INFLUENCE OF MILLING ATMOSPHERE ON AMORPHIZATION OF CHROMIUM AND CR-CU POWDERS BY BALL MILLING

Elemental chromium and Cr-Cu powder mixtures were milled in a vibrational ball mill under air-, nitrogen- and oxygen-containing argon atmospheres. Structural changes during milling and the thermal stabilities of the milled powders were examined by means of X-ray analysis, transmission electron microscopy and differential scanning calorimetry. The amounts of the gaseous elements absorbed by the powder particles during milling were also determined from the pressure reduction in the vial. It was found that large amounts of nitrogen were absorbed during milling in argon-air and argon-nitrogen atmospheres, resulting in significant grain refinement and subsequent amorphization of chromium and Cr70Cu30 powders. On annealing, crystallization of the amorphized powders occurred accompanied by the formation of a stable Cr2N phase. Amorphization did not occur in either the chromium or the Cr-Cu powders when milled in a pure argon or Ar-O2 atmosphere. A thermodynamic explanation for the grain refinement and the amorphization was proposed.