Phase Transitions in Metallic Multilayers

摘要

Dimensionally induced phase transitions in thin films and multilayers is of considerable interest because it affords the ability to tailor material properties by engineering their structure at the nanoscale. Recently, a series of interesting phase transitions have been reported in Ti/Al multilayers wherein Ti transforms from its bulk stable hcp structure to an fcc structure at small layer thicknesses (～5nm) and Al transforms from fcc to hcp at ～2.5 nm. These reported structural transitions were based on transmission electron microscopy (TEM) studies on sputter deposited Ti/Al multi layered specimens thinned in the cross-section geometry. Subsequently, a classical thermodynamic model has been proposed for explaining phase stability in multilayered materials as a function of the composition and bilayer thickness. Using this model it has been possible to rationalize the effect of cross-section thinning on the phase stability an terms of the anisotropic ingress of hydrogen into the Ti/Al multilayers. The model presents an elegant framework for rationalizing the effect of alloying/impurity additions on the phase stability. Furthermore, the scope of model has been expanded by considering its applicability to other systems including non close-packed ones such as Co/Cr multilayers. Recent experiments have explored in detail the formation of fcc Ti and the coherent to incoherent transition in Ti/Al multilayers as a function of composition and bilayer thickness. In addition, the initial thermodynamic model has been modified to account for the effect of coherency strains on the phase stability. These results will be discussed and a new phase stability diagram proposed for Ti/Al multilayers.