Kinetic, chemical and mechanical factors affecting mechanical alloying of Ni-bcc transition metal mixtures

The influence of enthalpy of mixing and elemental mechanical characteristic s on the kinetics of mechanical alloying (MA) of 80 at.% Ni-20 at.% bcc tra nsition metals (Fe, Cr, W, Nb, and Ta) has been studied. The features and s tructures of milled powders were characterized by X-ray diffraction, optica l microscopy, and particle size analysis. Powder caking of the grinding med ia in varying amounts depending On the materials alloyed and the alloying t ime - occurs when these elemental combinations are mechanically alloyed. Di fferences, if any, between the structures of 'free' and 'caked' powders wer e also determined. As expected, for systems with low enthalpies of mixing ( Ni/Fe, Ni/Cr, Ni/W) crystalline solid solutions form during MA. Moreover, t he compositions of these solid solutions are the same in both free and cake d powders. Also as anticipated, for systems with high enthalpies of mixing (Ni/Nb, Ni/Ta), extended MA produces amorphous phases that form from precur sor solid solutions. In distinction to systems with low enthalpies of mixin g, the structures of the free and caked powders differ for systems with hig h mixing enthalpies. Caked powders were inhomogeneous, consisting of powder in varying degrees of solid solution and, if the milling time was sufficie ntly long, also some amorphous powder. However, the free powder was almost entirely noncrystalline With extended milling, the fraction of free powder increases suggesting that the formation of the amorphous phase takes place on the surface layer of the coated grinding media. Subsequent to its format ion, the amorphous phase is abraded from the coated surfaces. The kinetics of solid solution and/or amorphous phase formation is discussed in terms of the differing mechanical characteristics of the bcc transition elements, a s well as the system thermodynamics. (C) 2000 Elsevier Science S.A. All rig hts reserved.