Local influence of material processing on phase transitions in NiTi shape memory alloys investigated by IR thermography

Materials Science and Engineering A 378 (2004) 175–179

Local influence of material processing on phase transitions in NiTi shape memory alloys investigated by IR thermography J. Gibkes a,∗ , W. Siegert b , D. Dietzel a , I. Delgadillo-Holtfort a , B.K. Bein a , J. Pelzl a a

Solid State Spectroscopy, Institute of Experimental Physics III, Ruhr-University, Bochum D-44780, Germany b Faculty of Mechanical Engineering, Institute of Materials, Ruhr-University, Bochum D-44780, Germany Received 2 June 2003; received in revised form 22 October 2003

Abstract Local variations of the phase transition temperatures are important for the functionality of actuators based on NiTi shape memory alloys. Mechanical distortions induced by process treatment can suppress the phase transition from the austenitic to the martensitic structure. In this work, local changes of the phase transition have been investigated by time-dependent IR thermography. For the quantitative interpretation of the thermographical measurements, information about the IR emissivity is important. In this work a new thermal wave method based on two detection techniques is presented, which can be used to measure both the IR emissivity and the thermal transport properties. © 2004 Elsevier B.V. All rights reserved. Keywords: IR thermography; Phase transition; Emissivity

1. Introduction In many applications, shape memory alloys (SMA) are used for mechanical switch mechanisms. An example of a decoupling mechanism, which is analysed in this work, is that of an actuator to be used to hold the solar panels of a space satellite during transport into space. After transport, the actuator is used only once to expand the solar panels (one way effect). Problems may arise due to mechanical distortions induced by process treatment during fabrication, which can suppress the phase transition from the martensitic to the austenitic structure in parts of the actuator. To study the effects of mechanical distortions, the local changes of the phase transition have been investigated by time-dependent infrared thermography. For the quantitave interpretation of the measured temperature profiles, the emissivity ε of the sample surface has to be known. In the following, a new technique is presented to measure the emissivity by comparing the results of different photothermal methods. Additionally, a brief description of the interpretation of the thermographic measurements is presented, developed for the quantitative analysis of the phase transitions. ∗ Corresponding author. Tel.: +49-234-32-28618; fax: +49-234-32-14336. E-mail address: [email protected] (J. Gibkes).

0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.343

2. Investigation of the temperature evolution at positions with and without phase transition The local variations of the phase transition temperature are important for the functionality of actuators based on NiTi shape memory alloys. Mechanical distortions induced by process treatment can contribute to suppress the phase transition from the martensitic to the austenitic structure. We have measured the effect of phase transitions on the temperature evolution of an electrically heated actuator (Fig. 1c) by time-dependent infrared thermography. For the temperature measurement, an IR camera from JENOPTIC (Varioscan 3011) with a temperature range from −40 to 1200 ◦ C, a temperature resolution of 30 mK, a geometrical resolution of 1.5 mrad and a time resolution of 0.8 s has been used. The camera is a single detector camera with a nitrogen-cooled MCT (HgCdTe) detector which is working in the wavelength spectrum between 8 and 12 ␮m. From the time-dependent IR images registered by the camera we can extract the temperature evolution at different positions of the sample surface (Fig. 1a and b). At the positions P1 and P2 the actuator material was affected by mechanical distortions related to the mechanical treatment process, whereas the positions P3 and P4 are not affected by mechanical treatment.

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P3

50-1-S-B3-05 120

T / ˚C

P1 80

P4

P2

40

(a) 20 P4 -P1

15

∆T / ˚C

P2 - P1

perature difference may then mainly be due to the absence of the phase transition at P1. Similarly, the convection effects at positions farther away from the outer edges may be similar, and the temperature difference between two considered points may then be related to the absence or a reduced effect of the phase transition. The suppression of the phase transition at the positions P1 and P2 has also been observed by scanning thermal microscopy (SThM) measurements on the nanometer scale [1]. Using time-dependent IR thermography, it is possible to map the different regions at which the phase transition occurs or not on the ␮m scale, a length scale which for engineering applications is much more useful. For a more quantitative interpretation of the measured temperature evolution, however, the emissivty ε of the sample surface, which in principle may also change with the phase transition, has to be determined.

10

5

P4 - P2

0

(b) 0

40

80

120

160

t / a.u.

4 3 21

(c) Fig. 1. (a and b) Temperature evolution (a) and temperature differences (b) as a function of time, measured at different positions of the actuator. (c) Schematic of the actor and of the positions where our traces in Fig 1a and b have been measured.

Initially, when the heating process starts, the four measured positions show similar temperature rises. After the onset of the phase transitions above approximately T ≈ 70 ◦ C between about t ≈ 40 and 80 a.u., the rate of temperature increase is reduced at P3 and P4, whereas at P1 and P2 the rate of temperature increase seems to be less affected, indicating the suppression of the phase transitions. Heat convection effects, which may be more efficient at positions close to the outer edges, contribute to complicate the interpretation of the measured temperature evolution. To eliminate or reduce such edge convection effects, temperature differences are considered in Fig. 1b. To this finality we assume, that the convection effects at the edge points P1 and P4 may be of the same order of magnitude, and that the observed tem-

3. Determination of the emissivity ε by thermal wave techniques For measuring the relevant optical properties, such as the emissivity ε a new technique has been developed, which is based on the concept of thermal waves. The sample is heated periodically, e.g. by an intensity-modulated laser beam. The heat diffuses into the sample and leads to a timeand space-dependent temperature distribution, which is described by the heat diffusion equation and can be considered as a thermal wave. For an opaque semi-infinite homogeneous solid heated by a laser beam of large diameter, the temperature oscillation is given by:
 ηI0 z δT(z, f) = √ exp − √ 2e 2πf α/(πf) 
π z (1) − × cos 2πft − √ 4 α/(πf) where α = k/(ρc) is the thermal diffusivity, e = (kρc)1/2 the effusivity, I0 the intensity of the incident light beam, f the modulation frequency of heating, and η the photothermal efficiency. The new method to determine the emissivity relies on the measurement of this temperature oscillation by combining two different detection techniques for thermal waves, namely IR detection and photoacoutic detection of thermal waves. The signal measured by modulated IR radiometry is mainly proportional to the emissivity ε in the measured wavelength interval in the IR spectrum and to the periodic temperature oscillation at the surface z = 0 [2]: δM(T, f) = 4Cε(T)γD σSB T 3 δT(f)

(2)

In Eq. (2), σ SB is Stefan–Boltzmann’s constant, γ D the detection efficiency for modulated IR emission [2], T the time-averaged sample surface temperature, and the quantity C is the responsivity of the used electronic components.

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Using photoacoustic detection the measured signal is proportional to the pressure oscillations [3] in the closed photoacoustic cell:  γ −1 t δp(f, t) = 2 dt δF(z = 0, t ) (3) lg 0 which can be described by the time integral over the periodic heat flux δF(x = 0, t) at the solid surface–gas interface. Here, the quantities γ and lg are the adiabatic exponent of the gas and the length of the gas column, inside the photoacoustic cell, respectively. To eliminate the different frequency characteristics of modulated IR radiometry and photoacoustics, which are due to the different signal generation mechanisms and detection equipment, the signals measured by the two detection techniques first have to be calibrated with the help of a common reference sample. The normalized signal amplitude and signal phase are then given by: Ss (f) Sr (f)

(4)

φn (f) = φs (f) − φr (f)

(5)

Sn (f) =

Here, the index n refers to the normalized amplitudes and phases, s to the signals of the sample of unknown properties and r to the signals measured for the reference body of known properties. With the help of the normalized photoacoustic and IR radiometric signals measured in the transmission configuration of thermal waves [4], where excitation and detection of the thermal waves take place at the opposite surfaces of a sample of finite thickness, it is possible to determine the effective integral thermal properties, namely the thermal diffusivity and the effusivity of the sample. The emissivity ε of the sample can be determined separately by comparing the photoacoustic and the IR response. Both signals rely on the same thermal wave amplitude determined by the thermal properties α and e and the photothermal efficiency η which governs the heat absorption process. While the photoacoustic signal is independent of the emissivity ε, the modulated IR amplitude essentially depends on this quantity ε. For the simultaneous measurement of the thermal wave response by means of the two different detection schemes, IR radiometry and photoacoustics, a special device has been developed, the schematic of which is shown in Fig. 2. For the excitation of the thermal waves, the beam of an argon ion laser (spectra physics) with a beam power of about 1 W and a diameter of about 2 mm is intensity-modulated with the help of an acousto-optical modulator (Laser Components, LM 080). Modulated heating of the sample in the frequency interval between 0.03 and 100 kHz can take place at the sample’s front surface or alternatively at its rear surface, as shown in Fig. 2. The IR radiometric signals and the photoacoustic signals are detected at the sample’s front surface. The closed small gas volume, necessary for microphone detection of the photoacoustic response, is limited by the sample’s

Laser

177

Modulator Sample

IR-detector IR-lens

Digital Driver IR-filter Microphone Lock-in Lock-in

S Computer

ϕ

Complex signal

Interface

Fig. 2. Schematic of the experimental setup for combined photoacoustic and IR-radiometric measurements, enabling the determination of the IR emissivity ε.

front surface, the inner cylindrical wall of the photoacoustic cell, and an IR transparent window (BaF2 ), allowing both heating in the visible spectrum and detection of the IR radiation emitted by the sample’s front surface. The IR signal is measured with the help of a MCT detector (Judson-Infrared, JD15-D12), allowing a maximum detectable wavelength interval of 2–12 ␮m. The photoacoustic response is measured with the help of a microphone (Sennheiser, KE4-211). Two lock-in amplifiers are used for the simultaneous analysis of the signals of the microphone and of the MCT detector. The used lock-in amplifiers are two-phase lock-in amplifiers (SR 830), enabling to measure both the amplitude and the phase shift of the two signals. For the investigation of the temperature dependence of the emissivity, high temperature units have been constructed which allow IR radiometric and photoacoustic measurements up to 1000 ◦ C [5,6]. The method and the device have successfully been tested with coated and non-coated metal samples of different optical and IR optical properties. In Fig. 3 the normalized photoacoustic (䊉) and IR radiometric signals (䊏) are approximated by theoretical solutions for the solid of limited thickness, using the same values for the thermal properties. The emissivity is directly obtained from the shift of the radiometric signals relative to the photoacoustic signals. The deviations between measured data and theoretical approximation at the higher frequencies, above about 9 Hz, are due to the small transmission signals, which with increasing frequency become comparable with the noise signals. The temperature values, which have been deduced from the camera signals at the different positions of the NiTi actuator and which have been compared with the temperatures measured by thermocouples, have shown that the differences of the emissivity at the various positions are negligible. This is in agreement with the results obtained from combined modulated IR and photoacoustic measure-

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surface temperature Ts (t) is known from the thermographic measurements, the absorbed net heat flux Fs (t) contributing to the effective heating process of the sample and the resulting rear surface temperature Tr (t) can be calculated by solving two coupled integral equations [8]. The power balance at the front surface before phase transition is given by:

6

Ni54.9Ti45.1 5

Finc (t) − Fref (t) = Frad (t, Ts ) + Fcon (t, Ts ) + Fs (t, Ts ) (6)

ln(sn)

4

where Finc is the incident, Fref (t) the reflected heat flux, and Frad and Fcon are the loss terms due to IR emission and convection. Based on the measured surface temperature Ts (t) the loss terms due to IR emission Frad (t, Ts ) and convection Fcon (t, Ts ) can be calculated. If we additionally assume that the incident and reflected radiation, Finc (t) − Fref (t), remain unchanged, the difference Finc (t) − Fref (t) has been determined, the power balance of the front surface during the second phase of the heating process can be analyzed and resolved for the surface heat sinks or sources by:

3

2

IR PA

1

0 0

1

2

3

4

1/2

Fsour/sink (t, Ts ) = +Finc (t) − Fref (t) − Frad (t, Ts )

(f / Hz)

− Fcon (t, Ts ) − Fs (t, Ts )

Fig. 3. Emissivity effect on the IR signals (normalized amplitudes after calibration with the signals measured for a common reference sample). Comparing the frequency-dependent signals obtained by the two measurements yields the emissivity ε = 0.39 of the Ni54.9 Ti45.1 sample.

ments for the austenitic and martensitic phase of other NiTi samples of similar composition.

Emissivity, ε

(7)

Similarly, the heat sources and heat sinks extending below the surface at later stages of the surface heating process can be treated analytically. These calculations, which are in progress, will allow to quantify the heat sink at the martensitic phase transition providing an indication for the size of

Actuator (Ni50.2 Ti49.8 )

Ni54.9 Ti45.1 (2 mm thick)

Ni54.9 Ti45.1 (0.2 mm thick)

Metal reference sample (Mo polished)

0.40

0.39

0.45

0.08

4. Quantitative interpretation of the heat sink effect Proceeding from inverse solutions of the surface heating problem relying on an external radiation source [7,8], the power balance at the heated surface (Fig. 4.) can be analyzed with respect to the heat sinks or heat sources, related to the phase transition process. If the time evolution of the front Finc

T (t) s

Fref Frad =

z

4

Fcon

Fs (t)

Fsour/sink Fig. 4. Schematic of the boundary conditions with surface heat sources/sinks.

the region of the sample which does not undergo the phase transition.

5. Conclusions The time evolution of the surface temperature measured by time-dependent IR thermography proves that particularly in those areas of the actuator, which were strongly affected by mechanical processing, the phase transition is suppressed. This observation points towards a damaging processes based on local stress generation. For a reliable quantitative interpretation a new measuring procedure has been developed that allows to determine the local emissivity ε. Applying a mixed numerical-analytical inversion procedure for surface heating processes of samples of limited thickness, and by proceeding from the measured surface temperature evolution, the phase transition-induced heat sinks can be quantified.

J. Gibkes et al. / Materials Science and Engineering A 378 (2004) 175–179

Acknowledgements This work was performed in the frame of Sonderforschungsbereich 459 . References [1] J. Gibkes, W. Siegert, B.K. Bein, J. Pelzl, Investigation of spatial inhomogeneities of the thermal properties of a NiTi shape memory alloy device-Untersuchung von räumlich inhomogenen thermischen Eigenschafren in eivem NiTi - Formgedächtrus-Bauelement, to be published in Materialmissenschaften and Werkstoffe (Materials Science and Engineering Technology) Wiley-VCH, 2004. [2] J. Bolte, J.H. Gu, B.K. Bein, Background fluctuation limit of IR detection of thermal waves at high temperatures, High Temp. High Pressures 29 (5) (1997) 567–580.

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