用机械合金化方法制备纳米晶Ni-Zn铁氧体

用机械合金化方法球磨金属氧化物,通过机械化学反应合成了Ni-Zn铁氧体.XRD,DSC,TG等分析发现,在600℃空气中退火后样品完全转变成单相的尖晶石结构.球磨后粉末样品的平均颗粒尺寸为15～32 nm,在不同温度退火后晶粒有所长大.在空气中低于1000℃的温度下退火后的样品具有较高的比饱和磁化强度,最高σs=63.4 emu·g-1,可与块状样品相比.也讨论了真空退火对样品磁性的影响,800℃真空退火后样品的分子磁矩计算值为3.4～3 7μB.     

Alloying behaviour in nanocrystalline materials during mechanical alloying

The alloying behaviour in a number of systems such as Cu-Ni, Cu-Zn, Cu-Al, Ni-Al, Nb-Al has been studied to understand the mechanism as well as the kinetics of alloying during mechanical alloying (MA). The results show that nanocrystallization is a prerequisite for alloying in all the systems during MA. The mechanism of alloying appears to be a strong function of the enthalpy of formation of the phase and the energy of ordering in case of intermetallic compounds. Solid solutions (Cu-Ni), intermetallic compounds with low ordering energies (such as Ni₃Al which forms in a disordered state during MA) and compounds with low enthalpy of formation (Cu-Zn, Al₃Nb) form by continuous diffusive mixing. Compounds with high enthalpy of formation and high ordering energies form by a new mechanism christened as discontinuous additive mixing. When the intermetallic gets disordered, its formation mechanism changes from discontinuous additive mixing to continuous diffusive one. A rigorous mathematical model, based on iso-concentration contour migration method, has been developed to predict the kinetics of diffusive intermixing in binary systems during MA. Based on the results of Cu-Ni, Cu-Zn and Cu-Al systems, an effective temperature (T _eff) has been proposed that can simulate the observed alloying kinetics. TheT _eff for the systems studied is found to lie between 0·42–0·52T ₁.     

Thermodynamic analysis of nanocrystalline solid solutions formation in copper-lead-tin ternary immiscible system during mechanical alloying

In this paper, copper-15 wt.%-lead-1 wt.%-tin (Cu-15wt %Pb-1wt %Sn), copper-15 wt.%-lead-2 wt.%-tin (Cu-15wt %Pb-2wt %Sn) and copper-15 wt.%-lead-3 wt.%-tin (Cu-15wt %Pb-3wt %Sn) powder mixtures were mechanically alloyed in order to study the solid solubility extension during the alloying process. Nanocrystalline supersaturated solid solutions have been prepared in copper-lead-tin (Cu−Pb−Sn) ternary immiscible system by mechanical alloying (MA). Based on the thermodynamic model, the Gibbs free energy changes in these alloys during the formation of solid solutions are calculated to be positive, which means that there are no driving force to form solid solutions in copper-lead-tin ternary immiscible system. It was found that a large fraction of grain boundaries and a high density of dislocations play a significant role in the solid solubility extension in copper-lead-tin ternary immiscible system.     

Formation mechanism of nanocrystalline high-pressure phases in silicon during nanogrinding

The phase transformations of Si under nanogrinding have been studied by transmission electron microscopy and Raman spectroscopy. Nanocrystalline high-pressure phases (Si-III/Si-XII) were found in the amorphous layer of the subsurface of heavily ground Si. The sequence of the phase transformation in nanogrinding has been found to be different to that in nanoindentation. The formation mechanism of the nanocrystalline high-pressure phases in nanogrinding is proposed based on experimental results.     

Formation of intermetallic phases during mechanical alloying and annealing of Cr + 20 wt % al mixtures

We have studied phase-formation processes in mixtures of Cr and Al (20 wt %) powders in the course of mechanical alloying (MA) and the phase transformations of the samples during subsequent annealing at temperatures of up to 800°C. The resultant x-ray amorphous intermetallic phases were identified by a differential dissolution method, which allows one to follow the formation of x-ray amorphous and partially crystallized phases. During MA of Cr + Al mixtures, the first to form is x-ray amorphous Cr₄Al, which then converts to partially crystallized Cr₂Al through reaction with aluminum. The peritectoid decomposition of Cr₄Al during heating of the MA samples is accompanied by heat release at 330–350°C. Heating to 420°C leads to the formation of Cr₅Al₈. At 800°C, Cr₅Al₈ reacts with Cr to form Cr₂Al.     

Structure evolution in the Cu-Fe system during mechanical alloying

High-resolution electron microscopy was used to examine the structure evolution of Cu-60 at % Fe powder mixture during mechanical alloying. Fracture and refinement of particles, the lamellar structure formed by cold-welding, and nanocrystals, were all observed at atomic scale. The X-ray diffraction patterns show that the Bragg peaks from the b c c phase decrease obviously in intensity after 3 h milling and entirely disappear after 5 h milling. Lattice images of the products obtained after 3 h milling reveal that there are Nishiyama-Wasserman orientation relationships between the b c c and f c c phases, i.e. (001)//(110), [1 0]//[1 2] and [110]//[ 11] . It is likely that for a mechanically alloyed iron-rich powder mixture, ball milling induces a reverse martensitic transformation of b c c Fe(Cu) to f c c Fe(Cu) phase. The greatly extended f c c phase range is closely related to this transformation. After 5 h milling, nanocrystals with sizes about 10 nm are formed.     

Metastable phases formation induced by mechanical alloying

Elemental aluminium, titanium and iron powders with compositions of Al₉₀Ti₁₀, Al₅₅Ti₄₅, Al₆₅Ti₂₅Fe₁₀, respectively, were mechanically alloyed in a planetary ball mill. The sequence of phase formation was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). Various metastable phases were experimentally observed: supersaturated solid solution Al(Ti) for Al₉₀Ti₁₀, amorphous phase and L1₂-Al₃Ti compound for Al₅₅Ti₄₅, amorphous phase and supersaturated solid solution Al(Ti,Fe) for Al₆₅Ti₂₅Fe₁₀, and an fcc crystalline phase was inevitably found in those alloys. The formation of the fcc crystalline phase has been critically assessed. The results suggest that the fcc crystalline phase seems to be metastable and it cannot be solely attributed to the contamination from the milling atmosphere underthe present experimental conditions.     

Phase transformations in nanocrystalline alloys synthesized by mechanical alloying

The effect of nanometer grain size and extensive grain boundary regions in nanocrystalline alloy systems was investigated for the chemical order-disorder, structural, precipitation, and spinodal phase transformations. The kinetic paths for approach to the chemically ordered phase from the disordered phase in FeCo-Mo alloys were observed to be the same at different temperatures due to grain boundaries acting as short-circuited diffusion paths for atom movements. The structure of Fe₃Ge was bcc for small crystallite size and the equilibrium fcc phase developed only after a critical grain size was attained. This was understood as a manifestation of the Gibbs Thomson effect. The precipitation phase transformation in Fe-Mo alloys proceeded by a rapid movement and clustering of the Mo atoms to the grain boundaries that was correlated to the size of the nano grains, and subsequent formation of the Mo rich lambda phase directly in the grain boundary regions. The composition fluctuation domains for spinodal decomposition in nanophase Fe-Cr alloys were observed to be linearly correlated to the growth of grains.     

Synthesis of nanocrystalline alloys and intermetallics by mechanical alloying

Nanocrystalline Al₃Ni, NiAl and Ni₃Al phases in Ni-Al system and theα, β, γ, ɛ and deformation induced martensite in Cu-Zn system have been synthesized by mechanical alloying (MA) of elemental blends in a planetary mill. Al₃Ni and NiAl were always ordered, while Ni₃Al was disordered in the milled condition. MA results in large extension of the NiAl and Ni₃Al phase fields, particularly towards Al-rich compositions. Al₃Ni, a line compound under equilibrium conditions, could be synthesized at nonstoichiometric compositions as well by MA. The phases obtained after prolonged milling (30 h) appear to be insensitive to the starting material for any given composition > 25 at.% Ni. The crystallite size was finest (∼ 6 nm) when NiAl and Ni₃Al phases coexisted after prolonged milling. In contrast, in all Cu-Zn blends containing 15 to 85 at.% Zn, the Zn-rich phases were first to form, and the final crystallite sizes were coarser (15–80 nm). Two different modes of alloying have been identified. In case of NiAl and Al₃Ni, where the ball milled product is ordered, as well as, the heat of formation (ΔH _f) is large (> 120 kJ/mol), a rapid discontinuous mode of alloying accompanied with an additive increase in crystallite size is detected. In all other cases, irrespective of the magnitude of ΔH _f, a gradual diffusive mode of intermixing during milling seems to be the underlying mechanism of alloying.     

Grain growth in nanocrystalline NbAl3 prepared by mechanical alloying

Grain growth and its kinetics were studied on an intermetallic compound, NbAl₃ powder prepared by mechanical alloying of elemental Nb and Al powders for 1.8 Ms in an argon atmosphere at ambient temperature. The initial and grown grain sizes were measured from the X-ray line broadening of as-alloyed and annealed powders. Isochronal annealing of mechanically alloyed powders from 573 to 1373 K indicated that substantial grain growth occurs only in a temperature range of 1048 to 1173 K and ceases at 1273 K regardless of anneal time. Accordingly isothermal annealing of 1.8 to 18 ks was carried out at 1048, 1073 and 1098 K to obtain the grain growth kinetic that is described by In (dD/dt) = In(r_o/3) –2.0 In D where D is the measured grain size and r _o a constant. This r _o depends on temperature according to r _o=r_o exp (– Q/kT) where Q is the activation energy for grain growth, k the Boltzmann constant and T the absolute temperature. Arrhenius plots of r _o against the reciprocal of temperature yield a straight line, from whose slope the activation energy for grain growth is deduced to be 162±2 kJ mol^–1. Of significance is the fact that the ultimate grain size at 1273 K is approximately 70 nm, which will not grow by further annealing even at 1373 K.On leave from Ibaraki University, Japan.     

Amorphization and nanocrystalline Nb3Al intermetallic formation during mechanical alloying and subsequent annealing

In this paper the formation as well as the stability of Nb₃Al intermetallic compounds from pure Nb and Al metallic powders through mechanical alloying (MA) and subsequent annealing were studied. According to this method, the mixture of powders with the proportion of Nb-25 at% Al were milled under an argon gas atmosphere in a high-energy planetary ball mill, at 7, 14, 27 and 41 h, to fabricate disordered nanocrystalline Nb₃Al. The solid solution phase transitions of MA powders before and after annealing were characterized using X-ray diffractometry (XRD). The microstructural analysis was performed using scanning electron microscopy (SEM) as well as transmission electron microscopy (TEM). The results show that in the early stages of milling, Nb(Al) solid solution was formed with a nanocrystalline structure that is transformed into the amorphous structure by further milling times. Amorphization would appear if the milling time was as long as 27 h. Partially ordered Nb₃Al intermetallic could be synthesized by annealing treatment at 850 °C for 7 h at lower milling times. The size of the crystallites after subsequent annealing was kept around 45 nm.     

Formation of nanocrystalline phases during thermal decomposition of amorphous Ni-P alloys by isothermal annealing

The microstructure and the average grain size were investigated by x-ray diffraction and transmission electron microscopy for nanocrystalline (n) Ni-P alloys with 18, 19, and 22 at.% P. A detailed study of the nanocrystalline states obtained along different heat treatment routes has been performed: (1) a-->ni by isothermal annealing of the melt-quenched amorphous (a) Ni-P alloys; (2) ni-->nii by isothermal annealing of the nanocrystalline ni state; (3) ni-->nii by linear heating of the ni state. The heats evolved during the structural transformations were determined by differential scanning calorimetry. From these studies, a scheme of the structural transformations and their energetics was constructed, which also includes previous results on phases obtained by linear heating of the as-quenched amorphous state of the same alloys. Grain boundary energies also have been estimated. In some cases it was necessary to assume a variation of the specific grain boundary energy during the phase transformation to understand the enthalpy and microstructure changes during the different heat treatments.     

Synthesis of nanocrystalline magnetite by mechanical alloying of iron and hematite

The synthesis of magnetite has been studied by mechanical alloying in an inert atmosphere of a stoichiometric mixture of micrometric particle size iron and hematite powders. The final products have been characterised by chemical analysis, SEM, TEM, XRD, Mössbauer spectroscopy as well as specific surface and magnetic measurements. The magnetite obtained in this way exhibits a high magnetic hardness. The formation of a wüstite layer on the magnetite core, because of the reaction between magnetite and iron contamination coming from the bowls and grinding balls, tends to decrease the coercive force of magnetite. The formation of this phase would be avoided by controlling the grinding time.     

Properties and in vivo investigation of nanocrystalline hydroxyapatite obtained by mechanical alloying

Mechanical alloying has been used successfully to produce nanocrystalline powders of hydroxyapatite (HA) using three different procedures. The milled HA was studied by X-ray diffraction, Infrared, Raman scattering spectroscopy and Scanning Electron Microscopy (SEM). We obtained HA with different degrees of crystallinity and time of milling. The grain size analysis through SEM and XRD shows particles with dimensions of 36.9, 14.3 and 35.5 nm (for (R1), (R2) and (R3), respectively) forming bigger units with dimensions given by 117.2, 110.8 and 154.4 nm (for (R1), (R2) and (R3), respectively). The Energy-Dispersive Spectroscopy (EDS) analysis showed that an atomic ratio of Ca/P=1.67, 1.83 and 1.50 for reactions (R1), (R2) and (R3), respectively. These results suggest that the R1 nanocrystalline ceramic is closer to the expected value for the ratio Ca/P for hydroxyapatite, which is 5/3≅1.67. The bioactivity analysis shows that all the samples implanted into the rabbits can be considered biocompatible, since they had been considered not toxic, had not caused inflammation and reject on the part of the organisms of the animals, during the period of implantation. The samples implanted in rabbits had presented new osseous tissue formation with the presence of osteoblasts cells.     

Structural and microstructural characterizations of nanocrystalline hydroxyapatite synthesized by mechanical alloying

Single phase nanocrystalline hydroxyapatite (HAp) powder has been synthesized by mechanical alloying the stoichiometric mixture of CaCO₃ and CaHPO₄ powders in open air at room temperature, for the first time, within 2 h of milling. Nanocrystalline hexagonal single crystals are obtained by sintering of 2 h milled sample at 500 °C. Structural and microstructural properties of as-milled and sintered powders are revealed from both the X-ray line profile analysis and transmission electron microscopy. Shape and lattice strain of nanocrystalline HAp particles are found to be anisotropic in nature. Particle size of HAp powder remains almost invariant up to 10 h of milling and there is no significant growth of nanocrystalline HAp particles after sintering at 500 °C for 3 h. Changes in lattice volume and some primary bond lengths of as-milled and sintered are critically measured, which indicate that lattice imperfections introduced into the HAp lattice during ball milling have been reduced partially after sintering the powder at elevated temperatures. We could achieve ~ 96.7% of theoretical density of HAp within 3 h by sintering the pellet of nanocrystalline powder at a lower temperature of 1000 °C. Vickers microhardness (VHN) of the uni-axially pressed (6.86 MPa) pellet of nanocrystalline HAp is 4.5 GPa at 100 gm load which is close to the VHN of bulk HAp sintered at higher temperature. The strain-hardening index (n) of the sintered pellet is found to be > 2, indicating a further increase in microhardness value at higher load.     

Formation mechanism of titanium silicide by mechanical alloying

The syntheses of five titanium silicides (Ti₃Si, TiSi₂, Ti₅Si₄, Ti₅Si₃, and TiSi) by mechanical alloying (MA) have been investigated. Rapid, self-propagating high temperature synthesis (SHS) reactions were involved in producing the last three materials during room temperature high-energy ball-milling of elemental powders. Such reactions appeared to occur through ignition by mechanical impact in the fine powder mixture formed after a critical milling period. From in-situ thermal analyses, each critical milling period for the formation of Ti₅Si₄, Ti₅Si₃, and TiSi was observed to be 22, 35.5 and 53.5 minutes, respectively. However, the formation of Ti₃Si and TiSi₂ did not occur even after 360 minutes of milling of as-received Ti and Si powder mixture, due to the lack of homogeneity of the powder mixture. Other ball-milling procedures were employed for the syntheses of Ti₃Si and TiSi₂ using different sizes of Si powder and milling medium materials. Ti₃Si was synthesized by milling a Ti and 60 minutes premilled Si powder mixture for 240 minutes. -TiSi₂ and TiSi₂ were produced by high energy partially stabilized zirconia (PSZ) ball-milling for 360 minutes in a steel vial followed by jar-milling of a Ti and 60 min premilled Si powder mixture for 48 hr. The formation of Ti₃Si and TiSi₂ occurs through a slow solid state diffusion reaction, and the product(s) and reactants coexist for a certain period of time. The formation of titanium silicides by MA and the reaction rate appeared to depend on the homogeneity of the powder mixture, milling medium materials, and heat of formation of the product involved.