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Architectured hierarchical porous materials exist widely in nature. Inspired by their unique mechanical and functional responses, extensive efforts have been made to develop architectured hierarchical porous metallic materials for specific industrial applications where fast mass transport and specific surface area are both critical, such as energy storage and catalysis. More specifically, the rapid development of additive manufacturing (AM) technologies has enabled the design and fabrication of a wide variety of architectured hierarchical porous metals in recent years. This review discusses the developments to date of AM-enabled hierarchical porous metals with or without the combination of other manufacturing processes. AM opens avenues for the creation of unprecedented hierarchical porous metals with novel properties or functionalities. [ABSTRACT FROM AUTHOR]

This paper investigates the evolution of the β phase in laser deposited Ti-6Al-4V via track of the cyclic phase transformation processes by dilatation experiments and characterization of the laser deposited microstructures by transmission electron microscopy (TEM) and an energy dispersive X-ray spectrometer (EDS). The formation temperature of the β phase in as-received mill-annealed Ti-6Al-4V decreased with increasing number of thermal cycles at the heating and cooling rates of 50 °C/s, while the vanadium (V) content in the β phase increased with decreasing β phase formation temperature. The distribution of the V content in the β phase in the laser deposited Ti-6Al-4V showed an increasing gradient from the top layer (i.e., the last deposited) to the fourth layer from the top, which is related to the thermal cycles and the formation temperature of the β phase. The formation mechanism of the β phase was discussed based on the experimental observations. [ABSTRACT FROM AUTHOR]

Grain refinement leads, in general, to a decreased tendency to hot tearing, a more dispersed and refined porosity distribution, and improved directional feeding characteristics during solidification. Reduced as-cast grain size can also lead to improved mechanical properties and wrought processing by reducing the recrystallized grain size and achieving a fully recrystallized microstructure. It is now well established that the two key factors controlling grain refinement are the nucleant particles including their potency , size distribution and particle number density , and the rate of development of growth restriction , Q , generated by the alloy chemistry which establishes the undercooling needed to trigger nucleation events and facilitates their survival. The theories underpinning our current understanding of nucleation and grain formation are presented. The application of the latest theories to the light alloys of Al, Mg and Ti is explored as well as their applicability to a range of casting and solidification environments. In addition, processing by the application of physical processes such as external fields and additive manufacturing is discussed. To conclude, the current challenges for the development of reliable grain refining technologies for difficult to refine alloy systems are presented. [ABSTRACT FROM AUTHOR]

The occurrence of the β (bcc) to α m (hcp) massive transformation in Ti–6Al–4V (wt.%) during additive manufacturing (AM) by selective electron beam melting (SEBM) has been identified. A variety of patch-shaped massive grains including both grain boundary (GB)-crossing and non-GB-crossing instances were characterised and their formation temperature and growth rate were estimated. In contrast to conventional Ti–6Al–4V, the massive grains in SEBM-fabricated Ti–6Al–4V consist of ultrafine lamellar α and β phases (100 nm wide β strips) due to in-situ decomposition of the massive grains during AM. The resulting ultrafine α-β lamellae obey the typical Burgers orientation relationship established for the lamellar α-β structure in conventional Ti–6Al–4V. These findings show that the β (bcc) to α m (hcp) massive transformation significantly contributes to development of the Ti–6Al–4V microstructure during AM by SEBM, providing an alternative microstructural design strategy for AM of Ti–6Al–4V and other titanium alloys. [ABSTRACT FROM AUTHOR]

Efficient lattice unit cell topologies often necessitate the manufacture of unsupported horizontal struts. The fabrication of unsupported horizontal struts by laser powder bed fusion (LPBF) is inherently challenging, where the associated research data is limited for solid or dense cross-sections while no data exists for hollow struts. By examining LPBF-fabricated Ti-6Al-4V hollow struts of various diameters and wall thicknesses over increasing span lengths, a robust LPBF additive manufacturability range has been established for Ti-6Al-4V hollow struts. To achieve successful fabrication, three laser scan strategies were explored, proposed, and systematically compared. Horizontal solid struts and vertical hollow struts of equivalent diameters (and wall thickness) were similarly assessed over the same span lengths as comparators to quantify the manufacturability. This research provides previously unavailable design data, required for LPBF additive manufacturing of lattices with horizontal strut elements. • Ti-6Al-4V horizontal hollow and solid struts were successfully manufactured at a submillimetre wall thickness. • The manufactured hollow struts achieved a low frequency of defects, enabling the establishment of acceptable LPBF guidelines. • Unique laser scan strategies were implemented for a range of wall thicknesses, strut diameters, and unsupported span lengths. • The experimental findings expand the capabilities of LPBF for thin walled structures. [ABSTRACT FROM AUTHOR]

Room temperature tensile ductility is an important property of titanium (Ti) and titanium alloys for structural applications. This article reviews the dependency of tensile ductility on oxygen for α-Ti, (α+β)-Ti and β-Ti alloys fabricated via traditional ingot metallurgy (IM), powder metallurgy (PM) and additive manufacturing (AM) or three-dimensional printing methods and recent advances in understanding the effect of oxygen on ductility. Seven mechanisms have been discussed based on case studies of individual titanium materials reported in literature. The dependency of ductility on oxygen is determined by both the composition and microstructure of the titanium alloy. For Ti-6Al-4V (wt-%), as sintered Ti-6Al-4V shows a critical oxygen level of about 0·33 wt-% while additively manufactured Ti-6Al-4V exhibits different critical levels ranging from about 0·22% to well above 0·4% depending on microstructure. Rare earth (RE) elements are effective scavengers of oxygen in titanium materials even just with a small addition (e.g. 0·1 wt-%), irrespective of the manufacturing method (IM, PM and AM). High cycle fatigue experiments revealed no initiation of fatigue cracks from the resulting RE oxide particles over the size range from submicrometres to a few micrometres. A small addition of RE elements offers a practical and affordable approach to mitigating the detrimental effect of oxygen on ductility. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fabrication processes, materials, and achievable geometrical parameters are explored. • The reported mechanical properties and unique deformation responses are assessed. • Discrepancies within specific unit cell definitions are discovered. • Applications and future outcomes for hollow-walled lattices are recommended. • The Gibson-Ashby predictive model is assessed for hollow-walled lattices. The rapid growth of additive manufacturing (AM) technologies has enabled the emergence of geometrically sophisticated materials or structures with tailored and/or enhanced mechanical responses. In addition to dense-walled lattice structures, innovation within the past decade has identified that hollow-walled lattice topologies exhibit the multifaceted potential of competitive strength and rigidity, whilst displaying unique deformation behaviours, indicating that they may be an important subsequent step in lattice evolution. Hollow-walled sections facilitate density and geometrical parameters well below what is achievable by dense-walled sections, providing additional hierarchies of architecture at micrometre to even nanoscale proportion. Their wall thickness can range from 20 nm to 800 µm while the relative density can span three orders of magnitude between 0.01% and 30%. Despite nearly a decade of research into hollow-walled lattice topologies, no meta-analysis exists to provide an informative overview of these structures. This research addresses this deficiency and provides a data-driven review of hollow-walled lattice materials. It elucidates how these hollow-walled lattices deviate from the current limitations of dense-walled lattices and the underlying mechanisms that dictate their performance, with data accumulated from an exhaustive collection of literature sources. A range of new insights into their design and manufacture is discussed for their future research and applications in different engineering fields. [ABSTRACT FROM AUTHOR]

Hollow-strut metal lattices are novel cellular materials. Compared to their solid-strut counterparts, their powder bed fusion additive manufacturing (PBF-AM) features remain largely uninvestigated. This work focuses on characterizing the hollow-strut internal channel and nodal profiles, the defects and microstructures of the hollow-strut thin walls, and the inner surface conditions of the LPBF-manufactured body-centred cubic (BCC) Ti-6Al-4V hollow-strut lattices with different relative densities. BCC lattices are selected because of the low inclination angle (35.26°) of their constituent struts. These low-inclination hollow struts are designed using a recent model developed for PBF of inclined solid struts, together with considerations to prevent powder occlusion and ensure easy removal of powder particles. Detailed characterization indicates that our design considerations resulted in high-quality hollow-strut BCC Ti-6Al-4V lattices, which provide useful design insights for PBF-AM of hollow-strut metal lattices. In terms of microstructure, the Ti-6Al-4V hollow-strut thin walls (≤ 0.5 mm thick) exhibited different microstructures compared with Ti-6Al-4V solid struts, due to the heat accumulation effect in the inner channels. The implications are discussed for in-situ microstructure control. • The low-inclination hollow struts of additively manufactured BCC Ti-6Al-4 V lattices are designed using a strut model, together with considerations to avoid powder occlusion. • The build quality, internal defects, node connection and surface roughness are systematically evaluated, indicating that our design considerations resulted in high-quality hollow-strut. • The Ti-6Al-4 V hollow-strut thin walls (≤ 0.5 mm thick) exhibited different microstructures compared with solid struts due to the 'double-powder-bed' effect. [ABSTRACT FROM AUTHOR]

Metal hollow-strut lattices manipulate the strut and node sections to achieve higher strength. Although these metamaterials have recently surpassed the structural efficiency of conventional solid-strut lattices of comparable density and topology, they often observe localised yielding and fragmentation through the nodes that limit their structural efficiency. To quantify the effect of this localised failure and to develop mitigating strategies, we have integrated solid and hollow strut and node sections in singular Ti–6Al–4V simple cubic hybrid-strut lattices for a deployable relative density range of 15–30%. Fabricated through laser powder bed fusion these hybrid-strut lattices are stronger (32%), stiffer (15%), and tougher (65%) than solid-strut lattices of equivalent density. Interestingly, the metamaterials with hollow beams can transition from a bending-dominated to a stretch-dominated deformation response with increasing density, while the lattices with solid beams only observed a uniformly stretch-dominated deformation response. The combination of solid and hollow strut and node sections in the hybrid-strut lattice establishes a simple yet effective strategy to enhance structural efficiency and control of failure response without increasing density or manipulating unit cell topology. [ABSTRACT FROM AUTHOR]

Within the realm of metal additive manufacturing, laser powder bed fusion (PBF-LB) has maintained a dominant role by offering exceptional geometric freedom, fine feature resolution and fine microstructure features. However, low productivity still presents a bottleneck in the adaptation of PBF-LB in most industrial contexts. In recent literature, build-up rates have shown to improve notably when thicker powder layers are employed. However, systematic analyses linking processing parameters, productivity, microstructural state and mechanical properties are lacking. This study aims to fill this gap for Ti6Al4V alloy with powder layer thicknesses in the range of 60 µm to 300 µm, specifically using system-agnostic process parameters applicable to the majority of currently available commercial AM systems. The results revealed that Youngs modulus (∼110 GPa) and yield strength (∼1.1 GPa) remain comparable to 'conventional' PBF-LB Ti6Al4V throughout the investigated range of layer thickness. At the same time elongation to failure decreases from 11.4 ± 2.7% at 60 µm to 8.4 ± 1.1% at 180 µm and finally to 2.0 ± 0.3% at 300 µm, which was microscopically correlated with increased occurrence of lack-of-fusion porosity in layers exceeding 180um. It was also demonstrated that the changes in the parent β phase texture arising from process changes could have contributed to decreased ductility at thicker layers. Ultimately, while productivity increases with layer thickness up to 8.76 mm3/s at 300 µm, the achievable build rate appears to plateau around 300 µm layer height and require further expansion of the laser power characteristics to enable additional gains without compromising mechanical performance. [Display omitted] • Productivity and performance of PBF-LB Ti6Al4V with 60 - 300 μm powder layer thickness were systematically explored. • In contrast to volumetric energy density, a machine- and material- agnostic Fourier number approach was employed. • With increased layer thickness, ductility was adversely affected by gradually increased porosity and prior-β texture changes. • Productivity gains stagnated for 240 - 300μm layer thicknesses due to limited power (400 W) of conventional PBF-LB systems. [ABSTRACT FROM AUTHOR]

• A classification of conformal lattice design methods is proposed. • The pros and cons of eleven conformal lattice design methods are reviewed based on seven criteria. • The choice of each design approach depends on specific circumstances, including design cost and time. • Additive manufacturability of complex conformal metal lattices is discussed, and solutions are proposed. Conformal lattice materials (cell sizes ranging from nanometres to millimetres), including conformal metal lattice metamaterials, are cellular materials or structures that conform to all or part of the physical space of a product with topologically complete boundary cells. Enabled by powder bed fusion (PBF) additive manufacturing (AM), conformal metal lattice metamaterials provide an innovative solution for lightweight engineering or integration of structure and function. A key step in their fabrication is to generate a conformal lattice model suitable for PBF AM. This research reviews their design methods and evaluates each method using seven criteria. These include (i) the sequence of geometric modelling and lattice topology generation (sequential or simultaneous), (ii) integrity of lattice cell topology at boundaries, (iii) compatibility with lattice cell types, (iv) applicability to complex geometry, (v) ease of coding, (vi) accessibility via common software tools, and (vii) ability to define strut inclination angles in a complex conformal design space. On this basis, various laser PBF (LPBF) manufacturability issues of conformal metal lattices are considered, and two Ti-6Al-4V conformal lattices are fabricated using LPBF and evaluated. This review provides a necessary foundation for future research and applications of conformal lattice metamaterials in various engineering fields. [ABSTRACT FROM AUTHOR]

A batch of six Ti-6Al-4V plates (5 mm thick, 45 mm wide, 220 mm high) were fabricated by selective electron beam melting (SEBM) and half of them were subsequently processed by hot isostatic pressing (HIP) at 920 °C for 2 h under 100 MPa in argon. The aim of this study was to investigate the texture change induced by HIP and its influence on tensile performances. Detailed texture analyses revealed that applying a default HIP treatment to SEBM Ti-6Al-4V thin plates intensified the texture component of 〈0002〉 (normal to the plate flat surface) in the α-phase due to variant selection. Consequently, this resulted in a noticeable reduction in deformation constraint for prismatic slip, which, in turn, changed the tensile fracture mode from a mixed type corresponding to both normal and shear stresses to a shear-dominated type. The change in tensile properties after HIP contained the influence of HIP-induced changes in texture. Unlabelled Image • The texture component of 〈0002〉 was intensified by hot isostatic pressing. • This texture enhancement was due to the preferential growth of primary α-laths. • A significant reduction in deformation constraints was obtained. • A change in fracture mode was observed. [ABSTRACT FROM AUTHOR]

Titanium alloy Ti-1Al-8V-5Fe (Ti-185) has essentially remained an experimental alloy since its invention because of the formation of iron-stabilized beta-flecks in conventional manufacturing. Laser directed energy deposition (L-DED) has the potential to avoid this issue while realizing in-situ ageing. This work investigates the L-DED fabrication of Ti-185 alloy. We show that the precipitate phases along the build height of the Ti-185 plate sample have a decisive influence on its tensile properties. Due to the formation of the embrittling isothermal omega-phase (ω iso), the top region of the Ti-185 plate sample exhibited zero plasticity, while the middle region, which is free of the omega-phase (ω), demonstrated significant tensile ductility (20 ± 2 %) and strength (1042 ± 13 MPa). The formation of each phase is elucidated using simulated temperature evolution profiles in the plate sample. Furthermore, we show that converting the isothermal omega-phase into the athermal omega-phase (ω ath) through beta-annealing and water quenching restores tensile ductility. [Display omitted] [ABSTRACT FROM AUTHOR]

Massive transformations occur in both additively and conventionally manufactured titanium (Ti) alloys. Unlike martensitic transformations, massive transformations can result in patch-like massive phases (α m) that traverse the parent prior-β grain boundaries (GBs). However, the conditions favouring the formation of these trans-GB α m -phases in Ti alloys remain largely unexplored. Through characterising the trans-GB α m -phases in α-β Ti alloys fabricated by additive and conventional processes, we find that their formation always occurs when two neighbouring prior-β grains share or nearly share a {110} pole, without exception. These trans-GB α m -phases exhibit concentrated {0001} poles while their { 11 2 ¯ 0 } poles spread widely. In addition, as metastable phases, they tend to decompose into ultrafine α-β lamellae. The role of relative orientation of adjacent parent grains in massive transformations and the implications for microstructural innovations in α-β Ti alloys are discussed. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract A systematic study has been made of the microstructural development in as-cast 30 mm thick Al-3Ti-1Sc (wt%) plate samples that were processed by electron beam (EB) melting at different scanning speeds of 3, 5, 8, 12, 15 and 20 mm/s. The composition of the alloy is designed to capitalize on the potential for significantly extended solubility values of both Sc and Ti in Al when cooled at high cooling rates. The resulting microstructures are characterised using transmission electron microscopy (TEM) and other analytical means, assisted with Thermo-Calc predictions and Vicker's microhardness measurements. EB scanning speed plays a key role in determining the phase formation and microstructural development in the Al-3Ti-1Sc alloy when the beam is applied an accelerating voltage of 50 kV and a current of 30 μA. The major microstructural features in the re-solidified zone include (i) the formation of the primary tetragonal Al 3 (Ti,Sc) phases and their retention or subsequent transformation to stable cubic Al 3 (Ti,Sc) phases; (ii) the complete suppression of the primary tetragonal Al 3 (Ti,Sc) phases at the scanning speed of 20 mm/s; and (iii) the formation of cubic Al 3 (Ti,Sc) precipitates in the α(Al) matrix supersaturated with both Sc and Ti. These experimental findings are informative for both EB processing of Al-Ti-Sc alloys and the design of new Al alloys for additive manufacturing processes. [ABSTRACT FROM AUTHOR]

Additively manufactured Ti-6Al-4V lattices display unique mechanical and biological properties by virtue of their engineered structure. These attributes enable the innovative design of patient-specific medical implants that (i) are conformal to the intended surgical geometry, (ii) mimic the mechanical properties of natural bone, and (iii) provide superior biological interaction to traditional implants. Selective electron beam melting (SEBM) is an established metal additive manufacturing (AM) process that has enabled the design and fabrication of a variety of novel intricate lattices for implant applications over the last 15 years. This article reviews the technical and clinical characteristics of SEBM Ti-6Al-4V lattices, including (i) the SEBM process and its capabilities, (ii) the structures of human bones with an exhaustive list of corresponding mechanical properties from literature, (iii) the mechanical properties of SEBM Ti-6Al-4V lattices of various designs and their shortcomings when compared to human bones, (iv) microstructural control of SEBM Ti-6Al-4V lattices for improved performance, (v) the lattice manufacturability and associated geometric errors, and (vi) clinical cases. Existing literature on the mechanical response of SEBM Ti-6Al-4V lattice structures is exhaustively evaluated for documentation quality using established theoretical models. This extensive data-set allows novel insights into the effect of lattice design on mechanical response that is not possible with the individual data; and provides a comprehensive database for those who are actively involved in patient-specific SEBM implant design. On this basis, outstanding challenges and research opportunities for SEBM Ti-6Al-4V lattices in the biomedical domain are identified and discussed. [ABSTRACT FROM AUTHOR]

The β prior → α m massive transformation was reproducibly observed during additive manufacturing (AM) of Ti-6Al-4V by selective electron beam melting (SEBM). Inside the massive phase grains, ultrafine α mpd − β mpd lamellar structures formed through in-situ decomposition by α m → α mpd + β mpd (mpd: massive phase decomposition), which can be used to further improve the ductility and yield strength of SEBM-fabricated Ti-6Al-4V. Two orientation relationships (ORs) were experimentally determined between α mpd and β mpd , although massive transformation does not necessarily depend on ORs. The experimental ORs are consistent with predictions made using the edge-to-edge matching (E2EM) model and correspond to low energy interfaces. In addition, the stability of the α mpd − β mpd lamellar structures was discussed. These findings improve the current understanding of microstructural formation in SEBM-fabricated Ti-6Al-4V, and could further stimulate design/fabrication of novel microstructures in AM-based Ti alloys for enhanced properties. [ABSTRACT FROM AUTHOR]

The “Holy Grail” of metal additive manufacturing is to manufacture reliable high-performance metal parts with no or a minimal need of post processing. However, Ti-6Al-4V parts made by selective laser melting (SLM) often suffer from poor ductility and low toughness because of the predominant acicular α′ martensite contained in columnar prior-β grains. In practice, post heat treatment is necessary. To overcome this deficiency, we have explored designing innovative SLM processing routes to turn the unfavoured α′ martensite, via in-situ decomposition, into lamellar (α+β) microstructures with tuneable characteristic length scales. Such lamellar (α+β) microstructures lead to superior mechanical properties which markedly exceed ASTM standards and outperform the majority of Ti-6Al-4V fabricated by other additive manufacturing processes. Furthermore, we find that the lattice parameter of the β phase in the (α+β) lamellae falls into a specific range of 3.18–3.21 Å. Hence the lattice parameter of β phase can serve as an indicator to predict whether significant martensite decomposition has taken place in situ in Ti-6Al-4V made by SLM. This work marks an important step forward in the understanding of how to tailor microstructure in situ for the development of high-performance Ti-6Al-4V parts by SLM. [ABSTRACT FROM AUTHOR]

Intricate hollow-strut metal lattice materials are an emerging class of novel metallic cellular materials enabled by additive manufacturing. This work shows that hollow-strut Ti-6Al-4V lattice materials are consistently stronger and stiffer (up to 60% better) than their solid-strut counterparts of the same relative density, both experimentally and through finite element analysis (FEA). The underlying reasons are investigated using analytical models derived from the Timoshenko-beam theory, which considers deformation by concurrent stretching, bending and shear, rather than the single-mode deformation mechanism assumed by the Gibson-Ashby model. Hollow-strut lattices exhibit higher resistance to bending than solid-strut lattices at the same strut length and relative density, thereby leading to increased strength and stiffness. Hollow-strut metal lattices offer an unusual option for lightweight designs, with better mechanical properties at the same or lower density than solid-strut metal lattices. [Display omitted] [ABSTRACT FROM AUTHOR]

Ti–1Al–8V–5Fe alloy was fabricated using electron beam powder bed fusion. No Fe-stabilized β-flecks were observed. In-situ precipitation hardening occurred in columnar prior-β grains. Nanoscale α-lath precipitates exhibited noticeable deformation while microscale α-laths became a major source of microcracks, which should be avoided. The as-fabricated Ti–1Al–8V–5Fe alloy showed significant tensile properties. [ABSTRACT FROM AUTHOR]

One of the critical issues in orthopaedic regenerative medicine is the design of bone scaffolds and implants that replicate the biomechanical properties of the host bones. Porous metals have found themselves to be suitable candidates for repairing or replacing the damaged bones since their stiffness and porosity can be adjusted on demands. Another advantage of porous metals lies in their open space for the in-growth of bone tissue, hence accelerating the osseointegration process. The fabrication of porous metals has been extensively explored over decades, however only limited controls over the internal architecture can be achieved by the conventional processes. Recent advances in additive manufacturing have provided unprecedented opportunities for producing complex structures to meet the increasing demands for implants with customized mechanical performance. At the same time, topology optimization techniques have been developed to enable the internal architecture of porous metals to be designed to achieve specified mechanical properties at will. Thus implants designed via the topology optimization approach and produced by additive manufacturing are of great interest. This paper reviews the state-of-the-art of topological design and manufacturing processes of various types of porous metals, in particular for titanium alloys, biodegradable metals and shape memory alloys. This review also identifies the limitations of current techniques and addresses the directions for future investigations. [ABSTRACT FROM AUTHOR]

Additive manufacturing (AM) offers a radical net-shape manufacturing approach for titanium aluminide alloys but significant challenges still remain. A study has been made of the AM of a high niobium-containing titanium aluminide alloy (Ti–45Al–7Nb–0.3W, in at% throughout the paper) using selective electron beam melting (SEBM). The formation of various types of microstructural defects, including banded structures caused by the vaporization of aluminum, was investigated with respect to different processing parameters. To avoid both micro- and macro-cracks, the use of higher preheating temperatures and an intermediate reheating process (to reheat each solidified layer during SEBM) was assessed in detail. These measures enabled effective release of the thermal stress that developed during SEBM and therefore the avoidance of cracks. In addition, the processing conditions for the production of a fine full lamellar microstructure were identified. As a result, the Ti–45Al–7Nb–0.3W alloy fabricated showed outstanding properties (compression strength: 2750 MPa; strain-to-fracture: 37%). SEBM can be used to fabricate high performance titanium aluminide alloys with appropriate processing parameters and pathways. [ABSTRACT FROM AUTHOR]

In α-phase dominant Ti alloys, the α/α boundaries are the main interfaces in the microstructure in terms of both their distribution and total area. This work reports identification of unusual large zones of Category I triple-α-variant clusters in laser metal (powder) deposited (LMD) Ti-4Al-2V alloy. These clusters lead to a continuous distribution of the Type 2 α/α boundary ([ 11 2 ¯ 0 ] / 60 ∘) in the microstructure, which possesses lower boundary energy than the Types 3–6 α/α boundaries, where the distribution of Type 1 α/α boundary is negligible. The formation of these large zones of Category I clusters can be understood using the autocatalysis nucleation and subsequent chain reaction theory suggested by J W Christian based on self-accommodation. These experimental observations imply that it is perhaps plausible to produce low energy α-variant boundary configurations through exploiting the confluence of alloy composition and additive manufacturing conditions. [Display omitted] [ABSTRACT FROM AUTHOR]

Oxygen remains to be a central issue in the manufacture of titanium (Ti) and its alloys from powder. The strong and short-range repulsion between interstitial oxygen and screw dislocation core strengthens titanium but compromises its resistance to cracking. It is therefore challenging to produce strong and ductile (tensile) Ti materials from high oxygen-content powder. This article reviews the tensile properties of high oxygen-content α-Ti, (α+β)-Ti alloys, and β-Ti alloys made from powder. A minimum tensile ductility value of 6% is used to define ductile Ti according to existing technical specifications for Ti-6Al-4V and other powder metallurgy materials. Two post-processing methods, solution heat treatment and thermomechanical processing, are identified to be effective remedies to convert non-ductile high oxygen-content Ti alloys into ductile ones. The underlying ductility-improvement mechanisms are discussed, which vary with alloy system. In particular, redistribution of oxygen between α and β phases by solution heat treatment offers a practical remedy to substantially improve tensile ductility, e.g., from 6% to 19% for the Ti-0.94 wt%O alloy. It is shown that appropriate post processing can enable a wide variety of Ti alloys made from powder to tolerate ≥0.5 wt%O for ≥6% tensile ductility for structural applications. • Redistribution of oxygen by solution heat treatment can reverse the detrimental effect of oxygen for Ti. • Thermomechanical processing can enable high oxygen-content Ti alloys to offer high tensile ductility (>15%). • Alloy composition affects exploitation of high oxygen Ti powder by PM, MIM and AM. • Alloy design plus post processing allows fabrication of strong and ductile Ti alloys from high oxygen Ti powder. [ABSTRACT FROM AUTHOR]

• A concise review of recent advances in Ni-free superelastic Ti alloys. • A detailed discussion of architectural design enabled superelasticity without resort to reversible phase transformation. • An in-depth analysis of additive manufacturing for Ni-free superelastic Ti alloys. • A comprehensive assessment of the properties of both dense and porous Ni-free superelastic Ti alloys vs. human bones. • An overview of representative biomedical applications of superelastic Ti alloys. Superelastic titanium (Ti) alloys are a group of unique functional metallic materials capable of recovering a substantial amount of mechanical strain thereby offering superior resilience. Such strain recovery is significantly greater than that exhibited by conventional elasticity and has been demonstrated to be clearly beneficial and necessary for a vast range of biomedical and dental applications. For example, the age-related physiological deterioration of bones signifies the necessity of employing superelastic implants. Currently, NiTi alloy remains to be the premier choice of superelastic alloys in the broad biomedical sector. However, recently reinforced views on the toxic, carcinogenic and allergenic properties of nickel have resulted in intensified concerns. This has encouraged the design and fabrication of Ni-free superelastic Ti alloys. In addition, enabled by additive manufacturing (AM) or 3D printing, hierarchical micro-architectured lattice meta-materials can exhibit exceptional superelasticity without undergoing phase transformations, upending the conventional perception and unlocking brand-new pathways to exploiting metal superelasticity. This article discusses recent developments in Ni-free superelastic Ti alloys and the determining factors affecting their superelastic recoverable strain. The importance of implant superelasticity relative to the elastic and "superelastic" properties of human bones is examined. Also discussed are the advances in Ni-free Ti-based superelastic alloy design and superelasticity-demanding medical and dental applications. The impact of the AM-enabled micro-architectural design on the development of superelastic structures or superelastic meta-materials is deliberated. Future research priorities are suggested. [ABSTRACT FROM AUTHOR]

Deep-powder-bed additive manufacturing (AM) can lead to distinctive microstructural features. In this study, 300-mm long cylindrical rods (12-mm diameter) of Ti–6Al–4V were vertically built to the limit height of a commercial selective electron beam melting (SEBM) system for quantitative three-dimensional (3D) characterization of the defects by X-ray micro-computed tomography (μ-CT). Detailed μ-CT data from 18,337 consecutive slices revealed a strong dependence of defect characteristics on build height, including defect volume, population, sphericity, major axis length, depth and orientation angle. The first 100-mm build exhibited the worst presence of defects by each measurement, while the middle and last 100-mm builds contained much fewer defects, especially the last 100-mm build, which was free of lack-of-fusion defects (sphericity < 0.5). As a result, the first 100-mm build displayed 50% lower reduction of area and 20% lower strain-to-fracture than the last 100-mm build, while the tensile strengths varied within just ±3%. An outer 3-mm thick ring and a central 1.5-mm diameter region were found to contain substantially less defects along the 300-mm build height. The dependence of defect features on build height was attributed to the existence of an upward temperature gradient during SEBM. The 3D defect features revealed by μ-CT along the build height provide important implications for deep-powder-bed AM by SEBM. [ABSTRACT FROM AUTHOR]