Martensitic transformation and deformation twinning are investigated in Ti-24Nb-4Zr-8Sn (wt. %) alloy with in situ tensile deformation in scanning and transmission electron microscopes. Deformation-induced β to α″ martensitic transformation occurs at a stress plateau stage of ∼200 MPa and deformation twinning takes place subsequently at later strain hardening stage. In situ transmission electron microscopy observations combined with finite element method calculation reveal that three deformation mechanisms, α″ martensitic transformation, {112}<111>β deformation twinning and kinks of α″ lamellae, are activated with the increase of applied stress/strain. Among them, deformation-induced β to α″ phase transformation dominates the early-stage deformation. As the stress increasing, {112}<111>β deformation twinning is activated and becomes the dominating deformation mechanism afterward. Interactions between dislocation slips and martensite/twin lamellae are likely responsible for the strain hardening. After the deformation over the ultimate tensile strength, slip bands initiate and become the major deformation mechanism during necking until fracture.Download high-res image (315KB)Download full-size image

Cyclic tensile loading tests and transmission electron microscopy investigation are conducted on a Ti-24Nb-4Zr-8Sn (wt%) alloy. Under tensile strain less than 3.3%, most of the deformation strain recovers after unloading but significant energy dissipation occurs during the loading-unloading cycle. Reversible migration of twin boundaries between α″ martensite variants, in virtue of dislocation movement on the twin boundaries, has been revealed by time resolved high-resolution transmission electron microscopy. This twin boundary migration contributes to the energy dissipation effect and consequently the damping property of the titanium alloy.

•α″ phase transformation was observed with in situ TEM in a β-Ti alloy.•Two variants of α″ phase formed in deformation bands along different directions.•The β to α″ phase transformation may contribute pseudo-elasticity of the alloy.Transmission electron microscopy observations during in situ tensile tests revealed α″ phase transformation in a β phase Ti-24Nb-4Zr-7.9Sn (wt%) alloy. Single variant α″ phase is formed in deformation bands under tensile stress and the deformation bands vanish after the applied stress is released. With increasing strain, two variants of the α″ phase are observed in deformation bands along different directions, respectively. When these deformation bands intersected with each other, these two variant α″ phases could remain after unloading of the external stress. The α″ phase transformation in single variant deformation bands likely contributes to the nonlinear pseudo-elasticity of this alloy.

•Deformations are studied for a Co-rich Ni based superalloy with different γ׳ sizes.•APB shearing changes to SF shearing and Orowan bypassing with the increasing γ׳ size.•Microtwins are formed with high density of stacking faults after deformation.A Co-rich nickel based superalloy with different size of γ׳ precipitates is obtained through varied aging time. Microstructures after tensile deformation at intermediate temperature are investigated by transmission electron microscopy. With the increasing γ׳ precipitate size, the deformation mode of the alloy changes from cutting the precipitates by 1/2〈110〉 dislocation pairs to cutting by 1/6〈112〉 partial dislocations. Microtwinning occurs with high density of stacking faults, attributing to the low stacking fault energy of the alloy. With further increase of the precipitate size, the alloy starts to be deformed by Orowan bypassing mechanism.

In this work, statistical analyses are conducted to measure the stacking fault density and stacking fault energy in a Ni–Co-based superalloy at different deformation temperatures and strain rates. High stacking fault density and low stacking fault energy indicate that the dynamic strain aging effects are caused by Suzuki segregation, which is enhanced by serious intersections of stacking faults in the superalloy. These results provide new insight into understanding stacking fault effects on dynamic strain aging in engineering alloys.

Macroscopically zero-strained twin lamellae are observed in a coarse-grained Ni–Co-based superalloy after plastic deformation. The twin lamellae presented as three layers of overlapping stacking faults with zero overall Burgers vector. The atomic stacking sequence in the twin lamellae is consistent with pairs of parallel stacking faults at neighboring or next-neighboring glide planes. The twinning is facilitated by the low stacking fault energy of the superalloy and the lengthening of stacking faults by shear stress from nearby stacking faults.

In order to faithfully investigate the microscopic and mesoscopic structures of bulk metallic glass (BMG) materials, inhomogeneous contrast in nanometer to micrometer scale in BMG specimens has been throughoutly studied by transmission electron microscopy (TEM), atomic force microscopy (AFM) and scanning electron microscopy (SEM). It is found that ion-milling induced surface roughening and pattern formation are the source of the inhomogeneous contrast in TEM images of BMG samples. Additionally, high resolution TEM and X-ray energy dispersive spectroscopy investigations rule out structure or composition changes in the specimens. The dynamic roughening process during ion-milling as well as saturated pattern sizes are revealed from Zr-, Cu-, Fe- and Mg-based BMGs. The understanding of this inhomogeneous etching in BMG specimens is critical for recently intensive TEM studies on microscopic and mesoscopic inhomogeneities of BMGs.Highlights► Ion-milling induced surface roughening and patterning found in bulk metallic glasses. ► Thickness variation causes the inhomogeneous contrast in TEM images of BMG samples. ► The dynamic roughening process was determined from Zr-, Cu-, Fe- and Mg-based BMGs.

Variable resolution fluctuation electron microscopy (FEM) experiments are implemented with hollow-cone dark-field transmission electron microscopy. Medium range order lengths of zirconium and iron based bulk metallic glasses and amorphous silicon nitride are determined from the FEM results. It shows that maximum normalized intensity variances of FEM images occur when their nominal resolution approaches the correlation length Λ of the amorphous materials. Additionally, differences in the length and magnitude of medium range order are compared between metallic and covalent bond amorphous materials.Highlights► Variable resolution fluctuation electron microscopy (FEM) was successfully conducted in the hollow-cone dark-field imaging mode. ► Medium range order lengths of Zr- and Fe-based bulk metallic glasses were obtained from FEM results. ► The maximum normalized variance of FEM image was found occurring when the experimental nominal resolution approaches the correlation length of amorphous materials.

Cu3Sn phase with the D019 structure is observed in Cu–Sn alloy by transmission electron microscopy. The structure is determined by electron diffraction, high-resolution transmission electron microscopy and high-angle annular dark-field imaging techniques. To the best of our knowledge this structure has not been reported before in Cu3Sn compounds. Transmission electron microscopy revealed high-density antiphase boundaries lying on the {0 1¯ 1 0} planes with translation vectors of (a/2)〈2¯ 1 1 0〉 in the D019 Cu3Sn.

The accuracy of maximum entropy reconstruction of Z-contrast STEM images has been evaluated with the effects of experimental variables and noise taken into account by the means of image simulation. As the specimen contains atom species of greatly different atomic numbers, special attention is given to the reliability of the position and composition of lighter atoms that are determined from Z-contrast images in the presence of heavier atoms. When the noise is moderate (SNR >2.5), the position of atom columns can be measured within an accuracy of 0.03 nm. With a higher signal-to-noise ratio (SNR >5) the composition of lighter atoms can be resolved reliably from the Z-contrast images. However, when image noise increases, the relative intensity of lighter atoms may deviate from the actual value in the specimen object function.

Aiming to determine the contrast mismatch factor i.e. the Stobbs factor between the experimental and simulated high-resolution transmission electron micrographs, we have systematically compared the experimental images and simulations of a cleaved silicon sample for a series of focal settings and specimen thicknesses. For zero-loss energy filtered images, a mismatch factor of about 1.5–2.3 is measured for the image contrast, where the mismatch factor is focal dependent and higher mismatch appears around the focus value of 10 nm. Attention is also given to the effects of the sample vibration and drift to the image contrast and pattern of the high-resolution micrographs.

•Nonstoichiometric dislocation cores with oxygen vacancies in LSMO films were quantitatively characterized.•MnLa antisite defects at dislocation cores were discovered.•The special configuration of the dislocation core was proposed to result from the strain accommodation.Oxide thin films with perovskite structures possess multifunctional properties, while defects in the films usually have significant influences on their physical properties. Here, the atomic structure and chemistry of a[100] dislocation cores in epitaxial La2/3Sr1/3MnO3 films were investigated by aberration-corrected scanning transmission electron microscopy combining with atomically resolved electron energy-loss spectroscopy imaging. The results demonstrated an edge dislocation terminated with Mn columns and significant nonstoichiometry at the dislocation core region. Quantitative analysis using core-loss spectrum indicates that La/Mn and O/Mn ratios are decreased at the dislocation core. Antisite defects with Mn ions at La-sites were directly determined at the dislocation cores with electron energy-loss spectroscopy. The structure of the dislocation core is discussed on the basis of high-angle annular dark-field imaging and electron energy loss spectroscopy results.