MAGNETICALLY INDUCED DECOMPOSITION IN CO-CR THIN-FILM AND BALK ALLOYS

It has been reported in recent years that composition modulations on a ten nanometer scale with unknown origin occur in Co-Cr alloy thin fil ms. In the present investigation, we intend to analyze the origin of t hese concentration modulations. Sputtered thin films as well as bulk s amples both with compositions of about 20 at. % Cr were analyzed by tr ansmission electron microscopy and by field-ion microscopy equipped wi th atomic-probe mass spectrometry, The experimental results show that composition modulations are present in both types of allays, with the concentration varying between 8 and 40 at. % Cr. The length scale of t he composition modulations is approximately 10 and 40 nm for the thin film and the bulk samples, respectively. The results are interpreted i n terms of the thermodynamics and kinetics of the system. Calculations of the thermodynamic functions of the system show that at high Co con centrations, the occurrence of magnetism leads to a downwards curvatur e of the Gibbs energy vs concentration curves. As a result of this mag netic stabilization at Go-rich concentrations, a metastable miscibilit y gap occurs that explains the tendency for phase separation into ferr omagnetic and paramagnetic phases. The diffusivity of the system was e xamined using multilayer thin-film diffusion couples, and the results indicate that grain-boundary diffusion is responsible for the fast dec omposition process observed. It is concluded that decomposition lakes place by a discontinuous precipitation process chat starts from grain boundaries.