Search

Fe x Au 100− x alloys have been produced for the first time by mechanical alloying. X-ray diffractograms show FCC peaks. From the X-ray diffracion peak-widths we estimate the final grain size, which vary with x from 112 nm (for x =15) to 32 nm (for x =30). Lattice parameter decreases with concentration (minimum 0.401 nm at x =30), but above Vegard's law values. Susceptibility measurements show cluster-glass behaviour. Critical temperatures are consistently lower than similar alloys produced by arc melting followed by fast quenching. A magnetic phase diagram is presented. Giant magnetoresistance is present in all samples, with a maximum at x =25. This effect is caused by the dispersion of small iron clusters produced by the mechanical work.

The repeated application of mechanical deformation of metallic materials has been proved to be an effective technique for producing bulk nano-scaled regulated structures. These materials have been shown to have unique properties characteristic for nano-materials. This paper reports the experimental results on the mechanical strength, magneto-resistivity, and thermo-electricity obtained by repeated pressing and rolling of alternately stacked thin metallic foils in the Cu–Fe system. In the samples which have the layer thickness greater than 35 nm, the Hall–Petch relation is primarily obeyed. For the samples with the layer thickness less than 35 nm, the strength and hardness deviate from this relation. Large magneto-resistivity change (GMR) have been confirmed and a noted change in thermo-electricity (EMF) dependent on the layer thickness, as observed for the Ag–Fe system, have also been confirmed.

Fe x Cu100−x magnetoresistive alloys were produced by mechanical alloying. X-ray diffraction shows fcc structure. The room-temperature Mossbauer spectra evolves from an asymmetrical doublet below x=25%, to a broad magnetic hyperfine field distribution above this concentration. Quadrupole splitting of the doublet varies between 0.48 and 0.57 mm/s, and its isomer shift from 0.16 to 0.29 mm/s. Low-temperature Mossbauer spectroscopy displays a B hf distribution. Magnetization measurements display different features depending on concentration, from mictomagnetism to ferromagnetism. Low-temperature magnetoresistance is measured. Samples with x∼20% exhibit larger magnetoresistivity ratios. Bulk and hyperfine magnetic properties are correlated in order to explain magnetoresistivity features of these samples.

We have successfully fabricated Fe/Ag multilayers that simultaneously possess the layer thickness of 4 nm and the layer number of above 10 000 by applying the repetition of pressing and rolling directly from a macroscopic stack of metal sheets. The clear correlation between the Ag layer thickness and MR ratio is observed, indicating that the critical layer thickness exhibiting magnetoresistance is 100 nm.

The synthesis of intermetallic compounds can be carried out through a repeated cold-pressing (RP) technique for binary metallic systems whose large negative heat of formation ensures the solid state reaction. In some cases, those reactions get started on the surface of highly energized regions, called hot spots. In this work, the intermetallic tin-tellurium compound SnTe has been prepared in air and at room temperature by RP of elemental Sn (granular) and Te (shots) precursors. The elemental structure is gradually cold-deformed until the pressing action supplies it with enough mechanical energy (microstrain) as to take the metallic mixture into a violent interaction, via hot spots formation and announced by means of an explosive reaction. Hot spots of 1–2 μm in diameter were visible on the metals surface when the powder particle size was 10–15 nm. That occurred with accumulation of 0.25–0.35% total strain, after 30 times pressing (1.25 g of the elemental mixture at 343 MPa). This indicates a close relationship of the reaction’s ignition point with both the crystallite size and stored strain in the precursor powders. Hot spots trigger the explosive intermetallic synthesis.

TiB/Ti composite were produced by SHS (self propagating high temperature synthesis) under conditions of PHIP (pseudo-hot isostatic pressing). The synthesis conditions were optimized to produce highly dense materials conforming a high volume fraction of TiB. Self-propagating reaction of the Ti and B powders occurred above the α- to β-Ti transformation temperature (1155 K). The composites consist of Ti matrices with dispersed TiB reaction products of 2–30 μm size. Near fully dense composites with a TiB/Ti volume ratio of 33–44 vol.% were synthesized for B/Ti atomic ratios of 0.3–0.4 at pseudo isostatic pressures of 100 MPa. The Vickers hardness and fracture stress of the composites are 5.6–7.1 and 2.1–2.3 GPa, respectively, and the wear of the composite is approximately 7–9% of the Ti and Ti–6Al–4V.

Bulk Fe/Ag multilayers with layer thickness of about 10 nm have been successfully fabricated by repeated press-rolling directly from a macroscopic stack of metal sheets. The press-rolled multilayers exhibited giant magnetoresistance of 13% in the current perpendicular to the plane geometry and 4% in the current in the plane geometry at 5 K.

The preparation and characterization of metastable phases of the Al–Fe alloy system by mechanical alloying are reported. In Al-rich (up to 10 at.% Fe) alloys, the supersaturated f.c.c. solid solution of Fe in Al (up to 1 at.% Fe) is formed. Almost complete amorphization is confirmed in the composition range 17–33 at.% Fe. The metastable disordered b.c.c. solid solution of about 10 nm in grain size has also been formed by ball-milling for over 180 h in Fe-rich (above 50 at.% Fe) alloys. Examination of lattice parameter and magnetization have shown that the composition range and degree of disorder are comparable to those formed by crushing and sputter deposition.

Multilayer structured alloys which have a layer thickness less than 10 nm have been produced by the application of repeated pressing and rolling to the initially macroscopically thick layered sample. The TEM observation of a Ag/Fe sample revealed a regular alternate lamination of Ag and Fe with the layer thickness in the range of 10 nm. The measured electrical resistivity showed a large dependence on the applied magnetic field which is characteristic to the composite structure of magnetic and non-magnetic metals in the thickness range of nanometers.

Thin metallic titanium foil was used instead of titanium powders in Al Ti alloy to check the possibility whether bulk materials could be applied to mechanical alloying (MA) processing for overcoming material limitations. On the basis of the results of microstructural evolution and structure examination by X-ray analysis, the final MA powders in the powder-foil system of an Al Ti alloy exhibit structures almost identical to those in the powder-powder systems. The low-energy ball milling processing needed to produce an ultraline dispersion of fractured titanium particles in aluminum powder particles was found to be reached after a milling time of 200 h.

The influence of low energy ball milling on the crystallite size, lattice strain, and storage of deformation energies of elemental metal powders is studied. The formation of nanosized grains (5–25 nm) and enhancement of lattice strain up to 0.4% is found. Excess enthalpies of up to 25% of the heat of fusion are reached.

Ball-milling of graphite powder was performed by the use of a stainless steel ball-mill and an agate ball-mill. The crystal structure of the ground graphite was studied by transmission electron microscopy and X-ray diffractometry. A spectrometric approach to the change of structure during the milling process was also carried out by the carbon Kα X-ray-emission band spectroscopy (XES). The carbon Kα XES spectra were compared with those of carbon materials such as ball-milled diamond powder, i-diamond film and amorphous carbon powder. In the case of the stainless steel ball-milling, microcrystalline and/or onion-like structure were formed. The XES spectrum suggested that sp3 hybrid orbital was formed after 2000 h of milling. While, in the case of an agate ball-milling, it was suggested that the 2p orbital became narrow as a result of further finely-ground pulverization of graphite crystallite after 1750 h of milling.

Mechanical alloying (MA) of iron and graphite was performed for C-10 at%Fe alloy by the use of a conventional ball mill. The structure of mechanically alloyed samples were examined by X-ray diffraction, transmission electron microscopy. A spectrometric approach to the change of structure during the milling process was carried out by carbon Kα X-ray-emission band spectroscopy and electron energy loss spectroscopy. Finely dispersed iron particles with a grain size of about 10 nm into graphite powder particles were obtained after 1000 h of MA. Spectrometric results suggested that fracturing of graphite of graphite crystal progressed by three steps ; fracturing along the hexagonal plane, fracturing the hexagonal networks and amorphization. The electron state of carbon atom in the ball milled sample was energized to form sp 3 hybrid orbital, after applying pressuring.

γ-Titanium aluminide (TiAl) is well known to have excellent potential for high temperature structural applications. The self propagating high-temperature synthesis (SHS) process has been expected to provide new fabrication routes for intermetallic materials. In recent years, considerable research on the reactive sintering process has been carried out for near-net shaping of intermetallics. In this study, we present a new fabrication route based on a combination of pseudoHIP and SHS to produce TiAl intermetallic compounds with the controlled microstructure and improved deformability. Reactive bonding of TiAl compounds also can be achieved by the SHS process, in particular, by use of pseudoHIP-SHS. It should be emphasized that the combination of pseudoHIP and SHS is effective in utilizing the heat release and transient liquid phase involved in SHS to obtain near-net shape products of TiAl intermetallic compounds with a high material density.

Some of the works related to the technique of mechanical alloying performed by the authors' group in the past 7 years are reviewed here under four topics : (1) Repeated rolling or pressing v.s. ball-milling ; (2) Thermodynamic proof of non-equilibrium state ; (3) Functional materials manufacturing ; (4) Chaos and mechanical alloying.

The reduction of crystalline size and amorphization of graphite and diamond during agate and stainless ball-milling are investigated by Raman spectroscopy. The ultimate crystalline size of graphite, estimated by the Raman intensity ratio, of 2.5 nm for the agate ball-mill is smaller than that of 3.5 nm for the stainless ball-mill, while the milling time to reach the ultimate size for the former is about 10 times larger than for the latter, indicating more stability of the nanocrystalline graphite. After reaching the ultimate crystalline size, a significant broadening of the Raman spectra, which indicates the completion of amorphization, is detected only for the stainless ball-milled graphite at ∼ 500 h of milling. Also the increase rate of the Raman peakwidth for the stainless ball-milled graphite before amorphization is higher than that for the agate ball-milled graphite, indicating a larger introduction of disorder from the start of milling. Amorphization of diamond is also observed for the stainless ball-mill. The difference in the results between the agate and the stainless ball-mill is discussed in terms of the effect of impurity mixed from the milling apparatus on the stability of nanocrystalline carbon materials.

The thermal behaviour of Al- x at.%Mo ( sx =, 10, 17, 20, 27, 50 and 75 powders mechanically alloyed in a conventional ball mill was investigated by differential scanning calorimetry, differential thermal analysis and X-ray diffraction. The temperature and heat of the solid state reactions leading to the formation of intermetallic phases markedly varied with milling time owing to significant microstructural changes occurring during milling. The intermetallic phases Al 2 Mo, Al 5 Mo, Al 4 Mo, Al 8 Mo 3 and AlMo 3 were identified as reaction products. The solid state reactions were discussed in terms of nanocrystalline structures created by mechanical alloying. After 1000 h of mechanical alloying, powders of various nominal compositions exhibited remarkably different microstructural parameters, i.e. crystallite size, intercrystalline (amorphous phase) volume fraction and intercrystalline thickness as well as intercrystalline composition. The most important factors affecting the change in thermal behaviour with milling time were suggested to be the reduction in crystalline size and increase in the intercrystalline volume fraction.

The thermal behaviour of powders obtained by mechanically alloying elemental crystalline nickel and molybdenum powders for various times was investigated using differential scanning calorimetry (DSC) and linked to structural data obtained by X-ray diffraction. The two observed exothermic heat effects were correlated with the milling time. The heat of relaxation reached a value of about 8.5 kJ mol−1 after 100 h of milling and slowly increased as the fraction of amorphous phase increased. The heat of crystallization reached a maximum value close to 5 kJ mol−1 after 300 h of milling and then slowly decreased. The formation of the amorphous phase by mechanical alloying occurred in three distinct stages: (1) the formation of very fine composite powders, (2) the nucleation and rapid growth of the amorphous phase, and (3) a steady-state reaction governed by the gradual dissolution of molybdenum crystallites into the amorphous matrix. The kinetic parameters for the transformation of the amorphous phase were derived from isothermal DSC. These parameters depend on the milling time. For the later stage of the mechanical alloying process, two-step crystallization was found. The compositionally dependent amorphous phase seems to be the factor most responsible for the changes in transformation kinetics.

Mechanical grinding (MG) of icosahedral Al 65 Cu 20 Fe 15 quasicrystal has been performed by ball milling for up to 200 hours. Specimens, taken after different milling times, were examined by X-ray diffraction and transmission 57 Fe Mossbauer spectroscopy. It is found that MG led to structural changes from quasicrystalline to amorphous phase. Two possible transformation mechanisms with the corresponding fitting models for evaluation of the Mossbauer spectra are proposed. The model consisting of two blocks of the P(Δ) distributions of the electric quadrupole interaction is superior to that one which assumes only one P(Δ) suggesting a creation of an amorphous phase and its gradual increase in the specimen's volume during the ball milling of icosahedral quasicrystal.

Mechanical alloying (MA) of nickel and graphite powders was performed in the composition range Ni1-xCx (x = 0.10 to 0.90) by use of a conventional ball mill. The structure of me-chanically alloyed samples was examined by X-ray diffraction, transmission electron micros-copy (TEM), scanning electron microscopy (SEM), and differential scanning calorimetry. A remarkable supersaturation of carbon in face-centered cubic (fcc) nickel phase was observed. A metastable phase Ni3C was formed by a prolonged MA treatment. For the purpose of com-parative study, MA of cobalt and graphite powders was also performed in composition Co1-xCx (x = 0.10, 0.15, and 0.30). The supersaturation of carbon in fcc cobalt and formation of a metastable carbide Co3C were confirmed.

The microscopic nature of mechanically alloyed powders has been investigated using the 57Fe and 119Sn Mossbauer spectroscopy. The following alloy systems have been treated by a low-energy ball mill; Al-Fe, Fe-Sn, Ag-Fe, Fe-C and Fe-B. To understand the elementary process of kneading, the repeated rolling method has been employed for the Al-Fe system and compared with the results from the ball-milling method. Pure Fe powders have also been ball-milled in order to understand the effects of heavy deformation using the same conditions and investigated by the 57Fe Mossbauer spectroscopy. Hyperfine interaction parameters obtained show that atomic dispersion and the formation of an amorphous, non-equilibrium solid solution and intermetallic compounds occur during mechanical alloying. From the experimental findings for each powder the characteristic features of the mechanical alloying process have been discussed.

Mechanical alloying (MA) for Fe-C-Si mixtures of elemental powders was performed by a conventional ball-mill under an argon gas atmosphere. The phase formation on the ball-milling process was studied by X-ray diffractometry and differential scanning calorimetry.An addition of silicon suppressed the formation of iron-carbides to promote the amorphization. For high silicon concentration, intermetallic compounds such as FeSi, β-FeSi2 and SiC were formed after indicating the partial amorphous formation. The inhomogeneous reaction, the slow reaction, which is a characteristic in MA of metal-nonmetal systems, was confirmed for Fe-C-Si system.

It is known that multilayered laminates with the layer thickness in nano-meter order have exceptional mechanical, magnetic and electrical properties which are often far superior to those of conventional materials. One of the most noted properties is the giant magnetoresistance (GMR), which was first reported in the Fe/Cr multilayer [1]. The multilayered laminates have been prepared by crystal growth techniques such as vacuum deposition, sputtering and electrodeposition. Also the formation of nano-scale multilayers by repeated rolling together with pressing of alternately stacked metal sheets has been reported for the Fe/Ag system [2, 3]. The present paper reports the experimental results of the Fe/Cu system. One of the greatest advantages by the repeated pressing-rolling is to fabricate metallic multilayers in a bulk form. In this study, Fe sheets of 20 μm and Cu sheets of 18 μm were cut into the size of 15× 20 mm. These Fe and Cu sheets were alternately stacked 200 times (400 sheets total) and bound tightly between two stainless steel plates by screws, and then annealed at 873 K for 1 h in a vacuum furnace. To prevent the sample from welding with the steel plates, alumina plates were placed between them. Then, the sample was hot-pressed at 673 K in vacuum to reduce the thickness to about 1/2 and annealed again at 873 K for 1 h in vacuum to remove defects and strain. The annealed sample was rolled at room temperature in air to reduce the thickness to about 1/50. The rolled thin strip of the sample was cut into pieces of equal size and stacked again to form a rectangular block to be subjected to the next same cycle for the further thinning of layers. Fig. 1 shows a schematic drawing of the processing procedures. The magnetoresistance (MR) was measured by the four-probe method, and presented by the fractional resistivity change, 1ρ/ρ. The direction of the applied magnetic field was parallel to the layer plane. Overall crystallographic orientation relationship of the Fe and Cu layers were examined by means of the pole figure X-ray analysis. Fig. 2 shows the overall reduction in sample thickness plotted against the average layer thickness observed by the microscope (either by SEM or TEM). Solid line indicates the expected layer thickness calculated from the overall thickness reduction of the sample. For the pressing-rolling of 1 cycle, observed average layer thickness is in a good agreement with the expected value. For the pressing-rolling of 2 cycles and 3 cycles, however, observed average layer thickness gets far away from the expected values. Such a difference between the observed and expected layer thickness

A wide composition range of aluminum-copper alloys has been prepared by mechanical alloying (MA). The microstructure and phase formation have been investigated by X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy. The primary grain size of the powders subjected to ball milling for the aluminum-rich compositions is less than 30 nm. The solid solubility of coppe in aluminum can reach 2.7 at. pct Cu and that of aluminum in copper can reach 18.0 at. pct Al. Over a wide composition range, formation of a nonequilibrium distorted body-centered cubic (bcc) phase which possesses some ordering has been confirmed.

Mechanical alloying of elemental aluminium and molybdenum powders was performed by conventional low-energy ball milling. Seven compositions (Al-3, 10, 17, 20, 27, 50 and 75 at% Mo), as well as elemental aluminium and molybdenum powders, were milled up to 1000 h. The structural changes during milling were followed by X-ray diffraction. In all cases milling produced refinement of the microstructure, and prepared powders were microcrystalline with grain sizes of the order of nanometres. A supersaturated solid solution of molybdenum in aluminium was formed. Only for the Al-75 at% Mo powders was a solution of aluminium in molybdenum observed.

Mechanical alloying of iron and graphite powders was performed in composition range Fe1 − xCx (x = 0.17−0.90) by the use of a conventional ball mill. The structures of mechanically alloyed samples were examined by X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy, 57Fe Mossbauer spectroscopy and differential scanning calorimetry. The results from X-ray, TEM and Mossbauer measurements suggested the partial formation of amorphous phase. Amorphization was notable on the sample ball milled for about 200 h. After subsequent milling, formation of metastable carbides Fe3C for the powders with x = 0.17−0.25 and Fe7C3 for x = 0.29−0.70 was detected. Formation of fine paramagnetic particles was detected by Mossbauer spectroscopy for the powders having carbon content x = 0.80−0.90.

Mechanical alloying in the AgCu system by the ball-milling and repeated rolling methods was performed. A super-saturated f.c.c. solid solution was formed in the entire composition range by ball milling. The lattice parameter change due to the super-saturated solid solution formation matched quite well with the reported result obtained by rapid quenching. Upon heating, the super-saturated solid solution decomposed into base silver and copper solutions, resulting in a large reduction of electrical resistivity.

Mechanical alloying (MA) of Fe-Mo binary system was performed by the low energy ball mill method. Elemental Fe and Mo powders were ball-milled up to 1000h. We characterized the reaction products by X-ray diffraction analysis, high resolution electron microscopy (HREM), and EDX analysis. From the X-ray diffraction patterns of MA powders, it was found that an amorphous phase was obtained in the composition of 30-50at%Mo and single bcc phase was obtained in the composition of 10-20at%Mo and 60-90at%Mo. The amorphization started from an early stage of MA and the amorphous region extended as MA continued. HREM investigation of Fe-30at%Mo and Fe-50at%Mo revealed that fine grains whose particle sizes are several nano-meters exist in the amorphous matrix. By the quantitative analysis by EDX, these grains were found to be Mo rich in composition.

Utilizing57Fe and119Sn Mossbauer spectroscopy, the microscopic behavior of metallic powders has been investigated during and after the mechanical alloying (MA) process. A conventional ball-milling method is employed and the following combinations of metallic powders have been studied for their MA; Al−Fe, Fe−Sn and Ag−Fe. Mossbauer spectra show the occurrence of mutual atomic dispersion during the MA process even for the Ag−Fe system which is known as mutually immissible even in the liquid state. Formation of an amorphous phase in the Al−Fe system has been detected.

Mechanical alloying (MA) of elemental aluminium and chromium powders has been performed using a conventional ball-mill. The MA process produces composite metal powders and homogeneously alloyed powders. During continuous heating at the rate of 0.33 K sec−1 Al-15 at% Cr samples ball-milled for 800 and 1000 h showed two exothermal peaks. The first peak which appeared at the lower temperature corresponds to amorphization of the MA powders. It was confirmed by X-ray and transmission electron microscopy that the heattreated powders, quenched from a temperature just above the first peak, were amorphous phase. Amorphous Al-Cr alloys were formed using elemental powders by MA and subsequent heat treatment.

Mechanical grinding (MG) of rapidly solidified crystalline powders has been performed by the use of a conventional ball-mill. It produced amorphous AI-15 and 20at% Cr alloys. Starting powders for MG were prepared from the ribbons by the single roller method and the component of them was mainly icosahedral quasicrystal. This treatment made it possible to produce the amorphous phase by MG even in the alloy system where liquid quenching did not result in the formation of amorphous phase. The amorphous phase obtained in AI-20 at % Cr alloy was stable up to about 820 K at the continuous heating rate of 0.33 K sec−1 and the activation energy for crystallization was about 240 kJ mol−1.

This letter reports an investigation on the consolidation of amorphous Fe 80.5 Si 6.5 B 12 C 1 (alloy 1) and Co 70.3 Fe 4.3 Si 16.4 B 9 (alloy 2) powders with soft magnetic properties

A pseudo HIP (PHIP) process has been developed for the purpose of consolidation of powder compacts to true density. This paper reports the application of this PHIP process to the reaction synthesis of the NiAl intermetallic compound. Compacts of the elemental powder mixture of Ni and Al or the powder mixture after subjected to the mechanical alloying treatment for various length of time are tested as starting material for PHIP. Significant improvement in mechanical properties of consolidated NiAl compounds have been noted by the mechanical alloying treatment prior to the reaction synthesis by PHIP.

Pure α-Fe powder was ground down mechanically by use of a conventional type ball-mill. Kneading of α-Fe powders by 2000 h milling creates nanometer-size crystalline grains showing a broad X-ray diffraction pattern. 57Fe Mossbauer absorption spectrum obtained from the ground powder consists of a pure bcc Fe component showing a sharp absorption dip and broad components with smaller hyperfine magnetic fields at 30 K. The intensity of the broad components decreases slightly at 10 K and a component having a large hyperfine field newly appeared to suggest the existence of finely dispersed iron oxides.