Accessing the phase transformation and deformation behavior of metastable stainless steels through cyclic nanoindentation

Austenitic metastable stainless steels are a materials group distinguished by their excellent mechanical properties, offering high potential for further improvement by thermo-mechanical treatments. Under deformation, these steels undergo a complex deformation and phase transformation. Their mechanical properties at macroscale, such as strength, ductility or fatigue behavior, have been largely investigated, yet they are not always predictable, as they highly depend on the microstructural characteristics of the material. In order to achieve a better understanding at the microstructural level, this work aims at the investigation of the deformation mechanisms in metastable stainless steels at sub-grain level and the interaction between grains. Therefore, monotonic and cyclic nanoindentation tests were performed in order to increase the cumulative deformation in a controlled way with the number of cycles. The emerging deformation mechanisms under the indents and on the surface, as well as the resulting morphology and mechanical and magnetic properties of the different phases, were characterized through different advanced microscopy techniques. It was found that, even after a high number of nanoindentation cycles, a loading-unloading hysteresis is present, indicating a reversible plastic behavior (which is believed to be due to formation of unstable dislocations at maximum load). The apparent hardness of the material drops with increasing cycles due to the high plasticity of austenitic stainless steels. Gradual phase transformation was triggered and the load–displacement curves exhibited features, such as pop-ins and changes in the slope and hysteresis size, probably related to the propagation of the induced martensite to the neighboring grains and the resulting stress relaxation. This behavior was found to be highly dependent on the crystalline orientation of the respective indented austenite grains. Nucleation of martensite at shear band intersections was detected by TEM investigation of a horizontal lamella, as well as by MFM, while a FIB tomography revealed the shape and location of the nucleated martensitic zones

LACDIF, a new electron diffraction technique obtained with the LACBED configuration and a Cs corrector: Comparison with electron precession

International audienceBy combining the large-angle convergent-beam electron diffraction (LACBED) configuration together with a microscope equipped with a Cs corrector it is possible to obtain good quality spot patterns in image mode and not in diffraction mode as it is usually the case. These patterns have two main advantages with respect to the conventional selected-area electron diffraction (SAED) or microdiffraction patterns. They display a much larger number of reflections and the diffracted intensity is the integrated intensity. These patterns have strong similarities with the electron precession patterns and they can be used for various applications like the identification of the possible space groups of a crystal from observations of the Laue zones or the ab-initio structure identifications. Since this is a defocused method, another important application concerns the analysis of electron beam-sensitive materials. Successful applications to polymers are given in the present paper to prove the validity of this method with regards to these materials

The Effects of Nitrogen on Kinetics and Products of Austenite Decomposition in Low-alloy Steel

International audienceIn order to better understand the effect of nitrogen on austenite decomposition in low alloy steel, specimens of 23MnCrMo5 homogeneously enriched in nitrogen, carbon or a mix of both were isothermally transformed at temperatures ranging from 200°C to 750°C. The transformation progress was monitored in situ by HEXRD and post mortem microstructures were characterized using SEM and TEM, including HRSTEM and ACOMTEM. At all temperatures, nitrogen reduces the incubation time of the transformation of austenite into ferrite and speeds up its completion. High volume fractions of CrN, both inter-and intragranular, were observed at all temperatures with different morphologies and very wide size distribution. It has also been shown that other nitrides precipitate during the various stages of the treatment, notably AlN, VN and MnSiN2. In nitrogen containing specimens transformed above BS, ferrite forms as equiaxed grains with interspersed pearlite islands. Between BS and MS, ferrite occurs as entangled laths featuring an extremely high density of nanotwins. The entanglement of the microstructure is reminiscent of acicular ferrite, albeit at a much finer scale, and it is thought to result from the particle stimulated nucleation of ferrite on CrN. As a comparison, bainite obtained in specimens containing carbon only was coarser, not entangled and did not show any twinning

Nitrogen-induced nanotwinning of bainitic ferrite in low-alloy steel

International audienceThe isothermal decomposition of austenite between BS and MS was investigated after carburizing and carbonitriding in low-alloy steel 23MnCrMo5. The microstructure of the products differed significantly between carburizing and carbonitriding. The carburized alloy produced an expected bainitic microstructure consisting mainly of bainitic ferrite with some cementite and retained austenite. In contrast, the microstructure of the nitrogen-containing sample showed finer, entangled ferrite plates, retained austenite and nitrides. The ferrite plates contain numerous transformation twins, which can be as thin as only a few nanometers. This original result proves that abundant twinning can be found in bainite as well as in martensite

Microstructure and functionality of a uniquely graded super duplex stainless steel designed by a novel arc heat treatment method

A novel arc heat treatment technique was applied to design a uniquely graded super duplex stainless steel (SDSS), by subjecting a single sample to a steady state temperature gradient for 10 h. A new experimental approach was used to map precipitation in microstructure, covering aging temperatures of up to 1430 °C. The microstructure was characterized and functionality was evaluated via hardness mapping. Nitrogen depletion adjacent to the fusion boundary depressed the upper temperature limit for austenite formation and influenced the phase balance above 980 °C. Austenite/ferrite boundaries deviating from Kurdjumov–Sachs orientation relationship (OR) were preferred locations for precipitation of σ at 630–1000 °C, χ at 560–1000 °C, Cr2N at 600–900 °C and R between 550 °C and 700 °C. Precipitate morphology changed with decreasing temperature; from blocky to coral-shaped for σ, from discrete blocky to elongated particles for χ, and from polygonal to disc-shaped for R. Thermodynamic calculations of phase equilibria largely agreed with observations above 750 °C when considering nitrogen loss. Formation of intermetallic phases and 475 °C-embrittlement resulted in increased hardness. A schematic diagram, correlating information about phase contents, morphologies and hardness, as a function of exposure temperature, is introduced for evaluation of functionality of microstructures.  Funding details: VINNOVA; Funding details: 2016-02834; Funding details: 20140130; Funding text: The financial support from the KK-foundation for the research school SiCoMaP ( 20140130 ) and the DUWELTOOL project ( 2016-02834 ) funded by Vinnova is acknowledged.</p