Stability of nanotwinned martensitic structures in Ni-Mn-Ga magnetic shape memory alloys
DIPC Seminars
- Speaker
-
Martin Zeleny, Brno University of Technology, Czech Republic
- When
-
2018/11/30
12:00 - Place
- DIPC, floor 1
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Much attention has been paid to the Ni-Mn-Ga magnetic shape memory alloys
because they exhibit interesting properties such as a giant magnetic field-
induced strain (MFIS) [1]. The MFIS is related to the high mobility of twin
boundaries in connection with a large magneto- crystalline anisotropy [2]. It
occurs below the martensitic transformation temperature at which a high-
temperature cubic phase with L21 structure, austenite, transforms to a phase
with lower symmetry, martensite. Several types of martensites have been
observed in the Ni Mn-Ga system [3]. The modulated five-layered (10M) or
seven-layered (14M) phases exhibit MFIS up to 10%. Giant MFIS has never been
reported for the third martensitic phase which has a purely tetragonal lattice
without modulation (NM) [2]. However, recent electronic structure calculations
predict orthorhombic structure with four-layered modulation (4O) as the ground
state at 0 K [4] but so far there is no experimental report.
The martensitic transformation to low-symmetry structures is associated with
characteristic features in the electronic DOS [5]. Using first-principles
calculations combined with the generalized solid state nudged elastic band
method [6] we determined the minimum energy path leading to the different
modulated phases of martensite (4O, 10M, 14M) and to NM phase. In
stoichiometric Ni2MnGa alloy there is no energy barrier on the path to the 10M
phase. Geometry of the lattice in the initial part of the path confirms that
this transformation is driven by a softening of the TA2 phonon branch [5]
corresponding to the shift of (110) planes. In later part of the path
individual NM nanotwins are stabilized by band Jahn-Teller effect.
Transformation paths to other structures including NM martensite exhibit more
or less significant barriers in the beginning, hindering such a transformation
from austenite although these structures exhibits lower total energy. This
finding corresponds to experiment and demonstrates that the kinetics of the
transformation is decisive for the selection of the particular low-symmetry
structure of martensite [7]. On the other hand, in off-stoichiometric alloys
the barrier-less transformation was found between austenite and NM martensite
corresponding to tetragonal distortion of the lattice due to Jahn-Teller
effect.
[1] Ullakko, K. et al., Appl. Phys. Lett. 69, 1966 (1996).
[2] Söderberg, O. et al., Handb. Magn. Mater. 16, 1 (2006).
[3] Niemann, R. & Fähler, S. , J. Alloy. Compd. 703, 280 (2017).
[4] Zelený, M. et al., Phys. Rev. B 94, 224108 (2016).
[5] Zayak, A. T. et al. ,Phys. Rev. B 72, 054113 (2005).
[6] Sheppard, D. et al., Chem. Phys. 136, 074103 (2012).
[7] Zelený, M. et al., Sci. Rep. 8, 7275 (2018).
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