Unorthodox determination of the modulation function in 10M modulated martensite of Ni-Mn-Ga(-Fe)

P. Veřtát, L. Straka, O. Heczko

 ­­­FZU - Institute of Physics of the Czech Academy of Sciences, 182 21 Praha 8, Czech Republic.

vertat@fzu.cz

The structure of low-symmetry phases (martensites) of Ni-Mn-Ga-based magnetic shape memory (MSM) alloys still represents a unique crystallography challenge. Not only do these alloys possess complex and practically inevitable twined microstructure [1,2], but they also feature still not fully understood structural modulation [3]. The modulated nature of the Ni-Mn-Ga-based alloys is not only of academic interest as the presence of modulation seems to be strongly connected with extreme mobility of twin boundaries and thus magnetically induced reorientation of martensite (resulting in giant magnetic field induced strain).

We recently showed that the modulation within the 10M martensite phase continuously evolves from commensurate to incommensurate [3]. Still, the not-very-common presence of high-intensity high-order satellites (up to the 6th order), Figure 1, remained puzzling.

We continued our previous studies by analysis of the structural modulation based on the comparison of our high-resolution XRD and ND data with simulations employing DISCUS [4]. Our results indicate a strongly anharmonic character of the modulation function and reveal its direct interaction with the twinned microstructure, particularly with the a/b twins.

Figure 1. Representative q-scan in the (110)* direction. High-intensity satellites of up to the 6th order are marked s1..s6. For clarity, satellites belonging to the (400), (620), (2-20), and (840) reflections are marked red, blue, violet, and green, respectively. Peaks marked with grey asterisks originate from the sample holder. Adapted from [3].

 

1. L. Straka, J. Drahokoupil, P. Veřtát, J. Kopeček, M. Zelený, H. Seiner, O. Heczko, Acta Mater., 132, (2017), 335-344.

2. L. Straka, J. Drahokoupil, P. Veřtát, M. Zelený, J. Kopeček, A. Sozinov, O. Heczko, Scientific Reports, 8, (2018), 11943.

3. P. Veřtát, H. Seiner, L. Straka, M. Klicpera, A. Sozinov, O. Fabelo, O. Heczko, J. Phys.: Condens. Matter, 33, (2021), 265404.

4. T. Proffen, R. B. Neder. J. Appl. Crystallogr., 30, (1997), 171–175.

This work was supported by the Czech Science Foundation grant no. 21-06613S.