Papers by Author: Kei Ameyama

Abstract: The harmonic structure composites with Ti-Ni alloy and Cu were fabricated by mechanical milling (MM) / spark plasma sintering (SPS) process and were investigated mechanical and thermal properties in detail. Fine Ti-Ni alloy powder and coarse Cu powder were mechanically milled using planetary ball mill equipment at cryogenic temperature. The MM powder was sintered by using the SPS apparatus at 1073 to 1273 K. Tensile tests carried out at 383 K as mechanical properties evaluation. Thermal expansion to 1073 K was evaluated using thermomechanical analyzer equipment. Microstructural observation of the MM powders and SPS compacts was achieved using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SPS compacts is the harmonic structure composite having the network structure with Ti-Ni alloy and the dispersive area with Cu. Such a Ti-Ni/Cu harmonic structure composite exhibits unique mechanical properties. T ensile strength and elongation increase with increasing the sintering temperature in the Ti-Ni/Cu harmonic structure composite. The coefficient of linear thermal expansion of the Ti-Ni/Cu harmonic structure composite lies between that of Ti-Ni alloy and Cu, and a sufficient reduction in the coefficient of linear thermal expansion is confirmed.

Abstract: The Harmonic Structure (HS) design was implemented in a high-entropy CrMnFeCoNi alloy compact to study its deformation characteristics at elevated temperatures, with particular emphasis on comparison with the homogeneous (Homo) compacts. The HS compact was prepared by powder metallurgy, employing a mechanical grinding process with a planetary ball mill in an argon atmosphere. The rotational speed was set at 150 rpm, and the milling time was either 180 or 360 ks. The resulting powders were then exposed to spark plasma sintering at 1223 K for 1.8 ks under 50 MPa. Subsequently, the compacts were subjected to high-temperature compression tests at 1073 K or 1173 K, at varying initial strain rates over a range of temperatures. These tests were conducted after the sintering process was completed. Homo exhibited a work hardening at the initial stage of deformation, followed by a slight decrease in flow stress, which then remained nearly constant. In contrast, HS exhibited a distinctive softening in flow stress following initial work hardening. A thorough examination of the microstructure during the softening process revealed that adjacent Shell/Core units caused grain boundary sliding in the Shell region. Furthermore, each Core exhibited a rotation of approximately 2.3 degrees and a lateral displacement of 1.5 μm. Observation of the softening phenomenon during high-temperature deformation was confirmed through TEM analysis, revealing that this softening resulted from dynamic recrystallization within the Shell region. Consequently, dynamic recrystallization in the Shell was postulated, followed by rotation of the Shell-Core unit through grain boundary sliding of the UFG structure.

Abstract: AlCoCrFeNi high entropy alloys (HEA) have superior strength and corrosion resistance at both room and high temperatures and are expected to application in elevated temperature environments. However, it is not clear the relationship between the harmonic structure and the mechanical properties of these HEAs at elevated temperatures. The harmonic structure is composed of dispersed coarse grains and fine grains that are networked around them. In this study, the harmonic structure AlCoCrFeNi HEA was fabricated by mechanical milling (MM) / spark plasma sintering (SPS) process and the microstructure and elevated temperature mechanical properties of AlCoCrFeNi HEA are investigated in detail. AlCoCrFeNi mixed powders with average particle sizes of 14.6 and 82.4 μm were treated with MM. The MM powders were consolidated by SPS at 1173 to 1373 K. Mechanical properties were evaluated by compression tests at room temperature to 1073 K. Microstructural observation was performed using a scanning electron microscope, electron back scattered diffraction and energy dispersive X-ray spectrometer. The conventional SPS compacts have modulated structure with BCC and B2 phase and grain boundary precipitates with FCC phase. While the MM-SPS compacts have a similar structure of the conventional compacts at dispersed region and an equiaxed nanograins including a σ phase at network region. MM compacts with harmonic microstructure demonstrate high compression strength compared to conventional compacts at room temperature to 673 K. However, conventional microstructure compacts have higher strength than harmonic structure above 873 K. These results suggest that the harmonic structure has unique deformation behavior at elevated temperatures.

Abstract: The Harmonic Structure [1] is a novel design concept that facilitates the engineering of metallic materials to achieve enhanced mechanical performance. The Harmonic Structure is composed of soft, coarse-grained regions, designated as the Core, which are surrounded in three dimensions by an interconnected network of hard, ultra-fine grain regions, referred to as the Shell. The interaction in these core/shell regions produces a synergistic effect during plastic deformation, resulting in superior mechanical properties that are of great significance. The distinctive network configuration of the Harmonic Structure enhances the dislocation density within the coarse-grain regions in contact with the interface through stress partitioning, thereby accelerating the work hardening rate and consequently enhancing the strength. This phenomenon is referred to as Hetero Deformation Induced (HDI) strengthening [2]. The fabrication of HS material is achieved through the application of mechanical milling (MM) to the powder, which results in the formation of a deformed layer on the surface of the powder and the creation of bimodal structured particles. However, a notable constraint of the MM process is its extended time requirement to attain the desired bimodal structure. In contrast, the bi-modal milling (BiM) technique involves the controlled mechanical milling of coarse and fine powders in conjunction with each other, with the objective of forming a layer of fine powders of a specified thickness over the coarse particles. The most advantageous aspect of bi-modal milling (BiM) is not only its reduced processing time, but also its superior ability to control the thickness of the surface deformation layer.

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: Harmonic structure has a heterogeneous microstructure consisting of bimodal grain size together with a controlled and specific topological 3D distribution of fine and coarse grains. These microstructural features of the harmonic structure materials lead to unique mechanical properties. In this study, harmonic structure was designed using the severe plastic deformation powder metallurgy process at room and cryogenic temperatures on pure nickel. There is no difference in appearance between mechanically milled (MM) powder at room and cryogenic temperatures. The compacts of the MM powder show the harmonic structure with a network fine grained area and the dispersed coarse grain area. The MM at cryogenic temperature affects the compact of the MM powder milled for 86.4 ks and its effects include an increase in shell fraction and a decrease in core grain size. Moreover, the harmonic structure materials show a synergy extra hardening in Hall-Petch relation. It is noteworthy that the harmonic structure materials exhibit a higher Hall-Petch coefficient than the homogeneous compacts despite of the same material.

Abstract: The high-temperature deformation behavior and microstructural changes of harmonic structure composites with WC-Co alloys and high-speed steel (HSS) were investigated in detail. A harmonic structure composite was fabricated by consolidating the mechanically milled powder having WC-Co and HSS powder. The harmonic structure composite demonstrates the microstructure composed of network area (WC-Co) and dispersed area (HSS). The harmonic structure composite shows a sufficient compressive strength in the compression tests at 773 K, but the compression strength decreases at temperatures of above 873 K. The 0.2% proof stress at high temperature almost unchanged even if the network area fraction changed. Furthermore, the network area plays an important role in the high temperature deformation of harmonic structure composites. These results suggest that the formation of voids for WC-Co boundary sliding and poor sintering is an important factor in stress reduction in the high-temperature compression of harmonic structure composites.

Abstract: Diffraction contrast tomography using ultrabright synchrotron radiation X-rays was performed on an austenitic stainless steel with a bimodal harmonic structure in which a network structure of fine grains (Shell) surrounds a coarse grain structure (Core). Then, not only were the shape and position of each grain reconstructed, but the change in excess dislocation density during the fatigue process, Δρ, was also measured. The results show that Δρ depends on the diffraction plane, Schmidt factor, and grain size, and that the change in Δρ during the fatigue process of a harmonic structured material is less than that of a conventional material. This result indicates that the network of fine grains in the harmonic structure supports microdeformation and suppresses the deformation of coarse grains. Furthermore, it was found that Δρ of grains unrelated to crack initiation increased continuously with the number of cycles, whereas that around the crack initiation site decreased with crack initiation.

Abstract: (α + γ) two phase stainless steel (Fe-21%Cr-4.8%Ni-1.5%Mo) powder was processed by high pressure torsion (HPT) and consolidation at room temperature. The received powder had fully α single phase due to the rapid cooling during gas atomizing process. Specimens after HPT process were heat treated at 1173K for 3.6ks. It was revealed that the decomposition of α phase to γ took place during the heat treatment. Detailed microstructure observation showed that an equiaxed (α + γ) micro-duplex structure was developed and its average grain size was approximately 3.2 micrometers. The same heat treatment given to the material without HPT resulted in a coarse two phase microstructure.Therefore, it is considered that an ultra fine grained microstructure was caused by increasing of nucleation sites for γ phase due to severe plastic deformation (SPD) of HPT process. Electron backscatter diffraction patterns (EBSD) analysis indicated that α phase has a {110}/ND strong texture, that is, the α phase seems to have single orientated coarse grain structure. The γ precipitates indicated a {111}/ND strong texture, and the crystallographic orientation relationship of Kurdjumov-Sachs was observed.