Materials Science Forum Vol. 1107

Abstract: The Harmonic Structure (HS) is a recently introduced concept that paves the way for engineering metallic materials to achieve superior mechanical performance. They consist of soft, coarse-grained regions surrounded in three dimensions by an interconnected network of hard, ultra-fine grained regions. In addition, from a structural materials point of view, high entropy alloys have attracted attention due to their unique mechanical properties. In the present study, the HS design was applied to a high entropy CrMnFeCoNi alloy (also called "Cantor alloy"). The HS-designed Cantor alloy was successfully fabricated by mechanical milling, which is one of the surface severe plastic deformation processes, and the subsequent sintering process. The mechanical properties of these HS and homogeneous (Homo) Cantor alloy compacts were investigated by high-temperature compression tests in the temperature range of room temperature (RT) and 1173K, under initial strain rates of 0.01 s^-1, 0.001 s^-1, and 0.0001 s^-1. The stress-strain curves of the HS compacts showed a large initial increase in stress and then a rapid decrease with strain, while that of the Homo compact showed a gentle increase and a gradual decrease. EBSD observation of the deformed compacts revealed that the HS compacts were probably deformed not only by dynamic recrystallization, but also by grain boundary sliding during deformation. The strain rate sensitivity value m of the HS compacts was 0.541 (true strain: 0.2) at 1173 K. In other words, the HS compacts exhibited pseudo-superplastic deformation at these temperatures.

Abstract: Harmonic Structure (HS) materials, a class of heterogeneously structured materials, are known to exhibit unique and superior mechanical properties. The HS consists of soft, coarse-grained regions (Core) that are three-dimensionally surrounded by an interconnected network of hard, ultrafine grained (UFG) regions (Shell). The unique UFG network structure of the Harmonic Structure increases the dislocation density of the core regions in contact with the Shell, resulting in increased strength and work hardening rate in the Core regions. These contribute to the high strength of the HS materials and suppress the plastic instability of the Shell regions, resulting in higher ductility of the HS materials. In the present research, the HS design is applied to a high-entropy CrMnFeCoNi alloy, also known as the Cantor alloy, to study the microstructure change during high temperature deformation at 1073 K and 1173 K. Although the alloy exhibits high strength and high ductility at cryogenic temperature due to the twinning deformation, the high temperature properties are not clear, especially in the case of the HS design. As a result, the alloy with or without HS design did not show twinning deformation at these temperatures, and it is noteworthy that the alloy with HS showed preferential recrystallization in the UFG network region, and thus the recrystallized UFGs played an important role in grain boundary sliding to demonstrate the pseudo-superplastic deformation behavior.

Abstract: Copper/diamond composites show promise as potential thermal management materials for electronic devices due to their excellent thermophysical properties. In this study, copper/55vol%Ti-coated diamond composites were fabricated by hot pressing at 800oC and varying pressures of 300MPa, 400MPa, 500MPa, and 685MPa. The results illustrated that the thermal conductivity of the copper/Ti-coated diamond composites initially increased and then decreased as the pressing pressure increased. Among these hot-pressed composites, the composite hot-pressed at 500MPa exhibits the highest thermal conductivity of 466W/mK. This is attributed to the uniform diamond distribution, highly covered TiC interface on the diamond, and the strong interfacial bonding between the copper and the diamond. Hot pressing is a feasible alternative to fabricate copper/diamond composites with high relative density and high thermal conductivity, the pressing pressure plays a vital role in the microstructure and the final properties of the copper/Ti-coated diamond composites.

Abstract: We have prepared multi-layered iron-based 32522-type (Ca, Sr)₃Al₂O_5-yFe₂As₂ superconductor with the double perovskite-based “325” structure using high-pressure technique. In this paper, we have investigated iso-valent cation Sr substitution effect on Ca site for 32522-type (Ca, Sr)₃Al₂O_5-yFe₂As₂. By the Sr atom substitution for Ca site, superconducting transition temperature (T_c) was slightly increasing, the critical current density (J_c) was kept under magnetic field near T_c and the irreversibility field (H_irr) value was also increasing.

Abstract: The synthesis of ceramic composites consisting of cerium and titanium-doped zirconium (ZCT) oxide was achieved by the solid-state reaction technique. The ZCT composite ceramic powder undergoes sintering at various temperatures, including room temperature (RT), 1000°C, 1100°C, 1200°C, and 1300°C. Extensive study has been conducted on ceria-based materials in the field of catalysis, owing to their vast array of uses. Nevertheless, there was a limited amount of research conducted on the impact of ceria in the solid-state reaction approach. The current study employed a solid-state reaction method to fabricate ceramic composites comprising ZrO₂, CeO₂, and TiO₂. Various sintering temperatures were employed in the process. This study aimed to evaluate the impact of the sintering effect of ZCT ceramic oxides on several aspects, including crystal structure, surface morphology, optical properties, and electrical properties. The ZCT ceramic oxide underwent sintering at room temperature (RT), 1000°C, and 1100°C, resulting in the formation of a monoclinic crystal structure. However, sintering at 1200°C and 1300°C led to the presence of mixed phases, characterized by both monoclinic and tetragonal crystal structures, as observed through X-ray diffraction (XRD) analysis. When the sintering temperature is increased from 1000 to 1300°C, there is a modest drop in the band gap of a ZCT material from 3.43eV to 3.25eV. frequency(1mHZ-200kHz) dependence of dielectric constant, dielectric loss and ac electrical conductivity of the synthesized composites were carried out. The results indicate that dielectric constant and loss decreases with frequency rises and reaches a constant value at higher frequencies. The electrical conductivity of all ZCT samples exhibits an increase as the frequency is raised, whereas it reaches a minimum at lower frequencies.

Abstract: Ceramics and metals are the two vastly explored classes of materials whose individual characteristics and targeted applications differ significantly. Continuous thrust for space exploration and energy generation demands materials with a wide range of properties. To tackle this demand, ceramic-metal combined structures that club heat, wear, and corrosion resistance of ceramics to the high toughness, good strength, and better machinability of metals are desirable. While various processing routes to combine ceramics and metals have been developed through the years, solutions to address problems associated with the interface, thermal property mismatch, and poor adhesion need to be explored. In this context, Functional Graded Materials (FGMs) have attracted particular attention by virtue of their ability to avoid sharp interfaces and local stress concentrations. Out of all, Additive Manufacturing (AM) routes, particularly the Directed Energy Deposition (DED) technique, is emerging as a productive technique capable of fabricating a wide range of metal-ceramic graded structures. This paper specifically discusses metal-ceramic FGMs ́ capability as a potential high-temperature material with customized multifunctional material properties. It further outlines the primary concerns with the realization of metal-ceramic graded structures and major techniques developed to mitigate problems encountered in processing them. Specific emphasis is laid on the powder-based Laser DED (L-DED) technique of FGM fabrication owing to its control over complex geometries and microstructural engineering.

Abstract: Utilizing both charge and spin degrees of freedom of electrons simultaneously in magnetic semiconductors promises new device concepts by creating an opportunity to realize data processing, transportation and storage in one single spintronic device. Unlike most of the traditional diluted magnetic semiconductors, which obtain intrinsic ferromagnetism by adding magnetic elements to non-magnetic semiconductors, we attempt to develop room temperature magnetic semiconductors via a metal-semiconductor transition by introducing oxygen into three different ferromagnetic amorphous alloy systems. These magnetic semiconductors show different conduction types determined primarily by the compositions of the selected amorphous ferromagnetic alloy systems. These findings may pave a new way to realize magnetic semiconductor-based spintronic devices that work at room temperature.

Abstract: In order to increase strength while maintaining the ductility of material, pure titanium was improved through the thermomechanical treatment that combines rolling and heat treatment. The tensile properties of pure titanium treated by rolling and heating were investigated. Test material was JIS Grade 2. This material has a higher corrosion resistance. However, the strength of JIS Grade 2 is lower than that of JIS Grade 3. JIS Grade 2 with high strength while maintaining corrosion resistance is being developed. Techniques for improving the properties of materials with simple compositions are important. Thermomechanical treatment is used as a method for improving material properties. In the present study, the effect of thermomechanical treatment on the material properties of JIS Grade 2 was investigated. Rolling was performed at room temperature and the reduction ratio ranged from 70 to 90 %. The heating temperature was in the range of 300 to 700 °C. Heat treatment from 400 to 500 °C showed an increase in tensile strength while maintaining ductility. When the heat treatment temperature was 450 °C, the strength and elongation were approximately 600 MPa and 25 %. Tensile stress of JIS Grade 4 and the tensile strain of JIS Grade 1 were exhibited.

Abstract: We developed a new 1-12-type magnetic material of (Sm,Zr)(Fe,Co)_11.3Ti_0.7 composition that exhibits magnetic properties superior to Nd-Fe-B magnets. In the new 1-12 magnets, the amorphous alloys of above composition show a change in XRD pattern with increasing of heat treatment temperature from a single 1-9 phase to 1-9 and 1-12 mixed phases, and finally to a single 1-12 phase, and the magnetic properties also change accordingly. In this study, we established a method for calculating the formation ratio of the 1-12 phase in the samples from the peak shift of the diffraction peaks of the 1-12 phase base on the peaks of the 1-9 phase. As a result, it was revealed that the formation ratio of the 1-12 phase in the samples, whose XRD pattern of 1-9 and 1-12 mixed phase, has a wide distribution, ranging from about 20 to 80 %. With the development of the phase transition from 1-9 to 1-12 phases, the lattice constant a of 1-12 phase increases, and inversely the lattice constant c of 1-12 phase decreases. Furthermore, it was revealed that the formation ratio of the 1-12 phase was about 83 % for the sample indicating the maximum coercivity H_c = 5.46 kOe.

Abstract: The amorphous-forming ability (AFA) and the magnetic properties of the amorphous Fe-B-C-Si soft magnetic alloys have been investigated. Though the ternary Fe-B-C alloys exhibit high magnetization, their AFA is poor, which has prevented their practical application. It was confirmed that the addition of Si to the Fe-B-C alloys significantly improves AFA. Furthermore, it has been found that the composition range of the Fe-B-C-Si alloys with sufficient AFA can be identified by focusing on the enthalpy of mixing (ΔH_mix) and δ, which is related to the ratio of atomic radius between the constituent elements, of the alloys. The Fe-B-C-Si amorphous alloys that combine the high saturation magnetization of 175−177 A m²/kg with sufficient AFA to produce thick sheets whose thickness of 70 μm or more have been successfully developed by using the relationship between ΔH_mix, δ and AFA as a guideline. The Fe-B-C-Si amorphous alloys are expected to be applied to core materials for various magnetic components.