DISLOCATION MECHANISM OF TWINNING IN Ni–Mn–Ga

Abstract

Tensile tests were performed in situ in a transmission electron microscope to investigate the twinning mechanism in non-modulated NiÀMnÀGa martensite. The reorientation of the twin variants occurs via twinning dislocations. Their generation and movement were followed; the glide plane and Burgers vector were verified. Individual twinning dislocations were visualized.NiÀMnÀGa is an interesting and widely investigated shape memory alloy showing remarkable properties, such as magnetoresistance, 1 magnetocaloric behavior, 2 thermally induced shape memory effect, 3 superelasticity 4 and, particularly, magnetic-field-induced-strain. 5 In alloys that transform upon cooling into modulated martensites, the martensitic twin structure is coupled with magnetic domains in such a way that the magnetic field can reorient martensitic twin variants, which results in very large strains. 6 The magnetic-fieldinduced-strain (magneto-plasticity) in NiÀMnÀGa is directly related to the reorientation of one martensite variant to another twin variant. 5 The mobility of the twin boundaries plays a decisive role in these processes. A microscopic model of the reversible motion of the twin boundary in the ferromagnetic shape memory alloys was proposed by Müllner et al.,7,8 with their concept being based on the movement of twinning dislocations. 9 Although the magnetoplasticity in NiÀMnÀGa has been described and discussed thoroughly in the literature, detailed investigations of the twin variant reorientation by transmission electron microscopy (TEM) are very rare. 10À12 The in situ TEM straining method used in the present study is highly suitable for this purpose as it enables the direct observation of the stress-induced processes. The experiments were intended to be carried out on modulated as well as nonmodulated martensites. However, they could be successfully performed only on the non-modulated samples. The results were partly published in Refs. 10 and 11. The samples of 5M martensite were too brittle for tensile tests inside the electron microscope, and only preliminary results were obtained for 7M martensite. The non-modulated tetragonal NiÀMnÀGa martensite exhibits superelasticity and shape memory effect while the magnetic-field-induced-strain was reported only 0.17%. 13 Nevertheless, we believe that an understanding of the stress induced twin variant reorientation in the non-modulated martensite will provide insight into the magnetically induced reorientation processes in the modulated martensites as well.The experiments were performed on a Ni 52:4 Mn 27:3 Ga 20:3 alloy, with M s ¼ 398 K, A f ¼ 408 K, T c ¼ 380:2 K. The structure of the alloy at room temperature was tetragonal martensite, a ¼ 0:549 nm, c ¼ 0:661 nm. The samples, 1:67 mm  5:5 mm  0:07 mm, were prepared from a plate with [001] normal in the austenite state, and thinned in the central part in a double jet polisher (30% solution of HNO 3 in methanol, À40 ○ C) to form a hole. The samples were strained in tension at room temperature in a JEM 1200EX microscope equipped wit...

Ab initiostudy of Ni2MnGa under shear deformation

“…However, the motion of twin boundary is not so simple because the activation energy of the shear induced coherent motion process is not of the same order of magnitude as the magnetocrystalline anisotropy energy which was shown by quasistatic DFT simulations [33,34]. The mechanism includes also twinning dislocations slip along the twinning interfaces, which was confirmed by an experimental transmission electron microscopy measurement [35][36][37]17] and also by DFT calculations in combination with the PeierlsNabarro model [32].…”

Transition from many domain to single domain martensite morphology in small-scale shape memory alloys

“…At the nanometer size scale, also, SMAs have been studied in situ using transmission electron microscopy (TEM), for example to investigate the suppression of phase transformation indicated by ex situ mechanical measurements [36]. The technique has also been employed to study the morphological evolution of martensite plates as they nucleate and grow and how this evolution is affected by dislocations [37,38] grain boundaries [39] and precipitates [40][41][42]. Lastly, in situ studies have been used to study interfaces [43], determining selection rules for stress-induced martensite phases and variants [44][45][46][47] and observing dislocation substructure evolution during cycling [48][49][50][51].…”

Non-Modulated Martensite Microstructure With Internal Nanotwins In Ni-Mn-Ga Alloys

“…Most importantly, the martensite plates in the lamellar structure are not twin related [14]. A consequence of this finding is that detwinning modes, responsible as has been lately shown for deformation of such a martensite structure [15,16], cannot change one martensite plate into another. Thus, deformation detwinning must take place within individual plates at a smaller length scale.…”

“…Recent in-situ straining of NM Ni-Mn-Ga shows that the deformation process in Ni-Mn-Ga alloys is governed by detwinning at the nanoscale level changing one region of nanotwins into another [14][15][16]. The full crystallographic description of deformation detwinning in Ni-Mn-Ga single crystals performed by means of the correspondence matrix method can be found in [20].…”