Jiří Man

Atomic force microscopy of surface relief in individual grains of fatigued 316L austenitic stainless steel

Abstract Surface relief adjacent to the persistent slip bands (PSBs) in polycrystalline 316L stainless steel cycled with constant plastic strain amplitude up to 60% of fatigue life was studied using atomic force microscopy with respect to the crystallographic orientation (determined by electron backscattering diffraction—EBSD) and the size of individual grains. Surface relief is formed mostly by ribbon-like extrusions whose height in a grain was found to be proportional to the thickness of the corresponding PSB. The extrusions grow predominantly in the direction of the active Burgers vector. The height of an extrusion in the direction of the active Burgers vector is proportional to the grain size. No systematic dependence of the extrusion height in the active slip direction on the grain orientation was observed. The experimental results are discussed in terms of the recent vacancy models of surface relief evolution.

Study of surface relief evolution in fatigued 316L austenitic stainless steel by AFM

Abstract Atomic force microscopy was applied to study the surface relief evolution at emerging persistent slip bands (PSBs) in individual grains of polycrystalline 316L stainless steel cycled with constant plastic strain amplitude. Quantitative data on the changes of the surface topography of persistent slip markings (PSMs) and on the kinetics of extrusion growth during the fatigue life of smooth specimens were obtained. The extrusion height can be assessed by a direct observation of the metal surface and the intrusion depth using the plastic replica technique. The first PSMs appear after the initial hardening (0.1–0.25% of Nf). PSBs can be active in the whole period of fatigue life in grains in which no cracks develop. The initial rapid growth of extrusions is followed by a period of stable linear growth up to the end of the fatigue life. The width of extrusions, corresponding to the thickness of the emerging PSBs quickly stabilizes and remains constant during the whole fatigue life. The experimental results are discussed in terms of recent vacancy models of surface relief evolution.

AFM evidence of surface relief formation and models of fatigue crack nucleation

Atomic force microscopy and high resolution scanning electron microscopy were used to find the true surface relief corresponding to persistent slip markings emerging from persistent slip bands on fatigued polycrystalline austenitic and ferritic stainless steels. The persistent slip markings are formed by extrusions and intrusions. The shape of extrusions and intrusions was documented and the kinetics of the extrusion growth was studied in both steels. The experimental data were discussed and compared with the predictions of the recent models of fatigue crack nucleation.

The shape of extrusions and intrusions produced by cyclic straining

Abstract Detailed observations of the identical area on the surface of fatigued austenitic 316L steel and on its inverse copy via plastic replica were performed using atomic force microscopy (AFM) and high-resolution scanning electron microscopy. It was shown that the AFM technique has some limitations in observation and in quantitative evaluation of the surface relief and that the real geometry of extrusions and intrusions can be distorted when adopting AFM. Only the combination of the above experimental techniques allows assessing the true shape of extrusions and intrusions produced by cyclic straining.

Surface Relief and Internal Structure in Fatigued Stainless Sanicro 25 Steel

High-resolution images of persistent slip markings developed on the surface of Sanicro 25 stainless steel during cyclic loading obtained from the FIB-produced surface lamella in TEM simultaneously with the underlying dislocation structure are reported. Extrusions, intrusions, incipient cracks, and the dislocation arrangement corresponding to the bands of intensive cyclic slip are documented and discussed in relation to the models of surface relief formation in cyclic loading.

Orientation dependence of surface relief topography in fatigued copper single crystals

Abstract Surface relief of cylindrical copper single crystals oriented for single slip and cyclically strained in plastic strain control up to fracture was studied using a scanning electron microscope (SEM) and interference light microscope. Both individual persistent slip bands (PSBs) and macrobands were observed. Their length, width and height were evaluated in dependence on the position on the specimen. The macroband height was found to be a periodic function of the position along the specimen circumference with a period of 180 °. In the surface line where the primary Burgers vector is a tangent to the specimen surface even the negative protrusions were observed and the average macroband height was approximately zero. The maximum macroband height was observed at the surface line resulting from the intersection of the surface with the plane parallel to the primary Burgers vector and passing through the specimen axis.

Microstructure of austenitic stainless steels of various phase stabilities after cyclic and tensile deformation

Abstract The microstructures of two metastable high-alloyed CrMnNi cast TRIP steels and a stable AISI 316L austenitic stainless steel were studied in detail after tensile and cyclic deformation. Electron backscattered diffraction was employed to localize the martensitic phase transformation and electron channelling contrast imaging to describe the typical dislocation arrangements. These were complemented by transmission electron microscopy and by scanning transmission electron microscopy performed in a scanning electron microscope. The TRIP steel with the lowest austenite stability shows a more pronounced martensitic phase transformation realized from the austenite via the intermediate formation of ∊-martensite. Martensitic phase transformation also occurred in the stable 316L austenitic stainless steel with a small volume fraction of α′-martensite, but only with cyclic deformation at low temperatures and/or at very high plastic strain amplitudes.

R-Curves and fracture toughness transition behaviour at static, rapid and impact loading of Cr-N-Mo-V reactor pressure vessel steel

Abstract The fracture toughness of Cr-Ni-Mo-V reactor pressure vessel steel was investigated over a wide temperature range. In the upper shelf fracture toughness region, R -curves were determined using large SEN bending specimens and small side-grooved precracked Charpy sepecimens. Two approaches were used for the evaluation of the fracture toughness scatter in the transition and in the lower shelf fracture toughness region: the exponential function with a least-squares regression procedure for a fracture toughness-temperature data and Weibull statistics for a fracture toughness data set obtained at the given temperature. The investigation of the influence of loading rate, ranging from 1·0 MPa m 1 2 s −1 (static) to 3 × 10 4 MPa m 1 2 s −1 (most rapid), on the behaviour of fracture toughness has shown a shift of cleavage/fibrous transition temperature by 60°C. In contrast to that, at impact loading using a precracked Charpy specimen (K I = 2 × 10 5 MPa m 1/2 s −1 ) the shift of this transition temperature was only 30°C, and a sudden transition of fracture toughness occurred from the lower shelf to the upper shelf toughness region.

Fatigue Crack Initiation in Crystalline Materials – Experimental Evidence and Models

Recent observations relevant to the early stages of the fatigue damage of crystalline materials are reviewed. Experimental evidence on the localization of the cyclic plastic strain and on the surface relief formation in cyclic loading of 316L austenitic stainless steel is presented. The focused ion beam is used for exposing three-dimensional evidence of persistent slip markings (PSMs). PSMs consist of extrusions and parallel or alternating intrusions which develop during cyclic loading. Monte Carlo simulations of vacancy generation within persistent slip band (PSB) and their migration to the matrix where they annihilate on the edge dislocations are used to simulate the growth of extrusions and intrusions. The results of the simulations are compared with experimental data and discussed in terms vacancy models of fatigue crack initiation.

Influence of Crystalline Elasticity on the Stress Distribution at the Free Surface of an Austenitic Stainless Steel Polycrystal. Comparison with Experiments

This numerical study focuses on the recent observations of Man et al. [4] showing welloriented grains presenting no Persistent Slip Marking even if PSMs are observed in 86% of the surface grains in 316L austenitic stainless steel cycled at room temperature up to 60% of fatigue life. Scanning Electron Microscopy (SEM) permits us to build Finite Element (FE) meshes of the observed aggregates and to assign to the modelled grains the crystallographic orientations measured by Electron Back Scattering Diffraction (EBSD). Then, 3D FE computations using crystalline elasticity allow the evaluation of mean grain stress tensors and resolved shear stresses. The results could explain qualitatively the anomalous behaviour of the studied well-oriented grains which is partly due to the particular orientations and shapes of the neighbour grains. This study highlights the influence of crystalline elasticity and neighbour grains in microplasticity and crack nucleation.