[en] Highlights: • Sigma phase was detected in solution annealed 316L by combining mechanical testing and in-depth microstructural analysis • Simulated diffraction patterns are used to validate low quality indexations of phases using EBSD • Additions to standard qualification procedures of steels containing high amounts of Cr and Mo are recommended MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) is an accelerator driven system, currently under development at SCK CEN in Mol, Belgium. This nuclear system will use liquid Lead-Bismuth eutectic alloy as a spallation target for fast neutron production and as coolant. The ideal structural material for a liquid metal cooled reactor should be unsusceptible to both liquid metal embrittlement and liquid metal corrosion, while possessing high toughness. Nuclear grade austenitic stainless steels similar to AISI 316L have therefore been chosen as the main candidate structural materials for MYRRHA. In the framework of the qualification of those candidates, a specific batch of this material has shown unexpectedly poor mechanical properties, which triggered the need of in-depth microstructural analysis. The behaviour was attributed to the unexpected and undesired presence of intermetallic σ-phase. The σ-phase was identified with a high confidence by combining the data for chemical composition from energy-dispersive X-ray spectroscopy and crystallographic information from electron backscatter diffraction by comparing simulated Kikuchi diffraction patterns with experimentally recorded ones. At first sight the optical appearance of σ-phase resembles δ-ferrite islands, which results in the risk of overlooking this when only classical material qualification methods are used. When left undetected, testing this material including the brittle σ-phase in a liquid metal environment, in combination with miniature mechanical test specimens, could lead to misinterpretation of embrittlement of the austenitic matrix.

[en] The occurrence of shear bands in the rotated Goss ({1 1 0}〈1 1 0〉) orientation of Fe–1.2 wt.% Si alloy was investigated by the hypothesis of the geometric softening mechanism. The reduction of Taylor factor and the orientation evolution of in-band crystals were calculated by applying the full-constraints Taylor crystal plasticity model. The presence of in-grain shear bands and particularly the Cube-oriented crystal volumes in shear bands was predicted by the models and confirmed by experimental electron backscattered diffraction observation.

[en] The softening effect in metals due to ultrasonic vibration is used in many industrial applications. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic treatment either imposes additional stress waves to supplement the quasi-static applied load or causes heating of the metal. In both cases the intrinsic deformation resistance and/or mechanisms of the metal are assumed to be unaltered by the ultrasound. In this study, the effect of an in situ ultrasonic treatment on the microstructure of low-carbon steel (Fe–0.051C–0.002Si–0.224Mn–0.045Al (wt.%)) under tensile deformation is reported. Detailed microstructural analyses reveal that the ultrasonic treatment intrinsically alters the deformation characteristics of the metal. The deformation microstructure underneath the area of treatment in the deformed samples was investigated by a combination of optical microscopy, scanning electron microscopy, crystal orientation mapping by electron backscattered diffraction and X-ray diffraction. The results show that the dislocation density and the fraction of low-angle grain boundaries decrease significantly, accompanied by preferential grain rotation. The softening effect of the ultrasound is found to drive recovery associated with a significant reduction in subgrain formation during deformation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, the reduction in subgrain formation is shown to occur due to the combined application of the quasi-static loading and the ultrasound, but is not a simple addition of the two factors acting separately. The effect of the ultrasound can be attributed to its ability to enhance dislocation dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances, thereby increasing the probability of annihilation

[en] The role of grain and phase boundary misorientations during nucleation of ferrite in austenite has been investigated. Electron back-scatter diffraction (EBSD) was performed on a high-purity iron alloy with 20 wt.% Cr and 12 wt.% Ni with austenite and ferrite stable at room temperature in order to identify the crystallographic misorientation between austenite grains and between ferrite and austenite grains. It is observed that the specific orientation relationships between ferrite and austenite play a dominant role during solid-state nucleation of ferrite. Ferrite grains nucleate on grain faces independently of the misorientation between austenite grains, although random high-angle grain boundaries have a slightly higher efficiency. Different types of nucleation mechanisms are found to be active during ferrite formation at grain faces. A slight deformation of the austenite matrix was found to triple the number of ferrite nuclei during isothermal annealing

[en] The microtexture of secondary α plates in Ti-4.5Fe-6.8Mo-1.5Al has been investigated by electron backscatter diffraction (EBSD) to obtain more insight in the nucleation and variant selection of these α plates. A statistical analysis of the EBSD data shows that for most β grain boundaries the variant selection of the α plates is in agreement with a commonly used variant selection criterion yielding that the α-{0 0 0 1} pole is nearly parallel to the closest β-{1 1 0} poles of the two adjacent β grains. For a small angle between the β-{1 1 0} poles nucleation is predominantly observed at both sides of the grain boundary, while with increasing angle some β grain boundaries exhibit nucleation of α plates at only one side. In the β grain interior many so-called Type 2 α-α grain boundaries are observed which are thought to originate from autocatalytic nucleation when a new α plate is formed at an existing α-β interface

[en] Third generation advanced high strength steels produced via quenching and partitioning (Q&P) treatment are receiving increased attention. A 0.25C–3Mn–1.5Si–0.023 Al steel was subjected to Q&P processing (with varying partitioning temperature and time) resulting in the formation of complex multi-phase microstructures. The effect of Q&P parameters on the microstructure and morphology of microconstituents was analyzed. Mechanical properties of the material and of its individual microconstituents were studied via tensile testing and nanoindentation on individual microconstituents, which were identified a priori by electron back-scattered diffraction analysis. Special attention is paid to the effect of the morphology of retained austenite on its transformation stability. In situ tensile tests and digital image correlation analysis were performed to study deformation behavior of the Q&P processed steel at the micro-scale with respect to the local microstructure. The effect of local microstructure and properties of individual phases on the degree of strain partitioning is discussed

[en] Failure in engineering materials like steels is strongly affected by in-service deleterious alterations in their microstructure. White Etching Layers (WELs) are an example of such in-service alterations in the pearlitic microstructure at the rail surface. Cracks initiate in the rails due to delamination and fracture of these layers and propagate into the base material posing severe safety concerns. In this study, we investigate the microscale fracture behaviour of these WELs. We use in situ elastic-plastic fracture mechanics using J-integral to quantify the fracture toughness. Although usually assumed brittle, the fracture toughness of 21–25 MPa√m reveals a semi-brittle nature of WELs. Based on a comparison of the fracture toughness and critical defect size of WELs with the undeformed pearlitic steels, WELs are detrimental for rails. In the micro fracture tests, WELs show crack tip blunting, branching, and significant plasticity during crack growth due to their complex microstructure. The fracture behaviour of the WELs is governed by their microstructural constituents such as phases (martensite/austenite), grain size, dislocation density and carbon segregation to dislocations and grain boundaries. We observed dislocation annihilation in some martensitic grains in the WELs which also contributes to their fracture behaviour. Additionally, the strain-induced transformation from austenite to martensite affects the crack growth and fracture.

[en] Highlights: • Soaking time greatly affects the microstructure of steel during UFH treatment. • Recovery and recrystallization processes define the matrix microstructure. • The fraction of recrystallized ferrite and martensite increases with soaking time. • Cementite spheroidization during UFH affects the microstructure evolution. • TKD allows to analyze the nanoscale elements of the complex microstructure. -- Abstract: This work focuses on the effect of soaking time on the microstructure during ultrafast heat treatment of a 50% cold rolled low carbon steel with initial ferritic-pearlitic microstructure. Dilatometry analysis was used to estimate the effect of heating rate on the phase transformation temperatures and to select an appropriate inter-critical temperature for final heat treatments. A thorough qualitative and quantitative microstructural characterization of the heat treated samples is performed using a wide range of characterization techniques. A complex multiphase, hierarchical microstructure consisting of ferritic matrix with embedded martensite and retained austenite is formed after all applied heat treatments. In turn, the ferritic matrix contains recrystallized and non-recrystallized grains. It is demonstrated that the ultrafast heating generally results in finer microstructure compared to the conventional heating independently on the soaking time. There is a significant effect of the soaking time on the volume fraction of martensite of the ultrafast heated material, while in the samples heated with conventional heating rate it remains relatively unchanged during soaking. Recrystallization, recovery and phase transformations occurring during soaking are discussed with respect to the applied heating rate.

[en] In the present work, 0, 1, 2 and 3 wt.% Li was added to conventional Al-0.9Mg-0.5Si alloy. The samples were extruded and aged to investigate the effect of Li addition on microstructure, texture and mechanical properties. The density of conventional alloy was reduced up to 7.8% while the ultimate tensile strength (UTS) increased by 62% with 3% Li addition. Electron backscatter diffraction (EBSD) revealed that Li addition effectively refined the grain size of the modified alloys. TEM/EDX and XRD analysis revealed the synergistic effect of Li addition which promoted the formation of nano-sized δ′(Al_3Li) precipitates when Li content is higher then 1%. The ageing trend first decreased for 1 wt.% Li addition and then increased with increasing Li content from 2 to 3 wt.% at the expense of ductility. The intensity of texture increased with the gradual increase in Li content from alloy-1 to 4. - Highlights: • Study of 0, 1, 2 and 3 wt.% Li on Al–Mg–Si alloys in extruded and T6 condition. • Density reduced to 7.8% with UTS increased by 62% for 3% Li addition. • Texture intensity increased with increase in Li content from alloy-1 to 4. • Property enhancement attributed to a refinement of δ′ (Al_3Li) precipitates.

[en] A methodology to distinguish martensite formed in the first quench (M1) from martensite formed in the second quench (M2) of the Quenching and Partitioning process is presented, enabling the study of the structural characteristics of both microstructural constituents. Investigations show that M1 displays larger block size and less lattice imperfections than M2, differences that can be related to their respective carbon contents. - Highlights: • An approach to distinguish “old” from “new” martensite in Q and P steels is presented • Methodology allows separate characterization of microstructure and crystallography “Old” martensite has larger block size and more perfect lattice than the “new” one • The differences between the old and new martensite depend on their carbon contents