Kenta Yamanaka

Bio: Kenta Yamanaka is an academic researcher from Tohoku University. The author has contributed to research in topics: Alloy & Microstructure. The author has an hindex of 27, co-authored 117 publications receiving 2193 citations. Previous affiliations of Kenta Yamanaka include Kobe Steel & Hitachi.

Relationship between the microstructure and mechanical properties of an equiatomic AlCoCrFeNi high-entropy alloy fabricated by selective electron beam melting

Abstract: Because of their superior properties, high-entropy alloys (HEAs) are considered promising novel structural materials that can substitute conventional alloys. From the viewpoint of future applications, it is important to explore methods for producing complex shaped products with HEAs. In this study, selective electron beam melting (SEBM) was employed for fabricating equiatomic AlCoCrFeNi HEA specimens, and their microstructures and mechanical properties were evaluated by comparing them with those of a conventionally cast specimen. Both cast and SEBM specimens dominantly consisted of a nano-lamellar mixture of disordered body-centered-cubic (BCC) and B2 (ordered BCC) phases. The face-centered cubic (FCC) phase was also precipitated at the grain boundaries of the B2/BCC mixture phases on the SEBM specimens. The fraction of the FCC phase at the bottom part of the SEBM specimen was higher than that at the top part. The preheating procedure—a process unique to SEBM—is responsible for the precipitation of the FCC phase, because of the long-term exposure at sufficiently high temperatures. As a result, the hardness of the SEBM specimens gradually decreased as we approached the bottom part of the specimens due to the increased fraction of the FCC phase, which had lower hardness than the B2/BCC phases. Further, the SEBM specimen exhibited much higher plastic deformability than the cast specimen, without significant loss of strength.

Ultrafine Grain Refinement of Biomedical Co-29Cr-6Mo Alloy during Conventional Hot-Compression Deformation

Abstract: In order to examine the microstructural evolution during hot-compression deformation of the biomedical Co-29Cr-6Mo (weight percent) alloy without the addition of Ni, hot-compression tests have been conducted at deformation temperatures ranging from 1050 °C to 1200 °C at various strain rates of 10−3 to 10 s−1. The grain refinement due to dynamic recrystallization (DRX) was identified under all deformation conditions by means of field-emission scanning electron microscopy/electron backscattered diffraction (FESEM/EBSD) and transmission electron microscopy (TEM) observations. Although the DRX grain size (d) of the deformed specimens considerably decreased with an increasing Zener–Hollomon (Z) parameter at strain rates ranging from 10−3 to 0.1 s−1, a grain size coarser than that predicted from the d-Z relation was obtained at strain rates of 1.0 and 10 s−1. An ultrafine-grained microstructure with a grain size of approximately 0.6 μm was obtained under deformation at 1050 °C at 0.1 s−1, from an initial grain size of 40 μm. The grain refinement to a submicron scale of biomedical Co-Cr-Mo alloys has been achieved with hot deformation by ~60 pct due to DRX, in which the bulging mechanism is not operative. The ultrafine grains obtained due to DRX without bulging is closely related to the considerably low stacking-fault energy (SFE) of the Co-Cr-Mo alloy at deformation temperatures.

Metal Additive Manufacturing: A Review of Mechanical Properties

Abstract: This article reviews published data on the mechanical properties of additively manufactured metallic materials. The additive manufacturing techniques utilized to generate samples covered in this review include powder bed fusion (e.g., EBM, SLM, DMLS) and directed energy deposition (e.g., LENS, EBF3). Although only a limited number of metallic alloy systems are currently available for additive manufacturing (e.g., Ti-6Al-4V, TiAl, stainless steel, Inconel 625/718, and Al-Si-10Mg), the bulk of the published mechanical properties information has been generated on Ti-6Al-4V. However, summary tables for published mechanical properties and/or key figures are included for each of the alloys listed above, grouped by the additive technique used to generate the data. Published values for mechanical properties obtained from hardness, tension/compression, fracture toughness, fatigue crack growth, and high cycle fatigue are included for as-built, heat-treated, and/or HIP conditions, when available. The effects of test...

A Promising New Class of High-Temperature Alloys: Eutectic High-Entropy Alloys

Abstract: High-entropy alloys (HEAs) can have either high strength or high ductility, and a simultaneous achievement of both still constitutes a tough challenge. The inferior castability and compositional segregation of HEAs are also obstacles for their technological applications. To tackle these problems, here we proposed a novel strategy to design HEAs using the eutectic alloy concept, i.e. to achieve a microstructure composed of alternating soft fcc and hard bcc phases. As a manifestation of this concept, an AlCoCrFeNi 2.1 (atomic portion) eutectic high-entropy alloy (EHEA) was designed. The as-cast EHEA possessed a fine lamellar fcc/B2 microstructure, and showed an unprecedented combination of high tensile ductility and high fracture strength at room temperature. The excellent mechanical properties could be kept up to 700°C. This new alloy design strategy can be readily adapted to large-scale industrial production of HEAs with simultaneous high fracture strength and high ductility.