N. Cowlam

A study of Cu50Fe50 produced by mechanical alloying and its thermal treatment

A 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 γ‐Fe. The annealing of these MA treated specimens effects a decomposition of the extended solid solution into FCC copper and both α‐ and γ‐iron allotropes. This decomposition process is discussed in relation to spinodal decomposition and to nucleation‐and‐growth mechanisms.

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...

The study of self-diffusion in crystalline and amorphous multilayer samples by neutron reflectometry

Abstract Real time, in-situ, annealing experiments on the CRISP reflectometer at ISIS neutron source, RAL, Chilton, have been made to measure the self diffusion of nickel in special isotope-enriched polycrystalline natNi:62Ni multilayers and in amorphous a-62Ni55Zr45:a-natNi55Zr45 multilayers where grain boundary diffusion is absent.

A search for non-collinear ferromagnetism in INVAR

Measurements have been made to test the proposal that the INVAR effect is associated with a non-collinear ferromagnetic state. Neutron scattering experiments with polarization analysis of the incident and scattered beams have been made to obtain the absolute spin-dependent scattering cross-sections from the archetypal INVAR alloy Fe65Ni35. The measured average spin-flip cross-section for this alloy has been found to be close to zero, independent of the scattering vector Q and effectively constant as the sample temperature is reduced from 300 to 4.2 K and the magnetic field increased from 1.4 to 2.0 T. All this suggests that the INVAR sample is a collinear ferromagnet. In addition, the measured spin-flip cross-section is also in poor agreement with calculated curves based on models of the proposed non-collinear state. The magnitudes of the magnetic moments on the iron and nickel atoms in the collinear state have been obtained from the intensities of the Bragg peaks in the non-spin-flip cross-sections. The diffuse scattering between the Bragg peaks has also been analysed to determine the magnetic disorder present. These data have been discussed in the context of different models of the magnetic structure and the results also support the conclusion that Fe65Ni35 is a collinear ferromagnet.

Size-quantization in extremely small CdS clusters formed in calixarene LB films

CdS nanoparticles have been formed within Y-type Langmuir-Blodgett (LB) films of cadmium salts of calix(8)- and calix(4)-arene by reaction with H2S. UV-vis absorption spectra of the LB films, measured at room temperature, show well-resolved transitions between size-quantization levels in CdS clusters. The size of the CdS particles, obtained by Gaussian fitting of the experimental spectra, is 1.5 +/- 0.3 nm, which is much less than those reported for fatty acid LB films. The particle size does net depend either on the type of calixarene or the number of LB layers. LB films were also characterised by X-ray diffraction and ellipsometry which show the film thickness do not change substantially after treatment with H2S. The mechanism of CdS nanoparticles formation is discussed

Metallic glasses: structure and properties

Abstract The experimental determination of the structures of metallic alloy glasses and the modelling of their structures are reviewed. Some current issues in metallic glass research are discussed, including structural simulation by computer methods and the subject of structural anisotropy in metallic glass ribbons and wires. The different types of magnetic order which can exist in metallic glasses are discussed, using the example of Fe-Zr glasses.

The crystal growth and atomic structure of As2(Se, S)3 compounds

Abstract The experimental conditions required for the growth of crystals of arsenic chalcogenides have been investigated, as a preliminary to the preparation of crystals of the mixed compounds As2(Se, S)3. Crystals of these compounds As2(Se1−xS x )3 have been prepared for 0<x<0.5, the size of the crystals produced decreasing rapidly with sulphur content. The crystal structure and site occupation of these mixed compounds have been examined by X-ray and neutron diffraction methods. The mixed compounds have the same monoclinic structure as the parent materials, but with neither a random solid solution nor an As2Se2S superlattice. Instead it appears that the S-As-S (or Se-As-Se) chain structure is important in both aspects of our study. In the crystal growth this structure provides a growth spiral which favours long needle-shaped crystals. On the other hand the geometrical similarity between this chain and the cradle-shaped molecule of AsS also appears to influence the site occupation in the mixed compounds, ...

The structure of Fe-Ni amorphous alloys

Abstract The magnetic structures of amorphous Fe-Ni alloys have been studied using fully spin polarised neutron scattering techniques with the IN20 spectrometer at ILL. The results show a large spin-flip scattering which can arise only from a non-collinear structure. In contrast to previous work it is found that Ni atoms carry a magnetic moment and that this is misaligned from the Fe moments by either local crystal field effects or antiferromagnetic exchange interactions.

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

Abstract Solid state amorphization reactions in CuTi 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 CuTi 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.

Mechanical alloying of immiscible Cu70TM30 alloys (TM = Fe,Co)

Abstract Four series of copper-based alloys with compositions Cu70Co30, Cu70Fe30Cu70Co15Fe15 and Cu70(CoFe)30 were synthesized by mechanical alloying (MA). The interest in these materials is as parents of alloys which will show the giant magnet resistance (GMR) effect. The structural evolution was studied as a function of the MA time by means of Mossbauer spectroscopy, X-ray and neutron diffraction techniques and differential scanning calorimetry (DSC). In spite of the positive heat of mixing of the alloy constituents, the complete dissolution of the minority phases (iron and/or cobalt) into the copper f.c.c. lattice was observed for each specimen after 16 h of MA. This alloying was accompanied by changes in lattice parameter ─Δa 0/a 0─ of up to 0.5% with respect to pure copper. Analysis of the crystallization products arising from DSC treatments up to 600°C revealed a separation into copper f.c.c. and iron-based b.c.c. phases. In the case of the ternary Cu70(FeCo)30 alloys, the FeCo intermetallic compoun...