Solid State Phenomena Vol. 383

Abstract: The use of Fe-Si electrical steels in transformer and electrical motor cores requires the material to be cold rolled into thin sheets, as this process efficiently achieves the final dimensions while ensuring desirable magnetic and mechanical properties. For Fe-6.5 wt%Si, hot rolling of this alloy goes well. However, direct cold rolling of the Fe-6.5 wt% Si alloy cause metal cracking due to its inherent brittleness behavior. In this work, we are interested in evaluating the effect of the quenching medium after annealing at 1000°C/1h on the formation of B2 and DO3 ordered phases, which are the main sources of brittleness in the alloy, and on the alloy's mechanical properties. The results show that the quenching in ice-brine reduces the hardness of the material to around 340 HV, with the presence of a cooling rate gradient for each quenching medium, which then causes a micro-hardness gradient. Is shown that an ordering gradient is at the origin of such a harness variation. The size of the ordered domains observed by TEM and the volume fraction of ordered phases measured by XRD decrease after quenching in ice brine, and goes from a continuous form (smoot bending) for quenching in liquid nitrogen to a point form. It is concluded that, the choice of a quenching medium that efficiently removes heat from the sample without changing its physical state can limit the formation of ordered phases in Fe-6.5 wt%Si electrical steel.

Abstract: This study investigates the fatigue performance of additively manufactured H13 hot work tool steel (AM-H13 TS) produced using the laser powder bed fusion (L-PBF) process with two distinct build orientations: vertical (V-BO) and diagonal with 45° (D-BO). A fixed volumetric energy density of 57.3 J/mm³ was employed for fabrication. The study compares the as-built AMH13 TS to its surface-finished counterpart, focusing on fatigue life and damage under fully reversed tension-compression loading conditions. The surface finishing processes involved electropolishing using commercial DLyte 100HF+ equipment, followed by mechanical surface refinement. The surface topography and roughness characteristics of the as-built and post-polished specimens were comprehensively analyzed using laser confocal scanning microscopy (LCSM). Scanning electron microscopy (SEM) was utilized to examine the microstructural features and fatigue mechanisms. The as-built AM-H13 TS exhibited high surface roughness due to the presence of satellites and partially melted particles, which are inherent to the L-PBF process. The surface-finishing approach substantially mitigated these surface imperfections, resulting in significantly improved surface quality and reduced roughness. As a result, the fatigue performance of surface-finished AM-H13 TS showed remarkable enhancement. The fatigue limit increased fivefold, from 100 MPa in the as-built condition to 500 MPa after surface finishing. SEM analysis revealed that the improved fatigue strength was primarily attributed to the reduction in surface roughness and the elimination of surface flaws, which acted as crack initiation sites in the as-built condition.

Abstract: This study presents the characterization of 316L stainless steels fabricated by selective laser melting (SLM), focusing on the influence of printing parameters on microstructure and mechanical properties. The choice of process parameters is crucial for achieving desired material properties, as it directly affects the microstructure and mechanical behavior, which is important when optimizing for potential applications in several fields, such as aerospace and automotive. In this study, different scanning speeds were tested to identify optimal settings, followed by the evaluation of the effects of orientation relative to the build plate and hatching strategies to enhance performance. To assess the impact of these factors, tensile tests, microhardness measurements, and X-ray diffraction (XRD) analyses were conducted. Tensile tests revealed that higher laser scanning speed generally reduces ultimate tensile strength and elongation, likely due to an increase in porosity and a less homogeneous fusion of layers. The analysis of samples printed with different orientation relative to the build plate highlighted a strong mechanical anisotropy, with the samples printed vertically exhibiting lower tensile strength and ductility compared to horizontally printed samples. Microhardness testing further confirmed an anisotropy in material properties. XRD analysis reveals a preferential orientation of austenitic grains depending on building direction. This, in turn, influences the anisotropic behavior. These findings highlight the critical role of process parameters in tailoring the microstructure and mechanical performance of SLM-produced parts, thereby providing insights into the optimization of additive manufacturing for specific applications.

Abstract: This study investigates the heat management in Duplex Stainless Steel (DSS) wall specimens produced by the Wire Arc Additive Manufacturing (WAAM), by optimizing the thermal cycles during their construction. A reduction in the inter-pass time was found to induce heat accumulation, which causes wall flaring, affects the ferrite/austenite balance, and leads to a reduction in the hardness of the mechanical part produced by DSS. In contrast, the introduction of an external cooling system enhances the geometry and helps control the temperature and microstructure of the produced parts, thereby achieving the required hardness. Therefore, the use of the external cooling system enables the production of parts with desirable mechanical properties in a shorter time.

Abstract: Fatigue crack generation and propagation processes in oxygen-free copper for power equipment were investigated in a time series to search for new parameters that indicate the fatigue damage degree. The damage behavior of crystal grains was observed by optical microscopy, electron backscattered diffraction (EBSD) analysis and elastic strain analysis. The obtained results suggest that the change in grain orientation spread (GOS) and grain average misorientation (GAM) values is possible to detect the fatigue crack generation. Moreover, it was found that the change in the plastic strain range is also possible to detect it.

Abstract: The present study aims to investigate the mechanical behaviour of pure cobalt in plasticity at varying temperatures, through in situ thermomechanical loadings under Electron Backscattered Diffraction (EBSD) in order to gain a deeper understanding of the various mechanisms that occur. EBSD analysis allows for the determination of microstructural parameters at a refined scale including grain size, grain misorientation, crystallographic and morphologic texture, and phase ratios, which can be employed to establish a correlation between microstructural changes and deformation mechanisms with temperature. Analyses were conducted in situ using either a furnace that can reach 1000 °C, or a 10 kN thermomechanical device, which enables simultaneous heating and mechanical loading. Both test types were automated in the Scanning Electron Microscope (SEM) by correlating the stage movement with the region of interest, enabling EBSD mappings to be acquired always at the same location. Mappings were post-processed via the spherical indexing process, which yields high-quality indexation (with a reduced number of points exhibiting a Confidence Index of less than 0.1), even at high strain levels. Such experiments conducted on cobalt demonstrated austenitic and martensitic transformations between hexagonal close packed and face centred cubic phases with temperature. Indeed, approximately 31.6 % of the initial face centred cubic phase has transformed into the hexagonal phase for 8 % strain during an in situ tensile test. This transformation is initiated in plasticity by dislocation motions in basal planes and subsequently accelerated by the concurrent activation of mechanical twinning. Additionally, an in situ thermal treatment in the SEM enabled the accurate determination of the phase transformation temperature: 460 °C during heating and 350 °C during cooling, corresponding to the points where the cubic phase fraction reaches a 50 % relative change.

Abstract: Recovery usually softens strain hardened coarse grained metallic materials but it can increase the strength of ultrafine grained materials. The present work shows evidence that dynamic recovery can produce strain-hardening behavior in ultrafine grained aluminum processed by high pressure torsion. Mechanical testing reveals an increase in flow stress during low strain rate tensile tests and a decrease in strain rate during creep tests. No significant change is observed in the grain size. It is shown that this effect can be used to increase the uniform elongation of these materials.