Materials Science and Engineering A 438–440 (2006) 931–934 Structure of the layered martensitic phases of Ni–Mn–Ga alloys J. Pons a,* , R. Santamarta a , V.A. Chernenko b , E. Cesari a a Departament de F´ ısica, Universitat de les Illes Balears, Ctra. de Valldemossa, km 7.5, E-07122 Palma de Mallorca, Spain b Institute of Magnetism, Vernadsky str., 36-b, Kiev 03142, Ukraine Received 8 May 2005; received in revised form 31 October 2005; accepted 14 February 2006 Abstract The present work contains a high-resolution transmission electron microscopy study (including image simulations) of the layered martensitic structures formed in Ni–Mn–Ga alloys. The main purpose is to distinguish two structural models proposed in the literature. For the seven-layered martensite, the observations confirm its nano-twinned nature, which is inherent to the description as stacking of nearly close-packed planes derived from {110} aust with (5 ¯ 2) sequence, i.e. the so-called 14M structure. The observed stacking sequence, however, is notably distorted (perfect (5 ¯ 2) stacking is present only in small areas), although, it preserves a predominant periodicity of seven planes. Equivalent results have been obtained for the 10-layered structure and a less frequent 12-layered phase, for which (5 ¯ 5) and (7 ¯ 5) stacking sequences, respectively, have been predominantly observed. In case of the five-layered martensite, the image simulations and atom projections obtained from the two structural models fit well with the experimental images, which makes it very difficult to discern between them. Other ferromagnetic shape memory alloys present similar structures as the Ni–Mn–Ga system. Particularly, a “new” six-layered martensitic structure has been found in Ni–Fe–Ga alloys. The first observations reveal its nano-twinned nature as well, with a predominant (4 ¯ 2) stacking sequence. © 2006 Elsevier B.V. All rights reserved. Keywords: High-resolution transmission electron microscopy (HRTEM); Ferromagnetic shape memory alloys (FSMA); Martensitic phase transformation; Layered structures 1. Introduction The observation of large magnetic field induced strains caused, in the last years, a growing interest in the Ni–Mn–Ga and other ferromagnetic alloys exhibiting thermoelastic martensitic transformations [1–4]. Different martensitic phases have been reported, some of them having layered structures with periods of 5, 7 or 10 planes. The layered structures have been interpreted in the literature, in two different ways: as modulated structures with shuffling of the atomic planes derived from {110} aust by a function with the corresponding periodicity [5,6], or as stacking of nearly close-packed basal planes derived from {110} aust , as in Ni–Al alloys [7,8]. These two approaches have originated a quite big dispersion in the nomenclature of the different marten- sitic structures. From the first approach, the structures are usually denoted as 5M, 7M and NM (or T), for the five-, seven- and non- layered phases, respectively. Using the second approach, these * Corresponding author. Tel.: +34 971 173217; fax: +34 971 173426. E-mail address: jaume.pons@uib.es (J. Pons). phases are denoted as 10M, 14M and 2M, respectively, while the 10-layered phase is named as 10O. In our previous paper [9], the two approaches for the struc- tural description of the layered martensites were discussed and compared, and it was concluded that the atomic positions are close enough to be practically undistinguishable from diffraction techniques using photographic recording (as electron diffraction or the X-ray techniques used in refs. [5,6]). In the present work, a high-resolution transmission electron microscopy (HRTEM) study of the layered martensites has been performed in order to discern from the two structural models. 2. Experimental procedure Three Ni–Mn–Ga alloys with compositions Ni 54.3 Mn 20.5 Ga 25.2 (alloy 1, numbers indicated of %), Ni 45.7 Mn 37.2 Ga 17.1 (alloy 2) and Ni 51.0 Mn 27.9 Ga 21.1 (alloy 3) were used in the present work, together with a Ni–Fe–Ga alloy of composition Ni 55 Fe 18 Ga 27 (alloy 4). The selected compositions show the dif- ferent martensitic structures at room temperature, which allowed the study by high-resolution transmission electron microscopy 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.179