Crystal structure of martensitic phases in Ni-Mn-Ga shape memory alloys

The crystal structures of the different martensitic phases observed in a wide variety of Ni-Mn-Ga alloy compositions have been studied in detail. Similarly to the Ni-Al alloys, the non-modulated martensite can be well described by the L1(0) lattice, although it must be doubled in order to account for the L2(1) type of order of the parent phase. Concerning the well known five- and seven-layered martensites, two approaches taken from the literature are analysed and discussed. The first approach, widely accepted in Ni-Al alloys, describes the structure as long period stacking of {110}(p) close packed planes (10 M and 14 M structures), while the second one considers lattices modulated by shuffling. The two approaches are shown to be very similar and, in many cases, indistinguishable by the diffraction techniques using photographic recording. However, some physical arguments are given to interpret the five- and seven-layered structures with the second and first approaches, respectively. The structure of the new 10-layered martensite is studied, being described by a (5 (5) over bar) stacking sequence of close-packed planes. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.