Challenging martensites in Ni-Mn-Ga-based alloys: Can minor changes in structural modulation affect physical properties?

P. Veřtát^1,2, L. Straka², M. Klicpera³, O. Sozinov⁴, O. Heczko²

¹ Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, 120 00 Praha 2, Czech Republic.

² FZU - Institute of Physics of the Czech Academy of Sciences, 182 21 Praha 8, Czech Republic.

 ³ Faculty of Mathematics and Physics, Charles University, 121 16 Praha 2, Czech Republic.

⁴ Material Physics Laboratory, LUT University, 538 50 Lappeenranta, Finland.

vertat@fzu.cz

The structure of martensites of Ni-Mn-Ga-based magnetic shape memory (MSM) alloys represents a unique crystallography challenge. Not only do these alloys possess complex and practically inevitable twined microstructure (for some compositions even close to merohedral twins), but they also feature a structural modulation that evolves with temperature. Some of our new single-crystal studies even indicate that we might be dealing with twinned composite crystals, i.e., there is more than one ordering. To make things even more complicated, SEM and HRTEM indicate the presence of nanotwinning (at least close to the a/c twin boundaries) [1-3]. Despite the challenging nature of our martensites, all the said phenomena seem to be precursors for the extremely high mobility of the twin boundaries (very unusual property itself) which are essential for the magnetically induced reorientation of the martensite (MIR).

Aiming to solve one of the most difficult topics in the field – the commensurate or incommensurate question, we focused on the evolution of the 10M modulated martensite with temperature. We used a simple modulation approach to describe the detected changes in modulation satellites. We found a universal rule that seems to be valid for a wider range of Ni-Mn-Ga-based alloys. Our studies indicate a commensurate 10M modulated structure in the vicinity of the austenite phase. Upon cooling, the modulation evolves into incommensurate and continues to evolve until it attains yet another commensurate metastable state followed by the intermartensitic transformation to 14M modulated martensite [4]. Observed changes in modulation exhibit a thermal hysteresis, Fig. 1. Interestingly, these minor changes in modulation can be corelated with the changes of physical properties, such as electrical resistivity, Fig. 2, and elastic moduli.

Figure 1. Evolution of the magnitude q of the modulation vector q with temperature within 10M modulated martensite of the Ni₅₀Mn₂₇Ga₂₂Fe₁ alloy. Data from the conventional XRD and two ND experiments in ILL Grenoble (D9 and D10 instruments). Transformation temperatures are represented by vertical lines and phases are marked: A – austenite, 10M – five-layered modulated martensite, 14M – seven-layered modulated martensite. The commensurate metastable states are schematically marked by red vertical lines. Adapted from [4].

Figure 2. Deviation from linear dependency of electrical resistivity with temperature within 10M modulated martensite of the Ni₅₀Mn₂₇Ga₂₂Fe₁ alloy. Measured resistivity curve within the studied region of the 10M martensite phase is presented in the inset [4].

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

2. P. Veřtát, L. Straka, J. Drahokoupil, O. Heczko, Acta Physica Polonica A, 134, (2018), 859-862.

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

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

This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS22/183/OHK4/3T/14, and by Czech OP VVV project MATFUN—CZ.02.1.01/0.0/0.0/15_003/0000487. We acknowledge the Institut Laue-Langevin and the project LTT20014 financed by the Ministry of Education, Youth and Sports, Czech Republic, for the provision of neutron radiation facilities and kindly thank O. Fabelo for assistance with the ND measurements.