Alloys, Volume 3, Issue 1 (March 2024) – 6 articles

Residual stress is caused by non–uniform deformation caused by non–uniform force, heat and composition, which is of great significance in engineering applications. It is assumed that the residual stress is always the upper limit of the elastic limit, so the reduction of the flow stress will reduce the residual elastic stress. It is particularly important to control the flow stress in metal materials. Compared with traditional methods, the use of electropulsing treatment (EPT) technology stands out due to its energy–efficient, highly effective, straightforward and pollution–free characteristics. However, there are different opinions about the mechanism of reducing flow stress through EPT due to the conflation of the effects from pulsed currents. Herein, a clear correlation is identified between induced stress levels and the application of pulsed electrical current. It was found that the decrease in flow stress is positively correlated with the current density and the duration of electrical contact and current action time. We first systematically and comprehensively summarize the influence mechanisms of EPT on dislocations, phase, textures and recrystallization. An analysis of Joule heating, electron wind effect, and thermal–induced stress within metal frameworks under the influence of pulsed currents was conducted. And the distribution of electric, thermal and stress fields under EPT are discussed in detail based on a finite element simulation (FES). Finally, some new insights into the issues and challenges of flow stress drops caused by EPT are proposed, which is critically important for advancing related mechanism research and the revision of theories and models. Full article

In this work, the refractory complex concentrated alloy (RCCA) 3.5Al–4Cr–6Ge–1Hf–5Mo–36Nb–22Si–1.5Sn–20Ti–1W (at.%) was studied in the as cast and heat treated conditions (100 h or 200 h at 1500 °C). There was strong macrosegregation of Si in the 0.6 kg button/ingot of the cast alloy, in which A2 solid solution, D8_m βNb₅Si₃, C14-NbCr₂ Laves phase and Ti_ss and a ternary eutectic of the A2, D8_m and C14 phases were formed. The partitioning of Ti in the as cast and heat treated microstructure and its relationships with other solutes was shown to be important for the properties of the A2 solid solution and the D8_m βNb₅Si₃, which were the stable phases at 1500 °C. The near surface microstructure of the alloy was contaminated with oxygen after heat treatment under flowing Ar. For the aforementioned phases, it was shown, for the first time, that there are relationships between solutes, between solutes and the parameters VEC, Δχ and δ, between the said parameters, and between parameters and phase properties. For the contaminated with oxygen solid solution and silicide, trends in relationships between solutes, between solutes and oxygen content and between the aforementioned parameters and oxygen content also were shown for the first time. The nano-hardness and Young’s modulus of the A2 solid solution and the D8_m βNb₅Si₃ of the as cast and heat-treated alloy were measured using nanoindentation. Changes of nano-hardness and Young’s modulus of the A2 solid solution and D8_m βNb₅Si₃ per solute addition for this multiphase RCCA were discussed. The nano-hardness and Young’s modulus of the solid solution and the βNb₅Si₃, respectively, were 9.5 ± 0.2 GPa and 177.4 ± 5.5 GPa, and 17.55 ± 0.5 GPa and 250.27 ± 6.3 GPa after 200 h at 1500 °C. The aforementioned relationships and properties of the two phases demonstrated the importance of synergy and entanglement of solutes, parameters and phases in the microstructure and properties of the RCCA. Implications of synergy and entanglement for the design of metallic ultra-high temperature materials were emphasised. Full article

The isothermal oxidation of a Fe₃₅Mn₂₁Ni₂₀Cr₁₂Al₁₂ high entropy alloy (HEA) was investigated in dry air for 50 h at 500, 600, and 700 °C after 90% cold rolling. The Fe₃₅Mn₂₁Ni₂₀Cr₁₂Al₁₂ HEA behaves according to the linear oxidation rate with rate constants of 1 × 10⁻⁶, 3 × 10⁻⁶, and 7 × 10⁻⁶ g/(cm²·s) for oxidation at 500 °C, 600 °C, and 700 °C, respectively. The activation energy for oxidation of the HEA was calculated to be 60.866 KJ/mole in the 500–700 °C temperature range. The surface morphology and phase identification of the oxide layers were characterized. The formation of MnO₂, Mn₂O₃, Mn₃O₄, Cr₂O₃, and Al₂O₃ in the oxide layers along with Fe₂O₃ is the key to the oxidation mechanism. The elemental mapping and line EDX scans were performed to identify the oxidation mechanisms. Full article

Gold and silver are, for all their chemical similarities, optically very different. Small Ag clusters show a localized surface-plasmon resonance (LSPR), whereas in Au clusters smaller than about 300 atoms, the resonance is absent due to the coupling with the interband transitions from the d electrons. This opens the possibility of tuning the cluster properties depending on their composition and chemical configuration. Earlier work on AgAu alloy clusters has shown that the outermost shell of atoms is crucial to their overall optical properties. In the present contribution, we consider the optical spectroscopic properties associated with the structural rearrangement in 55-atom AgAu alloy clusters in which the core transforms from pure silver to pure gold. Calculations using time-dependent density-functional theory are complemented by an in-depth study of the subtle effects that the chemical configuration has on the details of the materials’ d bands. Although the cluster surface remains alloyed, the geometrical changes translate into strong variations in the optical properties. Full article

Bimetallic nanoclusters have attracted great interest due to their ability to enhance the catalytic properties of nanoclusters through synergetic effects that emerge from the combination of the metal nanocluster with different transition metal (TM) species. However, their indefinite composition and broad distribution hinder the insightful understanding of the interaction between these invasive metals in bimetallic doped nanoalloys. In this study, we report a density functional theory calculation with the PBEsol exchange-correlation functional for 16-atom Ti_N−1TM (TM = Ni, Ir, Pd) nanoalloys, which provides new insights into the synergetic effect of these invasive metals. The probe into the effect of these metal impurities revealed that the replacement of a Ti atom with Ni, Ir and Pd enhances the relative stability of the nanoalloys, and the maximum stability for a lower bimetallic composition is reached for Ti₄Ir, Ti₅Pd and Ti₇Ni. The most stable nanoalloy is reached for the Ti₁₂Ir cluster in comparison with the Ti₁₂Pd and Ti₁₂Ni clusters and pure Ti₁₃ monoatomic nanocluster. This stability trend is as revealed well by both the binding energy and the dissociation energy. The average HOMO-LUMO gap for the bigger clusters revealed that the valence electrons in the HOMO can absorb lower energy, which is indicatory of a higher reactivity and lower stability. The quantum confinement is higher for the smaller clusters, which illustrates a higher stability and lower reactivity for those systems. Full article

As part of a comprehensive study on eco-friendly processing techniques, the influence of the heat treatment environment on the case hardening of AISI 1018 steel using pulverized cassava leaf was studied. The process was carried out at two different temperatures (850 °C and 950 °C) and under three environmental conditions: Process 1, the control experiment, was carried out in air only; in Process 2, the medium comprised pulverized cassava leaves; and in Process 3 a combination of pulverized cassava leaves plus barium carbonate (BaCO₃) was used as an energizer (CBC mixture). Vickers microhardness testing and scanning electron microscopy were used to evaluate the effect of the processing environment on the case hardening of the steel. As expected, regardless of the processing temperature, Process 1 resulted in little or no hardening of the steel surface. However, notable case hardening occurred when the steel specimens were subjected to either Process 2 or Process 3. Furthermore, the inclusion of barium carbonate in Process 3 significantly enhanced the case hardening effectiveness of the cassava leaf in terms of the rate of and maximum hardness achieved. A maximum enhancement was observed at 950 °C. After 1 h, the increase in hardness was 160% and 280% for Process 2 and Process 3, respectively. Upon increasing the processing time to 5 h, the increase in hardness due to Process 2 was raised to 254%, while that of Process 3 remained at approximately 280%. The diffusivity of AISI 1018 was calculated using the microhardness data. The diffusivity was highest in Process 2 samples with values of 1.568 × 10⁻⁹ m²/s at 850 °C and 1.893 × 10⁻⁹ m²/s at 950 °C. Effective case hardening of AISI 1018 steel was carried out using the medium of cassava leaf, without the addition of barium carbonate (BaCO₃) as an energizer. Full article