Recoverable stress-induced martensitic transformation in a ferromagnetic CoNiAl alloy H.E. Karaca a , Ibrahim Karaman a, * , D.C. Lagoudas b , H.J. Maier c , Y.I. Chumlyakov d a Department of Mechanical Engineering, Texas A&M University, MS 3123, College Station, TX 77843, USA b Department of Aerospace Engineering, Texas A&M University, College Station, TX 77843, USA c Lehrstuhl f€ ur Werkstoffkunde, University of Paderborn, 33095 Paderborn, Germany d Siberian Physical-Technical Institute, Tomsk 634050, Russia Received 5 June 2003; received in revised form 22 July 2003; accepted 24 July 2003 Abstract The stress-induced martensitic transformation characteristics of a new CoNiAl alloy were investigated under compression. Pseudoelasticity, stages of transformation, and thermal cycling under constant stress were revealed. The present CoNiAl alloy is a candidate material not only for magnetic but also for conventional and high-temperature shape memory alloy applications. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Shape memory alloys; CoNiAl; Martensitic phase transformation; Ferromagnetic materials; Pseudoelasticity 1. Introduction In recent years, ferromagnetic shape memory alloys (FSMAs) have attracted increasing interest since Ullakko et al. [1] observed a large magnetic field induced strain (MFIS) in NiMnGa. Later Murray et al. [2] and Tickle and James [3] reported 6% and 4.3% MFIS respectively. Other FSMAs reported to date are FePd [4,5], FePt [6] and NiFeGa [7,8]. However, these materials are ex- pensive and in most cases too brittle for practical applications. Similar to conventional shape mem- ory alloys, FSMAs undergo martensitic phase transformation with the application of stress or by changing temperature. Main requirements to ob- tain large MFIS [9,10] are: (1) low twin boundary energy [9], (2) high strength matrix (to prevent dislocation slip), and (3) high microcrystalline anisotropy energy [10]. More recently, the potential of Co based alloys such as CoNiGa [11,12] and CoNiAl [12–15] as FSMAs have been revealed. CoNiAl alloys seem promising for FSMA applications because of the possibility for obtaining enough ductility for forming through thermal treatments [16–18], rel- atively cheap constituents and the ability to control transformation temperatures and the Cu- rie temperature ðT c Þ independently over a large composition range [13]. Upon cooling from the * Corresponding author. Tel.: +1-9798623923; fax: +1- 9798622418. E-mail address: ikaraman@mengr.tamu.edu (I. Karaman). 1359-6462/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00470-6 Scripta Materialia 49 (2003) 831–836 www.actamat-journals.com