Search

This work is aimed at quantifying mechanochemical effects in reactive solid–gas systems. It deals in particular with the kinetics of H 2 absorption processes on a Mg 2 Ni/Ni composite under thermal and mechanical activation. In a first series of experiments, powders were exposed to H 2 under static conditions for selected time intervals and successively subjected to mechanical treatment. In a second series, the latter was instead started in coincidence with the exposure of powders to H 2 . The effects of mechanical processing on the H 2 absorption processes have been quantified via the accurate description of the absorption kinetics with suitable model curves. A tentative rationalization of experimental observations is proposed.

A systematic investigation on different systems has been performed in order to ascertain the possible role of local temperature rise at impacts in the mechanism of mechanically induced phase transformation processes. High-purity commercial powders have been mechanically treated under different milling conditions to study their kinetic behavior. Phase transformations have been correlated to the minimum temperatures needed to trigger the transformation processes under thermal activation conditions. Deep contradictions have arisen in connection with the so-called “hot spots” theory.

We report on the structural evolution and thermodynamic features of Zr-based amorphous and nanostructured powders subjected to mechanically induced hydrogen absorption. Zr 55 Cu 30 Al 10 Ni 5 and Zr 69.5 Cu 12 Al 7.5 Ni 11 compositions were studied. The process began with a milling induction period, during which neither gaseous uptake nor structural modifications were observed, then, according to an s-shaped kinetics, sorption reached the maximum rate, approaching finally the asymptotic value of 1.5 H/M wt.%. Specific kinetic features were depending on the starting materials. As a result of the absorption, the amorphous alloys of both the compositions promoted the segregation of crystalline δ-ZrH 1.66 phase, which was paralleled by the formation of an amorphous hydride of depleted Zr composition, with relative abundance of the two hydrides 45:55. The different thermodynamic stabilities of the two hydrides were evidenced by calorimetric analyses. While desorption started below 400 K in the metastable one, conversely the crystalline δ-ZrH 1.66 decomposed only above 1070 °C. Faster absorption kinetics and different structural reconstruction path characterised the hydrogenation of the Zr 55 Cu 30 Al 10 Ni 5 nanocrystalline alloy. Starting from a mixture of two nanosized fcc phases, with similar (Cu, Al, Ni)Zr 2 composition, a single fcc H–(CuAlNi)Zr 2 phase was formed, which reversibly desorbed H 2 , restoring the two initial phases.

In mechanical treatments by ball milling, the mechanical energy is transferred to powders when these are trapped between colliding milling tools. Under such processing conditions, the number of collisions is therefore the natural quantity to which refer the kinetics of any given transformation. A statistical model is here developed to relate the transformation rate to the amount of powder trapped at each collision. The phenomenological model is applied to mechanically induced amorphization processes of intermetallic compounds and binary mixtures. Provided that accurate experimental methodologies are employed to estimate the amorphization rate and the number of collisions, the comparison between experimental and theoretical kinetic curves permits to obtain a deeper insight into the observed kinetic behavior.

The present work discusses the preparation of nanostructured ceramics by mechanical processing. Coarse-grained powders of different oxides, carbides, borides and silicides have been subjected to mechanical treatment inside a commercial ball mill under controlled experimental conditions. Ceramic powders underwent a microstructural refinement process down to the nanometer range, accompanied by the accumulation of structural disorder. The analysis of X-ray diffraction patterns according to the Rietveld method permitted the estimation of both the average crystallite size and strain content as a function of the milling time. Opposite trends are observed, the decrease of the crystallite size being paralleled by the increase of the strain content. The processes of crystallite size decrease and strain accumulation were then phenomenologically characterized in terms of apparent rate constants and asymptotic final values of crystallite size and strain. It has been thus possible to point out an intimate connection between the processes of crystallite size decrease and strain accumulation.

We focus on the hydriding kinetics of Mg2Ni/Ni nanocrystalline and Zr55Cu30Al10Ni5 amorphous powders under reactive milling conditions. A methodological approach, based on mechanochemical parameters, is presented to quantify the different behaviour shown by the two systems. A decelerating course characterized the H2 absorption rate in the Mg based powders. Although they were able to absorb H2 under static conditions, a speeding up of the kinetics was observed under milling. The mechanochemical gain, g m , that is the rate increase at the milling outset, was found to depend on the specific milling intensity I M . By contrast an incubation period preceded the absorption in the case of the Zr based amorphous powders. Furthermore, S-shaped trends described the reactive behaviour in this case even if the kinetics was still dependent on I M . Irrespective of the milling intensity it was found that the same mechanical work, that is the energy dose, was required to reach the maximum hydriding rate.

The effect of mechanical activation of reactants on the self-propagating high-temperature synthesis of TaSi 2 and Ta 5 Si 3 was investigated. SHS experiments were performed on mixtures of compositions Ta:Si = 1:2 and Ta:Si = 5:3 which had been milled for various times. For both compositions, mechanical activation had a large influence on the characteristics of the combustion process, particularly on the combustion peak temperature and wave velocity. The product composition depended on the initial stoichiometry and milling time. For the 5:3 stoichiometry, the product was single phase Ta 5 Si 3 (in its hexagonal and tetragonal forms) only when the powders were milled for sufficiently long milling times (>120 min). The 1:2 stoichiometry produced TaSi 2 in a pure form, regardless of milling conditions. A mechanism for the combustion process is proposed on the basis of the microstructural evolution in quenched reactions.

It is well-known that the mechanochemical processing of binary mixtures of transition metals can induce the formation of nanostructured and amorphous phases. Previous kinetic studies have shown that the phase transformation in binary mixtures follows sigmoidal conversion curves irrespective of the intensity of mechanical processing. The rate of such transformations is instead strongly influenced by both the intensity of the mechanical treatment and the physical properties of the reactant powders. In the present work, we give additional information on the amorphisation kinetics of binary mixtures by exploring the effect of composition on the transformation kinetics. Cu–Ti and Fe–Ti mixtures with composition within the amorphisation ranges have been subjected to mechanical processing at fixed intensity. The kinetics of phase transformation has been followed by quantitative X-ray diffraction. The experimental data obtained demonstrate that the amorphisation rate has a strong dependence on the product of the mass fractions of the single reactants.

A phenomenological approach is presented to analyse the reaction kinetics and compare the efficiency of processes driven by mechanochemical and thermal activation. Solid phase systems as well as gas–solid processes are examined: the synthesis of equiatomic CoFe nanostructured solid solution, the hydrogen absorption by Mg2Ni alloy and the CO hydrogenation over supported CoFe powder catalyst. In spite of the low efficiency of the mechanical processing technique, related to the discontinuous treatment, however, when elementary impact events are taken into account, mechanically activated processes show remarkably higher conversion rate values than their thermal counterparts. Such result can be related to the occurrence of mechanochemical effects.

Analytical electron microscopy and X-ray diffraction techniques have been used to study the amorphization process of the TiAl system induced by mechanical alloying. The results show that the entire transformation develops from the diffusion of Al into the Ti crystal lattice, beginning as soon as a well-developed layered structure is formed in the powders in the early milling stages (after 6 h). Collapse of the highly distorted host lattice occurs after milling for 20 h and gives rise to a homogeneous amorphous structure at 24 h. Further milling time (34 h) produces an f.c.c. nanocrystalline phase with a TiAl at. ratio of approximately 3 and with the lattice parameter a=0.42 nm.

Some titanium-based alloys were examined by X-ray and neutron diffraction and differential scanning calorimetry (DSC) as part of a continuing research programme on amorphous alloys produced by mechanical alloying (MA). Large differences were found between the X-ray and neutron diffraction patterns on the same materials, which can be understood in terms of the relative visibilities of the alloy constituents for the two radiations. The neutron diffraction patterns provide unequivocal evidence that the alloys contain both amorphous and crystalline phases. A characteristic series of broad Bragg peaks was observed in the neutron diffraction patterns of amorphous CuTi, PdTi and AlTi alloys which was not evident when using X-rays. These peaks were identified with a mixed oxide-nitride Ti(O, N) impurity phase. DSC analysis of selected milling products revealed the transformation of the metastable phases. It also provided enthalpy data for a thermodynamic description of the milling process. These observations provide support to our previous findings on the significance of gaseous impurities of the parent metals in the MA process.