SCRIPTA Vol. 188, 1 Nov. 2020, P1-5

FeNiCoAlTaB shape-memory alloys are desirable alternatives to other smart metallic materials owing to their low base metal and processing costs. These alloys, however, require strong texture via thermomechanical treatments or directional solidification to obtain good functional properties. Here, we demonstrate the fabrication of FeNiCoAlTaB shape-memory wires by a rapid, continuous wire-casting process. These untextured wires exhibit 7% superelastic strains and 1.2 GPa tensile strength after training, and show good cyclability and low stress hysteresis at room temperature. These properties far surpass previous reports in untextured polycrystalline ferrous alloys, an improvement which we attribute to their unique oligocrystalline grain structure.

SCRIPTA Vol. 188, 1 Nov. 2020, P10-15

Oxide Dispersion Strengthened (ODS) steels are candidates for nuclear applications. ODS are produced by mechanical alloying of Fe-14Cr, Y2O3 and TiH2 powders and consolidation at 1100°C, resulting in finely dispersed nano-oxides. Their precipitation kinetics has been quantitatively determined by in-situ Small Angle X-ray Scattering during continuous heating up to 1100°C. Clusters, found in the as-milled state start growing at 450°C until 1100°C, while almost no coarsening was recorded during subsequent isothermal annealing. The nano-oxides resulting from these in-situ experiments were found to be representative of those in materials processed by hot isostatic pressing and hot extrusion.

SCRIPTA Vol. 188, 1 Nov. 2020, P16-20

A novel non-equiatomic NbTaTi-based refractory complex concentrated alloy (RCCA) capable of being cold-rolled up to a reduction in thickness of over 90% directly from the as-cast state is studied. Instead of conventional heat-treatment at temperatures greater than 0.8Tm for long durations, the excellent cold-workability in this RCCA allows for tunable microstructural features, including fine-grained, coarse-grained, and heterogeneous lamella (HL) structures, through proper heat-treatment at relatively lower temperatures for shorter duration. By tailoring microstructures, balanced tensile properties are achieved in this RCCA, through back-stress strengthening of HL structures. This study demonstrates an energy-saving and efficient way to fabricate high-performance RCCAs.

SCRIPTA Vol. 188, 1 Nov. 2020, P21-25

The deformation behavior of CrMnFeCoNi high entropy alloy was investigated by in situ neutron diffraction at an ultralow temperature of 15 K. Analysis of the diffraction peak widths showed an extremely high dislocation density at 15 K, reaching ~10¹⁶ m^-2. In addition, the dislocation density was found to closely follow the development of texture caused by deformation. In contrast to deformation by dislocation slip at room temperature, the ultralow-temperature deformation also involved stacking faults, twinning and serrations. The deformation pathway at ultralow temperature is outlined which is responsible for the extraordinary strength–ductility combination.

SCRIPTA Vol. 188, 1 Nov. 2020, P44-49

Structure and chemistry of metal/metal-oxide interfaces are critical for many catalytic processes and sensing. Pristine interfaces of Pt and γ-Al2O3 were fabricated using high-energy ion implantation and thermal processing. Amorphous regions of alumina develop in single crystal α-alumina during Pt+ implantation and an 800 °C thermal treatment crystalizes amorphized alumina to γ-Al2O3 and allows Pt ions to precipitate within the developing γ-alumina, yielding Pt nanoparticle tetrahedra terminated by {111} surfaces. The phase of alumina that developed and the distribution, morphology, and orientation of Pt nanoparticles was determined using x-ray diffraction, Rutherford backscattering spectrometry, transmission electron microscopy and scanning transmission electron microscopy.

SCRIPTA Vol. 188, 1 Nov. 2020, P54-58

The present Viewpoint set aims at providing a summary of the recent advancements in the fundamental understanding of high entropy alloys (HEAs) as well as igniting new ideas and activities in this rapidly evolving field of use-inspired basic research. The universality of the core effects in HEAs, ranging from configurational entropy contributions to cocktailing effects are still under a passionate debate and in particular the peer-reviewed articles are meant to provide original perspectives. The various contributions are strongly opinion-based in a variety of areas including diffusion, phase transformations, deformation behavior, corrosion, metastability, structural as well as functional properties. In addition, the impact of the original metallic HEAs onto the field of oxides and ceramics has been illustrated and the role of entropy in high-entropy oxides is critically discussed.

SCRIPTA Vol. 188, 1 Nov. 2020, P59-63

英文摘要

SCRIPTA Vol. 188, 1 Nov. 2020, P74-79

Misfits of γ-γ' based Al₁₀Co₂₅Cr₈Fe₁₅Ni₃₆Ti₆ and its Mo- and Hf-variations are studied up to a temperature of 980 °C and compared with Ni- and Co-based superalloys. The trace elements decrease (Hf) or increase (Mo) the edge radii of the γ' cuboids without changing their sizes. Atom probe measurements revealed that the Hf alloy prefers the γ' phase while Mo prefers the γ matrix, leading to a lattice parameters enhancement of both phases, as could be revealed by synchrotron X-ray diffraction. The misfit is influenced in opposite ways: Hf increases the positive misfit, while Mo reduces it at all investigated temperatures.

SCRIPTA Vol. 188, 1 Nov. 2020, P88-91

We have investigated the effects of solute oxygen on the kinetics of diffusionless isothermal omega (DI-ω) transformation in β-titanium vanadium alloys. This transformation constitutes a third category of ω transformation beside the athermal and isothermal modes. Thermal analysis, hardness and internal friction measurements were conducted after quenching oxygen-containing and near-oxygen-free alloys with ~ 21 at%V from the β-stable temperature. At this level of vanadium concentration, the athermal ω transformation is not expected. It is found that the DI-ω transformation more rapidly progresses in the low oxygen alloy and the relaxation strength of the elementary process of {111}β collapse is significantly reduced.

SCRIPTA Vol. 188, 1 Nov. 2020, P118-123

Plastic deformation of HfNbTaTiZr body-centered cubic high-entropy alloy (BCCHEA) was investigated at 77 K. Tensile stress-strain curves showed strong temperature dependence of yield strength. Characteristics of dislocation slip at 77 K was analyzed using slip traces which confirmed active slip on {112} planes. The apparent activation volume (V*) suggested thermally activated kink/kink-pair nucleation at 77 K (V*~7b³). V* values of BCCHEA and pure Nb were nearly same at 77 K. Low-temperature plasticity of the BCCHEA showed characteristics of pure BCC metals, suggesting chemical-complexity of HEAs may not alter the Peierls potential of a/2<111> screw dislocations.

SCRIPTA Vol. 188, 1 Nov. 2020, P124-129

Diffusion triples or multiples are commonly considered as high-throughput experiments to study phase equilibrium and diffusion. But when extracting diffusion coefficients, they are treated as diffusion couples with one-dimensional (1D) composition variation, never taking advantage of their two-dimensional (2D) composition distributions. In this work, we present an efficient numerical inverse approach to directly evaluate composition-dependent atomic mobilities and full matrix of interdiffusivities based on a novel 2D diffusion simulation scheme, which was applied to the Co–Fe–Ni alloys by analyzing only one diffusion triple over a wide composition range. Its reliability was confirmed by comparing with traditional methods.

SCRIPTA Vol. 188, 1 Nov. 2020, P130-134

In this study, we have employed a repetitive laser scanning strategy to fabricate a three-dimensional functionally graded NiTi alloy by SLM method. The microstructure shows gradient characterized by the increase of volume fraction for B19’ phase. An exceptional strain-hardening associated with continuous increase of mechanical recoverable strain is obtained in such material. The gradient of functionality is attributed to the superimposition of multi-deformation mechanisms originating from the microstructural gradient. The deformation mechanisms are dominated by reorientation of B19’ variants at early stage of deformation and then by stress-induced martensitic transformation at higher level of stress.

SCRIPTA Vol. 188, 1 Nov. 2020, P140-145

Here, novel medium-entropy alloys with chemical compositions of Co_17.5Cr_12.5Fe₅₅Ni₁₀Mo₄C₁ and Co_17.5Cr_12.5Fe₅₅Ni₁₀Mo₃C₂ (at%) exhibiting excellent tensile properties at both room and liquid nitrogen temperatures have been developed. Precipitation of carbides changes the chemical composition and phase stability of the matrix, resulting in the controlled deformation-induced martensitic transformation from face-centered cubic to body-centered cubic of the alloys. The carbide precipitation, lattice distortion, and martensitic transformation led the alloy to have ultra-high yield strength of ~1 GPa and ultimate tensile strength of ~2 GPa with extra work hardening at liquid nitrogen temperature.

SCRIPTA Vol. 188, 1 Nov. 2020, P146-150

Cobalt-based superalloy microlattices were created via (i) three-dimensional-extrusion printing of inks containing a suspension of Co-, Ni- and W-oxide particles, (ii) H2-reduction of the oxides and sintering to a homogenous Co-Ni-W alloy, (iii) Al pack-cementation to deposit Al on the microlattice struts, followed by Al-homogenization. The resulting Co-(18–20)Ni-(5–6)W-(10–13)Al (at.%) microlattices, with 27-30% relative density and 350 μm diameter struts, display a peak in yield strength at 750°C, consistent with their γ/γ′ aged microstructure. Oxidation resistance is strongly improved compared to Al-free printed Co-Ni-W lattices, via the formation of an Al₂O₃surface layer. However, the resulting Al depletion within the struts reduces creep resistance at 850°C, which nevertheless remains similar to Ni-based superalloy foam, with 35% relative density.

SCRIPTA Vol. 188, 1 Nov. 2020, P169-173

The behavior of solute atoms in an undersaturated Fe-3.3 at.%Ni model alloy upon heavy and light particle irradiations was investigated by Atom Probe Tomography. Whereas first irradiations, performed using 27 MeV Fe ions at 400 °C, highlight significant Ni enrichment on point defect clusters, solute-rich features detected after 2.8 MeV H+ ion (400 °C) and 1 MeV electron (320 °C) irradiations are only associated with pre-existent microstructural defects. Nanoindentation tests show no hardening in light particle irradiated samples, strongly suggesting that no dislocation loop formed. These observations put forward the importance of point defect clusters resulting from displacement cascades in Ni clusters’ formation.