Phase formation and thermal stability of mechanical alloyed iron-tantalum

Iron-rich iron-tantalum solid solutions were prepared by mechanical alloyin g of elemental powder mixtures. At low Ta concentrations the alloys form a crystalline solid solution with bcc structure. At higher contents additiona lly an amorphous phase appears. The phase fractions and their compositions were determined by x-ray diffraction and Mossbauer spectroscopy. The therma l stability of the supersaturated solid solutions was investigated by diffe rential scanning calorimetry measurements. Upon annealing, the solid soluti ons are found to establish a metastable equilibrium by precipitation of the amorphous phase before transformation to the equilibrium phases sets in. T he results are discussed on the basis of the phase stabilities of the parti cipating stable and metastable phases. It is concluded that the phase forma tion in this system during milling can be described by a smeared-out polymo rphous transition to the amorphous phase with increasing solute content rat her than by a metastable phase equilibrium. The possibility of grain bounda ry wetting with the amorphous phase is discussed. (C) 1999 American Institu te of Physics. [S0021-8979(99)00807-5].