Metastable phase formation in the immiscible Cu–Co system studied by thermodynamic, molecular dynamics andab initiocalculations together with ion beam mixing

Abstract

For the equilibrium immiscible Cu-Co system with a positive heat of formation of +10 kJ mol −1 , ab initio calculations were used to predict the physical properties of the metastable D0 19 and L1 2 structures for the Cu 75 Co 25 phases and the D0 19 structure for the Cu 25 Co 75 alloy. Based on the ab initio calculation results, an n-body Cu-Co potential was constructed and proven to be realistic. Applying the constructed Cu-Co potential, molecular dynamics simulations predict that the amorphous phase could be obtained at around Cu 60 Co 40 and its atomic distribution could be inhomogeneous. Experimentally, by using ion beam mixing with 200 keV Xe + ions, an amorphous Cu 60 Co 40 phase with inhomogeneous morphology was indeed obtained at a dose of 1 × 10 15 Xe + cm −2 . Increasing the irradiation dose to 4 × 10 15 Xe + cm −2 , a mixture of Cu-rich and Co-rich metastable phases was obtained. Besides, a mixture of FCC and HCP structures was observed in the Cu 82 Co 18 multilayered sample and an HCP structure was observed in the Cu 26 Co 74 multilayered sample. It was found that the lattice constants of the FCC and HCP phases determined by diffraction analysis were quite compatible with those predicted by the ab initio calculations.

Surfactant controlled interfacial alloying in thermally evaporated Cu/Co multilayers

“…It is well-known that Cu-Co is equilibrium immiscible system with positive heat of formation of +10 kJ/mol and therefore does not form any stable intermetallic compounds [1]. However, when thin layers of Cu and Co are staked together in the form of thin film multilayers, their interfacial alloys may be formed.…”

“…This happens due to a large difference between the surface free energy ( ) of Cu and Co. It is reported in the literature that metastable structure of a mixed CuCo phase may exist at the interfacial regions in Cu/Co thin film multilayers [1][2][3]. In case of Cu/Co multilayers, growth of Co-on-Cu is thermodynamically unfavorable because Co > Cu [4].…”

Growing imbedded Ni3C-rich layer with sharp interfaces by means of ion beam mixing of C/Ni layers

“…Ion beam mixing is far from being an equilibrium process and thus it can overcome the problem above. Really, IBM is frequently applied to produce various nonequilibrium phases like amorphous alloys with negative or positive heat of mixing [1][2][3][4][5][6][7][8][9]. To explain these phenomena it is frequently argued that the fast quenching is responsible for the process.…”

“…There are, however, explanations that are more refined. It was observed that multilayer systems of immiscible elements could be converted by IBM to amorphous nonequilibrium alloys [6,7]. In these cases the total energy of the system decreases despite of the positive heat of formation of the alloy, since the mixing removes the high energy interfaces.…”

“…A huge variety of metastable states can be observed ranging from miscible amorphous state [5] to the immiscible amorphous states [6,7]. The observed kinetics vary greatly too, the mass transport in limiting cases might be symmetric or asymmetric [8,9].…”