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Additive manufacturing (AM) of metals, also known as metal 3D printing, typically leads to the formation of columnar grain structures along the build direction in most as-built metals and alloys. These long columnar grains can cause property anisotropy, which is usually detrimental to component qualification or targeted applications. Here, without changing alloy chemistry, we demonstrate an AM solidification-control solution to printing metallic alloys with an equiaxed grain structure and improved mechanical properties. Using the titanium alloy Ti-6Al-4V as a model alloy, we employ high-intensity ultrasound to achieve full transition from columnar grains to fine (~100 µm) equiaxed grains in AM Ti-6Al-4V samples by laser powder deposition. This results in a 12% improvement in both the yield stress and tensile strength compared with the conventional AM columnar Ti-6Al-4V. We further demonstrate the generality of our technique by achieving similar grain structure control results in the nickel-based superalloy Inconel 625, and expect that this method may be applicable to other metallic materials that exhibit columnar grain structures during AM. 3D printing of metals produces elongated columnar grains which are usually detrimental to component performance. Here, the authors combine ultrasound and 3D printing to promote equiaxed and refined microstructures in a titanium alloy and a nickel-based superalloy resulting in improved mechanical properties. [ABSTRACT FROM AUTHOR]

Ti-6Al-4V (Ti-64) swarf is produced every day in commercial operations but its direct use for the fabrication of strong and ductile Ti-6Al-4V alloy is seldom reported. This study consolidated as-received Ti-64 swarf into near fully dense Ti-6Al-4V alloy by hot pressing. The microstructures and tensile mechanical properties of the hot-pressed Ti-64 alloy and the effect of subsequent solution treatment and ageing (STA) were studied. The hot-pressed Ti-64 achieved tensile yield strength (TYS) of 928-998 MPa, ultimate tensile strength (UTS) of 1076-1139 MPa and strain to fracture of 6.92-7.80%. The STA increased the TYS to 1103-1143 MPa and UTS to 1188-1223 MPa, accompanied by a decline in the strain to fracture to 5.32-6.48%. These tensile property data suggest that it is possible to directly consolidate Ti-6Al-4V swarf into a strong and ductile Ti-6Al-4V alloy by hot pressing for a variety of potential applications. [ABSTRACT FROM AUTHOR]

Abstract New non-equiatomic Ti (25+x) -Zr 25 -Nb 25 -Ta (25-x) (x = 0, 5, 10, 15, 20, in at%) medium entropy alloys (MEAs) have been designed using the atomic mismatch approach and fabricated through a conventional arc-melting process. These novel MEAs were derived from a recently developed equiatomic Ti-Zr-Nb-Ta MEA by gradually replacing its Ta content with Ti. Each non-equiatomic MEA solidified as a single solid-solution phase, which was characterised in detail and compared with Pandat™ simulation and empirical rules. Systematic tensile mechanical property data revealed the existence of a brittle-to-ductile transition for Ti-Zr-Nb-Ta MEAs, i.e., when 15 at% of Ta in the equiatomic Ti 25 -Zr 25 -Nb 25 -Ta 25 MEA was replaced by Ti to become a Ti 40 -Zr 25 -Nb 25 -Ta 10 MEA. The transition occurs corresponding to a small reduction in atomic mismatch from 4.72% to 4.65% but a signficant drop in nanoindentation hardness from 4.2 GPa to 3.5 GPa. In particular, both the as-cast Ti 40 -Zr 25 -Nb 25 -Ta 10 and Ti 45 -Zr 25 -Nb 25 -Ta 5 MEAs exhibited excellent tensile strain to fracture (>18%) and tensile strength (>900 MPa) with much reduced density compared to the brittle Ti 25 -Zr 25 -Nb 25 -Ta 25 MEA. They are both among a very small number of strong and ductile (tensile strain >15%) HEAs reported to date. Their tensile mechanical properties can be further tuned by adjusting the atomic mismatch of the resulting single solid-solution phase in conjunction with the improved understanding of the microstructures of these MEAs. [ABSTRACT FROM AUTHOR]

Abstract A biocompatible Ti-12Mo alloy was fabricated by metal injection moulding (MIM) using non-spherical titanium, molybdenum powders and a purposely designed binder. The density, microstructure and tensile properties were characterized. This was followed by a detailed assessment of its in vitro corrosion and biocompatibility performances, compared with that of two commonly used titanium-based materials extra low interstitial (ELI) Ti-6Al-4V and commercially pure (CP) titanium. The MIM-fabricated Ti-12Mo alloy can achieve a wide range of mechanical properties through controlling sintering process. Specimens sintered at 1400 °C are characterized by fairly uniform near-β microstructure and high relative density of 97.6%, leading to the highest tensile strength of 845.3 ± 21 MPa and elongation of 4.15 ± 0.2% while the highest elastic modulus of 73.2 ± 5.1 GPa. Owing to the formation of protective TiO 2 -MoO 3 passive film, the MIM-fabricated Ti-12Mo alloy exhibits the highest corrosion resistance including the noblest corrosion potential, the lowest corrosion current density and the highest pitting potential in four different electrolytes. The in vitro cytotoxicity test suggests that the MIM-fabricated Ti-12Mo alloy displays no adverse effect on MC3T3-E1 cells with cytotoxicity ranking of 0 grade, which is nearly close to ELI Ti-6Al-4V or CP Ti. These properties together with its easy net-shape manufacturability make Ti-12Mo an attractive new dental implant alloy. Graphical abstract fx1 Highlights • Ti-12Mo can be fabricated from non-spherical powders by metal injection moulding (MIM). • MIM-fabricated Ti-12Mo alloy can tolerate high oxygen with high tensile properties. • MIM-fabricated Ti-12Mo is much more corrosion resistant than common Ti dental alloys. • MIM-fabricated Ti-12Mo exhibits excellent in vitro biocompatibility. • MIM capability and corrosion resistance make Ti-12Mo a promising dental alloy. [ABSTRACT FROM AUTHOR]

Microwave (MW) radiation has attracted increasing attention in the fabrication and/or synthesis of titanium (Ti) and Ti alloys from powder since 1999 when the first study was reported by Gedevanishvili et al. This article provides a comprehensive review of MW processing of Ti and Ti alloys. It begins by discussing the critical technical issues associated with MW processing of Ti powder, including the heating response of Ti powder to MW radiation, temperature measurement by infrared pyrometry and calibration, and interstitial absorption and control. This is followed by a detailed review of the sintering of a range of powder metallurgy (PM) Ti and Ti alloys for structural, biomedical, and shape memory applications. As a new development in the field, MW heating and sintering of titanium hydride (TiH2) powder for the fabrication of PM Ti are discussed in terms of the heating response, interstitial contamination, microstructure, and tensile properties of the as-sintered Ti. The challenges that face MW sintering of PM Ti from either Ti powder or TiH2powder are reviewed, and solutions are proposed. Based on the heating and isothermal sintering characteristics by MW radiation, recommendations are made for the applications of MW processing of Ti and Ti alloys. [ABSTRACT FROM PUBLISHER]

The surface oxide film on Ti powder consists of TiO, TiO, and TiO and starts to dissolve into the titanium matrix underneath from ~ 943 K (670 °C). LaB can scavenge O prior to the active dissolution of the surface oxide film. The scavenging process begins by forming an interfacial LaBO layer due to reaction between surface oxide layer and LaB, followed by diffusion of oxygen through the LaBO layer. The unique oxygen-scavenging feature of LaB produces better tensile elongation. [ABSTRACT FROM AUTHOR]

Strong and ductile powder metallurgy (PM) Ti-10V-2Fe-3Al alloy has been fabricated by spark plasma sintering (SPS) of titanium and V-Fe-Al master alloy powder blends at 1100°C for 30 min under 30 MPa. The homogenization of vanadium, which dictates the realization of a uniform microstructure of the Ti-10V-2Fe-3Al alloy, was markedly accelerated by SPS. The mechanism is attributed to the intensive Joule heating effect produced by the direct current passing through the electric conducting powder blends, rather than through spark plasma discharge, because homogenization occurred mainly after near full identification had been achieved. The chemical and microstructural homogeneity ensured the achievement of excellent tensile properties of PM Ti-10V-2Fe-3Al in the as-sintered state, with tensile strength >1250 MPa and elongation >10%. [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]

Owing to their relatively low Young's modulus, high strength, good resistance to corrosion, and excellent biocompatibility, β-titanium (Ti) alloys have shown great potential for biomedical applications. In β-Ti alloys, carbon can exist in the form of titanium carbide (TiC) as well as interstitial atoms. The Ti-C binary phase diagram predicts a carbon solubility value of 0.08 wt.% in β-Ti, which has been used as the carbon limit for a variety of β-Ti alloys. However, noticeable grain boundary TiC particles have been observed in β-Ti alloys containing impurity levels of carbon well below the predicted 0.08 wt.%. This review focuses its attention on trace carbon (≤0.08 wt.%) in biomedical β-Ti alloys containing niobium (Nb) and molybdenum (Mo), and it discusses the nature and precipitation mechanism of the TiC particles in these alloys. [ABSTRACT FROM AUTHOR]

Lowering the cost of feedstock powder has been a major issue for wider applications of additive manufacturing (AM) of titanium (Ti) and its alloys. A novel and inexpensive Ti sponge material was selected as a precursor and processed using a CSIRO proprietary powder manipulation technology (PMT). The manipulated powder was characterized in terms of the particle size distribution (PSD), roundness, flowability in the Hall Funnel flowmeter, static angle of repose (AOR), apparent density and tap density. In addition, a universal powder bed (UPB) system was used to characterize the manipulated powder behavior after raking. Two benchmark powders, virgin Arcam Ti-6Al-4V powder and used Arcam Ti-6Al-4V powder, were assessed for a comparison. PMT processing of the Ti powder precursor produced near spherically shaped Ti powder in the size range of 75-106 µm, which performed very similarly to the used Arcam powder in the UPB system. The CSIRO PMT offers a cost-effective manipulation process to produce Ti powder promising for AM applications, while the UPB system allows a quick assessment of the powder spreading behavior in AM processes. [ABSTRACT FROM AUTHOR]

In situ heating, synchrotron radiation X-ray diffraction has been used to study the alloying and phase transformation behavior of TiH-6Al-4V and Ti-6Al-4V alloys. Accelerated alloying and phase transformation were observed in the powder compact of the TiH-6Al-4V alloy subjected to a high heating rate. In addition, an oxygen-stabilized TiFeO phase, which is present as sub-micron or nanoscaled particles, has been identified in the TiH-6Al-4V alloy. The implications of these experimental findings have been discussed in terms of alloying, improved densification and oxygen scavenging in titanium and titanium alloys. [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]

Abstract: The formation of grain boundary (GB) brittle carbides with a complex three-dimensional (3-D) morphology can be detrimental to both the fatigue properties and corrosion resistance of a biomedical titanium alloy. A detailed microscopic study has been performed on an as-sintered biomedical Ti–15Mo (in wt.%) alloy containing 0.032wt.% C. A noticeable presence of a carbon-enriched phase has been observed along the GB, although the carbon content is well below the maximum carbon limit of 0.1wt.% specified by ASTM Standard F2066. Transmission electron microscopy (TEM) identified that the carbon-enriched phase is face-centred cubic Ti2C. 3-D tomography reconstruction revealed that the Ti2C structure has morphology similar to primary α-Ti. Nanoindentation confirmed the high hardness and high Young’s modulus of the GB Ti2C phase. To avoid GB carbide formation in Ti–15Mo, the carbon content should be limited to 0.006wt.% by Thermo-Calc predictions. Similar analyses and characterization of the carbide formation in biomedical unalloyed Ti, Ti–6Al–4V and Ti–16Nb have also been performed. [Copyright &y& Elsevier]

A detailed transmission electron microscopy (TEM) study has been made of the microstructures of two as-sintered CP-Ti materials. We show that iron content at the impurity level of 1280 ppm, less than the limit (2000 ppm) for CP-Ti ASTM Grade 1, is sufficient to lead to the formation of a grain boundary (GB) β-Ti phase in the as-sintered microstructure due to segregation. The Fe-stabilized GB β-Ti phase contains ~7 at. pct Fe and ~1.5 at. pct Cl. In addition, nano-precipitates of ω-Ti exist in the Fe-stabilized GB β-Ti phase. A phosphorus (P)-enriched Ti-P-based phase was also identified, which has a tetragonal crystal structure with lattice parameters of ( a = b = 8.0 ± 0.2 Å and c = 2.7 ± 0.2 Å) and is new to the existing database for Ti-P-enriched phases. As-sintered CP-Ti materials are thus not necessarily a single α-Ti phase material. These impurity-induced phases may exert potential impacts on the properties of sintered CP-Ti. [ABSTRACT FROM AUTHOR]

The effectiveness of microwave (MW) sintering has been demonstrated on many ceramic systems, a number of metallic systems, and metal-ceramic composites, but remains ambiguous for Ti powder materials. This work presents a detailed comparative study of MW and conventional sintering of Ti powder compacts in vacuum. It is shown that MW radiation is effective in heating Ti powder compacts with the assistance of MW susceptors; it delivered an average heating rate of 34 K/min (34 °C/min), compared to 4 K/min (4 °C/min) by conventional vacuum heating in an alumina-tube furnace. Microwave radiation resulted in similar densification with well-developed sinter bonds. However, MW-sintered samples showed higher bulk hardness, a harder surface shell, and coarser grains. The difference in hardness is attributed to the difference in the oxygen content, supported by X-ray photoelectron spectroscopy analyses. The mechanisms of MW heating for metal powder compacts are discussed in the context of the sintering of Ti powder materials and attributed to three combined effects. These include heat radiation from the MW susceptors at low temperatures, enhanced MW absorption due to the transformation of the TiO film on each Ti powder particle to oxygen-deficient Ti oxides, which are MW absorbers; and the volumetric heating of Ti powder particles by eddy currents. [ABSTRACT FROM AUTHOR]

An introduction to the journal is presented wherein the editors discuss various articles published within the issue on topics including effects of oxygen content on tensile and fatigue performance of titanium alloys, influence of different cooling rates, and solidification grain size and morphology for additive manufacturing of titanium alloys.

Nickel (Ni) improves the room temperature ductility, hot workability and sinterability of γ-TiAl based alloys. This study investigates the microstructure of a powder metallurgy nickel-doped Ti–45Al–5Nb–0.2C–0.2B–1.25Ni alloy based on detailed transmission electron microscopy (TEM) studies. The alloy was sintered at 1375 °C for 120 min, followed by cooling to 900 °C for a subsequent ageing treatment of 360 min. Micrometric τ 3 -Al 3 NiTi 2 particles formed as a result of the solidification of a Ni-containing sintering liquid during subsequent cooling. Ageing of the as-sintered Ti–45Al–5Nb–0.2C–0.2B–1.25Ni alloy at 900 °C resulted in the precipitation of both plate-shaped and lath-shaped nanometric τ 3 phase in the α 2 lamellae. The constrained coincidence site lattice (CCSL) theory was used to understand the morphology and crystallography of the nanoscale τ 3 -Al 3 NiTi 2 precipitates. The compression yield strength of the Ti–45Al–5Nb–0.2C–0.2B–1.25Ni alloy increased from 750 ± 20 MPa to 850 ± 20 MPa after ageing. [ABSTRACT FROM AUTHOR]

An addition of ≤0.5 wt% lanthanum boride (LaB 6 ) to powder metallurgy commercially pure Ti (CP-Ti), Ti–6Al–4V and Ti–10V–2Fe–3Al (all in wt%) resulted in improved sintered density, substantial microstructural refinement, and noticeably increased tensile elongation. The addition of LaB 6 led to scavenging of both oxygen (O) and chlorine (Cl) from the titanium powder during sintering, evidenced by the formation of La 2 O 3 and LaCl x O y . The pinning effect of La 2 O 3 , LaCl x O y and TiB inhibited prior-β grain growth and resulted in subsequent smaller α-laths. The formation of nearly equiaxed α-Ti phase is partially attributed to the nucleation effect of α-Ti on TiB. The improved sintered density was caused by B from LaB 6 rather than La, while excessive formation of La 2 O 3 and TiB with an addition of >0.5 wt% LaB 6 resulted in a noticeable decrease in sintered density. The improved tensile elongation with an addition of ≤0.5 wt% LaB 6 was mainly attributed to the scavenging of oxygen by LaB 6 , partially assisted by the improved sintered density. However, an addition of >0.5 wt% LaB 6 led to the formation of large La 2 O 3 aggregates and more brittle TiB whiskers and therefore decreased tensile elongation. Balanced scavenging of O is thus important. The optimal addition of LaB 6 was 0.5 wt% but this may change depending on the powder size of the LaB 6 to be used. [ABSTRACT FROM AUTHOR]

Highlights: [•] A small addition of Ni (⩽1.25at.%) transforms the alloy from essentially unsinterable to fully sinterable at 1375°C. [•] A τ3 phase (Al3NiTi2) forms during heating before reaching 1150°C which enables subsequent liquid formation. [•] The as-sintered Ni-doped alloy shows excellent compression properties. [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]

Metal Injection Moulding (MIM) is a well-developed net or near-net shape manufacturing technique for stainless steel, copper and other metallic materials. This process has also received increasing attention over the last decade as a promising technique for the manufacture of small and intricate titanium parts for a range of applications in biomedical, aerospace, automotive and other industries. Historically, the necessity to use expensive fine spherical (< 45 μm), low-oxygen titanium powder has hindered the industrial application of titanium MIM from an economic perspective. However, recent efforts have shown promise in adapting low-cost non-spherical hydride-dehydride (HDH) titanium powder to the MIM process. HDH powder is considerably less expensive than fine spherical powder and thus there is significant potential in expanding the application of titanium MIM. This paper reviews recent developments in MIM of titanium and its alloys as well as the outstanding challenges with a special focus on MIM of HDH titanium powder. [ABSTRACT FROM AUTHOR]

This work reports the use of a novel strategy, high magnetic field (HMF, 3T) heat treatment (HT), to modify the microstructure of Ti–6Al–4V (Ti64) alloy fabricated by laser-powder bed fusion (L-PBF) to improve mechanical properties. Applying an HMF-HT to L-PBF Ti64 at 800–900 °C increased the sub-grain boundaries in the α-phase from ∼7% to ∼21%. The distribution of high kernel average misorientation (KAM) values is consistent with the distribution of the sub-grain boundaries in the α-phase. This finding was further supported by analysis using the modified Williamson-Hall approach and transmission electron microscopy (TEM) characterization. HMF-HT accelerated the coarsening and globularization of the α-phase, resulting in thick α-laths (3.40 μm) and the formation of globular α-phase particles. The HMF-HT at 900 °C further increased the β-phase volume fraction from 6.5% to 13.4% with a larger lattice parameter of 3.2143 Å due to the resulting lower average V content (confirmed by TEM analysis). In addition, the HMF-HT rendered the α-variants to better align with the <11–20> α direction, where the prismatic and basal slips in the <11–20> α direction are the easiest slip modes to activate in the α-variants. These combined effects enhanced the tensile ductility of Ti64 to 15.4 ± 1.3%. HMF-HT can be used to effectively tune the microstructure and mechanical properties of L-PBF α-β titanium alloys. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: Recent systematic experimental studies involving the benchmark Ti–6Al–4V alloy fabricated from powder have established that there exists a critical level of oxygen around 0.33mass%, beyond which the tensile ductility of the alloy drops dramatically until reaching total brittleness. To understand the fundamental mechanisms behind this critical oxygen content, three-dimensional atom probe tomography, transmission electron microscopy and other analytical means have been used to identify and characterize the fine microstructural changes induced by the increased oxygen content beyond the critical level. Three fine microstructural features were identified in as-sintered Ti–6Al–4V when the interstitial oxygen content was increased from 0.25mass% to 0.49mass%. These are: (i) the formation of fine acicular α precipitates in the β phase; (ii) the formation of α2-type (Ti3Al) nanometric clusters in the α matrix; and (iii) grain boundary α–β–α-layered structures between the α grains. The impacts of these microstructural changes on the tensile ductility are discussed. [Copyright &y& Elsevier]

Highlights: [•] Compared to cold die compaction, warm die (200°C) compaction of −100 mesh hydride–dehydride Ti powder at pressures 200–1000MPa increases the green density by 5.0–9.4% and the sintered density 2.0–4.4% theoretical density. [•] Warm die compaction facilitates the fabrication of near pore-free Ti (>99.5% theoretical density) by pressureless sintering despite that the sintering kinetic is essentially unchanged. [•] Warm die compaction is most effective in fabricating unalloyed Ti, then blended elemental Ti alloys but ineffective for pre-alloyed Ti alloys, affected by the yield strength of constituent powder. [ABSTRACT FROM AUTHOR]

Highlights: [•] Boron modified the morphology of α-Ti laths to equiaxed grains in as-sintered Ti alloys. [•] Most of TiB particles in as-sintered Ti alloys were located inside α-Ti grains. [•] Specific orientation relationships between TiB and α-Ti were identified. [•] A unique peritectoid reaction may have enhanced heterogeneous nucleation of α-Ti on TiB. [ABSTRACT FROM AUTHOR]

An addition of 1.5at% Co to Ti–48Al–2Cr–2Nb (in at%) transformed the alloy from essentially unsinterable to fully sinterable at 1300°C. This, together with a simple powder coating process developed recently, has allowed near-net shape fabrication of the alloy for the first time by cold compaction and pressureless sintering. The addition of Co results in the formation of an intermediate face centred cubic (fcc) CoAl2Ti phase prior to 1220°C during heating. It subsequently reacts with an α phase leading to the formation of a Co-containing, wettable sintering liquid through a two-step process, CoAl2Ti+α→Liquid at 1256.2°C and CoAl2Ti+α→γ-TiAl+Liquid at 1267.2°C, and therefore full densification of the alloy. Without Co, sintering of the Ti–48Al–2Cr–2Nb alloy powder at 1300°C is controlled by the slow self-diffusion of Ti and interdiffusion of Ti and Al according to the activation energy determined. Transmission electron microscopy (TEM) identified an fcc CoAl2Ti phase and a hexagonal close packed (hcp) Co-enriched Ti(Al, Co, Cr, Nb) phase in the final as-sintered Ti–48Al–2Cr–2Nb–1.5Co alloy. They both form during cooling at 1240°C through Liquid+α→CoAl2Ti+Ti (Al, Co, Cr, Nb). The tensile and compressive properties of the as-sintered Ti–48Al–2Cr–2Nb–1.5Co alloy were compared to the original General Electric (GE) Ti–48Al–2Cr–2Nb alloy fabricated by casting or metal injection moulding. [ABSTRACT FROM AUTHOR]

Abstract: A small addition (≤0.5wt%) of cerium silicide (CeSi2) to powder metallurgy (PM) commercially pure Ti (CP-Ti), Ti–6Al–4V and Ti–10V–2Fe–3Al (all in wt%) results in substantial microstructural refinement and noticeably improved ductility with marginally improved sintered density. CeSi2 is unstable and decomposes between 1423K and 1473K. The Si goes into solid solution in β-Ti and is responsible for the improved sintered density while the Ce scavenges both oxygen (O) and chlorine (Cl) from the Ti powder and therefore improves tensile ductility. The resulting CeO2 and CeCl x O y particles generally exist along or close to the prior-β grain boundaries. The substantial microstructural refinement in terms of both the prior-β grain size and the subsequent α-Ti lath size is attributed to the grain boundary pinning effect of the CeO2 particles. The optimum concentration of CeSi2 is approximately 0.5wt%, beyond which both the sintered density and tensile elongation drop with increasing addition of CeSi2. CeSi2 can be a practical form of Ce addition to PM Ti alloys for impurity scavenging, microstructural refinement and tensile ductility improvement. [Copyright &y& Elsevier]

Abstract: The effect of a small addition of boron (B) on the sintering, sintered microstructure and mechanical properties of a Ti-7Ni alloy (wt.%) compacted from −100 mesh hydride-dehydride (HDH) Ti powder has been investigated systematically. An addition of ⩾0.62%B to the Ti–7Ni alloy resulted in fast, near full-densification (⩾99% relative density) within 15min at 1200°C, compared to 95.6% relative density obtained after 120min at 1200°C without B, compacted at 400MPa in both cases. In addition, the pre-eutectoid α phase changed from long parallel α-laths (up to∼500μm) to near-equiaxed α-Ti throughout the microstructure with an addition of ⩾0.31%B. Elemental B was found to be more effective than TiB2 as a sintering aid for Ti-7Ni. A small addition of B increased both the tensile strength and yield strength of Ti–7Ni but at the expense of tensile ductility. The mechanisms behind each of these observations were discussed. [Copyright &y& Elsevier]

Abstract: Titanium aluminide based alloys are difficult to consolidate by pressureless sintering. A systematic study has been made of the effect of small additions of copper on the sintering densification, microstructure and mechanical properties of gamma Ti–48Al–2Cr–2Nb (in at% throughout). The density of the Ti–48Al–2Cr–2Nb alloy sintered at 1375°C for 120min increased consistently from 74% theoretical density (TD) without Cu to >98%TD with an addition of up to 2at%Cu. The enabling effect of a small addition of Cu is due to the formation of a wetting liquid, supported by dilatometry, differential scanning calorimetry (DSC) and Thermal-Calc analyses. Rapid sintering shrinkage occurred around 1251°C and 1370°C corresponding to liquid formation. Transmission electron microscopy (TEM) identified the formation of a Cu- and Cr-enriched hexagonal close-packed Ti(Al, Cr, Cu, Nb) phase with a=0.520Å and c=0.803Å. Its presence increased with increasing Cu content. The compressive, tensile and flexure properties of the as-sintered Ti–48Al–2Cr–2Nb–2Cu alloy were assessed. The tensile properties are similar or superior to those of the Ti–48Al–2Cr–2Nb alloy fabricated by metal injection moulding and sintering while the compression strength is about twice that of the as-cast Ti–48Al–2Cr–2Nb. [Copyright &y& Elsevier]

In situ synchrotron radiation revealed how the scavenging of oxygen by yttrium hydride (YH2) occurred in powder metallurgy commercially pure Ti (CP-Ti) and Ti–6Al–4V. YH2 decomposes at >900°C in CP-Ti and at >1100°C in Ti–6Al–4V. Elemental Y coexists with Y2O3 in both CP-Ti and Ti–6Al–4V up to 1300°C. Ex situ analytical means further confirmed that the scavenging product is Y2O3. Together with available diffusion data, mechanisms are proposed to understand the decomposition of YH2 and the scavenging of oxygen in CP-Ti and Ti–6Al–4V. [ABSTRACT FROM AUTHOR]

A detailed transmission electron microscopy analysis has revealed that small additions of yttrium hydride to a powder metallurgy Ti–2.25Mo–1.5Fe alloy resulted in the formation of both chlorine-free yttrium oxides and essentially oxygen-free yttrium chlorides. The oxides and chlorides showed distinctly different morphologies and spatial distribution. Yttrium acted as a potent getter for both oxygen and chlorine. Additionally, the β-Ti phase was free of nanoscaled α-Ti in the presence of yttrium. These microstructural changes contribute to the substantially increased ductility (∼90%). [ABSTRACT FROM AUTHOR]

Abstract: Thermodynamic predictions suggest that silicon has the potential to be a potent sintering aid for Ti–Ni alloys because small additions of Si lower the solidus of Ti–Ni alloys appreciably (>200°C by 1wt.% Si). A systematic study has been made of the effect of Si on the sintering of a Ti–3Ni alloy at 1300°C. The sintered density increased from 91.8% theoretical density (TD) to 99.2%TD with increasing Si from 0% to 2%. Microstructural examination reveals that coarse particles and/or continuous networks of Ti5Si3 form along grain boundaries when the addition of Si exceeds 1%. The grain boundary Ti5Si3 phase leads to predominantly intergranular fracture and therefore a sharp decrease in ductility concomitant with increased tensile strengths. The optimum addition of Si is proposed to be ≤1%. Dilatometry experiments reveal different shrinkage behaviours with respect to different Si contents. Interrupted differential scanning calorimetry (DSC) experiments and corresponding X-ray diffraction (XRD) analyses clarify the sequence of phase formation during heating. The results provide a useful basis for powder metallurgy (PM) Ti alloy design with Si. [Copyright &y& Elsevier]

Abstract: A comprehensive study has been made of the sintering and microstructural evolution of Ti–10V–2Fe–3Al compacted from titanium and master alloy powder blends. The densification of Ti–10V–2Fe–3Al at ≤1300°C occurs by solid-state sintering with apparent activation energy of 163±13kJ/mol, which falls into the reported activation energy range for the self-diffusion of titanium. The sintered density depends primarily on the titanium powder size while the sintered microstructure and mechanical properties depend mainly on the master alloy type or diffusion of vanadium. The real challenge for the fabrication of Ti–10V–2Fe–3Al by sintering is not densification, which is trivial with relatively fine titanium powder. Rather, it is the realisation of a desired microstructure through master alloy selection or design of the sintering pathway, which determines the distribution of the principal alloying element V, a slow diffuser in β-Ti. This distinguishes it from the sintering of lean alloys, where the focus has predominantly been on densification. The use of 10V–2Fe–3Al master alloy produced more uniform microstructures and therefore much better ductility than the use of 85V–15Al at similar densities. [Copyright &y& Elsevier]

In situ transmission electron microscopy (TEM) has been used to witness the nanocrystallization of amorphous Ti42.5Cu40Zr10Ni5Sn2.5. A crystallization front exists to separate the TEM sample into two parts with different thermal stabilities. The number density of the crystallization products varies significantly, with the precipitate sizes ranging from a few nanometres to ∼100nm. Detailed TEM analysis suggests that oxygen is the most likely reason for realizing the unusual nanocrystallization. External thermal analysis also indicates that oxygen affects the crystallization. [Copyright &y& Elsevier]

This research reports a promising bio-applicable new Ti35Zr15Nb25Ta25 (at.%) medium entropy alloy (MEA) designed with an atomic mismatch value (δ) of 4.0% through replacing 10 at.% Zr with 10 at.% Ti in its parent equiatomic TiZrNbTa MEA (δ = 4.84%). The as-cast MEA shows a single body centred cubic (BCC) phase with ~81% of its solidification grain boundaries (SGBs) having misorientation angles of > 30°. In the as-cast state, the MEA exhibited tensile ductility of 17 ± 0.6% and yield strength of 842 ± 11 MPa, above the minimum ductility−yield strength (10%-759 MPa) requirement for mill-annealed medical-grade Ti–6Al–4V (wt.%) alloy. The oxide film composition and thickness of the MEA are characterized. The MEA is substantially stronger and lighter than the most biocompatible metal, tantalum (Ta), while its Ta content (45.74 wt.% > the critical 40 wt.%) is sufficient to allow the formation of a continuously connected tantalum oxide (Ta 2 O 5) film for it to exploit the bio-advantages of Ta. Both micro-shear bands and twin structures (three types of twin boundaries ∑3, ∑11 and ∑33a) were identified in regions near the tensile fracture surfaces of the MEA, providing insights into the ductility of the alloy in the as-cast state. [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]