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Abstract The effects of ω precipitates and orthorhombic (α″) martensitic transformation on stress-strain behavior were studied in metastable β Ti-10V-2Fe-3Al alloy single-crystal in submicron scales. We successfully distinguished the size effects induced by ω precipitates and α″ phase transformation through compressing the metastable β Ti-10V-2Fe-3Al along the orientation of <011> β , which is unfavorable for α″ phase transformation. Two individual exponential curves express well the relationship between the strengthening exponents, which were regressed using the flow stresses at different ranges of plastic strains of 0.2–13% and the pillar widths, and the corresponding plastic strain. It is worth noting that the strengthening exponent jumps from 0.256 to 0.293 at 8% plastic strain. Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars. The first (ω precipitate-related) size effect has been predicted with the proposed models, and the second (martensitic transformation-induced) size effect is interpreted with the modified Liu's equation. [ABSTRACT FROM AUTHOR]

Shape memory alloys (SMAs) have attracted considerable attention for their high damping capacities. Here we investigate the damping behavior of Ti 50 (Pd 50− x D x ) SMAs (D=Fe, Co, Mn, V) by dynamic mechanical analysis. We find that these alloys show remarkably similar damping behavior. There exists a sharp damping peak associated with the B2–B19 martensitic transformation and a high damping plateau ( Q −1 ~0.02–0.05) over a wide ambient-temperature range (220–420 K) due to the hysteretic twin boundary motion. After doping hydrogen into the above alloys, a new relaxation-type damping peak appears in the martensite phase over 270–360 K. Such a peak is considered to originate from the interaction of hydrogen atoms with twin boundaries and the corresponding damping capacity ( Q −1 ~0.05–0.09) is enhanced by roughly twice that of the damping plateau for each alloy. Moreover, the relaxation peaks are at higher temperatures for the TiPd-based alloys (270–370 K) than for the TiNi-based alloys (190–260 K). We discuss the influence of hydrogen diffusion, mobility of twin boundaries and hydrogen–twin boundary interaction on the temperature range of the relaxation peak. Our results suggest that a martensite, with appropriate values for twinning shear and hydrogen doping level, provides a route towards developing high damping SMAs for applications in desired temperature ranges. [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]

Abstract: Microstructural evolution in primary α phase-containing Ti–10V–2Fe–3Al alloy subjected to cold forging under different applied strains was studied. Experimental results showed that, even at a strain of 0.1, stress-induced α″ martensites were abundantly produced within the β matrix, resulting in alternative α″/β lamellae. Shear bands initiated and grew across α″/β lamellae as the strain increased to 0.35. When the strain increased to 1.2, the volume fraction of shear bands significantly increased and the grains were almost occupied by the shear bands. Interestingly, nanocrystallines were observed inside shear bands. While in the primary α phase, slip was always the predominant plastic deformation mode and dislocations were accumulated to a high density within the strain range from 0.1 to 0.35. When the strain was up to 1.2, the dislocation density was further increased and α/β interface boundary became ill-defined. However, no grain refinement was observed in the α phase. The microstructure refinement in the β matrix could be attributed to that stress-induced α″ martensitic transformation promoted the initiation, thickening and coalescence of shear bands. The plastic deformation combined with martensitic phase transformation could provide a potential effective technique to produce nanocrystalline materials. [ABSTRACT FROM AUTHOR]

The functional properties of shape memory alloys are strongly sensitive to the composition of alloys. In this study, the effect of Ti/Ni ratio and Hf/Zr ratio on the martensitic transformation behavior, shape memory effect and microscopic structures of two different quaternary TiNiHfZr alloys (Ti x Hf 15 Zr 5 Ni 80-x and Ti 31.5 Hf y Zr 20-y Ni 48.5) have been investigated systematically. It is found that increasing Ti/Ni ratio of Ti x Hf 15 Zr 5 Ni 80- x alloys dramatically increases the martensitic transformation temperature and a maximum value (5%) of the recovered strain during shape recovery on heating is obtained for the alloy with x = 30. On the contrary, changing Hf/Zr ratio of Ti 31.5 Hf y Zr 20- y Ni 48.5 alloys does not result in big change of martensitic transformation temperature or recovered strain. The evolution of macroscopic properties of Ti x Hf 15 Zr 5 Ni 80- x alloys with x can be understood by considering the balanced effect between the Ni content change of the matrix, and the volume fraction of Ti 2 Ni-like precipitates, which are often brittle and not desired for SMAs. Our results suggest that the Ti x Hf 15 Zr 5 Ni 80-x (x > 30.5 at. %) alloys are promising shape memory alloys for high temperature applications above 250 °C. [ABSTRACT FROM AUTHOR]