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We designed an aluminum (Al)-based alloy with the α-Al (fcc) matrix strengthened by the T-Al6Mg11Zn11 (cubic) intermetallic phase using a large two-phase region of α and T phases in the Al–Mg–Zn ternary system. Thermodynamic analysis assessed a composition of Al–5Mg–3.5Zn (at%) with the α-Al phase reinforced with high fractions (approximately 10%) of T phase. We observed that the T phase preferentially precipitated at grain boundaries in the α-Al matrix, increasing the area fraction of the T phase at grain boundaries during aging. The granular precipitates of the T phase were dispersed rather homogenously in the α-Al matrix with a particular orientation relationship of (1−11)α // (1−21)T and [011]α // [111]T at temperatures above 300 °C. After aging at 200 °C, numerous fine precipitates with a mean size of ~20 nm in the grain interior were observed, which were likely the metastable phase associated with the T phase. The present alloy (pre-aged at 200 °C for 1 h) exhibited a high yield strength of approximately 260 MPa at 200 °C, much higher than those of the conventional Al alloys at elevated temperatures corresponding to service temperatures for compressor impellers in turbochargers.

The effects of a trace amount (~0.1 at.%) of added stabilizing elements (Nb, Ti, and Zr) on the tensile properties of 18Cr ferritic stainless steel sheets at strain rates ranging from 10−3–102 s−1 (from ordinary quasi-static deformation to dynamic deformation) were investigated. The additional Ti or Zr elements formed coarse carbide or nitride precipitates, which consumed interstitial solute elements (C and/or N) in the ferrite matrix. In the Nb-added steel, however, only fine NbN precipitates with a few hundred nanometers in size were observed locally, implying a small amount of solute Nb in the ferrite matrix. Ti or Zr additions had only a slight effect on the deformation resistance of 18Cr ferritic steel sheets and strain rate sensitivity (m) of the flow stress, however, the Nb addition increased the deformation resistance and significantly reduced the strain rate sensitivity. The measured activation volume (v*) values of the Ti- and Zr-added steels (62 b3 and 66 b3) were almost equivalent to one of the 18Cr base steel (64 b3). This result suggested that a general thermally activated process for screw dislocation motion is controlled by double-kink nucleation, which is slightly affected by Ti or Zr additions. The high v* value (106 b3) of the Nb-added steel indicated that the thermally activated process of plastic deformation would be associated with the substitutional solute Nb atoms (act as short-range obstacles) interacting with the moving dislocations. These results showed that the Nb element played a distinct role in plastic deformation from the other stabilizing elements of Ti and Zr.

In the present study, we examined changes in the microstructure and mechanical properties of AlSi10Mg alloy, initially fabricated using selective laser melting (SLM) combined with a powder-bed system, by applying heat treatments at temperatures of either 300 or 530 °C. The as-fabricated samples exhibited a characteristic microstructural morphology and {001} texture. Melt pools corresponding to the locally melted and rapidly solidified regions were found to be composed of several columnar α-Al grains surrounded by fine eutectic Si particles. A fine dislocation substructure consisting of low-angle boundaries is present within the columnar α-Al grains. At elevated temperatures, fine Si phase precipitates within the columnar α-Al phase and coarsening of the eutectic Si particles occurs. These fine Si particles inhibit grain growth in the α-Al matrix, resulting in the microstructural morphology and [001] texture observed in the heat-treated samples. The dislocation substructure disappears in the columnar α-Al grains. Furthermore, the formation of a stable intermetallic phase occurs, reaching microstructural equilibrium after long-term exposure. The as-fabricated specimen exhibits a high tensile strength of approximately 480 MPa. The strength is independent of the tensile direction, that is, normal and parallel to the building direction. In contrast, the tensile ductility is found to be direction-dependent, and is therefore responsible for a fracture preferentially occurring at a melt pool boundary. The direction-dependence of the tensile ductility was not found in the specimen that had been heat-treated at 530 °C. The present results provide new insights into the control of the direction-dependence of the tensile properties of AlSi10Mg alloys fabricated by SLM.

We attempted to produce a porous Ti–20 at% Al alloy by pressure-sintering of a mixed powder of Ti and Al with a NaCl space holder to achieve porosity of 60%. As-sintered specimens exhibited an inhomogeneous microstructure consisting of Ti particles surrounded by a Ti–Al alloy layer. However, a nearly single-phase Ti 3 Al microstructure appeared in the porous specimen heat-treated at 1300 °C, which shows high plateau-stress level of approximately 100 MPa resulting in high absorption energy. Thus, controlling the microstructure for the formation of the Ti 3 Al single-phase would be effective to achieve high energy absorption capacity of the porous Ti–Al alloy.

Single-crystal micropillar compression tests were performed in this study to investigate the mechanical properties and deformation mechanisms of Cr-based Laves phases. The results showed that, the activated slip system in the C15–Cr2Nb was exclusively the basal slip {111}⟨110⟩, transiting from the pyramidal slip {11 2 ¯ 2}⟨ 1 ¯ 1 ¯ 23⟩ to the prismatic slips {10 1 ¯ 0}⟨0001⟩ and {11 2 ¯ 0}⟨1 1 ¯ 00⟩ in the C14–Cr2Ta with the increased inclination angle. The average compressive strength of the C15–Cr2Nb changed between 10.0 GPa and 11.7 GPa, and that of the C14–Cr2Ta ranged within 11.6–12.4 GPa. The compressive strength changed due to the evolutions of activated slip systems in both Laves phases. The critical resolved shear stress (CRSS) of the C15–Cr2Nb was estimated as 4.4–4.6 GPa, which was lower than 5.1–6.1 GPa in the C14–Cr2Ta, and the higher CRSS of the C14–Cr2Ta resulted from higher shear modulus. The mechanical properties and deformation mechanisms obtained were helpful for better understanding and designing the Cr-based Laves phase alloys.

The strain-hardening of cell wall in porous metals decreases the energy absorption properties. In order to suppress the strain hardening, porous hybrid materials consisting of ductile Al and Ti phases and brittle Al3Ti phase were synthesized by reaction sintering between Al and Ti powders with NaCl space holder. Changes in the porous structure and microstructure with sintering were investigated. Area fraction of Al3Ti phase formed at Al/Ti interface increased following the Johnson-Mehl-Avrami-Koromogrov (JMAK) equation of three-dimensional diffusion-controlled growth with zero nucleation rate. The growth of Al3Ti phase led to the formation of pores and cracks in cell wall by Kirkendall effect and volume shrinkage. Effect of microstructure on compressive properties of the porous Al/Al3Ti/Ti hybrids were also investigated. Porous hybrids consisting of α-Al matrix exhibited the plateau region with positive slopes due to the strain hardening of α-Al phase. Porous hybrids consisting of Al3Ti matrix exhibited the plateau region with the slope of almost zero, resulting in high energy absorption capacity and efficiency. Fractography revealed that crack propagated in brittle Al3Ti phase but arrested at α-Al phase. These results were used to discuss the microstructure for improving energy absorption properties., ファイル公開：2022-03-03

An anomalously reduced size-dependent strength of commercial-purity aluminum (Al) single-crystal micropillars with diameters ranging approximately from 1 to 10 μm is reported. High-purity Al (99.99%) single-crystal micropillars exhibited an obvious size dependence of the resolved shear stress for slip. The measured shear stress resolved onto a primary slip system (τi) scaled by the shear modulus (G) and the pillar diameter (d) scaled by the Burgers vector (b) showed the following correlation: τi/G = 0.33(d/b)−0.63, which agreed well with previous works. However, the commercial-purity Al samples exhibited a lower power-law exponent (0.19) for their size-dependent strength, resulting in (τi/G) = 0.006(d/b)−0.19. TEM characterization revealed the local presence of Al–Fe intermetallic precipitates surrounded by relatively high-density dislocations in annealed commercial-purity Al samples. These results indicate the relatively high-density dislocations could be responsible for the reduced size-dependent strength, which was confirmed by the remarkably reduced size dependence of their resolved shear stress by prior cold rolling. The reduced size-dependent strength can be rationalized using the stochastic model of the dislocation source length. Thus, this study provides a new insight to allow the application of the micropillar compression test to commercially produced Al alloys.

Topology optimization approach was used for the design of Ti6Al4V ELI lattice structures with stiffness and density close to the human bone for implant applications. Three lattice designs with volume densities of 35 %, 40 % and 45 % and corresponding elastic modulus of 18.6 GPa, 23.1 GPa 27.4 GPa close to the human bone were generated. Laser powder bed fusion (LPBF) technique was used for the manufacturing of the specimens. Physical measurements and mechanical characterization of specimens were assessed by microCT analyses and compression test, perpendicular and parallel to the building direction of the specimens. LPBF Ti6Al4V ELI manufactured lattice structures showed deviations in wall thickness in comparison with the generated designs, leading to an increase in relative porosity but also a decrease in elastic modulus in comparison with the original designs. Horizontal walls of the lattice structures showed higher wall thickness in comparison with the vertical walls, leading to anisotropic behaviour of the lattice structures. Higher elastic modulus and compression strength were obtained when thicker walls were oriented along the loading direction of the compression test, showing a complete failure by dividing the specimens into two neighbouring halves. All specimens showed 45° diagonal shear fracture along the structure. On the other hand, higher energy absorption at first maximum compression strength peak was observed when samples were tested parallel to the building direction (when thinner walls were oriented along the loading compression direction). Results showed that designed lattice structures can possess the levels of human bones’ stiffness and therefore can reduce/avoid stress shielding on implant applications.

To elucidate the adhesion properties of anticorrosion hydroxide films formed on aluminum (Al)-alloy sheets prepared by the steam coating process, cracks that propagated inside the hydroxide film formed on Al–Mg–Si alloy (6061 grade) sheets were characterized. Continuous hydroxide films with different thicknesses of 0.7 μm and 1.4 μm formed at different temperatures of 180 °C and 200 °C, respectively. Microstructural characterizations revealed that these hydroxide films exhibited a dual-layer structure consisting of a polycrystalline γ-AlO(OH) layer (on the surface side) and a continuous amorphous layer (on the Al-alloy side). Bending tests caused numerous cracks inside the hydroxide films under tension and compression. On the tensile-strained surface, numerous vertical cracks penetrated through the hydroxide films, although their delamination was not observed. On the compressive-strained surface, local delamination of the upper γ-AlO(OH) layer occurred. However, cracks were not observed in the lower amorphous layer on the Al-alloy substrate deformed under compression. These results indicate that the dual-layer-structured hydroxide film prepared by steam coating exhibits remarkable adhesion to the Al-alloy substrate.

Open-cell structured porous titanium/ceramics composite was synthesized by a reactive precursor method using titanium and boron carbide (B 4 C) as reactant powders. Pore morphology was controlled by adding heat absorbing powder (titanium diboride: TiB 2 ) in the Ti+B 4 C blended powder. The effects of molar blending ratio of titanium and B 4 C and the amount of heat absorbing powder addition on the cell morphology (either open or closed) were investigated. Fine and homogeneous open-cell structure was achieved by adding appropriate amount of heat absorbing agent powder (>15 vol%), and the relative density of the specimen after the reaction became closer to that of the precursor by increasing TiB 2 volume fraction. When the volume fraction of TiB 2 addition was 20%, the open-cell fraction was maintained as 1.0 regardless of the relative density of the precursor.