Synthesis and characterization of Fe–Mn–Si shape memory alloy by mechanical alloying and subsequent sintering

Abstract

An Fe‐Mn‐Si bulk alloy was produced from elemental powders by mechanical alloying (MA) and subsequent sintering. The shape memory effect, microstructure and mechanical property of the bulk alloy were investigated. The α phase transformed into the γ phase during MA. The MA played an essential role in stabilizing the γ phase, which is associated with the shape memory effect in this alloy system. The γ phase with small amounts of ε and α′ martensitic phases formed after subsequent sintering. After deformation, a γ→ε stress-induced martensitic phase transformation occurred. Shape recovery was observed after subsequent heating, associated with an ε→γ reverse martensitic transformation. The grain size of the bulk alloy was about 2–3 μm, and the yield strength was about 500 MPa. These results show that powder metallurgy, a combination of MA and subsequent sintering, has the potential to produce Fe–Mn–Si shape memory alloy.

Introduction

Fe–Mn–Si alloys are a prominent system with a characteristic shape memory effect (SME). The SME of the Fe–Mn–Si alloy system has been undergone continued development since its discovery by Sato et al. in 1982 [1], [2], [3]. Because the alloying elements (Fe, Mn and Si) are relatively inexpensive, this system has been expected to be a promising alternative to expensive Ti–Ni-based shape memory alloys. While the SME of the Ti–Ni-based alloys occurs due to a thermo-elastic martensitic phase transformation, the effect in the Fe–Mn–Si alloys originates from a stress-induced γ (fcc)→ε (hcp) martensitic transformation, followed by an ε→γ reverse transformation during heating. The stress-induced martensitic transformation occurs due to the motion of Shockley partial dislocations in the fcc structure; the Burgers vector a/6[11$\overline{2}$]_FCC glides between two (111) layers of the fcc lattice. The γ phase forms and is stabilized at room temperature by the substitution of Mn and Si atoms in the α (bcc) phase lattice (elemental α-Fe). The stress-induced martensitic transformation dominates, overwhelming slip deformation within 3–4% deformation of the Fe–Mn–Si shape memory alloy [3]. The composition and additional alloying elements have been optimized in order to maximize SME for applications. Basic composition was established as Fe–(28–33)Mn–6Si (wt%) after the discovery of the SME in this alloy system [3], [4]. The addition of Cr (Fe–28Mn–6Si–5Cr) produced a stronger SME [5]. Furthermore, a combination of Cr and Ni (Fe–16Mn–5Si–12Cr–5Ni) led to better corrosion resistance, while still exhibiting the SME [5]. It has also been found that the SME can be further improved by the formation of fine precipitates of NbC [6] or VN [7]. In the meantime, a variety of industrial applications has been considered. Recently, construction components such as pipe joints and rail couplings have been discussed as potential applications of the Fe–Mn–Si-based shape memory alloys [8]. In general the Fe–Mn–Si-based shape memory alloy is produced by melting and subsequent casting under high vacuum (or in air) followed by thermo-mechanical processing [1], [2], [3], [4], [5], [6], [7].

Powder metallurgy offers several advantages for manufacturing industrial products. It enables the production of alloys in near net shape, minimizing the additional machining required to form the final product, compared with casting techniques. Therefore, it is worth exploring the alternative production route for the Fe–Mn–Si shape memory alloys. Mechanical alloying (MA) is a prominent synthetic technique involving solid state reaction among powders due to high energy collisions [9]. This offers many advantages, such as reduced grain size, the formation of intermetallic compounds, the ability to synthesize alloys from elements with different melting points and the capability of producing nano-sized structures [9]. These characteristic advantages can lead to excellent mechanical properties as well as other unique properties. MA of the Fe–Mn–Si alloy system has been discussed from the viewpoint of alloying kinetics [10] and thermal stability [11], resulting in the formation of a stabilized γ phase in the alloyed powder. We reported [12] the synthesis of Fe–Mn–Si alloy by MA and subsequent sintering, and discussed repeatability of the MA. However, further characterization and analysis of the SME and mechanical properties have not yet been reported. A better knowledge of this alternative production route could offer a broader range of applications for the Fe–Mn–Si shape memory alloy. The aim of this study is to attempt to produce Fe–Mn–Si shape memory alloy by MA and sintering from elemental powders, and to investigate the SME, microstructure and mechanical properties of the resulting alloy.