Influence of the twin microstructure on the mechanical properties in magnetic shape memory alloys

Abstract

The microstructure evolution, i.e. reorientation of martensite variants, is an important deformation mechanism in shape-memory alloys. This microstructure evolution occurs by the motion of twin boundaries and the nucleation and annihilation of twins in the hierarchical microstructure. An appropriate discrete disclination model for the description of the internal elastic fields and microstructure evolution is introduced for representative volume elements. The model is applied to an experimentally characterized microstructure, i.e. conjugation boundary, and the predicted mechanical response is verified by comparison to experimental measurements. The influence of the twin microstructure on the homogenized stress-strain curve is studied. It is found that regular twinned microstructures have a low strain energy and a high resistance against deformation. These simulations also reason the origin of the microstructural stability of conjugation boundaries.Commonly, the non-modulated structure (NM), the 10-layered modulated structure (10M) and the 14-layered modulated structure (14M) are observed in the martensitic structures of NiMnGa alloys [5]. The maximum MFIS is 10% in case of a 14M structure and 6% in case of a 10M structure. 12% MFIS was reported by Sozinov et al. in 2013 for NM Ni-Mn-Ga-Co-Cu [6]. However, the stress induced strain of the NM structure is the highest (≈ 20%) [7] compared to the other structures.The maximum MFIS is realized in case of an initial single-variant state. Magneto-thermomechanical training biases the twin microstructure [8], leading to a predominant twin variant; whereas very effective training leads to the formation of a single-variant crystal [9]. Untrained, self-accommodated martensite is composed of a hierarchically twinned microstructure, where higherorder, smaller twins form within lower-order twins [4,10]. It has been observed experimentally that samples that underwent effective training and that initially show large MFIS tend to fail by mechanical

GD3: generalized discrete defect dynamics

“…the X 1 axis. This configuration is very similar to that obtained in 2D simulations of twins in shape memory alloys (Reinholz et al 2016), except that it is more symmetric here. The initial in plane internal shear stress field is shown in the Fig.…”

“…In general, ID can adopt the form of dislocations (Vattré et al 2014b), disclinations Reinholz et al 2016;Rösner et al 2011) and disconnections (Hirth and Pond 1996;Pond 2006, 2016). Disconnections, consisting of a dislocation and a step (Hirth and Pond 1996), were experimentally reported in several serrated interfaces.…”

“…Further, while some have used disclinations as discrete static objects, the detailed crystallography of disclination lines has not yet been postulated. Thus far, networks of discrete wedge disclination lines were used to predict twin evolution in NiMnGa alloys (Reinholz et al 2016).However a general and 3D consideration of disclination motion was not proposed. This question includes that of the natural habit planes of disclinations as well as of their out-of-plane motion.…”

Features of a nano-twist phase in the nanolayered Ti3AlC2 MAX phase

“…As the rotational counterpart of dislocations, disclinations are line defects which introduce a discontinuity of the elastic rotation field, referred to as the Frank vector. The concept of disclinations is appropriate for the description of elastic fields of tilt boundaries with high misorientations [45] and for nanotwin microstructures [46,47]. Regarding the cookie shape NTP considered in this work, the stress field due to the network of overlapping dislocation arrays can be equivalently and more simply described by using two disclination loops delimiting the NTP.…”

Orientation relationships between nanotwins inside type II microtwins in Ni-Mn-Ga alloy

“…Cong, Zuo and their co-workers reported that the ORs between nanotwins across the interfaces of variant pairs generally deviated by a few degrees from either a perfect twin relationship or an identical orientation [21][22][23]. With respect to specific variant pairs, Muntifering et al investigated the A-D type [24,25], and Ouyang et al focused on the A-C type [26,27]. Although the detailed crystallography of the martensitic variant pairs A-D and A-C are quite different, generally the ORs between the variants in both types can be expressed as rotations around a common lattice direction.…”