Modulated martensite: why it forms and why it deforms easily

Abstract

Diffusionless phase transitions are at the core of the multifunctionality of (magnetic) shape memory alloys, ferroelectrics and multiferroics. Giant strain effects under external fields are obtained in low symmetric modulated martensitic phases. We outline the origin of modulated phases, their connection with tetragonal martensite and consequences for their functional properties by analysing the martensitic microstructure of epitaxial Ni-Mn-Ga films from the atomic to macroscale.Geometrical constraints at an austenite-martensite phase boundary act down to the atomic scale. Hence a martensitic microstructure of nanotwinned tetragonal martensite can form. Coarsening of twin variants can reduce twin boundary energy, a process we could observe from the atomic to the millimetre scale. Coarsening is a fractal process, proceeding in discrete steps by doubling twin periodicity. The collective defect energy results in a substantial hysteresis, which allows retaining modulated martensite as a metastable phase at room temperature.In this metastable state elastic energy is released by the formation of a 'twins within twins' microstructure which can be observed from the nanometre to millimetre scale. This hierarchical twinning results in mesoscopic twin boundaries. Our analysis indicates that mesoscopic boundaries are broad and diffuse, in contrast to the common atomically sharp twin boundaries of tetragonal martensite. We suggest that observed extraordinarily high mobility of such mesoscopic twin boundaries originates from their diffuse nature which renders pinning by atomistic point defects ineffective.

Collective Modes and Structural Modulation in Ni-Mn-Ga(Co) Martensite Thin Films Probed by Femtosecond Spectroscopy and Scanning Tunneling Microscopy

“…We show that the stacking sequence recorded by STM, which is close to the structure proposed by the adaptive martensite scenario [17,18], can be consistently explained by the higher-order coupling between the electronic order and the lattice. Strong support for this scenario is provided by the data obtained on films lightly doped with cobalt.…”

“…However, for the case of the 14M martensite, it was argued that the modulated phase can be constructed from nonmodulated martensitic unit cells [17,18]. This adaptive martensite scenario [17][18][19] recently received considerable attention.To determine which theory gives a more appropriate physical picture of the phase transition in Ni-Mn-Ga, we applied femtosecond time-resolved spectroscopy combined with scanning tunneling microscopy (STM) on a series of undoped and Co-doped Ni-Mn-Ga films. Alloying with cobalt results in a shift of T M far above room temperature and is thus of vital importance for technological applications [20].…”

“…In fact, the electronic band structure studies [11,15,16] revealed the possible Fermi surface nesting conditions for all of the observed structural modulations. However, for the case of the 14M martensite, it was argued that the modulated phase can be constructed from nonmodulated martensitic unit cells [17,18]. This adaptive martensite scenario [17][18][19] recently received considerable attention.…”

“…However, for the case of the 14M martensite, it was argued that the modulated phase can be constructed from nonmodulated martensitic unit cells [17,18]. This adaptive martensite scenario [17][18][19] recently received considerable attention.…”

Ab initiostudy of Ni2MnGa under shear deformation

“…It alternatively interprets the modulated martensites as a periodically nanotwinned tetragonal NM phase with a nanotwin period corresponding to the modulation period [17]. Thus the modulated structures can be described by an alternating sequence of nanotwins with a width of three (101) lattice planes in one orientation and width of two planes in the other for 10M [18] structure or by alternating nanotwins of width five and two (101) lattice planes for 14M structure [19,20]. The alternating nanotwins result in monoclinic symmetry of both structures, which are denoted in literature as (3-2) 2 and (5-2) 2 , respectively.…”