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Single-shot neutron transmission diffraction
ELSEVIER
Physica B 234-236 (1997) 1160-1162
Single-shot neutron transmission diffraction Kay Meggers a, Hans Georg Priesmeyer b'*, Meinhard Stalder b, Sven Vogel b, Walter Trela c aSiemens AG, Miinchen, Germany blnstitut fiir Reine und Angewandte Kernphysik, Universitiit Kiel, Germany "Los Alamos National Laboratory, Los Alamos NM, USA
Abstract Low-energy neutron transmission diffraction has been successfully demonstrated to be a powerful experimental method to investigate changes in crystal structure development down to the millisecond range. Bragg edge diffraction measurements thus can be the basis for future experiments like strain radiography, determination of the strain-rate dependent elastic properties of engineering materials, temporal development of phase changes, structural properties of materials under extreme environmental conditions like high pressure or temperature states. At high instantaneous neutron bursts current-mode detection can be used, which enables both transient and stroboscopic measurements. Progress has been made in rapid data acquisiton and analysis as well as temperature measurement by resonance doping. Keywords: Pulsed neutrons; Transmission; Diffraction; Phase transitions; Residual stress
1. Neutron transmission diffraction Low-energy coherent neutron scattering from polycrystalline materials can be performed with highest resolution in neutron time-of-flight transmission geometry with the detectors in forward direction. Bragg edges are observed, which represent the discontinuity in the scattering cross-section, whenever certain lattice planes are excluded from scattering. At LANSCE in Los Alamos, crystal structural properties have been measured by every single neutron pulse. Since each pulse contains the full thermalized neutron spectrum, a large number of Bragg reflections is observed simultaneously. Real-time investigations of changes in material properties influencing position, width and height of the Bragg edges can be observed with a time resolution between 100 las and several ms. The
* Corresponding author.
measured spectra must be divided by the open beam spectrum in order to calculate transmissions and calibrated by measuring phase volume fractions in specimens using conventional neutron powder diffraction. Early experiences with this new method were reported in Refs. [1-3]. Progress has been made concerning single-shot measurements. A transient recorder capable to register successive single-shots over minutes has been designed and used to investigate thermal expansion and phase changes in steel, together with a simultaneous temperature measurement by the nuclear Doppler broadening of the 4.9 eV neutron resonance in a gold foil attached to the specimen.
2. The precision of Bragg-edge parameters For residual stress assessment positions of lattice spacings must be determined to better than 10 -4 . Residual stress states develop as a very complicated
0921-4526/97/$17.00 © 1997 ElsevierScienceB.V. All rights reserved PII S092 1-4526(97)00 1 55-5
K. Meggers et al. / Physica B 234-236 (1997) 1160-1162
1161
12000
(200) (211)
10000
(220) (310)
8000 (321) t-
(222)
6000
4000 . ~
bcc-structure single shot spectrum at 10 m source-detector distance 2000
2400
2800
3200
3600
t i m e - o f - f l i g h t channel Fig. 1. Typical current-mode detector output of c~-iron transmission collected within 7 ms.
Typical austenite-bainite isothermal transformation time development (globulitic grey iron at 350 C) 120
4
A
.o
' 80
.... ~ ............. ~..''''"
mean fraction sum: 99.8± 3.4%
. 0 . . E]_ El: .[~.i.-.- -_-_- O_-_~_O . . . . Bainite fraction
N--
E > O)
40
o3 ra.
;
~1~.-"
•
Austenite fraction
0 102
103
104
Transformation time (sec) Fig. 2. Isothermal phase transformation at 350°C in GGG.
10s
1162
K. Meggers et al. / Physica B 234-236 (1997) 1160-1162
highly coupled thermal-metallurgical-mechanical process during welding, casting or forging or as a result of other manufacturing processes. Recent research on the development of residual stresses includes their calculation, for which certain material constants must be known. These include the time-resolved buildup of incompatible phases as well as the interrelation between phase transformation kinetics and internal stress. The height of the Bragg edges yields information on phase volume fractions [4], but it is also important to investigate the precision limits of the method concerning the position and shape of Bragg-edges, in order to be able to measure macro- and microstrain in single shot mode. Fig. 2 shows the results of a diffusioncontrolled phase change from austenite to bainite structure. (This work was done in cooperation with a German car manufacturer, who could reduce the thermal treatment times by about a factor of four.) Bragg-edge positions can be determined to 6d/d ~< 6 x 10 -s, as could be shown by thermal expansion experiments. Edge height, shape and position are extracted from the transmission spectra. The shape of Bragg edges is generally a convolution of the contributions of the natural and mosaic spread of diffraction peaks, the resolution function and strain gradient influences. An evaluation procedure similar to conventional Rietveld refinement is being developed. The knowledge of the resolution function is needed for the fit.
3. Single-pulse transmission capability of the measurement of nuclear Doppler broadening In this experiment, resonance Doppler broadening [5] was extended to current mode detection. Many of the possible applications of single pulse transmission (e.g. solid state phase changes, devel-
opment of transformation stresses, thermal stresses during cooling after welding) require the knowledge of the instantaneous temperature of the specimen. A series of experiments has been made on ferritic steel using the resonance-doping method, where minute amounts of gold (0.03 at%) were measured together with the specimen. Both resonance Doppler broadening and Bragg edge shift depend on the temperature of the specimen. While the position of the Bragg edges is influenced by the combined effects of thermal lattice expansion and strain, a resonance is only subjected to thermal Doppler broadening. The principle feasibility of the method has been demonstrated in the range from RT to ll00°C, but an accuracy 1% ~< AT/T <<.2% was only reached after 3 h of measuring time. Bragg edges may vary in height for two reasons: the Debye-Waller factor will decrease the diffracted intensity with increasing temperature, and diffusion-controlled phase changes may reduce the volume fraction of a certain phase. For 600°C in steel the intensity reduction can be fully attributed to the Debye-Waller factor.
References I-1] H.G. Priesmeyer, C.D. Bowman, V.W. Yuan, S.J. Seestrom, S.A. Wender, R. Richardson, O.A. Wasson, X. Xzu and P.A. Egelstaff, 'Fast transient neutron diffraction at LANSCE, Los Alamos Memo (1989) unpublished. 1-2] C.D. Bowman, P.A. Egelstaff and H.G. Priesmeyer, ICANS-XI KEK Report 90-25 (1991), AMRD Vol. II, eds. M. Misawa, M. Furusaka, H. Ikeda and N. Watanabe. [-3] H.G. Priesmeyer, J. Larsen and K. Meggers, J. Neutron Res. 2 (1994) 31. 1-4] K. Meggers, H.G. Priesmeyer, W. Trela and M. Dahms, Mater. Sci. Eng. A 188 (1994) 301. 15] P.H. Fowler and A.D. Taylor, LA 11393-C Conf. (UC 414) p. 46.
Physica B 234-236 (1997) 1160-1162
Single-shot neutron transmission diffraction Kay Meggers a, Hans Georg Priesmeyer b'*, Meinhard Stalder b, Sven Vogel b, Walter Trela c aSiemens AG, Miinchen, Germany blnstitut fiir Reine und Angewandte Kernphysik, Universitiit Kiel, Germany "Los Alamos National Laboratory, Los Alamos NM, USA
Abstract Low-energy neutron transmission diffraction has been successfully demonstrated to be a powerful experimental method to investigate changes in crystal structure development down to the millisecond range. Bragg edge diffraction measurements thus can be the basis for future experiments like strain radiography, determination of the strain-rate dependent elastic properties of engineering materials, temporal development of phase changes, structural properties of materials under extreme environmental conditions like high pressure or temperature states. At high instantaneous neutron bursts current-mode detection can be used, which enables both transient and stroboscopic measurements. Progress has been made in rapid data acquisiton and analysis as well as temperature measurement by resonance doping. Keywords: Pulsed neutrons; Transmission; Diffraction; Phase transitions; Residual stress
1. Neutron transmission diffraction Low-energy coherent neutron scattering from polycrystalline materials can be performed with highest resolution in neutron time-of-flight transmission geometry with the detectors in forward direction. Bragg edges are observed, which represent the discontinuity in the scattering cross-section, whenever certain lattice planes are excluded from scattering. At LANSCE in Los Alamos, crystal structural properties have been measured by every single neutron pulse. Since each pulse contains the full thermalized neutron spectrum, a large number of Bragg reflections is observed simultaneously. Real-time investigations of changes in material properties influencing position, width and height of the Bragg edges can be observed with a time resolution between 100 las and several ms. The
* Corresponding author.
measured spectra must be divided by the open beam spectrum in order to calculate transmissions and calibrated by measuring phase volume fractions in specimens using conventional neutron powder diffraction. Early experiences with this new method were reported in Refs. [1-3]. Progress has been made concerning single-shot measurements. A transient recorder capable to register successive single-shots over minutes has been designed and used to investigate thermal expansion and phase changes in steel, together with a simultaneous temperature measurement by the nuclear Doppler broadening of the 4.9 eV neutron resonance in a gold foil attached to the specimen.
2. The precision of Bragg-edge parameters For residual stress assessment positions of lattice spacings must be determined to better than 10 -4 . Residual stress states develop as a very complicated
0921-4526/97/$17.00 © 1997 ElsevierScienceB.V. All rights reserved PII S092 1-4526(97)00 1 55-5
K. Meggers et al. / Physica B 234-236 (1997) 1160-1162
1161
12000
(200) (211)
10000
(220) (310)
8000 (321) t-
(222)
6000
4000 . ~
bcc-structure single shot spectrum at 10 m source-detector distance 2000
2400
2800
3200
3600
t i m e - o f - f l i g h t channel Fig. 1. Typical current-mode detector output of c~-iron transmission collected within 7 ms.
Typical austenite-bainite isothermal transformation time development (globulitic grey iron at 350 C) 120
4
A
.o
' 80
.... ~ ............. ~..''''"
mean fraction sum: 99.8± 3.4%
. 0 . . E]_ El: .[~.i.-.- -_-_- O_-_~_O . . . . Bainite fraction
N--
E > O)
40
o3 ra.
;
~1~.-"
•
Austenite fraction
0 102
103
104
Transformation time (sec) Fig. 2. Isothermal phase transformation at 350°C in GGG.
10s
1162
K. Meggers et al. / Physica B 234-236 (1997) 1160-1162
highly coupled thermal-metallurgical-mechanical process during welding, casting or forging or as a result of other manufacturing processes. Recent research on the development of residual stresses includes their calculation, for which certain material constants must be known. These include the time-resolved buildup of incompatible phases as well as the interrelation between phase transformation kinetics and internal stress. The height of the Bragg edges yields information on phase volume fractions [4], but it is also important to investigate the precision limits of the method concerning the position and shape of Bragg-edges, in order to be able to measure macro- and microstrain in single shot mode. Fig. 2 shows the results of a diffusioncontrolled phase change from austenite to bainite structure. (This work was done in cooperation with a German car manufacturer, who could reduce the thermal treatment times by about a factor of four.) Bragg-edge positions can be determined to 6d/d ~< 6 x 10 -s, as could be shown by thermal expansion experiments. Edge height, shape and position are extracted from the transmission spectra. The shape of Bragg edges is generally a convolution of the contributions of the natural and mosaic spread of diffraction peaks, the resolution function and strain gradient influences. An evaluation procedure similar to conventional Rietveld refinement is being developed. The knowledge of the resolution function is needed for the fit.
3. Single-pulse transmission capability of the measurement of nuclear Doppler broadening In this experiment, resonance Doppler broadening [5] was extended to current mode detection. Many of the possible applications of single pulse transmission (e.g. solid state phase changes, devel-
opment of transformation stresses, thermal stresses during cooling after welding) require the knowledge of the instantaneous temperature of the specimen. A series of experiments has been made on ferritic steel using the resonance-doping method, where minute amounts of gold (0.03 at%) were measured together with the specimen. Both resonance Doppler broadening and Bragg edge shift depend on the temperature of the specimen. While the position of the Bragg edges is influenced by the combined effects of thermal lattice expansion and strain, a resonance is only subjected to thermal Doppler broadening. The principle feasibility of the method has been demonstrated in the range from RT to ll00°C, but an accuracy 1% ~< AT/T <<.2% was only reached after 3 h of measuring time. Bragg edges may vary in height for two reasons: the Debye-Waller factor will decrease the diffracted intensity with increasing temperature, and diffusion-controlled phase changes may reduce the volume fraction of a certain phase. For 600°C in steel the intensity reduction can be fully attributed to the Debye-Waller factor.
References I-1] H.G. Priesmeyer, C.D. Bowman, V.W. Yuan, S.J. Seestrom, S.A. Wender, R. Richardson, O.A. Wasson, X. Xzu and P.A. Egelstaff, 'Fast transient neutron diffraction at LANSCE, Los Alamos Memo (1989) unpublished. 1-2] C.D. Bowman, P.A. Egelstaff and H.G. Priesmeyer, ICANS-XI KEK Report 90-25 (1991), AMRD Vol. II, eds. M. Misawa, M. Furusaka, H. Ikeda and N. Watanabe. [-3] H.G. Priesmeyer, J. Larsen and K. Meggers, J. Neutron Res. 2 (1994) 31. 1-4] K. Meggers, H.G. Priesmeyer, W. Trela and M. Dahms, Mater. Sci. Eng. A 188 (1994) 301. 15] P.H. Fowler and A.D. Taylor, LA 11393-C Conf. (UC 414) p. 46.