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In-situ tensile tests were carried out on Fe22Mn0.6C and Fe22Mn0.6C3Al (wt%) twinning-induced plasticity (TWIP) steels specimens covered with gold micro-grids. High resolution atomic force microscopy (AFM) and scanning electron microscope (SEM) images were periodically captured. The latter were used for measurements of the plastic strain field, using digital image correlation (DIC). Although no meso-scale localization bands appeared, some areas were deformed three times more than average. Plastic deformation inside the grains was more heterogeneous in Fe22Mn0.6C, but at meso-scale, the degree of strain heterogeneity was not higher, at least up to 12% strain. Plastic deformation started from grain boundaries or annealing twin boundaries in both materials, due to a high elastic anisotropy of the grains. An original method based on DIC was developed to estimate the twin fraction in grains that exhibit a single set of slip/twin bands. Deformation twinning accommodated 60–80% of the plastic strain in some favorably oriented grains, from the onset of plastic flow in Fe22Mn0.6C, but was not observed in the Al-bearing steel until 12% strain. The back stress was important in both materials, but significantly higher in Fe22Mn0.6C. [ABSTRACT FROM AUTHOR]

The deformation mechanisms of two titanium batches with different oxygen contents were monitored during tensile tests performed along the rolling and transverse directions under an optical or scanning electron microscope, after EBSD mappings of grain orientations. Whereas the contribution of mechanical twinning was very limited, grain boundary sliding, sometimes leading to intergranular decohesion, as well as kink bands formation were observed. Based on the identification of the primary slip traces in a significant number of grains, the critical resolved shear stresses (CRSSs) for prismatic, basal and π 1 〈 a 〉 were estimated. Transmission electron microscopy was used to identify unambiguously dislocations of π 1 〈 c + a 〉 systems and to estimate the corresponding CRSS. The difference in oxygen content between T40 and T60 was found to modify the magnitude of the CRSSs, but to leave their relative values nearly unchanged. The evolutions in the work hardening rate were correlated with the active deformation mechanisms. [ABSTRACT FROM AUTHOR]

This study aims at explaining the absence of a threshold for crack propagation in an α/β titanium alloy during cyclic tests performed with constant Kmax and increasing Kmin, if Kmax is higher than 60–70% of KIc. Tensile, creep as well as fatigue crack growth tests are performed on specimens with various hydrogen content. SIMS analyses of hydrogen content around the tip of a crack developed in the abnormal regime are made. Solute hydrogen is shown to segregate at the crack tip and to enhance room-temperature creep, strain localisation and decohesion along α/β interfaces. [Copyright &y& Elsevier]

In situ tensile and reversed cyclic tests were run on a TWIP steel in a SEM, with High-Resolution Digital Image Correlation (HR-DIC) measurements of the plastic strain field in a few selected grains prone to twinning, with a spatial resolution between 150 and 250 nm, or under an AFM, with measurements of surface steps height at emerging deformation twins. Evidences of detwinning upon load reversal, as well as quantitative data on twinning/detwinning/retwinning were obtained. Detwinning and retwinning, which often were only partial, in spite of a fully reversed loading, did not seem to start at the onset of stress reversal. It required a sufficient variation of the stress, close to the twinning stress (estimated as 400–475 MPa) in absolute value, so that a mechanical hysteresis of the local twinned fraction occurred. Primary and secondary twinning along the same plane, inducing axial plastic strains in opposite directions, also allowed some grains to accommodate reversed plastic strain. Under fixed stress amplitude (±500 MPa), the twin fraction in all monitored grains saturated at values between 0.5 and 3.5%, from the 2nd cycle, while under fixed plastic strain amplitude (±0.5%), it increased in a ratchetting way during the whole cyclic hardening stage, reaching 0.5–5%. In both cases, however, the plastic strain amplitude accommodated by twinning/detwinning, which reached 0.35–0.42% in some grains during the 1st cycle, decreased down to less than 0.05% after 100 to 1000 cycles. [ABSTRACT FROM AUTHOR]

Abstract This study addresses the influence of as-cast spherulites, which may be present in the glassy matrix of a bulk metallic glass owing to the processing conditions, on its fracture behaviour. The fracture toughness of a Zr 55 Cu 30 Al 10 Ni 5 glass containing a low fraction as-cast spherulites (∼ 1 % volume fraction) is investigated by means of bending tests. The resulting mode-I fracture toughness values, around 72 MPa m , reveal the moderate detrimental effect of the spherulites. This result is compared to previous results for flawless and dendrite-containing glasses of the same composition, as well as with results from literature. The role played by the spherulites in crack propagation, under mode I and II, and fracture is finally addressed by means of fractography observations and finite element computations. [ABSTRACT FROM AUTHOR]

Abstract: In β-forged Ti6242, dwell periods during ambient stress-controlled fatigue substantially reduce the lifetime. Cracks initiate due to the coalescence of shear-induced cavities nucleated at α/β interfaces. This damage is most often due to pyramidal slip and enhanced by dwell periods. The influence of the hydrogen content on the tensile, creep and fatigue behaviour, damage mechanisms, and crack growth was investigated. Hydrogen reduced the proportional limit and the serrations on the tensile curves but increased the conventional yield stress at high strain rate, so that it either slowed down or enhanced room temperature creep, depending on the applied stress. When a H-induced reduction in creep occurred, H had a beneficial influence on the fatigue and dwell-fatigue lives, in spite of an acceleration of fatigue crack growth. Observations of the H distribution, using the microprint technique show that even though most of it resides in the β phase, some is transported into the α phase, especially in heavily deformed α laths. [Copyright &y& Elsevier]

Plastic deformation micro-mechanisms, dislocation structures and glide kinetics in two titanium batches with moderate and high oxygen contents (450 and 3200 wppm, respectively) are investigated by in situ tensile tests in a transmission electron microscope, at room temperature. In both materials, plastic deformation is accommodated with type screw dislocations gliding mainly in prismatic planes. The movement of screw segments is jerky, oxygen-dependent and strongly controlled by pinning on localized obstacles if the oxygen content is high. Dislocation multiplication is mainly controlled by the opening of loops produced by a double-cross slip mechanism at super-jogs. Evidences of composite glide between prismatic, first order pyramidal and basal planes are pointed out, proving the existence of intensive cross slip. [ABSTRACT FROM AUTHOR]