Search

Abstract The effect of precipitate structures of a B2-NiAl/L2 1 -Ni 2 TiAl hierarchical precipitate-strengthened ferritic alloy on the precipitate/matrix interface, elastic strain, and creep resistance has been investigated by systematic heat-treatments and creep experiments. The experimental results reveal that the highest creep strength was achieved in a heat-treated sample containing the largest coherent precipitate with fine internal networks, whereas the semi-coherent precipitate results in a sharp drop of the creep strength. It was found that the strong elastic strain field, affected by the internal structure within the precipitate, plays a crucial role in improving the creep resistance. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

Abstract The maximization of the mixing entropy with the optimal range of enthalpy in high-entropy alloys (HEAs) can promote the formation of a stable single solid-solution phase with the absence of competing intermetallic compounds. The resultant effects, such as lattice distortion, can contribute to excellent mechanical properties, which has motivated numerous efforts to develop and design single-phase HEAs. However, challenges still remain, particularly on quantifying the lattice distortion and relating it to materials properties. In this study, we have developed a NbTaTiV refractory HEA with a single body-centered-cubic (BCC) structure using an integrated experimental and theoretical approach. The theoretical efforts include thermodynamic modeling, i.e., CALculation of PHAse Diagram (CALPHAD). The microstructural evolutions have been investigated by systematic heat-treatment processes. The typical dendrite microstructure was observed, which is caused by the elemental segregation during the solidification in the as-cast condition. The structural inhomogeneity and chemical segregation were completely eliminated by the proper homogenization treatment at 1200 °C for 3 days. The homogeneous elemental distribution was quantitatively verified by the Atom Probe Tomography (APT) technique. Importantly, results indicate that this HEA exhibits the high yield strength and ductility at both room and high temperatures (up to 900 °C). Furthermore, the effects of the high mixing entropy on the mechanical properties are discussed and quantified in terms of lattice distortions and interatomic interactions of the NbTaTiV HEA via first-principles calculations. It is found that the local severe lattice distortions are induced, due to the atomic interactions and atomic-size mismatch in the homogenization-treated NbTaTiV refractory HEA. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

Precipitate features, such as the size, morphology, and distribution, are important parameters determining the mechanical properties of semi- or fully-coherent precipitate-hardened alloys at elevated temperatures. In this study, the microstructural formation and evolution of recently-developed Fe-Ni-Al-Cr-Ti alloys with superior creep resistance have been systematically investigated using transmission-electron microscopy (TEM), scanning-electron microscopy (SEM), and atom-probe tomography (APT). These alloys were designed by adding 2 or 4 wt percent (wt. %) Ti into a NiAl-hardened ferritic alloy with a nominal composition of Fe-6.5Al-10Cr-10Ni-3.4Mo-0.25Zr-0.005B in wt. %. These alloys were, then, subjected to a homogenization treatment at 1473 K for 0.5 h, followed by aging treatments at 973 K for 1–500 h. In the homogenization-treated case, both alloys contain a primary L2 1 -type Ni 2 TiAl precipitate, but with the distinct size and morphology of the precipitates and precipitate/matrix interface structures. In the subsequent aging treatments, the 2 wt % Ti alloy establishes a hierarchical-precipitate structure consisting of a fine network of a B2-type NiAl phase within the parent L2 1 -type Ni 2 TiAl precipitate, while the 4 wt % Ti alloy retains the single Ni 2 TiAl precipitate. It was found that the hierarchical structure is more effective in remaining the coherent interface during the growth/coarsening of the precipitate. The formation of the different types of the precipitates, and their effects on the microstructural evolution are discussed, and the driving forces for these features are identified from the competition between the interface energy and elastic interactions due to the lattice misfit and misfit dislocations. [ABSTRACT FROM AUTHOR]

Systematic variations of chemical compositions in TiNbHfTaZr HEA system were designed for an investigation of compositional complexity and chemical composition effects on lattice distortion. The lattice distortion was quantitatively analyzed by using the pair distribution function (PDF) method. The levels of lattice distortion were proportional to the atomic size misfit rather than the compositional complexity in the TiNbHfTaZr HEA system. The more pronounced lattice distortion in the TiNbHfTaZr HEA system increases the solid solution strengthening. The study suggested that the atomic size misfit is one of the foremost considerations on lattice distortion and solid solution strengthening in the TiNbHfTaZr HEA system. [Display omitted] [ABSTRACT FROM AUTHOR]

Here, we report on an economical, viable, one-step method for fabrication of Higher Manganese Silicide MnSi 1.75 (HMS). Fabrication of the HMS was followed by spark plasma sintering (SPS) at different temperatures (850, 900, 950 and 1000 °C), after which the thermoelectric and mechanical properties were investigated. It was found that the electrical conductivity significantly increased with increasing SPS temperature. This was attributed to the simultaneous increase of carrier concentration and mobility, while a reduction in the Seebeck coefficient was attributed to the formation of secondary phase in the HMS samples. A low thermal conductivity of 1.86 W/mK at 723 K was obtained for the sample sintered at 850 °C, which was attributed to increased scattering of heat-carrying phonons due to pores or voids, and grain boundaries. In addition, a maximum ZT of 0.27 was obtained at 723 K for the HMS-1000 sample, which is comparable with other previously reported data for undoped HMS material. Nevertheless, the HMS-1000 sample exhibited compressive strength of 1208 MPa and fracture toughness of 1.99 MPa m1/2, which is significantly higher than other state-of-the-art thermoelectric materials. The proposed fabrication method could be adopted for mass market applications requiring superior mechanical and thermoelectric properties. • Higher manganese silicides were fabricated using inexpensive single step process. • The electrical conductivity significantly increased due to the simultaneous increase of carrier concentration and mobility. • A low thermal conductivity was obtained due to increased scattering of phonons at pores/voids and grain boundaries. • High compressive strength 1208 MPa and best fracture toughness 1.99 MPa m1/2 were achieved. [ABSTRACT FROM AUTHOR]

Abstract Bismuth antimony telluride based thermoelectric (TE) materials have been intensively developed and synthesized using different mechanisms, for the room temperature TE applications. In particular, bismuth antimony telluride based TE alloys are very sensitive to deviations in their composition, and to contamination during the materials synthesis. Oxygen contamination during Bi-Sb-Te based materials synthesis is one of the critical factors that alters or diminishes thermoelectric-transport properties. Thus, in this study, how the oxide formation mechanism on the powder surface and bulks of p-type Bi 0.5 Sb 1.5 Te 3 alloys affected the microstructural features and thermoelectric properties were elucidated quantitatively. While applying heat treatment (HT) to Bi 0.5 Sb 1.5 Te 3 powder, the constituent elements interacted with the ambient atmosphere and formed a new oxide phase which acted as a barrier to carrier transport. At the initial stage (300 °C) of heat treatment, only the powder surface was oxidized due to the reaction of outer surface atoms with atmospheric air and moisture. While increasing in temperature during HT, this surface oxygen contamination diffused further inside the powder through the grain boundaries. More diffusion and spreading occurred throughout the matrix at 450 °C. The increment of oxygen content from 0.05 to 0.82 wt% drastically decreased the electrical conductivity by 67%, and thermal conductivity by 7% at the heat treatment temperature of 450 °C. This reduction behavior is mainly due to severe scattering of the carriers/phonons at the new formation of oxide (Sb 2 O 3) phase near grain boundaries and within the matrix. At a glance, a small increase in the oxygen content wouldn't significantly influence the thermoelectric properties; however, at a certain level of oxide formation (0.82 wt%), severe effects could occur due to the intensified scattering or trapping of carriers by the oxide barrier formation at the grain boundaries. Graphical abstract Image 1 Highlights • Determined the oxygen concentration of different heat-treated GA powder and SPS bulks. • Elucidation the relation between the mechanism of oxide formation and the thermoelectric properties of Bi 0.5 Sb 1.5 Te 3 alloys. • Identified Sb 2 O 3 phase at the grain boundary and within the grains. • Demonstrated the thermoelectric performance in Bi 0.5 Sb 1.5 Te 3 alloys with oxygen content. [ABSTRACT FROM AUTHOR]

'AB' intermetallic compound (IC) type (TiZrHf)-(NiCuCo) high-entropy metallic glass precursor (HE-MGP) has been designed under considering physical and thermodynamic relations between constitutive elements, such as atomic size difference, electronegativity difference, mixing enthalpy and entropy, and phase stability, for development of nanocrystalline single-phase high-entropy alloy. The IC type HE-MGP exhibits high thermal stability and polymorphic crystallization behavior. From the systematic investigations on crystallization kinetics and microstructural evolution, it was found that the HE-MGP crystallized into nanocrystalline single-phase high-entropy alloy with B2 structure by polymorphic crystallization, and nucleation and growth rates for crystallization processing strongly depend on the annealing temperature. Our approach and discovery provide an effective way to develop the nanocrystalline single-phase high-entropy alloys by designing IC type HE-MGP. [Display omitted] [ABSTRACT FROM AUTHOR]

In this study, we systematically reviewed the strategy of the tailoring intrinsic color characteristics of chromatic and achromatic metallic materials. Its scientific contribution involves defining and expanding the range of coloration of metallic materials, including single-phase solid-solution alloys and multiphase alloys that includes specific phases with chromatic colors. In chromatic single-phase alloys, the modulation of optical properties depends on the transition energy of the solid solution phase. The additional alloying elements significantly influence the optical properties, i.e., the reflectivity in the visible spectrum, by varying the transition energy of the solid solution phase. Another coloring alloy design strategy varies and realizes the intrinsic color based on the multiphase system. In particular, the microstructural design based on the multiphase forming alloy system, including the chromatic coloring phase, effectively improves the chromaticity of the alloys by blending effect from the constitutive phases. A methodological framework is subsequently suggested to realize chromatic characteristics based on the juxtaposition method. The novel alloy design method using the juxtaposition method can be considered an important strategy for developing novel coloring alloys with high chromatic color durability from achromatic metallic materials. [ABSTRACT FROM AUTHOR]

The analyses of phase composition, microstructure and the mechanical properties were conducted for as-cast Ta x Al 0.7 CoCrFeNi 2.1 (x = 0.3, 0.5, 0.6 and 0.7) eutectic high entropy composites (EHECs). With the x value increases from 0.3 to 0.6, the phases of dendrite changed from L1 2 to B2 and volume fraction of ultrafine-eutectic matrix consisting of Face-centered cubic and Laves phases increased. When x = 0.7, the Laves phase appears as the primary dendrite and the matrix region changed into the bimodal eutectic structure (L1 2 + Laves and B2 + Laves). Thermo-physical parameters could be used for understanding phase formation in EHECs. Experimental and theoretical comparison using thermo-physical parameters shows a reasonable consistency. Depending on the microstructural variation, the yield strength increased from 1304 MPa (x = 0.3) to 2154 MPa (x = 0.6) and plasticity decreased from 17% (x = 0.5) to 4% (x = 0.6). In order to understand the nano-to-micronscale deformation mechanisms depending on the microstructural variations, the systematic investigations of post-deformed samples (x = 0.5 and 0.6) were carried out by multi-scale SEM, TEM, and AFM analyses. [Display omitted] [ABSTRACT FROM AUTHOR]

Here, novel lightweight Al alloys were designed with an objective to achieve diverse color range and improved mechanical properties. The alloy design strategy was based on a scientific color expression method controlling the microstructural attributes, such as constitutive phases, and their morphology (length-scale). The intrinsic difference in colors between the constitutive phases, their spatial arrangement/dimension, and fraction play a critical role in determining chromatic characteristic of the alloys. Our current research demonstrates the feasibility of the present color alloy design concept with three groups which allows blue color diversification and mechanical properties for ultra-light aluminum-magnesium-silicon alloys. Unlabelled Image • Metal color can be realized by alloy design using intermetallic compounds. • Color analysis can be accepted for alloys by using the CIE Lab color coordinate system. • 3 Alloy groups can provide a variety of options for color and mechanical properties. • Presenting the function of jewelry by color implementation in metal [ABSTRACT FROM AUTHOR]