Phase Stabilities in Nano-scaled Metallic Multilayers

An experimental study is underway to investigate the structural stabilities in nanoscale metallic multilayers involving phase combinations with non-closed packed structures. Thus, it is well-known that phase combinations exhibited by multilayered materials may vary as a function of scale, i.e. the bilayer thickness, and also as a function of volume fraction of the constituents in a given bilayer. The proposed work is aimed at developing a detailed understanding of the factors which influence these phase selections. Emphasis is placed on systems with structural transitions which involve non-close packed structures (e.g. bcc) since these systems are more general examples compared with those involving only close-packed phases (fcc and hcp).
Specifically, a thermodynamic model describing structural stability as a function of both scale and volume fraction is applied to the case of phase selection in Co-Cr, Zr-Nb and Ti-Nb multilayers. A most useful product of the application of this model is the “biphase diagram”, which is a new phase diagram describing phase selection as a function of scale and volume fraction of constituents of given bilayers. The proposed research assesses the applicability of these new phase diagrams for the cases of the three candidate multilayer systems. This assessment involves a validation of the predictions afforded by the biphase diagrams. It is the predictive capability of the biphase diagrams which causes them to be extremely important to the material designer’s toolbox.
In addition to studies of the structural stabilities in nominally pure metallic multilayers, a significant part of the proposed research is an investigation of the possibility of manipulating the phase selection exhibited by a given multilayer system. Thus, it is intended to use pre-alloyed components in place of nominally pure layers to induce changes in the thermodynamics governing phase selection, and hence cause attendant changes to values of critical thicknesses for given phase transitions. This is a most exciting possibility, as applications of multilayer systems will most certainly require certain phases to be selected at specific bilayer thicknesses for the optimization of various properties. In addition, it is intended to introduce, through pre-alloying of layers, new types of biphase diagrams which will offer the materials designer the optimum assistance in the application of multilayers to components.

Finally, it is important when providing the most accurate physical picture of the phenomenon of structural stabilities in multilayers that the mechanisms of phase transitions in the various samples be understood. In-situ experiments have been designed specifically to permit direct observations of these phase transformations to be made.