M. Harmelin

Crystal-amorphous phase transition induced by ball-milling in silicon

Abstract We present an experimental study of the first reported crystalamorphous phase transition induced by ball-milling in a pure element: a silicon powder. Scanning electron microscope observations show some particular spherical particles with smooth shapes. Only minor contamination with iron and chromium (less than or equal to 0.2 at.%) has been determined by chemical EDX microanalyses. The analyses of the X-ray patterns reveal the coexistence of silicon microcrystallites and an amorphous phase. Transmission electron microscope investigations (selected area diffraction patterns) show that such mixing is on the micrometer scale. The various enthalpic evolutions which have been studied by differential scanning calorimetry experiments reveal three major contributions which may be successively interpreted starting from room temperature up to 725°C as a release of the strain energy (exothermic event), followed by an equilibration between the in situ differential scanning calorimetry annealed silicon crystallites and the amorphous phase (endothermic effect), and subsequently by the crystallization of the amorphous phase (exothermic contribution). The influence of the ball-milling conditions on the dynamic equilibrium between the crystalline and the amorphous phases has been studied.

Experimental investigation and thermodynamic calculation of the Al–Mg–Zn system

Abstract On the basis of a critical assessment, experimental investigations by EPMA on ternary Al–Mg–Zn alloys were specifically performed to provide missing data of the ternary solubilities of the Al–Mg and Mg–Zn phases as well as to improve the knowledge of the extensions of the homogeneity ranges of the ternary τ- and Φ-phases. A thermodynamic description for the Al–Mg–Zn system was obtained taking into account those experimental data together with constitutional, thermodynamic and crystallographic literature information. The binary intermetallic phases are modelled to have ternary solubilities. The ternary τ-phase is modelled according to its crystal structure with cubic symmetry as (Mg) 26 ( Mg , Al) 6 ( Al , Zn , Mg) 48 (Al) 1 in the compound-energy-formalism. The Φ-phase is described by the sublattice formula Mg 6 ( Al , Zn ) 5 .

Metastable phase transformations induced by ball-milling in the CuW system

Abstract For the first time, based on X-ray diffraction, scanning electron microscopy/energy dispersion X-ray analyses, differential thermal analysis and differential scanning calorimetry experiments, the crystal to non-equilibrium phase transition induced by ball-milling has been evidenced in the CuW system. This system exhibits a total immiscibility in both solid and liquid states. Starting from elemental copper and tungsten powders, a partial solubility of copper into the cubic tungsten lattice as well as of tungsten into the f.c.c. copper lattice is induced by ball-milling. Such an enhancement of the solubility is revealed by an effect on the lattice parameter of both Cu(W) and W(Cu) crystalline solid solutions over the whole investigated composition range, i.e. Cu 5 W 95 to Cu 95 W 5 (wt.%). Such an experimental investigation of the crystal to amorphous phase transition induced by ball-milling in a system which exhibits a positive heat of mixing either in the solid or in the liquid states supports our previous results leading to the conclusion that the mechanisms of the phase transitions induced by ball-milling are different from those of the so-called classical solid-state amorphization.

Far-from-equilibrium phase transition induced by mechanical alloying in the CuFe system

Abstract By means of X-ray diffraction patterns, scanning electron microscopy observations, chemical energy-dispersive analyses and thermal stability studies (differential scanning calorimetry and thermomagnetometry), the far-from-equilibrium phase transition induced by mechanical alloying in the FeCu system has been investigated. On the Cu-rich side of the phase diagram a supersaturated f.c.c. crystalline (Cu) solid solution is detected, while on the Fe-rich side a supersaturated b.c.c. (Fe) solid solution is detected. The sticking of the Cu particles which occurs during milling (leading to a Cu end-product depletion) is avoided by the addition of an ethanol fluid to the initial Fe and Cu powder mixture, resulting in a better homogenization of the end-product (in terms of composition). In this case an amorphous phase has been found in addition to the former supersaturated crystalline phases.

Experimental investigation of the MgAl phase diagram from 47 to 63 at.% Al

The purpose of this work was to investigate the MgAl phase diagram in the composition range from 47 to 63 at.% Al which has not previously been consistently determined. The rhombohedral e phase n ear 56 at.% Al is found to form at 410°C following a peritectoid reaction (γ + β → e) and not a peritectic reaction as reported by Schumann and Voss (Giessereiforschung. 33 (1981) 43). The respective homogeneity ranges of the γ phase in the Al-rich part and of the β phase have been determined. Besides the γ, β and e phases, the existence of a high temperature phase (named λ) between 435 and 445°C is suggested. The existence of the ξ phase and of the relevant invariant reactions reported by Schumann and Voss are not confirmed by the present results.

Crystal-to-amorphous phase transition induced by mechanical alloying in the GeSi system

Abstract We report on the crystal-to-amorphous phase transition induced by mechanical alloying in a mixture of germanium and silicon powders. Scanning electron micrographs show that some particles exhibit particularly smooth shapes. The X-ray diffraction patterns reveal the coexistence crystalline diamond Ge 1 − x Si x solid solution and an amorphous phase. Neither silicon nor germanium crystalline phases have been detected. Some particles about 100 nm in diameter exhibit a pure amorphous structure (transmission electron microscopy bright field images and selected-area diffraction patterns). Differential thermal analysis confirms the existence of an amorphous GeSi phase by an exothermic crystallization peak on heating. The influences of the mechanical alloying conditions and of the initial composition on the end product have been investigated. The container contamination, which has been determined by scanning electron microscopy and energy-dispersive X-ray microanalyses, has been found not to exceed 0.4 at.% Fe (no significant chromium contamination has been detected).

The iron-neodymium phase diagram

The authors report the development of an advanced technology for preparing high purity, oxygen-free, homogeneous and well-controlled metallic alloys by levitation melting, for carrying out quantitative high temperature differential thermal analysis (DTA) measurements, and for producing rapidly solidified alloys (cooling rates > 10{sup 6}{degrees}C s{sup {minus} 1}) by the planar flow casting process. They have undertaken to reinvestigate the binary Fe-Nd diagram in order to clarify the existence of the Fe{sub 2}Nd phase and to determine the effect of the rapid solidification on the phase formation.

Determination of the activation energies for nucleation and growth of crystal nuclei in metallic glasses

A mathematical procedure is proposed in order to determine separately the activation energy for nucleation,En, and for growth,Eg, from isothermal crystallization experiments on metallic glasses. Differential scanning calorimetry (DSC) is used in the isothermal mode to estimate the crystalline fraction as a function of time. The model deals only with polymorphic and eutectic growth. Cu60Zr40 amorphous alloys produced with different quenching rates are taken as an example for demonstrating the ability of the proposed method. It is shown that the number of pre-nuclei can be related to the conditions of the initial quench.

Le diagramme de phases fer-neodyme (Fe-Nd)

Abstract The phase diagram of the binary Fe-Nd system has been determined using X-ray diffraction and differential thermal analysis. The master alloys were prepared from high-purity metals by the levitation melting procedure. For pure neodymium, the melting point was found to be 979.0 ± 3.5 ° C and the α → β transition to be at 856.0 ± 3.0 ° C with an enthalpy change of 3580 ± 280 J (g at.)−1. The limit of solid solubility of Fe in Nd was determined to be

Identification by DSC and DTA of the oxygen and carbon contamination due to the use of ethanol during mechanical alloying of Cu-Fe powders

0.12 at.% Fe [0.05 wt.% Fe] and the α → β transition of the Fe-Nd solid solution to be at 849.0±1.0°C. In agreement with Schneider et al. (Z. Metallkde., 77 (1986) 755; 78 (1987) 694) the presence of only one binary compound, rhombohedral Fe17Nd2, could be verified. Fe17Nd2 melts non-congruently (Fe17Nd2 → liq.+Feγ) at 1210±16 °C. The Fe2Nd compound previously reported by Terekhova et al. [2] was not observed. The eutectic reaction (Fe17Nd2+ α-Nd→liq.) was found to occur at 684.5 ± 0.5 °C and 76.5 at.% Nd.