Enhanced shape recovery in cryogenically treated martensitic Ti–Ni alloys Arijit Sinha a,n , Bholanath Mondal b , Bikas C. Maji c , Partha Protim Chattopadhyay d a School of Materials Science and Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, India b Department of Central Scientific Services, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India c Materials Science Division, Bhabha Atomic Research Centre, Mumbai 400085, India d Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, India article info Article history: Received 16 March 2013 Received in revised form 30 April 2013 Accepted 3 May 2013 Available online 30 May 2013 Keywords: Shape memory alloys Cryogenic quenching Martensitic transformations Thermomechanical processing Aging Shape memory abstract The present investigation concerns the effect of cryogenic treatment on mechanical properties and shape recovery behavior of the Ti-rich Ti–Ni alloy. The sample quenched in liquid nitrogen (CQ) has resulted in higher volume fraction of martensite (B19′) phases and exhibited higher amount of recoverable strain (∼10%) in the region of iso-stress tensile deformation in comparison to the samples quenched in ice water. Cold rolling of the sample at near liquid nitrogen temperature (CR), with and without subsequent aging treatment, has resulted in the bimodal distribution of martensite plate size and negligible amount of recoverable strain under iso-stress tensile deformation. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Among the commercially available shape memory alloys (SMAs), equiatomic Ti–Ni alloy finds its use in the wide range of functional applications, owing to its attractive mechanical proper- ties and higher recoverable strain [1–6]. In Ti-rich composition the microstructural evolution under reversible transformation cycle is critically sensitive to the composition and thermo-mechanical treatment schedule [7–15]. Phase diagram of Ti–Ni alloy exhibits formation of the ordered phase with B2 structure at near equia- tomic composition. However, in the Ti-rich side, the alloy is susceptible to precipitation of Ti 2 Ni phase under favorable condi- tion [16–20]. The precipitation alters the local composition of the matrix and hence, influences the shape recovery behavior [21,22]. The effect of deformation strain is also crucial in respect of microstructural evolution. Part of the deformation energy may result in the non-uniform rise in temperature of the alloy resulting in the transformation of some amount of B19' to B2 [22]. At the same time, some amount of B2 phase present in the matrix may undergo deformation induced transformation to B19' phase [23,24]. Plastic deformation also stabilizes martensite in the matrix against the reversible transformation on the withdrawal of the load [25–31]. Recent studies have reported the enhancement of recovery strain for one-way and two-way shape memory effect, under B2- R-B19' transformation condition, depending on the room tem- perature microstructure [32,33]. More recently, exceptionally high recovery strain (both one-way and two-way) has been achieved in the equiatomic and Ni-rich (Ti–50.7 at% Ni) alloys with varying microstructures obtained by low-temperature thermomechanical treatment followed by annealing at suitable temperature [34]. The present authors have also reported similar enhancement of shape recovery in the case of cryogenically quenched Ti-rich alloys. The present study aims to correlate the microstructural evolu- tion with the corresponding mechanical properties and shape recovery behavior of the martensitic Ti–Ni alloy quenched and cold rolled at cryogenic temperature, followed by aging treatment. 2. Experimental procedures Cold-drawn and tempered Ti–(∼49 at%)Ni alloy (Ф 0.988 in.) obtained from the Special Metals Corporation (New Hartford, NY) was used for this investigation. The samples were homogenized at 900 1C for a duration of 60 min followed by quenching in ice water and in liquid nitrogen medium (LN 2 ). The characteristics transfor- mation temperatures were determined by differential scanning Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A 0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.05.036 n Corresponding author. Tel.: +91 33 2668 4561; fax: +91 33 2668 2691. E-mail address: arijitsinha2@yahoo.co.in (A. Sinha). Materials Science & Engineering A 580 (2013) 273–278