Scripta Materialia最新文献

Dislocation loops are one type of irradiation defects that severely degrade the mechanical properties of nuclear materials. In this study, we found that hydrogen atoms in irradiation environment significantly modify the loop properties including loop types and shapes during in-situ hydrogen irradiation. <100> loops have been energetically stable from 300 °C in H⁺ irradiated iron whereas the stability of <100> loops is delayed until 500 °C in Fe⁺ irradiated iron. Meanwhile, both <100> loops and 1/2<111> loops exhibit polygonal shapes with sharp corners at 400 °C after H⁺ irradiation, similar to loops predicted at 900 °C without hydrogen. Our results highlight the importance of considering the hydrogen effects in dislocation loop evolution and stability.

Hydrogen-enhanced decohesion (HEDE) is a proposed mechanism of hydrogen-induced grain boundary (GB) fracture in metals and has been widely calculated from first principles over the past decade. However, the effect of GB-segregated solutes on HEDE is complex and rarely quantified. This study presents a quantitative numerical estimation method based on statistical thermodynamics using first-principles calculations of multiple hydrogen trappings at a GB and its fracture surfaces with segregated solutes. This method accurately estimates the lattice-dissolution-hydrogen-dependent HEDE, including the interactions caused by the segregated solute: the decohering or cohesion-enhancing effect of the solute itself, solute-hydrogen interaction, solute-affected hydrogen-hydrogen interaction, and mobile hydrogen effect. We present a trial calculation to examine how the attractive interaction between solute and hydrogen influences HEDE, showing that HEDE can be induced at lower hydrogen concentrations if not canceled by other interactions.

The effect of the complex stress on the mobility of an edge dislocation on the first pyramidal plane in magnesium is investigated through molecular dynamic simulation (MDs). A novel dislocation with greatly improved mobility is obtained by applying the combined compressive normal stress and shear. The Peierls stress of the new dislocation is reduced to less than a tenth of the original and the mobility factor increases almost twice. Based on the analyses of atomic configuration and the energy barrier of vacancy formation, the mobility improvement is ascribed to the dislocation core reconstruction and its related dissociation. These new findings are also validated in other HCP metals e.g. Ti and Zr.

By using quasi-in-situ EBSD, we successfully probed the nucleation and growth of recrystallized grains during sub-solvus annealing at 1300 °C of an as-cast single-crystal superalloy after pre-compression. It clearly indicated that recrystallization nucleated in the interdendritic region through low-angle grain boundary migration under the experimental condition rather than subgrain coalescence or thermal twinning. Despite being hindered by the undissolved γ′ precipitates in the interdendritic region, these newly formed high-angle grain boundaries could still migrate rapidly. However, the migration velocity will slow quickly as the stored energy decreases. During 10.0-min annealing, the grain boundaries could migrate 14.4–27.4 μm, but the recrystallized grains were still confined in the interdendritic region without abnormal growth. These results provide novel perspective on the comprehension of recrystallization nucleation in single-crystal superalloys and the migration of newly formed high-angle grain boundaries under the hinderance of γ′ precipitates, and are valuable for developing process to control recrystallization.

During isothermal ageing of metastable β-Ti alloys decomposition of the β matrix occurs through the formation of either the ω or α phases. Recent literature has shown that the presence of ω_iso can facilitate α nucleation forming an α variant with a known orientation relationship to the β and ω phases. However, the mechanisms behind such a transformation are unclear, especially in systems with a low ω-β misfit. In this study, the effect of 450 °C ageing on a low misfit alloy, Ti-26Nb (at.%), has been studied using electron backscattered diffraction (EBSD). Isothermal ω precipitates were observed to coalesce along preferred growth directions. EBSD identified the presence of α within the core of the coalesced ω precipitates. The relationship between this α and the parent β was inconsistent with the expected Burgers orientation relationship and did not follow any known crystallographic relationship between α and ω, suggesting a novel transformation pathway.

The fracture behavior of a duplex stainless steel processed by laser powder bed fusion (LPBF) from mixed 25Cr + 5 wt.% Ni powders (i.e., 25Cr-5Ni) was compared to 25Cr that was processed without Ni but additionally heat-treated (i.e., 25Cr-HT) to give similar phase fractions of austenite and ferrite but with different micro and mesostructures. The 25Cr-5Ni alloy exhibited high yield strength (828–958 MPa) and good fracture toughness (122–167 MPa√m) while requiring no post heat-treatment after LPBF fabrication. In contrast, the 25Cr-HT alloy gave somewhat lower strength and higher fracture toughness in a classic strength-toughness trade-off due to the elimination of the high dislocation density from the LPBF process. While powder mixing induces somewhat more anisotropic properties due to a coarser mesostructure phase distribution, the results demonstrate an approach to develop LPBF processed duplex stainless steels that don't require costly post heat treatments.

The ability to control the stress-induced phase transformation of the shape memory alloy, NiTi, is an important technological challenge that must be understood for their wide application in devices that can exploit their reversible strain properties. This study elucidates the direct relationship between dislocation density and the martensitic, B19' & R-phase transformations, including its formation temperature from interrupted annealing of rolled NiTi samples. Deformation is shown to determine the enthalpy change required for the B2→R→B19' transformation, with associated transformation temperatures being modifiable via dislocation density and recovery processes. Recovery is shown to be rapid, highly heterogeneous and sensitive to crystal orientation. Grains with a 〈100〉 direction close to the macroscopic rolling direction recover more rapidly than 〈110〉 and 〈111〉 orientated grains. Considered to be governed by processing induced residual stresses and resultant crystallographic dependent annihilation/slip pathways, there are opportunities to tune B2→R→B19' transformation on either a grain-averaged or an orientation dependant per-grain basis.

Post-irradiation evaluation was performed on a 316 stainless steel baffle former bolt harvested after 40 years of service in a pressurized water reactor. Microstructure analysis revealed the presence of defect-free dislocation channels and strain-induced twins, indicative of loading at a stress level close to yield stress at least once while in service. Primary radiation-induced Ni/Si precipitates were likely destroyed during channel and twin formation, and secondary, significantly coarser Ni/Si precipitates formed inside the newly formed dislocation channels and along ∑3 boundaries during the continued irradiation. Complex chemistry inside the strain-induced features may overlap with dislocation pileups and impact localized corrosion and material long-term performance.

The application range of 2:17 Sm-Co magnets is limited by poor mechanical properties. This study investigates the effects of adding Sm₂O₃ during ball milling and introducing O₂ during jet milling on the mechanical and magnetic properties of 2:17 Sm-Co magnets. With increasing oxygen content, the flexural strength in both Sm₂O₃-added and O₂-added magnets was enhanced. Flexural strength in Sm₂O₃-added magnets increased by 18.5 % with oxygen content (x) rising from 0.07 wt.% to 0.46 wt.%, while in O₂-added magnets, a 43.0 % increase was observed with oxygen content (x) from 0.08 wt.% to 0.41 wt.%. The uniform dispersion of Sm₂O₃ and its grain refinement effect contribute to superior mechanical properties in O₂-added magnets. It can be foreseen that compensation with a certain Sm content can achieve a combination of magnetic and mechanical properties. This study provides insights for developing magnets with exceptional overall performance.

Density Functional Theory calculations of self-interstitial atom clusters in bcc Fe unexpectedly show that from ∼9–14 self-interstitial atoms, an intriguing new family of sessile ⟨100⟩ clusters, surrounded by ⟨110⟩ dumbbells, are more stable than highly mobile clusters of parallel ⟨111⟩ dumbbells. The ⟨110⟩ edge dumbbells find a favorable location in terms of strain energy on the tensile side around the edges of the ⟨100⟩ center, thus stabilizing the clusters. These sessile clusters might explain resistivity recovery results that suggested an absence of glissile self-interstitial clusters up to large cluster sizes in irradiated Fe, while smaller self-interstitial atom clusters likely would have been present. The mechanism of non-parallel edge interstitials stabilizing an otherwise higher energy interstitial loop is also found in some fcc metals.