Crystallographic Structure Analysis of a Ti-Ta Thin Film Materials Library fabricated by combinatorial Magnetron Sputtering.

Ti-Ta thin films exhibit properties that are of interest for applications as micro actuators and as biomedical implants. A Ti-Ta thin film materials library was deposited at T = 25 °C by magnetron sputtering employing the combinatorial approach, which led to a compositional range of Ti87Ta13 to Ti14Ta86. Subsequent high-throughput characterization methods permitted a quick and comprehensive study of the crystallographic, microstructural and morphological properties which strongly depend on the chemical composition. SEM investigation revealed a columnar morphology having pyramidal, sharp tips with coarser columns in the Ti-rich and finer columns in the Ta-rich region. By grazing incidence x-ray diffraction four phases were identified, from Ta-lean to Ta-rich: omega phase, alpha'' martensite, beta phase and a tetragonal Ta-rich phase (Ta(tetr)). The crystal structure and microstructure were analysed by Rietveld refinement and clear trends could be determined as a function of Ta-content. The lattice correspondences between beta as the parent phase and alpha'' and omega as derivative phases were expressed in matrix form. The beta <--> alpha'' phase transition shows a discontinuity at the composition where the martensitic transformation temperatures fall below room temperature (between 34 and 38 at.% Ta) rendering it first order and confirming its martensitic nature. A short study of the alpha'' martensite employing the Landau theory is included for a mathematical quantification of the spontaneous lattice strain at room temperature (epsilon_hat(max) = 22.4(6) % for pure Ti). Martensitic properties of Ti-Ta are beneficial for the development of high-temperature actuators with actuation response at transformation temperatures higher than 100 °C.