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Description
α″-martensitic Ti alloys display customizable thermal expansion, making them attractive for applications in precision optics, metrology, and semiconductor lithography, where dimensional stability under varying temperature is critical. The bulk thermal expansion of polycrystalline components is governed by the magnitude and anisotropy of the lattice thermal expansion, volume fraction and crystallographic texture of the α″ phase, all of which are customizable through the type and the concentration of β-stabilizing alloying elements and through thermomechanical processing.
Nb, a β-stabilizer in Ti alloys, directly influences the lattice thermal expansion of α″-martensite and its transformation temperatures, making binary Ti-Nb alloys the primary focus of thermal expansion studies to date. Ta is likewise a β-stabilizer but has received far less attention, despite its higher melting point and its potential to enhance the stability of α″-martensite. The present work examines how thermomechanical processing influences microstructure, lattice-level and bulk thermal expansion of α″-martensitic TiNbTa alloys, by comparing water-quenched and cold-rolled states. Three Ti alloys with varying Nb and Ta content were arc-melted, homogenized, and water-quenched to produce α″-martensitic microstructures. They were then cold-rolled to induce crystallographic texture leading to negative thermal expansion along the rolling direction. Laboratory X-ray diffraction (XRD) was employed to characterize the resulting texture. In-situ high-energy XRD at the German Electron Synchrotron (DESY) was used to track the evolution of phase fractions, lattice parameters and lattice thermal expansion during heating. Transformation temperatures were identified by differential scanning calorimetry, and macroscopic thermal expansion was measured by dilatometry. Cold-rolling induces a pronounced shift in bulk thermal expansion: whereas water-quenched alloys exhibit 5–6 ppm/°C, the strong ⟨010⟩α″ texture developed along the rolling direction yields a negative thermal expansion of ~−30 ppm/°C. Altogether, this methodology establishes the impact of processing by water-quenching versus cold-rolling on the macroscopic thermal expansion and thermal stability of these novel ternary alloys.