Search Results (49)

To enhance the mechanical properties of NbMoTaW refractory high-entropy alloys (RHEAs), Si was added at varying concentrations (x = 0, 0.25, and 0.5) via vacuum induction levitation melting (re-melted six times for homogeneity). The microstructure and mechanical properties of NbMoTaWSi_x (x = 0, 0.25, and 0.5) RHEAs were characterized using scanning electron microscopy (SEM), universal testing, microhardness testing, and tribological equipment. Experimental results manifested that Si addition induces the formation of the (Nb,Ta)₅Si₃ phase, and the volume fraction of the silicide phase increases with higher Si content, which significantly improves the alloy’s strength and hardness but deteriorates its plasticity. Enhanced wear resistance with Si addition is attributed to improved hardness and oxidation resistance. Tribological tests confirm that Si₃N₄ counterfaces are optimal for evaluating RHEA wear mechanisms. This work can provide guidance for the fabrication of RHEAs with excellent performance. Full article

High-entropy silicide (MeSi₂) coating was prepared by the slurry method and silicon infiltration method using Mo, Cr, Ta, Nb, W, and Si elemental powders as raw materials. The coating consisted of four layers, including a porous MeSi₂ layer, a (CrTa)Si layer, a TaSi₂ layer, and a Ta₅Si₃ layer from outside to inside. At 600 °C, Si was preferentially oxidized to form SiO₂ oxide film. The mass gain rate of the coating was 0.2 mg/cm² over a period of 100 h oxidation, eliminating the phenomenon of low-temperature pulverization. At 1200 °C, MeSi₂ coating had a protection time of 20 h. During the oxidation process, the coating generated metal oxides, forming a thin SiO₂ oxide film. TaSi₂ and Ta₅Si₃ gradually transformed into Ta₂O₅, and the coating eventually failed. Full article

A novel TiB₂-CeO₂-modified (Nb,Mo,Cr,W)Si₂ coating was prepared on a Nb-5W-2Mo-1Zr alloy substrate using two-step slurry sintering and halide-activated pack cementation to address the limitations of a single NbSi₂ coating in meeting the service requirements of niobium alloys at elevated temperatures. At 1700 °C, the static oxidation life of the coating exceeded 20 h, thus indicating excellent high-temperature oxidation resistance. This was due to the formation of a TiO₂-SiO₂-Cr₂O₃ composite oxide film on the coating surface, which, due to low oxygen permeability, effectively prevented the inward infiltration of oxygen. Additionally, the dense structure of the composite coating further enhanced this protective effect. The composite coating was able to withstand over 1600 thermal shock cycles from room temperature to 1700 °C, and its excellent thermal shock performance could be attributed to the formation of MoSi₂, CrSi₂, and WSi₂ from elements such as Mo, Cr, and W, which were added during modification. In addition to adjusting the difference in thermal expansion coefficients between the layers of composite coatings to reduce the thermal stress generated by thermal shock cycles, the formation of silicide compounds also improved the overall fracture toughness of the coating and thereby improved its thermal shock resistance. Full article

Abstract: The refractory complex concentrated alloy (RCCA) 5Al–5Cr–5Ge–1Hf–6Mo–33Nb–19Si–20Ti–5Sn–1W (at.%) was studied in the as-cast and heat-treated conditions. The partitioning of solutes in the as-cast and heat-treated microstructures and relationships between solutes, between solutes and the parameters VEC and Δχ, and between these parameters, most of which are reported for the first time for metallic UHTMs, were shown to be important for the properties of the stable phases A15–Nb₃X and the D8_m βNb₅Si₃. The nano-hardness and Young’s modulus of the A15–Nb₃X and the D8_m βNb₅Si₃ of the heat-treated alloy were measured using nanoindentation and changes in these properties per solute addition were discussed. The aforementioned relationships, the VEC versus Δχ maps and the VEC, Δχ, time, or VEC, Δχ, Young’s modulus or VEC, Δχ, nano-hardness diagrams of the phases in the as-cast and heat-treated alloy, and the properties of the two phases demonstrated the importance of synergy and entanglement of solutes, parameters and phases in the microstructure and properties of the RCCA. The significance of the new data and the synergy and entanglement of solutes and phases for the design of metallic ultra-high temperature materials were discussed. Full article

The Nb-16Si-24Ti-2Al-2Cr alloy was prepared by plasma rotating electrode process (PREP) technology and the hot-pressing (HP) method, and the effects of sintering temperature on the microstructure, mechanical properties and fracture behavior were investigated. The HP alloys sintered at temperatures below 1400 °C are composed of Nbss (Nb solid solution), Nb₃Si and Nb₅Si₃ phases. When the sintering temperature reaches 1450 °C, the Nb₃Si phase is completely decomposed into Nbss and Nb₅Si₃ phases. Meanwhile, the microstructure coarsens significantly. Compared with the cast alloy, the HP alloy shows better mechanical properties. The fracture toughness of the alloy sintered at 1400 °C reaches 20.2 MPa·m^1/2, which exceeds the application threshold. The main reason for the highest fracture toughness is attributed to the decomposition of large-sized brittle Nb₃Si phase and the formation of a fine microstructure, which greatly increases the number of phase interfaces and improves the chance of crack deflection. In addition, the reduction in the size and content of silicides also reduces their plastic constraints on the ductile Nbss phase. Full article

In this work, the refractory complex concentrated alloy (RCCA) 3.5Al–4Cr–6Ge–1Hf–5Mo–36Nb–22Si–1.5Sn–20Ti–1W (at.%) was studied in the as cast and heat treated conditions (100 h or 200 h at 1500 °C). There was strong macrosegregation of Si in the 0.6 kg button/ingot of the cast alloy, in which A2 solid solution, D8_m βNb₅Si₃, C14-NbCr₂ Laves phase and Ti_ss and a ternary eutectic of the A2, D8_m and C14 phases were formed. The partitioning of Ti in the as cast and heat treated microstructure and its relationships with other solutes was shown to be important for the properties of the A2 solid solution and the D8_m βNb₅Si₃, which were the stable phases at 1500 °C. The near surface microstructure of the alloy was contaminated with oxygen after heat treatment under flowing Ar. For the aforementioned phases, it was shown, for the first time, that there are relationships between solutes, between solutes and the parameters VEC, Δχ and δ, between the said parameters, and between parameters and phase properties. For the contaminated with oxygen solid solution and silicide, trends in relationships between solutes, between solutes and oxygen content and between the aforementioned parameters and oxygen content also were shown for the first time. The nano-hardness and Young’s modulus of the A2 solid solution and the D8_m βNb₅Si₃ of the as cast and heat-treated alloy were measured using nanoindentation. Changes of nano-hardness and Young’s modulus of the A2 solid solution and D8_m βNb₅Si₃ per solute addition for this multiphase RCCA were discussed. The nano-hardness and Young’s modulus of the solid solution and the βNb₅Si₃, respectively, were 9.5 ± 0.2 GPa and 177.4 ± 5.5 GPa, and 17.55 ± 0.5 GPa and 250.27 ± 6.3 GPa after 200 h at 1500 °C. The aforementioned relationships and properties of the two phases demonstrated the importance of synergy and entanglement of solutes, parameters and phases in the microstructure and properties of the RCCA. Implications of synergy and entanglement for the design of metallic ultra-high temperature materials were emphasised. Full article

Si-Co diffusion coatings were prepared on a Y-modified TiAl-Nb alloy using the pack cementation process. The structures of the coatings prepared at different temperatures (1050, 1080, and 1120 °C) and pack Co contents (5, 10, and 20 wt.%) were comparatively studied. The coatings possessed the typical structure of a (Ti,X)Si₂+Ti₅Si₄ (X represents Nb and Al elements) outer layer with a Co-rich superficial zone, a Ti₅Si₄+Ti₅Si₃ middle layer, and a TiAl₂ inner layer. Increasing the co-deposition temperature in the range of 1050–1120 °C led to a larger coating thickness but a more-porous coating structure, while increasing the pack Co contents in the range of 5–20 wt.% caused a lower coating growth rate. The formation of the Si-Co diffusion coating followed an orderly process of depositing Si first and then Co. The Si-Co diffusion coating had much better anti-oxidation performance than both the TiAl-Nb substrate and pure silicide coating. After undergoing oxidation at 1000 °C for 100 h, the oxidation parabolic rate of the Si-Co diffusion coating was approximately 6.16 × 10⁻³ mg²/cm⁴h¹, which was lower than those of the TiAl-Nb substrate by about two orders of magnitude and pure silicide coating by about one order of magnitude. Full article

Near-alpha titanium alloys are widely used in aeroengine blades due to their excellent specific strength and mechanical properties. The mechanical properties of near-α titanium alloys are closely related to the evolution of the microstructure and precipitates. In this paper, the microstructure and mechanical properties of a new type of multi-component near-α titanium alloy sheet after rolling, 700 °C aging, and 800 °C aging were studied. The results show that the strength of the alloy after aging at 700 °C increases from 1156 MPa to 1304 MPa, respectively, but decreases to 1246 MPa with the aging temperature increasing. The ductility of the alloy aged at 700 °C is lower than that of the rolled state, but the ductility increases slightly with the aging temperature increasing. The effect of aging heat treatment on the microstructure and precipitation behavior of alloy plates has been studied and compared with alloys before aging. After heat treatment, the content of primary α decreases from 25% to 5%, respectively. Two kinds of silicide precipitate at different positions, with the large-size spherical silicide being (Ti, Zr, Nb)₅Si₃, and the small-size fusiform silicide being (Ti, Zr, Nb)₆Si₃, respectively. Ti₃Al was precipitated in the primary α phase, during the aging process. The silicides exhibit the strengthening effect on the alloy, but the effect weakens when the silicides grow up. The loss in ductility is mainly attributed to the precipitation of the α₂ phase after aging treatment. However, ductility is improved after applying higher aging temperatures as the size of the α₂ phase becomes smaller, and the distribution of them tends to become dispersed. Full article

To improve the heat resistance of titanium alloys, the effects of Y content on the precipitation behavior, oxidation resistance and high-temperature mechanical properties of as-cast Ti-5Al-2.75Sn-3Zr-1.5Mo-0.45Si-1W-2Nb-xY (x = 0.1, 0.2, 0.4) alloys were systematically investigated. The microstructures, phase evolution and oxidation scales were characterized by XRD, Laser Raman, XPS, SEM and TEM. The properties were studied by cyclic oxidation as well as room- and high-temperature tensile testing. The results show that the microstructures of the alloys are of the widmanstätten structure with typical basket weave features, and the prior β grain size and α lamellar spacing are refined with the increase of Y content. The precipitates in the alloys mainly include Y₂O₃ and (TiZr)₆Si₃ silicide phases. The Y₂O₃ phase has specific orientation relationships with the α-Ti phase: (002)_Y2O3 // ($\overline{1}$$\overline{1}$20)_α-Ti, [110]_Y2O3 // [$\overline{4}$401]_α-Ti. (TiZr)₆Si₃ has an orientation relationship with the β-Ti phase: (02$\overline{2}$$\overline{1}$)_(TiZr)6Si3 // (011)_β-Ti, [$\overline{1}$2$\overline{1}$6]_(TiZr)6Si3 // [04$\overline{4}$]_β-Ti. The 0.1 wt.% Y composition alloy has the best high-temperature oxidation resistance at different temperatures. The oxidation behaviors of the alloys follow the linear-parabolic law, and the oxidation products of the alloys are composed of rutile-TiO₂, anatase-TiO₂, Y₂O₃ and Al₂O₃. The room-temperature and 700 °C UTS of the alloys decreases first and then increases with the increase of Y content; the 0.1 wt.% Y composition alloy has the best room-temperature mechanical properties with a UTS of 1012 MPa and elongation of 1.0%. The 700 °C UTS and elongation of the alloy with 0.1 wt.% Y is 694 MPa and 9.8%, showing an optimal comprehensive performance. The UTS and elongation of the alloys at 750 °C increase first and then decrease with the increase of Y content. The optimal UTS and elongation of the alloy is 556 MPa and 10.1% obtained in 0.2 wt.% Y composition alloy. The cleavage and dimples fractures are the primary fracture mode for the room- and high-temperature tensile fracture, respectively. Full article

Refractory metal (RM) M₅Si₃ silicides are desirable intermetallics in metallic ultra-high temperature materials (UHTMs), owing to their creep properties and high Si content that benefits oxidation resistance. Of particular interest is the alloyed Nb₅Si₃ that forms in metallic UHTMs with Nb and Si addition. The choice of alloying elements and type of Nb₅Si₃ that is critical for achieving a balance of properties or meeting a property goal in a metallic UHTM is considered in this paper. Specifically, the different types of alloyed “normal” Nb₅Si₃ and Ti-rich Nb₅Si₃, namely “conventional”, “complex concentrated” (CC) or “high entropy” (HE) silicide, in metallic UHTMs with Nb and Si addition were studied. Advanced metallic UHTMs with additions of RMs, transition metals (TMs), Ge, Sn or Ge + Sn and with/without Al and with different Ti, Al, Cr, Si or Sn concentrations were investigated, considering that the motivation of this work was to support the design and development of metallic-UHTMs. The study of the alloyed silicides was based on the Nb/(Ti + Hf) ratio, which is key regarding creep, the parameters VEC and Δχ and relationships between them. The effect of alloying additions on the stability of “conventional”, CC or HE silicide was discussed. The creep and hardness of alloyed Nb₅Si₃ was considered. Relationships that link “conventional”, CC or HE bcc solid solution and Nb₅Si₃ in the alloy design methodology NICE (Niobium Intermetallic Composite Elaboration) were presented. For a given temperature and stress, the steady state creep rate of the alloyed silicide, in which TMs substituted Nb, and Al and B substituted Si, depended on its parameters VEC and Δχ and its Nb/(Ti + Hf) ratio, and increased with decreasing parameter and ratio value, compared with the unalloyed Nb₅Si₃. Types of alloyed Nb₅Si₃ with VEC and Δχ values closest to those of the unalloyed Nb₅Si₃ were identified in maps of alloyed Nb₅Si₃. Good agreement was shown between the calculated hardness and chemical composition of Nb₅Si₃ and experimental results. Full article

Near-α Ti alloys find themselves in advanced aeroengines for applications of up to 600 °C, mainly as compressor components owing to their superior combination of ambient- and elevated-temperature mechanical properties and oxidation resistance. We evaluated, ranked, and selected near-α Ti alloys in the current literature for high-temperature applications in aeroengines driven by decision science by integrating multiple attribute decision making (MADM) and principal component analysis (PCA). A combination of 12 MADM methods ranked a list of 105 alloy variants based on the thermomechanical processing (TMP) conditions of 19 distinct near-α Ti alloys. PCA consolidated the ranks from various MADMs and identified top-ranked alloys for the intended applications as: Ti-6.7Al-1.9Sn-3.9Zr-4.6Mo-0.96W-0.23Si, Ti-4.8Al-2.2Sn-4.1Zr-2Mo-1.1Ge, Ti-6.6Al-1.75Sn-4.12Zr-1.91Mo-0.32W-0.1Si, Ti-4.9Al-2.3Sn-4.1Zr-2Mo-0.1Si-0.8Ge, Ti-4.8Al-2.3Sn-4.2Zr-2Mo, Ti-6.5Al-3Sn-4Hf-0.2Nb-0.4Mo-0.4Si-0.1B, Ti-5.8Al-4Sn-3.5Zr-0.7Mo-0.35Si-0.7Nb-0.06C, and Ti-6Al-3.5Sn-4.5Zr-2.0Ta-0.7Nb-0.5Mo-0.4Si. The alloys have the following metallurgical characteristics: bimodal matrix, aluminum equivalent preferably ~8, and nanocrystalline precipitates of Ti₃Al, germanides, or silicides. The analyses, driven by decision science, make metallurgical sense and provide guidelines for developing next-generation commercial near-α Ti alloys. The investigation not only suggests potential replacement or substitute for existing alloys but also provides directions for improvement and development of titanium alloys over the current ones to push out some of the heavier alloys and thus help reduce the engine’s weight to gain advantage. Full article

In this work, the RM(Nb)IC alloy Nb–30Ti–10Si–5Cr–5Sn–3Fe–2Al–2Hf (NV2) was studied in the as-cast and heat-treated conditions; its isothermal oxidation at 700, 800 and 900 °C and its room temperature hardness and specific strength were compared with other Sn-containing RM(Nb)ICs—in particular, the alloy Nb–24Ti–18Si–5Cr–5Fe–5Sn (NV5)—and with RCCAs and RHEAs. The addition of Fe (a) stabilised Nb_ss; A15–Nb₃X (X = Al, Si and Sn) and Nb₃Si; metastable Nb₃Si-m’ and Nb₅Si₃ silicides; (b) supported the formation of eutectic Nb_ss + Nb₅Si₃; (c) suppressed pest oxidation at all three temperatures and (d) stabilised a Cr- and Fe-rich phase instead of a C14–Nb(Cr,Fe)₂ Laves phase. Complex concentrated (or compositionally complex) and/or high entropy phases co-existed with “conventional” phases in all conditions and after oxidation at 800 °C. In NV2, the macrosegregation of Si decreased but liquation occurred at T >1200 °C. A solid solution free of Si and rich in Cr and Ti was stable after the heat treatments. The relationships between solutes in the various phases, between solutes and alloy parameters and between alloy hardness or specific strength and the alloy parameters were established (parameters δ, Δχ and VEC). The oxidation of NV2 at 700 °C was better than the other Sn-containing RM(Nb)ICs with/without Fe addition, even better than RM(Nb)IC alloys with lower vol.% Nb_ss. At 800 °C, the mass change of NV2 was slightly higher than that of NV5, and at 900 °C, both alloys showed scale spallation. At 800 °C, both alloys formed a more or less continuous layer of A15–Nb₃X below the oxide scale, but in NV5, this compound was Sn-rich and severely oxidised. At 800 °C, in the diffusion zone (DZ) and the bulk of NV2, Nb_ss was more severely contaminated with oxygen than Nb₅Si₃, and the contamination of A15–Nb₃X was in-between these phases. The contamination of all three phases was more severe in the DZ. The contamination of all three phases in the bulk of NV5 was more severe compared with NV2. The specific strength of NV2 was comparable with that of RCCAs and RHEAs, and its oxidation at all three temperatures was significantly better than RHEAs and RCCAs. Full article

In this paper, we present a systematic study of the as-cast and heat-treated microstructures of three refractory metal intermetallic composites based on Nb (i.e., RM(Nb)ICs), namely the alloys EZ2, EZ5, and EZ6, and one RM(Nb)IC/RCCA (refractory complex concentrated alloy), namely the alloy EZ8. We also examine the hardness and phases of these alloys. The nominal compositions (at.%) of the alloys were Nb-24Ti-18Si-5Hf-5Sn (EZ2), Nb-24Ti-18Si-5Al-5Hf-5Sn (EZ5), Nb-24Ti-18Si-5Cr-5Hf-5Sn (EZ6), and Nb-24Ti-18Si-5Al-5Cr-5Hf-5Sn (EZ8). All four alloys had density less than 7.3 g/cm³. The Nb_ss was stable in EZ2 and EZ6 and the C14-NbCr₂ Laves phase in EZ6 and EZ8. In all four alloys, the A15-Nb₃X (X = Al,Si,Sn) and the tetragonal and hexagonal Nb₅Si₃ were stable. Eutectics of Nb_ss + Nb₅Si₃ and Nb_ss + C14-NbCr₂ formed in the cast alloys without and with Cr addition, respectively. In all four alloys, Nb₃Si was not formed. In the heat-treated alloys EZ5 and EZ8, A15-Nb₃X precipitated in the Nb₅Si₃ grains. The chemical compositions of Nb_ss + C14-NbCr₂ eutectics and some Nb₅Si₃ silicides and lamellar microstructures corresponded to high-entropy or complex concentrated phases (compositionally complex phases). Microstructures and properties were considered from the perspective of the alloy design methodology NICE. The vol.% Nb_ss increased with increasing Δχ_Nbss. The hardness of the alloys respectively increased and decreased with increasing vol.% of A15-Nb₃X and Nb_ss. The hardness of the A15-Nb₃X increased with its parameter Δχ, and the hardness of the Nb_ss increased with its parameters δ and Δχ. The room-temperature-specific strength of the alloys was in the range 271.7 to 416.5 MPa cm³g⁻¹. The effect of the synergy of Hf and Sn, or Hf and B, or Hf and Ge on the macrosegregation of solutes, microstructures, and properties of RM(Nb)ICs/RCCAs from this study and others is compared. Phase transformations involving compositionally complex phases are discussed. Full article

Electron beam directed energy deposition (EB-DED) is a promising manufacturing process for the fabrication of large-scale, fully dense and near net shape metallic components. However, limited knowledge is available on the EB-DED process of titanium alloys. In this study, a near-α high-temperature titanium alloy Ti60 (Ti-5.8Al-4Sn-4Zr-0.7Nb-1.5Ta-0.4Si) was fabricated via EB-DED. The chemical composition, microstructure, tensile property (at room temperature and 600 °C), and creep behavior of the fabricated alloy were investigated and compared with those of the conventional wrought lamellar and bimodal counterparts. Results indicated that the average evaporation loss of Al and Sn was 10.28% and 5.01%, respectively. The microstructure of the as-built alloy was characterized by coarse columnar grains, lamellar α, and the precipitated elliptical silicides at the α/β interfaces. In terms of tensile properties, the vertical specimens exhibited lower strength but higher ductility than the horizontal specimens at both room temperature and 600 °C. Furthermore, the tensile creep strain of the EB-DED Ti60 alloy measured at 600 °C and 150 MPa for 100 h under as-built and post-deposition STA conditions was less than 0.15%, which meets the standard requirements for the wrought Ti60 alloy. The creep resistance of the EB-DED Ti60 alloy was superior to that of its wrought bimodal counterpart. Full article

Low-density materials show promising prospects for industrial application in engineering, and have remained a research hotspot. The ingots of Al₁₅Zr₄₀Ti₂₈Nb₁₂Cr₅, Al₁₅Zr₄₀Ti₂₈Nb₁₂Mo₅ and Al₁₅Zr₄₀Ti₂₈Nb₁₂Si₅ high-entropy alloys were prepared using an arc melting method. With the addition of the Cr, Mo, and Si, the phase structures of these alloys changed to a dual phase. The Cr and Mo promote the formation of the B2 phase, while the Si promotes the formation of a large amount of the silicides. The compression yield strengths of these alloys are ~1.36 GPa, ~1.27 GPa, and ~1.35 GPa, respectively. The addition of Si and Cr significantly reduces the compression ductility, and the Al₁₅Zr₄₀Ti₂₈Nb₁₂SiMo₅ high-entropy alloy exhibits excellent comprehensive mechanical properties. This work investigated the influence of Cr, Mo, and Si on the phase structures and properties of the low-density Al-Zr-Ti-Nb high-entropy alloys, providing theoretical and scientific support for the development of advanced low-density alloys. Full article