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The design freedom offered by additive manufacturing (AM) enables the fabrication of components with internal surfaces that are challenging to access post-manufacture. This is of concern, as the surface condition can markedly deteriorate fatigue performance. Additionally, the adaptation of surface finishing methods for AM components with topologically optimized designs can be a costly practice. It is therefore desirable to consider deploying AM parts with no or minimal surface processing for targeted applications. This requires an in-depth understanding of the formation of various types of AM surfaces, including the variation in surface condition and controlling factors, and their influence on mechanical performance. The last few years have seen significant research advances in these aspects. Ti-6Al-4V is the most extensively studied alloy for AM. The research data available now allows an informative treatment of this topic for both practical applications and future research. Using laser powder bed fusion (LPBF) of Ti-6Al-4V as a model AM − alloy system, this article examines (i) the characteristics of various types of LPBF surfaces including horizontal, vertical, inclined, upward, downward, internal isolated, and slotted surfaces; (ii) the design features and LPBF variables that affect the surface topography; (iii) the capabilities of existing post-AM surface processing methods; and (iv) the influence of AM surface topography on mechanical properties by focusing on the fatigue performance. On this basis, design considerations are recommended for AM of consistent surfaces, and priority surface-related research issues are identified. The purpose is to establish an essential knowledge base for improved commercial designs for LPBF for suitable dynamically loaded applications, with no or minimal surface processing. While centering on LPBF of Ti-6Al-4V, the insights derived are expected to be applicable to other AM processes or metallic materials. [ABSTRACT FROM AUTHOR]

The difficulty of achieving full densification of TiAl powder by pressureless sintering makes spark plasma sintering (SPS) an attractive alternative. In this study, Ti-48Al-2Cr-Nb alloy was fabricated by SPS from gas-atomized TiAl powder. Near-full densification was achieved after 4 min at 1100°C under applied pressure of 80 MPa, accompanied by the formation of an unexpected bright-contrast phase when observed under backscattered scanning electron microscopy. Detailed characterization revealed that the bright-contrast phase was composed of oxygen- and carbon-enriched α-Ti and chromium-enriched β-Ti. A two-particle model was used to simulate the current passing through the particles during SPS. The model predicts that when a high density of current passes through the narrow particle–particle contact area, it can produce a high-temperature zone (~ 1-μm-thick surface layer), in which the temperature is high enough to allow the decomposition of the surface oxide layer and melting of the TiAl alloy underneath. These complex changes lead to the formation of the unexpected phase. [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]

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]

The effects of gas flow pattern on the distortion in Al-7Zn-2.5Mg-1Cu alloy were experimentally examined by sintering three rectangular samples, equally spaced 2-40 mm, in each batch at 620°C for 45 min under flowing nitrogen. A three-dimensional (3D) computational fluid dynamics (CFD) model was developed to investigate the gas flow behaviour surrounding three samples during isothermal sintering. Streamlines, gas flowrate into each cavity between two adjacent samples and wall shear stress distribution along the surfaces of each sample were found dependent on sample separation distance. The flow patterns could affect the distortion levels of the three samples by changing the oxygen content in the local sintering atmosphere and the evaporation rates of Mg and Zn from the sintering surfaces. The 3D CFD model developed was also applied to the experimental design of the sintering of six samples to ensure minimum distortion as well as maximum densification. [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]

The distortion in a sintered 7xxx aluminum alloy, Al-7Zn-2.5Mg-1Cu (wt. pct), has been investigated by sintering three rectangular bars in each batch at 893 K (620 °C) for 0 to 40 minutes in nitrogen, followed by air or furnace cooling. They were placed parallel to each other, equally spaced apart at 2 mm, with their long axes being perpendicular to the incoming nitrogen flow. Pore evolution in each sample during isothermal sintering was examined metallographically. The compositional changes across sample mid-cross section and surface layers were analyzed using energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy depth profiling, respectively. The two outer samples bent toward the middle one, while the middle sample was essentially distortion free after sintering. The distortion in the outer samples was a result of differential shrinkage between their outer and inner surfaces during isothermal sintering. The porous outer surface showed an enrichment of oxygen around the large pores as well as lower magnesium and zinc contents than the interior and inner surface of the same sample, while the inner surface was distinguished by the presence of AlN. The differential shrinkage was caused by different oxygen contents in local sintering atmosphere and unbalanced loss of magnesium and zinc between the outer and inner surfaces. [ABSTRACT FROM AUTHOR]

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]

A systematic study has been conducted of the sintering, sintered microstructure and tensile properties of a range of lower cost Ti-Fe-Si alloys, including Ti-3Fe-(0-4)Si, Ti-(3-6)Fe-0.5Si, and Ti-(3-6)Fe-1Si (in wt pct throughout). Small additions of Si (≤1 pct) noticeably improve the as-sintered tensile properties of Ti-3Fe alloy, including the ductility, with fine titanium silicides (TiSi) being dispersed in both the α and β phases. Conversely, additions of >1 pct Si produce coarse and/or networked TiSi silicides along the grain boundaries leading to predominantly intergranular fracture and, hence, poor ductility, although the tensile strength continues to increase because of the reinforcement by TiSi. Increasing the Fe content in the Ti- xFe-0.5/1.0Si alloys above 3 pct markedly increases the average grain size and changes the morphology of the α-phase phase to much thinner and more acicular laths. Consequently, the ductility drops to <1 pct. Si reacts exothermically with Fe to form Fe-Si compounds prior to the complete diffusion of the Fe into the Ti matrix during heating. The heat thus released in conjunction with the continuous external heat input melts the silicides leading to transient liquid formation, which improves the densification during heating. No Ti-TiFe eutectoid was observed in the as-sintered Ti-Fe-Si alloys. The optimum PM Ti-Fe-Si compositions are determined to be Ti-3Fe-(0.5-1.0)Si. [ABSTRACT FROM AUTHOR]

The effects of sample position and gas flow pattern on the sintering of a 7xxx aluminum alloy Al-7Zn-2.5Mg-1Cu in flowing nitrogen have been investigated both experimentally and numerically. The near-surface pore distribution and sintered density of the samples show a strong dependency on the sample separation distance over the range from 2 mm to 40 mm. The open porosity in each sample increases with increasing separation distance while the closed porosity remains essentially unchanged. A two-dimensional computational fluid dynamics (CFD) model has been developed to analyze the gas flow behavior near the sample surfaces during isothermal sintering. The streamlines, velocity profile, and volume flow rate in the cavity between each two samples are presented as a function of the sample separation distance at a fixed nitrogen flow rate of 6 L/min. The CFD modeling results provide essential details for understanding the near-surface pore distribution and density of the sintered samples. It is proposed that the different gas flow patterns near the sample surfaces result in variations of the oxygen content from the incoming nitrogen flow in the local sintering atmosphere, which affects the self-gettering process of the aluminum compacts during sintering. This leads to the development of different near-surface pore distributions and sintered densities. [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]

A detailed transmission electron microscopy study of the structure of aluminium nitride formed during sintering of powder injection moulded aluminium is presented. A polycrystalline layer formed on Al particle surfaces exposed to a nitrogen atmosphere. This layer consisted of fine, rod-like crystallites of hexagonal AlN typically aligned normal to the Al surface. A double layer of AlN separated by a thin layer of Al was observed at the interfaces between Al grains. In this report, the structure of the nitride is characterised and its influence on sintering is discussed. [ABSTRACT FROM AUTHOR]

The article offers information on the meeting of Minerals, Metals & Materials Society( TMS) Powder Materials Committee held in March 2016, which discussed sintering and related phenomena. Topics include emergence of quantitative sintering theory from 1945 to 1955, effect of pre-annealing on sintering of stainless steel fiber felt, and role of high-entropy alloys (HEAs).

An introduction is presented in which the editor discusses several reports within the issue on topics including sintering process, laser sintering and fabrication of stainless steel fiber felts by sintering.

An introduction is presented in which the editor discusses various reports within the issue on topics including the geometric model of sintering of unequal sized particles, powder metallurgy of hollow fly ash, and microstructure and electrical properties of silicon carbide (SiC) dispersed molybdenum silicide composite.

NdB 6 can effectively react with the surface titanium oxide films to form an initial NdBO 3 layer at about 670 °C, just prior to their dissolution. Two thirds of all the oxygen scavenged during sintering occurred in the α-Ti phase region through this reaction. NdBO 3 layer continues to grow by diffusion of oxygen until 1130 °C, beyond which NdBO 3 decomposes into Nd 2 O 3 dispersoids and Nd 2 O 3 further reacts with chlorine to form NdCl x O y . The unique oxygen-scavenging process prior to the active dissolution of oxide film makes NdB 6 more effective in scavenging oxygen than other rare earth hydrides, resulting in better tensile elongation. The addition of NdB 6 also leads to the improved sintered density, substantial microstructural refinement and dispersion strengthening by Nd-containing compounds and TiB. The optimal addition level of NdB 6 is 0.3 wt.%, beyond which large Nd 2 O 3 aggregates and an increased number of TiB whiskers were formed. This is detrimental to the ductility. [ABSTRACT FROM AUTHOR]

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]

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]

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]

Abstract: Metallic glass powder of Al86Ni6Y4.5Co2La1.5, prepared by nitrogen gas atomization, was consolidated into 10mm diameter bulk metallic glass (BMG) samples using spark plasma sintering (SPS). The use of a high heating rate is critical to ensure oxide-free, well-sintered clean interfaces between the powder particles and therefore desired mechanical properties, compared to oxide-enveloped, poorly-sintered interfaces that result from the use of a slow heating rate. The marked influence of heating rate is attributed to the surface-cleaning effect of SPS associated with the increased electrical discharge. [Copyright &y& Elsevier]

Abstract: Abnormal crystallization was observed in Al86Ni6Y4.5Co2La1.5 metallic glass powder at about 20 °C below its glass transition temperature (T g, 271.5 °C) during spark plasma sintering (SPS). The crystallization product, identified to be a hexagonal-structured Al5Co2 phase, was not detected in the same powder when annealed in a differential scanning calorimeter (DSC). Nor was it detected in other Al-based metallic glasses of similar compositions which were annealed around their T g temperatures by conventional heating. SPS is effective to introduce a unique nanometric intermetallic phase in the amorphous matrix. The abnormal crystallization is attributed to the high applied pressure and non-thermal effects of SPS. [Copyright &y& Elsevier]

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]

We report that pure microwave (MW) radiation, without the assistance of MW susceptors, can rapidly heat titanium hydride (TiH2) powder to 1300°C for isothermal sintering. The release of hydrogen, however, induces low-temperature plasmas which reflect incident MWs, leading to cyclic temperature fluctuations. Commercially pure titanium fabricated by MW radiation of TiH2 powder exhibits much higher sintered densities, tensile strength and ductility together with finer residual pores, compared to that sintered from hydride–dehydride titanium powder. [ABSTRACT FROM AUTHOR]

TiC nanoplatelet-reinforced titanium composites were synthesized through a novel fabrication approach that combines resol nanosphere (10–30nm) coating with conventional powder metallurgy. The resulting TiC nanoplatelets, 28–130nm thick, are self-assembled, well aligned in each individual grain but randomly orientated throughout the microstructure. The as-sintered Ti–TiC composites exhibit outstanding compressive properties, with ultimate strength=2.54GPa, yield strength=1.52GPa and strain to fracture=44.4%, stronger than all other advanced Ti materials reported to date. [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]

Abstract: The hcp-structured α-Ti is one of the two basic phases (α-Ti and β-Ti) in Ti materials. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to analyse the α-Ti phase in both as-sintered and as-cast α-Ti materials. The structure factor of the (0001) plane of the α-Ti phase is 0. No diffraction should thus occur from the (0001) plane of the α-Ti phase when characterised by either x-ray diffraction or electron diffraction. However, electron diffraction of the as-sintered α-Ti materials recorded unambiguous diffraction from the (0001) plane as well as (000 l) where l =2n +1 (n is integer). A detailed analysis has clarified that this is a result of double diffraction arising from the two strong neighbouring () and () diffraction planes. The same phenomenon also occurred to the characterisation of as-cast α-Ti materials. [Copyright &y& Elsevier]

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]

Abstract: A comprehensive transmission electron microscopy study has been made of the secondary phases and their interfaces with the matrix in an alloy of Al–2Mg–2Si–0.25Cu sintered in nitrogen. AlN was detected both at the Al–Mg2Si interface and inside Mg2Si grains as strings of nanocrystallites. Mg2Si did not exist at the sintering temperature; it was the solidified residue of the sintering liquid. The observation confirms the formation of AlN during liquid phase sintering of aluminium alloys in nitrogen. The likely pore filling processes are discussed in the light of the distribution of the AlN nanocrystallites. Two Al-, Si- and O-rich secondary phases were also identified, suggesting that, in addition to Mg, Si may have also played a role in disrupting the Al2O3 film that enveloped each Al powder particle. These findings improve the fundamental basis for understanding the sintering of aluminium alloys in nitrogen and the role of Si. [ABSTRACT FROM AUTHOR]

Reducing porosity can help to improve the mechanical properties of an as-sintered material. An alternative is, however, to tailor the size, distribution and morphology of the residual pores without having to further reduce porosity. This study investigates the sintering of commercially pure titanium (CP Ti) in a flowing argon (Ar) atmosphere and compares the results with sintering in a vacuum of 10−2 Pa. The CP Ti sintered in Ar at 1300 °C for 2–3 h exhibited a marginally lower density but clearly better tensile ductility than Ti sintered in vacuum. The sensitivity of tensile ductility to residual pores was analyzed. Samples sintered in Ar exhibited much lower sensitivity than those sintered in vacuum. The superior ductility arises from the beneficial effect of sintering in Ar, which resulted in a greater number of but smaller residual pores with lower pore aspect ratios and finer matrix grains due to the grain-growth inhibiting role of the residual pores. The reason is attributed to the entrapped Ar in the closed pores during sintering, which is insoluble in Ti at the sintering temperature. The ever-increasing internal pore pressure prevents further pore shrinkage after reaching a critical size. The final pore size range is predicted and compared with experimental observations. Sintering of titanium in Ar is advantageous over sintering in vacuum in terms of both pore size distribution and pore morphology within a reasonable isothermal sintering period (e.g., 3 h at 1300 °C). Therefore, it can be more attractive for non-fatigue critical applications than sintering in vacuum. • Titanium sintered in argon exhibits better tensile ductility than sintered in high vacuum and similar strengths. • Sintering in argon produces smaller while more residual pores and finer grains, benefiting ductility. • The initial pore size and entrapped insoluble argon dictate the final pore size. • Existing theoretical models for pore evolution with entrapped gas agree with experimental observations. • Sintering of titanium in argon offers advantages over sintering in vacuum for non-fatigue critical applications. [ABSTRACT FROM AUTHOR]