Short Duration Heat Treatments before Aging Increase Mechanical Hysteresis of Pseudoelastic NiTi Alloy

Jacopo Romanò,Fabio Lazzari,Simone Pittaccio,Lorenzo Garavaglia

doi:10.1007/s11665-022-06634-6

Abstract

High temperature (550 °C) and short duration (10, 15, 20 s) heat treatments were performed on pseudoelastic NiTi alloy in addition to the conventional aging process in order to increase the mechanical hysteresis of the material. Pre-treated and aged samples were compared with plain aged ones through mechanical testing, powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The introduction of the pre-treatment, in combination with the aging process, was shown to increase the mechanical hysteresis of the aged samples up to ~30% after stabilization. XRD analysis showed how the introduction of the pre-treatment introduces a recovery of the residual stresses in the material microstructure, as well as the nucleation of different sets of metastable precipitates (such as TiNi3). We thus observe that the combination of pre-treatment and aging affects the alloys microstructure, causing a series of effects that synergise in increasing the material hysteresis.

Highlights

Nickel Titanium (NiTi) alloy is a smart metallic material capable, like other shape memory alloys (SMAs), of undergoing a solid-state transformation between an austenitic and a martensitic phase
This hysteresis area is a major source of the damping capacity of NiTi, which allows it to find successful application in several fields where vibrations and dynamic loads are a matter of concern, such as civil engineering (Ref 3), Jacopo Romanoand Fabio Lazzari, National Research Council of Italy - Institute of Condensed Matter Chemistry and Technologies for Energy (CNR-ICMATE), Via Previati 1/E, 23900 Lecco, Italy; and Politecnico di Milano - CMIC, Piazza Leonardo da Vinci 32, 20133
We present the Results of our analysis, and, in the Discussion, we propose an explanation for the effects of the proposed pre-treatment on the microstructural phenomena during ageing, and how these in turn affect the hysteresis of NiTi

Summary

Introduction

Nickel Titanium (NiTi) alloy is a smart metallic material capable, like other shape memory alloys (SMAs), of undergoing a solid-state transformation between an austenitic and a martensitic phase. If the applied stress is released, the material returns to its initial state, without any residual deformation, the whole process is not completely reversible thermodynamically, as some energy is lost between the forward and reverse transformation, as highlighted by the difference of the stress levels between the loading and unloading plateaux. This hysteresis area is a major source of the damping capacity of NiTi, which allows it to find successful application in several fields where vibrations and dynamic loads are a matter of concern, such as civil engineering (Ref 3), aerospace (Ref 4) or automotive industry (Ref 5). In all of these applications, the tuning and optimization of the pseudoelastic properties of NiTi are a crucial issue

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