Abstract
The microstructure and texture evolution of 10M Ni-Mn-Ga melt-spun ribbons were thoroughly evaluated by high-energy synchrotron radiation and electron backscatter diffraction. The as-spun ribbons were subjected to annealing treatment in order to tailor microstructure, atomic order degree, and crystallographic texture. The optimum annealing treatment at 1173 K for 72 h produced a homogenous <100> fiber texture and induced grain growth to the size that spans the entire ribbon thickness. This in turn reduced microstructural constraints for twin variant reorientation in the direction perpendicular to the ribbon surface. On the other hand, a homogenous radial microstructure ensured in-plane stress/strain compatibility giving rise to strain accommodation during variant reorientation. Particular attention was also given to the evaluation of atomic order, which to the largest extent controls the characteristic transformation temperatures. It also lowered the twinning stress to a level sufficiently low for martensitic variant reorientation under magnetic field. As a result, 1.15% magnetic field-induced strain without the aid of mechanical training in the self-accommodated state was achieved.
Highlights
Ni-Mn-Ga Heusler alloys have attracted considerable attention since the discovery of magnetic field-induced strain (MFIS) by Ullakko et al [1]
For the melt-spun ribbons and ribbons heat-treated at 573 and 873 K peaks are visibly broadened and split, which may relate to microstructural inhomogeneity, comprising of equiaxed grains as well as larger elongated slab-like shaped grains, atomic disorder and/or internal stresses since the chemical composition has been verified as uniform (SEM/EDS)
Diffusion processes occur ensuring more structural and microstructural homogeneity which is evidenced by almost sharp differential scanning calorimetry (DSC) peaks for high temperature annealed samples
Summary
Ni-Mn-Ga Heusler alloys have attracted considerable attention since the discovery of magnetic field-induced strain (MFIS) by Ullakko et al [1] This property can be leveraged to design materials such as actuators, sensors, energy harvesting devices, and bio-medical pumps for drug delivery with multi-responsive functions changing their shape and dimensions under the influence of external magnetic fields [1,2,3,4,5,6,7,8,9,10]. The 10M structure is chosen since so far it is characterized by the lowest twinning stress, as it provides the most stable and reliable operation over a large temperature range To attain these goals, the microstructure, crystallographic texture, and degree of order are carefully monitored with the purpose of increasing the twin boundary mobility within particular grains. 1.15% MFIS without the aid of mechanical training in the self-accommodated state is achieved
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