Microstructure and Thermomechanical Characterization of Fe-28Mn-6Si-5Cr Shape Memory Alloy

Antonio Collazo,Raúl Figueroa,Carmen Mariño-Martínez,Carmen Pérez

doi:10.3390/met11040649

Abstract

Iron-based shape memory alloys (SMAs) have been widely studied during the last years, producing new formulations with potential applications in civil engineering. In the present paper, the microstructure and the thermomechanical behavior of the Fe-28Mn-6Si-5Cr memory alloy has been investigated. At room temperature, the presence of ε-martensite and γ-austenite was confirmed using optical and electron microscopy techniques. The martensitic transformation temperatures (As, Af, Ms, and Mf) were determined by differential scanning calorimetry, together with an X-ray diffraction technique. The use of these techniques also confirmed that this transformation is not totally reversible, depending on the strain degree and the number of thermal cycles. From the kinetics study of the ε → γ transformation, the isoconversion curves (transformation degree versus time) were built, which provided the information required to optimize the thermal activation cycle. Tensile tests were performed to characterize the mechanical properties of the studied alloy. These kinds of tests were also performed to assess the shape memory effect, getting a recovery stress of 140 MPa, after a 7.6% pre-strain and a thermal activation up to 160 °C.

Highlights

Shape memory alloys (SMAs) are a variety of smart materials that can recover their original shape through temperature variation, after the material has been deformed beyond its elastic limit [1]
The appropriate thermomechanical processing depends on the alloy composition, the recovery strain can be improved by up to around 5% [3,19,20,21,22]. These results suggest that, for specific Fe-SMA, the optimum treatments have to be researched, in order to acquire the desired shape memory effect (SME) and mechanical properties
The microstructural characterization corroborated the presence of the two phases, εmartensite and γ-austenite at room temperature

Summary

Introduction

Shape memory alloys (SMAs) are a variety of smart materials that can recover their original shape through temperature variation, after the material has been deformed beyond its elastic limit [1]. The ironbased shape memory alloys emerged as an alternative to the Ti-Ni alloys due to their low cost, easy manufacturing process and better mechanical properties (higher stiffness and strength). They are specially suitable for applications that require high shape memory stress, i.e., constrained recovery applications; typical examples are pipe joins, rail couplings or pre-stressing reinforcing elements in civil engineering [2,3,4]. The SME in Fe-Mn-Si alloy systems was discovered by Sato et al in 1982 [5]; different alloying elements were subsequently added to improve this SME, making the systems more suitable for structural applications [6,7,8] Nowadays, research in this field is growing, in order to obtain smart materials which are more competitive for civil engineering purposes

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