Metastable phase formation in an immiscible Cu-Ta system studied by ion-beam mixing, ab initio calculation, and molecular dynamics simulation

Ion-beam mixing experiments show that amorphous phases are formed in the Cu70Ta30, Cu65Ta35, Cu60Ta40, and Cu50Ta50 multilayered samples, while a crystalline structure is retained in the Cu75Ta25 sample. These results suggest that the composition limit for forming an amorphous phase is around 30 at.% Ta, which confirms the same value of 30 at.% Ta predicted by recent atomistic modeling. For the metastable crystalline Cu75Ta25 phase, ab initio calculations identify its relatively stable structure to be L12 (fcc type) and determine some associated physical properties, which agree well with those derived from a proven realistic Cu-Ta potential through molecular dynamics simulations. Interestingly, an fcc Cu75Ta25 phase is indeed observed in ion-beam mixing experiments and its lattice constant determined by diffraction analysis matches well with those from theoretical calculations.