Yorinobu Takigawa

Crystal grain refinement in high- and medium-entropy alloys is an effective approach to obtaining good strength as well as ductility. An FeCoNi medium-entropy alloy with a wide range of grain sizes was fabricated by electrodeposition. The alloy had a metastable bcc structure with Fe–36 at.% Co–21 at.% Ni composition and heterogeneous distribution of grain sizes ranging from 20 nm to 15 µm. Moreover, the alloy possessed a tensile strength of 1.2 GPa, and a fracture elongation of 6.3 % without secondary phase. The excellent mechanical properties of the alloy can be attributed to its bcc phase and heterogeneous grain sizes extending to the nanoscale. GRAPHICAL ABSTRACT

In this study, a Ti–6Al–4V alloy composite with uniaxial anisotropy and a hierarchical structure is fabricated using electron beam powder bed fusion, one of the additive manufacturing techniques that enable arbitrary fabrication, and subsequent heat treatment. The uniaxial anisotropic deformation behavior and mechanical properties such as Young’s modulus are obtained by introducing a unidirectional honeycomb structure. The main feature of this structure is that the unmelted powder retained in the pores of the honeycomb structure. After appropriate heat treatment at 1020 °C, necks are formed between the powder particles and between the powder particles and the honeycomb wall, enabling a stress transmission through the necks when the composite is loaded. This means that the powder part has been mechanically functionalized by the neck formation. As a result, a plateau region appears in the stress–strain curve. The stress transfer among the powder particles leads to the cooperative deformation of the composites, contributing to the excellent energy absorption capacity. Therefore, it is expected that the composite can be applied to bone plates on uniaxially oriented microstructures such as long bones owing to its excellent energy absorption capacity and low elasticity to unidirectionally suppress stress shielding.

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