in situ experiments with synchrotron high-energy x-rays and neutrons

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DOI: 10.1002/adem.201000297
In Situ Experiments with Synchrotron High-Energy X-Raysand Neutrons**

By Peter Staron*, Torben Fischer, Thomas Lippmann, Andreas Stark, Shahrokh Daneshpour,Dirk Schnubel, Eckart Uhlmann, Robert Gerstenberger, Bettina Camin, Walter Reimers,Elisabeth Eidenberger, Helmut Clemens, Norbert Huber and Andreas Schreyer

High-energy X-rays offer the large penetration depths that are often required for determination ofbulk properties in engineering materials research. Photon energies of 150 keV and more areavailable at synchrotron sources, depending on storage ring and insertion device. In addition,synchrotron sources can offer very high intensities on the sample even at these energies. They can beused not only to obtain high spatial resolution using very small beams, but also high timeresolution in combination with a fast detector. This opens up possibilities for a wide range of in situexperiments. Typical examples that are already widely used are heating or tensile testing in thebeam. However, there are also more challenging in situ experiments in the field of engineeringmaterials research like e.g. dilatometry, differential scanning calorimetry, or cutting. Nevertheless,there are a number of applications where neutron techniques are still favorable and both probes,photons and neutrons, should be regarded as complementary. A number of in situ experiments wererealized at the GKSS synchrotron and neutron beamlines and selected examples are presented in thefollowing.

[*] Dr. P. Staron, T. Fischer, Dr. T. Lippmann, Dr. A. Stark,Dr. S. Daneshpour, D. Schnubel, Prof. N. Huber,Prof. A. SchreyerInstitute of Materials Research GKSS Research Centre MaxPlanck-Str. 121502 Geesthacht, (Germany)E-mail: [email protected]

Prof. E. Uhlmann, R. GerstenbergerInstitute for Machine Tools and Factory Management TUBerlin Pascalstr. 8–910587 Berlin, (Germany)

Dr. B. Camin, Prof. W. ReimersInstitut fur Werkstoffwissenschaften und -technologienMetallische Werkstoffe TU Berlin Ernst Reuter-Platz 1 10587Berlin, (Germany)

Dr. E. Eidenberger, Prof. H. ClemensDepartment of Physical Metallurgy and Materials TestingMontanuniversitat Leoben Franz Josef-Str. 18 8700 Leoben,(Austria)

[**] The authors thank Rene Kirchhof and Hilmar Burmester fromGKSS for technical assistance at the HARWI II beamline. G.Kozik from GKSS is gratefully acknowledged for designing andproviding the in situ SANS furnace and for technical assistanceduring the SANS measurements.

658 wileyonlinelibrary.com � 2011 WILEY-VCH Verlag GmbH & Co

Engineering materials science strives for both, under-

standing the basic principles behind materials properties and

development and optimization of processing steps required

for production. In both fields not only static but also dynamic

properties are of interest. An example is the cold or hot

deformation of materials where not only the degree of

deformation but also the deformation speed and temperature

determines the microstructure and, thus, the properties of the

material. As a consequence, often static material studies do not

give a sufficient answer to crucial questions. Here, ‘‘static

studies’’ means studies that are performed after the material

treatment is finished (‘‘post mortem’’) and the sample, or in

most cases only parts of it, can be prepared for studies like,

e.g., transmission electron microscopy (TEM) observations.

Instead of static studies, the possibility of dynamic studies

would often be more appropriate for answering specific

questions. Among these questions are, e.g., how exactly a

material has evolved from state A to state B, which often cannot

be answered adequately from studies of state A and state B

alone. However, the change of a material from state A to state B,

for example inmetal processing, requires some sort of treatment,

like e.g. high temperatures, or mechanical forces, or a

combination of both. In many cases the process environment

hinders in situ studies because it cannot be combined with the

probe to be used for the study. Nevertheless, a variety of in situ

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P. Staron et al./In Situ Experiments with Synchrotron High-Energy . . .

Fig. 1. Sketch of the dilatometer induction coil in the X-ray beam.

experiments already exists, among which furnaces and

mechanical testing units are probably the most common ones

used in diffractometers or microscopes.

Nevertheless, there are also in situ experiments that do not

yet exist because of the difficulties and costs of the required

instrumentation or beam properties. Additionally, available

spatial and time resolutions are often limiting such studies.With

the advent of 3rd generation synchrotron sources like PETRA III

at DESY, Hamburg, some of these restrictionsmay belong to the

past. Therefore, GKSS has started realizing a number of in situ

experiments, making use of the new opportunities. In the

following, a first experiment at the in situ quenching and

deformation dilatometer with differential scanning dilatometry

(DSC) unit called FlexiTherm will be described. Moreover, first

results of an in situ tensile test and an in situ cutting experiment

will be introduced. Finally, an in situ study of precipitation

kinetics using small-angle neutron scattering (SANS) will be

presented. The synchrotron experiments were performed at the

GKSS beamline HARWI II, which is at the second generation

synchrotron source DORIS III at DESY, Hamburg.[1] In the

future, these experimentswill benefit from the largely improved

brilliance at the new GKSS high-energy materials science

beamline HEMS at PETRA III at DESY, Hamburg.

In Situ Dilatometry

Motivation

g-TiAl-based alloys are promising as high temperature

lightweight materials for turbines and engines at tempera-

tures up to 800 8C because of their excellent strength and low

density (4 g cm�3). Suitable production processes like casting

or forging have to be developed for an application of such

alloys. This development requires a detailed knowledge of the

phases present at the processing temperatures. However,

investigating the constitution at high temperatures is difficult

by microstructural characterization and phase analysis at

room temperature. The reason is that different phase

transformations might have occurred on cooling even if high

cooling rates have been applied. Furthermore, it is also

important to study the kinetics of phase transformations at

temperatures relevant for processing. The latter can be

studied, e.g. by diffraction, by dilatometry, or by calorimetry.

Usually, diffraction is not performed in combination with

these two techniques of thermal analysis. The lack of such

simultaneous measurements, however, made a coherent

interpretation of the results difficult, especially in complex

multiphase materials such as g-TiAl-based alloys.

To overcome these problems, a commercial quenching and

deformation dilatometer with an additional DSC unit was

prepared to be installed in the X-ray beam at the GKSS

beamlines at DESY for simultaneousmeasurement of heat flux

or thermal expansion curves and diffraction patterns.

FlexiTherm

A Bahr 805A/D dilatometer was equipped with windows

for the X-ray beam (Fig. 1). The X-ray beam with a

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cross-section of 1mm� 1mm passes through the heating

coil. At photon energies of 100 keV samples with 4 or 5mm

diameter can be used, which is the standard dimension for the

used dilatometer. The samples can be heated inductively and

quenched by blowing on with water-cooled gas. A maximum

heating rate of 4 000K s�1 and a maximum cooling rate of

2 500K s�1 (employing hollow samples) can be achieved.

Results

Conventional g-TiAl-based alloys consist of tetragonal

g-TiAl (L10 structure; P4/mmm) and small amounts of

hexagonal a2-Ti3Al (D019 structure; P63/mmc). In order to

improve their workability as well as their mechanical

properties various elements can be alloyed. These additional

alloying elements can also promote the formation of ternary

intermetallic phases. However, up to now the exact pathways

of formation and transformation of these phases are not fully

understood up to now.[2]

Here, we present the results of a first test illustrating the

capabilities of the combinedmeasuring techniques. An as-cast

Ti–43Al (in at%) sample was inductively heated in Ar

atmosphere up to 1 200 8C and subsequently cooled with a

rate of 40Kmin�1. The diffraction rings were recorded with a

Mar555 flat panel detector every 2min.

In the beginning of the measurement, only reflections of

a2-Ti3Al occur, as shown in Figure 2. This indicates that the

single-phase high-temperature state was frozen due to a high

cooling rate after casting. At about 800 8C additional

reflections of g-TiAl appear, which can be attributed to the

onset of thermally activated diffusion processes. Thus, the

metastable non-equilibrium a2 decomposes to a2þ g lamellar

colonies.[3] At the same time, the thermal expansion

significantly changes. Obviously, the smaller volume of the

precipitated g compensates for the thermal expansion of the

sample. During further heating, the ordered hexagonal

a2-Ti3Al transforms to disordered hexagonal a-Ti(Al) (A3

structure; P63/mmc), indicated by the vanishing superlattice

reflections. Subsequently, g starts to transform to a and, as a

consequence, the thermal expansion rate increases. This

behavior proceeds reversely during cooling. However, due

to the slow cooling rate, Ti–43Al remains two-phase down to

room temperature.

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Fig. 2. Development of the elongation and diffraction pattern of a Ti–43 at% Al samplewith temperature and time.

These results illustrate the correlation of phase transforma-

tions with changes in the thermal expansion rate. Discontin-

uous volume changes, as observed during decomposition of

the metastable a2-phase, can cause high internal stresses and

lead to damage. However, the results will help designing

suitable heat treatments in order to minimize these stresses.

Another experiment performed with the FlexiTherm measur-

ing equipment is described in this issue.[4]

Fig. 3. Stress field results sy–sx (in MPa) close to the crack tip at an overload of 10.2 kN takeclose to the crack tip. The dots mark the measurement positions.

660 http://www.aem-journal.com � 2011 WILEY-VCH Verlag GmbH & C

In Situ Study on Single Overload of Fatigue-Cracked Specimens

Motivation

Reliable prediction of fatigue life has become a central topic

in the design of aerospace components and assemblies ever

since the 1970 s.[5,6] Since fatigue damage occurs locally at the

crack tip, the knowledge of the stress–strain fields around

the crack tip is essential for the development of reliable life

prediction models. Due to a highly non-linear evolution of

deformation and damage, the local stress–strain field depends

on the previous loading history. As a consequence, fatigue life

predictions are mainly based on a number of simplifications

and assumptions reducing the reliability of the life prediction

method. In this context, X-ray diffraction at a synchrotron

source provides direct insight into the stress–strain fields in

the vicinity of the crack tip, which is essential for a deeper

understanding of the mechanisms of fatigue damage. In

addition, such experiments can be used to quantitatively

investigate load interaction effects during fatigue crack

growth, which is an important problem in the damage

tolerant design and focus of current research.

The occurrence of an overload (high-low load sequence)

can lead to strong fatigue crack growth retardation.[7] To

provide a deeper understanding of the mechanisms of

retardation of the fatigue crack tip due to an overload, stress

fields were determined in specimens containing a fatigue

crack under external load. In conjunction with the finite

element (FE) simulation, such experiments will be used in

subsequent work to enhance existing life prediction methods.

Experimental

Compact-Tension (CT) specimens made of Al alloy

AA6056-T6 sheet material with 6.2mm thickness were used

[Fig. 3(a)]. To investigate load interaction effects on fatigue

crack propagation, different overloads during cyclic loading

n from simulation (top part) and diffraction patterns (bottom part). (a) overview; (b) area

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with a frequency of 10Hz were applied. The maximum cyclic

load was 5.1 kN and the load ratio R was 0.1. Overloads

ranged from 7.65 to 17.15 kN.

Diffraction measurements under static loading condition

were conducted on the CT specimens at the GKSS beamline

HARWI II at DESY using a hydraulic tensile testing machine

(INSTRON 8800) mounted in the beam. In total 160 diffraction

patterns have been measured along the crack path by a biased

area scan within an area of 31mm� 32mm [Figure 3(a)]. The

experiments were performed at a photon energy of 70 keV.

The beam cross-section was 1mm� 1mm to reach a good

lateral resolution. A Mar555 area detector was used to record

several complete diffraction rings.

Results

The small beam cross-section in combination with the

relatively large grain size of the AA6056-T6 sheet lead to noisy

results when using the normal data evaluation procedure

based on only four small sectors of the measured diffraction

rings. To overcome this difficulty, a data evaluation routine

was programmed to fit the strains in the global coordinate

axis directions using the information of the whole diffraction

rings.

In future work our investigations will be extended to

welded specimens. Since welding changes both microstruc-

ture and precipitation state, it also alters locally the stress-free

lattice parameter d0. To avoid long-winded preparation of

stress-relieved reference specimens, it is important to sup-

press the influence of d0 on the stress results. By calculating the

difference of the stress components in y and x directions [see

Fig. 3(a)] the resulting stress field sy–sx is independent of d0. In

view of planned work on welds, this approach was already

applied to the base material specimen used in the present

study.

A 3D FE analysis was performed to simulate crack tip

stress–strain fields in the CT-specimen under the given load.

Considering the symmetries, only a half of the specimen was

modeled. The FEmodel consists of linear hexahedral elements

available in ABAQUS. The analysis was conducted using a

plasticity model with non-linear isotropic hardening and

von-Mises yield criterion.

As an example, Figure 3 shows a comparison between

measured and simulated stress fields sy–sx for an applied load

of 10.2 kN. The dots in Figure 3(b) indicate the actual

measurement positions. The contour color plot was created as

a natural neighbor interpolation based on these points. The

simulation results were extracted in a similar way from the

fine-meshed region as shown in the upper half of Figure 3(a).

In general, a good agreement between simulation and

experimental results is observed, especially for the areas a

bit further away from the crack tip. As expected, Figure 3(b)

reveals high-gradient regions of stresses confined to the crack

tip for both simulation and experiment. A lack of data points

at the vicinity of the crack tip in combination with noisy raw

data limits the capability of capturing stresses at this highly

stressed region. However, the experimental data are essential


for calibration and validation of the simulation approach. To

obtain higher resolution within the region of interest and to

provide the additionally needed strain field, the simulation

results can be used to extrapolate the experimental data near

the crack tip. Furthermore, the 3D simulation provides the

depth-dependent distribution of the stress and strain fields,

which is integrated through the thickness of the specimen by

the synchrotron beam. Such 3D information is very useful to

predict the different crack propagation behaviors at the

specimen surface and inside the specimen. A more detailed

evaluation of the data will be published elsewhere.

In Situ Cutting Experiment

Motivation

The cutting process with a geometrically defined cutting

edge is characterized by a highly complex chip formation

process which is influenced by many factors. Numerous

analytical models have been developed to describe the

material flow through the chip formation zone, enabling the

calculation of mechanical and thermal state variables, such as

strain, stress, and temperature. The validity of these models

relies on a number of restricting preconditions, such as the

assumption of a plane strain state and a homogeneous stress

state within the entire shear plane. These postulations

represent a significant simplification of the general cutting

problem. However, experimental observations are difficult

because of the restricted accessibility to the chip formation

zone, the high process speed, and the non-stationary

conditions of the chip formation process.

Therefore, a first experiment for the evaluation of the

applicability of X-ray diffraction for the study of the cutting

process was performed at the GKSS beamline HARWI II

at DESY.[8] The experimentally determined strains and

stresses were compared with results from a FEM cutting

simulation.

Experiment

The cutting experiments were conducted with orthogonal

kinematics as well as a cutting depth of h¼ 0.3mm and a chip

width of b¼ 1.6mm. The setup was integrated in a tension/

compression test machine. The workpiece moved with a

cutting speed of nc¼ 0.01mmin�1. Since practically used

cutting speeds are significantly higher, the cutting speed

which was applied in this experiment is called quasistatic.

However, a long exposure time of 45 s was required to

increase the signal-to-noise ratio. The steel S235JR was used as

workpiece material because of its stationary chip formation.

Complete diffraction rings were recorded on a Mar345 image

plate detector using a photon wavelength of 100 keV. The

beam cross-section was 100 mm� 100 mm. The setup and

measurement positions are sketched in Figure 4(a).

Results

The strain tensor components e11(hkl), e22(hkl), and e12(hkl)

were calculated from the diffractograms for all reflections and


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Fig. 4. (a) Sketch of the experimental setup for in situ strain measurements duringorthogonal cutting. (b) Effective stresses determined from the measured strain data atthe eight positions indicated in a), compared with simulation results.

measuring positions. The calculation of the stress tensor

components was carried out under the assumption of

both plane strain and plane stress states. FEM cutting

simulations were conducted with the software DEFORM 3D

in order to evaluate the experimental results. The

geometry model of the cutting wedge and the workpiece

as well as the cutting parameters complied with the

experimental conditions. Coulomb friction with m¼ 0.2

was assumed between the workpiece and the rigid cutting

tool. The results show a very good qualitative and, partially,

also quantitative consistency of experimental and simula-

tion data.[8] As an example, the effective stresses after

von-Mises at the eight measurement positions are shown

in Figure 4(b).

The results, however, give evidence that this first experi-

ment suffered from insufficient spatial resolution (beam

cross-section) to capture the high stress gradients. Also the

number of data points was insufficient. It is thus planned to

continue these experiments at the new HEMS beamline at the

PETRA III ring of DESY, which is a third generation

synchrotron providing an intensity increased by a factor of

about 10 000. Further experiments with a decreased beam

focus of approximately 10 mm� 10 mm are therefore possible.

The increased intensity also allows for measurements at

increased cutting speed. A cutting speed of nc¼ 0.1mmin�1 is

expected to be possible with the presented experimental

setup.

662 http://www.aem-journal.com � 2011 WILEY-VCH Verlag GmbH & C

In Situ Study of Precipitation Kinetics Using Neutrons

Motivation

SANS has been a standard tool for studying precipitation

kinetics for decades.[9,10] It enables the determination of size

distribution and volume fraction of precipitates as a function

of time and temperature. Typical sample volumes can be

around 0.1 cm3, thus leading to good statistics as well as to

high sensitivity to small changes in the precipitate dispersion.

This makes SANS a complementary tool to TEM and atom

probe tomography (3DAP). The early stages of precipitation

can be investigated by isothermal in situ heating experiments.

A high heating rate is required for reaching the treatment

temperature within short time. For the study of ferromagnetic

material, the sample has to be magnetized to saturation for

avoiding a strong background scattering of magnetic

domains. Therefore, a furnace is required that can be used

in a magnet. A new compact furnace with a relatively high

heating rate for use in a 2 T electromagnet was developed at

GKSS. The prototype furnace was tested by studying

precipitation in an Fe–25 at% Co–9 at%Mo alloy strengthened

by precipitates of the intermetallicm-phase ((Fe,Co)7Mo6) with

sizes of a few nanometer.[11] Only little was known about

details regarding the early stage of precipitate formation in

this alloy system before these investigations were conducted.

Experimental Details

The furnace uses ceramic heating elements between which

a coin-shaped sample is fixed. Its water-cooledAl housing had

Al windows for the neutron beam. The evacuated furnace was

placed between the pole shoes of an electromagnet at the

SANS-2 instrument at GKSS and the sample was magnetized

to saturation. The temperature was measured with a

thermocouple placed into a hole drilled into the sample.

The heating rate of the prototype furnace was 5K s�1; thus, the

temperature of 493 8C was reached in 100 s (later versions of

the furnace achieved significantly higher heating rates). A

neutron wavelength of 0.58 nm and a detector distance of 1m

were used. The data acquisition time at the beginning of the

reaction was 30 s.

Results

Since the statistics of the SANS signal during the early

stages was weak, the integrated small-angle scattering

intensity Q was calculated from the experimental scattering

curves.[12] Q is a measure for the product of the precipitate

volume fraction f and the square of the scattering contrast Dh.

The ratio R of integrated intensities calculated from magnetic

and nuclear scattering curves, R¼Qm/Qn, thus reflects the

ratio of magnetic and nuclear scattering contrasts squared,

since both signals originate in the same precipitate structure.

From the fact that Qm initially has a higher slope than in later

stages, while Qn does not show this change in slope it was

concluded that strong compositional changes in the newly

formed precipitates lead to building up of the magnetic

contrast during the first 150 s (Fig. 5).

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Fig. 5. Integrated SANS intensities Qm and Qn after baseline subtraction and ratioR¼Qm/Qn for Fe–25 at% Co–9 at% Mo. The arrow marks the time when 493 8C arealmost reached.[14]

In the literature, the formation of precipitates in Fe-Co-Mo

alloys was often described to proceed via spinodal decom-

position.[13] From the present results it is concluded that,

should spinodal decomposition play a role in this alloy at

493 8C, it is most likely during the first 150 s where the strong

increase of R indicates strong compositional changes. After

150 s the precipitates should be close to their final composition

and only smaller compositional changes seem to occur as

indicated by the small changes in R.

Additional in situ SANS experiments at different tem-

peratures help to understand the early decomposition stages

in the investigated material.[14] Small-angle X-ray scattering

experiments at a synchrotron source can givemuch better time

resolution due to the high intensity of the source, but miss the

possibility of using themagnetic properties for analysis, e.g. of

the ratio R. The combination of SANS with 3DAP and TEM as

well as suitable decomposition models leads to a compre-

hensive description of the precipitation reaction in the

investigated material.[15]

Conclusions

Besides the experiments reported in this paper there is also

the possibility to use an in situ friction stir welding (FSW)

instrument called FlexiStir. It is a transportable FSW unit

equipped with a state-of-the-art welding head, which is not

only used for in situ experiments. First results have been

obtained already, which, however, will be published else-

where. Other in situ experiments are still in design or under

construction. Among them is a nano-indentation experiment

(FlexiIndent) that is planned for use with the 3DXRD and

diffraction contrast tomography techniques. These experi-

ments will be performed at the new grain mapper at the

HEMS beamline at PETRA III. Like FlexiStir, also an in situ

laser beam welding unit (FlexiLas) is under consideration,


which would be much easier to put into the beam than the

FSW unit. Also in situ tomography experiments can be

performed, which will mainly be limited by the speed at

which a sample can be rotated.

In the future, the high brilliance of the third generation

synchrotron source PETRA III will enable further in situ

experiments in the field of engineeringmaterials science,making

use of the high penetration power of high-energy photons.

Received: September 27, 2010

Final Verson: November 16, 2010

Published online: January 26, 2011

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T. Lippmann, L. Lottermoser, J. Herzen, T. Fischer,

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[2] F. Appel, M. Oehring, J. D. H. Paul, Adv. Eng. Mater.

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[10] G. Kostorz, in: Treatise on Materials Science and Technology –

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Academic Press, New York, USA 1979, pp. 227.

[11] E. Eidenberger, E. Stergar, H. Leitner, P. Staron,

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A 2009, 97, 331.

[12] P. Staron, E. Eidenberger, M. Schober, M. Sharp,

H. Leitner, A. Schreyer, H. Clemens, J. Phys.: Conf.

Ser. 2010, 247, 012038.

[13] M.Doi, H. Tanabe, T.Miyazaki, J. Mater. Sci. 1987, 22, 1328.

[14] E. Eidenberger, M. Schober, P. Staron, D. Casliskanoglu,

H. Leitner, H. Clemens, Intermetallics 2010, 18, 2128.

[15] E. Eidenberger, M. Schober, E. Stergar, H. Leitner,

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in situ experiments with synchrotron high-energy x-rays and neutrons

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