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  1. Abstract

    In recent research, additions of solute to Ti and some Ti-based alloys have been employed to produce equiaxed microstructures when processing these materials using additive manufacturing. The present study develops a computational scheme for guiding the selection of such alloying additions, and the minimum amounts required, to effect the columnar to equiaxed microstructural transition. We put forward two physical mechanisms that may produce this transition; the first and more commonly discussed is based on growth restriction factors, and the second on the increased freezing range effected by the alloying addition coupled with the imposed rapid cooling rates associated with AM techniques. We show in the research described here, involving a number of model binary as well as complex multi-component Ti alloys, and the use of two different AM approaches, that the latter mechanism is more reliable regarding prediction of the grain morphology resulting from given solute additions.

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    Free, publicly-accessible full text available December 1, 2024
  2. The formation of isothermal ω phase precipitates and its influence on subsequent fine-scale α precipitation is investigated in a metastable β-titanium alloy, Ti-10V-2Fe-3Al. Atom-probe tomography and high-resolution transmission electron microscopy reveal that the rejection of Al, a potent α stabilizer, from the growing isothermal ω precipitates at 330°C, aids in the formation of α precipitates. Additionally, the presence of α/ω and α/β interfaces conclusively establish that these α precipitates form at the β/ω interface. Interestingly, the local Al pile-up at this interface results in a substantially higher than equilibrium Al content within the α precipitates at the early stages of formation. This can be rationalized based on a novel three-phase β+ω+α metastable equilibrium at a lower annealing temperature (330°C, below the ω solvus). Subsequent annealing at a higher temperature (600°C, above the ω solvus), dissolves the ω precipitates and re-establishes the two-phase β+α equilibrium in concurrence with solution thermodynamic predictions. 
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    Free, publicly-accessible full text available September 1, 2024
  3. null (Ed.)
    Abstract For decades the poor mechanical properties of Ti alloys were attributed to the intrinsic brittleness of the hexagonal ω-phase that has fewer than 5-independent slip systems. We contradict this conventional wisdom by coupling first-principles and cluster expansion calculations with experiments. We show that the elastic properties of the ω-phase can be systematically varied as a function of its composition to enhance both the ductility and strength of the Ti-alloy. Studies with five prototypical β-stabilizer solutes (Nb, Ta, V, Mo, and W) show that increasing β-stabilizer concentration destabilizes the ω-phase, in agreement with experiments. The Young’s modulus of ω-phase also decreased at larger concentration of β-stabilizers. Within the region of ω-phase stability, addition of Nb, Ta, and V (Group-V elements) decreased Young’s modulus more steeply compared to Mo and W (Group-VI elements) additions. The higher values of Young’s modulus of Ti–W and Ti–Mo binaries is related to the stronger stabilization of ω-phase due to the higher number of valence electrons. Density of states (DOS) calculations also revealed a stronger covalent bonding in the ω-phase compared to a metallic bonding in β-phase, and indicate that alloying is a promising route to enhance the ω-phase’s ductility. Overall, the mechanical properties of ω-phase predicted by our calculations agree well with the available experiments. Importantly, our study reveals that ω precipitates are not intrinsically embrittling and detrimental, and that we can create Ti-alloys with both good ductility and strength by tailoring ω precipitates' composition instead of completely eliminating them. 
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  4. null (Ed.)
  5. A nano-scaled shuffle-induced modulated structure and heating-induced ordering have been characterized in a metastable β-titanium alloy, Ti-5Al-5Mo-5V-3Cr, and their inter-relationship was investigated through in-situ and ex-situ conventional and aberration-corrected scanning/transmission electron microscopy and atom probe tomography. The nano-scaled O′ phase with a disordered orthorhombic-modulated structure formed by the transverse phonon was characterized for the first time to be stable from room temperature to ∼200°C. The O″ phase with an ordered orthorhombic structure formed from the β phase during aging above the O′ phase solvus temperature, where Al segregation in nano-scaled regions led to ordering of every third {011} planes. 
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  6. Strain-transformable Ti-based alloys are known to display a superior combination of strength, ductility and strain-hardening and attracted considerable interest on recent years. They generally still display, however, a limited yield strength that can be possibly overcome by further precipitation strengthening of the developed systems. In that work, we developed a design strategy to reach a forged dual-phase (α+β) microstructure with TRIP/TWIP properties in a Ti–10V–2Fe–3Al alloy. The results showed an excellent combination of mechanical properties due to the strain-transformable deformed β-matrix. The investigation on the deformation mechanisms in the Ti–10V–2Fe–3Al alloy was accurately performed by means of both in-situ synchrotron XRD, mechanical testing followed by SEM/EBSD mapping and “post mortem” TEM microstructural analyses. Combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects were shown to be intensively activated in the alloy. The particular role of stain-induced martensite α″, acting as a relaxation mechanism at the α∕β interfaces, as well as the strong interactions between mechanical twins and primary α nodules were particularly highlighted. 
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