Two-way shape memory effect of Ni_50.1Mn_24.1Ga_20.3 Fe_5.5 shape memory alloy polycrystalline fiber

Abstract

Polycrystalline Ni-Mn-Ga-Fe fibers were prepared by melt-extraction technique. The internal stress and defects introduced by rapid solidification were modified by step-wise chemical ordering annealing. The microstructure and phase microstructure of fibers were characterized using Field Emission Scanning Electron Microscope (FESEM) and Transmission Electron Microscopy (TEM) and XRD diffraction technique. The phase transition behavior and two-way shape memory effect (TWSME) were tested using Dynamic Mechanical Analyzer (DMA). The results show that after heat treatment, the chemical ordering degree is increased, and the martensite twin boundaries are straight. Complete recoverable strains of parent and martensite phases during thermal cycles are achieved. During the two-way shape memory cycles, two characteristics of thermoelastic martensite transition are displayed in heat-treated fibers: reversibility and thermal hysteresis. In the thermal cycle test, the total strain reaches 1.32% when the fiber is loaded to 198 MPa. The thermomechanical tensile measurement shows that the phase transition temperature follows the Clausius-Clapeyron equation. Compared with other alloys such as Ti-Ni and Cu-Al-Ni, Fe-doped fiber exhibits a smaller strain-stress dependence, which is beneficial to the constant strain output.

Keywords

• ferro-magnetic shape memory fiber
• martensite transformation
• shape memory effect
• ordered heat treatment