EBSD- and TEM-investigations of microstructure in the austenitic steel X6CrNiNb18-10 under cyclic loading

E. Soppa, D. Willer, D. Kuppler

Materialprüfungsanstalt Universität Stuttgart

38th MPA-Seminar October 1 and 2, 2012 in Stuttgart

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Abstract To provide a more precise quantification of available safety margins in components under

cyclic loading a thorough understanding of the phenomena accompanying fatigue and

leading to crack formation in the stainless steel X6CrNiNb18-10 is necessary. A deformation

induced transformation of austenite to ’-martensite in a metastable stainless steel at room

temperature is thereby of great importance for the crack initiation and crack growth

mechanisms. As these processes emerge on the nano- and micro-level the application of

experimental methods that can deliver reliable information with high resolution is decisive.

The Electron Backscatter Diffraction (EBSD) is a modern and highly effective method that

serves to determine the grain orientations and identify phases in the microstructure.

Moreover the visualisation of plastic deformation leading to a local lattice deflection caused

by dislocation pile-up is also possible by this method.

Within the frame of the research project [2] the fatigue behaviour of the Nb stabilized

austenitic steel X6CrNiNb18-10 at room temperature was investigated. By a combination of

interrupted low cycle fatigue tests and EBSD-technique (in SEM) the deformation induced

martensitic transformation was studied during different stages of the specimen lifetime.

Based on the analysis of thin foils of the fatigued steel by Transmission Electron Microscopy

(TEM) the presence of cubic body-centered ’-martensite in the austenitic matrix was

confirmed. The transformation of a paramagnetic austenite to a ferromagnetic ’-martensite

is directly connected with the change of crystallographic and in particular with mechanical

properties of the material under cyclic loading. Also fatigue behaviour of the X6CrNiNb18-10

at room temperature depends on the martensitic transformation. Fatigue cracks form in the

phase boundary between austenite and martensite or in fully martensitic areas. The cracks

further propagate in the martensitic phase which forms permanently at the crack tip where

stresses and strains are high enough to move forward the transformation.

1. Introduction / Motivation A deeper knowledge of the phenomena accompanying fatigue and leading to the crack

formation is necessary for the reliable assessment of the component lifetime. Deformation

induced martensitic transformation in metastable steels is thereby of great importance.

Because this process emerges firstly on the nano- and micro-levels, before total failure

occurs, the application of experimental methods which can deliver the information with a high

resolution is necessary. The EBSD-technique is a powerful method for the investigation of

grain orientations, phase determination and visualisation of plastic deformation in the

microstructure.

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2. Material The chemical composition of the stainless steel X6CrNiNb18-10 is given in Table 1.

Table 1: Chemical composition of the austenitic stainless steel X6CrNiNb 18-10 [1].

C Si Mn P S Cr Ni Nb Ta

0,043 0,410 1,90 0,019 0,0020 17,150 10,300 0,660 0,008

Fe – balance, (in weight%).

This material was heat treated at 1050°C for 10 minutes and quenched in water to ambient

temperature [1]. The resulting microstructure was austenite and finely dispersed niobium

carbides (NbC) (Fig. 1). In addition to fine carbides also individual coarse NbC particles (5 µm

– 20 µm) were present. Only 1-1.5% of -ferrite was found (Fig. 1). Moreover, in

approximately 30% of the grains twins were detected. The grain size according to DIN EN

ISO 643 was estimated as G=1,5 – 5.

Fig. 1: Microstructure of the X6CrNiNb 18-10 after heat treatment and before mechanical loading.

The X6CrNiNb18-10 austenitic steel contains 10.3% of Ni, which is not sufficient to stabilize

the austenitic phase at room temperature. The difference of the Gibbs’ free energy between

austenite and ’-martensite is a driving force for the phase transformation. The permanent

supply of the mechanical energy during cyclic loading provides the missing part of the

enthalpy which is necessary for the phase transformation at room temperature [7].

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3. Experimental methods 3.1 Specimens and metallographic preparation Smooth cylindrical unnotched specimens (Fig. 2) with two symmetrically placed flat narrow

bands on the specimen surface were mechanically polished to 1 µm. The last step of the

metallographic preparation was electrolytic polishing using an electrolyte STRUERS A2 and

platinum as a cathode. The best results were achieved at the following parameters: ~20 °C,

40 V, 10 – 15 sec. After polishing small (2-4 µm) flat etching pits remained on the surface.

Although they represented certain imperfections for EBSD-measurements, they were prefect

marks for the determination of plastic strains in the microstructure [2].

3.2 Cyclic loading The interrupted low cycle fatigue (LCF) tests were performed strain controlled with the

loading amplitude ε =1.5 % (R= -1) at room temperature in air.

Fig. 2: Geometry of the specimen used for LCF-tests. The total length of the specimen was 60 mm.

The specimen length was tailored to the dimensions of the vacuum chamber in SEM, so that

non-destructive EBSD-measurements on the same specimen after different numbers of

loading cycles were possible. After 10, 50, 100, 130, 180, 230 und 300 cycles the LCF-test

was interrupted and the specimen was scanned in SEM using EBSD-technique.

3.3 EBSD-technique The EBSD measurements were carried out in a FE-SEM (Auriga, Zeiss) with an acceleration

voltage of 25 kV. The EBSD System EDAX-TSL with the software OIM data Collection 5 was

in use. The measuring field size was usually x = 350 µm, y = 350 µm and the step size of 1

µm. For the interesting details in small areas a step size less than 1 µm was applied. The

Software EDAX-TSL OIM Analysis 5 was used for the processing of the EBSD-data. Only

one clean-up proceeding (neighbor-CI-Correlation with minimum confidence-Index CI = 0.1)

was necessary in order to reduce the number of not correctly indexed points caused by the

flat etching pits.

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3.4 TEM analysis For the TEM investigation thinned metal foils were used. This method is a powerful tool to

characterize the microstructure regarding precipitations, sub-grain and dislocation structures

and the determination of the dislocation density. By means of Selected Area Diffraction

(SAD) the crystal lattice structure of selected areas in the metal foil can be determined.

For the preparation of the thinned metal foils with transparent areas the following steps were

taken:

Preparation of cylinders with a diameter of 3 mm taken from the specimen by spark

erosion technique

Cutting off discs with a thickness of about 0.4 mm

Mechanical grinding of the discs from both sides to a thickness of 0.1 mm

Dimpling of the discs from one side to get centered thinned areas

Electrochemical thinning of the discs with the Tenupol method (electrolyte A8; 40V, 13°C)

just until the formation of a small hole with electron transparent areas around it with a

thickness of 50 to 200 nm.

The thinned metal foils were investigated using a transmission electron microscope JEOL

JEM 2000 FX (200kV accelerating voltage), which is equipped with an energy dispersive X-

ray system, model Tracor Northern, USA (TN-5500).

4. Results and discussion 4.1 Martensitic transformation under cyclic loading Fig. 3 a - d show the same measuring field at the specimen surface before loading, after 100

cycles and after 300 cycles respectively.

before loading after 100 cycles after 300 cycles

a.

b.

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c.

d.

0° - 3° 3° - 6° 6° - 9°

9° - 12° 12° - 15°

Fig. 3: Measuring fields at the specimen surface before loading, after 100 cycles and 300 loading

cycles: a) SE-images; b) IPF-images; c) phase distribution green=austenite, red=martensite;

d) misorientation maps (0 - 15°, 3 pixel).

The SE-images, IPF-images, austenite and martensite phase distribution and misorientation

maps are presented in Fig. 3 a-d. In the SE-image the etching pits are visible. On the

specimen free surface characteristic marks of plastic deformation continue to accentuate in

the course of cyclic loading. In some austenite grains (IPF-image, Fig. 3b) a distinct gradient

of colours occurs after 100 cycles. This effect is connected with rotation [8] or diffraction of

crystallites caused by plastic deformation. This effect becomes more pronounced after 300

cycles. Moreover, the formation of very fine sub-grains was observed.

Before cyclic loading the microstructure of the material consisted of pure austenite (Fig. 3c),

apart from the etching pits, which were not totally removed by the clean-up-procedure. Their

contribution to the total amount of martensite was only 0.8%. ’-martensite forms gradually in

the course of cyclic loading and amounts to 2.2% after 100 cycles and 6.9% after 300 cycles.

The location of martensite in the microstructure corresponds to the fine substructure (Fig.

3d). The misorientation maps show the increase of the local misorientation during cyclic

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loading (areas with a gradient of colours in IPF-image). The greatest value of the

misorientation was found in the fine martensitic sub-grains.

4.2 Detection of ’- martensite by electron diffraction The samples for the TEM investigations (thin foils) were taken from the fatigued macroscopic

specimens after loading with the strain amplitude =1.5%, R = -1 until ~100% of the

specimen lifetime.

’-martensite

austenite

Fig. 4: Formation of the „martensitic island“ in the austenitic matrix in the fatigued LCF specimen.

Fig. 4 shows the „martensitic island“ formed in the austenitic matrix during cyclic loading. The

nature of the band in which the martensite was formed is under current investigation [6]. The

size of this “island” and its globular shape correspond to the martensitic grains measured by

EBSD on the specimen surface. Similar “martensitic islands” were found in several places in

thin foils from the specimen interior. The evaluation of the diffraction images of the supposed

martensitic areas confirmed the bcc structure of this phase in contrast to the fcc structure of

the austenitic matrix. A lattice parameter of ’-martensite was estimated as a=2,8664 Å [3],

that corresponds to the one of ferrite [5].

5. Summary and conclusion Using EBSD technique in combination with interrupted LCF-tests the deformation induced

transformation of fcc austenite to bcc ’-martensite in the Nb stabilized stainless steel

X6CrNiNb18-10 under cyclic loading was studied. The steel X6CrNiNb18-10 with 10.3% of Ni

contains a metastabile austenite at room temperature. The mechanical energy delivered

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during cyclic loading to the material compensates the free enthalpie for the phase

transformation and causes the formation of ’-martensite.

’-martensite forms preferably at grain boundaries, defects and on the free surface. The first

martensitic grains as small (~1 µm) islands were detected after 50 cycles. This means that a

certain amount of accumulated plastic strain is necessary [4] to initiate the phase

transformation even at great loading amplitude. We are assuming that a particular “structure

of microdefects” must be formed before the transformation begins. ’-martensite forms small

(~1 µm) grains with different orientations even in the same parent austenitic grain. These

areas have also a very high density of interfaces, which distinguish areas with an orientation

misfit. The volume fraction of ’-martensite goes up to 9% after 300 cycles (almost 100% of

the lifetime) in the specimen loaded with the strain amplitude of =1.5%, R= -1. The

formation of bcc ’-martensite in the fcc austenitic stainless steel under cyclic loading at

room temperature was detected by both methods, EBSD and TEM.

Acknowledgments This work [2] was performed with the support of the German Federal Ministry of Economics

and Technology (BMWi) which is gratefully acknowledged. Dipl.-Ing. P. Kopp and Mr. R.

Scheck (MPA University of Stuttgart) are truthfully thanked for their assistance in the

experiments. Literature [1] DMV STAINLESS Deutschland GmbH.

[2] Final report, BMWi-research project 1501353 „Micromechanical and atomistic modelling of crack initiation and crack development in fatigued steels“, Materialprüfungsanstalt Universität Stuttgart (2011).

[3] ICCD, 1991: International Centre for Diffraction Data, pdf2, version 2.12a, Swarthmore, USA, 1991.

[4] Krupp, U.; C. West; H.-J. Christ: “Deformation-induced martensite formation during cyclic deformation of metastabile austenitic steel: Influence of temperature and carbon content”, Materials Science and Engineering A 481-482 (2008) 713-717.

[5] Menthe, E.: Bildung, Struktur und Eigenschaften der Randschicht von austenitischen Stählen nach dem Plasmanitrieren, 1999, H. Utz Verlag.

[6] Ongoing BMWi-project 1501391 “Examination of influencing factors on cyclic crack growth behaviour of cracked components” Materialprüfungsanstalt Universität Stuttgart (2010-2013).

[7] Schoß, V.: Martensitische Umwandlung und Ermüdung austenitischer Edelstähle, Gefügeveränderung und Möglichkeiten der Früherkennung von Ermüdungsschäden, Dissertation, TU Bergakademie Freiberg, 2000.

[8] Schwartz, A.J., M. Kumar, B.L. Adams, D.P. Field. Electron Backscatter Diffraction in Material Science, Springer Science + Business Media, LLC 2009, New York.

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