Ti−8Al−1Mo−1V合金等轴α相的静态粗化行为

为深入了解Ti−8Al−1Mo−1V钛合金等轴α相在退火过程中的演变行为,基于粗化指数n及α界面迁移激活能的计算对其静态粗化机制进行研究。n=6的结果表明,等轴α相具有特殊的粗化机制。计算得到的α界面迁移激活能为138 kJ/mol,接近纯钛的晶粒长大激活能。这表明等轴α的粗化主要是由钛原子穿越α/α界面的自扩散导致。基于研究结果,建立等轴α相在α+β相区的静态粗化动力学模型。最后,分析等轴α相粗化行为对拉伸性能的影响。

选区激光熔化成形颗粒增强铝基复合材料综述

选区激光熔化(SLM)是一种新兴逐层增材制造技术,能够生成具有三维复杂结构的高性能构件。颗粒增强铝基复合材料(PAMCs)由于结合铝基体和增强相的优异特性在很多领域被认为是一种重要材料。基于SLM和PAMCs的综合优势,新型SLMPAMCs在近年来被陆续开发和研究。因而,本文总结当前有关SLMPAMCs的研究进展。首先,着重分析SLM PAMCs的凝固行为。其次,对高性能SLM PAMCs设计和制造中涉及的重要问题进行全面的讨论,其中包括增强相的选择、工艺参数对成形和微观结构的影响以及缺陷演变和相调控。第三,对SLM PAMCs的性能和增强机制进行系统讨论。最后,对高性能SLM PAMCs未来发展方向提出建设性的观点。

Experimental and theoretical studies on two-dimensional vanadium carbide hybrid nanomaterials derived from V_(4)AlC_(3) as excellent catalyst for MgH_(2)

Hydrogen is considered one of the most ideal future energy carriers.The safe storage and convenient transportation of hydrogen are key factors for the utilization of hydrogen energy.In the current investigation,two-dimensional vanadium carbide(VC) was prepared by an etching method using V_(4)AlC_(3) as a precursor and then employed to enhance the hydrogen storage properties of MgH_(2).The studied results indicate that VC-doped MgH_(2) can absorb hydrogen at room temperature and release hydrogen at 170℃. Moreover,it absorbs 5.0 wt.%of H_(2) within 9.8 min at 100℃ and desorbs 5.0 wt.% of H_(2) within 3.2 min at 300℃.The dehydrogenation apparent activation energy of VC-doped MgH_(2) is 89.3 ± 2.8 kJ/mol,which is far lower than that of additive-free MgH_(2)(138.5 ± 2.4 kJ/mol),respectively.Ab-initio simulations showed that VC can stretch Mg-H bonds and make the Mg-H bonds easier to break,which is responsible for the decrease of dehydrogenation temperature and conducive to accelerating the diffusion rate of hydrogen atoms,thus,the hydrogen storage properties of MgH_(2) are remarkable improved through addition of VC.

Accelerating matrix/boundary precipitations to explore high-strength and high-ductile Co_(34)Cr_(32)Ni_(27)Al_(3.5)Ti_(3.5) multicomponent alloys through hot extrusion and annealing

Annealing-regulated precipitation strengthening combined with cold-working is one of the most efficient strategies for resolving the conflict between strength and ductility in metals and alloys.However,precipitation control and grain refinement are mutually contradictory due to the excellent phase stability of multicomponent alloys.This work utilizes the high-temperature extrusion and annealing to optimize the microstructures and mechanical properties of the Co_(34)Cr_(32)Ni_(27)Al_(3.5)Ti_(3.5) multicomponent alloy.Hot extrusion effectively reduces grain sizes and simultaneously accelerates the precipitation of coherent L12 nanoparticles inside the face-centered cubic(FCC)matrix and grain boundary precipitations(i.e.,submicron Cr-rich particles and L12-Ni 3(Ti,Al)precipitates),resulting in strongly reciprocal interaction between dislocation slip and hierarchical-scale precipitates.Subsequent annealing regulates grain sizes,dislocations,twins,and precipitates,further allowing to tailor mechanical properties.The high yield strength is attributed to the coupled precipitation strengthening effects from nanoscale coherent L12 particles inside grains and submicron grain boundary precipitates under the support of pre-existing dislocations.The excellent ductility results from the synergistic activation of dislocations,stacking faults,and twins during plastic deformation.The present study provides a promising approach for regulat-ing microstructures,especially defects,and enhancing the mechanical properties of multicomponent alloys.

Enhanced mechanical performance of gradient-structured CoCrFeMnNi high-entropy alloys induced by industrial shot-blasting

In this study,CoCrFeMnNi high-entropy alloys(HEAs)with a surface gradient nanostructure were produced using industrial shot blasting,which improved their mechanical properties compared to the untreated alloy.The severely plastically deformed(SPD)surface layer had a multi-scale hierarchical structure with a high density of stacking faults,deformation nanotwins,and amorphous domains.The depth of the SPD layer steadily increased as the shot-blasting time increased.The differences in the microhardness and tensile strength before and after shotblasting demonstrated the significant effect of the SPD layer on the mechanical performance.The microhardness of the homogenized HEA was~5 GPa.In comparison,the maximum microhardness of the specimens after 20 min of shot blasting was~8.0 GPa at the surface.The yield strength also improved by 178%,and a large ductility of~36%was retained.Additional nanograin boundary,stacking fault,and twin strengthening within the gradientnanostructured surface layer caused the strength to increase.During tensile deformation,strain concentration began at the surface of the specimen and gradually spread to the interior.Thus,the gradient-nanostructured surface layer with improved strain hardening can prevent early necking and ensure steady plastic deformation so that high toughness is achieved.

New para-magnetic(CoFeNi)(50)(CrMo)(50-x)(CB)x(x=20,25,30)non-equiatomic high entropy metallic glasses with wide supercooled liquid region and excellent mechanical properties

In this study,high entropy metallic glasses(HEMGs)were developed through a combination of concepts for designing metallic glasses(main element+transition metal+metalloid element)and high-entropy alloys(more than five elements,each element having an atomic concentration between 5 at.%and 35 at.%).The developed metallic glass alloys are composed of Co-Fe-Ni main elements,transition metals(Cr,Mo)and metalloid elements(C,B).Moreover,the present work reports the thermal,mechanical and magnetic properties of(CoFeNi)(50)(CrMo)(50-x)(CB)x alloys with x=20,25,30.The developed as-spun HEMGs exhibit typical paramagnetic properties even for a high amount of ferromagnetic elements(Co,Fe,and Ni)and have high elastic modulus(103–160 GPa)and hardness(14–27 GPa),thus possessing mechanical properties similar to well-known Co-based metallic glasses(Co-Cr-Mo-C-B system).In addition,some of the bulk samples prepared with a diameter of 2 mm form bulk metallic glasses with a high compressive strength around 3.5 GPa.The mechanisms determining the stability of the supercooled liquid,as well as the paramagnetic and mechanical properties for the developed non-equiatomic HEMGs,are discussed.

Selective Laser Melting of Al-7Si-0.5Mg-0.5Cu:Effect of Heat Treatment on Microstructure Evolution,Mechanical Properties and Wear Resistance

Al-7Si-0.5Mg-0.5Cu alloy specimens have been fabricated by selective laser melting(SLM).In this study,the effects of solution treatment,quenching,and artifi cial aging on the microstructural evolution,as well as mechanical and wear properties,have been investigated.The as-prepared samples show a heterogeneous cellular microstructure with two different cell sizes composed ofα-Al and Si phases.After solution-treated and quenched(SQ)heat treatment,the cellular microstructure disappears,and coarse and lumpy Si phase precipitates and a rectangular Cu-rich phase were observed.Subsequent aging after solution-treated and quenched(SQA)heat treatment causes the formation of nanosized Cu-rich precipitates.The asprepared SLMs sample has good mechanical properties and wear resistance(compressive yield strength:215±6 MPa and wear rate 2×10^(-13)m^(3)/m).The SQ samples with lumpy Si particles have the lowest strength of 167±13 MPa and the highest wear rate of 6.18×10^(1-13)m^(3)/m.The formation of nanosized Cu-rich precipitates in the SQA samples leads to the highest compressive yield strength of 233±6 MPa and a good wear rate of 5.06×10^(-13)m^(3)/m.

Effect of Powder Particle Shape on the Properties of In Situ Ti–TiB Composite Materials Produced by Selective Laser Melting

This work studied the preparation of starting powder mixture influenced by milling time and its effect on the particle morphology （especially the shape） and, consequently, density and compression properties of in situ Ti-TiB composite materials produced by selective laser melting （SLM） technology. Starting powder composite system was prepared by mixing 95 wt% commercially pure titanium （CP-Ti） and 5 wt% titanium diboride （TiB2） powders and subsequently milled for two different times （i.e. 2 h and 4 h）. The milled powder mixtures after 2 h and 4 h show nearly spherical and irregular shape, respectively. Subsequently, the resultant Ti-5 wt% TiB2 powder mixtures were used for SLM processing. Scanning electron microscopy image of the SLM-processed Ti-TiB composite samples show needle-shape TiB phase distributed across the Ti matrix, which is the product of an in-situ chemical reaction between Ti and TiB2 during SLM. The Ti-TiB composite samples prepared from 2 h and 4 h milled Ti-TiB2 powders show different relative densities of 99.5% and 95.1%, respectively. Also, the compression properties such as ultimate strength and compression strain for the 99.5% dense composite samples is 1421 MPa and 17.8%, respectively, which are superior to those （883 MPa and 5.5%, respectively） for the 95.1% dense sample. The results indicate that once Ti and TiB2 powders are connected firmly to each other and powder mixture of nearly spherical shape is obtained, there is no additional benefit in increasing the milling time and, instead, it has a negative effect on the density （i.e. increasing porosity level） of the Ti-TiB composite materials and their mechanical properties.

Mechanical behavior and deformation mechanism of shape memory bulk metallic glass composites synthesized by powder metallurgy

The synthesis of martensitic or shape-memory bulk metallic glass composites(BMGCs)via solidification of the glass-forming melts requires the meticulous selection of the chemical composition and the proper choice of the processing parameters in order to ensure that the glassy matrix coexists with the desired amount of austenitic phase.Unfortunately,a relatively limited number of such systems,where austenite and glassy matrix coexist over a wide range of compositions,is available.Here,we study the effective-ness of powder metallurgy as an alternative to solidification for the synthesis of shape memory BMGCs.Zr_(48)Cu_(36)Al_(8)Ag_(8)matrix composites with different volume fractions of Ni_(50.6)Ti_(49.4)are fabricated using hot pressing and their microstructure,mechanical properties and deformation mechanism are investigated employing experiments and simulations.The results demonstrate that shape-memory BMGCs with tun-able microstructures and properties can be synthesized by hot pressing.The phase stability of the glass and austenitic components across a wide range of compositions allows us to examine fundamental as-pects in the field of shape memory BMGCs,including the effect of the confining stress on the martensitic transformation exerted by the glassy matrix,the contribution of each phase to the plasticity and the mechanism responsible for shear band formation.The present method gives a virtually infinite choice among the possible combinations of glassy matrices and shape memory phases,expanding the range of accessible shape memory BMGCs to systems where the glassy and austenitic phases do not form simul-taneously using the solidification route.

Structural homology of the strength for metallic glasses

The structure-property relationship,one of the central themes in materials science,is far from being well understood for metallic glasses(MGs)due to the great complexity of their amorphous structures.Based on the analysis of published experimental data for 165 MGs from more than 15 different alloy systems,the present study reveals a universal dependence of mechanical properties(Young’s moduli,shear moduli and yield strength)on simple structural parameters(the inter-atomic distance and/or valence electron density)originating from the interatomic potential and Fermi sphere-Brillouin zone interaction.This work establishes a structure-property relationship for metallic glasses and provides insights into the fundamentals of the mechanical properties of disordered systems.

Achieving work hardening by forming boundaries on the nanoscale in a Ti-based metallic glass matrix composite

Achieving work hardening in metallic glass matrix composites(MGMCs) is the key to the extensive use of these attractive materials in structural and functional applications.In this study,we investigated the formation of nanoscale boundaries resulted from the interaction between matrix and dendrites,which favors the work-hardening deformation in an in-situ Ti41Zr32Ni6 Ta7 Be14 MGMC with β-Ti dendrites in a glassy matrix at room temperature.The microstructures of samples after tension were observed by highresolution transmission electron microscopy(HRTEM) and X-ray diffraction(XRD).The work-hardening mechanism of the present composites involves:(1) appearance of dense dislocation walls(DDWs),(2)proliferation of shear bands,(3) fo rmation of boundaries on the nanoscale,and(4) interactions between hard and soft phases.A theoretical model combined with experimental data reveals the deformation mechanisms in the present work,proving that the in-situ dendrites with outstanding hardening ability in the glass matrix can provide the homogeneous deformation under tensile loading at room temperature.

Enhanced π-π Interactions Between a C_(60) Fullerene and a Buckle Bend on a Double-Walled Carbon Nanotube

In situ low-voltage aberration corrected transmission electron microscopy(TEM)observations of the dynamic entrapment of a C_(60) molecule in the saddle of a bent double-walled carbon nanotube is presented.The fullerene interaction is non-covalent,suggesting that enhancedπ-πinteractions(van der Waals forces)are responsible.Classical molecular dynamics calculations confirm that the increased interaction area associated with a buckle is sufficient to trap a fullerene.Moreover,they show hopping behavior in agreement with our experimental observations.Our findings further our understanding of carbon nanostructure interactions,which are important in the rapidly developing field of low-voltage aberration corrected TEM and nano-carbon device fabrication.

Inheritance factor on the physical properties in metallic glasses

Material genetic engineering can significantly accelerate the development of new materials.As an important topic in material science and condensed matter physics,the development of metallic glasses(MGs)with specific properties has largely been the result of trial and error since their discovery in 1960.Yet,property design based on the physical parameters of constituent elements of MGs remains a huge challenge owing to the lack of an understanding of the property inheritance from constitute elements to the resultant alloys.In this work,we report the inherent relationships of the yield strengthσ_(y),Young’s modulus E,and shear Modulus G with the valence electron density.More importantly,we reveal that the electronic density of states(EDOSs)at the Fermi surface(E_(F))is an inheritance factor for the physical properties of MGs.The physical properties of MGs are inherited from the specific element with the largest coefficient of electronic specific heat(γ_(i)),which dominates the value of the EDOS at E_(F).This work not only contributes to the understanding of property inheritances but also guides the design of novel MGs with specific properties based on material genetic engineering.