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The effect of low temperature on tensile properties of a AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) was investigated in the present work. Transmission electron microscopy (TEM) showed that the initial as-cast microstructure consisted of B2 (ordered body centered cubic structure) and L12 (ordered face centered cubic structure) phases. Upon tensile testing at temperatures ranging from room temperature (RT) to −196 °C (77 K), the L12 phase became disordered, changing to a simple FCC (face centered cubic) crystal structure whereas the B2 phase maintained an ordered structure. An increase over 300 MPa was observed in the ultimate tensile strength (σUTS) of the −196 °C tensile tested sample compared to the room temperature tensile tested sample, while keeping almost similar amount of total elongation. TEM results indicated an increase in the dislocation activity in the FCC phase as well as B2 phase in the −196 °C tensile tested sample compared to the sample deformed at room temperature.

The development of texture during cold-rolling and recrystallization was investigated in a AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA). For this purpose, the as-cast alloy was cold-rolled to 90% reduction in thickness and annealed at temperatures ranging from 800 °C to 1200 °C. The microstructure of the as-cast EHEA showed a nano-lamellar mixture of L12 and B2 phases. The B2 phase was significantly harder than the L12 phase. Development of an ultrafine microstructure was observed after 90% cold-rolling. During cold-rolling, progressive disordering of the L12 phase was observed while the B2 phase maintained the ordered structure. The progressive disordering and development of a predominantly brass type texture in the L12 phase correlated well with profuse shear band formation during cold-rolling. The B2 phase showed the presence of the {111} component which was typical for cold-rolled B2 alloys. An ultrafine duplex structure of equiaxed L12 and B2 phases developed after complete recrystallization that showed significant resistance to grain growth up to very high annealing temperatures (?1300 °C). The remarkable resistance to grain growth compared to conventional or even other single or dual phase HEAs was due to the formation of a homogeneous duplex structure where growth of one phase was effectively retarded by the other phase. The strong presence of the ?-fiber components, but weak BR ({236} ) and D ({113} ) components in the recrystallization texture of the L12/FCC phase was due to the absence of strong preferential nucleation or growth. Presence of ND-fiber (ND// ) with strong {111} component in the recrystallization texture of the B2 phase indicated ease of nucleation from similarly oriented regions in the deformed microstructure.

Display Omitted We investigated the surface tribological properties of two release agents.The friction and adhesion forces were measured separately.Two kinds of release agents were shown different friction and adhesion forces.Our tribological method of evaluating the properties is shown to be effective. The adhesive and friction forces generated in releasing a mold from a resin in the presence of a release agent was investigated by using a TriboIndenter. We investigated the surface tribological properties of two ultraviolet-nanoimprint-lithography release agents, Optool DSX and trichloro(tridecafluoro-octyl)silane C8H4Cl3F13Si] (F13), to elucidate the mold-release mechanism. For both release agents, we measured the friction and adhesive forces acting on the substrate. The friction force of Optool DSX was found to be lower than that of F13, whereas F13 showed a lower adhesive force than Optool DSX. Our previous results revealed that the durability of Optool DSX as a release agent was better than that of F13. Examining our previous and current results, we saw that the trends in the contact angle and the number of defects with increasing imprinting counts coincide with the trends in the friction and adhesive forces, respectively, of the release agents. Our method of evaluating the surface properties of release agents in two kinds of tests is shown to be effective.

Nano-lamellar (L12 + B2) AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) was processed by cryo-rolling and annealing. The EHEA developed a novel hierarchical microstructure featured by fine lamellar regions consisting of FCC lamellae filled with ultrafine FCC grains (average size ~200–250 nm) and B2 lamellae, and coarse non-lamellar regions consisting of ultrafine FCC (average size ~200–250 nm), few coarse recrystallized FCC grains and rather coarse unrecrystallized B2 phase (~2.5 µm). This complex and hierarchical microstructure originated from differences in strain-partitioning amongst the constituent phases, affecting the driving force for recrystallization. The hierarchical microstructure of the cryo-rolled and annealed material resulted in simultaneous enhancement in strength (Yield Strength/YS: 1437 ± 26 MPa, Ultimate Tensile Strength/UTS: 1562 ± 33 MPa) and ductility (elongation to failure/ef ~ 14 ± 1%) as compared to the as-cast as well as cold-rolled and annealed materials. The present study for the first time demonstrated that cryo-deformation and annealing could be a novel microstructural design strategy for overcoming strength-ductility trade off in multiphase high entropy alloys., 高強度・高延性を有するハイ・エントロピー合金の作製 --ヘテロ・ナノ組織制御による優れた力学特性の実現--. 京都大学プレスリリース. 2018-02-19.

It becomes important to measure mechanical properties of local area of materials because of downsizing of industrial materials, and the fracture of materials often starts at the nanoscale defects. Therefore, the measurement of hardness of the local area of materials is one of key technologies to develop new materials. Nanoindentation is the depth sensing indentation method and can measure the mechanical properties of nanoscale area of materials. The hardness of materials under the controlled environment like high temperature is a big issue in energy or environment industries. Therefore, it becomes important to measure high temperature mechanical properties of nanoscale area of materials by nanoindentation. However, the shape of indenter may change when an indenter contacts to the material surface at high temperatures, and the nanoindentation at high temperatures may lead to inaccuracy of measurements. Nanoindentation does not measure indented are directly but converts the indentation depth to the indented area by area function. In order to correctly convert the indentation depth to the indented area, it is necessary to derive the area function of indented area at high temperatures. In this report, the area function is proposed that considers the change of indenter shape during repeating contact at heated materials. By using this proposed area function, nanoindentation hardness and the reduced modulus of sapphire were obtained at 303K, 473K, 673K, 873K and 1073K successfully. The nanoindentation can be used at high temperatures, if this proposed area function is used.

Nowadays, industrial products are downsized, and the structure of materials is controlled with the nanometer precision, and it becomes very important to measure the mechanical properties of local area of bulk material. Especially the hardness and the elastic modulus are important mechanical properties. The orientation dependence of hardness and reduced modulus of single crystal sapphire surface was investigated by nanoindentation. The conventional technique to measure the hardness of materials using an optical micrometer cannot evaluate mechanical properties of a local region of several µm or less. However, nanoindentation can measure mechanical properties of very small surface area of materials, and is expected to detect the micro structure dependence of mechanical properties. Nanoindentation uses very small indenter made of diamond, and measures the indentation depth. The measured depth is converted to the indented area size using the area function. The area function of the indenter can be obtained using a standard material (fused quartz) in advance. Therefore nanoindentation can measure the indented area size without using an optical micrometer. In this report, it was shown that the nanoindentation could detect the structure dependence of mechanical properties of materials. The specimen was a single crystal sapphire with c - axis surface, and the indenter was Berkovich type diamond tip. It was confirmed that the nanoindentation hardness was the lowest, and the reduced modulus was the largest, when the ridge line of indenter was oriented to the m - axis of single crystal. The nanoindentation could detect the structure dependence of a local area of mechanical properties materials.

The adsorption of individual acetylene molecules at a Pd(1 1 1) surface has been studied with scanning tunneling microscopy (STM) at 4.7 K. The adsorbed acetylene molecules appear in the STM images as a combination of a protrusion and a pair of depressions. To determine the adsorption site and configuration of the molecule, we performed rotational manipulation of the molecule by injecting tunneling electrons into the molecule as well as performing atomically resolved STM imaging. The STM images revealed that the molecules have a total of six equivalent adsorption orientations on Pd(1 1 1), indicating three equivalent rotational states on two (both fcc and hcp) 3-fold hollow adsorption sites. From the atomically resolved STM images of the surface with a monatomic step edge, we have distinguished the two types of 3-fold hollow sites and confirmed that the fcc site is more stable than the hcp site at a low temperature.

Inelastically tunneled electrons from a scanning tunneling microscope (STM) were used to induce vibrationally mediated motion of a single cis-2-butene molecule among four equivalent orientations on Pd(110) at 4.8 K. The action spectrum obtained from the motion clearly detects more vibrational modes than inelastic electron tunneling spectroscopy with a STM. We demonstrate the usefulness of the action spectroscopy as a novel single molecule vibrational spectroscopic method. We also discuss its selection rules in terms of resonance tunneling.