The four-step multiple stage transformation in deformed and annealed Ti 49 Ni 51 shape memory alloy P.C. Su, S.K. Wu * Department of Materials Science and Engineering, National Taiwan University, 1, Roosevelt Rd., Sec. 4, Taipei 106, Taiwan Received 7 August 2003; received in revised form 29 October 2003; accepted 30 October 2003 Abstract A four-step multiple stage transformation is observed in 20% deformed and 500 °C annealed Ti 49 Ni 51 shape memory alloy. Two extra B2 ! B19 0 transformation peaks appear before the previously described B2 ! R and R ! B19 0 peaks while cooling, and these correspond to one new peak, which appears after the original B19 0 ! B2 peak during heating. These two extra peaks are caused by the combined effect of severe cold-working and long-time annealing on Ti 49 Ni 51 alloy, and they come separately from the B2 ! B19 0 transformation occurring in regions with low and high dislocation densities, which are originally suppressed by cold-working. Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Cold working; Annealing; Differential scanning calorimetry; Shape memory alloys; Martensitic transformation 1. Introduction The transformation sequence of TiNi shape memory alloys (SMAs) has been widely investigated in the past decades. In the equiatomic TiNi SMA, the martensitic transformation occurs in a single stage from high tem- perature parent B2 phase to low temperature B19 0 monoclinic martensite. After certain thermomechanical treatments, or addition of a third element, the trans- formation sequence of TiNi SMAs can change into a two-stage B2 to premartensite R-phase to B19 0 during cooling [1]. In addition, aged Ni-rich TiNi alloys with scattered Ti 3 Ni 4 precipitates in the matrix can exhibit either B2 $ R $ B19 0 transformation sequence with two distinct steps during both cooling and heating runs, or a B2 ! R ! B19 0 and B19 0 ! B2 transformation sequence with two distinct steps during cooling and only one step during heating [2]. In the past decade, the martensitic transformation was found to be able to appear in more than two distinct steps and is said to be a multi-stage martensitic trans- formation (MST). This MST occurring in TiNi alloys has been widely observed using differential scanning calorimetry (DSC), although the cause of this behavior is still controversial. Todoroki and Tamura [3] first ex- plained the effect of cold working on the transformation sequence of TiNi SMAs. Lo et al. [4] also found a two- stage transformation in as-quenched Ti 40 Ni 50 Cu 10 with B2 ! B19 and B19 ! B19 0 transformations, where B19 is an orthorhombic martensite. Bataillard et al. [5,6] attributed the cause of MST to the stress fields formed around the coherent interfaces between B2 matrix and Ti 3 Ni 4 precipitates, whereas Morawiec et al. [7–9] ex- plained MST to be due to the changes of dislocation configuration by low-temperature annealing. In a recent study, Khalil-Allafi et al. [10] found two distinct steps of B2 ! R and R ! B19 0 , and an additional step with B2 ! B19 0 in the DSC cooling curves of annealed Ni- rich TiNi alloys. They attributed the cause of MST to the Ni-concentration inhomogeneity between Ti 3 Ni 4 precipitates and the difference in nucleation barriers for R-phase and B19 0 formations. However, they soon corrected these proposed reasons with the argument that the heterogeneous microstructure between regions with and without Ti 3 Ni 4 precipitates is responsible for the MST behaviors [11]. Recently, Chrobak et al. [12] stated that the MST in the early stage of annealed Ti 49:3 Ni 50:7 with 10% cold-working had two transformation peaks * Corresponding author. Tel.: +886-2-2363-7846; fax: +886-2-2363- 4562. E-mail address: skw@ccms.ntu.edu.tw (S.K. Wu). Acta Materialia 52 (2004) 1117–1122 www.actamat-journals.com 1359-6454/$30.00 Ó 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2003.10.044