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

The crystal structures of the different martensitic phases observed in a wi de variety of Ni-Mn-Ga alloy compositions have been studied in detail. Simi larly to the Ni-Al alloys, the non-modulated martensite can be well describ ed 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 literatu re 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 la ttices modulated by shuffling. The two approaches are shown to be very simi lar and, in many cases, indistinguishable by the diffraction techniques usi ng photographic recording. However, some physical arguments are given to in terpret the five- and seven-layered structures with the second and first ap proaches, 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 Scienc e Ltd. All rights reserved.