Intermetallics最新文献

The deformation behavior and microstructure evolution of a novel Mn-containing HEA were explored during the deformation temperature ranges of 800∼1100 °C, and strain rate ranges of 0.001∼1s⁻¹. The study results indicate that the true stress-strain curves after work hardening exhibit continuous softening type at low strain rate ranges of 0.001–0.01 s⁻¹, while softening then steady-state type at high strain rate ranges of 0.1∼1s⁻¹, respectively. The activation energy (Q) and stress exponent (n) are calculated to as 404.6 kJ/mol and 4.9016, respectively. As the strain increases, the flow instability is prone to expand towards the regions of lower strain rate and higher deformation temperature, and the optical processing window is determined at 1050–1100 °C and 0.001–0.002 s⁻¹. The strain and stress field distribution of the FEM simulations reveal the inhomogeneous deformation and the damage field distribution predict the crack region of deformation specimen. The critical (ε_c)/peak strain (ε_p) for DRX do not display a proportional relationship with strain rate and an inversely proportional relationship with deformation temperature, and the relationship between ε_c and ε_p can be expressed as: ε_c = (0.3–0.5) ε_p. Only the discontinuous dynamic recrystallization (DDRX) mechanism is activated in the whole range of deformation conditions, but it is not fully carried out. Because of the inhomogeneous stress distribution, the dynamic recrystallized grains show bimodal grain size. Furthermore, the recrystallized grains do not grow significantly due to the synergistic contribution of sluggish diffusion effect, high solution hardening effect and the trimodal B2 precipitate phase distribution.

In this study, the effects of the rapid solidification process on microstructures, transformation behaviors and superelastic properties of the multi-component (TiZrHf)₅₀Ni₂₅Co₁₀Cu₁₅ (at%) high-entropy shape memory alloy (HESMA) were investigated. The as-spun (TiZrHf)₅₀Ni₂₅Co₁₀Cu₁₅ fibers were prepared by a rapid solidification process. The solution-treated (TiZrHf)₅₀Ni₂₅Co₁₀Cu₁₅ alloy bulk specimen consisted of a (NiCoCu)-rich matrix, (TiZrHf)₂(NiCoCu)-type phase and carbide, while the as-spun fiber specimen consisted of (TiZrHf)-rich matrix and carbide. The (TiZrHf)₂(NiCoCu)-type phase is dissolved in the matrix of as-spun fibers due to the rapid solidification process. The martensitic transformation start temperature of the (TiZrHf)₅₀Ni₂₅Co₁₀Cu₁₅ alloy increased from 53.5 °C to 91.5 °C after the rapid solidification process. Both the (TiZrHf)₅₀Ni₂₅Co₁₀Cu₁₅ alloy bulk and fiber specimens showed clear superelasticity and the total superelastic recovery strain increased from 4.6 % to 5.7% after the rapid solidification process.

This study was initiated to improve surface hardness and wear resistance of a HfNbTaTiZr refractory high entropy alloy (RHEA) by gas nitriding at a medium temperature (600 °C) for 3 h. Structural characterizations conducted by X-ray diffractometer (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) equipped scanning electron microscope (SEM) revealed that nitriding led to formation of a 1.5 μm thick surface layer containing precipitates of oxides and nitrides of the alloying elements. Detection of oxides within the surface layer was attributed to the presence residual oxygen in the nitriding atmosphere. Nevertheless, the employed gas nitriding provided remarkably higher scratch resistance compared to the untreated state, as the results of increment in the surface hardness and development of larger compressive residual stress.

In this work, the properties of two high-entropy alloys (HEAs) Al₂₅Cr₂₀Fe₂₀Ni₃₅ and Al₂₀Cr₂₀Fe₂₀Ni₄₀ (at.%) were compared for potential application as bond coats in thermal barrier coatings (TBCs). For this reason, both their oxidation resistance under isothermal and thermal shock conditions, as well as diffusion phenomena occurring at the substrate-HEA diffusion couple interface, were investigated. It was determined that both alloys demonstrate good phase stability and oxidation resistance during prolonged exposure to air atmosphere at 1000°C. In the initial 100 h period, selective aluminum oxidation is responsible for the protective scale growth on both materials under both isothermal and thermal shock conditions. However, only the Al₂₀Cr₂₀Fe₂₀Ni₄₀ high entropy alloy maintains a single Al₂O₃ layer for over 1000 h of oxidation with cyclic temperature changes. On the other hand, diffusion studies indicate that mainly iron and chromium travel between the ferritic steel substrate and the HEAs. Aluminum diffusion is seemingly limited by the unexpected formation of a thin aluminum nitride layer at the interface between the materials. From all these results it can be concluded that the Al₂₀Cr₂₀Fe₂₀Ni₄₀ alloy shows more promise for application in TBCs based on high entropy materials than Al₂₅Cr₂₀Fe₂₀Ni₃₅.

Considering the service safety for TiAl alloy under thermal shock, the effect of thermal shock temperature and cycles on micro-structural degeneration and mechanical properties were studied. The results show that the vertical decomposition of α₂→β₀ phase transition occurs in α₂ lamellae if the single thermal shock temperature increases to 1000 °C or the thermal shock cycle increases to 10 times at 900 °C. Cracks form during the thermal shock progress as the single thermal shock temperature increases to 1000 °C or the number of thermal shocks is 5 times at 900 °C. Simultaneously, the flexural strength of samples decreases to 70% of the initial flexural strength under the same condition. It indicates that Ti–45Al–4Nb–1Mo-0.1B alloy fails as the single thermal shock temperature is above 1000 °C or the number of thermal shocks exceeds 5 times at 900 °C, and cracks generated during the thermal shock progress cause the dramatic deterioration in flexural strength. Meanwhile, the formed cracks are coarse, and the propagation path is relatively straight, which further deteriorates the flexural strength of the Ti–45Al–4Nb–1Mo-0.1B alloy. Besides, the nucleation and propagation of cracks change with the thermal shock temperature and cycles were analyzed. When the single thermal shock temperature is 1000 °C, the nucleation of cracks is not only in lamellar colony boundaries but also in α₂/γ lamellar interfaces and the propagation path of cracks is relatively straight compared with the formation of cracks after 5 thermal shocks at 900 °C, because the thermal stress increases, and then cracks directly pass through lamellar colonies when the angle between lamellar orientation and crack propagation direction is about 90°.

FeCoNi_1.5CuB_0.5Y_0.2 high-entropy alloys (HEAs) were prepared through cold isostatic pressing and microwave sintering, followed by treatment with high-pulsed magnetic field treatment. This study investigated the structure, microstructure, and mechanical properties of HEAs under different magnetic field processing parameters (magnetic induction intensity, number of pulses). The results indicated that the application of the pulsed magnetic field led to the transformation of a portion of the face-centered cubic phase, which served as the matrix, into the hexagonal close-packed phase and M₃B phase. This transition was accompanied by the proliferation and movement of dislocations, leading to increased dislocation density and improved plastic deformation ability of the alloy. At a magnetic induction intensity of 1T and 120 pulses, the alloy exhibited the highest strength and toughness. Particularly, HEAs exhibited a compressive strength, maximum compression ratio, and Vickers hardness of 1153.7 MPa, 25.4%, and 289.2 HV, respectively. These values indicated increases of 9.8%, 27.0%, and 4.9% over those of the untreated sample. These findings revealed that pulsed magnetic field treatment enhanced the strength and toughness of the material and preserved its high hardness. Overall, the high-pulsed magnetic field treatment emerged as a novel solid-state treatment method for HEAs, resulting in improved strength and toughness. This improvement can be attributed to the synergistic effects of various strengthening mechanisms, such as dislocation strengthening and fine-grain strengthening.

In this study, the effect of titanium (Ti) on the microstructure, mechanical properties, wear resistance and corrosion behavior of CoCrFeMnNi high-entropy alloy (HEA) was examined. The selective laser melting (SLM) method was used to produce HEAs without Ti addition (HEA-1) and with Ti additions of 3 and 5 wt% (HEA-2 and HEA-3, respectively). While the HEA-1 sample exhibited a single-phase face-centered cubic (FCC) structure, the HEA-2 and HEA-3 samples exhibited intermetallic phase structures (Sigma and Laves) along with FCC. The addition of Ti and the presence of intermetallic phases in the HEA-2 sample revealed an improvement in mechanical properties without reducing the ductility value of the structure. However, in parallel with the increasing Ti ratio, the formation of more brittle intermetallic phases in the microstructure of the HEA-3 alloy caused a significant increase in strength but a decrease in ductility. Microstructural examinations revealed that all alloys had a cellular/dendritic structure and the relative densities of the samples were above 99%. While the ultimate tensile stress (UTS) of the HEA-1 sample was 548 MPa and the UTS of the HEA-3 alloy was 832.1 MPa, elongation values were obtained as 48% and 2%, respectively. HEA-2 sample exhibited more ideal results with an elongation value of approximately 22% and UTS values of 760.8 MPa. It was observed that the addition of Ti significantly increased the wear resistance in sliding conditions due to the increase in the hardness of the alloy. The highest hardness and lowest wear rate were obtained with HEA-3 coded samples. The HEA-1 sample exhibited the best corrosion rate, with higher corrosion potential (E_corr) and lower corrosion current density (I_corr) values. The highest corrosion rate was observed in the HEA-3 sample.

Ni–Mn-Ga-Co-Gd microwires were fabricated by the glass-coated melt spinning method, resulting in a bamboo-grained structure. In this paper, the magnetocaloric effect of Ni–Mn-Ga-Co-Gd microwires is mainly investigated, which is represented by the isothermal magnetic entropy change calculated using Maxwell equation. Specifically, Ni₄₃Mn₃₀Ga_19.9Co₇Gd_0.1 microwires exhibit maximum entropy change of 11.09 J/kg·K under 5 T magnetic field, which is generally greater than that of the current Ni–Mn-Ga microwires. Additionally, Ni₄₃Mn₃₀Ga_19.9Co₇Gd_0.1 microwires show the temperature corresponding to the greatest magnetic entropy change nearer to room temperature compared to other Ni–Mn-Ga microwires. Furthermore, the larger specific surface area of microwires will facilitate their application in the field of magnetic refrigeration.

The magnetic properties of PrMn₂Ge₂ samples in both the as-cast bulk and melt-spun ribbon forms have been investigated in detail by a comprehensive set of x-ray and neutron powder diffraction, magnetic and heat capacity measurements and corresponding sets of data analyses. Thermal expansion measurements indicate the presence of magnetoelastic coupling effects around all transition temperatures in the bulk sample. The bulk modulus K₀ = 42.0 GPa and its first derivative K₀’ = 18.7 have been derived from the pressure-volume data. The Curie temperature from the intralayer antiferromagnetism (AFl) of PrMn₂Ge₂ to a canted spin structure (Fmc) is T_C^inter = 332 K for the bulk sample, decreasing to T_C^inter = 320 K for the ribbon sample. The critical components γ, β and δ of this second order magnetic transition as determined from Kouvel-Fisher analyses, indicate long range magnetic interactions around T_C^inter. Based on these critical exponents the magnetization, field and temperature data around T_C^inter collapse onto two curves obeying the single scaling equation $M\left(H,\epsilon \right)={\epsilon }^{\beta }{f}_{\pm }\left(\frac{H}{{\epsilon }^{\beta +\gamma }}\right)$. The Debye temperatures and the density of states at the Fermi level are ${\theta }_D=306K$ and $N\left(E_F\right)=3.47\text{state}/\text{eV}\text{atom}$ for the bulk sample and ${\theta }_D=320K$ and $\left(E_F\right)=2.18\text{state}/\text{eV}\text{atom}$ for the ribbon sample with the nuclear specific heat coefficient for bulk PrMn₂Ge₂ derived from the splitting of the nuclear hyperfine levels as C_N = 517 mJ mol⁻¹ K⁻¹. With a field change of ΔB = 5 T, the maximum values of the magnetic entropy changes -ΔS_max in the region around T_C^inter are -ΔS_max = 3.00 J/kg K and 2.35 J/kg K for the bulk and ribbon samples respectively, while the relative cooling power (RCP) for the ribbon-spun sample, RCP = 135.9 J/kg, is significantly higher than the value of RCP =