高能球磨Mg-Zr-Ni合金组织演变

为了改善Mg-Ni合金的电化学性能,采用高能球磨技术合成了Mg-Zr-Ni储氢合金,通过改变球磨条件和添加合金元素Zr,利用XRD物相分析和电化学测量技术,研究了Mg-Ni合金的组织演变过程及其对电化学容量的影响.结果表明,高能球磨Mg-Ni和Mg-Zr-Ni合金都经历了非晶态向纳米晶态的转变过程,用少量Zr替代部分Mg后,促进了高能球磨Mg-Zr-Ni合金的非晶化和纳米晶化的过程.与非晶态Mg(Zr)Ni相比,纳米晶的Mg(Zr)Ni中氢更易放出,放电曲线主要呈现高电位放电特征,添加Zr后合金的放电容量有所下降.     

Consolidation and Compression Property of Nanocrystalline NiAl Synthesized by Mechanical Alloying

The mixture of pure nickel and aluminum powders in the composition of Ni50AI50 was subjected to high energy ball milling. The high energy ball milling to 30 h resulted in the formation of a nanocrystalline NiAl with a grain size of about 10 nm. The as-milled NiAl powder was hot-pressed and the compact with a density of 90% was obtained. It was showed that heating to900℃ and holding for more than 2 h failed to induce the coarsening of the fine nanocrystalin the material, the tendency of grain growth was weak. Compression testing showed that thecompression ductility at room temperature reached about 7%. The ductility increased with theexperimental temperature while the strength decreased drastically at elevated temperature.     

Simultaneous improvement in the strength and corrosion resistance of Al via high-energy ball milling and Cr alloying

The corrosion resistance and mechanical properties of nanocrystalline aluminium (Al) and Al–20 wt.%Cr alloys, synthesized by high-energy ball milling followed by spark plasma sintering, were investigated. Both alloys exhibited an excellent combination of corrosion resistance and compressive yield strength, which was attributed to the nanocrystalline structure, extended solubility, uniformly distributed fine particles, and homogenous microstructure induced by high-energy ball milling. This work demonstrates the possibilities of developing ultra-high strength Al alloys with excellent corrosion resistance, exploiting conventionally insoluble elements or alloying additions via suitable processing routes.     

Bimodal grain size distribution: an effective approach for improving the mechanical and corrosion properties of Fe–Cr–Ni alloys

The effect of nanocrystalline grain size and bimodal distribution of nano- and microcrystalline grain sizes on the oxidation resistance and mechanical properties of Fe-based alloys has been investigated. Nanocrystalline and bimodal Fe–10Cr–5Ni–2Zr alloy pellets, prepared by mechanical alloying route, have been compared with conventional microcrystalline stainless steel alloys having 10 and 20 wt% Cr. Zr addition has been shown to improve the grain size stability at high temperatures. A significant improvement in the ductility of bimodal alloys with respect to nanocrystalline alloys was seen presumably due to the presence of the microcrystalline grains in the matrix. The high temperature oxidation of nanocrystalline and bimodal alloys at 550 °C shows superior oxidation resistance over microcrystalline alloy of similar composition (Fe–10Cr–5Ni) and comparable to that of microcrystalline alloy having twice as much Cr (Fe–20Cr–5Ni). Secondary Ion Mass Spectroscopy depth profiling confirms the hypothesis that nanostructure facilitates the enrichment of Cr at the oxide metal interface resulting in the formation of a passive oxide layer.     

Nanostructured Composite Materials with Improved Deformation Behavior

The recent progress in the development of nanostructured composites is described for Zr‐base multicomponent alloys as a typical example for such materials. These advanced composite materials are attractive candidates for structural as well as functional applications. The combination of high strength with high elastic strain of fully nanocrystalline and glassy alloys renders them quite unique in comparison to conventional (micro‐)crystalline materials. However, one major drawback for their use in engineering applications is the often limited macroscopic plastic deformability, despite the fact that some of these alloys show perfectly elastic‐plastic deformation behavior. To improve the room temperature ductility of either fully nanocrystalline or amorphous alloys, the concept of developing a heterogeneous microstructure combining a glassy or nanostructured matrix with second‐phase particles with a different length‐scale, has recently been employed. This review describes the composition dependent metastable phase formation in the Zr‐(Ti/Nb)‐Cu‐Ni‐Al alloy system, which in turn alters the mechanical properties of the alloys. We emphasize the possibilities to manipulate such composite microstructures in favor of either strength or ductility, or a combination of both, and also discuss the acquired ability to synthesize such in‐situ high‐strength composite microstructures in bulk form through inexpensive processing routes.     

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.     

The thermal stability of nanocrystalline cartridge brass and the effect of zirconium additions

The thermal stability of nanocrystalline cartridge brass (Cu–30 at.% Zn) and brass–Zr alloys were investigated. The alloys were produced by cryogenic ball milling and subsequently heat treated to a maximum temperature of 800 °C. The grain size of pure brass was found to be relatively stable in comparison to pure copper, and a high hardness was retained up to 600 °C. When 1 at.% zirconium was alloyed with the brass, the grain size was stabilized near 100 nm even at 800 °C. At the highest temperature, hardness was retained above 2.5 GPa for 1 and 5 at.% zirconium alloys, but the pure brass softened significantly. The stabilization is believed to be dominated by Zn–Zr interactions as a second phase of these two was observed in X-ray diffraction and transmission electron microscopy. Thermodynamic modeling indicates a zero grain boundary energy may be achieved depending on the mixing enthalpy value used (i.e., calculated vs. experimental) under ideal conditions, but microstructural features such as twinning and second phase particles are thought to be the dominant stabilization mechanism. Zr worked well in stabilizing the brass in the nanocrystalline state to nearly 90 % of its melting temperature.     

Nanostructured materials by mechanical alloying: new results on property enhancement

Mechanical attrition—the mechanical alloying or milling of powders—is a very versatile and potent method of obtaining nanocrystalline or ultrafine grain structures with enhanced properties. This article presents three examples of enhanced properties obtained by materials in which the grain size has been reduced to the nanoscale or ultrafine scale by ball milling and consolidation of powders. Very high strength/hardness—the highest hardness yet reported for crystalline Mg alloys—for a ball milled Mg₉₇Y₂Zn₁ alloy is due in part to the nanocrystalline grain structure, along with nanoscale precipitates. A ternary Cu-base alloy with a low stacking fault energy was found to have both high strength and good ductility in a nanocrystalline material synthesized by the in situ ball milling consolidation method. This is another example that shows nanocrystalline materials need not be brittle. It is shown that bulk thermoelectric materials with superior properties can be produced by the ball milling and consolidation of powders to provide an ultrafine grain structure.     

Supersaturated solid solutions and metastable phases formation through different stages of mechanical alloying of FeTi

Elemental equiatomic Fe-Ti powder mixture was mechanically alloyed in high energy ball mill. XRD, DTA and Mössbauer spectroscopy (at liquid nitrogen temperature) were utilized to monitor the kinetics as well as the accompanied structural and phase transformations through different stages of milling. Our experiments showed that formation of nanocrystalline FeTi compound proceeds via the formation of the supersaturated solid solutions β-Ti(Fe) and α-Fe(Ti) at the interface. After 36 hours of milling, the main part of powder mixture transformed not only to FeTi but also to Fe2Ti intermetallic compound. The transition of last part of super saturated solid solutions β-Ti(Fe) to those intermetallic phases was observed after annealing of this sample at 600 °C.     

Mechanical alloying synthesis and structural characterization of ternary Ni-Al-Fe alloys

Ternary Ni-Al-Fe alloys with different Fe additions have been synthesized by mechanical alloying of elemental powder mixtures. The effects of Fe-substitution for Al in an equi-atomic NiAl alloy on the mechanical alloying process and on the final Ni-Al-Fe alloys were investigated experimentally. Lower Fe additions have been found to prolong the milling time prior to explosive formation of the NiAl(Fe) compound, while ≥ 15 at % Fe addition has been found not only to eliminate the explosive reaction during milling but also to produce a Ni-based supersaturated solid solution (the 15 at % Fe addition results in the formation of an amorphous-like phase). The addition of Fe improves the plastic deformation ability of the alloys and hinders the tendency of fracture during milling, resulting in the formation of larger alloy particles. Upon heating, the as-milled samples with lower Fe addition remain as NiAl(Fe) compound, whereas the Ni-based supersaturated solid solutions decompose into a mixture of compounds of β-(Ni, Fe) (Al, Fe) + γ′-(Ni, Fe)₃Al. It is suggested that the ternary addition into the binary intermetallic compound might be a possible route to improve ductility by forming the dual phase of β + γ′. This revised version was published online in November 2006 with corrections to the Cover Date.     

Synthesis of the Supermalloy Powders by Mechanical Alloying

The mixture of the Ni, Fe and Mo elemental powders with the nominal composition of the Supermalloy was milled in a planetary mill under Ar atmosphere. Several milling times have been used ranging from 4 to 16 h. A heat treatment of 30 min, 1, 2 and 4 h at temperature of 350°C has been performed in vacuum in order to improve the alloying process and remove the internal stresses. The formation of the Fe-Ni-Mo alloys by mechanical alloying was evidenced by X-ray diffraction. The nanocrystalline Supermalloy powders have been obtained after 16 h milling and after 8 h milling followed by 4 h annealing. A typical grain size of 11 ± 2 nm have been obtained after 16 h milling. The chemical homogeneity composition and the morphology of the powder particles have been studied by X-ray microanalysis and scanning electron microscopy respectively.     

球磨CeMg12+100%Ni合金热力学及电化学贮氢性能

用球磨工艺制备CeMg12+100%Ni电极合金,研究了球磨工艺对合金结构及电化学性能的影响。结果表明球磨CeMg12+100%Ni合金具有非晶纳米晶结构,由Mg2Ni相以及Ni相组成,其含量随球磨时间的延长而增加,这在一定程度上改善了合金的电化学放电性能。球磨非晶纳米晶具有较好的结构稳定性,在吸氢后形成的氢化物相仍保持非晶纳米晶尺度,这有利于降低氢化物的热稳定性,改善电化学放电性能。     

Structural and microstructural phase evolution during mechano-synthesis of nanocrystalline/amorphous CuAlMn alloy powders

The formation mechanism of Cu–11.5Al–4Mn alloys by mechanical alloying (MA) of pure elemental powders was investigated. During milling, the powder sampling was conducted at predetermined intervals from 1 h to 96 h. The quantitative phase analyses were done by X-ray diffraction and the particles size and morphology were studied by scanning electron microscopy. Furthermore, the microstructure investigation and phase identification were done by transmission electron microscopy. Concerning the results, the nanocrystalline Cu solid solution were formed at short milling times and, by milling evolution, the austenite-to-martensite (2H) phase transformation occurred. Moreover, the formation of considerable amount of amorphous phase and its partial transformation to crystalline phases during the milling process were revealed. It was also found that, by milling development, the powder morphology changes from lamellar to semi-spherical and their size initially increases, then reduces and afterward re-increases.     

High-strength Zr-based bulk amorphous alloys containing nanocrystalline and nanoquasicrystalline particles

Abstract

It was recently found that the addition of special elements leading to the deviation from the three empirical rules for the achievement of high glass-forming ability causes new mixed structures consisting of the amorphous phase containing nanoscale compound or quasicrystal particles in Zr–Al–Ni–Cu–M (M ? Ag, Pd, Au, Pt or Nb) bulk alloys prepared by the copper mold casting and squeeze casting methods. In addition, the mechanical strength and ductility of the nonequilibrium phase bulk alloys are significantly improved by the formation of the nanostructures as compared with the corresponding amorphous single phase alloys. The composition ranges, formation factors, preparation processes, unique microstructures and improved mechanical properties of the nanocrystalline and nanoquasicrystalline Zr-based bulk alloys are reviewed on the basis of our recent results reported over the last two years. The success of synthesizing the novel nonequilibrium, high-strength bulk alloys with good mechanical properties is significant for the future progress of basic science and engineering. © 2000 Published by Elsevier Science Ltd.     

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.     

Phase formation study of PZT nanopowder by mechanical activation method at various conditions

In this work, attempt is made to prepare and study the phase evolution of nanocrystalline Pb(Zr_0.53Ti_0.47)O₃ powder with both planetary and high energy shaker mill in air atmosphere, using X-ray diffraction (XRD) and transmission electron microscopy (TEM). Three different series of samples with various processing conditions i.e., ball to powder weight ratio (BPR) of 10/1. 20/1 at 500 rpm and 10/1 ratio at 250 rpm, were prepared using a planetary mill. Further, another series of samples were also prepared using a high energy shaker mill with BPR = 10 at 900 rpm. The BPR was seen to have a marked effect on the phase formation of PZT nanocrystalline powder. While the formation of some nanocrystalline intermediate phases of wide compositional distribution in an amorphous matrix was confirmed for the samples prepared using planetary mill with BPR = 10 at 250 and 500 rpm, single phase PZT could be formed when prepared at BPR = 20 at 500 rpm. Further, it was also shown that the formation of single phase PZT nanocrystalline powder of an average crystallite size of 10 nm could take place after only 30 h of milling when activated by shaker mill with BPR = 10 at 900 rpm. The results obtained are related to the rate of the injected energy and diffusion processes taken place.     

Synthesis of nanocrystalline SiC at ambient temperature through high energy reaction milling

This study investigated the in-situ synthesis of nanosized crystalline SiC powders at room temperature through high energy ball milling of elemental silicon and carbon mixtures. Milling conditions including the mill design, the milling speed, the milling time and the ball-to-powder weight ratio (i.e. the charge ratio) necessary for the in-situ synthesis were studied. It was found that uniform formation of nanosized crystalline SiC powders within the powder charge could be achieved with a correctly designed attritor and the contamination could be minimized with proper selections of milling conditions. The crystalline β-SiC powders synthesized were themselves in nanosize scale, quite different from many previous studies which have shown that it is the internal grain structure of milled powders that is the “nanocrystalline” component of the powders (typically 5–20 nm), while the powders are themselves typically 0.1 μm to > 1 μm in size. Furthermore, it was found that the product structures generated by high energy reaction milling depended strongly on the milling speed, the charge ratio and the milling time.     

Consolidation,microstructure and mechanical properties of nanocrystalline metal powders

Nickel, iron and aluminum nanocrystalline powders prepared by the evaporation-condensation method were high pressure consolidated to densities close to the theoretical at room temperature and at 300°C. Microstructures of the materials obtained were characterized by employing scanning (SEM) and transmission (TEM) electron microscopy. Microhardness and yield stress of the materials fabricated from the elemental nanocrystalline powders were measured and compared to those of Ni, Fe and Al high pressure consolidated from the micron/submicron elemental powders, and of a nanocrystalline Ni-20 TiC composite processed via attrition milling of a nickel oxide (NiO)/TiC powder blend followed by the reduction of NiO and high pressure consolidation to full density.     

Effect of carbon on the density,microstructure and hardness of alloys formed by mechanical alloying

This work aimed to produce iron-based alloys containing resistant microstructures to improve the mechanical properties of the resulting alloy. The effects of both carbon content and compaction pressure on the microstructure, density and hardness of the alloys were examined. Iron-based alloys with initial carbon contents of 0.5%, 1%, 2% and 3% were produced by powder metallurgy following a process that involved ball milling elemental powders, cold pressing and sintering. The composition, density, microstructure, porosity, hardness and ductility of the alloys depended on both compaction pressure and carbon content. As the carbon content increased, the amount of the resistant microstructure bainite in the alloys also increased, as did their hardness. In contrast, the density and ductility of the alloys decreased with increasing carbon content. This study shows that formation of the resistant microstructure bainite in alloys fabricated by powder metallurgy is influenced by both the initial carbon content of the alloy and compaction pressure during cold pressing.     

Electrochemical characteristics of nano and microcrystalline Fe–Cr alloys

The electrochemical behavior of nano and microcrystalline Fe–10Cr and Fe–20Cr alloys was determined using potentiodynamic polarization in 0.5 M H₂SO₄. Disks of the alloys were prepared by high-energy ball milling followed by compaction and sintering. In the current study, nanocrystalline Fe–Cr alloys reveal significantly different electrochemical characteristics, typified by lower anodic current densities and more negative passivation potentials, compared with their microcrystalline counterparts. In addition to the differences in grain boundary density, compositional characterization of corrosion films carried out by X-ray photoelectron spectroscopy indicates a higher Cr content in the film developed upon nanocrystalline Fe–Cr alloys. Mechanisms for observed enhancement in the corrosion performance of the nanocrystalline Fe–Cr alloys are discussed.