Diffraction artefacts from twins and stacking faults, and the mirage of hexagonal, polytypes or other superstructures

Recently, a hexagonal phase has been reported in high carbon steels in several studies. Here, we show that the electron microscopy results used in these studies were erroneously interpreted. The extra-spots in the diffraction patterns and the odd contrasts in the high resolution images are not those a superstructure but result from double diffraction and streaking effects due to the presence of twins and stacking faults. We point out a similar unfortunate misunderstanding of these effects in papers reporting the existence of a 9R structure in aluminium or copper, or exotic forms of carbon in diamonds. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd.

Automated summary

Recent studies have reported a hexagonal phase in high carbon steels, but it has been shown that this structure is actually the result of double diffraction and streaking effects caused by twins and stacking faults, rather than a superstructure. Similar misunderstandings have also been found in papers reporting exotic structures in aluminum, copper, and diamonds.