Physical Characterization of Sintered NiMnGa Ferromagnetic Shape Memory Alloy.

Francesca Villa,Adelaide Nespoli,Carlo Fanciulli,Elena Villa,Francesca Passaretti

doi:10.3390/ma13214806

Abstract

The present work focused on the microstructural, thermal, electrical, and damping characterization of NiMnGa samples produced through a powder pressing and a sintering process; the effect of sintering times and of the starting powder size were evaluated. Moreover, an observation of the evolution of martensitic transformation typical of NiMnGa ferromagnetic shape memory alloy was conducted in comparison with the cast material behavior and in correlation with the material densification. The optimum powder size and sintering time for the process, i.e., 50 µm or lower and 72 h, were identified considering the investigated physical properties of the sintered samples in comparison to the cast material. The corresponding sample showed the best compromise between density, thermal and electrical properties, and damping and functional behaviour. In general, the outcomes of this study could be the basis of a useful tool for production processes that include a sintering step as well as being a starting point for the evaluation of an alternative low cost fabrication method of this alloy.

Highlights

NiMnGa alloys are functional materials that perform a coupling between magnetic and structural order and this feature leads to a large magnetic-field-induced strain (MFIS), up to 10% in single crystals, caused by the rearrangement of the twin boundaries induced by an applied magnetic field [1]
NiMnGa samples were prepared through pressing and sintering processes starting from milled powder with different granulometry
The studied process was demonstrated to be a valid simple and low cost route to obtain samples with interesting functional properties that could be suitable for optimization and for the development of applications for small scales devices

Summary

Introduction

NiMnGa alloys are functional materials that perform a coupling between magnetic and structural order and this feature leads to a large magnetic-field-induced strain (MFIS), up to 10% in single crystals, caused by the rearrangement of the twin boundaries induced by an applied magnetic field [1].In addition, materials with specific compositions could exhibit magnetic superelasticity or shape memory effect and magnetocaloric effect, when exposed to a magnetic field. NiMnGa alloys are functional materials that perform a coupling between magnetic and structural order and this feature leads to a large magnetic-field-induced strain (MFIS), up to 10% in single crystals, caused by the rearrangement of the twin boundaries induced by an applied magnetic field [1]. Ternary Ni50 Mn25 Ga25 alloy and off-stoichiometric compounds have attracted increasing interest in several application fields including sensor and actuator development, due to their functional properties which are strictly correlated to crystal structure and phase transformation [1]. The modulation of the grain structure through shape and size in NiMnGa alloys has attracted increasing interest in order to induce an improvement of their functional properties, in particular the magnetic field induction strain (MFIS). The first scientific focus for this development was the preparation of powders of the alloys and the study of the relation between the size, the structural properties, and the martensitic transformation [3]

Methods

Results

Conclusion