Experimental and Thermodynamic Analysis on Phase Equilibria of Ni-rich Region in Ni-Al-Ti System

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The effects of peak temperature and cooling rate on the susceptibility to intergranular corrosion of alloy 690 by laser beam and gas tungsten arc welding

This study investigates the effects of welding method, peak temperature, and cooling rate on the susceptibility to intergranular corrosion of alloy 690 weldments. The experimental results reveal that the laser beam welding process with cooling rate of around 212.6 °C/s can be produced with much less mass loss and a lower value of maximum reactivation current density/maximum anodic current density than with the gas tungsten arc welding process, where cooling rate is at around 17-20.6 °C/s. This is because the very rapid cooling rate during welding leads to an insufficient exposure time of around 2.1 s within the chromium (Cr)-carbide precipitation temperature range, suppressing Cr-carbide precipitation and Cr-depletion along grain boundaries in the weld decay region of the heat affected zone.     

Hot corrosion behavior of detonation gun sprayed Cr3C2–NiCr coatings on Ni and Fe-based superalloys in Na2SO4–60% V2O5 environment at 900 °C

The present work investigates the hot corrosion resistance of detonation gun sprayed (D-gun) Cr₃C₂–NiCr coatings on Superni 75, Superni 718 and Superfer 800 H superalloys. The deposited coatings on these superalloy substrates exhibit nearly uniform, adherent and dense microstructure with porosity less than 0.8%. Thermogravimetry technique is used to study the high temperature hot corrosion behavior of bare and Cr₃C₂–NiCr coated superalloys in molten salt environment (Na₂SO₄–60% V₂O₅) at high temperature 900 °C for 100 cycles. The corrosion products of the detonation gun sprayed Cr₃C₂–NiCr coatings on superalloys are analyzed by using XRD, SEM, and FE-SEM/EDAX to reveal their microstructural and compositional features for elucidating the corrosion mechanisms. It is shown that the Cr₃C₂–NiCr coatings on Ni- and Fe-based superalloy substrates are found to be very effective in decreasing the corrosion rate in the given molten salt environment at 900 °C. Particularly, the coating deposited on Superfer 800 H showed a better hot corrosion protection as compared to Superni 75 and Superni 718. The coatings serve as an effective diffusion barrier to preclude the diffusion of oxygen from the environment into the substrate superalloys. It is concluded that the hot corrosion resistance of the D-gun sprayed Cr₃C₂–NiCr coating is due to the formation of desirable microstructural features such as very low porosity, uniform fine grains, and the flat splat structures in the coating.     

EFFECT OF COOLING RATES ON SEGREGATION AND DENSITY VARIATION IN THE MUSHY ZONE DURING SOLIDIFICATION OF SUPERALLOY INCONEL 718

The microstructure and segregation of experimental samples of the superalloy Inconel 718 solidified at different cooling rates have been investigated by SEM and EDAX. The equilibrium phase fraction diagrams of Inconel 718 and the segregated liquid composition were calculated by Thermo-Calc. The experimental and calculated results were in good agreement with those of an Inconel 718 ingot, and they indicated that, at a cooling rate less than 3°C/min, heavy Nb and Mo segregation resulted in Laves phase formation directly from the liquid. The density of the liquid surrounding the Laves phase decreased and the region between the two Laves precipitates (inter-precipitate) often had the lowest density. A density difference (or micro inter-precipitate density inversion) existed between the inter-precipitate liquid and the original liquid at the same or higher temperatures where there were no liquid-precipitated Laves.     

Effect of Aging Heat Treatment on the Microstructure and Creep Properties of the Cast Ni-Based Superalloy at Low Temperature

Effect of aging heat treatment on the grain boundary microstructure and creep properties of a cast Ni-based superalloy was investigated. With increasing aging temperature from 750 to 1000 ℃, M_(23)C_6 carbides along the grain boundaries evolve from fine distributed block, continuous film into the coarse discrete block. Moreover, the M_(23)C_6 carbides are mainly enveloped within γ’ layers along grain boundaries during 1000 ℃ aging. Creep rupture lifetime and elongation at 760 ℃ and 645 MPa are improved with increasing the aging temperature. In particular, the creep rupture lifetime of the specimens aging at 1000 ℃ is one order of magnitude higher than that of the specimens aging at 750 ℃. The enhancement of ductility induced by the γ’ envelopes plays a significant role in the improvement of creep rupture lifetime.     

Hydrogen and moisture-induced scale spallation: Cathodic descaling of a single crystal superalloy

Moisture-induced delayed spallation (MIDS) of protective alumina scales at room temperature is a well known phenomenon. One mechanism proposes that water and Al_alloy react, produce hydrogen at the scale–metal interface, and enable spallation. To test this mechanism, preoxidized samples of a single crystal superalloy, Rene’N5 + Y, were subjected to standard cathodic hydrogen charging treatments known to produce hydrogen embrittlement in bulk Ni and Ni₃Al alloys. Cathodic hydrogen charging, at <1 mA and an estimated −0.45 V SCE, stripped the scales at the oxide-metal interface, resulting in an initial loss of ∼3 mg/cm² and little additional change with time. This was supported by macro-photos and SEM of the spalled surface. On the other hand, anodic polarization at <1 mA produced less, but steady, linear weight loss (0.3 mg/cm²), primarily by anodic dissolution of the metal. Hydrogen charging was thus shown to be detrimental to the alumina scale–metal bond, supporting the hydrogen factor in MIDS. These and other MIDS results show remarkable similarities to embrittlement of Ni subject to hydrogen charging at similar potentials and varying amounts of interfacial (grain boundary) sulfur segregation. The MIDS phenomenon is also discussed in terms of comparative static corrosion fatigue characteristics. It is not necessarily related to other diverse moisture effects occurring at high temperature.     

Effect of B and Cr on the high temperature oxidation behaviour of novel γ/γ′-strengthened Co-base superalloys

Isothermal oxidation was carried out on new γ/γ′-strengthened Co-base superalloys of the system Co–Al–W–B and Co–Al–W–B–Cr at 800 and 900 °C in air. ToF-SIMS investigations show that boron accumulates in the inner oxide layer which improves the oxidation behaviour and oxide layer adhesion considerably. B-rich precipitations can be observed in the alloy matrix, as well. Appropriate amounts of chromium as additional alloying element lead to further improvement of the oxidation resistance due to the formation of protective inner Al₂O₃ and Cr₂O₃ scales.     

Mar-M247高温合金900℃下的氧化行为研究

为探究燃气轮机用高温合金在服役环境中组织结构变化及其对合金氧化性能的影响,利用扫描电镜和能谱表征了MarM247镍基高温合金在900℃长时效后的显微组织,研究其在1000、3000、5000、8000h时效后的氧化性能。结果表明,时效1000h的氧化初期合金表面氧化物相变化较快,Al₂O₃氧化膜不完整,氧化膜增厚较快;随着氧化时间增加,富W氧化膜内生成大量孔洞和岛状不连续WO2,导致内层Al₂O₃膜界面发生剥落,外层氧化膜结构得到简化,内层Al₂O₃膜增厚且变得均匀;时效8000h后氧化层总厚度和内层Al₂O₃膜变化幅度明显减小,整个氧化膜结构趋于稳定。由于内层保护性Al₂O₃膜的增厚和结构均匀连续,使得合金的氧化性能提高。     

分光光度法测定高温合金中铁的不确定度评定

对分光光度法测量高温合金中铁含量的测量不确定度进行评定。通过建立分光光度法测定高温合金中铁含量的数学模型,分析了测量过程中不确定度的主要来源,如测量重复性,溶液浓度,溶液体积,试样溶液定容体积,分取溶液体积等,并对各不确定度分量进行了量化,结果表明：分取试样溶液体积（V1）和试样称量（m0）引入的不确定度所产生的影响很小,可以忽略。对铁含量为0.05%～3.00%的高温合金,置信度水平为95%时,测量结果的扩展不确定度为0.03%。