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Diffusionless phase transformation characteristics of Mn75.7Pt24.3
Journal of Alloys and Compounds 589 (2014) 412–415
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Diffusionless phase transformation characteristics of Mn75.7Pt24.3 H.E. Karaca a,⇑, S.M. Saghaian a, H. Tobe a, E. Acar a, B. Basaran a,b, M. Nagasako c, R. Kainuma c, R.D. Noebe d a
Department of Mechanical Engineering, University of Kentucky, Lexington KY 40506, USA University of Turkish Aeronautical Association, Etimesgut, Ankara, Turkey c Department of Material Science, Graduate School of Engineering, Tohoku University, Sendai, Japan d Structures and Materials Division, NASA Glenn Research Center, Cleveland, OH 44135, USA b
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Article history: Received 30 October 2013 Received in revised form 22 November 2013 Accepted 24 November 2013 Available online 3 December 2013 Keywords: MnPt Phase transformations Damping X-ray diffraction
a b s t r a c t Phase transformation, damping, and magnetic properties of a Mn75.7Pt24.3 (at.%) alloy were characterized. It was observed that Mn75.7Pt24.3 exhibits a stable phase transformation in the temperature range of 180– 200 °C with a small temperature hysteresis and maximum transformation strain of 0.5%. The crystal structures and lattice parameters of the transforming phases were determined, where both the high and low temperature phases have a face-centered cubic structure but with different lattice parameters. Finally, it was revealed that the alloy possesses high damping capacity (average Tan Delta of 0.16) during phase transformation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As reversible solid-state actuators, shape memory alloys (SMAs) have received considerable interest due to their ability to recover large shape changes under high loads. NiTi-based, Fe-based and Cu-based alloys have been acknowledged as the most common SMAs due to their attractive physical and mechanical properties [1]. The potential and current application areas for such alloys include electronic devices, medical tools, and home appliances [2]. Although most SMAs are limited to temperatures below 100 °C [3], many scientists have become interested in the development of SMAs that can operate at temperatures above 100 °C for potential service in the aerospace, automotive, and oil and gas industries [3– 5]. The main demand for high temperature SMAs (HTSMAs) stems from an interest in utilizing them as solid state actuators. Among all of the NiTi-based HTSMAs, NiTiHf seems to be the most promising candidate for a wide range of applications in the 100–250 °C temperature range due to its low cost and satisfactory shape memory properties [5]. Although the shape memory properties of NiTiHf can be improved with aging [6–8] and strongly dependent on the crystal orientation/texture [9] and addition of quaternary elements [10,11], there is still large demand for solid state actuators with very low hysteresis and stable behavior at high temperatures. Low thermal hysteresis is an important parameter for many practical applications of solid state actuators and sensors [12]. Thermal ⇑ Corresponding author. E-mail address: [email protected] (H.E. Karaca). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.174
hysteresis can be described as the dissipated energy due to reversible phase transformations, which negatively impacts the efficiency of the device. It should be noted that the temperature hysteresis is usually much greater than 20 °C in most of the conventional and high temperature SMAs [13]. In contrast to the most common shape memory alloy systems, c-Mn based alloys are relatively promising high temperature systems with very low thermal hysteresis. Mn–Cu alloys have less than 1 °C thermal hysteresis upon thermo-deformation with shear stresses reaching 160 MPa [14]. Hence, these alloys might provide several advantages for applications that require low hysteresis upon temperature cycling. In addition to the MnCu system, Mn-rich Mn–Pt alloys have been investigated primarily with regard to their magnetic properties [15–18]. According to magnetic susceptibility and neutron diffraction studies [17], Krén et al. have found a first order antiferromagnetic-antiferromagnetic (AF–AF) transformation in the Mn3Pt phase below the Néel temperature, TN. The magnetic structures below and above the AF–AF transition temperature Tt are triangular (D-phase) and collinear (F-phase), respectively. They have also reported the composition dependence of the transition temperatures TN and Tt. Both TN and Tt increase and the difference between them decreases with increasing Mn content of the alloys, resulting in the D-phase directly transforming to the paramagnetic phase at TN during heating, i.e., F-phase cannot be seen. The cubic Cu3Au-type ordered structure does not change at TN and Tt. However, they revealed a discontinuous change of the lattice parameter (obtained by XRD measurements) during the transformation between D-phase and the high-temperature phase (F-phase or
H.E. Karaca et al. / Journal of Alloys and Compounds 589 (2014) 412–415
paramagnetic phase), where the lattice parameter of the D-phase is smaller than that of the high-temperature phase. To the authors’ knowledge, the mechanical properties of binary Mn–Pt alloys have not been previously reported. Consequently in this study, the crystal structures of the transforming phases, phase transformation properties under compressive stress and damping capacity of a Mn75.7Pt24.3 (at.%) alloy were characterized. The composition was selected due to its high phase transformation temperatures (above 100 °C) and low thermal hysteresis [15] that make the alloy suitable for high temperature applications.
2. Experimental procedures An ingot of Mn75.7Pt24.3 (at.%) was produced by arc melting. The ingot was flipped and re-melted three times to better assure homogeneity. The ingot was then homogenized at 1000 °C for 24 h followed by furnace cooling. Compression samples (4 4 8 mm3) were cut using an electro discharge machine. A Perkin–Elmer PYRIS 1 Differential Scanning Calorimeter (DSC) was used to determine the transformation temperatures (TTs) using a heating/cooling rate of 10 °C/min. The damping behavior of the alloys was investigated by using a Perkin–Elmer Dynamic Mechanical Analyzer (DMA-7e) with a temperature rate of 5 °C/min. The configuration used was three point bending and temperature was measured by a thermocouple, which was placed near the sample but was not in contact with its surface. The measuring frequency was 1 Hz and the static and dynamic forces were 5000 mN and 1000 mN, respectively. X-ray diffraction (XRD) was carried out on a Bruker AXS D8 diffractometer using Cu Ka radiation. The magnetic measurements were made with a vibrating sample magnetometer (VSM) in a heating/cooling rate of 1 °C/min under 1.5 T magnetic field. The isobaric thermal cycling experiments were conducted using an MTS Landmark servohydraulic test frame with custom compression grips and 100 kN capable load cell. The axial strain was measured by a high temperature extensometer with a gauge length of 12 mm. Heating and cooling of the sample was achieved by conduction through the compression platens at a rate of 10 °C/min using a PID driven Omega temperature controller.
3. Result and discussion Fig. 1 illustrates the DSC response and the damping capacity of the Mn–Pt alloy. Upon cooling, the high-temperature phase transforms to low temperature D-phase and vice versa upon heating. Mn–Pt has transformation peak temperatures of 195 °C and 185 °C during heating and cooling, respectively, with transformation enthalpy of 12 J/g. Thermal hysteresis was only 10 °C, which makes this alloy system a promising candidate for thermal driven solid state actuators. The sample was thermally cycled twenty times in the DSC to observe the stability of the TTs. A key requirement for active materials is to maintain the same characteristics after many cycles of operation. It is clear from Fig. 1a that the transformation in Mn–Pt is very stable with thermal cycling, meeting the stability requirement.
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The damping coefficient (Tan Delta) and storage modulus (E0 ) as a function of temperature are shown in Fig. 1b. The value of Tan Delta is measured as the ratio between the loss and storage modulus. The internal friction (IF) peaks that appear in both cooling and heating cycles of Tan Delta correspond to the high-temperature phase to D-phase and its reverse transformation peaks in the DSC curve, respectively. The damping coefficient of Mn–Pt, as measured through Tan Delta, was 0.17 and 0.15 for the forward (Blue curve) and reverse (Red curve) transformation at peak temperatures of 177 °C and 198 °C, respectively. The peak temperatures obtained from DMA are higher than the ones determined from DSC. This can be attributed to the fact that the thermocouple does not touch the sample in DMA resulting in less accurate values. During the thermal induced phase transformation, the storage modulus reached a minimum value of 3500 MPa where the Tan Delta has the maximum value. The damping coefficient of this Mn–Pt alloy is slightly higher than the damping constants of equiatomic NiTi, NiTiNb and Ni2MnGa SMAs [19–22], which have Tan Delta values below 0.15. It is well known that the IF peak due to the thermoelastic martensitic transformation of SMAs can be decomposed into three terms: IFTr, IFPT and IFInt [21–25]. The transitory term (IFTr) is a kinetic term of damping capacity, which is related to the volume fraction of martensite per unit time and strongly dependent on experimental variables such as temperature rate, frequency of oscillation, and strain amplitude. The phase transition contribution (IFPT) is responsible for the damping peak during isothermal measurements and it can be negligible for this study. The last term, IFInt, is the intrinsic IF, which describes the characteristic contribution of the high and low temperature phases on damping and is strongly dependent on the microstructure of each phase. In this study, the characteristic IF peak at low temperature, attributed to the high-temperature phase to D-phase transformation, can be ascribed to IFTr and IFInt. Normally, the value of Tan Delta at low temperatures should be larger than the one at high temperatures due to mobility of twin boundaries [21,22,24], but as shown in Fig. 1b, the intrinsic term for Mn75.7Pt24.3 is zero for the two phases since the level of Tan Delta is zero for these phases, which indicates that the microstructure of the high-temperature phase and D-phase are very similar to one another. Consequently, the main contribution to the IF peak is the transitory term. Thus, the IF peaks at low and high temperatures are associated with the motion of D-phase and the high-temperature phase interfaces during the thermally induced phase transformation. XRD measurements of Mn75.7Pt24.3 were carried out between 30 °C and 700 °C. Fig. 2a shows the XRD profiles obtained at 30 °C for the D-phase and 300 °C for the high-temperature phase.
Fig. 1. (a) DSC and (b) Tan Delta (solid line) and storage modulus (dash line) responses of Mn75.7Pt24.3.
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Fig. 2. (a) XRD profiles obtained at 30 °C and 300 °C, (b) temperature dependence of the lattice parameter, and (c) TEM diffraction pattern taken at room temperature for the Mn75.7Pt24.3 alloy.
All the diffraction peaks can be indexed as the ordered c phase with a Cu3Au-type face-centered cubic structure with no structural change observed due to the transformation. However, it is clear that the 2h positions of the peaks are quite different for the profiles obtained at 30 °C and 300 °C. This is due to the drastic change in the lattice parameter of the ordered FCC c phase during the transformation. The lattice parameter of the D-phase at 30 °C is 0.3828 nm, while that of the high-temperature phase at 300 °C is 0.3881 nm. The lattice parameters calculated from the XRD results are shown in Fig. 2b as a function of temperature. The lattice parameter increases with increasing temperature and shows a discontinuous change due to the phase transformation. At 190 °C, the lattice parameter of the high-temperature phase is estimated to be aH = 0.3867 nm, which is good agreement with previous work [17]. The strain caused by the contraction during the phase transformation is calculated by Da/aH, where Da is the difference in the lattice parameters between the high-temperature phase and D-phase at 190 °C, which is about 0.49%. Thus, the volume change of Mn75.7Pt24.3 during transformation is 1.5%, which is similar to that of CoNiAl SMAs (1.8%) and quite higher than NiTi SMAs (0.3% or less) [26,27]. The crystal structure of the D-phase was also confirmed by TEM selected area diffraction performed at room temperature, Fig. 2c. The magnetization response of the Mn–Pt alloy with respect to temperature under 1.5 T is shown in Fig. 3. The forward transformation start temperature was approximately 185 °C, which is in a good agreement with the DSC and DMA results. The difference in saturation magnetization of the transforming phases shows that the low temperature D-phase has higher magnetization than the
Fig. 3. Change in magnetization with temperature under constant magnetic field.
high-temperature phase. Thus, Mn–Pt alloys also can be exploited for magneto actuator and sensor applications. Fig. 4 demonstrates the isobaric thermal cycling responses for the Mn–Pt alloy under applied constant compressive stress levels of 25 MPa to 200 MPa. The stress was isothermally applied above the TTs and then the sample was thermally cycled. After the cycle was completed, the stress was increased to the next level and the thermal cycling was repeated on the same sample. Under 25 MPa, upon cooling, the high-temperature phase starts transforming to D-phase at 194 °C and finishes at 192 °C with a transformation strain of 0.53%. During heating, D-phase transforms back to the high-temperature phase and the induced compressive strain was recovered. A notable observation from Fig. 4a is that the load-biased phase transformation properties for the reverse transformation of the Mn–Pt alloy were essentially independent of the applied stress level. Transformation strain was constant (0.53%) as the applied stress increased from 25 MPa to 150 MPa. Although both phases are cubic, the high-temperature phase has a larger lattice parameter than the D-phase, therefore compression aids the transformation in the forward direction, as shown in Fig. 2b. It should be noted that uniaxial compression experiments only measured the strain along the loading direction, but there is also compressive strain in other directions. The high-temperature phase to D-phase transformation temperature only varies between 194 and 196 °C as stress increased from 25 MPa to 200 MPa. Thus, the transformation temperatures are very stable with stress. The hysteresis was only 8 °C under a compressive stress of 25 MPa. Transformation strain is completely recovered without any residual strain under the applied stress up to 100 MPa. However, the reverse transformation from D-phase to the high-temperature phase is affected by the compression stress, as the lattice tries to expand against the applied load. The transformation becomes sluggish with the finish temperature of the reverse transformation, Af, extending to higher temperatures and transformation becoming much less sharp as the stress increases. It is not clear whether the residual strain in this case is due to plasticity or incomplete transformation to the high-temperature phase and further work is needed to resolve this issue. It should be noted that accumulation of irrecoverable strain at high stress levels might result in poor cycling stability. Fig. 4b illustrates the thermal cycling response under no external stress, determined by DMA. The strain is calculated as a fraction of change in length of the sample over the initial length, which is about 0.5% and is in good agreement with the results shown in Fig. 4a. Fig. 4b confirms the argument that the transformation strain of Mn–Pt alloys does not depend on applied stress but depends only on the change in lattice parameters of the transforming phases.
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Fig. 4. (a) Load-biased strain vs. temperature curves and (b) detailed strain response from DMA at 0 MPa due to the phase transformation in Mn75.7Pt24.3.
4. Conclusions The present work has demonstrated that Mn75.7Pt24.3 undergoes a cubic to cubic transformation at high temperature (180–200 °C). The transformation is characterized by low thermal hysteresis, stable transformation temperatures, and high damping capacity. The transformation strain for the forward transformation determined by DMA and load-biased thermal cycling experiments was 0.53% and independent of the applied load. Theoretical transformation strain is calculated to be 0.49% with a volume change of 1.5%. It can be concluded that the Mn–Pt alloy is a promising candidate for high temperature actuator and damping applications requiring low hysteresis, although its transformation strain is isotropic and small. Acknowledgements This work was supported by the NASA Fundamental Aeronautics Program, Aeronautical Sciences Project and the NASA EPSCOR program under Grant no: NNX11AQ31A and National Science Foundation (NSF) CMMI award #0954541. References [1] K. Otsuka, X. Ren, Intermetallics 7 (1999) 511–528. [2] K. Yamauchi, I. Ohkata, K. Tsuchiya, S. Miyazaki, Shape Memory and Superelastic Alloys: Technologies and Applications, first ed., Woodhead Publishing, Cambridge, 2011. [3] G.S. Firstov, J. Van Humbeeck, Y.N. Koval, J. Intell. Mater. Syst. Struct. 17 (2006) 1041–1047.
[4] R. Noebe, T. Biles, S.A. Padula, NiTi-Based High-Temperature Shape-Memory Alloys: Properties, Prospects, and Potential Applications, Adv. Struct. Mater. (2007) 145–186. [5] J. Ma, I. Karaman, R.D. Noebe, Int. Mater. Rev. 55 (2010) 257–315. [6] G.S. Bigelow, A. Garg, S.A. Padula II, D.J. Gaydosh, R.D. Noebe, Scr. Mater. 64 (2011) 725–728. [7] X.L. Meng, Y.F. Zheng, Z. Wang, L.C. Zhao, Scr. Mater. 42 (2000) 341–348. [8] H.E. Karaca, S.M. Saghaian, G.S. Ded, B. Basaran, H. Tobe, H.J. Maier, R.D. Noebe, Y.I. Chumlyakov, Acta Mater. 61 (2013) 7422–7431. [9] H.E. Karaca, S.M. Saghaian, B. Basaran, G.S. Bigelow, R.D. Noebe, Y.I. Chumlyakov, Scr. Mater. 65 (2011) 577–580. [10] H.E. Karaca, E. Acar, B. Basaran, R.D. Noebe, Y.I. Chumlyakov, Scr. Mater. 67 (2012) 447–450. [11] H.E. Karaca, E. Acar, G.S. Ded, B. Basaran, H. Tobe, R.D. Noebe, G. Bigelow, Y.I. Chumlyakov, Acta Mater. 61 (2013) 5036–5049. [12] J. Van Humbeeck, Mater. Sci. Eng.: A 273–275 (1999) 134–148. [13] K. Otsuka, C.M. Wayman, Shape Memory Materials, Cambridge University Press, 1998. [14] E.Z. Vintaikin, G.I. Nosova, Met. Sci. Heat Treat. 43 (2001) 160–162. [15] E. Krén, G. Kádár, L. Pál, J. Sólyom, P. Szabó, T. Tarnóczi, Phys. Rev. 171 (1968) 574–585. [16] E. Krén, G. Kádár, L. Pál, P. Szabó, J. Appl. Phys. 38 (1967) 1265–1266. [17] E. Krén, P. Szabó, L. Pál, T. Tarnóczi, G. Kádár, C. Hargitai, J. Appl. Phys. 39 (1968) 469–470. [18] E. Krén, G. Kádár, L. Pál, J. Sólyom, P. Szabó, Phys. Lett. 20 (1966) 331–332. [19] W. Cai, X.L. Lu, L.C. Zhao, Mater. Sci. Eng. A – Struct. Mater. Prop. Microstruct. Process. 394 (2005) 78–82. [20] Z.Z. Bao, S. Guo, F. Xiao, X.Q. Zhao, Prog. Nat. Sci. 21 (2011) 293–300. [21] Y. Chen, H.C. Jiang, S.W. Liu, L.J. Rong, X.Q. Zhao, J. Alloys Comp. 482 (2009) 151–154. [22] S.H. Chang, S.K. Wu, Scr. Mater. 59 (2008) 1039–1042. [23] J. Stoiber, J.E. Bidaux, R. Gotthardt, Acta Metall. Mater. 42 (1994) 4059–4070. [24] W. Dejonghe, R.D. Batist, L. Delaey, Scripta Metall. 10 (1976) 1125–1128. [25] J. San Juan, M.L. Nó, J. Alloys Comp. 355 (2003) 65–71. [26] K. Otsuka, T. Sawamura, K. Shimizu, C.M. Wayman, Metall. Trans. 2 (1971) 2583–2588. [27] S. Dilibal, H. Sehitoglu, R.F. Hamilton, H.J. Maier, Y. Chumlyakov, Mater. Sci. Eng.: A 528 (2011) 2875–2881.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Diffusionless phase transformation characteristics of Mn75.7Pt24.3 H.E. Karaca a,⇑, S.M. Saghaian a, H. Tobe a, E. Acar a, B. Basaran a,b, M. Nagasako c, R. Kainuma c, R.D. Noebe d a
Department of Mechanical Engineering, University of Kentucky, Lexington KY 40506, USA University of Turkish Aeronautical Association, Etimesgut, Ankara, Turkey c Department of Material Science, Graduate School of Engineering, Tohoku University, Sendai, Japan d Structures and Materials Division, NASA Glenn Research Center, Cleveland, OH 44135, USA b
a r t i c l e
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Article history: Received 30 October 2013 Received in revised form 22 November 2013 Accepted 24 November 2013 Available online 3 December 2013 Keywords: MnPt Phase transformations Damping X-ray diffraction
a b s t r a c t Phase transformation, damping, and magnetic properties of a Mn75.7Pt24.3 (at.%) alloy were characterized. It was observed that Mn75.7Pt24.3 exhibits a stable phase transformation in the temperature range of 180– 200 °C with a small temperature hysteresis and maximum transformation strain of 0.5%. The crystal structures and lattice parameters of the transforming phases were determined, where both the high and low temperature phases have a face-centered cubic structure but with different lattice parameters. Finally, it was revealed that the alloy possesses high damping capacity (average Tan Delta of 0.16) during phase transformation. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction As reversible solid-state actuators, shape memory alloys (SMAs) have received considerable interest due to their ability to recover large shape changes under high loads. NiTi-based, Fe-based and Cu-based alloys have been acknowledged as the most common SMAs due to their attractive physical and mechanical properties [1]. The potential and current application areas for such alloys include electronic devices, medical tools, and home appliances [2]. Although most SMAs are limited to temperatures below 100 °C [3], many scientists have become interested in the development of SMAs that can operate at temperatures above 100 °C for potential service in the aerospace, automotive, and oil and gas industries [3– 5]. The main demand for high temperature SMAs (HTSMAs) stems from an interest in utilizing them as solid state actuators. Among all of the NiTi-based HTSMAs, NiTiHf seems to be the most promising candidate for a wide range of applications in the 100–250 °C temperature range due to its low cost and satisfactory shape memory properties [5]. Although the shape memory properties of NiTiHf can be improved with aging [6–8] and strongly dependent on the crystal orientation/texture [9] and addition of quaternary elements [10,11], there is still large demand for solid state actuators with very low hysteresis and stable behavior at high temperatures. Low thermal hysteresis is an important parameter for many practical applications of solid state actuators and sensors [12]. Thermal ⇑ Corresponding author. E-mail address: [email protected] (H.E. Karaca). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.174
hysteresis can be described as the dissipated energy due to reversible phase transformations, which negatively impacts the efficiency of the device. It should be noted that the temperature hysteresis is usually much greater than 20 °C in most of the conventional and high temperature SMAs [13]. In contrast to the most common shape memory alloy systems, c-Mn based alloys are relatively promising high temperature systems with very low thermal hysteresis. Mn–Cu alloys have less than 1 °C thermal hysteresis upon thermo-deformation with shear stresses reaching 160 MPa [14]. Hence, these alloys might provide several advantages for applications that require low hysteresis upon temperature cycling. In addition to the MnCu system, Mn-rich Mn–Pt alloys have been investigated primarily with regard to their magnetic properties [15–18]. According to magnetic susceptibility and neutron diffraction studies [17], Krén et al. have found a first order antiferromagnetic-antiferromagnetic (AF–AF) transformation in the Mn3Pt phase below the Néel temperature, TN. The magnetic structures below and above the AF–AF transition temperature Tt are triangular (D-phase) and collinear (F-phase), respectively. They have also reported the composition dependence of the transition temperatures TN and Tt. Both TN and Tt increase and the difference between them decreases with increasing Mn content of the alloys, resulting in the D-phase directly transforming to the paramagnetic phase at TN during heating, i.e., F-phase cannot be seen. The cubic Cu3Au-type ordered structure does not change at TN and Tt. However, they revealed a discontinuous change of the lattice parameter (obtained by XRD measurements) during the transformation between D-phase and the high-temperature phase (F-phase or
H.E. Karaca et al. / Journal of Alloys and Compounds 589 (2014) 412–415
paramagnetic phase), where the lattice parameter of the D-phase is smaller than that of the high-temperature phase. To the authors’ knowledge, the mechanical properties of binary Mn–Pt alloys have not been previously reported. Consequently in this study, the crystal structures of the transforming phases, phase transformation properties under compressive stress and damping capacity of a Mn75.7Pt24.3 (at.%) alloy were characterized. The composition was selected due to its high phase transformation temperatures (above 100 °C) and low thermal hysteresis [15] that make the alloy suitable for high temperature applications.
2. Experimental procedures An ingot of Mn75.7Pt24.3 (at.%) was produced by arc melting. The ingot was flipped and re-melted three times to better assure homogeneity. The ingot was then homogenized at 1000 °C for 24 h followed by furnace cooling. Compression samples (4 4 8 mm3) were cut using an electro discharge machine. A Perkin–Elmer PYRIS 1 Differential Scanning Calorimeter (DSC) was used to determine the transformation temperatures (TTs) using a heating/cooling rate of 10 °C/min. The damping behavior of the alloys was investigated by using a Perkin–Elmer Dynamic Mechanical Analyzer (DMA-7e) with a temperature rate of 5 °C/min. The configuration used was three point bending and temperature was measured by a thermocouple, which was placed near the sample but was not in contact with its surface. The measuring frequency was 1 Hz and the static and dynamic forces were 5000 mN and 1000 mN, respectively. X-ray diffraction (XRD) was carried out on a Bruker AXS D8 diffractometer using Cu Ka radiation. The magnetic measurements were made with a vibrating sample magnetometer (VSM) in a heating/cooling rate of 1 °C/min under 1.5 T magnetic field. The isobaric thermal cycling experiments were conducted using an MTS Landmark servohydraulic test frame with custom compression grips and 100 kN capable load cell. The axial strain was measured by a high temperature extensometer with a gauge length of 12 mm. Heating and cooling of the sample was achieved by conduction through the compression platens at a rate of 10 °C/min using a PID driven Omega temperature controller.
3. Result and discussion Fig. 1 illustrates the DSC response and the damping capacity of the Mn–Pt alloy. Upon cooling, the high-temperature phase transforms to low temperature D-phase and vice versa upon heating. Mn–Pt has transformation peak temperatures of 195 °C and 185 °C during heating and cooling, respectively, with transformation enthalpy of 12 J/g. Thermal hysteresis was only 10 °C, which makes this alloy system a promising candidate for thermal driven solid state actuators. The sample was thermally cycled twenty times in the DSC to observe the stability of the TTs. A key requirement for active materials is to maintain the same characteristics after many cycles of operation. It is clear from Fig. 1a that the transformation in Mn–Pt is very stable with thermal cycling, meeting the stability requirement.
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The damping coefficient (Tan Delta) and storage modulus (E0 ) as a function of temperature are shown in Fig. 1b. The value of Tan Delta is measured as the ratio between the loss and storage modulus. The internal friction (IF) peaks that appear in both cooling and heating cycles of Tan Delta correspond to the high-temperature phase to D-phase and its reverse transformation peaks in the DSC curve, respectively. The damping coefficient of Mn–Pt, as measured through Tan Delta, was 0.17 and 0.15 for the forward (Blue curve) and reverse (Red curve) transformation at peak temperatures of 177 °C and 198 °C, respectively. The peak temperatures obtained from DMA are higher than the ones determined from DSC. This can be attributed to the fact that the thermocouple does not touch the sample in DMA resulting in less accurate values. During the thermal induced phase transformation, the storage modulus reached a minimum value of 3500 MPa where the Tan Delta has the maximum value. The damping coefficient of this Mn–Pt alloy is slightly higher than the damping constants of equiatomic NiTi, NiTiNb and Ni2MnGa SMAs [19–22], which have Tan Delta values below 0.15. It is well known that the IF peak due to the thermoelastic martensitic transformation of SMAs can be decomposed into three terms: IFTr, IFPT and IFInt [21–25]. The transitory term (IFTr) is a kinetic term of damping capacity, which is related to the volume fraction of martensite per unit time and strongly dependent on experimental variables such as temperature rate, frequency of oscillation, and strain amplitude. The phase transition contribution (IFPT) is responsible for the damping peak during isothermal measurements and it can be negligible for this study. The last term, IFInt, is the intrinsic IF, which describes the characteristic contribution of the high and low temperature phases on damping and is strongly dependent on the microstructure of each phase. In this study, the characteristic IF peak at low temperature, attributed to the high-temperature phase to D-phase transformation, can be ascribed to IFTr and IFInt. Normally, the value of Tan Delta at low temperatures should be larger than the one at high temperatures due to mobility of twin boundaries [21,22,24], but as shown in Fig. 1b, the intrinsic term for Mn75.7Pt24.3 is zero for the two phases since the level of Tan Delta is zero for these phases, which indicates that the microstructure of the high-temperature phase and D-phase are very similar to one another. Consequently, the main contribution to the IF peak is the transitory term. Thus, the IF peaks at low and high temperatures are associated with the motion of D-phase and the high-temperature phase interfaces during the thermally induced phase transformation. XRD measurements of Mn75.7Pt24.3 were carried out between 30 °C and 700 °C. Fig. 2a shows the XRD profiles obtained at 30 °C for the D-phase and 300 °C for the high-temperature phase.
Fig. 1. (a) DSC and (b) Tan Delta (solid line) and storage modulus (dash line) responses of Mn75.7Pt24.3.
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Fig. 2. (a) XRD profiles obtained at 30 °C and 300 °C, (b) temperature dependence of the lattice parameter, and (c) TEM diffraction pattern taken at room temperature for the Mn75.7Pt24.3 alloy.
All the diffraction peaks can be indexed as the ordered c phase with a Cu3Au-type face-centered cubic structure with no structural change observed due to the transformation. However, it is clear that the 2h positions of the peaks are quite different for the profiles obtained at 30 °C and 300 °C. This is due to the drastic change in the lattice parameter of the ordered FCC c phase during the transformation. The lattice parameter of the D-phase at 30 °C is 0.3828 nm, while that of the high-temperature phase at 300 °C is 0.3881 nm. The lattice parameters calculated from the XRD results are shown in Fig. 2b as a function of temperature. The lattice parameter increases with increasing temperature and shows a discontinuous change due to the phase transformation. At 190 °C, the lattice parameter of the high-temperature phase is estimated to be aH = 0.3867 nm, which is good agreement with previous work [17]. The strain caused by the contraction during the phase transformation is calculated by Da/aH, where Da is the difference in the lattice parameters between the high-temperature phase and D-phase at 190 °C, which is about 0.49%. Thus, the volume change of Mn75.7Pt24.3 during transformation is 1.5%, which is similar to that of CoNiAl SMAs (1.8%) and quite higher than NiTi SMAs (0.3% or less) [26,27]. The crystal structure of the D-phase was also confirmed by TEM selected area diffraction performed at room temperature, Fig. 2c. The magnetization response of the Mn–Pt alloy with respect to temperature under 1.5 T is shown in Fig. 3. The forward transformation start temperature was approximately 185 °C, which is in a good agreement with the DSC and DMA results. The difference in saturation magnetization of the transforming phases shows that the low temperature D-phase has higher magnetization than the
Fig. 3. Change in magnetization with temperature under constant magnetic field.
high-temperature phase. Thus, Mn–Pt alloys also can be exploited for magneto actuator and sensor applications. Fig. 4 demonstrates the isobaric thermal cycling responses for the Mn–Pt alloy under applied constant compressive stress levels of 25 MPa to 200 MPa. The stress was isothermally applied above the TTs and then the sample was thermally cycled. After the cycle was completed, the stress was increased to the next level and the thermal cycling was repeated on the same sample. Under 25 MPa, upon cooling, the high-temperature phase starts transforming to D-phase at 194 °C and finishes at 192 °C with a transformation strain of 0.53%. During heating, D-phase transforms back to the high-temperature phase and the induced compressive strain was recovered. A notable observation from Fig. 4a is that the load-biased phase transformation properties for the reverse transformation of the Mn–Pt alloy were essentially independent of the applied stress level. Transformation strain was constant (0.53%) as the applied stress increased from 25 MPa to 150 MPa. Although both phases are cubic, the high-temperature phase has a larger lattice parameter than the D-phase, therefore compression aids the transformation in the forward direction, as shown in Fig. 2b. It should be noted that uniaxial compression experiments only measured the strain along the loading direction, but there is also compressive strain in other directions. The high-temperature phase to D-phase transformation temperature only varies between 194 and 196 °C as stress increased from 25 MPa to 200 MPa. Thus, the transformation temperatures are very stable with stress. The hysteresis was only 8 °C under a compressive stress of 25 MPa. Transformation strain is completely recovered without any residual strain under the applied stress up to 100 MPa. However, the reverse transformation from D-phase to the high-temperature phase is affected by the compression stress, as the lattice tries to expand against the applied load. The transformation becomes sluggish with the finish temperature of the reverse transformation, Af, extending to higher temperatures and transformation becoming much less sharp as the stress increases. It is not clear whether the residual strain in this case is due to plasticity or incomplete transformation to the high-temperature phase and further work is needed to resolve this issue. It should be noted that accumulation of irrecoverable strain at high stress levels might result in poor cycling stability. Fig. 4b illustrates the thermal cycling response under no external stress, determined by DMA. The strain is calculated as a fraction of change in length of the sample over the initial length, which is about 0.5% and is in good agreement with the results shown in Fig. 4a. Fig. 4b confirms the argument that the transformation strain of Mn–Pt alloys does not depend on applied stress but depends only on the change in lattice parameters of the transforming phases.
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Fig. 4. (a) Load-biased strain vs. temperature curves and (b) detailed strain response from DMA at 0 MPa due to the phase transformation in Mn75.7Pt24.3.
4. Conclusions The present work has demonstrated that Mn75.7Pt24.3 undergoes a cubic to cubic transformation at high temperature (180–200 °C). The transformation is characterized by low thermal hysteresis, stable transformation temperatures, and high damping capacity. The transformation strain for the forward transformation determined by DMA and load-biased thermal cycling experiments was 0.53% and independent of the applied load. Theoretical transformation strain is calculated to be 0.49% with a volume change of 1.5%. It can be concluded that the Mn–Pt alloy is a promising candidate for high temperature actuator and damping applications requiring low hysteresis, although its transformation strain is isotropic and small. Acknowledgements This work was supported by the NASA Fundamental Aeronautics Program, Aeronautical Sciences Project and the NASA EPSCOR program under Grant no: NNX11AQ31A and National Science Foundation (NSF) CMMI award #0954541. References [1] K. Otsuka, X. Ren, Intermetallics 7 (1999) 511–528. [2] K. Yamauchi, I. Ohkata, K. Tsuchiya, S. Miyazaki, Shape Memory and Superelastic Alloys: Technologies and Applications, first ed., Woodhead Publishing, Cambridge, 2011. [3] G.S. Firstov, J. Van Humbeeck, Y.N. Koval, J. Intell. Mater. Syst. Struct. 17 (2006) 1041–1047.
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