Effect of Σ3 grain boundaries on microstructure and properties of oriented Fe-6.5 wt%Si in twin-roll strip casting
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Three different tests on twinning by merohedry from the literature have been applied to single-crystal data sets of five different inorganic crystal structures. Although the three test procedures differ significantly with regard to their efficiency, in both detecting the existence of twinning and estimating the volume fractions of the twin individuals, they represent useful tools in the early stages of a structure analysis and should be applied routinely in the preliminary stage of a structure determination whenever a twinning by merohedry is possible.
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Definitive evidence from both electron backscattered diffraction and transmission electron microscopy is shown for the existence of twinning as a rare deformation twinning mode in coarse-grained commercial purity titanium after room temperature ballistic impact testing at 103 s−1. Non-Schmid-based twin-type selection is demonstrated for and the conjugate deformation twinning modes in this material within grains where the c-axis is closely aligned to the loading direction. Limited Schmid-based twin variant selection is shown for and deformation twinning modes in this material. The occurrence of high area fractions of twinning has relevance for high strain rate plasticity modelling of grains of textured titanium, compressed parallel to their c-axes.
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An impact test was conducted on a single crystal of an Fe–3mass%Al alloy with a bcc structure. Microstructural changes in the sheared regions resulting from the impact test were examined. Deformation twinning was observed, with the twinning plane K1={332} and the twinning shear direction η1=⟨113¯⟩. The conjugate twinning direction η2 of this mode was discussed. Considering the shear direction required to induce deformation twinning, it can be concluded that η2=⟨110⟩ is appropriate for the twinning mode in the Fe–3mass%Al.
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The \{ 11{\overline 2}2\} and \{ 11{\overline 2}6\} twinning modes were recently discovered by Ostapovets et al. [ Philos. Mag. (2017), 97 , 1088–1101] and interpreted as \{ {10{\overline 1}2} \}–\{ {10{\overline 1}2} \} double twins formed by the simultaneous action of two twinning shears. Another interpretation is proposed here in which the two conjugate twinning modes result from a one-step mechanism based on a (58°, a + 2 b ) prototype stretch twin and differ from each other only by their obliquity correction. The results are also compared with the classical theory of twinning and with the Westlake–Rosenbaum model.
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Reports of Type II twins are quite rare for most crystal structures. When they do occur, they are usually one of a number of possible twinning modes observed in a particular material. However, for the triclinic phase devitrite, Na2Ca3Si6O16, which nucleates from commercial soda−lime−silica float glass subjected to suitable heat treatments, the only reported twinning mode to date is a Type II twinning mode. In this study, this Type II twinning mode is first examined by molecular dynamics simulation to determine the lowest energy configuration of perfect twin boundaries for the twin mode. This is then compared with the lowest energy configurations of perfect twin boundaries found for six possible Type I twinning modes for devitrite for which the formal deformation twinning shear is less than 0.6. The most favourable twin plane configuration for the Type II twinning crystallography is shown to produce reasonably low twin boundary energies and sensible predictions for the optimum locations of the twin plane, K 1, and the [1 0 0] rotation axis, η 1, about which the 180° Type II twinning operation takes place. By comparison, all the Type I twinning modes were found to have very energetically unstable atomic configurations, and for each of these twinning modes, the lowest energy configurations found all led to high effective K 1 twin boundary energies relative to perfect crystal. These results therefore provide a rationale for the experimental observation of the particular Type II twinning mode seen in devitrite.
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The fundamental theory of crystal twinning has been long established, leading to a significant advance in understanding the nature of this physical phenomenon. However, there remains a substantial gap between the elaborate theory and the practical determination of twinning elements. This paper proposes a direct and simple method - valid for any crystal structure and based on the minimum shear criterion - to calculate various twinning elements from the experimentally determined twinning plane for Type I twins or the twinning direction for Type II twins. Without additional efforts, it is generally applicable to identify and predict possible twinning modes occurring in a variety of crystalline solids. Therefore, the present method is a promising tool to characterize twinning elements, especially for those materials with complex crystal structure.
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