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Using a new and simple etching method to investigate the tablets of nacre, it is possible to observe a great deal of tablets in a large range. We found that most tablets have the same formal orientations after citric acid treatment. This phenomenon occurred not only in the same lamina but also along the direction perpendicular to nacreous plane. Based on the additional selected area diffraction patterns of transmission electron microscope, it is proposed that the formal orientations could indicate the crystal orientations of aragonite tablets. Statistical analyze shows that there is a main (type I tablets) and a secondary (type II tablets) preferred orientation with an angle of around 60° between them. Two possible modes of structural correspondence between protein sheets and aragonite lattice are assumed to interpret the orientation preference. On the other hand, the cluster character of type II tablets strongly supports that mineral bridges of the organic matrix have significant function in the crystal growth. At last, the mechanisms of structural correspondence and mineral bridges are combined to explain the formation process of nacreous layer in mussel. [Copyright &y& Elsevier]

The study describes the recoverable strain in Ti-24Nb-4Zr-8Sn (wt.%) alloy with cellular structure having rhombic dodecahedron and topology-optimised unit cell fabricated by electron beam melting. At a certain cyclic strain, the recoverable strain in rhombic dodecahedron structure was greater in comparison to topology-optimised structure because of different stress distribution on struts. The cellular structure of Ti-24Nb-4Zr-8Sn alloy with rhombic dodecahedron unit cell had larger recoverable strain in comparison to Ti-6Al-4V cellular structure. This behaviour is attributed to higher strength to modulus ratio (σ/E) and superelasticity of Ti-24Nb-4Zr-8Sn alloy. [ABSTRACT FROM AUTHOR]

In this work, a newly-developed L1 2 -type precipitation-strengthened Al 0.3 FeCuCrNi 2 high-entropy alloy (HEA) was subjected to friction stir processing (FSP), and the microstructural evolution, as well as its effect on the surface properties (hardness and wear behavior) were studied in detail. The results showed that the FSP of Al 0.3 FeCuCrNi 2 HEA leads to significant grain refinement through continuous dynamic recrystallization (CDRX), dissolution of the needle-like L1 2 /Cu-rich precipitates, increase in the intragranular dislocation density, and re-precipitation of the Cu-rich phases at grain boundaries. The improved strength due to grain refinement and increased dislocation density outweighs the strength deterioration occasioned by the dissolution of the initial precipitates, along with a slight increment in the surface hardness from 215.3 ± 8.2 HV before FSP to 237.4 ± 12.7 HV after FSP. Also, the FSP-induced microstructures result in a shift in the wear mechanism from initial abrasive wear and slight oxidative wear to severe oxidative wear. • A newly-developed L1 2 -strengthened Al 0.3 FeCuCrNi 2 high-entropy alloy was subjected to friction stir processing. • The initial grains are significantly refined by continuous dynamic recrystallization along with the dissolution of L1 2 phase. • FSP-induced microstructural changes lead to increased hardness and a shift in the wear mechanism. [ABSTRACT FROM AUTHOR]

By preparing interpenetrating phase composites (IPCs) using two-phase composites of metals and polymers, it is possible to preserve the inherent properties of the metal lattice while leveraging the exceptional characteristics of polymers. In this study, a Ti2448/polyurea composite with continuous interpenetrating phase was fabricated by infusing polyurea into a cross-laminated lattice prepared using electron beam melting (EBM) technology. The IPCs exhibit a tensile strength of 91 MPa, representing a 14 % increase over the theoretical strength. Moreover, the maximum hysteresis loss energy measures 349 J/m³, representing about ∼102 % increase over the maximum hysteresis loss energy of the lattice. The damage and failure modes of IPCs under tensile loading was revealed by finite element analysis. Our results would provide important data and valuable insights to advance the development of IPCs. [Display omitted] • Novel Ti2448/polyurea IPCs was fabricated by electron beam melting and infiltration technique. • Tensile strength of Ti2448/polyurea IPCs increases by 14 % over theoretical strength. • Hysteresis loss energy of Ti2448/polyurea IPCs increases by 102 % over Ti2448 crossed-lamellar lattice. • The tensile failure mechanism of IPCs has been elucidated. [ABSTRACT FROM AUTHOR]

Powder metallurgy (PM) is a versatile technique to fabricate the particulate reinforced aluminum matrix composites (AMCs) by combining Al alloy powders with various types of reinforcement particles. However, the AMCs fabricated by PM generally lack ductility due to processing-related factors such as porosity, oxygen content, and undesirable microstructural features initiated by the contamination during powder preparation or sintering. In this study, the 5 wt% SiC/Al–Mg-Sc-Zr nanocomposite produced by PM was subjected to friction stir processing (FSP) to obtain the dense, uniform and refined microstructures and the attendant improved mechanical properties. The effect of FSP passes on the microstructural features and mechanical properties of as-PM nanocomposite was investigated, and the strengthening and toughening mechanism of FSPed nanocomposite was revealed. The results showed that the FSP passes with the opposite moving direction of FSP tool between the consecutive passes, largely affected the microstructure of the stir zone. Increasing FSP passes significantly reduced the microdefects and homogenized the microstructure. After 4-pass FSP, the whole stir zone consisting of the dense, uniform and refined microstructure, was obtained. This resulted in an increase in yield strength from 92 MPa in the as-PM condition to 161 MPa after 4-pass FSP, and a simultaneous nearly double enhancement in fracture strain from 0.11 to 0.29. [ABSTRACT FROM AUTHOR]

The equiatomic CoCrFeNiMn high-entropy alloy (HEA) has demonstrated excellent strength-ductility-toughness synergy over a wide range of temperatures, especially at cryogenic temperatures, but poor wear resistance. Herein, we explored the feasibility of constructing ultrafine-grained (UFG) surface layer on the CoCrFeNiMn HEA by underwater friction stir processing (FSP) and evaluated its dry sliding wear behavior. During air-FSP of CoCrFeNiMn HEA, continuous and discontinuous dynamic recrystallization (CDRX and DDRX) compete with each other responsible for grain refinement. Water-FSP promotes the occurrence of DDRX and inhibits the growth of recrystallized grains, leading to the formation of an UFG surface layer with average grain size of 1.14 ± 0.78 μm. The surface hardness of base material is about 179 HV, which is increased to 216 HV for the air-FSPed sample and 275 HV for the water-FSPed sample. Nonetheless, the three samples exhibit similar wear behavior, mainly adhesive wear and oxidation wear. This is mainly attributed to a combination of the formation of oxides on the worn surfaces during sliding and the significantly reduced strain-hardening effect with decreasing grain size. This work provides a fundamental understanding of the microstructural evolution of CoCrFeNiMn HEA during FSP as well as its sliding wear behavior before and after FSP. • An ultrafine-grained surface layer was successfully developed on CoCrFeNiMn HEA by underwater friction stir processing. • The effect of water cooling on the microstructure evolution of CoCrFeNiMn HEA during FSP was elucidated. • The relationship between the FSP-induced microstructure and the wear behavior was well established for CoCrFeNiMn HEA. [ABSTRACT FROM AUTHOR]

Abstract The strength and plasticity of metallic materials usually exhibit a trade-off relation. This study reports a simultaneous improvement in the ultimate tensile strength (UTS) and uniform elongation (UE) of Ti-24Nb-4Zr-8Sn (Ti2448) fabricated by selective laser melting (SLM), relative to those produced via forging. Detailed microstructural characterization reveals that the outstanding tensile property may result from the bi-model structure that forms during the rapid cooling associated with SLM. Coarse grains are surrounded by fine grains within the melt pool, which causes a back stress during tension. The back stress provides additional strain-hardening capacity, which postpones the initiation of necking and then leads to the simultaneous improvement of the strength and plasticity (SISP) of the Ti2448 alloy. Furthermore, the tensile property of the SLM-fabricated sample is anisotropic which is strongly related to the irregular shape of the melt pool. Graphical abstract Unlabelled Image Highlights • Melt pools with a bi-model structure are formed in a biocompatible Ti alloy through selective laser melting. • The as-built Ti alloy exhibits a better combination of strength and plasticity, relative to traditionally forged counterpart. • For the first time, the back stress of the melt pool along different directions is studied through finite element modelling. [ABSTRACT FROM AUTHOR]

In recent years, cellular metallic materials have attracted significant interest for biomedical applications. However, mutually opposing requirements of porous architecture and mechanical strength in conjunction with the high energy absorption capability have restricted their use. Here, we illustrate that electron beam melting can fabricate functionally graded Ti-6Al-4V alloy interconnected mesh structures with a combination of low density (0.5–2 g/cm 3 ), high fatigue strength (∼70 MPa) and energy absorption (∼50 MJ/mg), which is superior to the ordinary uniform cellular structures. The underlying fundamental mechanisms governing the compressive and fatigue behavior of the graded cellular structures are elucidated via in situ tomography experiments and digital volume correlation analyses. It is underscored that during cyclic deformation, the stress can be continuously redistributed because of inhomogeneous mechanical properties and crack formation in constituent meshes, thereby resulting in variation of cyclic ratcheting rate for the graded cellular structures. [ABSTRACT FROM AUTHOR]

β-type titanium porous structure is a new class of solution for implant because it offers excellent combinations of high strength and low Young's modulus. This work investigated the influence of porosity variation in electron beam melting (EBM)-produced β-type Ti2448 alloy samples on the mechanical properties including super-elastic property, Young's modulus, compressive strength and fatigue properties. The relationship between the misorientation angle of adjacent grains and fatigue crack deflection behaviors was also observed. The super-elastic property is improved as the porosity of samples increases because of increasing tensile/compressive ratio. For the first time, the position of fatigue crack initiation is defined in stress-strain curves based on the variation of the fatigue cyclic loops. The unique manufacturing process of EBM results in the generation of different sizes of grains, and the apparent fatigue crack deflection occurs at the grain boundaries in the columnar grain zone due to substantial misorientation between adjacent grains. Compared with Ti-6Al-4V samples, the Ti2448 porous samples exhibit a higher normalized fatigue strength owing to super-elastic property, greater plastic zone ahead of the fatigue crack tip and the crack deflection behavior. [ABSTRACT FROM AUTHOR]

Further improving the wear and corrosion resistance of duplex stainless steel (DSS) without disrupting the balanced ferrite (α)/austenite (γ) ratio is worth exploring, but very challenging. Here, we explored the feasibility of utilizing solid-state friction stir processing in air (AFSP) and underwater (UFSP) to realize surface modification of DSS. Notably, we successfully developed an ultrafine-grained (UFG) structure with balanced α/γ ratio on the DSS by UFSP, which imparts both enhanced wear and corrosion resistance to the sample surface. Further, the associated microstructure evolution mechanism during FSP of DSS in both environments is systematically elucidated. The simultaneous improvement in both wear and corrosion resistance is well correlated with the resulting microstructure characteristics. This work provides a simple and cost effective pathway to achieve a synergistic increase in the wear and corrosion resistance of DSS. We anticipate that the UFSP should be applicable to many dual-phase metallic systems. • Surface ultrafine-grained, γ/α balanced duplex stainless steel was developed by underwater friction stir processing. • The associated formation mechanism of ultrafine-grained duplex structure was systematically elucidated. • The concurrently enhanced wear and corrosion resistance was well correlated with the resulting microstructures. [ABSTRACT FROM AUTHOR]

This study investigates the differences in the microstructure, defects and mechanical behavior of porous structures from a β-type Ti 24Nb 4Zr 8Sn manufactured by electron beam melting (EBM) and selective laser melting (SLM). The phases, size and shape of melt pool, volume and distribution of defects are analyzed and correlated to the compressive mechanical and fatigue properties. Due to different powder bed temperatures, the microstructure of EBM and SLM samples consists of α+β phases and a single β phase, respectively. The faster cooling rate during SLM promotes the formation of fine β dendrites, which leads to a higher compressive strength (50 ± 0.9 MPa) and lower Young's Modulus (0.95 ± 0.05 GPa) in comparison to the EBM parts (45 ± 1.1 MPa and 1.34 ± 0.04 GPa respectively). The large defects present within solid strut are likely a result of tin vaporization. The tin vapor is more easily trapped during the SLM process due to a smaller laser spot size and a faster cooling rate. This results in a 10 times increase in the number of defects. These defects have a limited influence on both the static properties and low stresses level fatigue strength, but it causes a reduced and variable fatigue life at high stresses level. [ABSTRACT FROM AUTHOR]