G. Le Caër

Evaluation of hyperfine parameter distributions from overlapped Mossbauer spectra of amorphous alloys

An improved version of the Hesse-Rubartsch method is described. This method is applied, together with an adapted peak shape, to the evaluation of the hyperfine field distribution in an amorphous Fe79.5Si1.5B19 alloy at room temperature. The detailed structure of this distribution is discussed.

Etude par spectrométrie Mössbauer des carbures de Fer Fe3C et Fe5C2

Abstract Mossbauer spectra of Fe 3 C and Fe 5 C 2 have been obtained from 4°K up to the Curie temperature of each carbide. The asymmetry parameter of the Fe I atom of Fe 3 C is close to 1. The splitting of site II of Fe 5 C 2 is observed. A comparison of the quadrupole splittings of Fe 3 C and Fe 5 C 2 , in the paramagnetic state, leads us to attribute sites I and II of Fe 5 C 2 to Fe I and Fe II , respectively. In the case where there is only a spin direction for Fe I , this direction is located in the planes or near the planes ( a , c ) or ( a , b ).

On the validity of 57Fe hyperfine field distribution calculations from Mössbauer spectra of magnetic amorphous alloys

Abstract The approximations which are made in 57Fe hyperfine field distribution calculations from magnetic amorphous alloys are discussed. A diagram which gives an immediate indication of the validity of a given hyperfine field distribution obtained using first order perturbation theory is proposed. The assumption about the distribution of the polar angles of the hyperfine field direction with respect to the EFG principal axes in transition metal-metalloid amorphous alloys is discussed. The asymmetries of the Mossbauer spectra of the latter alloys are also considered.

Influence of the nature of milling media on phase transformations induced by grinding in some oxides

Abstract Polymorphic transformations induced by dry ball milling in an argon atmosphere have been investigated in various oxides (TiO2, SnO2, Y2O3, WO3) by X-ray diffraction, Mossbauer spectroscopy and transmission electron microscopy (TEM). The transformations have been found to depend on the nature of the milling media, particularly when reduction reactions take place between grinding tools and oxide particles. When ground with steel tools, cubic yttria is transformed into a monoclinic modification as reported in the literature, while tin oxide is reduced. When ground with zirconia tools, cubic yttria with a bixbyite type structure is transformed into cubic yttria with a fluorite type structure, while monoclinic tungsten oxide is transformed into a cubic oxide with an ReO3 type structure. Plausible structural explanations are proposed. In all cases, nanometre-sized domains have been observed by TEM even after grinding times as short as some minutes.

MICROSTRUCTURAL AND KINETIC ASPECTS OF THE TRANSFORMATIONS INDUCED IN A FeAl ALLOY BY BALL-MILLING AND THERMAL TREATMENTS

Abstract A Fe–40Al (at.%) alloy powder having a B2 ordered structure was milled in a high energy planetary ball-mill. The microstructural evolution of the alloy was followed by analysing powder specimens milled for different times by X-ray diffraction, Mossbauer spectroscopy and magnetisation measurements. Grain refinement and chemical disordering were the main transformations resulting from milling. A complete destruction of the long-range order under the adopted conditions of milling was not achieved. From diffraction analyses it was possible to see how the concentrations of such defects as dislocations, planar faults, antiphase domain boundaries, etc., were modified by the heavy deformations involved with milling. Starting from the specimens milled for the longest time, considered in this study, isothermal annealing experiments were carried out to monitor the reverse transformations. The annealing temperatures were selected on the basis of differential scanning calorimetric and thermogravimetric magnetic measurements, which revealed that several transformations occur when treating the deformed powders. Recovery and reordering take place at temperatures ranging from 100 up to 250°C. A complete reordering is possible only at higher temperatures, i.e. 700°C, when recrystallisation is fully accomplished.

Polymorphic transformations of titania induced by ball milling

Abstract The high-pressure modification of TiO2 with an α-PbO2-type structure is formed transiently during room-temperature grinding of anatase. Rutile is the only phase present after prolonged ball milling. These experimental results are at variance with reports of a direct transformation of anatase into rutile by room-temperature grinding.

Influence of milling conditions on the FeAl intermetallic formation by mechanical alloying

Abstract The formation of FeAl intermetallic compound by mechanical alloying has been investigated as a function of milling time. Mixtures of elemental powders of Fe and Al are progressively transformed into a disordered solid solution characterized by an average composition of Fe-35at.%Al. Changes in powder morphology, the degree of reaction advancement, as well as crystallite size and microstress evolution have been described. It has been found that the iron crystallite size tends to about 15 nm and that FeAl grains of the same size are formed. Mossbauer spectroscopy (MS) study has provided additional information about intermediate solid solutions, the final composition and the influence of the milling velocity on FeAl formation. No free iron has been detected in powders processed for 24 h at the highest rotation speed (400 rev min −1). MS appears to be a unique technique that can provide unequivocal information about the state of mixed powders, especially for long milling times.

Structural evolution of sputtered amorphous Fe1-xCx films for 0˙19 ≤ x ≤ 0˙49

Abstract Sputtered amorphous Fe1-x Cx films with 0˙19 ≤ x ≤ 0˙49 have been studied. The mean magnetic moment per iron atom and the mean 57Fe hyperfine field decrease, while the mean 57Fe isomer shift increases, with increasing x up to a carbon content x r ≍ 0˙32, while they remain almost constant above x r. The crystallization mechanisms also change dramatically at x r; only the Fe7C3 carbide is observed above x r. All the experimental results are explained by assuming the existence of a two-phase domain in the amorphous state for x ≍x r, one phase being amorphous Fe-C with a constant carbon content x≍ x r, and the second an amorphous C phase with a negligible Fe content. The carbon content x r, is close to the maximum amount of carbon one can combine with iron to form a carbide: x = 0˙33, in Fe2C.

Mechanosynthesis of nanophase materials

Abstract Among the synthesis methods of nanocrystalline materials, high energy milling is promising at production scale. The mechanochemical synthesis (Mechanosynthesis) of nanophase materials can be realized by direct synthesis of compounds from the elemental powders or by several exchange, transfer and mixing reactions such as for: 1) most metal carbides; 2) intermetallic compounds (silicides, aluminides); 3) semiconducting III-V compounds (GaAs and AlAs); 4) metal-oxide M-RO composites by reduction of a metal M oxide with a suitable reductant R; 5) metal-sulhide M-RS composites by reduction of a metal M sulphide with R; 6) fluorides and nitrides by exchange reactions. Reactions are driven in a ball mill at almost room temperature.

Mechanical alloying during cryomilling of a 5000 Al alloy/AlN powder: the effect of contamination

Elemental powders of 80 vol% 5000 Al alloy (3 wt% Mg) and 20 vol% AlN were milled either in liquid nitrogen or at room temperature under argon. The milling of the mixture involved changes in the chemical composition: Fe contamination, oxidation and in-situ nitridation. The effects of milling time and temperature on the contamination of the system were analysed. Oxidation and in-situ nitridation were characterized by a LECO analyser and thermal desorption measurements, whereas Mossbauer spectroscopy was used to study the mechanical alloying of the Al alloy. The results show that a metastable solid solution of Mg and Fe in Al is formed during cryomilling. The occurrence of mechanical alloying in spite of the very low milling temperature is interpreted as a consequence of the fine crystallite size of the Al matrix.