SCRIPTA

Crystallographic orientations and growth directions of dendrites in thin Al-15 wt.%Cu (200 µm) and Al-29 wt.%Ge (500 µm) alloys were investigated employing in situ X-radiography and post-mortem X-ray tomography and electron backscatter diffraction. The Al-Cu alloy was solidified under microgravity conditions, while the Al-Ge sample was processed horizontally on ground. It was found that (i) dendrites nucleate on both sides of the surface in microgravity, but accumulate under normal gravity near the top surface due to buoyancy, (ii) dendrites grow along preferred in-plane crystallographic directions, which are <100> for Al-15 wt.%Cu and <100> , <110> and <210> for Al-29 wt.%Ge, indicating that the solid-liquid interfacial anisotropy of this Al-Ge composition is similar for these <xy0> directions, (iii) slightly inclined dendrite arms in Al-Cu grow along the sample boundary after contact, whereas tips of inclined dendrite arms in Al-Ge never touch the boundary, but develop new primary tips originating from secondary branches.

SCRIPTA

Describing microstructure evolution in tungsten requires a quantitative description of the anisotropic grain boundary energy. We present a grain boundary energy function for tungsten that specifies the energy of an arbitrary boundary given its five macroscopic crystallographic parameters. A comparison of measured grain boundary areas and the grain boundary energies given by the function at the Σ11, Σ17b, and Σ33a misorientations, which are problematic to determine by measurement or atomistic calculations, reveals inverse correlations that are similar to what have been observed in other metals.

SCRIPTA

Strain-induced martensitic transformations can improve the strength and ductility of some face centered cubic (FCC) metals and alloys. Irradiation-induced defects such as vacancies, dislocation loops, and voids activate martensitic transformations over a wider range of mechanical loading conditions than in pristine material. However, the mechanisms underlying irradiation-enabled martensite transformations remain unclear. In this work, we use molecular dynamics simulations to study the effect of vacancies and voids on strain-induced martensitic transformations in a model concentrated solid solution alloy Fe-50 at.% Ni. It has been found that single vacancies have no resolvable effect on the transformation because they reduce the stacking fault energy by a relatively insignificant margin and do so only if the vacancy is located on the stacking fault plane. Voids, however, activate the martensite transformation through shear strain accumulation around the void due to dislocation pileup. The larger the void, the more pronounced this effect.

SCRIPTA

Hardening of Ni–Cr–Mo corrosion-resistant alloys facilitates the expansion of their industrial applications; however, it remains challenging. Herein, we report significant hardening in a Co-, Cu-modified Ni–Cr–Mo alloy under appropriate cold swaging/aging conditions. Age-hardening occurred over a relatively short period (∼0.5 h) and became clear upon cold-swaging to an area reduction of ≥60%, exhibiting peak hardness at 500–600 °C. Exceptional hardness (HV599), unattainable in conventional Ni–Cr–Mo alloys, was obtained after aging. Scanning transmission electron microscopy revealed that multiple nanoscale segregation mechanisms, including Suzuki segregation at stacking faults and Cr-rich nanodomains within the severely cold-swaged face-centered cubic matrix, were responsible for the resultant hardening. Furthermore, segregation of Cu, alongside Cr and Mo, along the deformation-induced boundaries induced a fine dispersion of nanoscale Cu precipitates throughout the matrix, imparting further hardening. A superior combination of hardness and corrosion performance was realized through this strategy.

SCRIPTA

A low density1.25C-5Al-1.5Cr steel involving both cementite and κ-carbide in the divorced eutectoid transformation (DET) led to successful softening from 416 ± 3 HV20 to 285 ± 1 HV20. The planar disregistry between two carbides was calculated to be 5.26%, which suggested that the undissolved cementite particles dispersed in the steel provided heterogeneous nucleation site for κ-carbides during the DET process. The orientation relationship (OR) between κ-carbide and cementite are (100)κ//(11-0)θ and [011]κ//[111]θ. The study also revealed that only cementite was retained during intercritical austenitization and was effective as nucleating agent for κ-carbide during cooling. Finally, spheroidal κ-carbide was connected to cementite and dispersed in the ferrite matrix and was present after the DET reaction. An orientation relationship (OR) between κ-carbide and cementite is defined for the first time.

SCRIPTA

Four different configurations of single-atomic-layer-height disconnections on (101-2) twin boundary in magnesium are investigated using molecular dynamics simulations. It is found that a prismatic-basal (PB) single-atomic-layer-height disconnection is more stable than a basal-prismatic (BP) single-atomic-layer-height disconnection when each of them is connected with an I₁ stacking fault associated with a stacking sequence change of BABCB. In contrast, a BP single-atomic-layer-height disconnection is more stable than a PB single-atomic-layer-height disconnection when each of them is connected with an I₁ fault associated with a stacking sequence change of ABACA. A stable BP (or PB) single-atomic-layer-height disconnection can transform to a stable PB (or BP) single-atomic-layer-height disconnection when its step orientation changes with a change in the lying plane of the connected I₁ fault.

SCRIPTA

A new low-alloyed Mg-2Sm-0.8Mn-0.6Ca-0.5Zn (wt.%) alloy is prepared by low-temperature and low-speed extrusion. The as-extruded alloy has ultra-high yield strength (YS, 453 MPa) but poor elongation (3.2%) mainly due to the formation of a fine-grained structure containing high-density residual dislocations and Mn nanoparticles. More importantly, after subsequent simple annealing, the alloy exhibits an excellent combination of high-strength and high-ductility, with the YS of 403 MPa and elongation of 15.5%. The effective inhibition of grain growth by grain boundary (GB) co-segregation of Sm/Zn/Ca is crucial for the annealed alloy to maintain high strength. Appropriately decreased dislocation density, especially the evolution of immovable long S-<c+a> dislocations towards new GBs, is a key factor for the remarkable increase of ductility for the annealed alloy. Thus, we put forward a new strategy for developing low-alloyed Mg alloy with high strength-ductility mainly based on dislocation evolution and GB segregation.

SCRIPTA

Superplastic Pb-Sn alloys were produced via tube high-pressure shearing (t-HPS), in a single step starting from elemental solid bulks. A Pb-40 wt% Sn alloy showed an exceptional superplastic elongation as high as ∼1870% at a strain rate of 1.0 × 10⁻³ s⁻¹ at room temperature, thereby elevating the optimum strain rate for maximum elongation under these conditions by more than one order of magnitude over conventional cast Pb-Sn alloys. This unprecedented room temperature superplasticity is attributed to the equiaxed grains having uniform sizes of the order of one micrometer, and in particular to the well-mixed domains of Pb and Sn in nearly equal proportions. This microstructure cannot be attained in cast eutectic or hypoeutectic alloys through conventional thermomechanical processing, but instead it is a direct outcome of t-HPS-generated compositional patterning at room temperature.

SCRIPTA

In this work, we report an experimental and theoretical study of the (130) twin system in the U-14.0 at.% Nb alloy using aberration-corrected TEM in combination with first-principles calculations. We clearly give the atomic structure of (130) twin and its twin boundary energy for the first time. We evidence that the widely used (130) twin system in pseudo-orthorhombic system is in fact the (101) twin system in the monoclinic crystal system. Through first-principles calculations, we also find that the local atomic structure of Nb significantly affects the twin boundary energy due to the depleted charge zone at Nb atom and charge-rich zone around Nb atom. These results may help us better understand the deformation mechanism in low-symmetry U metal and U-Nb alloys.