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.
Highlights
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
In addition to superelastic and shape memory effects, Ti alloys can exhibit high ductility, transformation induced plasticity (TRIP), and twining induced plasticity (TWIP) effects, which can be achieved through the coexistence and reversable transformation between β and α′′ phases[6,7]
The cluster expansion (CE) results are represented by a ground state phase diagram whose X and Y axes respectively represent composition and free energy as shown in Fig. 1 for our prototypical alloys
Summary
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. These precipitates often co-exist with α, α, and α′′ phases These athermal-ω precipitates, within the β matrix, are typically not detrimental to plasticity if their volume fraction is not too high, and results in alloys that are ductile and often exhibit good strain-hardenability. Annealing such a quenched system, causes a compositional partitioning between the β and ω phases[3]. A slip deformation mode was observed upon aging and elemental partitioning of ω-phase containing Ti alloy, which showed a lower ductility than the quenched alloy Such studies have revealed that long-time aging creates high-volume fraction of embrittling ω particles. This urges the need to study the effect of chemical composition on the intrinsic mechanical properties of the ω phase
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