6. Crystallographic symmetry analysis in NiTi shape memory alloys

Schmahl, Wolfgang W.; Schiebel, Korbinian; Kadletz, Peter M.; Yin, Xiaofei; Hoelzel, Markus

doi:10.1515/9783110497342-006

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“…This discontinuity strongly hints at a first order phase transformation that depends on chemical composition and temperature and confirms the martensitic character of α″, where the transformation temperatures ( M f , M s , A s , A f ) increase with decreasing Ta-content. A first order phase transition is identified by a discontinuity of a first derivative of the free energy G , such as entropy or the (unit cell) volume; this often involves a breaking of symmetry in solid-solid phase transitions. − …”

Section: Discussionmentioning

confidence: 99%

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

Kadletz¹,

Motemani

Iannotta³

et al. 2018

ACS Comb. Sci.

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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.

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