ESOMAT 2009, 06030 (2009)
DOI:10.1051/esomat/200906030
© Owned by the authors, published by EDP Sciences, 2009
Deformation induced martensite formation in metastable austenitic
steel during in situ fatigue loading in a scanning electron
microscope
I. Roth1a, U. Krupp2, H. J. Christ10.EEHOHU1, C.-P. Fritzen1
1
University of Siegen, 57068 Siegen, Germany
2
University of Applied Sciences Osnabrueck, 49009 Osnabrueck, Germany
Abstract. Aim of the study is to identify quantitatively the influence of deformation-induced phase transformation
on the fatigue damage of a metastable austenitic steel during loading in the high cycle fatigue regime. Cyclic
deformation tests were carried out in situ in a scanning electron microscope (SEM) in combination with automated
electron backscatter diffraction (EBSD) used for phase analysis and crystallographic orientation mapping. The in situ
experiments were supported by ex situ cycling in a servohydraulic testing machine. The examined metastable
austenitic steel (AISI 304L) transforms diffusion less from the fcc austenite lattice to the bcc D PDUWHQVLWH ODWWLFH
either spontaneously at very low temperatures or at room temperature when a critical value of monotonic or
accumulated cyclic plastic strain is exceeded. The experiments showed that already after some initial 10,000 cycles of
fatigue loading at stress amplitudes close to the fatigue limit a nucleation of martensite occurs as needles near
activated slip systems as a consequence of localized plastic deformation. Once first microstructurally short cracks
have nucleated, strong martensitic transformation occurs within the plastic zone ahead of the crack tip. Due to the
higher specific volume the martensite is considered to shield the crack tip, i.e., transformation-induced crack closure
takes place. The role of deformation-induced phase transformation on (i) crack initiation and (ii) the mechanism of
fatigue microcrack propagation is discussed in detail in the present paper.
1. Material and Experimental Details
The metastable austenitic stainless steel (AISI 304L) was delivered as rods with 25.5mm diameter. The chemical
composition is given in Table 1. Before fabrication of specimens a solution annealing for 1h at 1050°C has been
carried out to coarsen and homogenise the microstructure resulting in an average grain size of 75μm. After the
heat treatment the microstructure was completely austenitic with a slight amount of linear arranged G ferrite.
Table 1. Chemical composition of the austenitic stainless steel investigated
Shape Alloy Fe C Cr Ni Si Mn Cu Mo
rod AISI 304 Base 0.03 18.1 8.75 0.62 1.85 0.54 0.37
Cylindrical shallow notched specimens were used [1] to carry out the short crack growth experiments in a MTS
servohydraulic testing system. In order to carry out in situ experiments in a Philips XL30 LaB6 scanning electron
microscope (SEM) a piezo driven testing system was newly developed for fatigue loading of flat specimen (Fig.
1.).
All tests were executed under load control at a load ration of R=-1 and a frequency of 5 Hz at room temperature.
The chosen stress amplitude of 'V/2 = 230 MPa was slightly above the fatigue limit (107).
a
email : ingmar.roth@uni-siegen.de
This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License
(http://creativecommons.org/licenses/by-nc/3.0/), which permits unrestricted use, distribution, and reproduction in any
noncommercial medium, provided the original work is properly cited.
Article available at http://www.esomat.org or http://dx.doi.org/10.1051/esomat/200906030
ESOMAT 2009
a) b) c) d)
Fig. 1. In situ fatigue testing system, a) piezo driven miniature testing system, b) 70° tilt for in situ electron backscatter
diffraction (EBSD), c) SEM chamber enlargement, c) flat in situ specimen
The ex situ experiments were interrupted after a certain number of cycles for crack investigation in the SEM. An
automated EBSD (TSL OIMTM) was used for determination of the local lattice orientation and for an analysis of
the phase transformation. It should be noted that smooth and deformation-free surfaces are necessary for OIM
investigation. Therefore the specimens were electrochemically polished before testing.
2. Formation of martensite under cyclic loading in the HCF regime
During cyclic loading at the stated stress amplitude stated, a locally high plastic activity arrises. Nearly every
grain shows slip markings. Close to activated slip systems of the highest Schmid factors (almost 0.5) a
transformation of austenite to lamellae of martensite can already be detected after some initial 10.000 cycles
even though a global threshold for transformation has not been reached [2] (Fig. 2.). From the EBSD phase data
the D PDUWHQVLWH FDQ EH GHWHUPLQHG DV WKH ERG\ FHQWUHG FXELF EFF ODWWLFH VWUXFWXUH 7KH H[LVWHQFH RI WKH
hexagonal close packed (hcp) H martensite could not be proven. Most likely, H martensite can be considered as a
transition phase in the transformation from the face centred cubic (fcc) austenite to DPDUWHQVLWH>@$QRWKHU
reason for not detecting H martensite could be the insufficient resolution of the SEM used for the very small
volume fraction of this martensitic phase.
a) b) c)
Fig. 2. Nucleation of D´ martensite near activated slip systems according to the Kurdjumov-Sachs relationship, a) SEM
micrograph, b) EBSD inverse pole figure map, c) EBSD phase map
The crystallographic relationship of austenite matrix and formed martensite was calculated from the EBSD
orientation data. Within some scatter it was found that a close-packed lattice plane of the fcc austenite is parallel
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to a close-packed lattice plane of the bcc martensite and within this parallel planes close-packed lattice directions
are parallel.
This kind of crystallographic orientation relationship is described by the Kurdjumov-Sachs (K-S) orientation
relationship [4] (Fig. 2., (1)).
1 1 1 J II 101 D´ >1 1 0@ J II >1 1 1@ D´ (1)
With 4 possible plains and 6 equivalent directions per plain this relation leads to 24 (12 twin related) variants of
martensite. The two variants illustrated in Fig. 2. are formed on the same schema and are twin related.
3. Crack initiation and short crack growth
At the stress amplitude applied, active cracks can be observed very early in fatigue life although the density of
cracks is very small. Only one or two cracks per sample side were found in the absence of any diffusionless
phase transformation. In the majority of cases the initiation sides are located at twin boundaries (TB) as it is
often observed in stable austenitic stainless steels [5,6].
The initial propagation takes place as shear-stress-controlled stage 1 crack propagation along the initiating twin
boundary. In this stage no formation of DPDUWHQVLWHWDNHVSODFHLQFRQQHFWLRQZLWKFUDFNJURZWK$IWHUOHDYLQJ
the TB the cracks propagate transcrystalline perpendicular to the loading direction on crystallographic crack
planes with low indices, such as {100} and {110} (Fig. 3.).
Fig. 3. Short crack in metastable stainless steel, left: EBSD phase map, right EBSD inverse pole figure map
This kind of crack propagation is described in ref. [1] as stage 1b or in refs. [5,7] as a plastic
blunting/resharpening mechanism stage 2 crack growth where two adjacent {111} slip systems are involved.
As a consequence of crack propagation in the described manner, formation of DPDUWHQVLWHLQWKHSODVWLF]RQHRI
the crack tips take place. Similar to the martensite lamellae described above, the crystallographic orientations
obey the Kurdjumov-Sachs relation. The extend of the transformed area and the crystallographic orientation are
fluctuating along the crack path.
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ESOMAT 2009
4. Transformation induced short crack closure
Transformation induced crack closure of long cracks in metastable stainless steels is a well-known phenomenon
[8,9]. The volume increase of 2-5% of the formed DPDUWHQVLWHWKDWQXFOHDWHVLQWKHSODVWLF]RQHRIORQg cracks
leads to compression stresses in the wake of propagating cracks. This leads to a premature contact of the crack
surfaces and hence to a reduction of the effective cyclic stress intensity factor.
a) b)
Fig. 4. Transformation induced crack closure, a) short crack is still open at zero loading, b) propagation through large
transformed areas results in a strong decrease in crack growth velocity
In the case of short cracks there are also indications of transformation induced crack closure similar to long
cracks. It was observed that the cracks are still open at zero load, when the zone of transformation at the crack tip
is large (Fig. 4. a). Furthermore the crack is retarded while propagating through these large transformed areas
(Fig. 4. b). As a subject of ongoing work an interferometric strain/displacement gauge has been applied for
measuring local displacements in order to study the phenomenon of transformation induced short crack closure
in more detail.
5. Summary
The influence of martensite formation on initiation and growth of short cracks in the high cycle fatigue regime
was surveyed. The main results can be summarised as follow:
1. After some first 10.000 cycles of fatigue loading, a nucleation of DPDUWHQVLWHWDNHVSODFHLQORFDODUHDV
of the microstructure although the global threshold for transformation is not reached.
2. The DPDUWHQVLWHIRUPVQHHGOHVQHDUDFWLYDWed slip systems because of high local plastic deformation.
3. The correlation of the crystallographic orientation of the fcc austenite and the bcc martensite lattice
follows the Kurdjumov-Sachs relationship.
4. Short cracks initiate mostly at twin boundaries in absence of any formerly developed DPDUWHQVLWH
5. After propagation along the initial twin boundary in stage 1 crack growth, the crack continues to grow
perpendicular to the loading direction by a crack growth mechanism where two adjacent {111} slip
systems are involved. This crack growth mechanism leads to crack planes with low indices such as
{100} and {110}.
6. In the plastic zone of the growing short cracks a formation of DPDUWHQVLWHRFFXUV'XHWRWKHYROXPH
expansion of 2-5% a crack closure effect seems to retard the short crack growth. By use of an
interferometric strain/displacement gauge for measuring local displacements at very high resolution this
phenomenon will be studied in more detail in the ongoing work.
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The financial support by the Deutsche Forschungsgemeinschaft under grant number KR 1999/11-2 is gratefully
acknowledged.
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