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Abstract Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars subjected to compress along <011> β in the Part I of this paper. In this Part II, a series of samples at different deformation stages were systematically analyzed by transmission electron microscopy (TEM) to understand the deformation mechanism for double size effects. Two distinct phase transformations were observed to take place at different stages of plastic deformation. In the initial stage, dislocation interaction with ω precipitates and deformation-induced ω variants transformation from one to another are predominant plastic deformation mechanisms when the total strain is less than 10%. As a result, high density of ω precipitates contributes to the weak size effect and the continuous stable strain-hardening behavior in the metastable β Ti-10V-2Fe-3Al alloy. In comparison, the martensitic transformation from β to α″ is induced when the critical strain reaches to 10%. The strain-induced martensitic transformation becomes primary deformation modes, which result in higher strain-hardening exponent and stronger size effect, in the second deformation stage. These results provide a new perspective of designing micro-electromechanical materials with an excellent combination of the enhanced yield strength and stable plasticity. [ABSTRACT FROM AUTHOR]

Abstract The effects of pre-deformation on the precipitation behaviors and properties in Cu-Ni-Si-Cr alloy were investigated. The results showed the electrical conductivity and the strength of the alloys after peak aging increased with the pre-deformation amount. Quantitative analyses demonstrated the deformation-induced defects improved the precipitation and the number density of precipitates but reduced their average radii. As a result, the conductivity improved due to the slight promotion of precipitation while the strength increased mostly from the enhanced precipitate strengthening and then from the unremoved work hardening. At elevated temperatures, the effects of pre-deformation were greatly weakened and then the difference among the alloys after different treatments disappeared. [ABSTRACT FROM AUTHOR]

In present work, the localized deformation, crack initiation and propagation behavior of Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si (TC21) titanium alloy was investigated through the in-situ scanning electron microscope (SEM) test and electron backscatter diffraction (EBSD). The results indicate that deformation in α phase is mostly accommodated by the prismatic slip. The localized deformation is concentrated at the interface of primary α lath. Some parallel slip bands with prismatic and pyramidal slip are prevalent in primary α lath, while the multiple slip bands with prismatic slip are dominant in equiaxed primary α phase. Higher ability to coordinate local plastic strain (nearly to 79.5%) is induced in equiaxed α than the primary α lath (7.9%). Most of the cracks easily initiate at the shear bands in primary α lath, which is a fatal site for the tensile deformation. The main crack propagates quickly along the shear bands or interfaces of α lath and then penetrates into the β trans matrix and also transfers across the equiaxed α phase along the slip bands. The results indicate that the high compatibility of deformation in equiaxed primary α phase delays the initial crack nucleation. Whereas, it does not show satisfied resistance against crack propagation in front of the main crack tip due to the abundant slip bands. [ABSTRACT FROM AUTHOR]

Deformed coarse-grained polycrystalline metals always unload elastically where permanent dislocation network well-developed in the loading regime hinders the movement of dislocations and allows only the elastic relaxation of stress. Such elastic unloading behavior is, however, unexpected in nanocrystalline metals because the dislocation network cannot effectively form inside nanometer-scale grains. In this work, we report the experimental finding of significant plastic deformation that emerges in the unloading regime in the compressive cyclic test at room temperature of nanocrystalline Cu. The magnitude of plastic strain produced during unloading depends strongly on loading and unloading rates. This plastic unloading behavior arises from the rapid absorption of dislocations accumulated during loading, which was quantitatively interpreted by performing the incremental unloading test and developing a relationship between the dislocation density and the loading and unloading rates based on the models of the statistical absorption of dislocations by grain boundaries and the dislocation emission from grain boundary ledges. Concurrently, the evolution of deformation structures during the cyclic deformation was also analyzed in terms of the interactions of gliding dislocation–twin boundaries. [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]

Different size effects and deformation mechanisms are revealed in α-titanium (Ti) single crystal nanopillars orientated for [112̄0] and [0001] based on molecular dynamics simulations. The strength–size relationship changes from “smaller is stronger” to “smaller is much weaker” when the width of nanopillars reduces from 19 nm to 3 nm. The “smaller is much weaker” is attributed to that the surface effect caused by a thermal vibration of surface atoms leads to the initiation and growth of surface dislocations. [ABSTRACT FROM AUTHOR]

Abstract: Effect of the initial grain size on plastic deformation behavior and grain refinement mechanism has been investigated in Ti–2Al–2.5Zr alloy subjected to multi-impact process. The results show that grain refinement is dominated by dislocation slip in the fine-grained sample. In contrast, deformation twinning becomes prevalent in the coarse-grained one, and contributes to grain refinement by means of twin subdivision and fragmentation. Dynamic recrystallization (DRX) takes place in both samples. But, it is delayed in the coarse-grained one due to relative low fraction of grain boundaries and energy dissipation of deformation twinning. Nevertheless, the final grain size in the refined area does not show an obviously difference between the two samples at a large strain, i.e., 1.61. Moreover, grain refinement is inhomogeneous in both samples. The heterogeneity in the coarse-grained sample is much stronger than that in the fine-grained one. It was ascribed to the superimposed effect of large initial grain size and subsequent deformation twinning. A criterion for achieving an optimum grain refinement was finally suggested. [Copyright &y& Elsevier]

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 size dependent hardness and strain-rate sensitivity of magnetron sputtered nanocrystalline Mo thin films with film thickness h ranging from 250 to 2000 nm were systematically investigated by using nanoindentation tests. Microstructure examinations manifested that all the as-deposited Mo thin films had a columnar-nanograined hierarchical structure with highly (110)-oriented growth texture, and the lateral columnar size went through a maximum of ~60 nm at h = 1500 nm sample with h. Interestingly, both the hardness and strain-rate sensitivity exhibited a monotonic increment with reducing the columnar size. The underlying strengthening and deformation mechanisms could be interpreted in light of the confined slip of dislocations in the columnar grains, rather than the thermal activation of screw dislocations. [ABSTRACT FROM AUTHOR]