Download Continuous stress annealing of amorphous ribbons for

Sensors and Actuators A 106 (2003) 117–120
Continuous stress annealing of amorphous ribbons
for strain sensing applications
Luděk Kraus a,∗ , Jan Bydžovský b , Peter Švec c
a
Institute of Physics ASCR, Na Slovance 2, CZ-18221 Praha 8, Czech Republic
b Slovak Technical University, Ilkovičova 3, SK-81219 Bratislava, Slovakia
c Institute of Physics SAS, Dúbravská cesta 9, SK-84228 Bratislava, Slovakia
Abstract
Stress-annealed amorphous Co69 Fe2 Cr7 Si8 B14 ribbons are suitable materials for strain sensors required in civil engineering applications.
The equipments for the continuous stress-annealing of ribbons and the automatic homogeneity test of magnetic properties are described.
The magnetic properties of stress-annealed ribbons are discussed. The use of ribbon in an inductive strain sensor is also illustrated.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Strain sensors; Amorphous alloys; Magnetic anisotropy; Magnetoelastic properties
1. Introduction
The extraordinary mechanical and magnetic properties
of amorphous alloys can be used in various sensing applications [1]. High magnetostrictive Fe-rich alloys are particularly suitable for very sensitive strain sensors [2] working
in small strain ranges. Using the as-quenched Co-Ni based
alloys with negative magnetostriction the measuring range
was substantially increased [3]. High elastic strength and
corrosion resistance of sensor materials are required for
outdoor applications in civil engineering. It has been proved
that for high-level strain measurements (up to 2 × 10−3 )
amorphous Co69 Fe2 Cr7 Si8 B14 alloys with small negative
magnetostriction are suitable [4]. To improve further the
sensor performance transversal magnetic anisotropy was induced by annealing under tensile stress [5]. For large-scale
material production the continuous stress-annealing is required. To ensure good reproducibility and uniformity of
magnetic properties along the ribbon length automated
equipments for continuous stress-annealing and quality
control were developed.
2. Experimental
Amorphous Co69 Fe2 Cr7 Si8 B14 ribbon, 6 mm wide, was
prepared by the planar flow casting. The amorphous struc∗ Corresponding author. Tel.: +420-2-66052174;
fax: +420-2-8689-0527.
E-mail address: [email protected] (L. Kraus).
0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0924-4247(03)00147-X
ture was checked by X-ray diffraction. The basic magnetic
properties, determined from quasistatic hysteresis loop
measurements, are summarized in Table 1. The influence
of stress-annealing on domain structure and hysteresis
properties was investigated on 40 cm long ribbon pieces
statically annealed for 1 h in a radiation furnace with a
temperature plateau about 14 cm along its axis. The temperature in the furnace was controlled by the temperature
controller/programmer (Eurotherm 902). Stress was applied by a weight attached to one of the cold ends of the
sample.
Domain structures on the shiny sides of 7 cm long samples were observed by scanning electron microscope JEOL
Superprobe JXA-733 using the type II magnetic contrast
of backscattered electrons [6]. The sample was tilted with
respect to the electron beam around the longitudinal or
transversal axis so that either longitudinal (L) or transversal (T) magnetic contrast could be observed. Tensile stress
up to 800 MPa was applied to ribbons by means of a
brass spring.
For the continuous stress-annealing the same radiation furnace was used with the ribbon continuously moving through
it. The schematic diagram of the equipment is shown in
Fig. 1. The ribbon is moved with a constant velocity by winding on a bobbin driven by the stepping motor, which is controlled by PC via RS 232 serial port. Two ways of applying
stress are used during annealing. The first one uses a weight
attached to the end of the ribbon. The weight moves down
until it reaches the floor. With this method the force on the
ribbon is constant, but the length of annealed sample is limited by the height at which the furnace is placed (about 2 m).
118
L. Kraus et al. / Sensors and Actuators A 106 (2003) 117–120
Table 1
Magnetic properties of as-quenched Co69 Fe2 Cr7 Si8 B14 ribbon
19.7 ␮m
0.56 T
2.9 A/m
−1 × 10−6
Ribbon thickness t
Saturation polarization Js
Coercive force Hc
Magnetostriction constant λs
The second method uses the friction applied to the second
bobbin, from which the ribbon is unwound (see Fig. 1). The
horizontal component of force acting on the bobbin is measured by the digital force gauge (Chatillon DFGS-R-ND).
Long samples can be annealed using this method. The disadvantage, however, is the slight variation of the frictional
force.
The homogeneity of magnetic properties of stress-annealed ribbons is measured by means of the automated hysteresis loop tracer schematically shown in Fig. 2. The
quasistatic hysteresis loops along the ribbon are measured
step-by-step at a constant distance between the measurements. Between the measurements the ribbon is moved by
means of the stepping motor in the same way as described
above. M-H tracer consists of the 30 cm long magnetizing solenoid and the 3 cm pick-up coil with 8000 turns.
The solenoid is fed by the power supply controlled by
the output voltage of the programmable voltage source
(Keithley 230). The current is measured by means of
DMM (Keithley 175A). The pick-up voltage is integrated
by the fluxmeter (Dr. Steingroever EF3). The program
written in LabVIEW, allows automatic measurements of
a large number of hysteresis loops for a set of predefined
parameters:
•
•
•
•
•
Fig. 2. The schematic diagram of automatic homogeneity M-H loop tester.
3. Properties of stress-annealed ribbons
Number of measured loops;
Displacement between the measurements;
Points per loop;
Maximum amplitude of magnetic field;
Shape of field versus time function, etc.
Stress-annealing induces magnetic anisotropy with
the easy plane perpendicular to the ribbon axis and the
anisotropy constant proportional to the stress applied during
the annealing. Transversal stripe domains are observed in
the stress-annealed samples and the magnetization reversal
takes place only by magnetization rotation. The domain
structures of stress-annealed samples show wide stripe domains perpendicular to the ribbon axis (see Fig. 3). The
domain walls show the zigzag structure, typical for the
easy-plane anisotropy [7]. As can be seen, applied tensile
stress up to 450 MPa has only little effect on the domain
structure.
The hysteresis loops of stress-annealed ribbon, measured
at various applied tensile stress σ are shown in Fig. 4. The
loops are linear practically up to 90% of saturation magnetization and show very little coercive force. The slope
of the hysteresis loop decreases with applied stress and the
effective anisotropy field, determined from the initial susceptibility, well satisfies the theoretical linear dependence
The measured loops are saved into ASCII format files.
HK = HK0 − 3
λs
σ,
Js
Fig. 1. Device for continuous stress-annealing.
(1)
L. Kraus et al. / Sensors and Actuators A 106 (2003) 117–120
119
Fig. 3. Domain structures of stress-annealed Co69 Fe2 Cr7 Si8 B14 ribbon (350 ◦ C/1 h/400 MPa). (a) σ = 0; (b) σ = 450 MPa.
where HK0 is the anisotropy field induced by stress-annealing.
An example of the anisotropy field HK measurement along
the continuously stress-annealed ribbon is shown in Fig. 5.
As can be seen, in the central part, where the temperature
of the furnace and the ribbon velocity were constant, the
anisotropy field is constant within 10%. It was found that
for the annealing temperature of 350 ◦ C the velocity less
than 20 cm/h (which corresponds to the annealing time of
∼42 min) ensures good homogeneity of magnetic properties
along the ribbon.
4. Two-coil strain sensor
Fig. 4. Effect of applied stress on hysteresis loops of stress-annealed
Co69 Fe2 Cr7 Si8 B14 ribbon.
Fig. 5. Anisotropy field measured along a continuously stress-annealed
ribbon. The inset shows the hysteresis loop measured at the position
x = 50 cm.
A linear dependence of the anisotropy field on the applied
stress Eq. (1) can be utilized for a strain sensor, where amorphous ribbon is used as the magnetic core of an induction
coil. The linearity of magnetizing characteristics and low
hysteresis of the stress-annealed ribbons, provide nearly linear dependence of dynamic reluctivity (inverse of magnetic
permeability) on applied stress. The prototype of two-coil
strain sensor is schematically shown in Fig. 6. The ribbon
core, 85 mm long, is placed in the primary winding (driving
coil) N1 = 460 and the secondary winding (sensing coil)
N2 = 120. The ends of the ribbon are glued to the supports
mounted on a brass beam. The sensor is strained by means
of the brass beam, which is fixed at both ends and bent by
a force applied to its center. The primary coil is supplied by
a power operational amplifier at a frequency of 5 kHz. For
materials with a linear dynamic magnetization curve, small
hysteresis and high permeability the voltage induced in the
sensing coil can be expressed by the relation [8]
U2 = C
ωJs2
Imag ,
Js HK0 − 3ελs E
(2)
120
L. Kraus et al. / Sensors and Actuators A 106 (2003) 117–120
Fig. 6. Basic electrical scheme with feedback to ensure constant induced voltage. Magnetizing current is used as signal for strain detection.
where ω is the angular frequency of the driving current, ε
the strain, E the Young’s modulus and C the constant, which
takes into account the demagnetizing effect and filling factor
of the secondary coil by the core. If the induced voltage U2
is kept constant the magnetizing current Imag is proportional
to the strain ε.
Both the waveform and the amplitude of magnetizing
current are controlled by a feedback loop, which ensures
that the voltage U2 induced in the pick-up coil remains
sinusoidal and constant when the flexure of the beam is
changed. The electronic circuit with the feedback loop is
schematically shown in Fig. 6. Different types of solid state
circuits were developed and tested for the driving amplifier. On the basis of the obtained experience three types
of portable strain-measuring devices were designed and
constructed.
5. Conclusion
Equipment for continuous stress-annealing of amorphous ribbons and an automated hysteresis loop tester
were developed. It was shown that magnetic properties of
stress-annealed Co69 Fe2 Cr7 Si8 B14 ribbon were homogeneous along its length. The techniques, described above,
can be used for a large-scale production and control of
amorphous ribbons utilized in inductive strain sensors for
civil engineering applications.
Acknowledgements
The work was supported by the NATO SfP Project No.
SfP-973649 “Quenched Materials” and Ministry of Education, Youth and Sports of the Czech Republic, Programme
KONTAKT ME 355 (2000).
References
[1] A. Hernando, M. Vázquez, J.M. Barandiarán, Metallic glasses and
sensing applications, J. Phys. E. Sci. Instrum. 21 (1988) 1129–1139.
[2] M. Wun-Fogle, H.T. Savage, A.E. Clark, Sensitive, wide frequency
range magnetostrictive strain gauge, Sens. Actuators A 12 (1987)
323–331.
[3] J.M. Barandiarán, J. Gutierrez, Magnetoelastic sensors based on soft
amorphous magnetic alloys, Sens. Actuators A 59 (1997) 38–42.
[4] L. Kraus, P. Švec, J. Bydžovský, Amorphous CoFeCrSiB ribbons for
strain sensing applications, J. Magn. Magn. Mater. 242–245 (2002)
241–243.
[5] J. Bydžovský, M. Kollár, V. Jančárik, P. Švec, L. Kraus, Strain sensors
for sensors for civil engineering applications based on CoFeCrSiB
amorphous ribbons, Czech J. Phys. 52 (Suppl. A) (2002) A117–
A120.
[6] K. Jurek, K. Závěta, Observation of type II magnetic contrast with
electron microprobe JXA-733, JEOL News 26E (1988) 17–19.
[7] K. Závěta, L. Kraus, K. Jurek, V. Kamberský, Zig-zag domain walls
in creep-annealed metallic glass, J. Magn. Magn. Mater. 73 (1988)
334–338.
[8] J. Bydžovský, M. Kollár, P. Švec, L. Kraus, V. Jančárik, Magnetoelastic
properties of CoFeCrSiB amorphous ribbons—A possibility of their
application, J. Electr. Eng. 52 (2001) 205–209.