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A significant increase in the recovery strain of a high-Zr β-Ti alloy from 2.25 % to 5.5 % when decreasing the deformation temperature from 300 K to 77 K is reported in this study. It is found that the super-elasticity of this alloy is independent of the β-grain size at 77 K. The results reveal that a coarse-grained specimen exhibited approximately the same super-elasticity as its ultra-fine grain counterpart at 77 K. The relative easiness of deformation-induced martensitic transformation and dislocation slip was substantially changed at 77 K, with a strong suppression of dislocation slip, which overshadowed the effect of grain refinement on the super-elasticity. [ABSTRACT FROM AUTHOR]

Stress induced nanoscale martensitic transformation (SINMT) as well as its effect on mechanical behaviors have been investigated for Ti2448 single crystal in compression along [100] orientation by decreasing sample size to micron-to nano scales which possess high stress due to "smaller, stronger" in metals to trigger SINMT. The transformation process of β (BCC)→O′ (Orthorhombic)→α" (Orthorhombic) involving { 011 } 0 1 ‾ 1 β shuffle followed by { 2 1 ‾ 1 } 11 1 ‾ β shear was directly observed by high resolution transmission electron microscopy (HRTEM). Real-time recording of phase transition by in-situ HRTEM in 90 nm pillar clearly reveals that this SINMT with a critical stress 1206 MPa is high-order-like (continuous) and reversible. Its competition and interaction with dislocation avalanche exhibited a strong size-dependence upon uniaxial compression, inducing a transition from dislocation avalanche to SINMT with decreasing of the pillar size from 2.5 μm to 90 nm, evidenced by scanning electron microscopy (SEM) and HRTEM. According to the results from uniaxial compression, scanning electron microscopy (SEM) and HRTEM, deformation behaviours and mechanical properties of Ti2448 pillars ranging from 2.5 μm to 90 nm exhibit strong size-dependence due to the competition and interaction between the dislocation avalanche and the SINMT. Owing to the "smaller, stronger" size effect, Ti2448 sub-micron pillars possess high stress to induce plenty of nanoscale α" martensites during loading which can effectively impede dislocation avalanche. Ti2448 sub-micron pillars (d < 1 μm) deform in homogenous mode and show an excellent combination of ultrahigh strength (1635 MPa) and plastic stability. By contrast, in micron scale (d ≥ 1 μm) dislocation avalanche dominates the deformation, leading to the plasticity instability. • Mechanical properties and deformation behavior of Ti2448 alloy in micro-to nanoscale is investigated. • Stress-induced nanoscale α" martensitic transformation (SINMT) was recorded by in-situ HRTEM observation. • Phase transition from β (BCC).→ O' (Orthorhombic) → α" (Orthorhombic)was directly observed by HRTEM. • Ultrahigh stress with good plastic stability is achieved by SINMT in sub-micron pillar. [ABSTRACT FROM AUTHOR]

In the paper, in terms of the new developed additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V alloy, different microstructural design strategies were proposed to tune the morphology and content of α phase to optimize the mechanical properties of the alloy and the corresponding deformation mechanism of the alloy were characterized and analyzed. The results show that after cyclic heat treatment and solution-aging, the best comprehensive mechanical properties (yield strength ∼ l009 MPa, and elongation ∼12%) are obtained. The better comprehensive tensile properties are attributable to the microstructure consisting 16% equiaxed α p and β t structure. The cycle heat treatment can effectively decrease the length-width ratio of lamellar α phase and improve spheroidization of primary α phase. The equiaxed primary α phase has higher ability to coordinate local plastic strain, leading to the activation of extensive prismatic slip and improving the ductility of the alloy. After aging-solution, large amount of fine α s precipitate from β phase. The high hetero-deformation induced stress at α s /β interfaces in β t structure promotes the activation of the pyramidal slip systems, thereby increasing the yield strength. The present findings provide significant guidance for additive manufactured titanium alloy having good combination of tensile ductility and strength. • In terms of the new developed additively manufactured Ti-6.9Al-6.8Zr-2.3Mo-2.2V alloy, different heat treatments were proposed to improve the mechanical properties. The best matching of strength and ductility (yield strength ∼ l015 MPa, and elongation ∼10.5%) was obtained after cyclic heat treatment and solution-aging. • Owing to the high hetero-deformation induced stress at αs/β interfaces, the pyramidal and slip systems were operated, resulting in an increase of yield strength. • The prismatic slip plays a dominant role in accommodating the external strain and promoting deformation. [ABSTRACT FROM AUTHOR]