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Purpose: This paper aims to present a broad review of near-a titanium alloys for high-temperature applications. Design/methodology/approach: Following a brief introduction of titanium (Ti) alloys, this paper considers the near-α group of Ti alloys, which are the most popular high-temperature Ti alloys developed for a high-temperature application, particularly in compressor disc and blades in aero-engines. The paper is relied on literature within the past decade to discuss phase stability and microstructural effect of alloying elements, plastic deformation and reinforcements used in the development of these alloys. Findings: The near-a Ti alloys show high potential for high-temperature applications, and many researchers have explored the incorporation of TiC, TiB SiC, Y2O3, La2O3 and Al2O3 reinforcements for improved mechanical properties. Rolling, extrusion, forging and some severe plastic deformation (SPD) techniques, as well as heat treatment methods, have also been explored extensively. There is, however, a paucity of information on SiC, Y2O3 and carbon nanotube reinforcements and their combinations for improved mechanical properties. Information on some SPD techniques such as cyclic extrusion compression, multiaxial compression/forging and repeated corrugation and straightening for this class of alloys is also limited. Originality/value: This paper provides a topical, technical insight into developments in near-a Ti alloys using literature from within the past decade. It also outlines the future developments of this class of Ti alloys. [ABSTRACT FROM AUTHOR]

In present work, a multiphase Ni 3 Al-based intermetallic alloy with complex components of γ'+γ dendrite comprising extremely high volume fraction (81 vol %) of γ′ phase and interdendritic β phase is investigated, the microstructure evolution and phase transformation behavior tailored by different solution cooling rates are studied by applying different cooling methods of water cooling (WC, 138 °C/s), air cooling (AC, 72 °C/s), and furnace cooling (FC, 0.05 °C/s) after solution treatment at 1200 °C for 10 h. Results show that the cubic degree, size and volume fraction of cuboidal primary γ′ as well as size of spherical secondary γ′ particles in the γ'+γ dendrite are improved by decreasing cooling rates, and the average sizes of the primary and secondary γ′ particles are found to meet logarithmic relationships with cooling rates. There are three kinds of precipitates in the interdendritic β phase resulted from different cooling methods, i.e., the rod-like Cr 3 C 2 carbides, the semi-spherical α-Cr particles, and the long acicular γ′ precipitates. The lower cooling rate decreases the number of rod-like Cr 3 C 2 carbides, increases the number and size of the semi-spherical α-Cr particles, and promotes the formation of long acicular γ′ precipitates in the interdendritic β-matrix. The semi-spherical α-Cr particles play the role of pinning dislocations in the β-matrix and have the orientation relationship of [‾110] α-Cr //[‾110] β-matrix. In addition, the precipitation of semi-spherical α-Cr particles in β-matrix is observed to accomplish in two sequential stages and grow in an onion-like ordered way. Stacking faults existed in partial of α-Cr particles due to the nanotwins in β-matrix. Microtwins are observed in the interdendritic β-matrix after water cooling, which have a bimodal configuration of nanotwins at the microtwinning boundaries and interior stacking faults. Besides, γ′-envelope formed between the γ'+γ dendrite and interdendritic β phase dependent on the solution cooling processes. Image 1 • Re-precipitation sizes of primary and secondary γ′ particles have the logarithmic relationship with solution cooling rates. • Microtwins in the β-matrix have a bimodal configuration of nanotwins at their boundary and interior stacking faults. • Precipitation pattern and growth mode of semi-spherical α-Cr particles in β-matrix phase after air cooling. • Precipitation behaviors influenced by solution cooling rates of precipitates in the interdendritic β. [ABSTRACT FROM AUTHOR]

Abstract The effect of dual aging treatment on phase morphology, composition, and microstructure evolution of a pre-deformed Ti 2 AlNb-based alloy were investigated in this study. The 1040 °C-pre-deformed alloys were dual aged at different temperature in O + β /B2 region followed by 750 °C-aged treatment. The experimental results show that coarse O lath and lamellar O + β /B2 were formed when the aging temperature is lower than 950 °C while globular O appeared when aging temperature is higher than 950 °C. The size and content of the O phase increase and decrease with the aging temperature, respectively. Widmanstätten O + β /B2 laths were obtained after secondary aging treatment at 750 °C when the first aging temperature is higher than 910 °C. Complete lamellar O + β /B2 precipitation and coarse O lath, in particular, were obtained in the 910 °C and 750 °C dual aged alloy with no Widmanstätten O + β /B2 laths appeared. The phase content is closely related to the aging temperature. When the 970 °C-aged alloy put into the secondary aging at 750 °C, coarse O laths which were fragmented or partially spheroidized and globular O during 970 °C-aged process were dissolved and the Widmanstätten O began to precipitate inside. Highlights • Effect of dual aging treatments on phase transformation and morphology of pre-deformed Ti 2 AlNb-based alloy was presented. • No Widmanstätten O + β /B2 laths appeared in the 910 °C and 750 °C dual aged alloy. • Dissolution and regeneration of O phase during aging treatment was discovered. [ABSTRACT FROM AUTHOR]

Cold-worked Inconel 718 alloy has successful applications in turbine engines due to its elevated strength. Otherwise, cold working will bring about the undesired ductility degradation accompanied with strength improving. Meanwhile, the susceptibility to δ precipitation is increased with modified morphology. To address these issues, this work aims expressing the effect of cold-rolling on hot ductility and the role of δ phase regarding differential morphologies in tensile behavior of Inconel 718 alloy at 650 °C. The results revealed that the ductility degradation can be attributed to the interactions of necking, microstructure instability and oxidation process tailored by cold-rolling. Nevertheless, appropriate cold-rolling is proved to enhance the resistance to crack propagation. The needlelike δ phase in strain-free structure tightly hindered the migration of horizontal GBs to enhance the strength but promote the intergranular brittle cracking, resulting in the degradation of ductility. In contrast, the δ phase with modified granular morphology in cold-rolled structures showed good deformation compatibility with horizontal GBs, playing a positive role in delaying necking occurrence and impeding crack propagation. In summary, the granular δ precipitation endowed cold-rolled Inconel 718 alloy with a superior combination of strength and ductility at 650 °C. [ABSTRACT FROM AUTHOR]

In DELTA processing of Inconel 718 alloy, δ phase plays a vital role in final grain refinement, whose precipitation during intermediate thermal cycles is tailored by prior wrought structure. Therefore, it is necessary to renew insights into the effect of wrought structure on the morphology and quantity of δ phase. From this point, this work was conducted to investigate the δ precipitation behaviors in wrought Inconel 718 alloy. The results show that the δ precipitation behaviors are tailored by prior dynamic recrystallization (DRX) degree. Under partial DRX, δ precipitation exhibits twofold characteristics of preferential GB-precipitation accompanied by sluggish intragranular precipitation; while modified into predominant homogeneous GB-precipitation by fully DRX, in spite of the presence of few intragranular δ precipitates. After hot deformation, the equilibrium δ content is improved and inversely proportional to DRX degree. After 24 h aging at 900 °C, the volume fraction of δ phase is 38.6% and 27.9% for partial and fully DRX, respectively, compared to 15.1% for strain-free state. The GB-δ nucleation process is accelerated due to the occurrence of DRX, and firstly finished in few minutes; differently, the intragranular δ nucleation is grain-dependent, that promoted within unrecrystallized grains but inhibited within DRX grains. The “encounter” phenomena are observed during δ growth, and some particular coalescence mechanisms are proposed. [ABSTRACT FROM AUTHOR]

The solid solubility of Si in a quaternary Al–4Mg–1Cu–0.5Si (wt%) alloy has been found to be only about 0.061 at% at 520 °C, and 0.093 at% at 560 °C, less than 1/10 of that in the ternary Al–Mg–Si system. The excess Si in the alloy remains as coarse Mg 2 Si dispersoids which produce no significant age-hardening effect. Interestingly, a small change of Si content (from 0.061 at.% to 0.093 at.%) in the solid solution of the alloy has been found to be effective in modifying precipitation microstructure by promoting the formation of fine Guinier–Preston–Bagaryasky (GPB) zones and postponing the formation of the S phase during ageing at 200 °C. Consequently, the quaternary alloy exhibits an enhanced age-hardening response during earlier-stage ageing. [ABSTRACT FROM AUTHOR]

Abstract: Refinement and modification performance of Al–Ti–B master alloy alone or with Al–P master alloy on primary Mg2Si in Al–12.67% Mg–10.33% Si alloy was investigated in this paper. The experimental results show that perfect effect can be obtained after addition of 1% Al–5Ti–1B master alloy into the Al–Mg–Si alloy. The morphologies of primary Mg2Si particulates change from dendritic to polygonal shape, and their average sizes decrease from ∼100 to ∼20μm. The ultimate tensile strength of the alloy increases. EPMA and TEM results show that TiB2 particles act as the nuclei of primary Mg2Si. Crystal lattice correspondence indicates TiB2 has a good lattice matching coherence relationship with Mg2Si, and the disregistry is only 4.64%. Furthermore, the addition of Al–5Ti–1B master alloy can significantly improve the refinement and modification effect of AlP on primary Mg2Si. It is also found that coupling particles of AlP and TiB2 exist in the center of primary Mg2Si. [Copyright &y& Elsevier]

The effect of the Ni/Cu-rich aluminides on the strength of Al–15%Mg 2 Si–Cu–Ni alloys at room and elevated temperatures was analyzed and revealed. The contribution of aluminides to alloys' strength is promoted with the temperature increasing from 25 °C to 350 °C. Both Mg 2 Si and aluminides strengthen Al matrix and carry the load at room temperature (25 °C). The thermal stable and interconnected aluminides play the leading role in load-carrying and load-transfer capacity at high temperature (350 °C), resulting in the pronounced contribution to alloys' strength. At 450 °C, aluminides lose load-bearing capacity and have no effect on the increase of alloys' strength. • Both Mg 2 Si and aluminides can effectively strengthen the Al matrix and carry the load at 25 °C. • The thermal stable and interconnected aluminides play the leading role in load-carrying and load-transfer capacity at 350 °C. • At 450 °C, the loss of load-bearing capacity of aluminides leads to a significant decrease of alloy's strength. [ABSTRACT FROM AUTHOR]

Precipitation evolution and strengthening effect of Al-15wt%Mg2Si-5wt%Cu alloy were studied by characterizing the change of morphology and tensile properties after ageing at 200 °C for various times. At the under-ageing condition (ageing for 0.5 h), metastable θ″, θ′, fine dots and σ phases are formed. After ageing for 1 h (peak-ageing condition), θ″ and θ′ are dominant phases, and the high number density of precipitates produces a significant improvement of the hardness (173 Hv) and ultimate tensile strength (356 MPa). As ageing time is prolonged (ageing for 24 h), plenty of σ phases are formed at the expense of θ"/θ′ phases and act as the main strengthening precipitates. The over-aged microstructure consequently causes a decrease in the hardness (134 Hv) and ultimate tensile strength (311 MPa) of the alloy. [ABSTRACT FROM AUTHOR]

Experiment reveals that the two-dimensional ordered phase of 3 × 3-Au/Pb alloy formed on Si(111)-[formula]-Pb surface is dominated by Au parallelogram tetramers (4P). However, theoretical results indicate that three-dimensional tetrahedral tetramers (4T) grown on such surface is more energetically favorable. Using first-principles methods, we show that the discrepancy originates from a lower potential energy barrier of formation of the metastable 4P phase than that of the formation of stable 4T phase in nucleation and subsequent growth processes from triangular trimers (3T) islands and from stronger competition between Au s and d states caused by spin-orbit coupling effects. [ABSTRACT FROM AUTHOR]

The microstructure evolutions during 1000 °C-1000 h long-term aging and relevant 800 ~ 840 °C/220 ~ 300 MPa creep behaviors of a novelly designed multiphase Ni 3 Al-based intermetallic alloy were investigated. Results showed this alloy presented excellent intermediate temperature creep properties even after 1000 °C-1000 h long-term aging. A kind of bimodal R-type γ′ rafting structures were observed in the 800 °C/220 MPa creep process of the long-term aged alloy, and the creep resistance levels of three kinds of γ′ rafting structures were evaluated to be in the order of bimodal R-type γ′ rafting structures > R-type γ′ rafting structure > N-type γ′ rafting structure. The 1000 °C-1000 h long-term aging can promote the inhomogeneous precipitation of Cr 4.6 MoNi 2.1 phase at the edge γ′+γ dendrite area, which can play the role of pinning dislocations and inhibiting the further rafting and coarsening of the R-type γ′ rafts during creep. Differed from Ni-based alloys and other Ni 3 Al-based alloys, this long-term aged alloy exhibited abnormal four-stage creep characteristics, which consisted of atypical short steady-state creep stage with step fluctuant feature and atypical long accelerated creep stage with creep rate increasing at a constant rate. [ABSTRACT FROM AUTHOR]

The microstructural evolution and phase transformation behavior of a designed multiphase Ni 3 Al-based superalloy in as-cast and spray casting states were investigated under thermal exposure at various temperatures. For the exposure temperatures range from 600 °C to 900 °C, the average size of γ′ precipitates in the dendritic dual (γ+γ′) areas of the two states alloys increases, and the morphology of γ′ changes to irregular shapes due to merging of neighboring particles and Ostwald ripening. After thermal exposure at 600 °C or 900 °C, bulky γ′ precipitates generate in the interdendritic regions of as-cast alloy, and the γ′ envelope develops a curved interface by aggregation. For the spray casting sample, high density of microtwins in γ′ precipitates are 'inherited' from the preceding L1 0 -type martensite phases after thermal exposure at 600 °C. However, the γ′ precipitates without any microtwins form after thermal exposure at 900 °C. • For as-cast alloy, bulky γ′ precipitates generate in β phase after thermal exposure at 600 °C or 900 °C. • For the spray casting sample, high density of microtwins in γ′ precipitates are 'inherited' after thermal exposure at 600 °C. • The γ′ precipitates without microtwins form in spray casting alloy after thermal exposure at 900°C. [ABSTRACT FROM AUTHOR]

The precipitation and evolution of intersected plate-like γ′ (γ′ P) phase within interdendritic β during long-term aging and its relevant influence on the creep behavior of a multiphase Ni 3 Al-based intermetallic alloy are investigated. Results show that the precipitation and growth of the γ′ P precipitates during 800 °C aging of original as-cast alloy are significantly rapid, which can be fully precipitated in 60 min to form the intersected morphology and continue to coarsen and grow during the extended aging and subsequent 800 °C creep processes. The γ′ P precipitates have the Bain orientation relationship (OR) of (100)γ′ P //(100) β and [0-11]γ′ P //[001] β with the interdendritic β-matrix. The precipitation of intersected γ′ P phase is sensitive to the elemental fluctuations in β-matrix, and the preceding high-temperature annealing treatment can significantly reduce the elemental inhomogenization and inhibit its generation within the interdendritic β. Precipitation of adequate intersected γ′ P phase can effectively inhibit the dislocation proliferation and crack initiation in the brittle interdendritic β during creep, resulted in the improved creep properties. Slightly rafted γ′ after long-term aging at 800 °C transitioned into R-type γ′-raft structure during creep at 800 °C/220 MPa, and can induce larger creep resistance than N-type γ′-raft structure. [ABSTRACT FROM AUTHOR]

Microstructure evolution, static coarsening mechanism and coarsening kinetics of a pre-deformed Ti 2 AlNb-based alloy during aging at 945 °C and 970 °C were investigated in this study. The corresponding microstructures were characterized by scanning electron microscopy and transmission electron microscopy. The results show that the pre-deformed alloy exhibited different morphologies aged at 945 °C and 970 °C. When the alloy was aged at 970 °C, the coarsening mechanism of the O precipitates was mainly controlled by the Ostwald ripening mechanism at first aging stage; after that, it was controlled by the boundary splitting mechanism and termination migration mechanism. Due to the difference in interface energy between the terminals and the long-axis direction of the O lath, diffusion process of solute atoms leads to the coarsening and fragmenting of O lath. The dimension of the O precipitates in the studied alloy continued to grow with the prolonged time during aging at 945 °C or 970 °C while the coarsening rate decreased. The static coarsening kinetic of the studied alloy was established in term of the modified Lifshitz-Slyozov-Wagner (LSW) theory. Static coarsening behavior of the pre-deformed alloy was mainly controlled by bulk diffusion at 945 °C and interfacial diffusion at 970 °C, respectively. • The thickness of O precipitates increased with aging temperature and time. • The microstructure consisted of lamellar O at 945 °C and changed from spherical to lath with aging time at 970 °C-aging. • Static coarsening behavior was controlled by Ostwald ripening, boundary splitting and termination migration mechanism. • Coarsening kinetics of the studied alloy can be established according to modified LSW theory. [ABSTRACT FROM AUTHOR]

In present work, the unique envelope structure (about 1.1 μm in width, and ~3 vol%) in an as-cast multiphase Ni 3 Al-based intermetallic alloy was characterized in detail, its formation mechanism during solidification and widening behavior during long-term aging process were analyzed, and relevant effects on creep behaviors of this Ni 3 Al-based intermetallic alloy under 800 oC/220 MPa were evaluated. Results show that the envelope structure located between the γ'+γ dendrite and interdendritic β phase is γ′-Ni 3 Al phase with ordered face-centered L1 2 structure, which plays a vital role in adjoining the heteroid γ'+γ dendrite/interdendritic β phases. The γ′-envelope possesses a coherent orientation relationship of [101] γ'-envelope //[01‾1] dendritic-γ with the dendritic-γ phase (face-centered A1 structure) due to their similar face-centered lattice type, while there exists a semi-coherent transition region between the γ′-envelope (face-centered L1 2 structure) and interdendritic β phase (body-centered B2 structure) to accommodate their relative lattice mismatching. The formation of γ′-envelope during solidification process is induced by nickel enrichment in the residual liquidoid after interdendritic β phase, and its widening during long-term aging at 800 oC is mainly controlled by the diffusion of Ni, Al, Hf and Mo atoms at the interfaces between γ'+γ dendrite/γ′-envelope/interdendritic β phase under thermal activations, involving the relevant phase transformation of β (NiAl) + γ (Ni) → γ' (Ni 3 Al). The widening of γ′-envelope resulted in a decrease of interdendritic β phase and an increase of γ′ phase in volume fraction, benefited in the improved creep rupture life and reduced steady-state creep rate. However, the γ′-envelope widening also plays a positive role in crack initiation during creep deformation, since besides the GBs and the semi-coherent interfaces of the γ′-envelope/interdendritic β, cracks are also found to initiate near the Cr 23 C 6 carbides surrounded by the widened γ′-envelope. [ABSTRACT FROM AUTHOR]

Grain size is industrially important for improving mechanical properties and formability. In order to investigate the effect of grain size on the hot workability and deformation mechanisms, Al-Zn-Mg-Cu alloys with different initial grain sizes, 241.2, 100.4 and 48.4 μm, were subjected to hot compression tests. Using the established constitutive equation, the rheological behavior is described and the influence of the initial grain on the thermal deformation behavior is analyzed. Electron back-scatter diffraction (EBSD) analysis was used to study the dynamic recrystallization (DRX) mechanisms of Al-Zn-Mg-Cu alloys with different initial grain sizes. The microstructures obtained under various deformation conditions were analyzed. The results show that decreasing the initial grain size enhances both the peak stress and the activation energy for thermal deformation. Meanwhile, the dynamic recrystallization mechanism varies depending on the initial grain size. Continuous dynamic recrystallization (CDRX) is the primary DRX nucleation mode for coarse-grained alloys. Nonetheless, discontinuous dynamic recrystallization (DDRX) is predominant for alloys with fine grains. • The flow stress behavior of alloys is influenced by T , ε ̇ and initial grain size. • Constitutive model was built and Q is calculated to predict the softening behavior. • The dominant mechanism for coarse-grained alloys is CDRX, while fine-grained is DDRX. [ABSTRACT FROM AUTHOR]

In order to further refine W particles and improve properties of traditional W-Cu alloys, an innovative freeze-drying and subsequent solid phase sintering technology was employed to fabricate ultrafine W-10/20/30Cu alloys in our work. Compared with W-Cu powder precursors synthesized by traditional technologies, the freeze-dried powders (35 nm) are finer and better dispersed, which possess ultrahigh sintering activity. After solid phase sintering, the relative density of prepared W-Cu alloys is as high as above 98%. Meanwhile, the Cu network possesses complete continuity and the W particles keep ultrafine size (178 nm), which is much smaller than W particles in W-Cu literatures. Compared with other traditional technologies, freeze-dried W-Cu alloys in our work possess the highest hardness (499 HV 0.2), taking into account high thermal conductivity (246.9 W/mk) and electrical conductivity (48.3% IACS), which provides a new insight for the further development of W-Cu alloy industries. [Display omitted] [ABSTRACT FROM AUTHOR]

Oxygen evolution reaction (OER) is crucial for electrochemical water splitting. However, corrosive issues in the alkaline environment impede its applications. Here, we explore the corrosion-resistant multiphase Ni 3 Al-based intermetallic alloy (Ni 3 Al alloy) as self-supported OER catalysts for the first time. A two-step treatment, annealing at high-temperature followed by acid oxidation is performed to improve the catalytic activity. Further investigations demonstrate that the annealing process can tune the size of γ′ precipitates, which will modify the strain effects caused by γ/γ′ lattice mismatch to boost the OER activity. The oxidation of Ni during the acid pretreatment can facilitate the formation of active high-valent Ni sites during the OER process, and further enhance the OER performance. The optimized electrode exhibits a low overpotential of 280 mV at 10 mA cm−2, a small Tafel slope of 73.5 mV dec−1 and long-term stability of 180 h in 1 M KOH solution. The findings in this paper will inspire the exploration of high stability OER catalysts and the functional application of Ni 3 Al alloy. • Ni 3 Al high-temperature alloy shows high OER activity and stability. • The boosted OER activity was ascribed to modified strain effect at γ/γ′ interface. • The formation of active high-valent Ni sites also promotes the catalytic activity. • Ni 3 Al alloy treated by acid oxidation has better catalytic performance for OER. [ABSTRACT FROM AUTHOR]

Microalloying of an Al-15%Mg 2 Si alloy (wt%) with 1%Cu produced an enhanced age-hardening response, increasing peak hardness and ultimate tensile strength due to a modified precipitation process. The solid solubility of Cu in Al-15%Mg 2 Si alloy effectively promotes the formation of the high number density of Guinier-Preston-Bagaryasky (GPB) zones combined with S precipitates. It suppresses β′ and β″ formation during ageing at 200 °C. Consequently, Al-15%Mg 2 Si-1%Cu alloy with the high density of GPB zones and S phase exhibited high peak hardness (121 HVN) and ultimate tensile strength (284 MPa), 42.4% and 49.5% higher than that of the Al-15%Mg 2 Si alloy, respectively. • The addition of 1% Cu in Al-15%Mg 2 Si alloy has a significant influence on precipitation during ageing at 200 °C. • Cu addition promotes the formation of GPB zones and S phase, instead of β′ and β″ precipitates. • The high number density of GPB zones and S phase produces an enhanced age-hardening response. [ABSTRACT FROM AUTHOR]

To investigate the effect of initial microstructure on the hot deformation behavior of a Ni 3 Al-based alloy, the isothermal compression tests were conducted on the studied alloy with two different microstructures at 1100 °C and 1200 °C with strain rate range of 0.01–1 s−1. The results show that the hot deformation behavior of the Ni 3 Al-based alloy is sensitive to the initial microstructure, particularly under low deformation temperature and high strain rate. The flow curves signified that the peak stress and the magnitude of the stress drop for the alloy with coarse γ′ precipitates are higher than those for the alloy with fine γ′ precipitates at a given temperature and strain rate. Besides, it is found that the critical conditions are affected by deformation temperature, strain rate and initial microstructure. Both the critical stress and strain of dynamic recrystallization significantly decreases with the increase of strain rate and decrease of deformation temperature. For different initial microstructures, the critical stress and strain for the alloy with coarse γ′ precipitates are higher than that for the alloy with fine γ′ precipitates. The critical ratios of the studied alloy were also determined. The dissolution of coarse γ′ precipitates easily occurs and the dissolution rate can be accelerated at the sufficiently high temperature, which leads to that the effect of initial microstructure on hot deformation behavior of the Ni 3 Al-based alloy is weakened. • Hot deformation behavior of Ni 3 Al-based alloy with two microstructures was studied. • Hot deformation behavior is sensitive to initial microstructure. • Critical stress and strain were determined at various deformation conditions. • Critical conditions affected by initial microstructure, strain rate and temperature. [ABSTRACT FROM AUTHOR]

In this paper, the interactions between interstitial oxygen atoms and substitutional atoms in Ti–Nb–O alloys were investigated by first-principles calculations based on the density functional theory. The binding energies and formation energies of oxygen atoms sited were calculated in various interstices with different geometries, element species, and atomic arrangements. The calculation results showed that there was a strong repulsive binding energy between the interstitial oxygen and the substitutional niobium atoms. On this basis, the energetically favored sites and the distribution of interstitial oxygen atoms in Ti–Nb–O alloys were discussed and demonstrated by an electron energy loss spectroscopy method. Furthermore, the diffusion activation energies of interstitial oxygen atoms based on the formation energy results were presented, and the diffusion behaviors of interstitial oxygen atoms in Ti–Nb–O alloys were also discussed. [ABSTRACT FROM AUTHOR]

The present work investigates the change in the microstructure and the improvement in the microhardness in the Mo-modified Ti 2 AlNb alloys induced by solution treatment. Typical Widmanstätten O + B2 structure was observed in the alloys directly aged in the O + B2 phase region, with a lower hardness value compared to that of the alloy without Mo addition. Well-arranged O + B2 colonies were observed in the specimen solution treated in single B2 phase region, and it exhibited a favorable hardness performance of up to 620 HV. Besides the precipitation hardening of the O phase, the solution of Mo in B2 phase should be responsible for the enhancement of hardness. Further microstructural observation proved that the substitution of Mo for Nb in the lattice of B2 phase induced the lattice distortion mainly the (200) and (211) planes, and the induced entanglement of dislocations also contributed to the enhancement of hardness. Image 1 • Well-arranged O + B2 structure is obtained in Ti 2 AlNb alloys by Mo addition. • Solution treatment and ageing enhanced hardness of Mo-modified Ti2AlNb alloys. • Lattice distortion of B2, especially along (211) face, is induced by adding Mo. • Orientation relationship of (211) B2 //(26 1 ¯ ) O is identified in the aged Ti 2 AlNb alloy. [ABSTRACT FROM AUTHOR]

The microstructure control of multiphase Ni 3 Al-based intermetallic alloy by traditional heat treatment is still limited, especially size and volume fraction of interdendritic β phase cannot be effectively reduced, due to the limitation of interdendritic β phase dissolution temperature close to the melting point of the alloy. In present work, the temperature gradient and solidification rate of the alloy under different laser remelting parameters were calculated by the Rosenthal equation. Afterward, the as-cast multiphase Ni 3 Al-based intermetallic alloy (19.4 vol% of interdendritic β phase) was remelted under different laser parameters of 3000–4500 W laser power and 5–20 mm/s laser scanning rate. And the effects of different laser remelting parameters on the microstructure, room temperature, and 980 °C high-temperature tensile properties of multiphase Ni 3 Al-based intermetallic alloy were studied. The results show that laser remelting treatment with the appropriate parameters (4000 W laser power and 10 mm/s laser scanning rate) can effectively reduce the size and volume fraction of interdendritic β phase and the morphology and size of γ′ phase in the γ′+γ dendrite, and significantly refine the microstructure of alloy, thus effectively improving the room temperature and 980 °C tensile properties of multiphase Ni 3 Al-based intermetallic alloy. Under the optimum laser remelting parameters of 4000 W-10 mm/s, the tensile strength at room temperature and 980 °C of the corresponding laser remelting alloy can reach 1215 MPa and 280 MPa, respectively, and the elongations after fracture of the corresponding alloy are 14.09% and 6.41%, respectively. [ABSTRACT FROM AUTHOR]

In the present study, to determine the mechanism of the breakaway phenomenon, high-temperature steam oxidation of zirconium alloys was carried out at 1000 ℃. Additionally, the oxide microstructures of samples were characterized using multiple techniques. The results show that the microchannels caused an undulated structure of the oxide/metal (O-M) interface, which is a function of time. Meanwhile, after breakaway oxidation, a formation mechanism of cracks, based on vacancies, was proposed according to the crack morphologies. Finally, taking the microchannels as the center, a mechanism is established to explain the high-temperature oxidation behavior of the Zr alloy. • The undulation structure of oxide/metal (O-M) interface is caused by the formation of microchannels at different stages. • Raman spectroscopy shows that zirconia phase distributions correspond to the local curvature of the O-M interface. • Two different mechanisms on cracks formation before and after breakaway oxidation are established. • A breakaway oxidation mechanism focusing on formation of microchannels vertical to the O-M interface is proposed. [ABSTRACT FROM AUTHOR]

In this work, eutectic Si modification grade formula was established and effect of Sr element on modification level and mechanical properties of Al-9Si-0.4Mg-0.1Cu alloy was studied. Thermal analysis, SEM, EPMA and TEM were applied to analyze cooling curves, microstructure and Si twinning of the alloys. The results showed that precision and accuracy of eutectic Si modification grade formula were significantly improved and quantitative evaluation modification level of eutectic Si could be achieved. Modification level of eutectic Si increased first and then decreased with Sr content increasing. Though twin plane re-entrant edge (TPRE) mechanism and impurity induced twinning (IIT) mechanism were effective under certain conditions, they cannot fully explain the modification mechanism of eutectic Si in Al-9Si-0.4Mg-0.1Cu alloy. Sr element caused constitutional undercooling at the front of solid-liquid interface, promoted the formation of fine Al 2 Si 2 Sr phases and nano-precipitates, resulting in the changes of size and morphology of eutectic Si. With the improvement of eutectic Si modification level, the UTS and El of the alloys increased. While, adding excessive Sr led to the formation of coarse Al 2 Si 2 Sr, which was detrimental to the improvement of modification level and mechanical properties of alloy. • Modification level function suitable for quantitatively evaluate modification grade of eutectic Si was established. • Fine Al 2 Si 2 Sr phases and nano-precipitates change the size and morphology of eutectic Si. • Coarse Al 2 Si 2 Sr reduces the modification level of eutectic Si and the mechanical properties of alloy. [ABSTRACT FROM AUTHOR]

Oxide dispersion strengthened tungsten (ODS-W) alloys are a class of important materials that widely used for high temperature applications due to their unique physical and mechanical properties. However, the insufficient ductility of ODS-W alloys at low or high temperatures limits their widespread application, especially as structural materials. It is always a great challenge to efficiently disperse oxide second phase nanoparticles within W matrix for achieving ODS-W alloy with excellent balance of strength and ductility. Here we report an innovative strategy for the fabrication of high-performance ODS-W alloy with strength of up to 1134 MPa and total elongation over 30% at 700 °C. The innovative route involves the preparation of W–Y 2 O 3 composite powder precursor via alkaline hydrothermal method and subsequent low-temperature sintering in hydrogen atmosphere. The internal flake structure with uniformly dispersed Y component and external coating of W component on this flake structure can be formed in alkaline hydrothermal product and then be passed on to the reduced W–Y 2 O 3 composite nanopowders. As a result, the distribution of second phase oxide particles in the low-temperature sintered alloys using these composite nanopowders as precursor is optimized significantly including an increase in the volume fraction of intragranular oxide particles and a decrease in the size of both intergranular and intragranular oxide particles. Besides, the average W grain size is also refined dramatically (410 nm). More importantly, the intragranular oxide particle / W matrix interface is regulated to be coherent relationship. The combined action of these microstructural factors is responsible for the significantly enhanced strength and ductility of ODS-W alloy. These results indicate that the newly developed alkaline hydrothermal method in our work is a promising way to fabricate high performance ODS tungsten based alloys. [ABSTRACT FROM AUTHOR]

• The fundamental role of atomic structure on thermal oxidation of alloys is revealed. • Al-Zr was selected as the model alloy system to circumvent preferential oxidation effect. • Structural order of parent alloys influences strikingly oxide compositions and oxidation kinetics. • Underlying mechanisms were disclosed by molecular dynamics simulations. • The study offers a new perspective in the field of surface engineering. To reveal the fundamental role of structural order of the parent alloy on the oxidation mechanism, thermal oxidation of amorphous and crystalline Al–Zr alloys with identical compositions were investigated in detail. An (Al 0.68 Zr 0.32)O 1.66 amorphous oxide emerged on crystalline Al 2 Zr, whereas an (Al 0.33 Zr 0.67)O 1.83 amorphous oxide formed on amorphous Al 68at.% Zr 32at.% under the same conditions. Oxidation kinetics was fast and linear for crystalline Al 2 Zr but slow and parabolic for amorphous Al 68at.% Zr 32at.%. The underlying mechanisms of such striking differences were disclosed by molecular dynamics simulations. The findings thus offer new prospects in the field of surface engineering. [ABSTRACT FROM AUTHOR]