L. Schiffini

The invariant laws of the amorphization processes by mechanical alloying: I. Experimental findings

Abstract Several Ti-, Zr-, Hf- and Nb-based alloys were synthesized using differing milling regimes. Metastable phases, either crystalline or amorphous, develop from the parent elements according to a general sigmoid-shaped behaviour ruled by an interface-controlled kinetic mechanism. The extent of the alloying reactions was related to the operating variables, experimentally determined in the course of the process. Although the transformation rates depended on the milling intensity, that is on the impact energy times the impact frequency, it was found that the reaction yield, defined by the ratio of the transformed fraction to the specific energy dose, is an invariant quantity characteristic of each system. The specific energy dose defines the mechanical work done on the system per mass unit of the reactants. A rationale for the observed behaviours was provided by the energy needed to reach a given level of the reactant dispersion. Ruling the total extent of the grain boundary area, and the overall kinetics of the alloying process, the work expended in the microstructure refinement was found to be another invariant property of the treated mixtures. The reaction yield is the reference parameter to compare milling trials on an absolute basis, so providing an opportunity towards a quantitative understanding of the mechanical alloying processes.

X-ray powder diffraction and Mossbauer study )of nanocrystalline Fe-Al prepared by mechanical alloying

Abstract Iron-aluminium alloys of composition Fe 50 Al 50 and Fe 75 Al 25 were produced by mechanical alloying (MA) of the pure elemental powders. A structural refinement of X-ray powder data on the mechanically alloyed products according to the Rietveld method has detailed the progressive dissolution of aluminium into the lattice of α-iron as a function of MA time. With respect to pure iron, a volume expansion of ≈ 3% is measured in both compositions mechanically alloyed for 32 h. In the iron and aluminium phases, the Debye-Waller static disorder increases as a function of MA time and the intrinsic shape of the peak profiles becomes predominantly Cauchy. These changes are accompanied by an increase in the average microstrain and by a reduction in the average crystallite size (which includes also the effect of dislocations). The Mossbauer spectra show that in the equiatomic case the initial sharp magnetic sextet of α-iron is progressively reduced and is replaced by a doublet, while for the Fe 75 Al 25 composition a broad magnetic sextet is eventually obtained. Thermal scans at 600°C of the specimens mechanically alloyed for 2, 4 and 8 h precipitate essentially the Al 5 Fe 2 phase. In the case of the Fe 50 Al 50 specimens, annealing of the powders mechanically alloyed for 16 and 32 h precipitates mainly the partially ordered FeAl intermetallic compound, whilst no ordering is obtained in the Fe 75 Al 25 case.

X-ray diffraction study of the amorphization process by mechanical alloying of the NiTi system☆

Abstract X-ray Fourier methods were used to follow fragmentation phenomena and structural defects during mechanical alloying processes in the Niue5f8Ti system. Furthermore, a methodological X-ray diffraction approach was developed to study the kinetics of the process up to complete amorphization of the powders. Results show that the effective crystallite size and defect concentration of the unreacted powders achieve asymptotic values after mechanical alloying for about 5 h, whereas the amorphization process is accomplished after 30 h. The limiting value of the effective sizes, calculated after correction for the strain contribution, is around 30 nm, which is one order of magnitude larger than the values reported in these materials from a line-broadening analysis using the Scherrer equation. As for the defect concentration, elemental titanium and nickel powders milled separately show approximately the same strain content as the Niue5f8Ti mixture treated under the same experimental conditions. However, the pre-induced disorder does not speed up the solid state reaction; in fact, when the single elements milled separately are reacted, the amorphization process is also completed in 30 h.

Thermal properties of mechanically alloyed Ni50Ti50 powders

Abstract Ni 50 Ti 50 amorphous powders were prepared by the novel technique of ball milling pure elements. Amorphization was complete after milling for about 35 h. X-ray diffraction was used to determine the amount of amorphous phase produced. Differential scanning calorimetry (DSC) was employed for thermal analysis. At various stages of alloying, two phenomena were detected. In the temperature range 450–650 K, the amorphization of part of the alloy occurs, giving a large exothermal signal. Peak shape analysis was used to estimate the coefficient of interdiffusion in the amorphous alloy. In the temperature range 680–780 K, crystallization occurs. The DSC peak changes its shape and area with milling time. Increases in the temperature of the maximum and of the heat crystallization were observed. The role of the specific heat difference between liquid and crystal phases for solid state amorphization is outlined.

Toward a Quantitative Understanding of the Mechanical Alloying Process

The mechanical alloying behavior of transition metal-based mixtures and the amorphization of equilibrium intermetallic compounds by milling were characterized on a quantitative basis as a function of the processing parameters. Different kinetic mechanisms rule the two processes. The former giving rise to a sigmoidal-shaped trend, while the latter is characterized by a continuous, gradual decrease of the conversion rate. It was found that the extent of the microstructure refinement determines the overall transformation rates and that the mechanical energy dose required to transform a given amount of the parent phases is a characteristic invariant quantity of the system undergoing amorphization. The atom fluxes at the impact, the sublayer distance affected, and the dimension of the amorphous domains formed were also determined. The emerged framework supports the view that diffusive phenomena and chemical driving forces play a minor role in the mechanical alloying processes. Conversely, the thermodynamic qualities of the reacting systems and the enthalpy gain remain the main factors in mechanically induced combustion reactions, provided a critical dispersion of the reactants is reached under milling.

Copper–cobalt f.c.c. metastable phase prepared by mechanical alloying

Abstract The solid state reaction of the normally immiscible copper and cobalt powders has been studied at the equiatomic composition with the mechanical alloying (MA) technique, supplemented by the milling of cobalt and copper elemental powders. The diffraction of unmilled pure cobalt powder shows the coexistence of the two face-centred-cubic (f.c.c.) and hexagonal-close-packed (h.c.p.) allotropes. After 1 h of milling the neutron diffraction pattern reveals only the highly distorted h.c.p. phase. Further, the degree of distortion in the h.c.p. phase is highly dependent on the crystallographic directions. Mechanical alloying the Cu-Co equiatomic mixture creates an almost entirely f.c.c. single phase after zh of treatment. The lattice parameter of the Cu(Co) extended solid solution decreases on increasing the milling time. Moreover, a thermal treatment at 700°C of the powders M A 16 h demixes the pure constituents. A comparison with previous data on the Cu-Co system prepared by rapid quenching, evaporatio...

Structural and elastic behavior of Fe50Al50 nanocrystalline alloys

Pure iron and aluminum powders were mixed in the equiatomic ratio and mechanically alloyed in a high‐energy ball mill for different times. Structure refinement of x‐ray powder diffraction data was performed to study the structural transformations induced by mechanical and subsequent thermal annealing treatments. The mechanical alloying (MA) process induces a progressive dissolution of aluminum phase into the bcc iron phase. After 32 h of MA a single‐phase Fe(Al) bcc extended solid solution, with lattice parameter a0=2.891 A, average coherent domain size 〈D〉≊50 A, and lattice strain 0.5%, was observed. The annealing of the specimens after MA up to 8 h favored the aluminum dissolution in α‐iron and the precipitation of the Al5Fe2 phase, whereas a nanostructured B2 FeAl intermetallic compound was observed in the annealed samples which were previously milled for 8, 16, and 32 h. In the same specimens a minority cubic phase Fe3AlCX, anti‐isomorphous with perovskite, derived from contamination of ethanol and in...

Some kinetic features of mechanical alloying transformation processes

Abstract The development of a methodological approach and experimental protocols have permitted the accurate evaluation of some of the key parameters of a ball milling process, such as the impact energy and the number of impacts. Thus, a detailed description of the milling regime in terms of energy transfer to the powders has been possible. On this basis, the crystal to amorphous state reaction of Cu–Ti mixture has been studied for widely differing milling conditions. The analysis of X-ray diffraction data has shown correlations between the parameters of the milling treatment which relate to energy and the structural evolution of the amorphous phase. An attempt has been made to relate the observed kinetic features to a rate law involving microscopic pulse energy and structural factors.

Crystal-to-glass transformation in the Ni-Ti system by mechanical alloying and consequent surface area

Abstract The structural transformation occurring by mechanically alloying nickel and titanium crystalline powders has been investigated by a variety of X-ray techniques, up to the complete formation of an amorphous Ni50Ti50 phase. Whether the starting pure metal powders are highly strained or not, the amorphization reaction takes the same milling time to arrive at completion, and the surface area of the final amorphous powder is about 20m2g−1. This value is about three orders of magnitude higher than that reported for amorphous flakes prepared by grinding melt-quenched ribbons. The result suggests that mechanically alloyed powders can be used conveniently for catalytic performance.

Thermal behaviour of CuTi and CuTiH amorphous powders prepared by ball milling

Abstract Solid state amorphization reactions in Cuue5f8Ti have been studied by means of DSC and structural techniques. The influence of hydrogen from the parent titanium powder on the amorphization and crystallization processes has been investigated. For Cuue5f8Ti a diffusion-controlled process can be inferred for solid state amorphization from the parabolic trend of the heat of crystallization, as a function of the milling time. The presence of hydrogen in the alloys is found to modify the crystallization behaviour of the amorphous phase. A DSC method for the determination of the amount of hydrogen present in the alloys is given.