On the chevron morphology of surface martensite

It is established that the annealing twin boundaries of austenite act as nucleation sites for the martensitic transformation. In the present study, we observe that in surface martensite the transformation only takes place for twin boundaries that are approximately vertical in respect with the sample free surface. This phenomenon is shown to be related to variant selection. The criterion for surface martensite formation introduced in a previous study is used here to account for the selection of the twin boundaries that promote the transformation. This criterion correctly captures the phenomenon and explains the highly symmetrical chevron morphology observed in surface martensite. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.