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Scripta Materialia 65 (2011) 903–906 www.elsevier.com/locate/scriptamat
Work output of the two-way shape memory effect in Ti50.5Ni24.5Pd25 high-temperature shape memory alloy K.C. Atli,a I. Karamana,b,⇑ and R.D. Noebec a
b
Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA Materials Science and Engineering Graduate Program, Texas A&M University, College Station, TX 77843, USA c NASA Glenn Research Center, Structures and Materials Division, Cleveland, OH 44135, USA Received 9 July 2011; revised 5 August 2011; accepted 8 August 2011 Available online 12 August 2011
A significant two-way shape memory effect (TWSME) was demonstrated, for the first time, in a TiNiPd high-temperature shape memory alloy (SMA) with TWSM strains as high as 2.6%. This TWSME was able to perform mechanical work, with a maximum work output of 0.12 J g1, well above the levels obtained from conventional SMAs. Microstructural changes obtained through severe plastic deformation processing did not improve the TWSME stability and resulted in a reduction of strain capability and work output. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: TiNiPd; High-temperature shape memory alloy; Training; Two-way shape memory effect; Work output
TiNiPd high-temperature shape memory alloys (HTSMAs) have attracted considerable interest over the last decade owing to their high transformation temperatures (ranging from 100 to 500 °C), high work output levels, ductility and workability [1–4]. Several applications have been proposed involving the use of these alloys as solid-state actuators in the automotive, aerospace and oil industries [1]. These actuators mostly operate based on the one-way shape memory effect (OWSME) combined with a biasing force to reset the HTSMA after each actuation cycle. Numerous studies have been conducted on the shape memory response of TiNiPd HTSMAs under an applied load [2–4]. However, there has been no investigation of the two-way shape memory effect (TWSME) of TiNiPd alloys. The only characterization of TWSME for any HTSMA system was performed by Meng et al. [5] on TiNiHf and the overall response of the material was poor. The exploitation of the TWSME for actuator applications would be advantageous in terms of design by rendering it possible for the actuator to remember both its lowtemperature and high-temperature shapes without the need for a biasing force.
⇑ Corresponding
author at: Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843, USA. E-mail:
[email protected]
TWSME has been attributed to the oriented internal stress fields of the dislocation arrays or retained martensite generated during thermomechanical training [6,7]. These internal stress fields are able to induce the same variants of martensite, when no external stress is applied, as the ones that are generated by the external training stress, thus causing the TWSME [7–9]. TWSME has previously been considered an unstable effect, which could easily be suppressed by applying an opposing force during transformation, thus having no capability of performing work [10]. However, Stalmans et al. [11] demonstrated that in a well-trained CuZnAl SMA, the TWSME was actually capable of performing 0.025 J g1 of work and 52 MPa was required to suppress it completely. Fukuda et al. [12] investigated the work output of the TWSME in a Ti49Ni51 (at.%) SMA, utilizing the B2 (austenite) to R-phase transformation. The TWSME was induced by aging under stress, which led to the formation of aligned Ni4Ti3 precipitates relative to the applied stress. The transformation took place under the influence of the coherency stress fields of these precipitates, resulting in specific martensitic variant selection under no external stress. The aged specimens could perform around 0.030 J g1 of work under opposing stress levels of 50–100 MPa. The functional stability of TiNiPd HTSMAs during multiple actuation cycles has been studied in some detail [2,4,13]. If the TWSME is to be utilized for actuation, its
1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.08.006
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stability under stress should also be investigated since most of the emerging actuator applications require the HTSMA to do work. Therefore, there is a need to characterize the magnitude and stability of the TWSME in HTSMAs during repeated actuation to develop a baseline for this behavior and begin to investigate methods for its improvement. The key issues addressed by the present study include: (1) whether TiNiPd HTSMAs can demonstrate TWSME despite their high transformation temperatures; (2) how effective training procedures are for obtaining TWSME in HTSMAs; (3) whether the TWSME in TiNiPd HTSMAs, utilizing the B2 (austenite) to B19 (martensite) transformation, can do mechanical work provided that transformation temperatures are much higher than those of conventional SMAs; (4) whether the defects that lead to TWSME can be sustained at high temperatures under opposing stresses, i.e. the level of the stability of TWSME; and (5) whether it is possible to enhance the TWSME by severe plastic deformation (SPD) processing prior to training. TWSME was induced in a Ti50.5Ni24.5Pd25 (at.%) HTSMA through a training procedure consisting of 100 thermal cycles between a lower cycle temperature (LCT) of 100–120 °C and an upper cycle temperature (UCT) of 280 °C under various stress levels. The stability of the TWSME was characterized during both stressfree and load-biased thermal cycling using the same LCT and UCT values. The latter tests were conducted to assess the work output of the TWSME under different opposing stresses. The effects of SPD on the stability and work output of the TWSME were also studied by training a sample previously processed by equal channel angular extrusion (ECAE). The goal was to reveal the effect of the microstructure induced by ECAE, i.e. small grain size and high dislocation density, on the work output of the TWSME. An ingot of Ti50.5Ni24.5Pd25 was prepared by vacuum induction melting, homogenized at 1050 °C for 72 h and furnace cooled. It was then hot-extruded at 900 °C, yielding a rod 12 mm in diameter. A 75 mm long section was cut from the as-extruded rod for subsequent ECAE processing at 425 °C for four passes. Further details of the ECAE processing are described elsewhere [13]. Dogbone-shaped tensile specimens with 8 3 1 mm3 gauge sections were extracted from the as-extruded and ECAE-processed billets using wire electrical
discharge machining. Thermomechanical experiments were conducted using an MTS servohydraulic test frame. An MTS high-temperature extensometer was used to measure the strain. Heating and cooling rates during testing were 10 ± 2 °C min1. Training of the as-extruded and ECAEprocessed samples were performed on a custom-built constant-stress testing frame. Training consisted of 100 thermal cycles at either 80, 150 or 200 MPa. The selection of 100 cycles was due to previous work on this alloy system [2,14], which indicated that at least 40 cycles were required to achieve a reasonably stable response. A capacitive displacement probe (CapacitecÒ HPC-75) was attached to the grips to measure the displacement. Strain was calculated by dividing the change in displacement between the grips to the initial gage length. Figure 1 shows the strain vs. temperature responses of the as-extruded and ECAE-processed samples during the 100 cycle training under 150 MPa. Both materials were thermally cycled to 280 °C to ensure complete reverse transformation while preventing any differences in response that could be attributed to different UCTs. Both materials exhibited a similar evolution of the shape memory response during training. The first cycle was characterized by a relatively high value of irrecoverable strain, eirr. Each subsequent cycle induced fewer defects, until a level of functional stability, with negligible eirr, was achieved at the end of 100 cycles. For the as-extruded material, eirr decreased from 0.31% during the first cycle to 0.01% at the end of 100 cycles, whereas for the ECAE-processed material, the decrease was from 0.10% to 0.01%. Hence, the total eirr accumulated during training was 2.25% and 0.65% for the as-extruded and ECAE-processed Ti50.5Ni24.5Pd25, respectively. These results were expected since it has already been reported that ECAE improves functional stability during repeated thermomechanical cycling as a result of grain refinement and work hardening [13]. However, at the same time, recovered transformation strain, erec, decreases due to a larger barrier against phase front motion. Another result of ECAE processing was the decreased number of training cycles required to reach a certain level of stability (Fig. 1b) compared to the as-extruded condition. However, the final levels of eirr per cycle were quite similar for both materials by the end of the 100 training cycles. Therefore, even though as-extruded Ti50.5Ni24.5Pd25 takes longer to reach a stable condition, both materials ultimately reach the same level
Figure 1. Comparison of strain vs. temperature evolution during training for 100 cycles under 150 MPa for the (a) as-extruded and (b) ECAEprocessed Ti50.5Ni24.5Pd25.
K. C. Atli et al. / Scripta Materialia 65 (2011) 903–906
of dimensional stability, while a much greater level of erec is achieved in the as-extruded material. Following the training cycles, samples were unloaded and heated above the stress-free austenite finish temperMPa . During heating, a relatively large strain ature, Ar¼0 f recovery took place (Fig. 2), which is associated with the reverse transformation of the post-trained oriented martensite. 10 stress-free thermal cycles were subsequently performed to assess the stability of the TWSME that was developed due to training. Both materials exhibited slight degradation in TWSME, as evidenced by a decrease in martensitic (cold) and austenitic (hot) shape strains. In 10 stress-free cycles, the cold-shape strains, measured at the LCT, and the hot-shape strains, measured at the UCT, for the as-extruded material decreased by 0.31% and 0.18%, respectively, while the ECAE-processed material exhibited 0.20% and 0.03% decreases in cold and hot-shape strains, respectively. When these values were normalized to the overall TWSM strain in each material, the cold-shape strain stability was identical, while the hot-shape stability was superior in the ECAE-processed material. The degradation in cold-shape strain is usually associated with the loss of internal stresses during thermal cycling, and thus with the conversion of oriented martensite to selfaccommodated martensite. On the other hand, a decrease in hot-shape strain is attributed to the recovery of retained martensite [15]. It is believed that the ECAE-processed material either developed less retained martensite during training or the retained martensite that was formed did not relax during the stress-free thermal cycling because of the structure of the ECAE-processed material (larger dislocation density and finer grains). The TWSM strain was calculated as the strain difference between the cold- and hot-shape strains for a given stress-free cycle. For the as-extruded material, erec of the first training cycle at 150 MPa was 2.48% and improved slightly by the end of training to 2.60%, while the TSWM strain during the first stress-free cycle was 2.39%. For the ECAE-processed material, erec during the first training cycle was 1.95% and by the last training cycle was 2.26%, while the TWSM strain during the first stress-free cycle was 1.71%. In terms of magnitude, the TWSM strain for the as-extruded material was 92% of erec in the fully trained material but represented only 75% of the erec of the trained ECAE-processed material.
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This indicates that the as-extruded Ti50.5Ni24.5Pd25 reacts very efficiently to the training procedure employed in this study. Conversely, it is much more difficult to train the ECAE-processed material due to its higher initial dislocation content. Thus it appears that the internal stress state due to the high initial dislocation density in the ECAE-processed material does not promote TWSME and actually works against the development of the dislocation structure that is responsible for this phenomenon. The stability of the TWSM strain was determined by comparing the TWSM strain at the first and tenth stressfree cycles. For the as-extruded Ti50.5Ni24.5Pd25, the TWSM strain decreased from 2.39% to 2.27% in 10 cycles, while the decrease was from 1.71% to 1.54% for the ECAE-processed material. For both materials, this represents about a 10% loss in TWSM strain after 10 cycles. One of the most important characteristics of an SMA actuator is the work output, which is the product of erec and applied stress during a thermal cycle. The trained samples exhibited TWSME by contraction and extension during reverse and forward transformations, respectively. If compressive stresses are applied to the samples during forward transformation, work may be done by the TWSME. To quantify the work output of the TWSME, load-biased thermal cycling tests were conducted. Both samples, trained under 150 MPa tensile stress and subsequently subjected to 10 stress-free cycles, were loaded in compression in 25 MPa increments, starting at 0 MPa (Fig. 3a and b). Loading was done at the UCT when the sample was in the austenitic state. As-extruded materials trained under 80 and 200 MPa tensile stresses were also tested to assess the effect of training stress on the work output of the TWSME. At each stress increment, samples were thermally cycled through full transformation while recording the strain vs. temperature response. Loading was done to 75 MPa, at which point the TWSME was almost suppressed for the as-extruded material and completely suppressed for the ECAE-processed material, suggesting the level of internal stress generated during training, at least in the uniaxial direction, was greater in the as-extruded material compared to the ECAE-processed material. Furthermore, the as-extruded material maintained its higher recovered strain level at all stresses compared to the ECAE-processed material, resulting in a larger work output (Fig. 3c).
Figure 2. 10 stress-free thermal cycles for the (a) as-extruded and (b) ECAE-processed Ti50.5Ni24.5Pd25 performed after training for 100 cycles at 150 MPa, demonstrating the magnitude and stability of the TWSME.
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Figure 3. Load-biased thermal cycling test results for the (a) as-extruded plus trained and (b) ECAE-processed and trained Ti50.5Ni24.5Pd25 performed after 10 stress-free thermal cycles. Samples were loaded under different compressive stress levels to assess the work output of the TWSME. (c) A comparison of the TWSME work output values for the as-extruded and ECAE-processed Ti50.5Ni24.5Pd25 trained under different stress levels, as well as the TWSME work output levels for a binary TiNi SMA [12] aged under stress and a trained Cu-based [11] SMA.
Figure 3c is a compilation of work output results obtained from Figure 3a and b, and similar test results on the Ti50.5Ni24.5Pd25 alloy after training at 80 and 200 MPa, along with data from the literature for other systems. If the work output is defined as W TWSME ¼ rOPP eqTWSM , where rOPP is the opposing compressive stress, eTWSM is the TWSM strain obtained under rOPP and q is the density, it is evident that WTWSME increased with increasing training stress for the as-extruded material. WTWSME values as high as 0.12 J g1 could be obtained from the as-extruded Ti50.5Ni24.5Pd25 under the present training conditions. These values are substantially higher compared to the reported TWSME work output values for TiNi [12] and Cu-based [11] SMAs. However, the work output for the TWSME is still less than what can be achieved under a biased OWSME, which is 0.491 and 0.743 J g1 for Ti50.5Ni24.5Pd25 under 150 and 200 MPa bias stresses, respectively [4]. But it is still a significant level, especially when it is considered that the stress needed to block the TWSME in the Ti50.5Ni24.5Pd25 alloy is enough to cause most aluminum alloys to yield. In summary, as-extruded Ti50.5Ni24.5Pd25 HTSMA was subjected to a training procedure consisting of 100 thermal cycles under different stress levels. The resulting TWSME was characterized in terms of its stability during both stress-free and load-biased thermal cycling. We demonstrated for the first time that TiNiPd HTSMAs can exhibit stable TWSME with very high TWSM strains as a result of an efficient training procedure. This TWSME was capable of mechanical work despite the high temperatures of operation. A maximum TWSME work output of 0.12 J g1 was obtained after training under 200 MPa, which was significantly higher than that reported for conventional TiNi and Cu-based SMAs. The effect of ECAE prior to training on the magnitude and stability of the TWSME was also studied. A stable TWSME with small degradations in cold- and hot-shape strains upon stress-free thermal cycling was obtained. Due to the nature of the training and the already induced defect structure and fine grains in the ECAE-processed material, the magnitude of the resulting erec and TWSME work output was far less than that developed in the as-extruded material. The microstruc-
ture induced by the ECAE process, i.e. small grain size and high dislocation density, significantly reduced the magnitude of the TWSME during work actuation, which indicates that a “clean” microstructure, free of any previous dislocation distribution, is essential for the generation of a maximum TWSME with high transformation strain and work output capability. This work was supported by the NASA Fundamental Aeronautics Program, Subsonic Fixed Wing Project through Cooperative Agreement No. NNX07AB56A, and the Supersonics Project. [1] J. Ma, I. Karaman, R.D. Noebe, Int. Mater. Rev. 55 (2010) 257. [2] G. Bigelow, S. Padula, A. Garg, D. Gaydosh, R. Noebe, Metall. Mater. Trans. A 41 (2010) 3065. [3] R.D. Noebe, S. Padula, G. Bigelow, O. Rios, A. Garg, B. Lerch, in: W.D. Armostrong (Ed.), Proceedings of SPIE: Active Materials: Behavior and Mechanics, San Diego, CA, 2006, pp. 617010-1–13. [4] K.C. Atli, I. Karaman, R.D. Noebe, A. Garg, Y. Chumlyakov, I. Kireeva, Metall. Mater. Trans. A 41 (2010) 2485. [5] X.L. Meng, Y.F. Zheng, W. Cai, L.C. Zhao, J. Alloys Compd. 372 (2004) 180. [6] J. Perkins, R.O. Sponholz, Metall. Trans. A 15 (1984) 313. [7] Y. Liu, P.G. McCormick, Acta Metall. Mater. 82 (1990) 1321. [8] L. Contardo, G. Gue´nin, Acta Metall. Mater. 38 (1990) 1267. [9] H. Scherngell, A.C. Kneissl, Scripta Mater. 39 (1998) 205. [10] K.N. Melton, Engineering Aspects of Shape Memory Alloys, Butterworth-Heinemann, Oxford, 1990, pp. 58–68. [11] R. Stalmans, J. Van Humbeeck, L. Delaey, Acta Metall. Mater. 40 (1992) 2921. [12] T. Fukuda, A. Deguchi, T. Kakeshita, T. Saburi, Mater. Trans. JIM 38 (1997) 514. [13] K.C. Atli, I. Karaman, R.D. Noebe, A. Garg, Y. Chumlyakov, I. Kireeva, Acta Mater. 59 (2011) 4747. [14] W. Cai, S. Tanaka, K. Otsuka, Mater. Sci. Forum 327-3 (2000) 279. [15] J.A. Monroe, I. Karaman, D.C. Lagoudas, G. Bigelow, R.D. Noebe, S.A. Padula, Scripta Mater. 65 (2011) 123.
