Determination of diffusion coefficients in immiscible systems: Cu-W as an example

Proper understanding of structural and compositional stability of metastable alloys synthesized in binary systems with positive heat of mixing must be based on the knowledge of the diffusion process. The study of diffusion in immiscible systems includes difficulties because the components do not mix with each other but rather separate. For this reason, diffusional data practically do not exist in immiscible systems in literature. In this work, an experimental method to measure diffusion coefficients in immiscible systems is introduced. This paper demonstrates that the investigation of decomposition in these systems provides an opportunity to observe and measure diffusion in metastable alloys. Our investigations were carried out in sputtered Ni/WCu/Ni tri-layers, where the tungsten-copper is one of the most heavily used metastable alloys. The samples were annealed at temperatures ranging from 100 degrees C to 450 degrees C for various times. The decomposition of WCu layer was characterized using high-resolution transmission electron microscopy (HR-TEM) and secondary neutral mass spectrometry (SNMS). From the evolution of the copper concentration distribution, the copper diffusion coefficients in the WCu metastable alloy as well as in the Ni layer were deduced. Moreover, an Arrhenius relation was also determined with a diffusion prefactor of D-0 = 8.4 x 10(-14) m(2) s(-1) and activation energy of Q = 57 kJ mol(-1).