DOI : 10.1051/jp4 : 2003904 Improved shape memory properties and internal structures in Fe-Mn-Si-based alloys containing Nb and C A. Baruj, T. Kikuchi, S. Kajiwara and N. Shinya Nationallnstitute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan Abstract. It bas recently been found that the shape memory properties of Fe-Mn-Si-based alloys are notably improved by addition of small amounts of Nb and C elements. In these newly modifies alloys, the so-called " training process " is replaced by a simpler thermomechanical treatment. For example, in an Fe-28Mn-6Si-5Cr- 0.5NbC alloy (mass %), a combination of pre-rolling at 870 K followed by 10 min ageing at 1070 K results in a shape recovery of 90% for 4% initial strain and a shape recovery stress of 295 MPa for 4.5% initial strain. In this work, we present the shape memory properties of the samples pre-rolled up to 200ha in Fe-28Mn-6Si-5Cr-0. 5NbC, and atomic force microscopy (AFM) and transmission electron microscopy (TEM) observations of the stress- induced martensite. Detailed discussion is made on the relations between the microstructures observed by AFM and TEM and the shape recovery and shape recovery stress, and the main factors responsible for drastic improvement of the shape memory properties in this alloy system are drawn. 1. INTRODUCTION The industrial use of Fe-Mn-Si-based shape memory alloys has been scarce due to the necessity of performing the so-called " training treatment " in order to obtain a good shape memory effect (SME) [1-3]. The training treatment consists of several cycles in which martensite is stress-induced at room temperature and subsequently reversed by heating the material above the reverse transformation temperature (Af). We have recently found a way to avoid the training treatment and still attain good shape memory properties in Fe-Mn-Si-based alloys containing Nb and C elements [4-7]. This improvement of SME results from the combination of two processes. The first one is the production of a high density of stacking faults by pre-rolling the fcc matrix at a high temperature where no appreciable amount of martensite is induced. The second process is the formation of fine coherent NbC precipitates in the fcc matrix by ageing. These precipitates interact with the stacking faults [7] producing a uniform distribution of nucleation sites for the hcp martensite that is essential for obtaining a good SME [2]. In this way, 90% of an initial 4% extension was recovered in pre-rolled samples of an Fe-28Mn-6Si-5Cr-0. 53Nb-0. 06C (mass %) alloy without performing any training. In addition, shape recovery stresses of 255 MPa and 295 MPa were obtained for 6% and 14% pre-rolled samples, respective ly [6, 7]. In the present paper, we extend the study on the effect of pre-rolling on SME in the Fe-Mn-Si- based alloys containing Nb and C in two ways. First, by studying the effect of increasing the amount of pre-rolling up to 20% on the shape memory properties of the alloy, secondly, by performing atomic force microscopy (AFM) and transmission electron microscopy (TEM) observations of the stress-induced martensite. 2. EXPERIMENTAL An Fe-28Mn-6Si-5Cr-0. 53Nb-0. 06C (mass %) alloy was prepared by induction melting. The details of the alloy preparation have been given elsewhere [5]. The alloy was first solution treated at 1470 K for 10 h. After the solution treatment, square-pillar shaped samples with about 15 mm thickness were rolled at 870 K by 6-20%. Extension test samples of 0. 7 mm thickness and 1-4 mm width with the gauge length of 15 mm were spark eut from the rolled material. These pre-rolled samples were subjected to ageing at 1070 K for 10 min in order to produce NbC precipitates. The alloy subjected to this treatment is hereafter