Ultrastrong nanotwinned titanium alloys through additive manufacturing

Uncovering the Intrinsic High Fracture Toughness of Titanium via Lowered Oxygen Impurity Content

Titanium (Ti) and its alloys are known to exhibit room-temperature fracture toughness below 130 MPa m1/2, only about one half of the best austenitic stainless steels. It is purported that this is not the best possible fracture resistance of Ti, but a result of oxygen impurities that sensitively retard the activities of plasticity carriers in this hexagonal close-packed metal. By a reduction of oxygen content from the 0.14 wt% in commercial purity Ti to 0.02 wt%, the mode-Ι fracture toughness of the low-oxygen Ti is measured to be as high as KJ Ic ≈ 255 MPa m1/2, corresponding to J-integral-based crack-initiation toughness of up to JIc ≈ 537 kJ m-2. This extraordinary toughness, reported here for the first time for pure Ti, places Ti among the toughest known materials. The intrinsic high fracture resistance is attributed to the profuse plastic deformation in a significantly enlarged plastic zone, rendered by the pronounced deformation twinning ahead of the crack tip along with ample twin-stimulated 〈c+a〉 dislocation activities, in the absence of impeding oxygen. Controlling the content of a property-controlling impurity thus holds the promise to be a readily applicable strategy to reach for unprecedented damage tolerance in some other structural alloys.

Multicomponent alloys designed to sinter

Powder sintering is a low-energy, net-shape processing route for many new products in the additive manufacturing space. We advance the viewpoint that for future manufacturing, alloys should be designed from materials science principles to sinter quickly at lower temperatures and with controlled final microstructures. Specifically, we illustrate the computational design of multinary Ni-base alloys, whose chemistries permit a low-temperature solid-state sintering scheme without any pressure- or field-assistance, as well as heat-treatability after sintering. The strategy is based on sequential phase evolutions designed to occur during sintering. The reactions involve rapid reorganization of matter to full density in cycles up to just 1200 °C, while conventional Ni alloys sintered in the solid-state require about ten times longer, or more than 250 °C degrees higher temperature. Our approach yields an alloy that benefits from precipitation hardening, has an increased strength ~ 50% higher than solid-state processed commercial Ni alloys, and yet exhibits extensive plasticity beyond 35% uniaxial strain. The results point to a generalizable design scheme for many other alloys designed for solid-state powder processing that can enable greater value from additive manufacturing.

Discovering Pyramidal Treasures: Multi-Scale Design of High Strength-Ductility Titanium Alloys

Mechanical properties of titanium alloys, one of humankind's most essential structural materials, suffer from the lack of 〈c + a〉 dislocations on pyramidal slip planes, failing homogeneous plastic strain accommodation. This mechanical treasure is not easily accessible in titanium alloys because of the required excessively high stress levels. The present work demonstrates that such a dilemma may be overcome by meticulously tuning the c/a ratio, the simplest crystallographic parameter of the hexagonal close-packed lattice, through Sn alloying. Combining this lattice-scale design concept with a cross-rolling based polycrystal-scale design solution, this study showcases a facile route to bimodal (α + β) titanium alloys with exceptional strength-ductility synergy.

Advanced Nanomaterials in Medical 3D Printing

3D printing is now recognized as a significant tool for medical research and clinical practice, leading to the emergence of medical 3D printing technology. It is essential to improve the properties of 3D-printed products to meet the demand for medical use. The core of generating qualified 3D printing products is to develop advanced materials and processes. Taking advantage of nanomaterials with tunable and distinct physical, chemical, and biological properties, integrating nanotechnology into 3D printing creates new opportunities for advancing medical 3D printing field. Recently, some attempts are made to improve medical 3D printing through nanotechnology, providing new insights into developing advanced medical 3D printing technology. With high-resolution 3D printing technology, nano-structures can be directly fabricated for medical applications. Incorporating nanomaterials into the 3D printing material system can improve the properties of the 3D-printed medical products. At the same time, nanomaterials can be used to expand novel medical 3D printing technologies. This review introduced the strategies and progresses of improving medical 3D printing through nanotechnology and discussed challenges in clinical translation.

Phase Engineering of Nanostructural Metallic Materials: Classification, Structures, and Applications

Metallic materials are usually composed of single phase or multiple phases, which refers to homogeneous regions with distinct types of the atom arrangement. The recent studies on nanostructured metallic materials provide a variety of promising approaches to engineer the phases at the nanoscale. Tailoring phase size, phase distribution, and introducing new structures via phase transformation contribute to the precise modification in deformation behaviors and electronic structures of nanostructural metallic materials. Therefore, phase engineering of nanostructured metallic materials is expected to pave an innovative way to develop materials with advanced mechanical and functional properties. In this review, we present a comprehensive overview of the engineering of heterogeneous nanophases and the fundamental understanding of nanophase formation for nanostructured metallic materials, including supra-nano-dual-phase materials, nanoprecipitation- and nanotwin-strengthened materials. We first review the thermodynamics and kinetics principles for the formation of the supra-nano-dual-phase structure, followed by a discussion on the deformation mechanism for structural metallic materials as well as the optimization in the electronic structure for electrocatalysis. Then, we demonstrate the origin, classification, and mechanical and functional properties of the metallic materials with the structural characteristics of dense nanoprecipitations or nanotwins. Finally, we summarize some potential research challenges in this field and provide a short perspective on the scientific implications of phase engineering for the design of next-generation advanced metallic materials.

Manufacturing of high strength and high conductivity copper with laser powder bed fusion

Additive manufacturing (AM), known as 3D printing, enables rapid fabrication of geometrically complex copper (Cu) components for electrical conduction and heat management applications. However, pure Cu or Cu alloys produced by 3D printing often suffer from either low strength or low conductivity at room and elevated temperatures. Here, we demonstrate a design strategy for 3D printing of high strength, high conductivity Cu by uniformly dispersing a minor portion of lanthanum hexaboride (LaB6) nanoparticles in pure Cu through laser powder bed fusion (L-PBF). We show that trace additions of LaB6 to pure Cu results in an improved L-PBF processability, an enhanced strength, an improved thermal stability, all whilst maintaining a high conductivity. The presented strategy could expand the applicability of 3D printed Cu components to more demanding conditions where high strength, high conductivity and thermal stability are required.

High fatigue resistance in a titanium alloy via near-void-free 3D printing

The advantage of 3D printing-that is, additive manufacturing (AM) of structural materials-has been severely compromised by their disappointing fatigue properties1,2. Commonly, poor fatigue properties appear to result from the presence of microvoids induced by current printing process procedures3,4. Accordingly, the question that we pose is whether the elimination of such microvoids can provide a feasible solution for marked enhancement of the fatigue resistance of void-free AM (Net-AM) alloys. Here we successfully rebuild an approximate void-free AM microstructure in Ti-6Al-4V titanium alloy by development of a Net-AM processing technique through an understanding of the asynchronism of phase transformation and grain growth. We identify the fatigue resistance of such AM microstructures and show that they lead to a high fatigue limit of around 1 GPa, exceeding the fatigue resistance of all AM and forged titanium alloys as well as that of other metallic materials. We confirm the high fatigue resistance of Net-AM microstructures and the potential advantages of AM processing in the production of structural components with maximum fatigue strength, which is beneficial for further application of AM technologies in engineering fields.

Remarkable Wear Resistance in a Complex Concentrated Alloy with Nanohierarchical Architecture and Composition Undulation

Sustained wear damages on the sliding surfaces of alloys are generally the culprit responsible for the failure of various mechanical systems. Inspired by high-entropy effects, here we deliberately deploy nanohierarchical architecture with composition undulation in a Ni₅₀(AlNbTiV)₅₀ complex concentrated alloy, which yields ultralow wear rate within the order of 10^-7 to 10^-6 mm³/Nm between room temperature and 800 °C. Such remarkable wear resistance heretofore represents one of the highest wear resistance reported for the bulk alloys or composites, and originates from the multi-type adaptive friction interface protection governed by intrinsically nano-coupled grains and nanoprecipitates. This cooperative heterostructure releases gradient frictional stress in stages upon wear at room temperature through the coexistence of multiple deformation pathways while activating a dense nanocrystalline glaze layer upon wear at 800 °C to minimize adhesive and oxidative wear. Our work uncovers a practical avenue for tailoring wear properties with multicomponent heterostructures over a wide temperature range.

Influence of the Coarse Grain Structure of a Titanium Alloy Ti-4Al-3V Formed by Wire-Feed Electron Beam Additive Manufacturing on Strain Inhomogeneities and Fracture

In this work, based on the multilevel approach, the features of the structure and properties of titanium alloy, formed during high-performance additive manufacturing by wire-feed electron beam technology, were studied. Methods of non-destructive X-ray control and tomography, along with optical and scanning electron microscopy, were used to study the structure at different scale levels of the sample material. The mechanical properties of the material under stress were revealed via the simultaneous observation of the peculiarities of deformation development, using a Vic 3D laser scanning unit. Using microstructural and macrostructural data, as well as fractography, the interrelations of structure and material properties caused by the technological features of the printing process and the composition of used welding wire were revealed.

Hierarchical triphase diffusion photoelectrodes for photoelectrochemical gas/liquid flow conversion

Photoelectrochemical device is a versatile platform for achieving various chemical transformations with solar energy. However, a grand challenge, originating from mass and electron transfer of triphase-reagents/products in gas phase, water/electrolyte/products in liquid phase and catalyst/photoelectrode in solid phase, largely limits its practical application. Here, we report the simulation-guided development of hierarchical triphase diffusion photoelectrodes, to improve mass transfer and ensure electron transfer for photoelectrochemical gas/liquid flow conversion. Semiconductor nanocrystals are controllably integrated within electrospun nanofiber-derived mat, overcoming inherent brittleness of semiconductors. The mechanically strong skeleton of free-standing mat, together with satisfactory photon absorption, electrical conductivity and hierarchical pores, enables the design of triphase diffusion photoelectrodes. Such a design allows photoelectrochemical gas/liquid conversion to be performed continuously in a flow cell. As a proof of concept, 16.6- and 4.0-fold enhancements are achieved for the production rate and product selectivity of methane conversion, respectively, with remarkable durability.