Materialia最新文献

Objective

Materials and methods

Results

Conclusion

Clinical relevance

Grain boundary diffusion of Ni in the equiatomic CoCrFeMnNi high-entropy alloy, produced by additive manufacturing, is measured using a radiotracer technique in an extended temperature interval of 350 to 703 K. A strongly non-monotonic temperature dependence of the Ni grain boundary diffusion coefficients (with a spectacular intermittent retardation of the diffusion rates with increasing temperature) is seen and explained by relaxation of a non-equilibrium state induced by rapid solidification during fabrication. The grain boundary excess energy of the non-equilibrium state of these grain boundaries, as estimated from the diffusion data, is found to be larger than 0.3 J/m${}^2$. This corresponds to an increase of about 30% of the interface energy compared to relaxed general high-angle grain boundaries. The temperature-induced evolution of the grain boundary state is analyzed in terms of the concomitant structure evolution, segregation, phase stability and precipitation in the multi-component alloy.

As-deposited Wire-Arc Additive Manufactured (WAAM) Ti-6Al-4V parts typically contain large columnar β-grains on a centimetre scale, with a strong 〈001〉 fibre texture, leading to anisotropic mechanical properties and unacceptable scatter in damage tolerance. Inter-pass deformation, introduced by the application of Ultrasonic Impact Peening (UIP) across each added layer, has been shown to be effective in refining the β-grain structure and achieving a weaker texture. The depth of deformation and the grain refinement mechanism induced by UIP have been investigated by combining advanced electron backscatter diffraction (EBSD) characterization with a ‘stop action’ observation technique. UIP facilitates a similar refinement mechanism and nearly the same depth of deformation as conventional machine hammer peening, with the advantages of a much higher strain rate, lower peak force, and two orders of magnitude lower impact energy, making it a faster and more economical process. β recrystallization is seen within the deformation zone during re-heating through the α → β transition. Although new recrystallized β-grains formed in the UIP surface-deformed layer to a shallower depth than that of remelting, recrystallization initiated ahead of the melt pool and the recrystallized grains grew downwards to a greater depth before remelting. These refined grains were thus able to survive and act as nucleation sites at the fusion boundary for epitaxial regrowth during solidification, greatly refining the grain structure.

Generally, the fatigue strength of ultra-high strength steel (UHSS) and high strength steel (HSS) arc-welded joints are comparable regardless of base metal's strength. Still, the micro-needle peening (MNP) method can improve the fatigue strength to the level of those of base metals. To understand the mechanism of this improvement, this paper investigates the microstructure of UHSS (tensile stress grade of 980 MPa) arc-welded joints treated with MNP and compares it to HSS (tensile stress grade of 440 MPa) joints. We focus on the presence of nanotwins, which exhibited a minimum thickness of 4.7 nm, observed in the UHSS joints following the MNP treatment. Importantly, these nanotwins demonstrated remarkable stability even under cyclic loading conditions (nominal stress σ_n = 600 MPa, N = 3 × 10⁶ cycles). This indicates that the nanotwins contribute to the significant improvement in fatigue strength demonstrated by MNP. However, the nanotwins were not observed in the HSS joints, suggesting sufficient driving stress is necessary for their occurrence. The dislocation pileup stress at the grain boundary during twinning was estimated by the thickness of the twin, which was 8.1 GPa. This value is of the same order of magnitude as the 3.7 GPa estimated by the Hall-Petch coefficient for ferritic steel. The lower levels of C, Si, and Mn can contribute to the lower pileup stress, resulting in absence of the nanotwins in the 440 MPa joint. Overall, this study provides insights into the microstructural changes induced by MNP treatment and their impact on the fatigue strength.

This study reports experimental measurements of spacing selection of confined dendrite growth in chill-cast stainless steel under transient cooling conditions. This phenomenon is also explored using phase-field simulations under non-steady state cooling conditions. Two phase-field models are employed, a ternary phase-field (PF) model where Ni and Cr are explicitly simulated, and a pseudobinary model which acts as a special case. Simulations reveal a strong finite-size dependence in the primary arm spacing (PAS) of cells and dendrites. This behavior is also observed experimentally within individual grains, where dendritic fronts evolve within parent grains that impose constraints on the solidification front. Quantifying PAS by a characteristic length ${\lambda }_1$, this metric is observed to exhibit a stick–slip behavior as the front advances, which corresponds to times in the solidification of slow (or no) change in ${\lambda }_1$ followed by a rapid increase in ${\lambda }_1$ caused by a succession of cell elimination events. The statistics of cell extinction are also analyzed, finding a correlation in the statistical time between extinction events and system size. Specifically, simulations reveal that the length of time of such PAS plateaus is stochastic, exhibiting a mean time that decays approximately exponentially with system size. As system size increases, this effect diminishes and a more monotonic relation between ${\lambda }_1$ vs. front speed is observed, consistent with classic geometric theories.

A systematic parametric study was conducted on alternating current (AC) electric field-assisted freeze-casting to enable a comprehensive understanding of tuning freeze-cast microstructure and compressive strength and provide insights into the role of AC field. A novel finding was that the AC field increased the viscosity of aqueous ceramic suspensions, where the viscosity increase was dependent on the ceramic loading of suspensions, dispersant concentration, and field duration. Viscosity increased with field duration for a fixed solid loading and dispersant concentration. It was suggested that AC field-induced dielectrophoretic (DEP) forces decreased interparticle distances and increased interparticle interactions in ceramic suspensions, hence viscosity. It was revealed that the increase in viscosity of ceramic suspensions due to the AC field could be reversed. It was demonstrated that simple magnetic stirring of the suspensions previously subjected to an AC field (which increased viscosity) reduced viscosity to the level of the as-prepared suspensions. For materials fabrication, an AC electric field was applied to aqueous ceramic suspensions for the desired duration, then turned OFF, followed by freeze-casting, which remarkably influenced freeze-cast sintered microstructure. The impact of the field on microstructure increased with solid loading, dispersant concentration, and field duration, and microstructure changes were associated with viscosity of suspensions prior to freeze-casting. With increasing viscosity, freeze-cast microstructure became increasingly dendritic, i.e., bridge density increased. A positive correlation was observed between bridge density and compressive strength for all the materials. Depending on the solid loading, dispersant concentration, and field duration, about 5- to 8-fold increase in strength was achieved.