Phase dependence of the Thermal Memory Effect in Polycrystalline Ribbon and Bulk Ni55Fe19Ga26 Heusler Alloys

TL;DR

This study investigates how the thermal memory effect in NiFeGa shape memory alloys depends on the microstructure, particularly the presence of the 14M modulated phase, and how it varies between ribbon and bulk forms.

Contribution

It reveals the phase dependence of the thermal memory effect and demonstrates the influence of microstructure and macrostructure on its manifestation in NiFeGa alloys.

Findings

Thermal memory effect is prominent in ribbon samples with 14M phase.

Degradation of the modulated structure reduces the TME.

High-resolution calorimetry is needed to detect weak TME signals.

Abstract

The thermal memory effect, TME, has been studied in Ni55Fe19Ga26 shape memory alloys, fabricated as ribbons via melt-spinning and as pellets via arc-melting, to evaluate its dependence on the martensitic structure and the macrostructure of the samples. When the reverse martensitic transformation is interrupted, a kinetic delay in the subsequent complete transformation is only evident in the ribbon samples, where the 14M modulated structure is the dominant phase. In contrast, degradation of the modulated structure or the presence of the gamma-phase significantly reduces the observed TME. In such cases, the magnitude of the TME approaches the detection limits of commercial calorimeters, and only high-resolution calorimeter at very low heating rate (40 mK h-1) can show the effect. Following the kinetic arrest and subsequent cooling, the reverse martensitic transformation was completed at several heating rates to confirm the athermal nature of the phenomenon.