Materials Science and Engineering A 378 (2004) 333–336 Improvement of shape memory properties of NbC containing Fe–Mn–Si based shape memory alloys by simple thermomechanical treatments A. Baruj * , T. Kikuchi, S. Kajiwara, N. Shinya National Institute for Materials Science, 1-2-1 Sengen, 305-0047 Tsukuba, Japan Received 30 June 2003; received in revised form 14 October 2003 Abstract A systematic study of effect of pre-deformation before aging on shape memory properties in Fe–Mn–Si based alloys containing Nb and C has been performed and it is found that the room temperature (RT) deformation as well as deformation at high temperature in the austenitic range can greatly improve shape memory effect (SME). The results on two representative NbC containing alloys, i.e., Fe–28Mn–6Si–5Cr–0.53Nb–0.06C and Fe–15Mn–5Si–9Cr–5Ni–0.53Nb–0.06C (mass%) are as follows: (1) about 90% shape recovery for 4% initial strain and 250–320 MPa shape recovery stress are obtained for pre-rolling by 10–30% at 870 K and (2) in the case of pre-rolling at room temperature, 75% shape recovery for 4% initial strain is obtained with 10% rolled samples and 240–320 MPa shape recovery stress is attained for 10–30% rolling. The improved shape recoveries are fairly close to those of the trained conventional shape memory alloys (SMAs) without NbC and their shape recovery stresses are nearly twice those of the trained ones. © 2004 Elsevier B.V. All rights reserved. Keywords: Shape memory effect; Iron-based shape memory alloy; Thermomechanical treatment; NbC precipitate; Pre-deformation 1. Introduction It is well known that low cost Fe–Mn–Si based shape memory alloys (SMAs) have a serious short coming, i.e., only a poor shape memory effect (SME) of about 50% recovery is obtained unless the special thermomechanical treatment called “training” is applied. This “training” treat- ment, which can improve SME to nearly 100%, consists of several cycles of deformation by stress-induced transforma- tion (fcc → hcp) and its reversion (hcp → fcc) by heating [1]. This kind of the treatment not only raises the production cost of SMA for practical application, but also seriously limits the applicable fields of industry. This is the main rea- son why Fe–Mn–Si based SMAs have not been employed in the industry, beyond some experimental applications, over the past two decades despite their good workablity, machin- ablity, and weldability. Very recently, however, Kajiwara et al. [2,3] have made a breakthrough to this problem; that * Corresponding author. Present address: Institute for Materials, Ruhr-University Bochum, 44780 Bochum, Germany. Tel.: +49-234-322-5933; fax: +49-234-321-4235. E-mail address: alberto.baruj@ruhr-uni-bochum.de (A. Baruj). is, they found that addition of small amount of Nb and C to the conventional Fe–Mn–Si based SMAs makes substantial improvement of SME by producing small NbC precipitates by aging. They later reported that further enhancement of SME can be achieved by pre-rolling of austenite before ag- ing treatment [4–6]. The key point of such improvement is that finely distributed small NbC particles with large elastic strains produce very thin martensite plates with a single vari- ant, which is an indispensable condition for good SME in this kind of alloys [7,8]. In the present work, we have made a fur- ther systematic study of effect of pre-rolling on SME in Fe– Mn–Si based SMAs containing Nb and C, using two alloys, i.e., Fe–28Mn–6Si–5Cr–0.53Nb–0.06C and Fe–15Mn– 5Si–9Cr–5Ni–0.53Nb–0.06C (mass%). The corresponding conventional SMAs without Nb and C, which served as ref- erence alloys, are two typical alloys of this type; namely, the former one, Fe–28Mn–6Si–5Cr, exhibits a relatively good SME, but a poorer corrosion resistance, while the latter, Fe– 15Mn–5Si–9Cr–5Ni, shows a better corrosion resistance, but a poorer SME. Much emphasis is placed on studying the effect of pre-rolling at room temperature (RT), because such a process is evidently much easier for engineering ap- plications than the pre-deformation at elevated temperature. 0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.357