Structure of the layered martensitic phases of Ni-Mn-Ga alloys

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 (52) sequence, i.e. the so-called 14M structure. The observed stacking sequence, however, is notably distorted (perfect (52) 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 (55) and (75) 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 (42) stacking sequence. (c) 2006 Elsevier B.V. All rights reserved.