Glass formation by mechanical alloying

Abstract

Amorphous metallic powders can be formed by mechanical alloying in a high energy ball mill. Starting from the elemental, crystalline powders, ball milling first produces powder particles with a characteristically layered microstructure. Further milling leads to an ultrafine composite in which amorphization by solid state reaction takes place. A study of the influence of the milling intensity on glass formation shows that the effective temperature during mechanical alloying can rise significantly. In addition to the use of X-ray diffraction and differential scanning calorimetry to study crystallization the atomic arrangement can be characterized by the measurement of structure-sensitive physical properties such as superconductivity, Mössbauer effect, structural relaxation and hydrogen absorption. As a result, the atomic structure of the mechanically alloyed amorphous metals is quite similar to that of melt-spun samples showing that the amorphous state should not be considered a frozen liquid but a metastable phase which can be equally approached by quenching from the liquid state and by a crystal-glass transition via interdiffusion. The glass-forming ranges for mechanical alloying can also be exactly determined by measuring physical properties as a function of composition. These experimentally obtained glass-forming ranges agree quite well with those derived theoretically, indicating that glass formation by mechanical alloying can be described as a metastable equilibrium process neglecting the existence of intermetallic phases which are prevented from formation by the kinetic restrictions of the mechanical alloying process.