Nanostructure in a Ti alloy processed using a cryomilling technique

A nanocrystalline Ti alloy with a uniform distribution of grains was synthesized using cryogenic mechanical milling. The effects of cryomilling parameters, such as milling time and ball to powder ratio (BPR), on the particle size, grain size, chemistry, and structure of cryomilled Ti powders were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The experimental results show that nanocrystalline Ti powders with a grain size of about 20 nm can be prepared using the cryomilling technique. Compared to SPEX milling at room temperature, cryomilling led to lower contamination levels of oxygen, nitrogen, and iron in the cryomilled Ti powder. The average particle size initially increased from the original 55 μm to a maximum value of 125 μm after 2 h of milling, and then decreased to 44 μm after 8 h of milling. Both the average particle size and the grain size decreased as the BPR increased.     

Recent Advances on High-Entropy Alloys for 3D Printing

Boosted by the success of high-entropy alloys (HEAs) manufactured by conventional processes in various applications, the development of HEAs for 3D printing has been advancing rapidly in recent years. 3D printing of HEAs gives rise to a great potential for manufacturing geometrically complex HEA products with desirable performances, thereby inspiring their increased appearance in industrial applications. Herein, a comprehensive review of the recent achievements of 3D printing of HEAs is provided, in the aspects of their powder development, printing processes, microstructures, properties, and potential applications. It begins with the introduction of the fundamentals of 3D printing and HEAs, as well as the unique properties of 3D-printed HEA products. The processes for the development of HEA powders, including atomization and mechanical alloying, and the powder properties, are then presented. Thereafter, typical processes for printing HEA products from powders, namely, directed energy deposition, selective laser melting, and electron beam melting, are discussed with regard to the phases, crystal features, mechanical properties, functionalities, and potential applications of these products (particularly in the aerospace, energy, molding, and tooling industries). Finally, perspectives are outlined to provide guidance for future research.     

Influence of Al content on thermal stability of nanocrystalline AlxCoCrFeNi high entropy alloys at low and intermediate temperatures

Thermal stability of mechanically alloyed nanocrystalline Al_xCoCrFeNi (x = 0, 0.3, 0.6, 1 mol) high entropy alloys (HEAs) has been investigated for the low and intermediate temperature range of 673–1073 K. Single phase FCC structure is observed in the as milled CoCrFeNi. A mixture of FCC and BCC phases is exhibited by × = 0.3, 0.6 and 1, alloys where the volume fraction of BCC increases with increasing Al content. Phase evolution in heat-treated Al_xCoCrFeNi HEAs proceeds via increasing BCC fraction at 673 K, followed by subsequent reduction at elevated temperatures. For each alloy, the major phase observed in as milled condition and it is retained even after prolonged exposure at the 1073 K. Al favors the formation of the BCC phase due to its high affinity to form ordered B2 structures with constituent elements Co, Fe and Ni. Thermal exposure of Al_xCoCrFeNi HEAs also leads to the formation of Cr₇C₃, owing to the higher negative free energy of carbide formation for Cr among other constituents. Transmission electron microscopy (TEM) investigations substantiated that nanostructure of milled powder is maintained even after the heat treatment. Grain growth factor for quinary HEAs is relatively lower than quaternary CoCrFeNi owing to their slower rates of diffusion.     

Synthesis of nanocrystalline M50 steel powders by cryomilling

The present paper reports on a study of the synthesis of nanocrystalline high speed tool steel M50 powders (4.5% Mo, 4.0% Cr, 1.0% V, 0.8% C, balance Fe) by cryogenic high energy ball milling (cryomilling). Pre-alloyed M50 steel is spray atomized, and subsequently cryomilled in liquid nitrogen for 25 hours. Elemental Al powder is added prior to cryomilling to promote the formation of nanoscale Al₂O₃ and AlN dispersoids to improve the thermal stability of the nanocrystalline M50 steel. High resolution transmission electron microscopy (HRTEM) reveals the formation of various carbides (V₈C₇, Fe₃C, and FeC), oxides (Al₂O₃, MoO₃, and V₃O₇), and a nitride phase (AlN) during cryomilling. Following one hour of heat treatment at 1373 K (0.77 T_m), an average grain size of 70 nm was retained for the M50 steel powders.     

Optical properties of Bi12SiO20 (BSO) and Bi12TiO20 (BTO) obtained by mechanical alloying

Mechanical alloying has been used successfully to produce nanocrystalline powders of BTO and BSO. The milled BTO and BSO were studied by x-ray powder diffraction, DTA, infrared and Raman scattering spectroscopy. After 7 hours of milling the formation of BTO and BSO was confirmed by x-ray powder diffraction. The infrared and Raman scattering spectroscopy results suggest that the increase of the milling time lead to the formation of ferroelectric BTO and BSO, as seen by x-ray diffraction analysis. These materials are attractive for various electro-optical devices, including optical data processing. They present a number of attractive features as reversible recording materials for real-time holography and image processing applications. This milling process presents the advantage, that melting is not necessary, and the powder obtained is nanocrystalline with extraordinary mechanical properties. The material, can be compacted and transformed in solid piezoelectric ceramic samples. The high efficiency of the process, opens a way to produce commercial amount of nanocrystalline piezoelectric powders. Due to the nanocrystalline character of this powder, their mechanical properties have changed and for this reason a pressure of 1 GPa is enough to shape the sample into any geometry.     

Cryomilling-induced solid dispersion of poor glass forming/poorly water-soluble mefenamic acid with polyvinylpyrrolidone K12

The effect of mechanical impact on the polymorphic transformation of mefenamic acid (MFA) and the formation of a solid dispersion of mefenamic acid, a poor glass forming/poorly-water soluble compound, with polyvinylpyrrolidone (PVP) K12 was investigated. The implication of solid dispersion formation on solubility enhancement of MFA, prepared by cryomilling, was investigated. Solid state characterization was conducted using powder X-ray diffraction (PXRD) and Fourier-transform infrared (FTIR) spectroscopy combined with crystal structure analysis. Apparent solubility of the mixtures in pH 7.4 buffer was measured. A calculation to compare the powder patterns and FTIR spectra of solid dispersions with the corresponding physical mixtures was conducted. Solid state characterization showed that (1) MFA I transformed to MFA II when pure MFA I was cryogenically milled (CM); and (2) MFA forms a solid dispersion when MFA was cryogenically milled with PVP K12. FTIR spectral analysis showed that hydrogen bonding facilitated by mechanical impact played a major role in forming solid dispersions. The apparent solubility of MFA was significantly improved by making a solid dispersion with PVP K12 via cryomilling. This study highlights the importance of cryomilling with a good hydrogen bond forming excipient as a technique to prepare solid dispersion, especially when a compound shows a poor glass forming ability and therefore, is not easy to form amorphous forms by conventional method.     

Composition and structure of nitrogen-containing dispersoids in trimodal aluminum metal–matrix composites

Trimodal aluminum (Al) metal–matrix-composites (MMCs), consisting of B₄C particulates, a nanocrystalline Al (NC-Al) phase, and a coarse-grain Al phase (CG-Al), has been fabricated. These MMCs exhibits extremely high compressive strength and tailorable ductility. Excellent thermal stability of NC-Al grains and high strength has been attributed partially to the nitrogen present within the trimodal Al MMCs, which is introduced during the cryomilling process in liquid nitrogen. This paper describes an investigation into the concentration and constituents of nitrogen within the trimodal Al MMCs. The structure of nitrogen-containing dispersoids was examined by analytical transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS) was employed to determine the total concentration of nitrogen. The nitrogen concentration increased linearly with an increase in cryomilling time up to 24 h. Both crystalline and amorphous aluminum nitrides with very fine size, down to 5 nm, as dispersoids, have been observed by analytical TEM. Correlations between the cryomilling time, nitrogen concentration, NC-Al grain size, and composite hardness are presented and discussed. The presence of nitrogen as nitride-dispersoids can contribute to the outstanding mechanical properties of trimodal Al MMCs by inhibiting NC-Al grain growth during the high temperature consolidation and deformation process, and by dispersion-strengthening.     

高熵合金中典型金属间化合物

高熵合金作为一种新型金属材料,因其具有优异的力学性能而受到越来越多研究者的广泛关注。在高熵合金中,金属间化合物从最初追求单相固溶体以避免形成有害相,发展到可作为有益的析出强化相或合金基体相(有序固溶体),丰富了高熵合金的组织调控策略,提升了高熵合金的力学性能。同时,也为高熵合金的发展起到了重要的推动作用。从高熵合金中相的形成规律出发,综述了高熵合金中典型金属间化合物及有序固溶体的研究现状,主要包括合金元素和热处理工艺等对典型金属间化合物形成规律和高熵合金力学性能的影响,并对高熵合金中金属间化合物的未来发展进行了展望。     

Thermal stability and oxidation behavior of Al-containing nanocrystalline powders produced by cryomilling

Al-containing nanostructured coatings provide excellent protection from high temperature corrosion. Aluminum oxide scales generally provide better oxidation resistance and yield lower oxidation rates than other oxide scale compositions. In this study, nanocrystalline 316L stainless steel containing 6 wt.% Al was synthesized using cryogenic milling (cryomilling). Complete alloying was obtained after 32 h of milling and the average grain size was found to be 7 nm. High temperature thermal stability and oxidation kinetics of the alloyed powders were examined. The powder demonstrated good grain growth stability at 500 °C, at which point, the powders had been heat treated for 120 h and the average grain size was found to be 11.4 nm. The oxidation kinetics of the powder were studied for 48 h at 500, 800, and 1,000 °C, respectively. For comparison, conventional 316LSS powder was also tested. Nanocrystalline 316LSS-6 wt.% Al showed lower weight gain than the conventional 316LSS powders. During the oxidation of nanocrystalline 316LSS-6 wt. % Al at 500 °C, protective aluminum oxide scale formed at the surface. At 800 °C and 1,000 °C, most of the nanocrystalline 316LSS-6 wt.% Al particles showed completed outer aluminum oxide scale. However, at 800 and 1,000 °C, some particles showed growth of chromium oxide scale underneath the aluminum oxide scale. In those samples, Al depletion was also observed due to a non-homogenous distribution of Al during cryomilling. The activation energy of the oxidation reaction was calculated and was found to be affected by the enhancement of the grain boundary diffusion in nanostructured particles.     

Defect induced weak ferroelectricity and magnetism in cubic off-stoichiometric nano bismuth iron garnet: effect of milling duration

The mechanical activation technique has been used to synthesize nanocrystalline multiferroic materials as it creates large density of crystal defects in these materials leading to novel magnetic properties of the nanostructured materials. In the present work, an attempt has been made to prepare nanocrystalline bismuth ferrite by mechanical activation process using high-energy planetary ball mill followed by sintering at 830 °C for 2 h. The milled powder was characterized using X-ray diffraction, scanning and transmission electron microscope, which revealed the formation of Bi_3?δFe₅O₁₂ instead of BiFeO₃. A giant dielectric peak has been observed at around the Curie temperature indicating magneto dielectric coupling which is a primary requisite for potential applications and transition temperature increases with the milling duration. Suppression in ferroelectric nature and enhancement in magnetic properties has been observed with the milling duration.     

Size effect studies on nanocrystalline Pb(Zr0.53Ti0.47)O3 synthesized by mechanical activation route

The nanocrystalline lead zirconate titanate (PZT) with the composition Pb(Zr_0.53Ti_0.47)O₃ has been synthesized by mechanical activation, without calcination at intermediate temperature. The PZT powder that has been mechanically activated for 10, 20 and 30 h shows crystallite size of 15, 12 and 8 nm, respectively. Size effect on PZT formation, tetragonality factor and dielectric constant has been investigated in detail. TEM and AFM studies have been carried out to confirm the formation of nanocrystalline PZT. The temperature dependence of dielectric constant at different frequencies was also measured, to study the size effects in PZT ferroelectrics. The nanocrystalline PZT obtained by this route has high dielectric constant at Curie temperature in comparison to that prepared conventionally.     

Preparation of nanocrystalline Ni–Fe strip via mechanical alloying–compaction–sintering–hot rolling route

Nanocrystalline structures offer opportunity for the development of soft magnetic materials, such as 80 wt% Ni–20 wt% Fe, with superior properties. In recent years, nanocrystalline 80Ni–20Fe (wt%) alloy has been prepared by mechanical alloying of elemental powders. However, retention of nanocrystallinity during consolidation of powder is the key issue to take advantage of improved magnetic properties. In the present work, it has been shown that near-full density bulk nanocrystalline 80Ni–20Fe strip can be prepared via a route consisting of mechanical alloying, cold compaction, sintering, and multi-step unsheathed hot rolling. A crack-free strip of nanocrystalline 80Ni–20Fe, having 99% theoretical density and a grain size of approximately 55 nm, was successfully prepared by sintering and hot rolling of mechanically alloyed powder preforms at 1140 °C. The bulk nanocrystalline 80Ni–20Fe material resulted in a very narrow hysteresis loop indicating a very small hysteresis loss. The present study shows that mechanical alloying–sintering–hot rolling route can be a promising method for producing bulk nanocrystalline materials.     

Synthesis,microstructural investigation and compaction behavior of Al0.3CrFeNiCo0.3Si0.4 nanocrystalline high entropy alloy

This research article focused on developing Al_0.3CrFeNiCo_0.3Si_0.4 nanocrystalline high-entropy alloy (HEA) by mechanical alloying. The initial powders mixture was ball milled for 1 hr (HEA-1 h), 5 hr (HEA-5 h), 15 hr (HEA-15 h) and 25 hr (HEA-25 h) at ball to powder mass ratio (BPR) of 15:1 and a speed of 300 rpm. The mechanical alloying time was varied from 1 to 25 hr to ensure the nanocrystalline nature and attainment of steady state in HEA powders. The structure of the developed HEAs was characterized by means of X-ray diffraction (XRD), Laser particle size analyzer (LPSA), and various electron microscopes (TEM and FEGSEM with EDS). HEA-25hr sample exhibited the crystallite size of 13.8 nm with lattice strain of 0.67% obtained from XRD which matched the result by TEM. The formation of a solid solution (SS) with a uniform elemental dispersion was observed with a major BCC stable structure and a minor FCC structure in HEA-25 h sample. The HEA-25 h sample revealed an average particle size of 386.2 nm (89.8% peak intensity) with Polydispersity Index (PDI) value of 0.364 which confirmed the uniform distribution of particles over a narrow range of particle size. The synthesized powders were consolidated to green compacts with a loading rate of 1 mm/min at different compaction pressures (25, 50, 75, 100, 150, 200, 400, 600, 800, 1000, and 1100 MPa) for examining the powder particles packing. Several compaction models (both linear and non-linear) were discussed to establish the density-pressure relationship of developed HEAs. The results revealed that the milling time has influenced the relative density. HEA-1 h sample was exhibited the relative density of 0.76 whereas HEA-25 h sample was produced the relative density of 0.6 indicating more strength and more amount of strain hardening occurs in MAed HEA-25 sample in addition to the entropy effect for the same composition.     

Synthesis of titanium oxynitride by mechanical milling

The synthesis of a titanium oxynitride phase by mechanically milling elemental titanium powder in air at ambient temperature has been studied. X-ray powder diffraction, transmission electron microscopy and chemical analysis showed that milling resulted in the formation of a nanocrystalline f c c oxynitride phase having the composition TiNO_0.5. The effects of milling time and subsequent heat treatment on the structure and lattice parameters are reported. The results indicate that mechanical milling significantly increases the reactivity of titanium with nitrogen.     

高熵合金力学性能的研究进展

高熵合金作为一类新型的合金,具有许多优于传统合金的性能,在诸多领域有广阔的应用前景。高熵合金现阶段的研究主要集中在其力学性能方面,它的高强度、高硬度、高耐磨性和耐腐蚀性等优点展现出了其作为在严酷条件下服役的结构材料的潜力,但目前对高熵合金的研究仍处于探索性阶段,所以研究高熵合金的力学性能具有重要的现实意义。主要综述了组分、制备工艺、热处理工艺、冷轧等对高熵合金的组织与力学性能的影响,并展望了高熵合金的应用前景。     

Mg2Ni纳米晶储氢材料的机械合金化制备工艺研究

利用机械合金化方法制备了Mg2Ni纳米晶粉末,根据不同球磨参数下样品的X射线衍射结果,分析了球磨过程中相组成、粉末晶粒度等的变化,研究了球磨时间、球磨转速、过程控制剂等工艺条件对粉末晶粒度、机械合金化程度的影响.结果表明:在较高的转速下进行循环变速运行并加入合适的过程控制剂可以使生成Mg2Ni的时间提前,完全合金化的过程缩短,得到的Mg2Ni晶粒度为10nm左右.对Mg2Ni纳米晶粉末进行了初步的贮氢性能研究,结果表明:机械合金化制备的样品在室温下即可吸氢,贮氢性能较之传统材料有较大改善.     

Fundamental deformation behavior in high-entropy alloys: An overview

High-entropy alloys (HEAs), as a new class of materials, are nearly equiatomic and multi-element systems, which can crystallize as a single phase or multi-phases. Most of the HEAs described in the literature contain multiple phases (secondary phases, nanoparticles, and so on), rather than a single solid-solution phase. Thus, it is essential to review the typical mechanical properties of both single-phase and multiphase HEAs thoroughly, with emphases on (1) the fundamental physical mechanisms and (2) the difference from conventional alloys. In this paper, mainly based on different mechanical properties, HEAs are classified into four types for the first time, i.e., (a) HEA alloy systems of 3d-transition metals only (Type 1), (b) HEA alloy systems of transition metals with larger atomic-radius elements (Type 2), (c) HEA alloy systems of refractory metals (Type 3), and (4) others (Type 4). Then a number of aspects of mechanical behavior are reviewed and discussed, including the elastic anisotropy, yield strength, high-temperature performance, serration behavior, fracture toughness, and fatigue responses, which may serve as a demonstrative summary for the current progress in the scientific research of HEAs. Several mechanisms that quantitatively explain the mechanical properties of single-phase and multiphase HEAs in terms of basic defects (dislocations, twinning, precipitates, etc.) are discussed. A number of future research activities are suggested, based on the emphasis on developing high-performance structural materials. The review concludes with a brief summary of major mechanical properties and insights into the deformation behavior of single-phase and multiphase HEAs. The comparison and contrast between HEAs and conventional alloys remain the most compelling motivation for future studies. With the integrated experimental and simulation investigations, a clearer picture of the fundamental deformation behavior of single-phase and multiphase HEAs could be explored.     

Short-range ordering and its effects on mechanical properties of high-entropy alloys

In this letter,we briefly summarize experimental and theoretical findings of fo rmation and characterization of short-range orderings(SROs)as well as their effects on the defo rmation behavior of high-entropy alloys(HEAs).We show that existence of SROs is a common yet key structural feature of HEAs,and tuning the degree of SROs is an effective way for optimizing mechanical properties of HEAs.In additional,the challenges concerning about formation mechanism and characterization of SROs in HEAs are discussed,and future research activities in this regard are also proposed.     

A novel refractory WMoVCrTa high-entropy alloy possessing fine combination of compressive stress-strain and high hardness properties

Refractory high-entropy alloys (HEAs) possess outstanding mechanical strength at room and high temperature but lack the room temperature ductility. A novel refractory equiatomic powder combination of WMoVCrTa was selected and verified for the feasibility of formation of solid solutions or else bulk-metallic glasses (BMGs) in the alloy based on the Guo et al.’s criteria and mismatch entropy criterion. The powder combination satisfies the above two criteria to crystallize in solid solution phases and inhibit the BMGs. Mechanical alloying characteristics of the powder mixture were determined. The particle size of the powder mixture decreased continuously during initial milling and later increased after 32 h of mechanical alloying. A homogeneous mixture was obtained after milling for 64 h. Crystallite sizes of the constituent elements in the powder mixture decreased continuously on progressive milling. A nanocrystalline powder was obtained by mechanical alloying. The powder milled for 64 h revealed a major BCC1, a minor BCC2 and small unknown phases. The lattice parameters of those BCC1 and BCC2 phases are 3.16 Å and 2.88 Å respectively. The alloy ingots were fabricated from the milled powder by vacuum arc melting technique followed by heat treatment. The ingot crystallizes in three phases such as a major BCC1, a minor BCC2 and a minor laves phase. The lattice parameters of these BCC1 and BCC2 phases are 3.05 Å and 2.85 Å respectively. Thereby, the BCC1 lattice of the milled powder contracts slightly after ingot fabrication. A fine combination of compressive stress and strain of 995 MPa and 6.2% respectively was achieved by the alloy at room temperature. Vickers hardness of the alloy was as high as 773 ± 20HV0.5. The density of the alloy was 11.52 g/cm³. The combination of excellent room temperature stress-strain and high hardness properties can enable the refractory HEA as a potential candidate for structural applications.     

高能球磨中促进粉体细化的主要因素研究

高能球磨技术作为一种制备包括纳米粉体在内的多种亚稳相材料的有效方法,已获得广泛应用.通常可以添加过程处理剂或在低温下球磨来抑制球磨过程中的冷焊,促使粉末细化.利用外加物理能场辅助高能球磨,使物理能与机械能协同作用到粉末,也是提高高能球磨效率的一种有效途径.根据不同条件下Fe粉的球磨细化结果,分析了高能球磨中过程处理剂、低温和外加能场辅助等主要促进粉体细化的因素及其作用.研究表明,采用等离子体辅助球磨仅仅10h,铁粉颗粒就被细化到了100nm.这种高的细化效率是因为等离子体辅助球磨中,等离子体对粉体产生的热爆效应和热应力的协同作用使粉体更易细化.