Synthesis of NiTi and FeTi alloys by mechanical alloying: formation of amorphous phases and extended solid solutions

Abstract

NiTi and FeTi powders prepared by mechanical alloying of elemental crystalline powders in a planetary ball mill have been investigated by X-ray diffraction, differential scanning calorimetry and saturation magnetization measurements. The NiTi powders become completely amorphous in the central composition range. For Ni 70Ti 30, the powder consists of an amorphous phase and crystalline intermetallic Ni 3Ti. The FeTi powders exhibit an amorphous phase from 30 to 70 at.% Fe and residual elemental α-Fe and Ti or crystalline intermetallic FeTi, respectively. The intermetallic phases form directly by milling, since the thermal conditions during extended processing are not sufficient for an in situ crystallization of amorphous powder. The coexistence of amorphous and intermetallic phases indicates that the mechanical alloying does not proceed under metastable equilibrium conditions. Outside the regime of the amorphous phase, mechanical alloying produces markedly extended solid solutions of at least 20 at.% Ti in Ni or Fe. The experimental results are compared with the glass-forming ability predicted by thermodynamic calculations. The amorphization depends strongly on a large negative enthalpy of mixing as a driving force for the interdiffusion reaction during milling. The enhanced solid solubility in both alloy systems can be explained by a small difference of the atomic radii of the individual components and a deformation-induced increase of the free enthalpy of the intermetallic phases.