Stability of Structure, Phase and Elemental Composition of ChemicallyDeposited and Electrodeposited Gold Layers in Multilayer SystemsAuchemically reduced; electrodeposited /Co-W/Cusputtered / Glass-Ceramics
The changes in the structure and elemental composition of Au and Co-W thin layers after thermal treatment of 200 h at 200-300 oC and 50 h at 450 oC were systematically studied using the X-ray microanalysis of scanning electron microscope XL-30 ESEN Philips. The phase stability was evaluated by X-ray diffraction (XRD). The electrodeposited Co-W layer (W 46.0-53.0 wt. % depending on the electrodeposition regime) is amorphous and maintains its amorphous state after heating at 200 oC for 200 h and subsequent heating (+) at 300 oC for 150 h. Co-W amorphous layers prevent interdiffusion of Au and Cu atoms and formation of intermetallics AuCu in the systems of layers Au chemically reduced; Au electrodeposited /Co-W/Cu sputtered / glass-ceramics. The chemically reduced 0.4 � m Au layers maintain their structure and phase in the systems of layers upon heating at 200 oC for 200 h and subsequent heating (+) at 300 oC for 50 h. The Au layer maintains its Au content 85-86 wt. % upon heating at 450 oC for 50 h, the cobalt content of the layer up to 8.6 wt. % and content of oxygen bonded to Au of 8.6 wt. %. The electrodeposited Au layers (0.8 - 1.0 � m) maintain their structure at heating at 200 oC and 300 oC for 200 h. After additional heating at 450 oC for 30-50 h gold content in the layer decreases from 98.4 wt. % to 85.0 wt. %, cobalt content increases to 7.6 wt. % and oxygen to 7.2 wt. %. Functional properties of the above multilayer systems deteriorate by mutual diffusion of metal atoms and formation of brittle intermetallics. The latter may give rise to the formation of monolithic films, cause the deterioration of mutual adhesion of the layers and complete reactions of the upper conducting Sn, Au layers with the base metal Ni, Cu, Ti, Sn (15, 16). This diffusion can worsen the properties of very thin (0.5-1.0 � m) metal layers electrodeposited on a Cu layer (1-2 � m) sputtered on glass-ceramics (15).
Microstructure-controlled interdiffusion of Cu/Co/Au thin films investigated by three-dimensional atom probe
The dependence of thin-film microstructure on the curvature of the substrate is investigated by field ion microscopy (FIM) and transmission electron microscopy (TEM). For this purpose, Cu-Au multilayers were deposited on W single crystals, which are prepared both as curved (radius of curvature ∼ 50 nm) and as planar substrates. 3D atom probe analysis shows that the curved substrates lead to heterogeneous reaction patterns by grain boundary reaction beside planar reaction patterns. This result is explained by local variations of microstructure. Grain boundary reaction is dependent on the atomic structure of the grain boundary and can lead to the early formation of intermetallics. At temperatures too low for significant volume diffusion, interdiffusion between enriched grain boundaries and neighbored grains is observed under the formation of ordered compounds. The elastic strain, which is caused by a considerable atomic size mismatch, is found to hinder interdiffusion and to cause alloying under grain boundary motion (DIGM). The introduction of a Co diffusion barrier between Au and Cu slows down interdiffusion, resulting in higher reaction temperatures. Thus, it is possible to observe that the grain boundary reaction in the Au layer disappears at elevated temperatures, at which volume diffusion of Cu in Au gets possible, whereas in the Cu layer the grain boundary reaction is still observed.
Mechanical behaviour and deformation mechanisms of Ni-Ti shape memory alloys in tension
We investigated the behaviour in tension and at room temperature of TiNi shape memory alloys in martensitic state. Our first experimental results show that two deformation mechanisms are involved in the behaviour of such materials: martensite platelets reorientation and irreversible plasticity. Moreover when irreversible plasticity appears, we observe an increase in the martensite to austenite transformation temperatures. The present study is especially focused on the deformation mechanisms and intends to identify the nature of the irreversible phenomena. It is shown that the reorientation process and the appearance of interface dislocations are in perfect agreement with a kinematic hardening. Finally a two loading surface model is proposed which allows not only to account for the overall behaviour but also to predict the transformation temperatures under an internal and/or external stress field.