1. Introduction
Duplex stainless steels are widely used in mechanical and chemical engineering due to their excellent combination of properties in terms of strength, ductility, and corrosion resistance. These advantageous material properties are achieved via the interaction of two phases, ferrite and austenite, which both exist in large volume fractions. Many manufacturing processes of metal components involve non-uniform plastic deformations and thereby cause the development of residual stresses. These ‘internal stresses’ are superimposed on the external loads during operation and can decisively influence the material behaviour, for example the service life of components subjected to cyclic loads [
1]. In plastically deformed duplex stainless steels, phase-specific micro-residual stresses are observed, which are superimposed on the macro-residual stresses. These kinds of microstresses develop because the ferritic and austenitic phases differ in their mechanical behaviour. The sign and magnitude of the phase-specific micro-residual stresses are affected by the degree of plastic deformation and the phase-specific elasto-plastic behaviour, which depends, i.e., on the specific material composition, crystallographic texture, and previous heat treatments [
2].
The most widely used method for the analysis of phase-specific residual stresses on polycrystalline materials is the
method using X-ray diffraction [
3]. The method is based on the measurement of diffraction lines from specific lattice plane families of type
of one phase under various sample inclinations
for a fixed azimuthal direction
. A classical sample-fixed coordinate system, indicating angles
and
, is shown in
Figure 1a. Following Bragg’s law
the lattice spacing
in the direction of the scattering vector
can be determined from the X-ray wavelength
and diffraction angle
, as schematically depicted in
Figure 1b. From
, the lattice strain
is derived considering the lattice spacing
corresponding to the stress-free state.
In general, the stress calculation from the measured strain data is based on the assumption that the stress tensor, averaged over the measurement volume, causes a linear
−
distribution. Linear distributions occur when there is a surface-parallel, uniaxial or biaxial stress state which is sufficiently homogeneous, i.e., shows no steep in-depth gradient within the information depth. Furthermore, the material volume irradiated by the X-rays must contain a sufficient number of randomly oriented grains, i.e., the grain sizes are very small in comparison to the irradiated sample volume [
4]. If shear stresses are present normal to the specimen surface, the
−
distribution shows an elliptical course. However, in cold-formed polycrystalline materials, pronounced non-linearities, i.e., oscillating courses in the
−
plots, can occur. This is due to the phase-specific crystallographic texture (elastic anisotropy) and the plastically induced microstresses (plastic anisotropy), which are also denoted as intergranular stresses [
5]. In situations that pronounced oscillations are observed, the
method can lead to erroneous results and should no longer be applied. It is known that near-surface residual stress depth gradients can also cause non-linear
−
distributions [
1]. Because in-depth stress gradients play no role in this study, this is not further discussed.
The lattice spacing
, determined by diffraction methods, always represents a selective mean value of those crystallite orientations whose lattice planes
are perpendicular to the respective measurement direction
. In terms of strain, this mean value can be separated into two parts: a mean strain part
, which is related to the mean stress tensor
, and a second part
, which is related to orientation-dependent microstresses caused by previous plastic deformations [
6]. At this point, it should be emphasised that the superscript ‘pl.’ does not indicate plastic strain; instead, it indicates elastic intergranular strain induced by plastic deformation. Generally, the relation between
and
is described by stress factors
, which can be calculated with knowledge of the single-crystal elastic anisotropy and the orientation distribution function (ODF) [
7]. The crystallite coupling within the polycrystal is thereby taken into account using appropriate mathematical models, e.g., according to Voigt [
8], Reuss [
9] or Eshelby/Kröner [
10,
11]. A detailed description of the
calculation approaches following different models is given in [
12]. The measured strain
in the direction
of a plastically deformed, polycrystalline material is calculated as follows [
6]:
Thus, the plastically induced intergranular strains are independent from the acting mean stress tensor and are instead due to the history of plastic deformation, which is usually not known.
Several works have already dealt with the experimental analysis of phase-specific stresses in plastically deformed duplex stainless steels, e.g., [
13,
14,
15,
16,
17,
18,
19,
20]. Although materials with comparable chemical compositions and similar phase fractions were examined, different conclusions were drawn regarding the phase-specific yield strength and the formation of phase-specific microstresses. On the one hand, this can be attributed to the fact that phase-specific strength is affected by the specific crystallographic texture (orientation strengthening) and by the differences in the precise phase-specific chemical composition (solid solution strengthening). On the other hand, different measurement and evaluation approaches were used, which can also lead to different stress results for such complex material states. A systematic study on the evolution of non-linear
−
distributions for several lattice planes
caused by plastic deformation and the accompanying error in residual stress evaluation has not yet been performed. Usually, only the residual stress state is analysed. Without a comparison of the same material state under additional external loading, the influences of texture and intergranular strains cannot be easily separated. Furthermore, the data are frequently obtained by diffraction experiments using conventionally generated X-rays (lab X-ray applications) in reflection mode. In this case, mostly only a limited
range is accessible due to the absorption at high inclination angles (
) or simply due to the geometric constraints of the applied measurement setup, and the oscillatory courses of
vs.
are not necessarily visible.
The use of high-energy synchrotron X-ray in transmission mode, however, enables the analysis of mean phase-specific stresses for metal samples having a thickness of up to a few millimetres. Here, the diffraction data contain integral information over the sample thickness; hence, depth gradients of the crystallographic texture or residual stresses are not resolved. In contrast, the information gained reflects an overall material response that is unaffected by local deviations due to near-surface effects. By means of a 2D detector, full diffraction rings of several lattice planes can be recorded for polycrystalline samples. After azimuthal segmentation of the diffraction pattern, the diffraction profiles can be analysed for various azimuthal directions. Using these means,
−
distributions can be evaluated based on a single exposure [
21,
22]. This approach allows for the determination of lattice spacings with polar angles of up to
. Thus, it is a valuable tool for the systematic analysis of the development of oscillatory
−
distributions with elastic and elasto-plastic deformations.
In the present work, the phase-specific lattice strain responses of a cold-rolled duplex stainless steel sheet and a ferritic stainless steel sheet were analysed under increasing tensile deformations up to a total strain of about . The aim was to analyse the development of oscillatory courses in vs. for several lattice planes for single-phase and two-phase materials and to obtain a better comprehension of the respective contributions of elastic and plastic anisotropy. Therefore, in situ loading experiments using 2D high-energy synchrotron X-ray diffraction were carried out at the P07B@PETRA III beamline at Deutsches Elektronen-Synchrotron (DESY) Hamburg, Germany. During uniaxial deformation in the elastic and elasto-plastic regime, entire Debye–Scherrer rings of several lattice planes for the ferritic phases were detected by means of a flat-panel detector. In the case of the duplex stainless steel, the Debye–Sherrer rings were also found for the austentitic phase. From these, the direction-dependent lattice spacings were evaluated.
The results are presented and discussed in the following order. At first, the continuous evolution of the phase-specific lattice strain is analysed for selected directions with respect to the loading direction. Thereafter, − plots of the individual lattice plane families of both materials are discussed for four particular load increments including the unloaded state. It is investigated whether linear regression over oscillating distributions leads to comparable stress results for different lattice planes if the entire range in (from 0.1) can be considered. The ferritic phase of the duplex stainless steel and the ferritic stainless steel exhibit the same crystal structure. In comparing the − distributions of the single-phase material and the two-phase material, it is investigated if the second phase has an influence on the non-linearities. Finally, the influences of intergranular strains and crystallographic texture on the − courses are separated by comparing the loaded and unloaded state.