J. Bigot

Cast bulk ZrTiAlCuNi amorphous alloys

Abstract Cylindrical bulk amorphous samples with diameters up to 10 mm have been prepared by casting Zr Ti Al Cu Ni alloys in a copper mould. In order to rank glass-forming ability as a function of alloy composition, alloys were also cast into wedge-shaped moulds; to a first approximation, the thickness of the amorphous region obtained can be taken as an indication of glass-forming ability. The compositions which lead to the production of bulk glasses all have reduced glass transition temperatures in excess of 0.65 and the extremely high glass-forming ability of these compositions is discussed. We suggest that both the Al and Ti contents are determining factors for the production of bulk amorphous samples and these are believed to reduce the driving force for, and hence the rate of, crystallisation. These amorphous alloys have been found to display high thermal stability and can be annealed for several minutes in the supercooled liquid region. They are ductile at room temperature and have a high value of yield stress.

Martensitic transformation in ultrafine FeNi powders

Abstract The austenite ↔ martensite transformation in ultrafine FeNi powders (25–35 wt.% Ni; particle size, about 10–200 nm) was investigated by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and differential scanning calorimetry. The ultrafine powders were prepared by levitation melting in liquid nitrogen. The as-prepared ultrafine powders were a mixture of martensite (b.c.c. α′) and austenite (f.c.c. γ). Each individual particle was a single crystal of either b.c.c. martensite or f.c.c. austenite. In contrast, bulk alloys of the same composition quenched into liquid nitrogen were found to be entirely martensitic. Austenite stabilization of some particles is probably related to the fabrication process and to the small size of the particles. It is suggested that austenite and martensite particles might be formed directly from the metallic vapours in the cold parts of the calefaction layer. On further heating, the reverse transformation of martensite to austenite occurs in the same temperature range for the ultrafine powders and for the bulk alloys. The martensite → austenite transformation was found to obey a shear mechanism.

A neutron diffraction determination of short-range order in a Ni63.7Zr36.3 glass

The total structure factors for amorphous Ni63.7Zr36.3 have been obtained by neutron diffraction for three different isotropic compositions of nickel. The use of a zero alloy leads to a direct determination of the Bhatia-Thornton structure factor SCC. The high value of the determinant of the system produces accurate partial structure factors. From which partial pair correlation functions have been calculated. The behaviour of the partial functions and the deduced partial coordination numbers show the existence of strong chemical short-range order, the nature of which is discussed.

On the stability and structure of Cu—Zr based glasses

Abstract Glassy Cu—Zr alloys with compositions ranging from Cu33Zr67 to Cu66Zr34 have been prepared. Their thermal behaviour, structure factor and density have been measured. These results are discussed in terms of an ideal substitutional solid solution and briefly compared to the predictions of the nearly free electron-like model for the stability of metallic glasses.

Preparation of ultra-fine metallic powders. A study of the structural transformations and of the sintering behaviour

Abstract Ultra-fine metallic powders—iron, nickel, cobalt, chromium, FeCo, FeNi—were prepared by levitation melting in a cryogenic liquid (argon or nitrogen). Most of the particles were spherical single crystals. The particle size varied from about 10 nm to 200 nm, the mean diameter being 30 nm or 70 nm depending on the experimental conditions. The austenite-martensite transformation in ultra-fine FeNi powders (25 wt.% Ni, 30 wt.% Ni and 35 wt.% Ni) was investigated. It is suggested that either martensite or austenite single particles might be formed directly from the metallic vapours of the calefaction layer. Austenite stabilization would result from the increasing difficulty of plastic accomodation. The dilatometric study of the sintering behaviour showed that shrinkage is much larger for ultra-fine powders than for the classical powder (5 μm).

Alloys of high glass-forming ability

Abstract A series of Zr-Ti-Al-Cu-Ni alloys, e.g., Zr 57 Ti 5 Al 10 Cu 20 Ni 8 , were found to have high glass-forming ability (GFA). Cylindrical bulk amorphous samples with diameters of 10 mm were produced by casting the melts in a copper mould. The Zr 65 Cu 17.5 Ni 10 Al 7.5 alloy, which was reported to have the highest GFA heretofore, was found to be amorphous only in the surface with radius dimensions of less than 1 mm under the same cooling conditions. The alloys were also cast in a chevron-shaped copper mould to compare the GFA of the alloys. Ti and Al were found to be the critical components favorable to the GFA of the Zr-Ti-Al-Cu-Ni alloys, which could greatly reduce the crystal growth rate of the alloys. Cu and Ni appeared to be the less critical components. However, it was necessary to modify the contents of all the components according to their effects in order to design an alloy of high GFA. The amorphous alloys displayed very high strength, thermal stability and superplasticity at elevated temperature.

Dodecahedral-shaped quasicrystalline precipitates in dilute Al-Mn solid solutions

Abstract Abstract The Al-Mn icosahedral phase is found to precipitate and grow at room temperature from rapidly quenched Al-4 at % Mn f.c.c. solid solutions, with the composition 13•5 ± 1•5at.%Mn, the surrounding matrix having a composition of 3•1 ± 0•3 at.% Mn. Dodecahedral-shaped precipitates with orientation relationships with the f.c.c. matrix are produced. The above information is rationalized in terms of symmetry arguments and growth processes. Comparison of the compositions obtained here with that of other work extends our knowledge of the Al-Mn metastable equilibrium between the icosahedral phase and the f.c.c. Al-Mn solid solution in the solid state.

Effects of Al on the glass-forming ability of Zr-Cu based alloys

Abstract Effects of Al on the glass-forming ability (GFA) of Zr-Cu alloys were investigated by studying the solidification characteristics of the Zr65Cu27.5Al7.5 alloy in comparison with those of the Zr65Cu35 alloy. It is presumed that the much higher GFA of the Zr65Cu27.5Al7.5 alloy than that of the Zr65Cu35 alloy is due to the much lower diffusion coefficient of Cu and Al in the Zr63Cu27.5Al7.5 melt, the low nucleation rate of the Zr2(Cu, Al) nucleating phase due to its small nucleation driven force and long nucleation reduction time resulting from the addition of Al. The higher GFA of the Zr65Cu27.5Al7.5 alloy does not result from the crystal growth difficulty caused by the long-range redistribution of Al at the solid-liquid interface. In fact, Al does not have long-range redistribution at the solid-liquid interface but is trapped by the growing solid to form Zr2(Cu,Al) and Zr(Cu,Al) phases under rapid quenching conditions.

Preparation of CUxZr1−x metallic glasses by sputtering and their thermal stability, electrical and optical properties☆

Amorphous CuxZr1−x (0.30 < x < 0.60) films were sputtered from an alloy target onto silica substrates held at room temperature. Their structure was checked by electron or X-ray diffraction depending on the thickness of the films. The crystallization process was followed by differential thermal analysis. The thermal stability of the samples was compared with that of samples obtained by rapid quenching from the melt. Measurements of the reflectance R and transmittance T were performed at normal incidence in air between 0.03 and 6.2 eV. The dielectric function was determined for thin films from R and T and for opaque samples from R using the Kramers-Kronig analysis. The effect of oxidation is discussed. Optical absorption spectra for the amorphous samples exhibit interband transitions at low energies as in pure zirconium and a broad peak around 3.5 eV, which could be due to transitions starting from the copper d band shifted to high energy with respect to its position in pure copper. These results agree with those from previous UV photoelectron spectroscopy measurements and could be an effect of compositional rather than of structural disorder.

Calorimetric evidence of structural changes in thermal aged CuZr and FeCoBSi amorphous alloys

Abstract Evidence of structural relaxation phenomena has been obtained by DSC in Cu-Zr and Fe-Co-B-Si amorphous alloys. Two major effects are described: an exothermic (irreversible) phenomenon which occurs during the first scan of the as-quenched sample, and an endothermic (reversible) one which is observed on the pre-annealed samples. Quantitative measurements of the endothermic phenomenon as a function of the annealing conditions are reported for several Cu-Zr alloys.