**1. Introduction**

Iron-based shape memory alloy (Fe-SMA, especially referring to Fe-Mn-Si class shape memory alloy) possesses shape memory effect (SME) [1,2], outstanding low-cycle fatigue (LCF) resistance [3], and some other desirable characteristics by which the material has proven its potential in the field of the construction industry. Fe-SMA was traditionally regarded as an ideal material used in fasteners and tie systems, e.g., pipe joints, rail couplings, and crane rail joint plates, where constrained stress is required [4]. The main purpose of using such materials is to simplify the construction process by its SME property which induces prestress conveniently. The so-called SME-induced prestress, also known as 'recovery stress', is associated with its unique deformation-induced martensitic transformation and subsequent heating–cooling process, where an approximate stress of 200–400 MPa can be generated.

Apart from its desirable prestressing capability, Fe-SMA also has excellent low-cycle fatigue resistance, which was first recognized by Sawaguchi in 2006 [3]. A series of studies have been carried out on this front and considerable achievements have been made. The engineering community has gained particular confidence with the completion of the 196-m skyscraper 'JP Tower Nagoya', where Fe-SMA seismic dampers were first employed in an

**Citation:** Zhang, Z.-X.; Zhang, J.; Wu, H.; Ji, Y.; Kumar, D.D. Iron-Based Shape Memory Alloys in Construction: Research, Applications and Opportunities. *Materials* **2022**, *15*, 1723. https://doi.org/10.3390/ ma15051723

Academic Editors: Francesco Iacoviello and Eunsoo Choi

Received: 20 December 2021 Accepted: 21 January 2022 Published: 25 February 2022

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actual project in seismic-prone cities. These works further expand the application boundary of Fe-SMA and inspire the interests of seismic engineers.

Fe-SMA has some extra benefits. It is reported that the corrosion-resistance of Fe-SMA is close to that of stainless steel due to the addition of Nickel and Chromium elements [5]. This makes Fe-SMA well suited to chloride environments, e.g., coastal/offshore engineering construction. In addition, in contrast to Nitinol (another popular class of SMA) which is less easily produced in large scale because of the demanding metallurgical process [6–8], Fe-SMA can be mass produced with conventional metallurgical equipment [9], and, more encouragingly, the cost of the raw materials is inherently low [10]. This facilitates practical use of Fe-SMA in the civil engineering sector, where the necessary size of elements/members is often large and the budget is often controlled.

This paper seeks to summarize the recent technological advances in the research and application of Fe-SMA in construction, covering diverse aspects including structural retrofitting and seismic damping. The paper is also keen to highlight the authors' unique reflections on the present issue and future needs in this field, in addition to the latest solutions. An overview of the basic mechanical properties of Fe-SMA is first presented, followed by the possible application scenarios emerging over the past 10 years. Challenges arising from the application of the new material are described, and where further studies are emphasized that are required to respond to the identified issues. Research needs and new application opportunities of Fe-SMA are also presented. While this paper is of scientific interest to the mechanical and material science community, much emphasis is placed on making the contents easier to learn by civil engineers. Therefore, in most cases, the presentation is structured following the civil engineering custom and terminology.
