Transformation twinning of Ni–Mn–Ga characterized with temperature-controlled atomic force microscopy

Abstract

The magnetomechanical properties of ferromagnetic shape memory alloy Ni-Mn-Ga single crystals depend strongly on the twin microstructure, which can be modified through thermomagnetomechanical training. Atomic force microscopy ͑AFM͒ and magnetic force microscopy ͑MFM͒ were used to characterize the evolution of twin microstructures during thermomechanical training of a Ni-Mn-Ga single crystal. Experiments were performed in the martensite phase at 25°C and in the austenite phase at 55°C. Two distinct twinning surface reliefs were observed at room temperature. At elevated temperature ͑55°C͒, the surface relief of one twinning mode disappeared while the other relief remained unchanged. When cooled back to 25°C, the twin surface relief recovered. The relief persisting at elevated temperature specifies the positions of twin boundaries that were present when the sample was polished prior to surface characterization. AFM and MFM following thermomechanical treatment provide a nondestructive method to identify the crystallographic orientation of each twin and of each twin boundary plane. Temperature dependent AFM and MFM experiments reveal the twinning history thereby establishing the technique as a unique predictive tool for revealing the path of the martensitic and reverse transformations of magnetic shape memory alloys.

Localized deformation in Ni-Mn-Ga single crystals

“…AFM and MFM are a powerful combination, as together they can reveal changes in the twin structure and magnetic axis orientation at the nanoscale in response to applied thermomechanical stresses. 15,16,18,23 MFM is particularly useful and instructive in the case of FSMAs such as Ni-Mn-Ga, as it provides the ability to readily identify twins and the orientation of the easy magnetization axis (c-axis) at the nanoscale, which is not possible in the case of non-ferromagnetic shape memory alloys such as NiTi (nitinol).…”

A switch-like magnetoresistance of ferromagnetic Ni–Mn–Ga ribbon during martensitic transformation

“…A negative-positive-negative switch-like behavior of MR with temperature is observed in ferromagnetic shape memory alloy Ni 53 Mn 23.5 Ga 23.5 ribbons [5,9,11]. Inherent lattice instability is the primary reason for the ability of ferromagnetic Ni-Mn-Ga alloys to exhibit a very large MFIS, and it is also the reason for the 6 negative-positive-negative switch-like behavior of MR with MT temperature [9,24,25].…”

Working Ni–Mn–Ga Single Crystals in a Magnetic Field Against a Spring Load

“…The blue Type II twins grew as the compression on the sample increased. The diagonal relief pattern indicates historic twins [15] which are unrelated to the currently active twins. The direction of the magnetic field H and the compression/expansion axis r is shown Shap.…”

“…Mem. Superelasticity historic twins and features in the micrograph that were unrelated to the active twins [15]. Specific work for one series of tests using a spring pair with a spring constant of 102 N/mm is shown in Fig.…”