Analysis of Electron Channeling Contrast of Stacking Faults in fcc Materials.

The characteristics of electron channeling contrast (ECC) of stacking faults in a [101] single-crystal 316L fcc (face-centered cubic) stainless steel have been evaluated. Channeling contrast was analyzed from a series of ECC images taken as a function of sample tilt at ~0.1° increments across the (111) Kikuchi band. The most relevant imaging parameters of the fault contrast, namely the number of fringes, fringe intensity, and fringe spacing, were analyzed under different channeling conditions. The present work shows that the channeling contrast of stacking faults exhibits strong dependence on the sign and magnitude of the deviation parameter, w (w = s ξg, where s is the excitation vector and ξg is the extinction distance). This effect has strong influence on channeling conditions for fault imaging and g.R analysis for determining the nature of a stacking fault. g.R analysis was evaluated by the method developed by Gevers et al. (1963) on g-reversion experiments of ECC images of a stacking fault configuration. The interplay between the stacking fault nature and ε-martensite is analyzed.

Quantitative analysis of electron channeling contrast of dislocations.

Quantitative analysis was performed for electron channeling contrast (ECC) of dislocations in a [001] single-crystal 316 L stainless steel sample in order to establish the range of deviation parameter, w, for optimal dislocation imaging. Channeling contrast was evaluated from a series of ECC images taken as a function of sample tilt at ∼ 0.1° increments across 〈200〉 Kikuchi lines. The deviation parameter, w, was estimated by quantitative analysis of several parameters, namely experimental dislocation contrast intensity and dislocation feature width, as well as calculated BSE signal. Our analysis reveals that optimum dislocation imaging conditions by ECC occur for positive values of w. Under these imaging conditions, dislocations displayed strong contrast and narrow dislocation feature width, y. In particular, we found that the value of y can be greatly reduced (about two times in the present work) by using large positive values of w (w > 1.0 ±â€¯0.5). Out of these channeling conditions, dislocations displayed weak contrast and larger y-values were measured.

Twinning behavior of orthorhombic-α" martensite in a Ti-7.5Mo alloy.

Deformation microstructure of orthorhombic-α" martensite in a Ti-7.5Mo (wt.%) alloy was investigated by tracking a local area of microstructure using scanning electron microscopy, electron back-scattered diffraction, and transmission electron microscopy. The as-quenched α" plates contain {111}α"-type I transformation twins generated to accommodate transformation strain from bcc-ß to orthorhombic-α" martensite. Tensile deformation up to strain level of 5% induces {112}α"-type I deformation twins. The activation of {112}α"-type I deformation twinning mode is reported for the first time in α" martensite in ß-Ti alloys. {112}α"-type I twinning mode was analyzed by the crystallographic twinning theory by Bilby and Crocker and the most possible mechanism of atomic displacements (shears and shuffles) controlling the newly reported {112}α"-type I twinning were proposed.

Quantitative analysis of {332}〈113〉 twinning in a Ti-15Mo alloy by in situ scanning electron microscopy.

We have performed quantitative analysis of {332}〈113〉 twinning in a ß-Ti-15Mo (wt.%) alloy by in situ scanning electron microscopy and electron backscattering diffraction (EBSD). Microstructure-twinning relations were evaluated by statistical analysis of the evolving twin structure upon deformation at room temperature. Our analysis reveals that at the early stages of deformation (ε < 1.5 to 2.0%), primary twinning is mainly determined by the applied macroscopic stress resolved on the twin system. Most of the primary twins (~70-80% of the analyzed twins) follow Schmid's law with respect to the macroscopic stress, and most of the growth twins (~ 85% of the analyzed twins) correspond to the higher stressed variant. In the grain size range studied here (40-120 µm), we find that several twin parameters such as number of twins per grain and number of twins per grain boundary area exhibit grain size dependence. We ascribe these effects to the grain size dependence of twin nucleation stress and apparent critical resolved shear stress for twinning, respectively.

Analysis of dislocation configurations in a [0 0 1] fcc single crystal by electron channeling contrast imaging in the SEM.

We have investigated the dislocation configurations in a [0 0 1] single crystal of a face-centered alloy (316L stainless steel) by the electron channeling contrast imaging (ECCI) technique in the scanning electron microscope under controlled diffraction conditions. Specific dislocations such as piled-up dislocations and Lomer-Cottrell dislocations were characterized by the analysis of dislocation contrast and dislocation line trace analysis. The ECCI technique also allows the sound estimation of the local resolved stress acting on gliding dislocations by the analysis of the radius of curvature following a transmission electron microscopy-based geometrical approach.

Analysis of FIB-induced damage by electron channelling contrast imaging in the SEM.

We have investigated the Ga+ ion-damage effect induced by focused ion beam (FIB) milling in a [001] single crystal of a 316 L stainless steel by the electron channelling contrast imaging (ECCI) technique. The influence of FIB milling on the characteristic electron channelling contrast of surface dislocations was analysed. The ECCI approach provides sound estimation of the damage depth produced by FIB milling. For comparison purposes, we have also studied the same milled surface by a conventional electron backscatter diffraction (EBSD) approach. We observe that the ECCI approach provides further insight into the Ga+ ion-damage phenomenon than the EBSD technique by direct imaging of FIB artefacts in the scanning electron microscope. We envisage that the ECCI technique may be a convenient tool to optimize the FIB milling settings in applications where the surface crystal defect content is relevant.

Plastic accommodation at homophase interfaces between nanotwinned and recrystallized grains in an austenitic duplex-microstructured steel.

The plastic co-deformation behavior at the homophase interfaces between the hard nanotwinned grain inclusions and the soft recrystallized matrix grains in a duplex-microstructured AISI 316L austenitic stainless steel is examined through the analysis of long-range orientation gradients within the matrix grains by electron backscatter diffraction and transmission electron microcopy. Our analysis reveals that the mechanical accommodation of homophase interfaces until a macroscopic strain of 22% is realized within a small area of soft grains (about four grains) adjacent to the homophase interface. The activation of deformation twinning in the first two grain layers results in the occurrence of a 'hump' in the orientation gradient profile. We ascribe this effect to the role of deformation twinning on the generation of geometrically necessary dislocations. The smooth profile of the orientation gradient amplitude within the first 10 grain layers indicates a gradual plastic accommodation of the homophase interfaces upon straining. As a consequence, damage nucleation at such interfaces is impeded, resulting in an enhanced ductility of the single phase duplex-microstructured steel.

Study of {332}<113> twinning in a multilayered Ti-10Mo-xFe (x = 1-3) alloy by ECCI and EBSD.

We have investigated the propagation of {332}<113> twins in a multilayered Ti-10Mo-xFe (x = 1-3) alloy fabricated by multi-pass hot rolling. The material contains a macroscopic Fe-graded structure (about 130 µm width) between 1 and 3 wt% Fe in the direction perpendicular to rolling. We observe strong influence of the Fe-graded structure in the twin propagation behavior. The propagation of {332}<113> twins that are nucleated in Fe-lean regions (~1 wt% Fe) is interrupted in the grain interiors at a specific Fe content, namely, about 2 wt% Fe. We ascribe this effect to the role of Fe content in solid solution on the stress for twin propagation. The interruption of twins in the grain interiors results in the development of characteristic dislocation configurations such as highly dense dislocation walls (HDDWs) associated to strain localization phenomena. The nucleation and propagation of these dislocation configurations is ascribed to the underlying plastic accommodation mechanisms of the stress field at the twin tips. We find that the crystallographic alignment of HDDWs is determined by the stress field at the twin tips and the deformation texture. The excellent plastic accommodation at the interrupted twin tips allows attaining the good ductility of the present material (total elongation of 28%).

Multi-scale correlative microscopy investigation of both structure and chemistry of deformation twin bundles in Fe-Mn-C steel.

A multi-scale investigation of twin bundles in Fe-22Mn-0.6C (wt%) twinning-induced plasticity steel after tensile deformation has been carried out by truly correlative means; using electron channelling contrast imaging combined with electron backscatter diffraction, high-resolution secondary ion mass spectrometry, scanning transmission electron microscopy, and atom probe tomography on the exact same region of interest in the sample. It was revealed that there was no significant segregation of Mn or C to the twin boundary interfaces.