M. X. Quan

Structural disorder and phase transformation in graphite produced by ball milling

Nanocrystalline graphite with a crystallite size of about 2 nm is formed after 8 h of ball milling. Further milling produces a mixture of nanocrystalline and amorphous phases. The nanocrystalline graphite is relatively stable against heating when compared with some nanocrystalline metals.

Deformation-induced microstructural changes in Fe40Ni40P14B6 metallic glass

The effect of mechanical deformation via high-energy ball milling an the structure of the Fe40Ni40P14B6 metallic glass was studied by means of x-ray diffactometry, transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). After 5 h of milling, TEM observations indicated that some nanocrystallites with a diameter of about 6 nm precipitated from surface layers of the amorphous ribbons, whereas the bulk remained amorphous. When milling time was increased to 11 h, bulk crystallization occurred. The amorphous Fe40Ni40P14B6 alloy crystallized into a mixture of gamma-(Fe,Ni) and (Fe,Ni)(3)(P,B). To understand the microstructural changes occurring in the amorphous ribbons before the onset of bulk crystallization, the isothermal crystallization behavior of as-deformed amorphous ribbons was studied. Compared with as-quenched amorphous ribbons, the local value of the Avrami exponent, derived from isothermal DSC data, increased from 3.5 to 4.1 for bulk crystallization. The thermal crystallization mechanism of deformed amorphous Fe40Ni40P14B6 ribbons changed from an eutectic-type reaction with simultaneous precipitation of gamma-(Fe,Ni) and (Fe,Ni)(3)(P,B) from the amorphous matrix to a primary-type reaction with precipitation of alpha-Fe(P,B) preceding the formation of gamma-(Fe,Ni) and (Fe,Ni)(3)(P,B). Our results suggest that several hours of mechanical milling cause surface crystallization and some atomic rearrangements in the amorphous alloy. The latter effect may be responsible for the observed primary-type reaction for crystallization of the deformed amorphous alloy.

Solid-state reaction in nanocrystalline Fe/SiC composites prepared by mechanical alloying

Different solid-state reactions, i.e. Fe + SiC → Fe3C + Fe(Si), and Fe + SiC → Fe3Si + Fe2Si + C, were found in mechanically alloyed nanocrystalline Fe/SiC composites induced by prolonged milling or heat treatment, respectively. The solid-state reaction between nanocrystalline iron and SiC upon heating is greatly enhanced when compared with that between bulk iron and SiC. It is believed that the prolonged milling-induced reaction is related to the changed thermodynamics and kinetics while the heat-treatment-induced reaction, completed during a short time, is attributable to the changed reaction kinetics.

Mechanically induced structural relaxation in an amorphous metallic Fe80B20 alloy

A melt-spun metallic Fe80B20 glass was subjected to high-energy ball milling. Microstructural changes of the glassy sample during milling were characterized by means of x-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy, and Curie temperature measurements. It was found that the metallic glass may relax towards a low energetic configuration by mechanical milling, leading to a reduction of the heat release associated with crystallization of the amorphous phase and an increase of the average hyperfine field as well as of the Curie temperature. These results can be attributed to the occurrence of a strong short-range order in the amorphous state. Our experimental observations suggest that mechanical milling may induce structural relaxation in the amorphous Fe80B20 alloy

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.

Preparation and thermal stability of supersaturated nanocrystalline Al-Ti alloys

Supersaturated nanocrystalline Al100-xTix alloys were mechanically alloyed in a planetary ball mill. For alloys with x = 5 and 10, a mixture of supersaturated solid solution Al(Ti) and an fee phase coexist while a single supersaturated solid solution Al(Ti) was obtained during mechanical alloying for x = 15 and 35. With increasing Ti content, from 5 to 35 at%, the grain size of the Al(Ti) solid solution decreased from 40 to 10 nm. The results suggest that fast diffusion along nanocrystalline grain boundaries is the main alloying process. DSC curves and TEM observations indicate that solute atom segregation can thermally stabilize the nanocrystalline solid solutions.

Effect of nickel addition on the combustion reaction of the Ti-C system during mechanical alloying

The explosive reactions or self-propagating high temperature synthesis (SHS) take place during milling Ni20Ti50C30 and Ni50Ti30C20 elemental powder mixtures, The coexistence of agglomerates and powders in products indicates the occurrence of melting and solidification, TiC phase and NiTi compound were obtained during millingNi(20)Ti(50)C(30), while no compound of nickel and titanium was observed when milling Ni50Ti30C20, the final product of which is TiC and Ni. It is suggested that the explosive reaction is ignited by the heat releasing from initial formation of TiC through heavy collisions of milling balls, and the reaction between Ni and Ti, as well as the existence of Ni-Ti liquid, make the following reaction self-sustained. The variation of the addition of nickel did not affect the reaction time in both compositions, but made the reaction temperature different due to the difference of composition of Ni and Ti. It is estimated that the temperature during the reaction in Ni20Ti50C30 rises above 1112 degrees C, while in Ni50Ti30C20, it might rise above 1349 degrees C. However, no phenomenon suggests the melting of pure elemental Ti; the formation of TiC is mainly controlled by the diffusion mechanism in SHS.

On the mechanically driven rapid crystallization of amorphous Si3N4 ceramics

The effect of high-energy ball milling on the structure of nanometer sized amorphous ceramics, a-Si3N4 and a-Si-N-C, respectively, has been investigated. At high milling intensity, a-Si3N4 may rapidly crystallize into a mixture of alpha-Si3N4 and beta-Si3N4 after the initial 4 s of milling whereas no structural changes were observed at low milling intensity. For a-Si-N-C, mechanical milling does not cause structural changes at both low and high intensity. It was found that extension of mechanical milling of these hard ceramics can introduce large volume fractions of contamination fragments from the milling media. We conclude that the observed structural changes occurring in a-Si3N4 may be due to mechanical effect, rather than local heating and/or impurity effect

Supersaturated Al(Ti) solid solutions with partial L12 ordering prepared by mechanical alloying

The authors report phase formation during mechanical alloying of Al rich Ti-Al powder blends. Their experimental results further support the idea that the synthesis of Al rich supersaturated solid solutions in the Al-Ti system occurs in the following two steps. First, the ordered L1{sub 2}-Al{sub 3}Ti intermetallic compound is formed at Al/Ti interfaces. Second, the ordered L1{sup 2}-Al{sub 3}Ti compound was partially disordered by mechanical deformation. Meanwhile, Ti or Al atoms dissolve into the partially disordered phase and a supersaturated solid solution is finally obtained. However, the disordering is not complete and the resulting alloys may exhibit partial L1{sub 2} ordering.

Nanocrystallization of Fe80B20 by ball milling

By means of X-ray diffraction (XRD), transmission electron microscopy (TEM), as well as differential scanning calorimetry (DSC) measurements, the sequence of structural evolution by ball milling of polycrystalline and amorphous Fe80B20 alloys was studied. Ball milling of polycrystalline Fe80B20 alloy results in a continuous refinement of the grain size to about 13 nm and a decrease of axial ratio c/a for the Fe2B phase. For the amorphous Fe80B20 alloy, with increasing milling time, the amorphous phase crystallizes into alpha-Fe and a metastable Fe3B phase, followed by the phase transformation from the resulting metastable Fe3B phase into alpha-Fe and the stable Fe2B phase, which are also nanocrystalline. The decreased axial ratio c/a for Fe2B phase indicates that chemical disordering is introduced during the mechanical deformation.