- Email: [email protected]
Effect of the demagnetisation factor on the Barkhausen noise signal
N.[--[
~i
Journalof
- ~ _ " magnetism
and magnetic maleriais
ELSEVIER
Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
Effect of the demagnetisation factor on the Barkhausen noise signal D.K. Bhattacharya a,*, S. Vaidyanathan b a National Metallurgical Laboratory., Jamshedpur 831007, India b lndira Gandhi Centre for Atomic Research, Kalpakkam, India Received 22 April 1996
Abstract In the presence of a demagnetisation factor, a splitting of the magnetic Barkhausen noise (MBN) signal distribution was observed in the form of two peaks during a slowly varying magnetisation sweep through specimens of an alloy steel. On the other hand, for closed field magnetisation, a single peak was observed at the coercivity point. The demagnetisation factor was introduced by solenoid coil magnetisation and by providing air gaps in specimens magnetised by a 'U'-shaped yoke. The results show the importance of a standard mode of magnetisation. The splitting is explained in terms of the unimpeded growth of nucleated domains in the early period of the magnetic sweep. Keywords: Barkhausen noise; Demagnetisation factor
1. Introduction The magnetic Barkhausen noise (MBN) signal generated by discontinuous changes in the magnetic moment when a varying magnetic field is swept over a ferromagnetic material, has been investigated during the last several decades and found to be sensitive to characteristics of the microstructure and residual stress [1]. During the magnetic sweep, apart from MBN, an acoustic signal called the acoustic Barkhausen noise (ABN) is generated due to localised elastic distortions. This signal has also been found to be important for the characterization of the microstructure and residual stress [1].
* Corresponding author. Fax: +91-657-42-6527.
In the course of the development of a system for Barkhausen noise signal acquisition and analysis [2], a survey was made of the designs that have been used by various investigators. It was found that two types of magnetization scheme have mainly been used, namely (a) a 'U'-shaped electromagnet with the specimen volume under investigation placed between the pole pieces of the electromagnet, and (b) a solenoid coil. The first scheme, which approximates a closed magnetic field, is shown schematically in Fig. 1. This scheme is preferred, presumably because this design facilitates practical applications on a large component. In Ref. [3] the use of a yoke was justified because of the lower demagnetisation factor. Refs. [3-6] are only a partial list of the reported investigations in which 'U'-shaped electromagnets have been used. On the other hand, although less in
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S 0 3 0 4 - 8 85 3 ( 9 6 ) 0 0 4 4 4 - 1
112
D.K. Bhattacharya, S. Vaidyanathan / Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
PROCESSING AND DISPLAY
Fig. 1. Block diagram of MBN acquisition and analysis setup using a yoke.
number, the use of a solenoid coil has been reported in investigations from several groups [7-9]. There is thus an absence of a standardised scheme. In the case of magnetic hysteresis loop generation, the requirement on the magnetisation scheme is standard. A closed magnetic field is a prerequisite for the generation of the correct loop. Otherwise, if there is a demagnetisation factor, there would be a tilting of the loop [10,11]. With the exception of the coercivity, the other parameters of the hysteresis loop would be affected. Therefore, toroids are used with a magnetising coil wound around it, rather than a cylinder placed within a solenoid coil. In contrast, there seems to be no standard requirement on the magnetisation scheme and no assessment as to whether there would be any effect on the MBN signals if the magnetisation is induced by a 'U'-shaped electromagnet, in which the demagnetization factor may be insignificant, or by a solenoid coil.
2. Domain activities during a magnetisation sweep Fig. 2 shows a B - H loop in which the different magnetic processes that take place during a magnetic sweep between Hmax and -Hma x are indicated. Of the various magnetic phenomena indicated, the following processes have been reported to give rise to Barkhausen noise signals: Domain nucleation: 180° domain wall motion: 90 ° domain wall motion: Domain annihilation:
MBN [7-9] ABN [8,9] MBN [3-7] ABN [7,9] ABN [7,12].
It can be seen that the conclusions are influenced by the location of the hysteresis loop at which the large intensities of the signal are observed. When the maximum intensity of MBN signals is observed at or near the coercivity point, discussion in terms of domain wall motion is logical. On the other hand, when large intensity signals are observed in the knee region (corresponding to large values of H ) of the hysteresis loops with zero or less intensity at the coercivity point, discussion in terms of domain nucleation is logical. To determine if the mode of magnetisation affects the pattern of the MBN signal, three experiments were conducted in this work in the following manner. In the first, the same specimen (10 mm diameter and 100 mm long) of 2.25Cr-lMo steel in an austenitised and air cooled condition was magnetised by a yoke and a solenoid coil. The detailed scheme of the magnetic sweep system, amplification of MBN signal and display is given in Ref. [2]. Fig. 3a shows the B - H hysteresis loop and the rms voltage plot of the MBN signal in the case of magnetisation by the yoke with the end faces of the specimens in close contact with the pole faces of the electromagnet. Fig. 3b shows the same plots when the magnetisation was performed using the solenoid coil. As expected, the B - H loop is tilted when the magnetisation is performed by the solenoid coil. The MBN rms voltage plot in this case has two peaks, both at a higher value of H. The signal is minimum at the coercivity point. On the other hand, for yoke magnetisation, the signal has one peak only and the peak is situated at the coercivity point.
(a)
B
O
A
~
(b)
B
H
Fig. 2. (a) Magnetisation through the hysteresis loop; (b) magnetisation process through half hysteresis sweep. U, reversible rotation of magnetic movements; V, domain nucleation; W, irreversible domain wall displacements; X, irreversible domain rotation and annihilation; Y, approach to saturation.
D.K. Bhattacharya, S. Vaidyanathan / Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
(a) SWEEP DIRECTION
Z
9
r"
Q Z
- 32000
2.2T
H + 32000 APPLIED FIELD STRENGTH (A/m)
H
113
positioned at the location of the air gap between the two specimen halves. The gap was changed to vary the extent of demagnetisation. Fig. 4 shows the rms voltage plots as a function of the gap between the specimen halves. It is seen that the MBN rms voltage plot is split in the presence of the air gap. The separation between the two peaks increases and the peak heights decrease as the air gaps increase. In the third experiment, austenitised, oil quenched, and stress relieved (1 h at 923 K) 2.25Cr-lMo
(b) z o
-32000
__ _ o32000 "H'APPLE ID ^FIELDSTRENGTH /~
: / , ml J
4SDV I~E [~o IN ~
Fig. 3. (a) B - H loop and MBN rms voltage plot for a 2 . 2 C r - l M o steel specimen magnetised by a yoke; (b) B - H loop and MBN rms voltage plot for the same specimen as in (a) but magnetised by a solenoid coil.
In the second experiment, the magnetisation was carried out only by the yoke. The specimen was cut into two equal halves, and the MBN signal was acquired keeping the two pieces between the pole pieces of the 'U'-shaped electromagnet yoke. The encircling probe for sensing the MBN signal was
~~ ~ ,= o, --'=
0.Ts
~
0.15ram
u u ~
mm
z ~
32000
-H
O
oH ° 3 2 0 0 0
APPLIED FIELD STRENGTHAim
Fig. 4. MBN rms voltage plot for a 2.25Cr-lMo steel specimen magnetised by a yoke and having different air gaps.
Fig. 5. (a) Microstructure of 2.25Cr-lMo steel austenitised, oil quenched and stress relief annealed at 923 K for 1 h; (b) microstructure of specimen heat treated as for (a) followed by ageing at 1023 K for 500 h; (c) microstructure of specimen heat treated as for (a) followed by ageing at 1023 K for 1000 h.
D.K. Bhattacharya, S. Vaidyanathan / Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
114-
=~
_
/If
(A/m) /~J
(A/m)
Fig. 6, B - H loop and MBN rms voltage plot for specimen with microstructure as in Fig. 5a.
300 ~ ~a
SWEEP x ~ DIRECTION /?,1~200 z ~ /Itp,~
t
i
-20000-10000
cI)
i
_ _ lO00h 500h
O
~
J
10000 20000 HAGNETICFIELD STRENGTH(A/m)
Fig. 7. MBN rms voltage plots for specimens with microstructures as in Fig. 5b,c.
specimens (Fig. 5a shows the microstructure) were aged at 1023 K for 500 h (Fig. 5b) and 1000 h (Fig. 5c). MBN rms voltage plots were obtained for these three microstructural conditions. Fig. 6 shows the B - H loop and the MBN rms voltage plot for the first microstructural condition. Fig. 7 shows the MBN rms voltage plots for the remaining two microstructural conditions. Fig. 6 shows one peak, the position of which coincides with the coercivity point. Fig. 7 shows two peaks. Although the B - H loop is not shown (for clarity), it was found that the two peaks correspond to the knee regions of the B - H loop with the minimum occurring at the coercivity points.
3. Discussion From examination of the information published in the literature, and the results of the experiments reported in this paper, it is seen that the pattern of the rms voltage plot of MBN during the sweep of a ferromagnetic steel by a varying magnetic field depends on the mode of magnetization. The use of a solenoid coil, or the introduction of an air gap in the
specimen, gives rise to two peaks corresponding to the knee regions of the loop (regions of large values of H at which domain nucleation and domain annihilation take place). The signal intensity at the coercivity point (where 180 ° domain wall motion is expected to take place) has been found to be minimum. From the tilting of the B - H loop in Fig. 3b and from the effect of the increasing air gap as shown in Fig. 4 it can be concluded that the demagnetisation factor is responsible for the occurrence of the two peaks. When the demagnetising factor decreases, the two peaks come closer and merge into a single peak at the coercivity point. It is therefore necessary to use a standard mode of magnetisation when MBN analysis is intended for microstructural analysis. Considering the closeness of the magnetic field and practical application, the use of a yoke should be standardised. The use of a solenoid coil should be avoided because, firstly, the signal would depend on the magnitude of the demagnetisation factor and, secondly, a solenoid coil cannot be used in most cases on a component. The splitting of the MBN signal distribution has another important implication. The question arises as to whether the MBN signal is mainly due to the motion of the 180 ° domain wall or is it mainly due to domain nucleation and annihilation? The same question has been asked by Guot and Cagan for the generation of ABN [12], i.e. is ABN generated due to the motion of the non-180 ° domain wall, or due to domain nucleation and annihilation? The question can be put in another manner: is the hysteresis loss contributed mainly by domain nucleation and annihilation? In Ref. [12], the presence of the demagnetisation factor split the ABN signal pattern. Otherwise, the ABN signal distribution had one peak only. An important point to consider here is that mere domain nucleation would not give rise to significant ABN or MBN. The total domain surface involved should be large in order to make the Barkhausen noise signal significant. A possible scenario could be that the nucleated domains grow fast if conditions are such that impediments to domain growth are less. To assess this possibility a third experiment was carried out. In this experiment, MBN signal distributions were compared between specimens having two types of microstructure: the first having a stress relieved martensitic microstructure and the second an
D.K. Bhattacharya, S. Vaidyanathan / Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
overaged martensite having large grains and dispersed small sized carbides with large interparticle distances. The purpose was to have impediments to domain wall motion after domains are nucleated in the first case, and to have fewer impediments for domain growth after nucleation in the second case. As seen from Fig. 5a the MBN signal distribution for the first case has one peak near the coercivity point. In Fig. 5b, splitting of the signal is observed. The differences in the signal distribution in the two figures can be explained as follows. In the first case, after domain nucleation the domains cannot grow until the applied magnetic field strength is reversed because of the presence of impediments to domain wall motion in the form of a complex dislocation network. Dislocations are two-dimensional defects that are largely responsible for the mechanical behaviour of materials. In an investigation by electron microscopy using Lorentz polarisation [13], dislocation tangles were found to be effective pinning points against the motion of domain walls. In the second case, the domains after nucleation at the grain boundaries can grow easily because the impediments to domain wall motion in the form of dispersed carbides is less. This rapid growth of domains gives rise to the right-hand peak. Once grown domains cover the grains, no significant changes in the domain patterns should take place until and unless sufficient magnetic field strength is present in the reverse sense for annihilation to take place. During domain annihilation, growth of some domains takes place at the expense of others. This would involve the motion of both 180° and 90 ° domain walls, and domain rotation. These processes would lead to both MBN and ABN. The splitting of the signal distribution during solenoid magnetisation can be explained in a similar manner. During the magnetic sweep, growth of the nucleated domains takes place when the magnetic force behind the growth is large. The possibility that the reverse magnetic forces can be higher at the surface than in the interior in the presence of a demagnetisation factor and while a magnetic sweep is imposed has been reported in Ref. [14]. This means that the change in the magnetic condition takes place in the surface layers of a specimen in advance of the interior. In such a case, the growth of nucleated domains is enhanced giving rise to the first peak. Once the domains grow and
115
cover the grain volume, domain dynamics is minimal until the domains are annihilated.
4. Conclusion It has been observed that a demagnetisation factor gives a pattern of magnetic Barkhausen noise (MBN) signal different from cases where the demagnetisation factor is insignificant. The demagnetisation factor may be introduced when magnetisation is performed by a solenoid coil. It may also be introduced by an air gap in the specimen when the magnetisation is performed by a yoke. In the presence of a demagnetisation factor two peaks are observed. The first peak is associated with domain nucleation and the second with domain wall annihilation. The minimum occurring at the coercivity point corresponds to domain wall displacement without important domain surface variations. In the absence of a demagnetisation factor the two peaks merge to give a single peak at the location of the coercivity point. Two peaks are also observed when there are fewer impediments to domain growth in the microstructure (larger grain size, less precipitate number density, etc.). A single peak is observed when impediments are significant. Three major conclusions can thus be drawn. (a) When MBN analysis is used for the characterisation of microstructure and residual stress, a standard scheme for magnetisation using a yoke should be used. In this case, the magnetisation would involve an insignificant demagnetisation factor. The use of a standard mode of magnetisation would make the comparison of data from different laboratories more meaningful. Secondly, yoke magnetization is a practical proposition. (b) In practice, splitting of the signal distribution pattern in the presence of an air gap can be taken as an indication of the presence of either a surface breaking crack, or large grain growth, or dispersion of secondary phase precipitates, etc. All these features indicate degradation in a material. Once such an indication is observed, the region can be investigated by other techniques such as in-situ metallography and magnetic particle inspection to determine the exact nature of the degradation. (c) Investigation of the domain pattern by the
116
D.K. Bhattachatya, S. Vaidyanathan / Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
bitter technique, in the presence or absence of a demagnetisation factor, should give an indication as to whether the domain activities are different in the two cases. In other words, whether a single peak can be taken as equivalent to the merger of two peaks.
References [1] D.K. Bhattacharya, Trans. Indian Inst. Met. 48 (1995) 495. [2] D.K. Bhattacharya, Ph.D. Thesis, Indian Institute of Science, Bangalore, 1995. [3] W.A. Theiner, B. Reimringer, P. Deimel, D. Kuppler and N. Schroeder-Obst, Nucl. Eng. Des. 76 (1983) 251. [4] R. Ranjan and D.C. Jiles, IEEE Trans. Magn. 23 (1981) 1869.
[5] L. Clapham, C. Jagadish and D.L. Atherton, Acta Metall. Mater. 39 (1991) 1555. [6] M. Dubois and M. Fiset, Mater. Sci. Technol. 11 (1995) 264. [7] D.J. Buttle, G.A.D. Briggs, J.P. Jakubovics, E.A. Little and C.B. Scruby, Philos. Trans. R. Soc. London A 320 (1986) 363. [8] J. Kameda and R. Ranjan, Acta Metall. 35 (1987) 1515. [9] R. Hill, R.S. Geng, A. Cowking and J.W. Mackersie, NDT&E Int. 24 (1991) 179. [10] B.D. Cullity, Introduction to Magnetic Materials (Addison Wesley, Reading, MA, 1972). [11] S. Chikazumi, Physics of Magnetism (Wiley, New York, 1964). [12] M. Guot and V. Cagan, J. Magn. Magn. Mater. 101 (1991) 256. [13] AJ. Birkett, W.D. Corner, B.K. Tanner and S.M. Thompson, J. Phys. D 22 (1989) 1240. [14] N.N. Zatsepin, A.V. Chernyshev and N.O. Gusak, Defektoskopyia (USSR) 2 (1980) 16.
~i
Journalof
- ~ _ " magnetism
and magnetic maleriais
ELSEVIER
Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
Effect of the demagnetisation factor on the Barkhausen noise signal D.K. Bhattacharya a,*, S. Vaidyanathan b a National Metallurgical Laboratory., Jamshedpur 831007, India b lndira Gandhi Centre for Atomic Research, Kalpakkam, India Received 22 April 1996
Abstract In the presence of a demagnetisation factor, a splitting of the magnetic Barkhausen noise (MBN) signal distribution was observed in the form of two peaks during a slowly varying magnetisation sweep through specimens of an alloy steel. On the other hand, for closed field magnetisation, a single peak was observed at the coercivity point. The demagnetisation factor was introduced by solenoid coil magnetisation and by providing air gaps in specimens magnetised by a 'U'-shaped yoke. The results show the importance of a standard mode of magnetisation. The splitting is explained in terms of the unimpeded growth of nucleated domains in the early period of the magnetic sweep. Keywords: Barkhausen noise; Demagnetisation factor
1. Introduction The magnetic Barkhausen noise (MBN) signal generated by discontinuous changes in the magnetic moment when a varying magnetic field is swept over a ferromagnetic material, has been investigated during the last several decades and found to be sensitive to characteristics of the microstructure and residual stress [1]. During the magnetic sweep, apart from MBN, an acoustic signal called the acoustic Barkhausen noise (ABN) is generated due to localised elastic distortions. This signal has also been found to be important for the characterization of the microstructure and residual stress [1].
* Corresponding author. Fax: +91-657-42-6527.
In the course of the development of a system for Barkhausen noise signal acquisition and analysis [2], a survey was made of the designs that have been used by various investigators. It was found that two types of magnetization scheme have mainly been used, namely (a) a 'U'-shaped electromagnet with the specimen volume under investigation placed between the pole pieces of the electromagnet, and (b) a solenoid coil. The first scheme, which approximates a closed magnetic field, is shown schematically in Fig. 1. This scheme is preferred, presumably because this design facilitates practical applications on a large component. In Ref. [3] the use of a yoke was justified because of the lower demagnetisation factor. Refs. [3-6] are only a partial list of the reported investigations in which 'U'-shaped electromagnets have been used. On the other hand, although less in
0304-8853/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S 0 3 0 4 - 8 85 3 ( 9 6 ) 0 0 4 4 4 - 1
112
D.K. Bhattacharya, S. Vaidyanathan / Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
PROCESSING AND DISPLAY
Fig. 1. Block diagram of MBN acquisition and analysis setup using a yoke.
number, the use of a solenoid coil has been reported in investigations from several groups [7-9]. There is thus an absence of a standardised scheme. In the case of magnetic hysteresis loop generation, the requirement on the magnetisation scheme is standard. A closed magnetic field is a prerequisite for the generation of the correct loop. Otherwise, if there is a demagnetisation factor, there would be a tilting of the loop [10,11]. With the exception of the coercivity, the other parameters of the hysteresis loop would be affected. Therefore, toroids are used with a magnetising coil wound around it, rather than a cylinder placed within a solenoid coil. In contrast, there seems to be no standard requirement on the magnetisation scheme and no assessment as to whether there would be any effect on the MBN signals if the magnetisation is induced by a 'U'-shaped electromagnet, in which the demagnetization factor may be insignificant, or by a solenoid coil.
2. Domain activities during a magnetisation sweep Fig. 2 shows a B - H loop in which the different magnetic processes that take place during a magnetic sweep between Hmax and -Hma x are indicated. Of the various magnetic phenomena indicated, the following processes have been reported to give rise to Barkhausen noise signals: Domain nucleation: 180° domain wall motion: 90 ° domain wall motion: Domain annihilation:
MBN [7-9] ABN [8,9] MBN [3-7] ABN [7,9] ABN [7,12].
It can be seen that the conclusions are influenced by the location of the hysteresis loop at which the large intensities of the signal are observed. When the maximum intensity of MBN signals is observed at or near the coercivity point, discussion in terms of domain wall motion is logical. On the other hand, when large intensity signals are observed in the knee region (corresponding to large values of H ) of the hysteresis loops with zero or less intensity at the coercivity point, discussion in terms of domain nucleation is logical. To determine if the mode of magnetisation affects the pattern of the MBN signal, three experiments were conducted in this work in the following manner. In the first, the same specimen (10 mm diameter and 100 mm long) of 2.25Cr-lMo steel in an austenitised and air cooled condition was magnetised by a yoke and a solenoid coil. The detailed scheme of the magnetic sweep system, amplification of MBN signal and display is given in Ref. [2]. Fig. 3a shows the B - H hysteresis loop and the rms voltage plot of the MBN signal in the case of magnetisation by the yoke with the end faces of the specimens in close contact with the pole faces of the electromagnet. Fig. 3b shows the same plots when the magnetisation was performed using the solenoid coil. As expected, the B - H loop is tilted when the magnetisation is performed by the solenoid coil. The MBN rms voltage plot in this case has two peaks, both at a higher value of H. The signal is minimum at the coercivity point. On the other hand, for yoke magnetisation, the signal has one peak only and the peak is situated at the coercivity point.
(a)
B
O
A
~
(b)
B
H
Fig. 2. (a) Magnetisation through the hysteresis loop; (b) magnetisation process through half hysteresis sweep. U, reversible rotation of magnetic movements; V, domain nucleation; W, irreversible domain wall displacements; X, irreversible domain rotation and annihilation; Y, approach to saturation.
D.K. Bhattacharya, S. Vaidyanathan / Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
(a) SWEEP DIRECTION
Z
9
r"
Q Z
- 32000
2.2T
H + 32000 APPLIED FIELD STRENGTH (A/m)
H
113
positioned at the location of the air gap between the two specimen halves. The gap was changed to vary the extent of demagnetisation. Fig. 4 shows the rms voltage plots as a function of the gap between the specimen halves. It is seen that the MBN rms voltage plot is split in the presence of the air gap. The separation between the two peaks increases and the peak heights decrease as the air gaps increase. In the third experiment, austenitised, oil quenched, and stress relieved (1 h at 923 K) 2.25Cr-lMo
(b) z o
-32000
__ _ o32000 "H'APPLE ID ^FIELDSTRENGTH /~
: / , ml J
4SDV I~E [~o IN ~
Fig. 3. (a) B - H loop and MBN rms voltage plot for a 2 . 2 C r - l M o steel specimen magnetised by a yoke; (b) B - H loop and MBN rms voltage plot for the same specimen as in (a) but magnetised by a solenoid coil.
In the second experiment, the magnetisation was carried out only by the yoke. The specimen was cut into two equal halves, and the MBN signal was acquired keeping the two pieces between the pole pieces of the 'U'-shaped electromagnet yoke. The encircling probe for sensing the MBN signal was
~~ ~ ,= o, --'=
0.Ts
~
0.15ram
u u ~
mm
z ~
32000
-H
O
oH ° 3 2 0 0 0
APPLIED FIELD STRENGTHAim
Fig. 4. MBN rms voltage plot for a 2.25Cr-lMo steel specimen magnetised by a yoke and having different air gaps.
Fig. 5. (a) Microstructure of 2.25Cr-lMo steel austenitised, oil quenched and stress relief annealed at 923 K for 1 h; (b) microstructure of specimen heat treated as for (a) followed by ageing at 1023 K for 500 h; (c) microstructure of specimen heat treated as for (a) followed by ageing at 1023 K for 1000 h.
D.K. Bhattacharya, S. Vaidyanathan / Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
114-
=~
_
/If
(A/m) /~J
(A/m)
Fig. 6, B - H loop and MBN rms voltage plot for specimen with microstructure as in Fig. 5a.
300 ~ ~a
SWEEP x ~ DIRECTION /?,1~200 z ~ /Itp,~
t
i
-20000-10000
cI)
i
_ _ lO00h 500h
O
~
J
10000 20000 HAGNETICFIELD STRENGTH(A/m)
Fig. 7. MBN rms voltage plots for specimens with microstructures as in Fig. 5b,c.
specimens (Fig. 5a shows the microstructure) were aged at 1023 K for 500 h (Fig. 5b) and 1000 h (Fig. 5c). MBN rms voltage plots were obtained for these three microstructural conditions. Fig. 6 shows the B - H loop and the MBN rms voltage plot for the first microstructural condition. Fig. 7 shows the MBN rms voltage plots for the remaining two microstructural conditions. Fig. 6 shows one peak, the position of which coincides with the coercivity point. Fig. 7 shows two peaks. Although the B - H loop is not shown (for clarity), it was found that the two peaks correspond to the knee regions of the B - H loop with the minimum occurring at the coercivity points.
3. Discussion From examination of the information published in the literature, and the results of the experiments reported in this paper, it is seen that the pattern of the rms voltage plot of MBN during the sweep of a ferromagnetic steel by a varying magnetic field depends on the mode of magnetization. The use of a solenoid coil, or the introduction of an air gap in the
specimen, gives rise to two peaks corresponding to the knee regions of the loop (regions of large values of H at which domain nucleation and domain annihilation take place). The signal intensity at the coercivity point (where 180 ° domain wall motion is expected to take place) has been found to be minimum. From the tilting of the B - H loop in Fig. 3b and from the effect of the increasing air gap as shown in Fig. 4 it can be concluded that the demagnetisation factor is responsible for the occurrence of the two peaks. When the demagnetising factor decreases, the two peaks come closer and merge into a single peak at the coercivity point. It is therefore necessary to use a standard mode of magnetisation when MBN analysis is intended for microstructural analysis. Considering the closeness of the magnetic field and practical application, the use of a yoke should be standardised. The use of a solenoid coil should be avoided because, firstly, the signal would depend on the magnitude of the demagnetisation factor and, secondly, a solenoid coil cannot be used in most cases on a component. The splitting of the MBN signal distribution has another important implication. The question arises as to whether the MBN signal is mainly due to the motion of the 180 ° domain wall or is it mainly due to domain nucleation and annihilation? The same question has been asked by Guot and Cagan for the generation of ABN [12], i.e. is ABN generated due to the motion of the non-180 ° domain wall, or due to domain nucleation and annihilation? The question can be put in another manner: is the hysteresis loss contributed mainly by domain nucleation and annihilation? In Ref. [12], the presence of the demagnetisation factor split the ABN signal pattern. Otherwise, the ABN signal distribution had one peak only. An important point to consider here is that mere domain nucleation would not give rise to significant ABN or MBN. The total domain surface involved should be large in order to make the Barkhausen noise signal significant. A possible scenario could be that the nucleated domains grow fast if conditions are such that impediments to domain growth are less. To assess this possibility a third experiment was carried out. In this experiment, MBN signal distributions were compared between specimens having two types of microstructure: the first having a stress relieved martensitic microstructure and the second an
D.K. Bhattacharya, S. Vaidyanathan / Journal of Magnetism and Magnetic Materials 166 (1997) 111-116
overaged martensite having large grains and dispersed small sized carbides with large interparticle distances. The purpose was to have impediments to domain wall motion after domains are nucleated in the first case, and to have fewer impediments for domain growth after nucleation in the second case. As seen from Fig. 5a the MBN signal distribution for the first case has one peak near the coercivity point. In Fig. 5b, splitting of the signal is observed. The differences in the signal distribution in the two figures can be explained as follows. In the first case, after domain nucleation the domains cannot grow until the applied magnetic field strength is reversed because of the presence of impediments to domain wall motion in the form of a complex dislocation network. Dislocations are two-dimensional defects that are largely responsible for the mechanical behaviour of materials. In an investigation by electron microscopy using Lorentz polarisation [13], dislocation tangles were found to be effective pinning points against the motion of domain walls. In the second case, the domains after nucleation at the grain boundaries can grow easily because the impediments to domain wall motion in the form of dispersed carbides is less. This rapid growth of domains gives rise to the right-hand peak. Once grown domains cover the grains, no significant changes in the domain patterns should take place until and unless sufficient magnetic field strength is present in the reverse sense for annihilation to take place. During domain annihilation, growth of some domains takes place at the expense of others. This would involve the motion of both 180° and 90 ° domain walls, and domain rotation. These processes would lead to both MBN and ABN. The splitting of the signal distribution during solenoid magnetisation can be explained in a similar manner. During the magnetic sweep, growth of the nucleated domains takes place when the magnetic force behind the growth is large. The possibility that the reverse magnetic forces can be higher at the surface than in the interior in the presence of a demagnetisation factor and while a magnetic sweep is imposed has been reported in Ref. [14]. This means that the change in the magnetic condition takes place in the surface layers of a specimen in advance of the interior. In such a case, the growth of nucleated domains is enhanced giving rise to the first peak. Once the domains grow and
115
cover the grain volume, domain dynamics is minimal until the domains are annihilated.
4. Conclusion It has been observed that a demagnetisation factor gives a pattern of magnetic Barkhausen noise (MBN) signal different from cases where the demagnetisation factor is insignificant. The demagnetisation factor may be introduced when magnetisation is performed by a solenoid coil. It may also be introduced by an air gap in the specimen when the magnetisation is performed by a yoke. In the presence of a demagnetisation factor two peaks are observed. The first peak is associated with domain nucleation and the second with domain wall annihilation. The minimum occurring at the coercivity point corresponds to domain wall displacement without important domain surface variations. In the absence of a demagnetisation factor the two peaks merge to give a single peak at the location of the coercivity point. Two peaks are also observed when there are fewer impediments to domain growth in the microstructure (larger grain size, less precipitate number density, etc.). A single peak is observed when impediments are significant. Three major conclusions can thus be drawn. (a) When MBN analysis is used for the characterisation of microstructure and residual stress, a standard scheme for magnetisation using a yoke should be used. In this case, the magnetisation would involve an insignificant demagnetisation factor. The use of a standard mode of magnetisation would make the comparison of data from different laboratories more meaningful. Secondly, yoke magnetization is a practical proposition. (b) In practice, splitting of the signal distribution pattern in the presence of an air gap can be taken as an indication of the presence of either a surface breaking crack, or large grain growth, or dispersion of secondary phase precipitates, etc. All these features indicate degradation in a material. Once such an indication is observed, the region can be investigated by other techniques such as in-situ metallography and magnetic particle inspection to determine the exact nature of the degradation. (c) Investigation of the domain pattern by the
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bitter technique, in the presence or absence of a demagnetisation factor, should give an indication as to whether the domain activities are different in the two cases. In other words, whether a single peak can be taken as equivalent to the merger of two peaks.
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