New approach to twin interfaces of modulated martensite

Abstract

In Ni–Mn–Ga ferromagnetic shape memory alloys, the crystallographic nature of martensitic variant interfaces is one of the key factors governing the variant reorientation through field‐induced interface motion and hence the shape memory performance. So far, the crystal structure studies of these materials – conducted by means of transmission electron microscopy – have suffered from uncertainties in determining the number of unit cells of modulated superstructure, and consequently improper interpretations of orientation correlations of martensitic variants. In this paper a new approach is presented for comprehensive analysis of crystallographic and morphological information of modulated martensite, using automated electron backscatter diffraction. As a first attempt, it has been applied for the unambiguous determination of the orientation relationships of adjacent martensitic variants and their twin interface characters in an incommensurate 7M modulated Ni–Mn–Ga alloy, from which a clear and full‐featured image of the crystallographic nature of constituent twin interfaces is built up. Certainly, this new approach will make it feasible not only to generalize the statistical analysis of martensitic variant distributions for various materials with modulated superstructure, but also to give insight into the crystallographic characteristics of martensitic variant interfaces and the variant reorientation mechanism of new advanced materials for interface engineering.

Determination of the orientation relationship between austenite and incommensurate 7M modulated martensite in Ni–Mn–Ga alloys

“…Fig. 1a displays the powder X-ray diffraction (XRD) pattern of the present alloy taken at room temperature [18], being consistent with that reported by Righi et al [11]. With the information for the atomic coordinates in the incommensurate modulated superstructure described by 10 adjacent subcells along the c-axis [11], the measured XRD profile was fitted using Powder Cell software [20].…”

“…225) with lattice parameter a A = 5.84 Å [19], whereas the martensite has the incommensurate 7M modulated monoclinic crystal structure (P2/m, No. 10) with lattice parameters a 7M = 4.2651 Å , b 7M = 5.5114 Å , c 7M = 42.365 Å , and b = 93.27° [18]. Fig.…”

“…However, owing to the complexity of structural modulation of the martensitic phases, studies of these materials have been confined to the crystal structures [9][10][11], magnetic properties [12,13], magneticfield-induced strains [1,2,14,15] and magneto-mechanical response [16,17]. Recently, the authors' group conducted a thorough analysis of the orientation correlations between adjacent martensitic variants and the crystallographic characteristics of twin interfaces for a Ni 50 Mn 30 Ga 20 (at.%) alloy by means of electron backscatter diffraction (EBSD) in a scanning electron microscope [18], using the superstructure information for the incommensurate 7M modulated martensite [11]. But a complete description of the phase transformation OR to provide theoretical guidance for possible microstructure control during the martensitic transformation and more thorough study on the transformation crystallography is still lacking.…”

A general method to determine twinning elements

“…Notably, the concept of transformation twinning is widely adopted for the elucidation of structural changes during martensitic transformation. Although the formation of twinned martensitic variants is driven by a deformation from the parent phase and may not have any relation to the simple shear deformation defined by the twinning shear, the detwinning process can be well predicted by these elements, especially for the newly developed ferromagnetic shape memory alloys (Gaitzsch et al, 2009;Li et al, 2010). In such a case, the twinned martensitic variants always form regular arrays of alternate lamellae with fixed thickness and the twin boundaries are highly glissile, where the detwinning shear determines the shape memory performance.…”

Directin situmeasurement of coupled magnetostructural evolution in a ferromagnetic shape memory alloy and its theoretical modeling

“…type (following the monoclinic notation used here) and a misorientation angle of ~86° between the easy magnetization axes across the twin boundaries [76][77][78]. Based on quasi-static compression experiments on singlecrystal Ni-Mn-Ga FSMA samples, the critical driving force required to initiate motion of type I twin boundaries was estimated earlier to be ~50-75 kJ/m 3 at low travelling velocities [79].…”