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In addition to the common stable and metastable phases in titanium alloys, the face-centered cubic phase was recently observed under various conditions; however, its formation remains largely unclarified. In this work, the effect of nonmetallic interstitial atoms O, N, C and B on the formation of the face-centered cubic phase of titanium was investigated with the density functional theory. The results indicate that the occupancy of O, N, C and B on the octahedral interstitial sites reduces the energy gap between the hexagonal-close-packed (HCP) and face-centered cubic (FCC) phases, thus assisting the formation of FCC-Ti under elevated temperature or plastic deformation. Such a gap further decreases with the increase in the interstitial content, which is consistent with the experimental observation of FCC-Ti under high interstitial content. The relative stability of the interstitial-containing HCP-Ti and FCC-Ti was studied against the physical and chemical origins, e.g., the lattice distortion and the electronic bonding. Interstitial O, N, C and B also reduce the stacking fault energy, thus further benefiting the formation of FCC-Ti. [ABSTRACT FROM AUTHOR]

The ductile-to-brittle transition is commonly observed in metastable β -titanium (Ti) alloys containing ω -precipitates, while the fundamental understanding on ω -embrittlement hitherto remains elusive. In this work, the prototypical Ti-20 wt% Mo metastable β -Ti alloy has been systematically investigated by coupling experiments and first-principles calculation to eliminate this puzzle. It is shown that the structural evolution of ω -phase controls the deformation mechanism transition of twinning-to-slip in Ti Mo alloy, being the origin of ductile-to-brittle transition of this alloy. The initial trigonal ω -structure continuously collapses to hexagonal ω -structure (structural collapse) whilst Mo-atoms are rejected out concurrently (stoichiometric varieties), both leading to hardening of ω -precipitates. This self-hardening of ω -precipitates was further rationalized in terms of the enhanced propensity for a covalent character of the atomic bond demonstrated by the electronic density of states (DOS) from first-principles calculation. Specifically, the self-hardening behavior of ω -precipitates promotes dislocation slip on isolated planes in lieu of correlative slip on successive planes inside ω1 -variant, while dislocations are completely blocked ahead ω2/ω3/ω4- variants. This in turn renders the transition from deformation twinning that contributes to great macro-plasticity to ordinary dislocation slip that contributes to localized deformation bands in the present Ti Mo alloy. Image 1 [ABSTRACT FROM AUTHOR]

Unlike the general belief that either dislocations cut or by-pass the ω -precipitates contributes to the high strength of Ti alloys, here we uncovered a new strengthening mechanism of lattice disordering of ω -precipitates by investigating the deformation behavior of body-centered-cubic (bcc) single crystalline Ti-10V-2Fe-3Al (Ti1023) alloy micropillars. This unique ω -precipitate disordering strengthening mechanism emerges in-between the precipitate cutting and by-passing mechanisms, because the dislocations pile-up stress reaches the ideal strength of ω -phase. The ω -precipitates with four variants manifest the strong deformation anisotropies, as verified by both the crystallographic orientation analysis and energy landscapes from the first principle calculations. The combination of precipitates cutting of ω 1-variant and precipitates disordering of ω 2/ ω 3/ ω 4-variants leads to the formation of precipitate-free channel in bcc (146) β -oriented Ti1023 alloy micropillars. Due to the presence of ω -precipitates, the bcc Ti1023 alloy micropillars exhibit stable plastic flow, associated with weak-size dependent superior strength. These findings provide fundamental understanding of the deformation characteristic of precipitate-hardening alloys, and have important implications for the design of small-scale materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • The ω -embrittlement of high misfit metastable β -Ti alloys was investigated. • Mechanically-driven ω -mediated amorphization is the underlying mechanism. • Covalent character of atomic bonding of ω -precipitates causes ω -lattice disordering. ω -embrittlement is ubiquitous in metastable β -titanium (Ti) alloys, while the fundamental understanding on the damage-fracture mechanism hitherto remains elusive. In this study, we systematically investigate ω -embrittlement of high misfit Ti-10Cr (wt.%) alloys by coupling experiments and first-principles calculation. It is found that brittle cleavage-like fracture prevails in tensile samples, irrespective of the quenching or subsequent aging states. Microscopically, cracks nucleation and propagation proceed along slip bands, inside which ω -lattices are first disordered and then the localized (β + ω)-amorphous-like structures are developed in the shape of white patches. The underlying mechanism of mechanically-driven localized amorphization is that due to the remarkable covalent character of atomic bonding of ω -precipitates caused by composition partitioning of the Cr element, ω -precipitates impart extremely high energy barrier opposed to dislocation gliding and render dislocations pile-up ahead of ω -precipitates, thus leading to their lattice disordering. It is unveiled that the hydrostatic pressure, serving as the driving force for dislocations pile-up, plays a critical role in this unusual cleavage-like fracture of Ti-10Cr alloys caused by mechanically-driven ω -mediated localized amorphization. Accompanied by the transition from the co-operation of deformation twining and ordinary dislocation slip in the quenched Ti-10Cr alloys to the exclusive ordinary dislocation slip in the long-time aged Ti-10Cr samples, it is unexpected that the resulting tensile fracture strength monotonically decreases to a stress level of ~ 100 MPa. These findings provide new insights into the damage and fracture behavior of high misfit β -titanium alloys, such as Ti-Cr alloys. [ABSTRACT FROM AUTHOR]

The ω -phase transformation in metastable β -titanium (Ti) alloys has attracted great attention in the past decade due to its intrinsic complexity and modifying mechanical properties. Interfacial twin boundary (ITB) ω -phase was reported to appear along {332} β or {112} β twin boundaries in metastable β -Ti alloys. The formation of such ITB ω -phase was proposed to arise from the reverse α″ to β martensitic transformation and subsequent stress relaxation along the twin boundaries. In this study, a new formation mechanism is revealed in Ti-10wt.%Cr alloy containing ω -precipitates in the initial microstructure. It is experimentally found that the formation of ITB ω -phase is closely correlated with the favored pre-existing one ω -variant at the expense of the other three-siblings, i.e. , reorientation of ω -variants. The ω -reorientation mechanism is further rationalized by first-principles calculation in terms of the energy barrier of transformation pathway between ω -variants. These findings advance our fundamental understanding to the ω -phase transformation and further plastic deformation behavior of Ti alloys. Unlabelled Image • A new formation mechanism of interfacial twin boundary (ITB) ω -phase has been revealed in metastable β -Ti alloys. • The formation of ITB ω -phase is experimentally ascribed to the reorientation of ω -variants. • The ω -reorientation mechanism is further rationalized in terms of the encountered energy barrier by first-principles calculation. [ABSTRACT FROM AUTHOR]