrast to the CrMnFeCoNi and CrCoNi alloys resulting in a relatively constant and slightly increasing SFE as the temperature decreased, which is confirmed via ab initio simulations. However, the deformation localization near the grain boundaries at 298 K changed into a homogeneous distribution throughout the whole grains at 77 K, leading to a highly sustained strain hardening rate. The reduced impact toughness is directly related to the decreased plastic zone size, which is due to the reduced dislocation width and significant temperature dependence of the yield strength.

Strong and ductile materials that have high resistance to corrosion and hydrogen embrittlement are rare and yet essential for realizing safety-critical energy infrastructures, hydrogen-based industries, and transportation solutions. Here we report how we reconcile these constraints in the form of a strong and ductile CoNiV medium-entropy alloy with face-centered cubic structure. It shows high resistance to hydrogen embrittlement at ambient temperature at a strain rate of 10 s, due to its low hydrogen diffusivity and the deformation twinning that impedes crack propagation. Moreover, a dense oxide film formed on the alloy's surface reduces the hydrogen uptake rate, and provides high corrosion resistance in dilute sulfuric acid with a corrosion current density below 7 μA cm. The combination of load carrying capacity and resistance to harsh environmental conditions may qualify this multi-component alloy as a potential candidate material for sustainable and safe infrastructures and devices.

Precipitation strengthening has been the basis of physical metallurgy since more than 100 years owing to its excellent strengthening effects. This approach generally employs coherent and nano-sized precipitates, as incoherent precipitates energetically become coarse due to their incompatibility with matrix and provide a negligible strengthening effect or even cause brittleness. Here we propose a shear band-driven dispersion of nano-sized and semicoherent precipitates, which show significant strengthening effects. We add aluminum to a model CoNiV medium-entropy alloy with a face-centered cubic structure to form the L2 Heusler phase with an ordered body-centered cubic structure, as predicted by ab initio calculations. Micro-shear bands act as heterogeneous nucleation sites and generate finely dispersed intragranular precipitates with a semicoherent interface, which leads to a remarkable strength-ductility balance. This work suggests that the structurally dissimilar precipitates, which are generally avoided in conventional alloys, can be a useful design concept in developing high-strength ductile structural materials.© 2021. The Author(s).

Boron doping with an adequate concentration is highly desirable for alloy development because it profoundly improves the material's interface cohesion via interfacial segregation. However, scientific and applicable potentials of soluble boron that resides at the alloy internal grains are generally overlooked. Here we report a strategy for overcoming the typically low strengths of face-centered cubic high-entropy alloys (HEAs) through exploiting soluble boron instead of the interfacial boron. We find that soluble boron increases stress strain field at the recrystallized HEA grain structure, leading to the generation of short-range order (SRO) in those deformation structure under load at 77 K. The highly increased degree of SRO at planar dislocation slip band that forms during straining, proved by electron microscopy and synchrotron X-ray diffraction, strengthens a typical non-equimolar Fe40Mn40Co10Cr10 (at%) HEA, particularly increasing yield strengths by similar to 32%, to similar to 1.1 GPa compared to those of boron-free reference materials with similar grain sizes. The advent of deformation-induced SRO domains causes severe lattice distortion (specifically, contraction), leading to the increased cryogenic yield strength of similar to 210 MPa, but generating micro-voids at grain boundaries. This study on deformation-induced SRO via boron advances the fundamental understanding of SRO impacts on HEA grains and mechanical properties at cryogenic temperatures, which may pave a general pathway for developing a wide range of ultrastrong alloys for cryogenic applications. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.

Multiferroic phase-transforming alloys demonstrate intriguing multicaloric effects, but they are intrinsically brittle and their elastocaloric effect shows poor cyclability, which remains a major challenge for exploiting more efficient multicaloric refrigeration. Here, by employing a novel strategy of strengthening grain boundary, the cyclability of elastocaloric effect in the prototype Ni-Mn-based multiferroic phase-transforming alloys is strikingly enhanced by two orders of magnitude. Ultrahigh cyclability of a large elastocaloric effect is achieved. This not only paves the way for exploiting multicaloric effects for more efficient cooling, but also provides a strategy for overcoming the cyclability issues in the ubiquitous brittle intermetallic phase-transforming materials. [GRAPHICS]IMPACT STATEMENT The cyclability of elastocaloric effect in the prototype Ni-Mn-based multiferroic phase-transforming alloys has been strikingly enhanced by two orders of magnitude through employing the strategy of strengthening grain boundary.

Growing attention has been placed on high-entropy alloys (HEAs) owing to their promising mechanical properties. Particularly, HEAs in which the main crystal structure is fcc are attracting significant attention. Although such alloys exhibit a good combination of strength and ductility, they cannot meet the increasing demands of applications because of limited yield strengths. In recent years, researchers have tried to improve yield strengths of HEAs by refining grains and introducing interstitial atoms. However, the processing cost is high and is often accompanied by the significant loss of ductility. In this study, we propose a simple processing route incorporating cold rolling at medium thickness reductions and short-time annealing at medium temperatures to obtain a heterogeneous structure in Fe-Mn based HEAs consisting of deformed grains with an average diameter of several tens of microns and recrystallized ultrafine grains. By simultaneously introducing multiple strengthening mechanisms, including the strengthening contributed by the microstructural characteristics of dense dislocations, grain refinement, precipitates, ε-martensite, α-martensite, and recovery twins, as well as the strengthening induced by deformation twinning and ε-martensite phase transition that occurs continuously during deformation, the yield strength of the alloy significantly increases compared with that of the fully recrystallized material and reaches 825 MPa. Simultaneously, due to the activation of significant deformation twinning and deformation-induced martensitic transformation, the uniform elongation of the alloy is about 28.6%. The proposed material fabrication method is simple, cost-effective, and can effectively improve the mechanical properties of Fe-Mn based HEAs, providing new insight into optimizing the mechanical properties of low stacking fault energy alloys of the fcc structure.

提出了一种简单的高熵合金加工工艺，即对Fe-Mn系高熵合金采用中等形变量冷轧和中温短时退火相结合的方法，获得了由晶粒尺寸为数十微米的形变晶粒和超细尺度再结晶晶粒组成的非均匀结构。通过向合金中同时引入由高密度位错、晶粒细化、析出相、ε-马氏体、α-马氏体和回复孪晶等微观结构特征及变形过程中持续发生的形变孪生、ε-马氏体相变引起的多种强化机制，使屈服强度较充分再结晶态显著提升并达到825 MPa。同时，在塑性变形过程中由于发生了显著的形变孪生和一定的由形变诱发的奥氏体向ε-马氏体转变，合金仍具有约28.6%的均匀延伸率，合金的综合力学性能得到有效提升。该工艺为优化以fcc结构为主的低层错能合金的力学性能提供了新思路。

对18Mn-3Al-3Si和21Mn-3Al-3Si高锰TRIP/TWIP效应共生钢动态变形过程中的变形行为, 应变硬化速率、真应力和应变硬化指数随真应变的变化, 以及应变硬化和基体软化间的相互作用等进行了研究, 采用OM, SEM, TEM和XRD等方法对变形前后的组织进行了分析. 结果表明, 高应变速率下, TRIP/TWIP效应共生钢应变诱发相变途径为&gamma;&rarr;&epsilon;&rarr;&alpha;; 高速变形对滑移的抑制、奥氏体向马氏体的相变和形变孪晶对奥氏体晶粒的细化是应变硬化的主要因素; 造成基体软化的原因是绝热温升效应、&epsilon;&rarr;&gamma;的逆相变和孪晶的动态再结晶.

研究了一种用于汽车车体的高强、高塑性中C-高Mn系孪晶诱发塑性(TWIP)钢, 有助于达到汽车减排、节能和安全的目的. 通过单向拉伸实验和OM观察, 分析研究了水淬工艺对TWIP钢的力学性能和微观组织的影响规律, 采用SEM和TEM对不同变形程度TWIP钢的精 细结构进行了分析. 结果表明, 随着水淬温度的提高, 退火孪晶体积分数和晶粒尺寸增大, 塑性、加工硬化性提高, 而试件的强度和屈强比降低, 可以获得抗拉强度960 MPa, 延伸率60.5%, 具有优异的综合力学性能(强塑积最高达6.096&times;10⁴ MPa?%)的试件; 具有大量退火孪晶的奥氏体在变形过程中产生大量的形变孪晶, 提高了TWIP钢的强度和塑性.

Toward an electronic level understanding of intergranular embrittlement and its control in steels, the effects of phosphorus and boron impurities on the energy and electronic properties of both an iron grain boundary and its corresponding intergranular fracture surface are investigated by the local density full potential augmented plane wave method. When structural relaxations are taken into account, the calculated energy difference of phosphorus in the two environments is consistent with its measured embrittlement potency. In contrast to the nonhybridized interaction of iron and phosphorus, iron-boron hybridization permits covalent bonding normal to the boundary contributing to cohesion enhancement. Insights into bonding behavior offer the potential for new directions in alloy composition for improvement of grain boundary-sensitive properties.