Abstract
Mechanical alloying in combination with chemical treatment was applied to produce metastable phases such as bcc Ni and Co by leaching aluminum from mechanically alloyed nickel and cobalt aluminides. For Ni and Al the MA process was characterized with 1 MeV TEM JEOL-1000, X-ray, DSC and magnetic measurements. Ni sub 2 Al sub 3 was formed during MA of Ni and Al. Since shear displacements during MA led to disordering of Ni sub 2 Al sub 3 (D5 sub 13 ) the formation of the closely related B2 structure with the composition Ni sub 35 Al sub 65 was also observed. The B2 structure was also founded for MA of Co and Al. The leaching of Al atoms from both B2 structures proceeds topotactically and induces no change in the X-ray diffraction patterns. Upon heating B2 type Ni and Co transform into the fcc structures. The magnetic properties of new B2 Ni and Co were studied in the temperature range of 4.2-800K. According to theoretical band calculations, B2 Ni should be a paramagnet and become ferromagnetic as its atomic radius expands above a critical value, while B2 Co is ferromagnetic. It was observed that B2 Ni is a strong paramagnet, unstable > 470K. The magnetic moment of Co is less in the ferromagnetic B2 phase than in fcc one. Nickel-Raney catalysts prepared through leaching of MA alloys are more active than conventional catalysts in liquid-phase hydrogenation of potassium maleate, nitrobenzene, phenylacetylene and some other unsaturated compounds.