Amorphization of the intermetallic phase CoZr by mechanical grinding

Abstract

The intermetallic phase CoZr was milled in a planetary ball mill. X-ray diffraction shows that the crystalline Bragg reflexes vanish totally with increasing milling period while simultaneously a broad maximum appears, attributed to a developing amorphous phase. To amorphize the intermetallic phase completely it takes a relatively long milling period (100 h), whereas a powder mixture of the elements of 50 at% Co and 50 at% Zr is already completely amorphized after 8 h of mechanical alloying. Both amorphous powders produced by different starting materials show identical properties by means of X-ray diffraction, measurement of the released crystallization enthalpy, the absolute specific heat capacity, and Mößbauer spectroscopy. TEM analysis of the intermetallic phase confirms the simultaneous presence of amorphous and remaining crystalline grains after short milling periods and the complete amorphization after long milling periods. A possible explanation for the amorphization process of the compound may be the accumulation of internal strain in the crystalline grains during the milling process. Another possible explanation may be the addition of iron impurities to the stoichiometric compound due to the wear debris of the milling balls and the milling vials of stainless steel.