FORMATION OF NANOPHASE NIOBIUM ALUMINIDE BY MECHANICAL ALLOYING

Mechanical alloying of an elemental powder mixture containing 51.5 mass%Nb, 43.0 mass%Al and 5.5 mass%Ti was carried out in an Argon-filled ball mill at ambient temperature for 90 to 1800 ks. MAed powders were examined by X-ray diffraction for changes in elemental particulate size and grain sizes of newly formed intermetallic phases. The elemental particulate size measured from the line-broadening decreased with increasing MA time up to 720 ks, while XRD identified the newly formed intermetallic phases, Nb2Al and NbAl3 at and beyond 720 ks. Their grain sizes remained at a few nanometers level (3-8 nm). Differential scanning calorimetry (DSC) of MAed powders revealed both exothermic and endothermic reactions. Combined with XRD analysis, it was regarded that the former is due to the devitrification of amorphous-like elemental mixture to form Nb2Al and the latter is the transformation of crystallized Nb2Al to NbAl3. Although the activation energy associated with the former reaction slightly decreases with the MA time (from 176 to 155 kJ/mol), that for the latter reaction remains constant at 242 kJ/mol regardless of the MA time. Accordingly it is concluded that, in MA processing, a mixture of elemental powders is homogeneously mixed together by reducing their particulate sizes to a few nanometers level to form an amorphous-like mixture, then forming Nb2Al at and beyond 720 ks and NbAl3 at and beyond 1440 ks. This Nb2Al, however, entirely transforms into NbAl3 upon heating to 868 K.