Combinational rate effects on the performance of nano-grained pseudoelastic Nitinols Abbas Amini a,n , Chun Cheng b , Alireza Asgari c a Institute for Frontier Materials, Deakin University, Waurn Ponds, VIC 3217, Australia b Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA c School of Engineering, Deakin University, Waurn Ponds, VIC 3217, Australia article info Article history: Received 2 April 2013 Accepted 16 April 2013 Available online 2 May 2013 Keywords: Shape memory Phase transitions Metals and alloys Latent heat transfer abstract Combinational loading-unloading rate effects were studied on the behavior of NiTi shape memory alloys (SMAs) under nanoindentation loads. While combinational loading rates showed negligible effects on the performance of NiTi SMAs, the combinational unloading rates did show significant effects on hysteresis energy. The heating-cooling phenomenon during the loading stage and the sole cooling during the unloading stage explain the effects. This study elucidates the nature of thermomechanical SMAs' behaviors during complex compressive loadings with the presence of solid-state phase transition. & 2013 Elsevier B.V. All rights reserved. 1. Introduction In NiTi SMAs the parent austenite phase and the martensite phase revert to each other during loading-unloading stages with the release or absorption of latent heat [1–5]. Due to the creation of a temperature gradient from a rate variation, the loading and unloading rates show significant effects on the performance of thermomechanical SMAs [6]. In this paper, a series of repeatable nanoindentation tests on NiTi SMAs is conducted to examine the effects of the combinational loading-unloading rates on the material's behavior along with their underlying physics. 2. Experimental procedure A NiTi SMA was used for this study with an austenite finish temperature of 19 1C measured using a differential scanning calorimeter (TA-Q1000). The alloy was kept in 100 1C for one hour and slowly cooled to room temperature to retain the austenite phase. The martensite phase was induced by the application of compressive stress with a superelastic behavior. The nominal alloy composition as determined by a high resolution X-ray diffraction spectroscopy (PW1825, Philips) was 56.4% Ni and 43.6% Ti by weight. The grain size was in the range of 50–100 nm as observed by a transmission electron microscopy (200 kV: JEM-2100F). The sheet was cut into pieces (5 mm  5 mm) and polished using a series of silicon carbide and diamond papers until the average surface roughness was less than 6 nm measured using a 3D surface profiler (SPM: NT3300). The nanoindentation tests were con- ducted at room temperature (23 1C) in a quasi-static loading mode using Hysitron TI 900 Triboindenter with a spherical 5.23 μm- radius diamond tip. 3. Results and discussions Fig. 1a represents some of the indentation curves of the super- elastic NiTi SMA with combinational loading rate set ups. In these experiments, a maximum indentation load of 4000 mN and an unloading rate 400 mNs -1 were constant for all tests. The loading rates are specified for four distinct loading sections as 0–1000, 1000–2000, 2000–3000 and 3000–4000 mN. 21 different loading rate set ups were examined for these loading sections using the four loading rates of 100, 200, 300 and 400 mNs -1 . For example, 1342 has a loading rate set up with the order of 100, 300, 400 and 200 mN/sec for the above four loading regions respectively. In these set ups, the unloading rate of 400 mNs -1 is equal to the maximum loading rate used in the loading stage. Except for a small decline in the slope in the partitions of loading curves with the lower rate, the trend of the indentation curves in Fig. 1a do not show significant differences in the total. Fig. 1b represents the hysteresis energy for the above 21 setups. With a total mild increase, the lowest hysteresis area belongs to the loading rate set up with a minimum start loading rate value (100 mNs -1 ). When in a combinational set up this initial loading rate increases, the dissipated energy also increases ∼0.005 nanoJoules. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.04.072 n Corresponding author. Tel.: +61 3 52479122. E-mail addresses: abbas.amini@deakin.edu.au, abbasust@gmail.com (A. Amini). Materials Letters 105 (2013) 98–101