中程尺度类晶体结构决定非晶合金慢二次弛豫

二次弛豫特别是慢β弛豫和结构不均匀性是非晶合金悬而未决的关键问题.尽管经历了几十年的深入研究,慢β弛豫与结构不均匀性之间的本征关联及其成分依赖性尚未建立.本文以镝基块体非晶合金为载体,通过微量掺杂不同类金属元素包括硼、碳、氮和硅,实现了β弛豫的有效调控,并且揭示了其原子尺度结构起源.通过对比研究混合焓、弹性不均匀度、相互作用强度和不完美序结构(或类晶体结构)对β弛豫的影响,发现β弛豫强度与中程类晶体结构的含量密切相关(与混合焓没有显著的对应关系),这表明原子的链状协同运动发生在含有类晶体结构的中程序软区.此外,镝元素和类金属元素之间的相互作用强度对于类晶体结构的形成至关重要.本工作对于理解β弛豫的原子结构起源和调控非晶合金性能具有重要意义.

Magnetic behaviors and magnetocaloric effects in rare earth high-entropy amorphous/nanocrystalline alloys

Rare earth high-entropy alloys(RE-HEAs)exhibit great potential to be applied as refrigerants due to their good comprehensive magnetocaloric properties.In this work,octary GdTbDyHoErTmCoAl and GdTbDyHoErTmCoNi RE-HEAs with amorphous/nanocrystalline structure exhibiting comparable magnetocaloric effect were synthesized.Both RE-HEAs show a second-order magnetic phase transition in the temperature range of hydrogen liquefaction.Due to the complex magnetic interactions,a spin glasslike behavior at low temperatures is observed in the RE-HEAs.A superior magnetocaloric effect is obtained in the nanocrystalline GdTbDyHoErTmCoNi high-entropy alloy that is multiphase attributed to a stronger magnetic exchange interaction when compared with the other that exhibits single amorphous structure.Despite heterogeneous microstructure,homogeneous chemical distributions are observed in the partially crystallized high-entropy alloy.In addition,the magnetocaloric effect and magnetic transition behavior of rare earth medium-and high-entropy alloys,including the RE-HEAs in this study,are summarized and discussed.The results in this work provide a helpful guide for the design of RE-HEAs for hydrogen liquefaction applications with excellent magnetocaloric effects.

Critical state-induced emergence of superior magnetic performances in an iron-based amorphous soft magnetic composite

Soft magnetic composites(SMCs)play a pivotal role in the development of high-frequency,miniaturization and complex forming of modern electronics.However,they usually suffer from a trade-off between high magnetization and good magnetic softness(high permeability and low core loss).In this work,utilizing the order modulation strategy,a critical state in a FeSiBCCr amorphous soft magnetic composite(ASMC),consisting of massive crystal-like orders(CLOs,∼1 nm in size)with the feature ofα-Fe,is designed.This critical-state structure endows the amorphous powder with the enhanced ferromagnetic exchange interactions and the optimized magnetic domains with uniform orientation and fewer micro-vortex dots.Superior comprehensive soft magnetic properties at high frequency emerge in the ASMC,such as a high saturation magnetization(Ms)of 170 emu g^(-1)and effective permeability(µ_(e))of 65 combined with a core loss(Pcv)as low as 70 mW cm^(-3)(0.01 T,1 MHz).This study provides a new strategy for the development of high-frequency ASMCs,possessing suitable comprehensive soft magnetic performance to match the requirements of the modern magnetic devices used in the third-generation semiconductors and new energy fields.

Polyamorphism mediated by nanoscale incipient concentration wave uncovering hidden amorphous intermediate state with ultrahigh modulus in nanostructured metallic glass

Comprehending the pressure-/temperature-induced structural transition in glasses,as one of the most fascinating issues in material science,is far from being well understood.Here,we report novel polyamorphic transitions in a Cu-based metallic glass(MG)with apparent nanoscale structural heterogeneity relating to proper Y addition.The low-density MG compresses continuously with increasing pressure,and then a compression plateau appears after∼8.1 GPa,evolving into an intermediate state with an ultrahigh bulk modulus of∼467 GPa.It then transforms to a high-density MG with significantly decreased structural heterogeneity above∼14.1 GPa.Three-dimensional atom probe tomography reveals concentration waves of Cu/Zr elements with an average wavelength of∼5-6 nm,which promote the formation of interconnected ringlike networks composed of Cu-rich and Zr-rich dual-glass domains at nanometer scale.Our experimental and simulation results indicate that steplike polyamorphism may stem from synergic effects of the abnormal compression of the Zr-Zr bond length at the atomic scale and the interplay between the applied pressure and incipient concentration waves(Cu and Zr)at several nanometer scales.The present work provides new insights into polyamorphism in glasses and contributes to the development of high-performance amorphous materials by high-pressure nanostructure engineering.

Liquid dynamics and glass formation of Gd_(55)Co_(20)Al_(25)metallic glass with minor Si addition

Liquid dynamics plays an essential role in glass formation.Here we observed a distinct change of liquid dynamics in Gd_(55)Co_(20)Al_(25)metallic glass induced by microalloying Si element.In the equilibrium melt,minor Si(0.5 at.%)addition leads to a more fragile liquid behavior and a smaller strength of liquid-liquid transition with the transition strength(F)decreasing from 0.76 to 0.35.However,in the supercooled liquid,Si-doped liquid exhibits a remarkable enhanced fragile-to-strong transition(FST),and the value of FST factor f increases sharply from 1.63 to 3.84,resulting in a stronger liquid behavior and more sluggish crystallization kinetics for Gd_(55)Co_(20)Al_(24.5)Si_(0.5)metallic glass.Moreover,minor Si addition promotes the formation of a crystal-like structure with a size of 1-2 nm.The interactions between the crystal-like structures and other local favored clusters frustrate the further growth of crystal-like phases,thus stabilizes the amorphous structure.As a result,the glass-forming ability(GFA)was largely improved.The critical diameter of Gd_(55)Co_(20)Al_(25)metallic glass increased from 2 to 7 mm with 0.5 at.%Si addition without deterioration of the magnetocaloric effect.This study provides valuable insight for understanding the distinct effect of microalloying on GFA of metallic glasses from the aspect of the evolution of the liquid.

Heterogeneous GdTbDyCoAl high-entropy alloy with distinctive magnetocaloric effect induced by hydrogenation

Developing novel magnetocaloric materials is of great significance for the applications of magnetic refrigeration.In this study,we designed a heterogeneous rare-earth-based high-entropy alloy(HEA)comprising amorphous matrix,local crystal-like cluster and nanocrystalline dihydride with average size of 7.5 nm through isothermal hydrogenation.This heterogeneous structure can significantly tune the magnetocaloric effect of alloy.After hydrogenation,the predominant exchange interaction transforms from ferromagnetic to antiferromagnetic with the disappearance of spin-glass-like behavior,and a complete second-order magnetic transition is obtained.Compared with the Gd_(20)Tb_(18)Dy_(18)Co_(20)Al_(24) high-entropy metallic glass with a small number of nanocrystals,the maximum magnetic entropy change of the hydrogen-containing HEA is increased from 8.8 to 13.6 J kg^(-1)K^(-1) under applied magnetic field change of 5 T accompanying unobvious hysteresis and decreased magnetic transition temperature from 59 to 8 K,which is more promising as magnetic refrigerant at cryogenic temperature.This work provides a novel concept of designing heterogeneous structure in terms of special cluster and preferential nanocrystalline to modulate the properties of metallic glasses.

变形和能量注入协同作用制备可控非晶纳米晶结构超高饱和磁化强度FeCoB粉末

非晶纳米晶磁粉芯(ANMPCs)是电子元器件在高频应用的关键软磁材料.但其相对较低的饱和磁化强度无法满足器件的小型化要求.强Fe-Co交换耦合非晶纳米晶粉是制备高磁化强度ANMPCs的潜在材料.本文采用机械球磨法制备了一系列(Fe_(0.8)Co_(0.2))_(87)B_(13)软磁粉末,这些粉末的非晶相、纳米晶相含量和粒径可调.在球磨过程中,出现了合金化、玻璃化和纳米化三个阶段以及粉末形貌和尺寸的非线性变化,进而导致ANMPCs软磁性能的非单调变化.磁粉具有超高饱和磁化强度达239 emu g^(-1),有效磁导率为33.球磨150 h后,FeCoB粉末非晶-纳米晶双相结构达到动态平衡,同时粒径达亚微米尺度,从而提高了粉末高频稳定性且降低了磁芯损耗.在变形和能量注入的协同作用下,机械球磨实现了含极高铁磁元素的非晶-纳米晶双相粉末可控制备、提高了高磁化强度ANMPCs综合软磁性能.