Achieving Ultrahigh Strength with Stable Plasticity by Stress-Induced Nanoscale Martensitic Transformation in Ti2448 Sub-Micron Pillars

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 [[EQUATION]] shuffle followed by [[EQUATION]] shear was directly observed by high resolution transmission electron microscopy (HRTEM). Real-time recording of phase transition by in-situ HRTEM in 90nm pillar clearly reveals that this SINMT with a critical stress 1206MPa 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 2.5μm to 90nm, 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 2.5μm to 90nm exhibit strong size-dependence due to competition and interaction between dislocation avalanche and SINMT. Owing to “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 <1μm) deform in homogenous mode and show excellent combination of ultrahigh strength (1635MPa) and good plastic stability. By contrast, in micron scale ( d ≥1μm) dislocation avalanche become predominant mechanism, which leads to plasticity instability