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Evidence of fractals in the preliminary stages of amorphisation by mechanical alloying
No full textThe evolution of the amorphous phase in the solid state reaction of alloys with composition Fe67Ti33 was followed with neutron and X-ray scattering techniques in the case of metal powders treated by mechanical milling (MM) and by mechanical alloying (MA). The X-ray and neutron diffraction data show that the milling time is related to the coordinate of reaction. In the case of neutron small-angle scattering, data analysis of the log I vs. log Q graphs suggests that the fractal concept is useful in describing the earliest stages of the solid state amorphization reaction
NEUTRON-DIFFRACTION AND NEUTRON SMALL-ANGLE SCATTERING STUDY OF THE AMORPHIZATION REACTION IN NI50MO50
No full textThe solid state amorphisation reaction in a Ni50Mo50 alloy has been investigated by neutron diffraction and small angle scattering for a series of specimens mechanically alloyed for 2, 4, 8, 16 and 32 h. In the initial stages of the reaction, deformation of the parent crystalline phases occur. After 8 h of treatment, a change in the samples is observed and those treated for 16 and 32 h consist of an amorphous NiMo phase with some free molybdenum. Small angle scattering graphs of log I(Q) vs. log Q show a sequence of changes which correspond to the sequence of changes seen in the diffraction data, The graphs for the 2 and 4 h samples are linear over an extended range of scattering vector, Q. The slope of the graphs is about -3.4 which is indicative of a rough surface having fractal character between the transformed and untransformed regions of the sample
A NEUTRON-SCATTERING STUDY OF THE AMORPHIZATION REACTION IN A NI50MO50 ALLOY
No full textThe solid state amorphization reaction in Ni50Mo50 alloys has been investigated by neutron diffraction and neutron small-angle scattering. A series of specimens has been mechanically alloyed for 2, 4, 8, 16 and 32 hours in a high-energy ball mill. The initial stages of the reaction are characterized by the deformation of the parent crystalline phases. After 8 hours of treatment a change in the samples is observed and those treated for 16 and 32 hours consist of an amorphous NiMo phase with some free molybdenum. The intensity of the small-angle scattering is much greater for the 2 hour specimen than for the parent, but decreases regularly for longer milling times. The Guinier plots show considerable curvature indicative of particles with a range of sizes. Graphs of log I (Q) versus log Q show a sequence of changes which correspond to the sequence of changes seen in the diffraction data. The graphs for the 2 and 4 hour samples are linear over an extended range of scattering vector Q. The slope of these graphs is about -3.4, which is indicative of a rough surface having the fractal dimension of 2.6. We tentatively associate this with the convoluted surfaces between the nickel and molybdenum particles which are drawn into an interpenetrating filamentary structure in the early stages of the mechanical alloying process
A study of nanocrystalline binary Fe80Cu20 and multicomponent Fe81Cu1Si9B6Nb3 alloys prepared by mechanical alloying.
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A study of Cu<sub>50</sub>Fe<sub>50</sub> produced by mechanical alloying and its thermal treatment
No full textA specimen of Cu50Fe50 equiatomic composition was mechanically alloyed (MA) by ball milling starting from the pure elements, which are immiscible according to the equilibrium phase diagram. Structural analysis by x-ray and neutron diffraction has shown that the mechanical process initially reduces the crystallite size of both elements as a function of the milling time. The diffraction data show that the bcc iron phase is subsequently consumed, due to progressive incorporation of the iron atoms into the fcc copper matrix. The Mossbauer spectra of a specimen MA for 16 h has a broad magnetic profile typical of a Fe-Cu extended solid solution, with some evidence of two local environments of the iron atoms and a small admixture of the gamma-Fe. The annealing of these MA treated specimens effects a decomposition of the extended solid solution into FCC copper and both alpha- and gamma-iron allotropes. This decomposition process is discussed in relation to spinodal decomposition and to nucleation-and-growth mechanisms
COPPER COBALT FCC METASTABLE PHASE PREPARED BY MECHANICAL ALLOYING
No full textThe 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 of the Cu-Co equiatomic mixture creates an almost entirely fc.c. single phase after 2 h of treatment. The lattice parameter of the Cu(Co)extended solid solution decreases on increasing the milling time. Moreover, a thermal treatment at 700-degrees-C of the powders MA 16 h demixes the pure constituents. A comparison with previous data on the Cu-Co system prepared by rapid quenching, evaporation and magnetron sputtering methods confirms the MA method to be an efficient synthesis tool of new metastable non-equilibrium phases
The study of nonequilibrium Fe-Ti alloys prepared by high-energy mechanical deformation
No full textWe have studied the nature of the solid-state amorphization induced by mechanical deformation (ball milling) in the case of the Fe2Ti phase [mechanical milling (MM)] and that of the iron and titanium phases mixed in the atomic proportion Fe67Ti33 [mechanical alloying (MA)]. Neutron and x-ray diffraction, Combined with Mossbauer spectroscopy at room temperature, of samples mechanically treated for different times indicate that the MM of the Fe2Ti compound leads to a highly fragmented and disordered (micro)crystalline phase. No changes of the Fe2Ti (MgZn2-type) structure are observed in the mechanically and thermally treated Fe2Ti alloys. Conversely, the MA of the pure elements shows a partial amorphization reaction. The chemical short-range order observed in the neutron-diffraction data indicates that a true alloying of the elements occurs. In addition, the thermograms of the MA specimens are different from those of the equivalent MM products. In fact, the phases observed after annealing at 600 degrees C of the specimens MA 2 and 4 h are a mixture of alpha-iron and of disordered Fe-Ti intermetallic. A fully amorphous structure is obtained after annealing the samples MA treated for 8 and 16 h. (C) 1996 American Institute of Physics
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