The {110}-nanotwinned phase in the vicinity of the martensitic transformation in Ni-Mn-Ga

P. Veřtát1,2, L. Straka2, J. Drahokoupil1,2, O. Heczko2

1Department of Solid State Engineering, FNSPE CTU in Prague, Trojanova 13, 120 00, Prague 2, Czech Republic

2Department of Advanced Structural Materials, Institute of Physics of the Czech Academy of Sciences, Na Slovance 2, 182 21 Praha 8, Czech Republic

vertapet@fjfi.cvut.cz

 

Martensite of the Ni-Mn-Ga magnetic shape memory Heusler alloys exhibits large magnetoelastic response and it is, therefore, of current interest for possible applications in actuator and magnetocaloric devices. The primary condition for the magnetic shape memory effect is a phase transformation from the high temperature and high symmetry cubic austenite to a low symmetry martensite phase. We focus on the non-stoichiometric compositions of the Ni-Mn-Ga resulting in modulated 10M martensitic structure.

Apart from the well-known martensitic transformation observable e.g. as a steep change in the AC magnetic susceptibility, electric resistivity and dilatation curves [1], we recently discovered a non-standard behaviour of the temperature dependence of electric resistivity, few tenths of kelvin below the transformation to austenite (Fig. 1). Similar effect on the evolution of the lattice parameters was observed by Richterová [2] making the initial set of measurements.

Figure 1. Electric resistivity as the function of temperature. Linear evolution during heating in martensite followed by irregular behaviour before the steep jump to austenite at TA.

 

We measured the temperature evolution of the lattice parameters of single-crystalline samples with very fine temperature step, using the PANalytical X’Pert PRO diffractometer and Peltier element for heating [3]. The divergent geometry and the twinned microstructure allow us to observe the 400 and 040 reflections simultaneously in one relatively quick (~ 20 s) scan with reasonable resolution (Fig. 2). Very slow heating (0.8 K/min or lower) allows us to observe this dynamic effect in detail.

In the temperature region analogous to the irregular region in the resistivity measurement, two reflections 400 and 040 of the original 10M martensite merged to one peak of the new phase marked as 10M’. This structural change prior transformation to austenite corresponds to changes in resistivity shown in Fig. 1. The heating was stopped just below the TA and immediately followed by cooling resulting in observable hysteresis of the occurrence of the 10M’ phase, as seen in Fig. 2.

Figure 2. 400 and 040 reflections as a function of temperature (background level corresponds to temperature on the right axis) upon heating (a) and cooling (b). [4]

 

Measured data were fitted in custom-made program based on MS Excel and VBA for applications, using solver.xlam add-in for the least squares method implementation. Four parameters of the pseudo-Voigt function used for each profile fitting were obtained – intensity at maximum, 2θ position, relative width and shape parameter [5]. Thermal evolution of the calculated lattice parameters a and b of the 10M phase and the a’ parameter of the 10M’ phase and corresponding maximal intensities of the diffraction lines during heating are shown in Fig. 3-4.

 

Figure 3. Development of the lattice parameters a and b of the 10M and a’ of the 10M’ structure during heating. Dominant phase is marked with more saturated colour.
Figure 4. Intensities of the diffraction lines during heating corresponding to Fig. 3.

 

Although the 10M’ phase seems to be present in the whole temperature interval, the intensity of the corresponding reflection before transition to 10M’ is approximately two orders lower than the intensity of the reflections corresponding to the dominant 10M phase. Therefore, the lattice parameter a’ in lower temperature region can only be estimated with major errors and is depicted just to complete the overall image. As the lattice parameter of austenite is far from depicted region (aa = 0.584 nm at T = 332 K), the 10M’ phase cannot be ascribed to austenite.

Upon cooling, the 10M’ phase remains stable in a broader temperature interval, then it transforms back to 10M structure at ~ 318 K. The reciprocal space mapping showed no significant difference between the 10M and 10M’ – in particular, the same satellite reflections were present in the maps.

After the theoretical calculations of the diffraction lines and detailed scanning electron microscopy (SEM) observations, the 10M’ was ascribed to be the {110}-nanotwinned form of the 10M monoclinic phase originating from the low energy of the a/b twin boundaries (see ref. [4] for details).

 

1. P. Veřtát, J. Drahokoupil, O. Perevertov, O. Heczko, Phase Transitions, 89, (2016), pp. 752-760.

2. K. Richterová, J. Drahokoupil, O. Heczko, Materials Structure, 20, (2013), pp. 108-110.

3. J. Drahokoupil, Materials Structure, 22, (2015), pp. 164-165.

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

5. R. Kužel, Materials Structure, 10, (2003), pp. 18-19.

 

This work was supported by the Grant Agency of the Czech Technical University in Prague, grant No. SGS16/245/OHK4/3T/14.