Constrained hierarchical twinning in Ru-based high temperature shape memory alloys.
The microstructure of high temperature shape memory alloys of the Ru-Ta and Ru-Nb family are investigated in detail. After cooling from the high temperature cubic 13 phase the off-stoichiometric alloys show a compound twinning in the tetragonal beta’ phase which prevails at room temperature and can be observed as two levels of laminates A and C . The stoichiometric alloys are characterized by a second phase transformation from beta’ to monoclinic beta ”, which adds another type of twinning. The latter belongs to the monoclinic P2/m structure and has been created out of one of the lost symmetry elements from the tetragonal P4/mmm phase during the beta’ -> beta ” transformation. The unusual characteristic of this new twinning, called B, is the fact that it has an intermediate size between the A and C twins that belong to the high-temperature transformation beta’ -> beta ”. This is due to a second constrained martensitic transformation. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.