FORMATION OF THERMODYNAMICALLY METASTABLE PHASES IN THE TA-CO, TA-NI,AND TA-CU SYSTEMS BY MECHANICAL ALLOYING

Metastable allows have been synthesized in a planetary ball mill by me chanical alloying of elemental crystalline powder blends between Ta an d one of the 3d-transition metals Co, Ni, and Cu. The progress of allo ying and the milling products are characterized by X-ray diffraction, optical metallography, electron microprobe analysis, and by differenti al scanning calorimetry. The deformation during the milling process re duces the crystallite size within the powder particles to nanometer sc ale after a few hours of ball milling. For longer milling times super- saturated solid solutions have been formed in all systems for Ta-lean powder blends. Formation of amorphous phases has been observed in all systems: For Ta-Co and Ta-Ni wide glass forming ranges have been found , whereas Ta-Cu shows (only) incomplete amorphization for Ta-Cu. Conta minations of Fe and Cr from the milling tools are particular important for Ta-rich starting compositions or long milling times. The thermal stability of the amorphous phases has been investigated. Crystallizati on temperatures, crystallization enthalpies, and crystallization produ cts are reported. The thermodynamic conditions for the systems Ta-Ni a nd Ta-Co, which both exhibit a negative heat of mixing and form severa l intermetallic compounds in equilibrium, are different from those of the system Ta-Cu, which is immiscible with a positive heat of mixing. The alloy formation is discussed with respect to thermodynamic and kin etic aspects.