Highly mobile twinned interface in 10 M modulated Ni–Mn–Ga martensite: Analysis beyond the tetragonal approximation of lattice L. Straka a,⇑ , O. Heczko b , H. Seiner c , N. Lanska d , J. Drahokoupil b , A. Soroka d , S. Fa ¨hler e , H. Ha ¨nninen a , A. Sozinov d a Aalto University School of Science and Technology, Laboratory of Engineering Materials, PL 14200, FIN-00076 AALTO, Finland b Institute of Physics, ASCR, Na Slovance 2, 182 02 Prague, Czech Republic c Institute of Thermomechanics, ASCR, Dolejs ˇkova 1402/5, 182 00 Prague, Czech Republic d AdaptaMat Ltd., Yrityspiha 5, Helsinki, FIN-00390, Finland e IFW Dresden, Institute for Metallic Materials, PO Box 270116, D-01171 Dresden, Germany Received 11 July 2011; received in revised form 8 September 2011; accepted 9 September 2011 Available online 15 October 2011 Abstract The huge strains that Ni–Mn–Ga magnetic shape memory alloys can achieve are usually described in a tetragonal unit cell approx- imation of a five-layered modulated (10 M) crystal structure. Here we analyze the impact of a slight orthorhombic and monoclinic dis- tortion of the 10 M structure in Ni 50.2 Mn 28.3 Ga 21.5at.% single crystal. Combining dedicated experiments to probe the microstructure, structure and mechanical properties with calculation using elastic continuum theory, we prove the existence of fine a/b-laminates within modulation macrotwins of the order of 100 micrometers in size. This complex twin microstructure containing a Type II macrotwin inter- face is associated with an extraordinarily low twinning stress of between 0.05 and 0.3 MPa, while Type I twins exhibit twinning stress of about 1 MPa. The findings provide important guidelines for designing the martensitic microstructure for more efficient actuators. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Microstructure; Twinning; X-ray diffraction; Heusler phases; Magnetic shape memory 1. Introduction Magnetic shape memory (MSM) alloys have the out- standing property that they can exhibit giant (up to 10% [1]) and simultaneously fast (1 kHz [2]) straining in a moderate magnetic field (<1 T). This magnetic-field- induced strain (MFIS), accompanied by a reasonable force output (1 MPa [3,4]), exceeds the strain reachable with the best magnetostrictive materials by about two orders of magnitude. The strain of the best ferroelectric materials is exceeded by up to about one order of magnitude [5]. The extraordinary properties of MSM alloys, and the new application possibilities they offer, have motivated intense research, which has resulted in active material in the form of single crystals [6,7], polycrystals [8], fibers [9], foam [10] and thin films [11,12]. Since the Ni–Mn–Ga alloys are not only the prototype MSM alloy but still the best MSM material available, here we focus on this system. The existence of ferromagnetic twinned martensite micro- structure is a precondition for the existence of the MFIS as the effect occurs by the rearrangement of the twinned micro- structure in a magnetic field [13]. More specifically, Ullakko et al. [14] identified the movement of twin boundaries as the underlying mechanism of MFIS. When applying a magnetic field, the twin variants that have their easy magnetization axis along the field direction are energetically favored. These variants grow under the field by the motion of twin boundaries at the expense of unfavorably oriented variants, which results in the giant strain or MFIS. This effect has also 1359-6454/$36.00 Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2011.09.020 ⇑ Corresponding author. Tel.: +358 9 470 25713; fax: +358 9 470 23518. E-mail address: ladislav.straka@tkk.fi (L. Straka). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 59 (2011) 7450–7463