The description of the short-range ordering (SRO) for binary systems was expanded to multi-component systems in the framework of the CALPHAD method. The present formulation estimates the effect of SRO in high entropy alloys (HEAs) based on the Gibbs energy functions from the CALPHAD-type thermodynamic assessments. It was applied to the BCC- and FCC-based HEAs and their subsystems. The excess Gibbs energy and excess entropy due to SRO in the FCC solutions were higher than those in the BCC solutions because of the difference in their coordination numbers. In the present calculation, the total contribution of the SRO to the Gibbs energy was approximately −1 kJ mol−1 and −0.2 kJ mol−1 in the BCC- and FCC-based HEAs at 1000 K, respectively. The variations in the excess Gibbs energy and excess entropy due to SRO in lower-order systems can disappear in the HEAs because of the mixing of multiple elements, as formally suggested in the previous work for the excess Gibbs energy in Cantor alloy.
Fig. 3 (a) WC-SRO and (b) excess Gibbs energies due to SRO in the BCC-based HEA (Mo–Nb–Ta–V–W) as a function of temperature.
The classification of the microstructures of Al–Si–Mg casting alloy in different cooling rates was accomplished by using our originally developed methods and machine learning techniques. The mechanical properties of the samples were slightly increased with the increase of cooling rates. The microstructures of the samples were similar because of the approximate cooling rates. High classification rates of about 80% to 90% were obtained using the software with machine learning techniques developed by us. The classification rate change with the number of images in training data was tested and a suitable number of images for training in the machine learning process was found.
Sm–Fe–M (M = Ti, V) sintered magnets with the ThMn12-type structure which consist of higher Sm content than the stoichiometric composition of ThMn12-type compound, were produced by the powder-metallurgy. Sm–Fe–Ti sintered magnets had a low level of coercivities of about 10 kAm−1. In contrast, Sm–Fe–V anisotropic sintered permanent magnets, particularly, Sm10.5Fe74.1V15.4 magnet exhibited magnetic properties with a coercivity of 648.7 kAm−1, a remanence of 0.73 T and a (BH)max of 97.6 kJm−3. Through microstructure observation of the grain boundary between the adjacent ThMn12-type grains using STEM, no grain boundary phases were observed in Sm10.0Fe81.7Ti8.3 magnet, whereas two types were observed in Sm10.5Fe74.1V15.4 magnet: an amorphous structure with a thickness of about 1 nm and an incomplete periodic structure with a thickness of about 2 nm. EDS analysis indicated that the composition ratios of Sm to the sum of Fe and V were about 1 to 1 and 1 to 1.7, respectively. It suggested that the existence of a grain boundary phase is important to raise coercivity.
Fig. 3 Demagnetization curves of Sm–Fe–V sintered magnets.
The effect of porosity on coercive force in iron powder cores with different porosities was analyzed quantitatively. The coercive force of the iron powder cores decreased 11.0 A m−1 with a decrease in porosity of 0.01. From in-situ observation by Kerr effect microscopy, nucleation of the reverse domain was observed in local areas along narrow gaps such as the contact interface between particles and fine pores among particles, and nucleation of the reverse domain did not occur at coarse pores. This indicates that the local decrease in the diamagnetic field with a decrease in porosity may be reduced in these areas, resulting in a decrease in coercive force. This result suggests that densification of iron powder cores can be an effective method for reducing coercive force.
This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 68 (2021) 20–27.
Improving the oxidation resistance of Mg2Si is important for its practical use in thermoelectric devices. The oxidation behavior of Sb-doped Mg2Si with and without added Al2O3 or Al under heating from 293 K to 1023 K in a 200 Pa water vapor atmosphere was observed by using an environmental scanning electron microscope (E-SEM). Compositional analysis before and after in situ oxidation in the E-SEM was performed by energy-dispersive X-ray analysis (EDX). The depth profile of the samples after thermal oxidation in the E-SEM was evaluated by X-ray photoelectron spectroscopy (XPS). The dimensionless figure of merit of Sb-doped Mg2Si with added Al2O3 or Al was also evaluated. The oxidation onset temperature was 603 K for Mg2Si, and 747 K to 793 K for Mg2Si with 0.8 to 4.5 mol% Al2O3 added or 4.0 at% Al added. The O concentration after oxidation at 873 K as measured by EDX (accelerating voltage: 3 kV) was 35.85 at% without Al2O3 or Al addition, but 11.55 at% to 13.80 at% with Al2O3 or Al addition. The Si concentration after oxidation at 873 K as measured by EDX (accelerating voltage: 3 kV) was 0.30 at% when no Al2O3 or Al was added to the Mg2Si, but 15.32 to 20.97 at% with Al2O3 or Al addition. Evaluation by XPS revealed a layer with a relatively high concentration of aluminum oxide or aluminum at a depth of about 20 nm from the surface of the Mg2Si sample with added Al2O3 or Al, respectively. This layer appears to suppress Mg2Si oxidation. The addition of Al2O3 or Al had a slightly positive effect on the thermoelectric properties of Mg2Si.
The application of high-density pulsed electric current (HDPEC) is one of the effective methods for the modification of material properties in metals. To evaluate fracture behavior modified by HDPEC, critical fracture parameters such as fracture strength, fracture toughness, and fracture profile of crack tip are important criteria. This work investigates the finite element analysis (FEA) based evaluation of improved fracture characteristics by the application of HDPEC in a SUS 316 austenite stainless steel. Tensile tests were first conducted to deduce the modified material properties with different conditions of HDPEC. A series of theoretical considerations was employed to estimate the modified fracture toughness. The relationship between critical fracture strength and critical crack length was numerically determined based on the estimated fracture toughness. The results in FEA showed that critical von Mises stress on the singularity at the crack tip increases as the effect of HDPEC increases. The evolution of increased fracture toughness with respect to conditions of HDPEC was specified. Crack opening profiles were simulated to assist the explanation. The evaluation of fracture parameters in this study proposes that the modified material properties by HDPEC play a positive role to resist crack propagation.
In our previous study, a thin wire of a Cu–0.29 mass%Zr alloy was produced by annealing after rolling and two subsequent intermediate annealing steps during wire drawing (IA wire). The produced IA wire had an ultimate tensile strength, σu, of 610 MPa and a small total elongation, εt, of 1.2% despite a small grain size, D, of 240 nm. In this study, a thin wire of the same alloy was produced by annealing after rolling and subsequent wire drawing (S wire). Even though the S wire had almost the same values of σu = 620 MPa and D = 230 nm as the IA wire, the S wire exhibited a larger εt = 2.9%. This study investigated the cause of this smaller value of εt for the IA wire. In both the IA and S wires, voids were formed by decohesion of the interface between the Cu matrix and particles consisting of a eutectic (Cu + Cu5Zr) during wire drawing; however, the fraction of eutectic particles with voids was larger in the IA wire than the S wire. The IA wire exhibited lower ductility as a result of easier coalescence of the voids during tensile testing. In addition, the larger fraction of eutectic particles with voids in the IA wire is attributed to the larger size of recrystallized grains generated by the intermediate annealing during wire drawing than that of the grains before intermediate annealing. As a result, dislocations accumulated significantly around the eutectic particles during the subsequent wire drawing, resulting in a high stress concentration.
This Paper was Originally Published in Japanese in J. Japan Inst. Copper 59 (2020) 76–79. The captions of all figures and table have been modified slightly.
Fig. 5 Fraction of eutectic particles with voids (fe) as a function of the inverse of the diameter (d−1) for the IA, S, and ECAP wires.
Metal additive manufacturing (AM) technologies are attracting attentions not only as a fabrication process of complicated three-dimensional parts but also as microstructure controlling processes. In powder bed fusion (PBF)-type AM, crystallographic texture can be controlled by scanning strategies of energy beam. To optimize microstructures, computer simulations for predicting microstructures play very important roles. In this work, we have developed simulation programs to explain the mechanism of the crystal orientation control. First, we simulated the shape of melt pool by analyzing the heat transfer using apparent heat conductivity when the penetration of laser beam through keyholes was taken into consideration because of the evaporation and accompanying convections. It was assumed that the primary crystal growth direction can be determined by the temperature gradient, and the crystals grow keeping the growth direction as generally recognized. The shapes of simulated melt pools agree well with experimental observations. The modified cellular automaton simulations successfully reproduced two typical textures with different preferential orientations along the building directions of 〈100〉 and 〈110〉 when the bidirectional scanning with and without a rotation of 90°, respectively, was accomplished between the layers.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 103–109.
Fig. 8 Comparison of analysis results (lower side) with experimental results (upper side: after Ishimoto et al.1)) of scan strategy X.
Titanium oxide can be converted directly to its metallic state. This extraordinary technology is realized in a CaCl2-based molten salt that can extract oxygen ions from the cathode. Several discussions have been exchanged in the field of electrochemistry and metallurgy in the last two decades. Recent papers on titanium refining are overviewed by selecting popular papers, especially those published in this journal, Materials Transactions. Basic and scientific studies on titanium and its refining are briefly analyzed, and some practical applications are introduced in related oxide reductions and the corresponding molten salts.
Zn was electrodeposited on an Fe electrode at a current density of 50–5000 A·m−2, charge of 4 × 104 C·m−2, and temperature of 313 K in an unagitated zincate solution containing 0.62 mol·dm−3 of ZnO, 4.0 mol·dm−3 of KOH or NaOH, and organic additives. The effects of KOH and NaOH on the deposition behavior of Zn in the solution containing the organic additives and on the microstructure of the deposits were investigated. In a solution containing a straight-chain polymer composed of a quaternary ammonium cation (PQ) and a quaternary ammonium salt with a benzene ring (QA), the current efficiency for Zn deposition in a high-current-density region (1000–5000 A·m−2) to produce glossy films was higher with KOH than that with NaOH. At high current densities above 1000 A·m−2, the Zn deposition approached the diffusion limitation of ZnO22− ions. With the addition of PQ and QA, the diffusion of ZnO22− ions was significantly suppressed, and the degree of suppression was smaller with KOH than that with NaOH. The polarization resistance at 200 A·m−2, which was investigated through alternating current impedance, revealed that the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH. Since the suppression effect of the additives on the Zn deposition was smaller with KOH than that with NaOH, the current efficiency for Zn deposition in the high-current-density region was larger with KOH. The upper limit of the current density needed to produce glossy films was smaller with KOH than that with NaOH, and spongy thin films were partially observed on platelet crystals obtained at high current densities in the KOH solution. The C content resulting from the additives in the deposited Zn was smaller with KOH because the adsorption ability of PQ and QA onto the cathode was smaller with KOH than that with NaOH.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 59–66. Caption of Fig 12 is slightly changed.
Fig. 6 Current efficiency for Zn deposition from the KOH and NaOH solutions with and without additives. [● KOH without additive, ▲ KOH with PQ and QA, ○ NaOH without additive, △ NaOH with PQ and QA]
CO2 tolerance of hydrogen storage alloys of AB2-type (C14 Laves phase) Ti0.515Zr0.485Mn1.2Cr0.8M0.1 (AB2-M, M = none, Fe, Co, Ni, and Cu) depends on dopant M. Since our goal is to clarify this mechanism, we determined the elemental analysis using X-ray absorption spectroscopy (XAS), scanning electron microscope (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), powder X-ray diffraction (XRD), and neutron powder diffraction (NPD) with Rietveld refinement in this study. As a result of XAS analysis, a strong evidence of all doped elements occupying B site in AB2 was obtained. SEM-EDX showed inhomogeneous composition with vacancy in B site and linear correlation of Ti/Zr and Mn/Cr ratio. The peak width in XRD patterns of AB2-M depends on the magnitude of homogeneity, therefore the Rietveld analysis using NPD patterns could not be well refined. Thus, homogeneity is not important but element of B site would be important for CO2 tolerance as well as AB5 type alloys.
Additive metal M occupies B site in AB2-type hydrogen storage alloys. Therefore, CO2 tolerance depends on B element.
Announcement Concerning Article Retraction The following paper has been withdrawn from the database of Mater. Trans., because a description based on a misinterpretation of the experimental results was found by the authors in advance of publication after acceptance. Mater.Trans. 52(2011) Advance view. Improvement in Fatigue Strength of Biomedical β-Type Ti-Nb-Ta-Zr Alloy while Maintaining Low Young’s Modulus through Optimizing ω-Phase Precipitation