Evolution of phases in Al55Ni30Pd15 alloy at temperatures up to 600 °C

The originally as-cast Al₅₅Ni₃₀Pd₁₅ alloy was investigated during continuous heating from room temperature to 600 °C and after annealing at 600 °C for respective 455, 2650, and 4050 h. In the investigation the synchrotron X-ray diffraction and the high-resolution scanning electron microscopy inclusive of the energy-dispersive X-ray spectroscopy were used. In all the investigated conditions β-(Ni,Pd)Al (Pm3m) and Al₃(NiPd)₂ (P3m1) phases were identified. After 2650 h of annealing the former phase was found to be separated into two isostructural modifications differing from one another in Pd- and Ni-contents. The annealing for 455 h contributed to the decrease of lattice parameters in the Al₃NiPd phase compared to the original as-cast condition.     

Microstructure and physical properties of PVD TiN/（Ti, Al）N multilayer coatings

Magnetron sputtered （Ti, Al） N monolayer and TiN/（Ti, Al） N multilayer coatings grown on cemented carbide substrates were studied by using energy dispersive X-ray spectroscopy （EDX）, scanning electron microscopy （SEM）, nanoindentation, Rockwell A indentation test, strength measurements and cutting tests. The results show that the （Ti, Al）N monolayer and TiN/（Ti, Al）N multilayer coatings perform good affinity to substrate, and the TiN/（Ti, Al）N multilayer coating exhibits higher hardness, higher toughness and better cutting performance compared with the （Ti, Al）N monolayer coating. Moreover, the strength measurement indicates that the physical vapour deposition （PVD） coating has no effect on the substrate strength.     

Mechanical properties of as-fabricated and 2300 °C annealed tungsten wire tested up to 600 °C

Recent efforts dedicated to the mitigation of tungsten brittleness have demonstrated that tungsten fiber-reinforced composites acquire pseudo ductility even at room temperature. Crack extension and fracture process is basically defined by the strength of tungsten fibers. Here, we move forward and report the results of mechanical and microstructural investigation of different grades of W wire with a diameter of 150 μm at elevated temperature up to 600 °C. The results demonstrated that potassium doping to the wire in the as-fabricated state does not principally change the mechanical response, and the fracture occurs by grain elongation and delamination. Both fracture stress and fracture strain decrease with increasing test temperature. Contrary to the as-fabricated wire, the potassium-doped wire annealed at 2300 °C exhibits much lower fracture stress. The fracture mechanism also differs, namely: cleavage below 300 °C and ductile necking above. The change in the fracture mechanism is accompanied with a significant increase of the elongation to fracture being ~ 5% around 300 °C.     

Precipitation evolution in Al–Zr and Al–Zr–Ti alloys during aging at 450–600 °C

The transformation of Al₃Zr (L1₂) and Al₃(Zr_1−xTi_x) (L1₂) precipitates to their respective equilibrium D0₂₃ structures is investigated in conventionally solidified Al–0.1Zr and Al–0.1Zr–0.1Ti (at.%) alloys aged isothermally at 500 °C or aged isochronally in the range 300–600 °C. Titanium additions delay neither coarsening of the metastable L1₂ precipitates nor their transformation to the D0₂₃ structure. Both alloys overage at the same rate at or above 500 °C, during which spheroidal L1₂ precipitates transform to disk-shaped D0₂₃ precipitates at ca. 200 nm in diameter and 50 nm in thickness, exhibiting a cube-on-cube orientation relationship with the α-Al matrix. The transformation occurs heterogeneously on dislocations because of a large lattice parameter mismatch of the D0₂₃ phase with α-Al. The transformation is very sluggish and even at 575 °C coherent L1₂ precipitates can remain untransformed. Mechanisms of microstructural coarsening and strengthening are discussed with respect to the micrometer-scale dendritic distribution of precipitates.     

The oxidation behavior of high Cr and Al containing Nb-Si-Ti-Hf-Al-Cr alloys at 1200 and 1250 °C

The oxidation behavior of two alloys containing different content of Al and Cr from the Nb-Si-Ti-Hf-Al-Cr system has been evaluated at 1200 and 1250 °C. The alloy compositions in atomic percent are Nb-24Ti-16Si-2Hf-2Al-10Cr (B1), and Nb-24Ti-16Si-2Hf-6Al-17Cr (B2). The oxidation kinetic of B1 alloy at 1200 and 1250 °C followed a mixed parabolic-linear law, while the oxidation kinetic of B2 alloy at 1200 and 1250 °C followed a parabolic law. The weight gain of B2 alloy was 18.9 mg/cm² after oxidation at 1200 °C for 100 h, which was a seventh of the value of that of B1 alloy. Besides, oxidation became more severe as temperature increased to 1250 °C. The oxide scales of B2 alloy consisted of CrNbO₄, TiNb₂O₇ and SiO₂, which were relatively compact and protective. In addition, the oxidation mechanism of Nb-Si based alloys were also discussed.     

RE-W cathode discharge properties in N2 and CO2 atmosphere at high temperature up to 1000 °C

Rare earth-tungsten (RE-W) cathode discharge properties in N₂ and CO₂ atmosphere at high temperatures were studied experimentally. The effect of temperature, atmosphere, cathode material and diameter on discharge current and corona inception voltage were investigated. The results show that the discharge current is larger and the corona inception voltage is lower in N₂ atmosphere than in CO₂ atmosphere. The range of the corona discharge voltage is narrower in CO₂ atmosphere. When the temperature is above 900 °C, the corona discharge becomes very unstable. As the temperature increases, the discharge current increases obviously and the corona inception voltage decreases significantly. Besides, the discharge current increases and the corona inception voltage decreases with the decrease of cathode diameter. The corona inception voltage also decreases significantly with the decrease of work function of different cathodes.     

Creep- and coarsening properties of Al–0.06 at.% Sc–0.06 at.% Ti at 300–450 °C

Upon aging at 300–450 °C, nanosize, coherent Al₃(Sc_1−xTi_x) precipitates are formed in pure aluminum micro-alloyed with 0.06 at.% Sc and 0.06 at.% Ti. The outstanding coarsening resistance of these precipitates at these elevated temperatures (61–77% of the melting temperature of aluminum) is explained by the significantly smaller diffusivity of Ti in Al when compared to that of Sc in Al. Furthermore, this coarse-grained alloy exhibits good compressive creep resistance for a castable, heat-treatable aluminum alloy: the creep threshold stress varies from 17 MPa at 300 °C to 7 MPa at 425 °C, as expected if the climb bypass by dislocations of the mismatching precipitates is hindered by their elastic stress fields.     

Oxidation behavior of ZrC-based composites in static laboratory air up to 1300 °C

Two ZrC-based composites, viz., ZrC + 20 vol.%SiC and ZrC + 20 vol.%SiC + 20 vol.%ZrB₂, were prepared by hot pressing. The oxidation behavior of the composites was studied in the temperature range of 800 °C–1300 °C in air. From the mass gain tests, the surface and cross-sectional SEM observation and EDS analysis of the oxidized samples, it was found that the ZrC + 20 vol.%SiC composite seemed to have a high oxidation resistance below 1000 °C, but the ZrC + 20 vol.%SiC + 20 vol.%ZrB₂ composite appeared a more excellent oxidation resistance up to 1200 °C. It is main reason that the oxidation rate of ZrC is higher than that of SiC below 1300 °C, meaning that SiO₂ from SiC is insufficient to form a dense tight protection film; however, the addition of ZrB₂ particles promotes the formation of more borosilicate above 1000 °C, which exerts more protective effect by self-healing mechanism.     

Enhanced sulphidation/oxidation resistance of Ti–45Al–8Nb alloy by multilayered coatings at 850°C for up to 675 h

The high temperature sulphidation/oxidation behaviour of three multilayered coatings CrAlYN/CrN etched by Y⁺, CrAlYN/CrN etched by Cr⁺ and CrAlYN/CrN etched CrAl⁺ and the uncoated γ-TiAl (Ti–45Al–8Nb (at-%) used as reference sample was studied at 850°C for 675 h. Sulphidation/oxidation test was performed in the environment of H₂/H₂S/H₂O, yielding pS₂?=?10^?1 Pa and pO₂?=?10^?18 Pa. Kinetic data obtained by discontinuous gravimetric method showed that the multilayered coatings effectively enhanced the sulphidation/oxidation resistance of γ-TiAl alloy. The corrosion resistance decreasing in order: CrAlYN/CrN etched by Y⁺>CrAlYN/CrN etched by CrAl⁺>CrAlYN/CrN etched by Cr⁺>γ-TiAl. Scale development studies using SEM, EDX and X-ray diffraction confirmed two regions of the coated materials: ‘affected,’ where coating cracked and developed non-protective TiO₂ scale, and ‘unaffected,’ where protective (Al,Cr)₂O₃ scale formed. The uncoated γ-TiAl material, after exposure, showed a typical multilayered structure consisting of layers of TiO₂ and Al₂O₃.     

Adaptive VN/Ag nanocomposite coatings with lubricious behavior from 25 to 1000 °C

A two-phase nanocomposite coating that consists of inclusions of silver in a vanadium nitride matrix (VN/Ag) was investigated as a potential adaptive coating with a reduced friction coefficient from 25 to 1000 °C. This nanocomposite structure was selected based on the premise that silver and silver vanadate phases would form on the surface of these coatings, reducing their friction coefficient in the (i) room to mid-range and (ii) mid-range to high temperatures, respectively. Silver and vanadium were expected to react with oxygen at high temperatures and create a lubricious silver vanadate film on the coating. The VN/Ag coatings were deposited using unbalanced magnetron sputtering and their elemental composition was evaluated using X-ray photoelectron spectroscopy. The tribological properties of the materials against Si₃N₄ balls were investigated at different temperatures. The lowest friction coefficients recorded for samples with identical compositions were 0.35, 0.30, 0.10 and 0.20 at 25, 350, 700 and 1000 °C, respectively. Post-wear testing Raman spectroscopy and X-ray diffraction (XRD) measurements revealed the formation of silver vanadate compounds on the surface of these coatings. In addition, real time Raman spectroscopy and high temperature XRD revealed that silver vanadate, vanadium oxide and elemental silver formed on the surface of these coatings upon heating to 1000 °C. Upon cooling, silver and vanadium oxide were found to combine at about 400 °C, leading predominantly to the formation of silver vanadate phases on the surface of these materials.     

Interdiffusion and reaction between Zr and Al alloys from 425° to 625 °C

Zirconium has recently garnered attention for use as a diffusion barrier between U–Mo nuclear fuels and Al cladding alloys. Interdiffusion and reactions between Zr and Al, Al-2 wt.% Si, Al-5 wt.% Si or AA6061 were investigated using solid-to-solid diffusion couples annealed in the temperature range of 425° to 625 °C. In the binary Al and Zr system, the Al₃Zr and Al₂Zr phases were identified, and the activation energy for the growth of the Al₃Zr phase was determined to be 347 kJ/mol. Negligible diffusional interactions were observed for diffusion couples between Zr vs. Al-2 wt.% Si, Al-5 wt.% Si and AA6061 annealed at or below 475 °C. In diffusion couples with the binary Al–Si alloys at 560 °C, a significant variation in the development of the phase constituents was observed including the thick τ₁ (Al₅SiZr₂) with Si content up to 12 at.%, and thin layers of (Si,Al)₂Zr, (Al,Si)₃Zr, Al₃SiZr₂ and Al₂Zr phases. The use of AA6061 as a terminal alloy resulted in the development of both τ₁ (Al₅SiZr₂) and (Al,Si)₃Zr phases with a very thin layer of (Al,Si)₂Zr. At 560 °C, with increasing Si content in the Al–Si alloy, an increase in the overall rate of diffusional interaction was observed; however, the diffusional interaction of Zr in contact with multicomponent AA6061 with 0.4–0.8 wt.% Si was most rapid.     

Behaviour and mechanisms of alkali-sulphate-induced hot corrosion on composite coatings at 900 °C

Three coating systems (one single MCrAlY, two composite coatings with/without the Cr-rich interlayer) have been prepared by the arc ion plating (AIP) and electroplating methods. Hot corrosion of the coatings at 900 °C by alkali sulphates shows that a composite coating with a chromium-rich interlayer exhibits the best corrosion resistance. The MCrAlY coating was severely deteriorated in 100 h since its non-productive (Ni,Co)Al₂O₄ and Cr₂O₃ scales induce internal sulfidation and oxidation. However, the microstructural involutions seldom occur within composite coatings, especially for the one with the Cr-rich interlayer. Mechanisms of hot corrosion of the three coatings are discussed.     

Hot corrosion properties of composite coatings in the presence of NaCl at 700 and 900 °C

Cyclic hot corrosion tests have been carried out on three coatings (one NiCoCrAlY and two composite coatings) at 700 and 900 °C. The kinetic curves and evolution of microstructure show that the composite coating with a Cr-base interlayer performs best. The Cr₂O₃ scale is more effective to protect the coating at 700 °C than that at 900 °C. The corrosion process is accelerated by NaCl via forming volatile MCl_x and inducing the formation of molten voids in the coating or extra oxidation at the interface of fusant/oxide scale, determined by the temperature and the compositions of the coating.     

The Gd–Ni–Al system: Phases formation and isothermal sections at 500 °C and 800 °C

Phase relations in the Gd–Ni–Al ternary system have been established for two isothermal sections (500 and 800 °C) based on X-ray powder diffraction analysis (XRD), Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM) coupled with Energy dispersive Microprobe Analysis (EPMA) on about 31 annealed alloys in the 50–100 at.% Al region. Seven intermetallic phases have been identified in samples annealed at 500 °C and 800 °C: Gd₃Ni₅Al₁₉ (oS108–Gd₃Ni₅Al₁₉), Gd₄Ni₆Al₂₃ (mS66–Y₄Ni₆Al₂₃), GdNiAl₄ (oS24–YNiAl₄), GdNiAl₃ (oP20–YNiAl₃), Gd₃Ni₇Al₁₄ (hP72–Gd₃Ni₇Al₁₄), GdNiAl₂ (oS16–CuMgAl₂) and GdNi₂Al₃ (hP18–GdNi₂Al₃). One additional ternary phase GdNi₃Al₉ (hR78–ErNi₃Al₉) has been found only in samples annealed at 800 °C. The isothermal sections have been determined and then compared, the crystal structures of the ternary phases have been confirmed and lattice parameters calculated.     

Oxidation behavior of silicon carbide at 1200 °C in both air and water–vapor-rich environments

Oxidation of SiC in both air and water–vapor–rich environments was carried out at 1200 °C to examine the effects of different oxidation conditions on the early-stage oxidation behavior of SiC. Two different types of SiC oxidation behavior were found, passive or active, depending on the oxidation environment. All the samples possessed amorphous oxide layers, regardless of the oxidation environment. Three Si oxidation states (SiO, Si₂O₃, and SiO₂) were observed in this layer. The amorphous Si₂O₃ state was dominant, and the ratio of the three different states changed with the test conditions.     

Scale composition and oxidation mechanism of the Ti–46Al–8Nb alloy in air at 700 and 800 °C

It is known that the oxide scale formed on TiAl alloys is generally composed of a mixture of alumina (Al₂O₃) and titania (TiO₂). The presence of niobium changes the activities of Ti and Al and influences the kinetics of oxidation and oxide layer composition. In this work, the Ti–46Al–8Nb alloy was subjected to cyclic oxidation in air at 700 °C (for 2 and 24 h) and 800 °C (for 300 h). Scale composition was analyzed by means of different techniques including X-ray photoelectron spectroscopy, X-ray diffraction and secondary ion mass spectroscopy. The scale consisted of several layers. The outer layer was built of alumina (amorphous or with very fine grains), whereas the inner layer – mainly of titania. After a longer exposure at a higher temperature (800 °C), niobium-rich precipitates and aluminum oxide grains were detected near to the alloy/scale interface and titanium nitride was found in the inner parts of the scale. Oxidation mechanism was studied by two-stage oxidation method using oxygen-18 and oxygen-16 isotopes combined with SIMS analyses. The distribution of oxygen isotopes over the alloy/scale interface indicated mixed inward/outward diffusion at the of reacting species. The experiments using Au markers showed that after longer oxidation time the inward diffusion was a predominant transport process.     

ZrB2–SiC(Al) ceramics with high resistance to oxidation at 1500 °C

In this paper, we study the oxidation behaviour of a ZrB₂/Al-doped SiC composite at 1500 °C. The composite was prepared by hot-pressing the mixture of ZrB₂ and polymer-derived SiC(Al). The oxidation behaviour was studied by measuring the weight change as a function of oxidation time and by observing the structure of the oxide layer. It is shown that the ZrB₂–SiC(Al) exhibits different oxidation behaviour and improved oxidation resistance as compared to the conventional ZrB₂–SiC without Al-doping. The improvement in oxidation resistance is attributed to that Al-doping could increase the bond strength of the Si–O and suppress the active oxidation of SiC.     

Effects of Ti,Zr, and Hf on the phase stability of Mo_ss + Mo3Si + Mo5SiB2 alloys at 1600 °C

Understanding the stability of the three-phase Mo_ss + Mo₃Si + Mo₅SiB₂ region is important for alloy design of Mo–Si–B-based refractory metal intermetallic composites. In this work, thermodynamic modeling is coupled with guided experiments to study phase stability in this three-phase region of the Mo–Si–B–X (X = Ti, Zr, Hf) system. Both the calculated and experimental results show that additions of Zr and Hf limit significantly the stability of the three-phase region because of the formation of the ternary phases MoSiZr and MoSiHf, while Ti addition leads to a much larger region of stability for the three-phase equilibrium.