Coexisting Multiple Martensites in Ni57−xMn21+xGa22 Ferromagnetic Shape Memory Alloys: Crystal Structure and Phase Transition

Lian Huang,Daoyong Cong,Yandong Wang,Mingguang Wang

doi:10.3390/met11101534

Abstract

A comprehensive study of the crystal structure and phase transition as a function of temperature and composition in Ni57−xMn21+xGa22 (x = 0, 2, 4, 5.5, 7, 8) (at. %) magnetic shape memory alloys was performed by a temperature-dependent synchrotron X-ray diffraction technique and transmission electron microscopy. A phase diagram of this Ni57−xMn21+xGa22 alloy system was constructed. The transition between coexisting multiple martensites with monoclinic and tetragonal structures during cooling was observed in the Ni51.5Mn26.5Ga22 (x = 5.5) alloy, and it was found that 5M + 7M multiple martensites coexist from 300 K to 160 K and that 5M + 7M + NM multiple martensites coexist between 150 K and 100 K. The magnetic-field-induced transformation from 7M martensite to NM martensite at 140 K where 5M + 7M + NM multiple martensites coexist before applying the magnetic field was observed by in situ neutron diffraction experiments. The present study is instructive for understanding the phase transition between coexisting multiple martensites under external fields and may shed light on the design of novel functional properties based on such phase transitions.

Highlights

NiMn-based Ni-(Co)-Mn-Z (Z = Ga, In, Sn, Sb) Heusler-type alloys have fascinating multifunctional properties, such as magnetic-field-induced strain (MFIS) [1,2], superelasticity [3,4], the magnetocaloric effect [5,6], the magnetoresistance effect [7] and magnetothermal conductivity [8]
It is commonly recognized that the large MFIS in Ni-Mn-Ga alloys originates from the rearrangement of martensite variants (RMV) via the motion of twin boundaries under external fields; that is, one variant grows at the expense of others under an applied magnetic field [1]
Compared with RMV, this mechanism can bring about a reversible phase transformation, but the required magnetic field for this is usually larger than 5 T; this finding was confirmed by our group using a high energy X-ray diffraction technique [13], 4.0/)

Summary

Introduction

By optimizing the composition or heat treatment, two or more coexisting martensites can be obtained due to their close formation energy; applying a small external field (e.g., temperature, stress, or a magnetic field) can result in transformations between these coexisting martensites in such physically parallel ferro-elastic systems, which may yield a high output of functional properties (such as a giant magnetocaloric effect, large magnetoresistance and large MFIS) with a very low field [14]. An in situ neutron diffraction technique was employed to study the structural evolution of coexisting multiple martensites under a magnetic field at 140 K These results will be instructive for designing novel magnetic shape memory alloys with low-field-triggered multifunctional properties

Experimental

Results and Discussion

High-energy patterns experimentally

Magnetic-Field-Triggered Transition between Coexisting Multiple Martensites

Conclusions

Ga22the coexisting