Crystallographic Structure Analysis of a Ti–Ta Thin Film Materials Library Fabricated by Combinatorial Magnetron Sputtering

Abstract

Ti-Ta thin films exhibit properties that are of interest for applications as microactuators 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 TiTa to TiTa. 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: ω phase, α″ martensite, β phase, and a tetragonal Ta-rich phase (Ta). The crystal structure and microstructure were analyzed by Rietveld refinement and clear trends could be determined as a function of Ta-content. The lattice correspondences between β as the parent phase and α″ and ω as derivative phases were expressed in matrix form. The β ⇌ α″ 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 α″ martensite employing the Landau theory is included for a mathematical quantification of the spontaneous lattice strain at room temperature (ϵ̂ = 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.

First-Principles Calculations of High-Pressure Physical Properties of Ti0.5Ta0.5 Alloy

“…3.260 3.295 [11], 3.286 [14] 3.278 [24], 3.274 [25] A series of structural optimizations are performed on the crystal structure of the Ti0.5Ta0.5 alloy under different applied pressures to get the corresponding volumes and equilibrium lattice constants, which are used to investigate the impacts of applied pressure on the unit cell volume and lattice constant. Subsequently, the dependencies of dimensionless ratios 0 V V and 0 a a on the applied pressure are obtained in the range of −10 to 50 GPa, where the negative pressure denotes the tension, as shown in Figure 3.…”

“…From the variation curves, it can be found that the two ratios decrease when applied pressure increases and the descent speed for 0 V V is significantly faster compared to 0 a a , suggesting that applying pressure can greatly reduce interatomic distance, and then strengthen electron interactions between adjacent atoms in the Ti0.5Ta0.5 alloy. Lattice constant a 0 (Å) 3.260 3.295 [11], 3.286 [14] 3.278 [24], 3.274 [25] A series of structural optimizations are performed on the crystal structure of the Ti 0.5 Ta 0.5 alloy under different applied pressures to get the corresponding volumes and equilibrium lattice constants, which are used to investigate the impacts of applied pressure on the unit cell volume and lattice constant. Subsequently, the dependencies of dimensionless ratios V/V 0 and a/a 0 on the applied pressure are obtained in the range of −10 to 50 GPa, where the negative pressure denotes the tension, as shown in Figure 3.…”

“…Behera et al [13] measured the enthalpy change of Ti-xTa alloys at the temperatures ranging 463 to 1257 K, and then clearly revealed that the enthalpy came mainly from the contributions of two parts: (i) one part from untransformed α and coexisting β phases, (ii) another part from the diffusional phase transformation from α phase to β phase. Furthermore, plenty of studies on TiTa alloys were also carried out by others [14][15][16][17].…”

Discovery ofω-free high-temperature Ti-Ta-Xshape memory alloys from first-principles calculations

“…A possible alternative to Ni-Ti as high-temperature shape memory alloys (HTSMAs) 14,15 are Ti-Ta alloys [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] . In these alloys, the transformation temperatures A s and M s increase with decreasing Ta concentration c Ta , and can be as high as 430 • C when c Ta is reduced to 20 at.% 30 .…”

First-principles characterization of reversible martensitic transformations

“…To overcome these limitations we have employed ab initio molecular dynamics (aiMD) simulations to access structural properties at finite temperature, and combined our ab initio data with a 2-4-6 Landau-Falk expansion of the free energy [12,13] to characterize the nature of reversible MTs and suggest necessary conditions to distinguish them from irreversible ones. We have applied our method to the shape memory alloy Ti-Ta [14][15][16][17][18][19][20][21][22][23] that features a reversible MT with a high (>100 • C) transition temperature. Our key findings include that, in this system, there is only a small interval of temperatures where austenite and martensite are both dynamically stable and that, in this interval, the two phases are separated by an extremely small free energy barrier.…”