Polycrystalline Austenitic Stainless Steel Research Articles

Three-dimensional configuration of crystal plasticity in stainless steel assessed by high resolution digital image correlation and confocal microscopy

In polycrystalline austenitic stainless steels, a comprehensive three-dimensional configuration of crystal plasticity in relation to microstructure is required to elucidate the deformation behaviors during mechanical loading. In the present work, a combination of backscattered electron image (BSE) based high-resolution digital image correlation (HR-DIC) and laser scanning confocal microscopy (LSCM) was employed to simultaneously investigate in-plane and out-of-plane deformations at the sub-grain scale in 316 stainless steel. The high spatial resolution and precise alignment provided by the three-dimensional deformation assessment enable the identification of specific slip and twinning systems. This is achieved through the minimization of the residual L2 norm between the in-plane components of experimental and theoretical strain tensors. Dominant slip and few mechanical twinning activities are initiated at the early-stage of yielding, and their frequency and intensity increase with the applied load. The hierarchical networks formed by nanotwins, along with the presence of grain and annealing twin boundaries, serve as obstacles for slip glide, leading to substantial twin–slip interaction and grain boundary strengthening. As a consequence of wide mechanical twins formed by parallel nanotwins coalescence and strain localization at the twin boundary, slip transmission occurs, resulting in an effective accommodation of external stress. Furthermore, the consistent observation of out-of-plane roughness, in alignment with twinning Burgers vector predictions, offers valuable insights into the significant deformation gradient occurring in the vicinity of boundaries. This deformation gradient is influenced by the neighboring mechanical twinning activities, indicating strong obstacle to twin transmission and high back-stresses resulting from the highly misoriented grain boundary.

Effect of boundary conditions on the evolution of lattice strains in a polycrystalline austenitic stainless steel

The effect of boundary conditions (constant load, constant strain and elastic follow-up) on lattice strain evolution during creep in a polycrystalline austenitic stainless steel was studied using in situ neutron diffraction at 550 °C. The lattice strains were found to remain constant under constant load control. However, under constant strain and elastic follow-up control, the lattice strains relaxed the most in the elastically softest lattice plane {200} and the least in the elastically stiffest lattice plane {111}. The intergranular stresses created between different grain families were constant during creep tests irrespective of the boundary conditions with the initial applied stresses of 250 MPa.

Effect of creep on the Bauschinger effect in a polycrystalline austenitic stainless steel

A combined thermodynamic/kinetic approach is adopted to derive the climb-controlled recovery of dislocation junctions on individual slip planes in a material during creep when dislocation core diffusion is the rate controlling kinetic process. The model is used in an integrated multi-scale crystal plasticity/creep self-consistent framework to evaluate the effect of creep pre-strain on the Bauschinger effect in a polycrystalline austenitic stainless steel. The model predictions are in good agreement with experimental findings of the effect of tensile creep on the subsequent low temperature response in tension and compression and on the development of residual stresses determined from neutron diffraction studies.

On the evaluation of the Bauschinger effect in an austenitic stainless steel—The role of multi-scale residual stresses

In this work, a physically based self-consistent model is developed and employed to examine the microscopic lattice response of pre-strained Type 316H polycrystalline austenitic stainless steel subjected to uniaxial tensile and compressive loading. The model is also used to explain the Bauschinger effect observed at the macroscopic length-scale. Formulated in a crystal based plasticity framework, the model incorporates detailed strengthening effects associated with different microstructural elements such as forest dislocation junctions, solute atoms and precipitates on individual crystallographic slip planes of each individual grain within the polycrystal. The elastoplastic response of the bulk polycrystal is obtained by homogenizing the response of all the constituent grains using a self-consistent approach. Micro-plasticity mechanisms and how these influence the Bauschinger effect are illustrated in terms of the role of residual stresses at different length-scales. Overall, predictions are in good agreement with experimental data of the Bauschinger effect and the corresponding meso-scale lattice response of the material, with the latter measured by neutron diffraction. The results demonstrate that transient softening of the material is related to residual stresses at different length scales. In addition, the (Type III) residual stress at the micro-scale slip system level extends the strain range over which the tensile and compressive reloading curves of the pre-strained material merge.

A multi-scale self-consistent model describing the lattice deformation in austenitic stainless steels

Differently oriented grains within a polycrystalline material exhibit different micro-mechanical lattice responses for a given macroscopic stress due to the local elastic and plastic anisotropy. A physical understanding of the lattice deformation mechanism during plastic flow is still lacking. In this study, a three-dimensional multi-scale self-consistent model is developed to examine the micro-mechanical deformation behaviour of F.C.C. polycrystalline austenitic stainless steels under uniaxial tensile loading at ambient temperature. The model is formulated in a crystal based plasticity framework and takes into account the detailed kinematics of dislocation slip and its influence on the evolution of the dislocation distribution on different crystallographic planes of individual grains within a polycrystalline material. The effect of athermal solute strengthening is also incorporated. Predictions of the microscopic lattice response developed within individual grains are compared with various available experimental results obtained using neutron diffraction (ND). There are significant differences in the way in which the lattice strain evolves with macroscopic strain obtained by different groups. This is explained in terms of the variation in initial residual stress state in the samples tested by these different groups.

Influence of substrate microcrystallinity on the orientation of laser-induced periodic surface structures

The research in this paper deals with the angular dependence of the formation of laser-induced periodic surface structures (LIPSS) by linearly polarized nanosecond laser pulses on polycrystalline austenitic stainless steel. Incident angles ranging from 45° to 70° lead to the generation of superimposed merely perpendicular oriented LIPSS on steel as well as on monocrystalline (100) silicon which was used as a reference material. Additional extraordinary orientations of superimposing LIPSS along with significantly different periodicities are found on polycrystalline steel but not on (100) silicon. Electron backscatter diffraction measurements indicate that the expansion of these LIPSS is limited to the grain size and affected by the crystal orientation of the individual grains. Atomic force microscopy imaging shows that LIPSS fringe heights are in good agreement with the theoretically predicted penetration depths of surface plasmon polaritons into stainless steel. These results indicate that optical anisotropies must be taken into account to fully describe the theory of light-matter interaction leading to LIPSS formation.

Effects of strain amplitude, cycle number and orientation on low cycle fatigue microstructures in austenitic stainless steel studied by electron channelling contrast imaging

Substructure analysis on cyclically deformed metals is typically performed by time-consuming transmission electron microscopy probing, thus limiting such studies often to a single parameter. Here, we present a novel approach which consists in combining electron backscatter diffraction (EBSD), digital image correlation and electron channelling contrast imaging (ECCI), enabling us to systematically probe a large matrix of different parameters with the aim of correlating and comparing their interdependence. The main focus here is to identify the influence of cycle number, initial grain orientation and local strain amplitude on the evolving dislocation patterns. Therefore, experiments up to 100 cycles were performed on a polycrystalline austenitic stainless steel with local strain amplitudes between 0.35% and 0.95%. EBSD and ECCI maps reveal the individual influence of each parameter while the others remained constant. We find that the dislocation structures strongly depend on grain orientation. Dislocation structures in grains with double-slip (〈112〉 // LD, 〈122〉 // LD and 〈012〉 // LD) and multiple-slip (〈111〉 // LD, M 〈011〉 // LD and 〈001〉 // LD) orientations with respect to the loading direction (LD) are characterized under the variation of strain amplitude and cycle number.

Self-consistent Modelling and the Evaluation of Lattice Deformation in a Polycrystalline Austenitic Stainless Steel

Individual anisotropic grains within a polycrystalline material exhibit different micro-mechanical responses for a given applied macroscopic stress, leading to a variation in the meso-scale intragranular lattice strain. Despite various experimental data for the lattice strain evolution measured by neutron diffraction, a physical understanding of how the lattice strain evolves during plastic flow is still incomplete. In this study, a self-consistent model is established, taking into account detailed dislocation kinetics and its influence on different evolution processes of the dislocation distribution on different crystallographic planes of each grain within a polycrystalline material. The model is used to evaluate the micro-mechanical behaviour of F.C.C. polycrystalline austenitic stainless steels. Predictions of the lattice strain developed within individual grains of a polycrystal are compared with experimental results obtained using neutron diffraction. This model captures the major trends in the evolution of the lattice strains and provides an explanation and interpretation of the internal processes that most influence the observed response.

Monotonic mechanical properties of plasma nitrided 316L polycrystalline austenitic stainless steel: Mechanical behaviour of the nitrided layer and impact of nitriding residual stresses

The impact of plasma nitriding at 400°C on the monotonic mechanical behaviour of 316L austenitic stainless steel at room temperature has been investigated. It is shown that the residual stresses in the nitrided layer lead to a tension–compression anisotropy whose magnitude depends on the residual stresses intensity and extension (i.e. thickness of the nitrided layer). Using the stress differential technique, average residual stresses in the nitrided layer ranging from −1.5 up to −3GPa were measured. The local mechanical behaviour of the nitrided layer has also been investigated through SEM in situ tensile tests. A quasi-brittle mechanical behaviour of the nitrided layer is observed with first evidences of crack initiation for plastic strains below 1%, whatever the nitrided layer. By increasing the total applied strain up to 20%, a progressive segmentation of the layer occurs. The crack initiation location mainly depends on the local nitrided thickness.

Influence of temperature on layer growth as measured by in situ XRD observation of nitriding of austenitic stainless steel

Investigating the formation of expanded austenite has resulted in several, different models trying to explain the particular diffusion and phase formation behaviour. However, only ex situ information, influenced by cooling and annealing processes of the samples after ion implantation has been available until now. Here, the time and temperature dependent layer growth is reported using in situ XRD measurements obtained from low energy broadbeam nitrogen ion implantation into polycrystalline austenitic stainless steel 304 in the temperature range from 300 to 500°C for a process time of up to 1h. Expanded austenite was observed at all temperatures without any CrN, in agreement with already published lifetime data for this metastable phase. The layer growth was derived from the time evolution of the substrate peak intensity. Using the temperature dependence of the layer growth, an activation energy of nearly 0.8eV was estimated for the nitrogen diffusion. In contrast, a complex behaviour was observed for the lattice expansion and peak width of the expanded peak, indicating additional dynamic annealing during implantation.

Crystal plasticity computation and atomic force microscopy analysis of the internal hydrogen-induced slip localization on polycrystalline stainless steel

A local approach combining atomic force microscopy measurements and finite-element calculations at the grain scale is proposed to study the effects of hydrogen on the slip morphology of polycrystalline austenitic stainless steel. The definition of a localization index highlighted the particularly visible effect of hydrogen on plastic slip localization in relation to the crystallographic orientation and the intragranular plastic strain level.

Effects of deformation mode on surface roughening of austenitic stainless steels

Free surface roughening of polycrystalline austenitic stainless steels during tensile and deep drawing deformation is investigated using scanning white light interferometry. A fractal analysis method is applied to analyse the surface profile data in two directions which are perpendicular to each other. It is shown that the surface profile is fractal within the length and is on the same order of magnitude as the grain size. The Hurst exponent H of the two deformation modes in different directions stabilises at H≈0·85. While the correlation lengths are different, the value in the longitudinal direction is about two times larger than the one in the transverse direction. The relationship between root mean square roughness and strain appears to be linear, and the roughening rate in tensile deformation is larger than the one in deep drawing deformation.

Microscale prediction of deformation in an austenitic stainless steel under uniaxial loading

Abstract In this study, the deformation behaviour of polycrystalline austenitic 316H stainless steel under uniaxial loading is investigated by means of in-situ neutron diffraction (ND) measurement and crystal plasticity-based finite element (FE) modelling. Data have been obtained for the macroscopic stress–strain response and the lattice strain evolution in the longitudinal and transverse direction relative to the uniaxial loading axis. Comparison between the model predictions and the ND measurements suggests that in most cases the FE model can predict the lattice strain evolution at the microscale and capture the general trends observed in the experiments. Both ND measurements and FE modelling simulations identify little effect of micromorphology effect on the longitudinal lattice strain evolution, while the transverse lattice strain response appears to be sensitive to the microstructure, in particular the initial crystallographic orientation of the material.

Anisotropy changes in hardness and indentation modulus induced by plasma nitriding of 316L polycrystalline stainless steel

The changes in anisotropic hardness and indentation modulus induced by plasma nitriding at 400 °C of a 316L polycrystalline austenitic stainless steel are analyzed. The dependence of hardness and elastic modulus modifications on the crystallographic orientation is investigated through instrumented indentation and electron backscattering diffraction. Both hardness and indentation modulus exhibit an inverted anisotropy compared to the untreated 316L, likely associated with the presence of the N atoms in interstitial sites.

Out-of-plane magnetic patterning on austenitic stainless steels using plasma nitriding

A correlation between the grain orientation and the out-of-plane magnetic properties of nitrogen-enriched polycrystalline austenitic stainless steel surface is performed. Due to the competition between the magnetocrystalline anisotropy, the exchange and dipolar interactions, and the residual stresses induced by nitriding, the resulting effective magnetic easy-axis can lay along unusual directions. It is also demonstrated that, by choosing an appropriate stainless steel texturing, arrays of ferromagnetic structures with out-of-plane magnetization, embedded in a paramagnetic matrix, can be produced by local plasma nitriding through shadow masks.

On the Correlation between the Plastic Deformation and the Fractal Dimension of the Stainless Steel 304 Mesostructure

In this work we study the elastic-plastic transition of the spatially extended polycrystalline austenitic stainless steel 304 (SEPC-ASS-304) advanced materials during an irreversible deformation process. Such transition was characterized by means of the fractal dimension computed of a sequence of digital images of the mesostructure of the SEPC-ASS-304 surface, obtained during the elastic-plastic transition. Our results show a correlation between the fractal dimension and the evolution of the granular flow during the deformation of such advanced material.

Stress evolution in CrN/Cr coating systems during thermal straining

CrN, Cr and CrN/Cr coatings were deposited at 350°C on monocrystalline Si(100) and polycrystalline austenitic stainless steel substrates by magnetron sputtering using Cr targets in Ar + N 2 atmosphere. The stress evolution in the coating systems were characterized using X-ray diffraction and wafer curvature technique in the temperature range of 25–550°C. Both techniques revealed larger stresses in coatings deposited on the steel substrates. The heat treatment reduces the deposition point defect concentration, which is reflected in a decrease of intrinsic stresses in Cr and CrN coatings. Additionally, roughness of the Cr films decreases. The stresses in the individual sublayers of the CrN/Cr bilayer coatings indicate that the constraint imparted by the CrN layer on the buried Cr layer prevents a stress relaxation in Cr since no stress hysteresis is observed during heating and cooling. The intrinsic stresses of CrN are − 3.4 ⁎ 10 9Pa on the steel substrate and − 1.7 ⁎ 10 9Pa on the Si(100) substrate. For Cr intrinsic stresses of − 1.35 ⁎ 10 9Pa are obtained on steel and − 0.7 ⁎ 10 9Pa on Si(100). The intrinsic stresses of CrN in the CrN/Cr bilayer system remains at − 3.4 ⁎ 10 9Pa on the steel but increases to − 2.8 ⁎ 10 9Pa on the Si(100) substrate. As a result of the annealing cycle, a stress relaxation of approximately 1.3 ⁎ 10 9Pa is obtained for CrN on steel but 3.4 ⁎ 10 9Pa are relaxed for CrN in the CrN/Cr/steel system.

Determination of Percolation Threshold for Random Boundary Network on the Basis of EBSD/OIM Observations

Grain boundary engineering through the control of grain boundary character distribution (GBCD) has been extensively employed as a powerful tool for achieving enhanced properties and for development of high performance both structural and functional polycrystalline materials. Many efforts were made firstly to increase the frequency of low-energy CSL boundaries of polycrystalline materials in grain boundary engineering. However, the connectivity of grain boundaries can be an important microstructural parameter governing bulk properties of polycrystalline materials as well as the GBCD. In the present work, the connectivity of random grain boundaries was quantitatively evaluated using both the triple junction distribution and random boundary cluster length on the basis of SEM-EBSD/OIM observations, and then these evaluated parameters were linked to intergranular corrosion of SUS304 stainless steel. We have found that the length of the maximum random boundary cluster drastically decrease with increasing CSL boundaries in the fraction ranging 60 – 80% CSL boundaries, which leads to percolation threshold occurring at approximately 70±5% CSL boundary fraction (at 30±5% random boundary fraction). The experimentally observed percolation threshold is much higher than theoretically obtained one based on randomly assembled network (at 35% resistant bonds for a 2D hexagonal lattice). In addition, the fraction of resistant triple junctions is found to increase with increasing the the CSL boundary fraction. An increase in the frequency of resistant triple junctions can enhance intergranular corrosion resistance of polycrystalline austenitic stainless steel even if the GBCD is the same.

電解研磨による微小サイズ引張試験片作製法の開発

The ratio of surface area to volume increases as a material gets smaller. For this reason, the mechanical properties of a micro-sized material are strongly influenced by the surface conditions. To accurately study how the mechanical properties are influenced by the size, we need to test specimens with negligible work-affected layers. However, conventional microfabrication methods are poorly suited for the manufacture of specimens that meet this condition. In order to overcome these problems, we developed a novel method for manufacturing micro-sized tensile specimens with negligible work-affected layers using a precision electrolytic polishing apparatus. A polycrystalline austenitic stainless steel (SUS304) wire with a diameter of 0.5 mm was selected as the material for study. The sample wire (anode) was vertically set at the center of a ring-shaped counter electrode and gripped by a collet chuck. Once the sample was set, a rectangular wave voltage for electrolytic polishing was applied in a flowing electrolytic solution. The apparatus was used to manufacture a tensile specimen with a diameter of 51 μm, i.e., a diameter smaller than the average grain size of the sample. Optical, laser, and scanning electron microscope observations revealed a smooth and mirrored specimen surface. Tensile testing of the specimen by a testing machine for micro-sized materials revealed a clear yield point on a stress-displacement curve. The yield and fracture strength of the specimen were 100 and 180 MPa, respectively, or about the same as those of a single crystal. The specimen deformation observed by scanning electron microscope mainly occurred at single crystal regions placed between grain boundaries.

Size Effects of the Micro-Sized Polycrystalline SUS304 Tensile Specimen Fabricated by Electrolytic Polishing Technique

The tensile specimens with diameters less than 100 micro m were fabricated from a 0.5 mm diameter wire of polycrystalline austenitic stainless steel (SUS304) with average grain size of 166 ~m using electrolytic polishing technique. Optical, laser and scanning electron microscope observations revealed a smooth and mirrored specimen surface. Tensile tests of these specimens were carried out by the testing machine for micro-sized materials. In this study, tensile properties of the specimens with diameters smaller than average grain size of the sample were investigated. Tensile tests showed the clear yield points and gradual work hardening behaviors on the stress-displacement curves. All specimens exhibited strengths like a single crystal. The yield and tensile strengths were about 100 and 180~240 MPa, respectively. These specimens exhibited anisotropic fracture surfaces after tensile tests. Scanning electron microscope observations revealed the deformations of these specimens mainly occurred at grains placed between grain boundaries. The specimen with diameter less than or equal to about maximum grain size of the sample exhibited strength like a single crystal.