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.     

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.     

Effect of Hf additions on phase transformation,microstructural stability,and hardness of nanocrystalline 304L stainless steels synthesized by mechanical alloying

304L stainless steels with Hf additions were nanostructured by mechanical alloying (MA) and annealed at temperatures up to 1100 °C. The results showed that face-centered cubic (fcc) phase in 304L transformed to body-centered cubic (bcc) phase during MA. The in-situ studies revealed that bcc-to-fcc phase transformation completed after 105 min annealing at 900 °C for 304L, whereas Hf addition increased the required time and temperature for the complete transformation. The grain size of 304L stainless steel was ~10 nm after MA and remained ~167 and ~293 nm after annealing at 900 and 1100 °C, respectively, with Hf addition in comparison to 960 nm average grain size of base 304L stainless steel after annealing at 900 °C. The hardness of 304L increased from ~200 HV to 408 HV after MA and remained 329 HV after annealing at 1100 °C with Hf addition as opposed to 195 HV hardness of 304L.     

Microstructure correlated electrical conductivity of Manganese alloyed nanocrystalline cubic zirconia synthesized by mechanical alloying

Fully stabilized cubic (c) ZrO₂ phase has been synthesized by mechanical alloying (MA) the stoichiometric powder mixture of elemental Mn (5–20 mol%) and monoclinic (m) ZrO₂ at room temperature. XPS study reveals that major part of metallic Mn is ionized to Mn²⁺ oxidation state during MA. Mn-alloyed c-ZrO₂ nanoparticles with ～18 nm particle size have been synthesized within 10 h of MA. Microstructures of the compounds have been precisely evaluated by analyzing the X-ray powder diffraction patterns employing Rietveld refinement and transmission electron microscopy images. A decrease in lattice parameter from 5.11 Å to 5.09 Å is correlated with an increase in oxygen vacancy from 14% to 26% with increasing Mn concentrations. Elemental compositions in the compounds are obtained from electron probe microanalysis. The role of Mn alloying in the polymorphic phase transformation (m → c) has been established with changes in structure and microstructure parameters. Electrical conductivities of all c-ZrO₂ compounds are measured in the temperature range 350–550 °C. Grain and grain boundary contributions to total conductivity are calculated from frequency dependent real and imaginary impedance. Conductivity of Mn alloyed c-ZrO₂ increases with increasing temperature and Mn concentrations. Electrical transport mechanism in the compound is studied by impedance and modulus spectroscopy. The relaxation frequency is found to be temperature, microstructure and composition dependent.     

Kinetics of grain growth in nanocrystalline magnesium-based metal–metal composite synthesized by mechanical alloying

Nanocrystalline (nc) Mg–5 wt%–Al and Mg–5 wt%–Al–10.3 wt%–Ti metal–metal composites have been synthesized by mechanical alloying (MA) for 20 h of milling duration. XRD was employed as analysis tool for the structural evolution during MA and thermal stability at elevated temperatures. Kinetics of grain growth of Mg–5 wt%–Al–10.3 wt%–Ti and Mg–5 wt%–Al systems under isothermal and isochronal annealing was investigated. The results showed that the grain growth behavior can be described by the parabolic kinetic equation of grain growth and the activation energy of mechanically alloyed metal–metal composite is higher than that of cast Mg alloy and that of its base metal of Mg–5 wt%–Al.     

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.     

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.     

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.     

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.     

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.     

NiAl-B composites with nanocrystalline intermetallic matrix produced by mechanical alloying and consolidation

Powder mixtures with equiatomic Ni–Al stoichiometry and with the addition of 5, 10, 20 and 30 vol% of boron were mechanically alloyed in a high-energy SPEX mill. Differential scanning calorimetry (DSC) was used for examination of the thermal behaviour of the milled powders. The mechanically alloyed powders and powders after DSC examinations were investigated by X-ray diffraction (XRD). For all the powder mixtures, a nanocrystalline NiAl intermetallic phase was formed during milling. With the increase of boron concentration in the mixtures, more intense refinement of the NiAl grain size during mechanical alloying was observed. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) examinations showed that the produced powders have composite structure, with boron particles uniformly distributed in the nanocrystalline NiAl intermetallic matrix. The density of the composite powders decreases with the increase of boron content, following the rule of mixture.The produced powders were subjected to consolidation by hot-pressing at 800 °C under the pressure of 7.7 GPa for 180 s. The produced bulk materials were investigated by XRD, SEM and EDS as well as characterised by hardness, density and open porosity measurements. It was found that during applied consolidation process the nanocrystalline structure of the NiAl matrix was maintained. The average hardness of the bulk composite samples is in the range of 10.58–12.6 GPa, depending on boron content, increases with the increase of boron content, and is higher than that of the NiAl intermetallic reference sample (9.53 GPa). The density of the bulk composite samples is the same as that of the corresponding powders after milling, decreases with the increase of boron content and is lower than that of the NiAl reference sample. To the best of our knowledge, the NiAl-B composites with nanocrystalline intermetallic matrix have been produced for the first time.     

Discharge properties of Mg2Ni-Ni alloy synthesized by mechanical alloying

The electrochemical characteristics of Mg₂Ni-Ni alloys prepared by mechanical alloying (MA) using a planetary ball mill were investigated. The discharge capacity depends on the molar ratio in Ni andMg₂Ni, and it has a maximum value of about 480 mAh/g at the equimolar ratio of Ni/Mg₂Ni. This discharge capacity (480 mAh/g) is about 74% of the theoretical one. The discharge capacity is reduced to about half within five cycles. Thus, the cycle time would be improved by inhibiting the oxidation of Mg₂Ni.     

Phase transformations and microstructural evolution of nanocrystalline Ti-18Zr-5Nb-3Sn-4Ta powders through mechanical alloying

ABSTRACT

The aims of the present study are to determine the possibility of obtaining a new, nanocrystalline Ti-18Zr-5Nb-3Sn-4Ta at.-% alloy by a ball milling method, and to observe the influence of various milling times on phase composition and structure of the obtained material. The X-ray diffraction results confirmed the alloy synthesis and revealed the presence of α- and β-Ti phases. The Rietveld refinement showed that the production method and the atomic radii of the elements used in the mechanical synthesis have influence on the structure. Scanning electron microscopy observation leads to the conclusion that the powder particles and the degree of aggregation change with increasing milling time.

This paper is part of a Thematic Issue on The Crystallographic Aspects of Metallic Alloys.     

Structural characterization of silicon-substituted hydroxyapatite synthesized by a hydrothermal method

Silicon-substituted hydroxyapatite (Si-HA) was prepared successfully by hydrothermal method. The crystalline phase, microstructure, chemical composition, morphology and thermal stability of Si-HA were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The results show that the substitution of the silicate groups for the phosphate groups causes some OH^- loss to maintain the charge balance and changes the lattice parameters of HA. Furthermore, the substitution of the silicate groups restrains the growth of Si-HA crystal. DSC analysis shows that the small amount of silicon incorporates into HA lattice does not influence the thermal stability of HA.     

Grain growth and recrystallization of nanocrystalline Al3Ti prepared by mechanical alloying

The grain sizes and lattice strains during mechanical alloying of Ti-75 at.% Al powder mixtures were studied using X-ray diffraction methods. Nanocrystalline L1₂-Al₃Ti was obtained after a certain time period of ball milling. Minimum grain sizes of 17 nm for Al and 28 nm for Ti have been determined using XRD. During subsequent thermal annealing processing, an obvious recrystallization resulting in significant reduction of grain size was observed. The recrystallization in nanocrystalline Al₃Ti was affected by both the temperature and the degree of order. The incubation period for recrystallization at 400°C was about 6 hours while those at 510 and 700°C were about 2 hours. The completion time of recrystallization in Al₃Ti at 400 and 700°C was about 15 hours and 8 hours at 510°C. It is clear that the recrystallization at 700°C was retarded as a result of the higher degree of order structure which limited the mobility of the boundaries. Phase transformation occurring within the recrystallization temperature range was observed to have little influence on the recrystallization itself. However, transformation products do have significant effects on it which is originated from the degree of order in the products. The recrystallization in this alloy system provides an excellent means to maintain the nanocrystalline microstructure during the necessary consolidation thermal cycle by decreasing the processing temperature and increasing the hold time considerably.     

Synthesis and characterization of nanocrystalline Mg2CoH5 obtained by mechanical alloying

The stoichiometric mixture of 2MgH₂ + Co was ball milled under a hydrogen atmosphere to synthesize nanocrystalline metal hydride Mg₂CoH₅. Upon milling, the mixture was analyzed by X-ray powder diffraction (XRD) and thermal methods employing the techniques of differential scanning calorimetry (DSC), thermogravimetry (TG) and differential thermal analysis (DTA). Hydrogen absorption and desorption measured by pressure-composition-temperature (P-C-T) curves indicated that the capacity loss was small after 20 consecutive cycling tests. The enthalpies associated with hydride formation and decomposition were measured to be –69.5 and –83.2 kJ mol^–1 H₂, respectively. At the temperatures of this study (553 to 653 K), hysteresis decreases with increasing temperature.     

Synthesis of nanocrystalline NiAl over a wide composition range by mechanical alloying

The paper reports the synthesis of nanocrystalline NiAl by mechanical alloying of pure metal mixture and a mixture of prealloyed powder with Ni/Al. A large number of compositions have been studied to establish the phase field of NiAl in the milled state. The phase field of NiAl in the ball milled condition was found to be much wider (10–68 at.% Ni) than its equilibrium phase field (45–59 at.% Ni). The metastable equilibrium achieved by mechanical alloying was identical for a given composition irrespective of the starting ingredients. The crystallite size of NiAl reached a minimum (5 nm) at the phase boundary of NiAl/Ni₃Al.