•Water vapor accelerated the consumption of the oxide-forming elements of nickel aluminide coatings.•Water vapor leaded to the further increment of surface corrosion products.•The surface corrosion ...was the most serious with 30% water vapor.•The intergranular corrosion was the most serious with 15% water vapor.•The generation of Cl2 was inhibited when the water vapor content increased to 30%.
Nickel aluminide coatings were prepared by aluminizing pure nickel at 700 ℃ for 6 h. The corrosion behavior of nickel aluminide coatings was investigated under a KCl deposit environment (95 %N2 +5%O2) alone or with 15 % or 30 % water vapor at 700℃. Without water vapor, the coatings suffered slight surface and intergranular corrosion attack. With water vapor increasing to 30 %, the generation of chlorine is inhabited and reduces the “chlorine-active corrosion”. However, with 30 % water vapor, the oxidation-decomposition reaction becomes more active, so more corrosion products accumulate on the surface and the degradation of coatings was more severe.
•Self-Propagating High–Temperature Synthesis initiated by concentrated solar energy.•Efficient and sustainable synthesis of Ni-Al intermetallic compounds laminates.•Advanced solar manufacturing ...process with reduced energy.•Laminate parts with good hardness, porosity and appearance were obtained.•CSE minimised heating times and increased the process productivity and performance.
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The intermetallic compounds are very interesting in a multitude of structural and functional applications, such as in turbomachines or engine parts, due to their good behaviour at high temperatures, as well as their high resistance to corrosion and oxidation. In the present work, the study of processing of the Ni-Al system intermetallics has been carried out by means of Self-Propagating High–Temperature Synthesis initiated by concentrated solar energy (SHS–CSE). Synthesis by this high-energy exothermic reaction often causes the appearance of porosity, lack of interlayer adhesion, volumetric expansion, and even the total loss of sample shape. To obtain high-quality systems, a study has been carried out on one-layer and multi-layer configurations from powder materials, considering the influence of the heating rate and time. The use of concentrated solar energy has allowed to obtain fully densified nickel parts at 1000°C in just 15 min, compared to the 1325°C and more than 12h required by conventional techniques to obtain lower relative densities (~97%). The best results in multilayer configurations have been achieved in Ni-Ni/Al-Ni system with the synthesis of intermetallic compounds type Ni2Al3, NiAl and Ni3Al. The control of volumetric expansion and porosity, full densified nickel layers perfectly adhered to the interlayer and hardness values up to 900 HV have been obtained in just 2 min. This work highlights the wide possibilities of the use of CSE in metal treatment, which allows efficient and sustainable synthesis of new systems and the improvement of the final properties of the parts.
Directional solidification (DS) of near-stoichiometric NiAl with a reinforcing refractory metal is of great interest for high-temperature structural applications. A three-dimensional model based on ...single-crystal plasticity theory is introduced for the description of the creep behavior of DS NiAl–9Mo (at.%) with a well-aligned fibrous microstructure. A hardening model considering also the transition from theoretical to bulk strength is motivated. To evaluate the model, NiAl–9Mo samples were directionally solidified using various growth rates. With the DS samples, creep experiments were performed at 900 °C and 1000 °C for different applied stresses. The model reproduces correctly a change of the applied stress, the temperature as well as a change in the fiber diameter. It is found that creep of the composite is mainly controlled by the plastic behavior of the fibers. A closer insight into the interactions between the fiber and the matrix is obtained by the simulation. Finally, by revealing the impact of each phase on the composite's behavior, the shape of the creep curve could be explained.
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In recent decades, spark plasma sintering (SPS) has become a widespread technique in the production of high-quality intermetallic-based materials. In this study, we used SPS to fabricate multilayer ...metal-intermetallic (MIL) composites from pure nickel and aluminum foils. The effect of sintering temperature and pressure on the structure and properties of the fabricated MIL composites is discussed. The sintering temperature was changed from 900 to 1100 °C, the pressure varied from 10 to 40 MPa. The structure of the materials was investigated using optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray energy-dispersive spectroscopy (EDX), and X-ray diffraction (XRD). Depending on the sintering regimes the intermetallic layers in the composites consisted of Ni2Al3, Ni3Al, and NiAl of cubic and tetragonal modifications. Tetragonal NiAl appeared in the form of microtwinned lamellar martensite. A change in SPS temperature from 900 to 1100 °C led to an increase in the volume fraction of nickel-enriched intermetallic compounds, which was accompanied by improving composites strength. With an increase of the SPS pressure from 10 to 30 MPa, the number of pores and cracks in composites decreased. At the same time, their bending strength increased by 2 times (from 450 to 900 MPa). Fractographic studies showed that the fracture mechanism of the intermetallic layers depends on their elemental composition and varies from brittle transcrystalline to wavy “feathery”.
•Multilayer metal-intermetallic composites were produced by spark plasma sintering.•Ni2Al3, NiAl, and Ni3Al compounds were formed in the intermetallic layers.•Increase in temperature led to an increase in the volume fraction of Ni-rich phases.•NiAl twinned lamellar martensite was formed after sintering at 1100 °C.•Increase in pressure and temperature increased tensile and bending strength.
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Three novel precipitation strengthened bcc alloys which exhibit a smooth microstructural gradient with composition have been fabricated in bulk form by induction casting. All three ...alloys are comprised of a mixture of disordered A2-(Fe, Cr) and L21-ordered (Ni, Fe)2AlTi type phases both as-cast and after long-term annealing at 900 ∘C. The ratio of disordered to ordered phase, primary dendrite fraction, and overall microstructural coarseness all decrease as Cr is replaced by Al and Ti. Differences in phase composition are quantified through domain averaged principal component analysis of energy dispersive spectroscopy data obtained during scanning transmission electron microscopy. Bulk tensile testing reveals retained strengths of nearly 250 MPa up to 900 ∘C for the alloys which contain a nanoscale maze-like arrangement of ordered and disordered phases. One alloy, containing a duplex microstructure with ductile dendritic regions and highly creep resistant interdendritic regions, shows a promising balance between high temperature ductility and strength. For this alloy, tension creep testing was carried out at 700, 750, and 800 ∘C for a broad range of loading conditions and revealed upper bound creep rates which surpass similar ferritic superalloys and rival those of several conventionally employed high temperature structural alloys, including Inconel 617 and 718, at much lower density and raw material cost.
•Effective thermal conductivity of porous NiAl samples manufactured by spark plasma sintering has been studied.•Micro-CT finite element framework with additional resistance phase allows to achieve ...the satisfied correspondence with experimental data.•Modified analytical Landauer relation with interfacial thermal resistance of necks enhances the model capability for sintered materials.
This work presents a comprehensive analysis of heat transfer and thermal conductivity of porous materials manufactured by spark plasma sintering. Intermetallic nickel aluminide (NiAl) has been selected as the representative material. Due to the complexity of the studied material, the following investigation consists of experimental, theoretical and numerical sections. The samples were manufactured in different combinations of process parameters, namely sintering temperature, time and external pressure, and next tested using the laser flash method to determine the effective thermal conductivity. Microstructural characterisation was extensively examined by use of scanning electron microscopy and micro-computed tomography (micro-CT) with a special focus on the structure of cohesive bonds (necks) formed during the sintering process. The experimental results of thermal conductivity were compared with theoretical and numerical ones. Here, a finite element framework based on micro-CT imaging was employed to analyse the macroscopic (effective thermal conductivity, geometrical and thermal tortuosity) and microscopic parameters (magnitude and deviation angle of heat fluxes, local tortuosity). The comparison of different approaches toward effective thermal conductivity evaluation revealed the necessity of consideration of additional thermal resistance related to sintered necks. As micro-CT analysis cannot determine the particle contact boundaries, a special algorithm was implemented to identify the corresponding spots in the volume of finite element samples; these are treated as the resistance phase, marked by lower thermal conductivity. Multiple simulations with varying content of the resistance phase and different values of thermal conductivity of the resistance phase have been performed, to achieve consistency with experimental data. Finally, the Landauer relation has been modified to take into account the thermal resistance of necks and their thermal conductivity, depending on sample densification. Modified theoretical and finite element models have provided updated results covering a wide range of effective thermal conductivities; thus, it was possible to reconstruct experimental results with satisfactory accuracy.
Platinum plays a crucial role in the superior high-temperature oxidation resistance of Pt-modified nickel aluminide (PtAl) coatings. However, PtAl coatings usually serve in thermo-mechanical coupling ...environments. To investigate whether Pt contributes to the high-temperature mechanical properties of PtAl coating, stress rupture tests under 1100 °C/100 MPa were performed on PtAl coatings with varying Pt contents. The different coatings were obtained by changing the thickness of the electroplated Pt layer, followed by a diffusion heat treatment and the aluminizing process in the present work. The results of the stress rupture tests indicated that an increasing Pt content resulted in a significant decrease in the stress rupture life of PtAl-coated superalloys under 1100 °C/100 MPa. Theoretical calculations and microstructural analysis suggested that an increased coating thickness due to the Pt content is not the main reason for this decline. It was found that the cracks generated close to the substrate in high-Pt-coated superalloys accelerated the fracture failure.
Antiphase boundaries (APBs) are crucial to understand the anomalous temperature dependence of the yield stress of Ni3Al. However, the required, accurate prediction of temperature-dependent APB ...energies has been missing. In particular, the impact of magnetism at elevated temperatures has been mostly neglected, based on the argument that Ni3Al is a weak ferromagnet. Here, we show that this is an inappropriate assumption and that – in addition to anharmonic and electronic excitations – thermally-induced magnetic spin fluctuations strongly affect the APB energies, especially for the (100)APB with an increase of nearly up to 40% over the nonmagnetic data. We utilize an ab initio framework that incorporates explicit lattice vibrations, electronic excitations, and the impact of magnetic excitations up to the melting temperature. Our results prompt to take full account of thermally-induced spin fluctuations even for weak itinerant ferromagnetic materials. Consequences for large-scale modeling in Ni-based superalloys, e.g., of dislocations or the elastic–plastic behavior, can be expected.
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