Adaptive Modulations of Martensites

Modulated phases occur in numerous functional materials like giant ferroelectrics and magnetic shape-memory alloys. To understand the origin of these phases, we employ and generalize the concept of adaptive martensite. As a starting point, we investigate the coexistence of austenite, adaptive 14M phase, and tetragonal martensite in Ni-Mn-Ga magnetic shape-memory alloy epitaxial films. We show that the modulated martensite can be constructed from nanotwinned variants of the tetragonal martensite phase. By combining the concept of adaptive martensite with branching of twin variants, we can explain key features of modulated phases from a microscopic view. This includes metastability, the sequence of 6M-10M-14M-NM intermartensitic transitions, and the magnetocrystalline anisotropy.

Figure 1

Comparison of lattice constants for the three different phases. The blue dashed lines mark the calculated lattice constants of the adaptive martensite phase. For each phase, film, and bulk [15] lattice constants are shown.

Figure 2

14M structure constructed by periodic $(5\overline{2}{)}_2$ twinning of tetragonal NM building blocks. One of the NM cells is exemplarily marked in grey. The green lines mark the nanotwin boundaries connecting NM cells. The directions of the three different 14M lattice parameters are sketched with brown color. The angles of the NM unit cells subtended with the 14M supercell (thick lines) are given. The conventional 14M unit cell within the bct reference system is marked in yellow.

Figure 3

Pole figure measurements of the $\{400\}$-planes of the pseudo-orthorhombic 14M [(a)–(c)] and the tetragonal NM martensite [(d)-(e)] from $\psi =0\dots 10°$. The fourfold symmetry verifies epitaxial growth on the MgO(100) substrate. The shift of reflections with respect to the center is due to a misalignment of the sample during measurement.