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'Metallic glasses' in keywords Facet   Publication Year 1996  [X]
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1996[X]
1Author    J. Blétry, D.E MRequires cookie*
 Title    Description of the Glass Transition by a Percolation Blocking of Local Chemical Order  
 Abstract    A model for the liquid-glass transition, based on a percolation blocking of local chemical order, is proposed. The case of metallic liquids and glasses, whose structure is dominated by first neighbour chemical arrangement, is first treated. The chemical ordering "reaction" of the liquid phase is studied at thermodynamical equilibrium and the increase of the chemical order parameter with decreasing temperature is calculated. Within a given composition interval, however, a geometrical percolation process is shown to block this reaction below a "percolation temperature" (correspond-ing to null cooling rate) where the liquid is irreversibly frozen into a glass. The liquid-glass "phase diagram" is established and kinetic arguments, involving "frustrated" finite clusters which are formed close to the percolation threshold, provide an evaluation of the experimentally measured "glass transition temperature" as a function of cooling rate. The validity of this one order parameter model is then discussed with the help of the irreversible thermodynamics theory of Prigogine. The formation of tetracoordinated glasses is explained by the formation of tetrahedral bonds, when the liquid temperature decreases, and represented by a "hole ordering" reaction. A general description of the structure of tetracoordinated glasses is thus achieved, which applies to amorphous silicon and germanium, 111 -V compounds, silica, amorphous water etc. Furthermore, an estimation of the temperature interval for the glass transformation of silica is obtained, which agrees well with experiment. The existence of frustrated clusters gives to glasses a composite structure in the "medium distance order", which could explain the "fractal nature" of glass fracture surfaces down to the nanometer scale. 
  Reference    Z. Naturforsch. 51a, 87 (1996); received September 6 1995 
  Published    1996 
  Keywords    Glass transition Percolation, Short range chemical order, Metallic glasses, Tetracoordi-nated glasses, Fractography 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0087.pdf 
 Identifier    ZNA-1996-51a-0087 
 Volume    51 
2Author    M. Sorescu, E. T. KnobbeRequires cookie*
 Title    Mössbauer Spectroscopy Evidence for Phase Selectivity in Excimer Laser Induced Amorphization of Thermally Annealed FeCoBSi Metallic Glass  
 Abstract    Pulsed excimer laser irradiation effects (/. = 308 nm, T = 10 ns, w = 3 J/cm 2 , N = 2 laser pulses/ spot, repetition rate 1 Hz) on the magnetic properties and phase composition of thermally annealed Fe 66 Co 18 B 15 Si 1 metallic glass have been studied by transmission and conversion electron Möss-bauer spectroscopy (CEMS). Radiation-driven surface modifications were examined by scanning electron microscopy (SEM). Excimer laser induced amorphization was observed in partially crystal-lized Fe 66 Co 18 B 15 Si 1 samples (7" A = 648 K, r A = 1 hour) and was found to be accompanied by the development of laser-induced magnetic anisotropy. In totally crystallized Fe^Co^B^Si! speci-mens (T A = 723 K, = 1 hour), the effect of excimer laser induced amorphization exhibited phase selectivity with respect to the crystalline components of the alloy system. When compared to bulk data, a different behavior of the surface magnetic texture and relative abundance of alloy phases was observed. The laser-induced phase transformation of thermally annealed Fe 66 Co 18 B 15 Si 1 specimens was shown to consist of partial amorphization and surface oxidation of the irradiated material. 
  Reference    Z. Naturforsch. 51a, 389—395 (1996); received February 3 1996 
  Published    1996 
  Keywords    Metallic glasses, Laser processing, Phase transformations, Magnetic properties, Möss-bauer spectroscopy 
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 TEI-XML for    default:Reihe_A/51/ZNA-1996-51a-0389.pdf 
 Identifier    ZNA-1996-51a-0389 
 Volume    51