No abstract available

[en] The results of a study on the occurrence of serrated [101^-2] twin interfaces in zirconium and titanium indicate that these serrations form on planes containing the twinning shear direction. During twin growth these serrations occur preferentially at the interface corresponding to the prominent corner produced at the free surface by the twinning shear. The occurrence of serrations is favoured, however, when the twinning shear results in a relatively small amount of surface tilt. When the surface tilt is relatively large, well-formed serrations in [101^-2] twin interfaces occur less frequently since they cannot be easily accommodated. It is proposed that the ease with which the deformation modes of these h.c.p. metals accomodate the serrations is an important factor in determining the conditions that favour the occurrence of serrations. This proposal would also explain why the occurrence of serrated twin interfaces is principally a surface phenomenon. (Auth.)

[en] Twinning processes during dynamic recrystallization were examined. The spatial frequency distribution of the twin grain boundaries in the recrystallized areas was determined and correlated with the various deformation degrees in the recrystallized areas by way of a deformation simulation according to Taylor. The measured twinning frequency in the last developed DRX areas is comparable with the frequency observed in the non-dynamically recrystallized initial material. So it is assumed that the twinning process is of similar significance to DRX and recrystallization in the autenitic steel. It could be revealed that twinning occurs during growth of the recrystallization nuclei, and therefore has an influence on the kinetics of the dynamic recrystallization. (orig./MM)

No abstract available

[en] This work presents a mechanism of deformation-twin-induced grain boundary failure, and demonstrates the mechanism using molecular dynamics simulations. Deformation twinning is observed as the dominant mechanism during tensile deformation of columnar nanocrystalline body-centered cubic Mo. As a twin approaches a grain boundary, local stress concentration develops due to the incompatible plastic deformations in the two neighboring grains. The magnitude of the stress concentration increases as the twin widens, leading to grain boundary cracking by nucleation and coalescence of microcracks/voids.

[en] In this work we present a new method to produce gold nanorods based on the bio-reduction process. The nanorods produced tend to show twins appearing either as bands or concentric forms, generating a new type of nanorods based on a decahedral structure. Examples of these two types of twins are presented here. The main conclusion is that the bio-reduction method to produce nanorods is a good alternative to the electrochemical methods

[en] Very regular twin structures can be formed in textured industrial Zr-2.5% Nb alloy by compression along a direction which does not - or only to a very limited extent - lie in basal planes of the crystallites. Single or co-operative twins arise in rolled rods or tubes through plastic deformation if this exceeds 2%. Those twins have the appearance of regular strips at an angle about 45⁰ to the rolling direction. Their length is determined by grain size or by dimensions of areas with stable crystallographic grain orientation. For sharper tube texture, the twin length exceeds that for rods. The thickness of the twins increases with the degree of deformation. The maximum twin thickness is limited by grain size, which is 4-7 μm for the material investigated. With increase in deformation, the form of the twins changes from strips to a lenticular shape and with further increase eventually to the shape of normal grains. This is the phenomenon of the transformation of twins into grains. The twin structure is stable and preserved after standard annealing. The above considerations for Zr-2.5% Nb may indeed be applicable to other α-Zr alloys. (orig.)

No abstract available

[en] Regularities and a formation mechanism of predominant orientations on longitudinal and transverse rolling of cadmium, zinc and magnesium sheets were studied by the methods of pole figures and metallography. It is shown, that the initial stage of longitudinal cadmium rolling is characterized with a feebly marked texture. With a reduction increase up to 30%, the texturing degree increases, and the texture formation process is completed, when the reduction reaches 90%. The initial stage of transverse rolling leads to a sharp change in predominant orientations. The reduction increase up to 40% intensifies the scattering of the predominant orientations, however, the further increase of reduction up to 70% again causes the formation of deflected texture similar to the texture of longitudinal rolling

[en] We consider a model of shape memory materials in which hierarchical twinning near the habit plane (austenite-martensite interface) is a new and crucial ingredient. The model includes (1) a triple-well potential (φ⁶ model) in local shear strain, (2) strain gradient terms up to second order in strain and fourth order in gradient, and (3) all symmetry allowed compositional fluctuation-induced strain gradient terms. The last term favors hierarchy which enables communication between macroscopic (cm) and microscopic (A) regions essential for shape memory. Hierarchy also stabilizes tweed formation (criss-cross patterns of twins). External stress or pressure modulates (''patterns'') the spacing of domain walls. Therefore the ''pattern'' is encoded in the modulated hierarchical variation of the depth and width of the twins. This hierarchy of length scales provides a related hierarchy of time scales and thus the possibility of non-exponential decay. The four processes of the complete shape memory cycle-write, record, erase and recall-are explained within this model. Preliminary results based on 2D molecular dynamics are shown for tweed and hierarchy formation. (orig.)