Interdiffusion Of Elements Research Articles

Effect of Re-base diffusion barrier on the oxidation performance of a gradient NiCoCrAlY coating

A gradient NiCoCrAlY coating with a Re-base diffusion barrier (RG-NiCoCrAlY) was prepared by sequential treatments of NiRe coelectrodeposition, arc ion plating and chemical vapor aluminization. For comparison, normal NiCoCrAlY and simply aluminized NiCoCrAlY coatings (G-NiCoCrAlY) were also prepared to assess their cyclic oxidation behavior at 1050 °C. The results indicated that the oxide scale formed on the normal NiCoCrAlY coating peeled off after 400 cycles, while the G-NiCoCrAlY and RG-NiCoCrAlY coatings developed and maintained continuously dense alumina scale after oxidation for 600 cycles, confirming the benefits of increasing the Al content in NiCoCrAlY. In contrast to G-NiCoCrAlY, the presence of Re-base diffusion barrier mitigated element interdiffusion, which not only reserved Al in the coating but also reduced the outward diffusion of refractory elements from the substrate, thus was capable of minimizing precipitation of topologically close-packed phase (TCP) in single-crystal superalloy. Instead of outward diffusion, refractory elements were dissolved in the diffusion barrier and became fixed. Undoubtedly, the best anti-cycling performance was achieved by the RG-NiCoCrAlY coating. The mechanisms of the effect of the Re-base diffusion barrier on oxide scale growth and elemental interdiffusion, which influence the overall oxidation behavior of the NiCoCrAlY coating, are discussed.

Microstructural evolution and element interdiffusion of NiAlHf and NiAlHfY coatings deposited on a Ni-based superalloy

The NiAlHf and NiAlHfY coatings were deposited on a nickel-based single crystal superalloy using arc ion plating technology. The microstructural evolution and element interdiffusion behavior of the NiAlHf and NiAlHfY coatings were investigated under a 1200 °C thermal exposure process. The results showed that the NiAlHfY coating had a higher residual Al content and fewer topologically close-packed (TCP) phases, which implied that Y effectively retarded the diffusion rate of related elements. Besides, the inward diffusion of Al and outward diffusion of Ni lead to a γ → γ´ phase change and the formation of different TCP phases, resulting from the main precipitating elements (Cr, Mo, Re, W, and Ta) having a lower solubility in γ´. Moreover, it is found that the σ and Laves phase precipitate in the interdiffusion zone, which is described by an intergrowth relationship, where the σ phase is a metastable phase and plays a key role in forming the Laves phase. Finally, the interdiffusion mechanism of the elements is proposed.

Effect of Mo and W interlayers on microstructure and mechanical properties of Si3N4–nickel‐base superalloy joints

AbstractSi3N4/nickel‐base superalloy (Inconel‐625) and Si3N4/Si3N4 joints with refractory metal (W and Mo) interlayers were vacuum brazed using a Ti‐active braze Cu‐ABA (92.75Cu–3Si–2Al–2.25Ti) at 1317 K for 30 min with the following interlayered arrangements: Si3N4/Mo/W/Inconel and Si3N4/Mo/W/Si3N4. The joints exhibited Ti segregation at the Si3N4/Cu‐ABA interface, elemental interdiffusion across the joint interfaces, and sound metallurgical bonding. Knoop microhardness profiles revealed hardness gradients across the joints that mimicked the interlayered arrangement. The compressive shear strength of Si3N4/Si3N4 joints both with and without W and Mo layers was ∼142 MPa but the strength of Si3N4/Inconel joints increased from ∼9 MPa for directly bonded joints without interlayers to 53.5 MPa for joints with Mo and W interlayers.

Lower Interfacial Resistance between a Ta-Doped Li7La3Zr2O12 (LLZTO) Solid Electrolyte and NiCl2 Cathode by a Simple Heat Treatment for a High-Specific Energy Thermal Battery.

Large current discharge is restricted because of the poor conductivity between the Ta-doped Li7La3Zr2O12 (LLZTO) solid electrolyte and electrode. The poor conductivity would be caused by the interfacial reaction between LLZTO and the cathode, which is detrimental to the secondary Li ionic or metal battery. In this case, we studied the interfacial reaction between LLZTO and a haloid cathode (NiCl2) for a thermal battery for the first time, and a lower interfacial resistance could be obtained by a simple heat treatment. Owing to the element interdiffusion of Cl- and O2- at a high temperature of 600 °C, the main reaction products are LaOCl, LiCl, and La2Zr2O7. This reaction reduces the interfacial resistance from 3 Ω to 2 Ω. After a pretreatment at 600 °C, the discharge specific energy could reach 1254 Wh kg-1 from 828 Wh kg-1 at 550 °C with a cut-off voltage of 1.8 V. These results suggest that the interfacial reaction could be significant for the battery by adding interfacial contact. It is an effective approach to decrease the interfacial resistance at high temperature for some specific system, such as a haloid cathode-solid electrolyte.

Oxidation and interfacial cracking behaviors of TBCs with double-layered bond coat on different substrate materials

The oxidation and interfacial cracking behaviors of thermal barrier coatings (TBCs) with double-layered bond deposited on two typical substrate materials K452 and M247 was comparatively studied. Composition of the substrate can affect the element interdiffusion of the inner bond coat and the driving force of interfacial cracking. The capacity of the inner bond coat to supply Al for the upper bond coat is critical to the lifetime of TBCs with double-layered bond coat. Adopting a substrate with higher resistance to the inward-diffusion of Al can facilitate the formation of the protective Al2O3 scale and significantly extend the lifetime of TBCs.

Development and Characterization of CrCoNi Medium Entropy Alloy Particles Reinforced Aluminum Matrix Composite

As for metal matrix composites (MMCs), the selection and application of reinforcements play a vital role in their comprehensive properties. In this work, the CrCoNi medium entropy alloy (MEA) was selected as reinforcement for Al matrix composites, and the effects of the content of the CrCoNi MEA on the mechanical properties and friction resistance were systematically investigated. It was found that the CrCoNi MEA can effectively improve the mechanical properties of the Al matrix composites, especially the 5 wt.% CrCoNi/Al composite can achieve a high strength without the sacrifice of ductility, due to the strengthened interfacial bonding between the Al matrix and the CrCoNi reinforcements caused by the element interdiffusion and the high mechanical performance of the CrCoNi MEA itself. In addition, the wear resistance of the composites can be enhanced by the inclusion of the CrCoNi MEA reinforcement, because the CrCoNi MEA can substantially improve the hardness of the composites and promote the formation of the oxidative protection film during the friction process. This work paves a new route for preparing Al matrix composites with high mechanical properties and friction resistance.

Enhanced structural stability and oxidation resistance of NiCoCrAlY bonding layer via Al addition

A novel Al doping was designed to enhance the structural stability and oxidation resistance of NiCoCrAlY bonding layer. Results showed that NiCoCrAlY without Al doping suffered from severe elemental inter-diffusion and rapid growth of (Ni, Co)Cr2O4 spinel during high-temperature exposure, leading to poor structural stability and a high oxidation weight gain rate. However, Al doping induced the formation of a continuously dense alumina, which effectively suppressed the element inter-diffusion. Preferential oxidation of Al also suppressed rapid growth of brittle (Ni, Co)Cr2O4 spinel. NiCoCrAlY with Al addition exhibited excellent structural stability and a low oxidation weight gain rate.

SiC-glass as a diffusion barrier at the interface of MoSi2 coating and Nb-Si based alloy

A SiC-glass interlayer was prepared as a diffusion barrier to suppress inter-diffusion between MoSi2 coating and an Nb-Si based alloy, and its oxidation behavior at 1250 ℃ and 1400 ℃ was investigated. The results revealed that the SiC-glass diffusion barrier effectively inhibited the inter-diffusion between silicide coating and substrate, and the IDZ thickness is 47% lower than that of the coating without diffusion barrier after oxidation at 1250 ℃ for 500 h. At 1400 °C, the SiC-glass diffusion barrier also can maintain a good stability and retard the inter-diffusion of elements between the coating and substrate effectively.

Enhanced oxidation resistance of an AlSi alloy coating via Cr barrier layer insertion

A novel Cr barrier layer was designed for suppressing the interdiffusion of an AlSi alloy coating on stainless steel. Without a Cr barrier layer, the same coating experienced a severe element interdiffusion upon exposure to high-temperature air. The rapid upward movement of the Fe atoms induced the formation of the (FeAl + Fe2Al5) alloy compounds and eventually led to severe volume shrinkage and the ultimate fracture of coating. The significant downward diffusion of the (Al + Si) atoms not only triggered numerous pores but also induced the accumulation of the FeAl alloy compound in the substrate resulting in extreme interfacial cracking. In contrast, a diffusion barrier layer comprising (Cr2O3 + SiO2) and Al2O3 was formed in the coating with a Cr layer during high-temperature exposure, effectively preventing the rapid elemental interdiffusion. Moreover, it inhibited the formation of the (FeAl + Fe2Al5) alloy compounds in the coating as well as prevented the accumulation of the FeAl in the substrate. The obvious upward diffusion of Cr atoms further healed these pores induced by the downward diffusion of (Al + Si) atoms. The strong barrier capability of this composite oxide layer and the healing effect of Cr diffusion eventually led to an improved oxidation resistance of the Cr-coated layer.

High entropy alloy reinforced aluminum matrix composites prepared by novel rotation friction extrusion process: Microstructure and mechanical properties

A novel rotation friction extrusion (RFE) process was taken to fabricate CrMnFeCoNi high entropy alloy (HEA) particles reinforced aluminum matrix composites (AMCs). The experimental methods and the first principle calculation based on density functional theory were employed to study the microstructure and mechanical properties of the composites. Results showed that the HEA particles reacted violently with the aluminum (Al) matrix during RFE. The average grain size of the AMCs was 3.6 μm, which was significantly lower than that of the RFEed Al (13.2 μm). Numerous second phases with FCC structure were formed, as a result of the large thermodynamic driving force and the enhanced element interdiffusion. The tensile strength of the AMCs showed substantially enhanced compared to that of RFEed Al, which could be ascribed to load transfer, dislocation strengthening, fine-grained strengthening and Orowan strengthening. The uniform elongation of the AMCs was lower than that of the RFEed Al, which was explained by the fine grain size of the AMCs resulting in high dislocation dynamic recovery rate.

Interfacial evolution and mechanical behavior of explosively welded titanium/steel joint under subsequent heat treatment process

Explosive welding (EXW) has advantages in joining metallurgically incompatible metals due to its high quality and simple process. Successful titanium/steel clad plates fabricated by explosive welding have been extensively reported in recent years. However, no research has been reported on the fabrication of explosively welded TA2/15CrNi3MoV clad plates and the effect of subsequent heat treatment processes on the clad plates. This study aims to clarify the microstructure evolution and mechanical behavior of explosively welded TA2/15CrNi3MoV clad plates during the subsequent heat treatment process. The explosively welded TA2/15CrNi3MoV clad plates were heat-treated in the temperature range of 700–1000 °C for 30 min. The results elucidated that the heat treatment process accelerated the interdiffusion of constituent elements (e.g., Fe, Ti, and C) at the TA2/15CrNi3MoV interface, which formed FeTi, Fe2Ti, and TiC. For the samples heat-treated at 700 °C, a continuous TiC layer was generated at the TA2/15CrNi3MoV interface, which hampered the formation of FeTi and Fe2Ti. However, for the samples heat-treated at 800 °C, the continuous TiC layer was broken up, which resulted in a significant increase in the Fe-Ti layer to 1.21 μm. This microstructure evolution dramatically dropped the tensile strength to 329.2 MPa. For samples heat-treated in the temperature range of 900–1000 °C, the thickness of the brittle Fe-Ti layer dramatically increased and caused a rapid drop in tensile strength.

Investigation of HVOF sprayed novel Al1.4Co2.1Cr0.7Ni2.45Si0.2Ti0.14 HEA coating as bond coat material in TBC system

The present study aims to investigate non-stoichiometric Al1.4Co2.1Cr0.7Ni2.45Si0.2Ti0.14 high entropy alloy (HEA) as a bond coat material for the TBC (Thermal Barrier Coating) system. The mechanical activated synthesized HEA was sprayed on a Ni-based superalloy substrate by High-velocity oxy-fuel (HVOF) spraying, and 8 mol% Yttria Stabilized Zirconia (YSZ) was deposited on HEA by Air Plasma Spray (APS). X-Ray Diffraction (XRD) analysis and Scanning Electron Microscopy (SEM) were used to investigate the phases and microstructure of the as-synthesized HEA powder, Ni superalloy/HEA-bond coat, and Ni superalloy/HEA-bond coat/YSZ topcoat. The mechanical properties of the coating like microhardness, Young’s modulus and residual stress between bond coat and YSZ top coat was evaluated using the Nano-Hardness Tester (NHT). The TBC system was investigated for cyclic oxidation at 1050 ℃ for 100 cycles, and its cross-sections were analyzed for TGO (Thermally grown oxides) layer composition, thickness and interdiffusion of elements. The properties of the TBC system containing HEA as a bond coat were compared with those of the conventional TBC system comprising of MCrAlY (AMDRY 365–4) as a bond coat. It was observed that HEA containing TBC displayed exceptional high temperature properties and were comparable to MCrAlY.

Interdiffusion mechanism at the interface between TiAl alloy and NiCoCrAlY bond coating

Surface coatings are the best ways to protect materials from deterioration due to various aspects including corrosion and wear. However, after long-term oxidation, the high temperature oxidation resistance of TiAl-4Nb-1.5Mo (B, Y) alloy coated with NiCoCrAlY bond coating is weakened by the interfacial interdiffusion. Herein, the microstructure evolution, elements interdiffusion, and mechanical properties at the interface between TiAl alloy and NiCoCrAlY bond coating are analyzed through an isothermal oxidation experiment at 950 °C. Results show that the coarsening and coalescence of the β phase occur in the bond coating during high temperature oxidation. Meanwhile, the thickness of the bond coating decreases linearly before oxidation for 100 h. As the oxidation time increases, the consumption rate of the bond coating slows down. Based on the Layer-by-layer phase composition analysis, it is found that the interdiffusion zone forms three layers in sequence: NiAl and Al (Ni, Co)2Ti, AlNi2Ti(Nb, Mo) and Ti3Al, and Al3NiTi2 and TiAl. The TiN phase segregated around the α-Cr as well as the enriched Y element led to generation of microcracks. The TiN phase forms at the phase boundary between the β and γ phases after 250 h of oxidation. The layer composed of AlNi2Ti(Nb, Mo) and Ti3Al phase exhibits a severe hardening phenomenon, which may reduce the long-term service life of the TiAl alloy coated with NiCoCrAlY bond coating.

Temperature and doping-dependent interplay between the direct and indirect optical response in buffer-mediated epitaxial germanium

The structural and optical properties of buffer mediated epitaxial germanium (Ge) layer were investigated and compared with bulk n-type and p-type Ge substrates. An interconnected dual-chamber molecular beam epitaxy (MBE) system was used to grow a 280 nm thin Ge epilayer on (100)GaAs substrate with an intermediate AlAs buffer layer. The lattice-matched, abrupt Ge/AlAs heterointerface was analyzed using cross-sectional transmission electron microscopic analysis, and no elemental interdiffusion was detected via secondary ion mass spectrometry. A strong direct gap transition, compared to the indirect gap transition, and a series of phonon-assisted transitions was observed by photoluminescence (PL) spectroscopy. In addition, the intensity of the direct gap recombination decreases with decreasing PL measurement temperatures, which was ascribed to the reduced density of Γ-valley electrons available for recombination at lower temperature. Furthermore, the intensity ratio between the direct and indirect optical transition drastically decreases with decreasing temperature in both n-type epitaxial and p-type bulk Ge. An empirical relation in both direct and indirect peak position with temperature was established. The observed strong luminescence in 280 nm thick epitaxial Ge at room temperature is vital for Ge-based photonic devices. In addition, the quality of the epitaxial Ge layer grown via MBE is on par with bulk Ge substrates.

Influence of La doping concentration and A-site deficiency on electrical conductivity of La substituted SrTiO3 and its chemical compatibility with ScSZ

The influence of La doping concentration and A-site deficiency concentration on the electrical conductivity of La substituted SrTiO3 and its high-temperature chemical compatibility with ScSZ was investigated. It was found that the chemical compatibility between (LaxSr1-x)1-yTiO3-δ ((LxS1-x)1-yT) and 1CeO2-10Sc2O3–89ZrO2 (ScSZ) gradually deteriorates with the increase of La doping concentration from 10% to 40%. When the La doping concentration is 30–40%, chemical reaction will occur between LxS1-xT and ScSZ and La2Zr2O7 (LZO) will be generated, and the diffusion of Sc3+and Zr4+ in ScSZ to LxS1-xT is aggravated. A-site deficient (LxS1-x)1-yT can effectively inhibit the interfacial reaction and element interdiffusion between it and ScSZ. The first principle computation results indicate that the influence of La doping concentration and A-site deficiency on the vacancy formation energy (VFE) of Ti in (LxS1-x)1-yT determines its chemical compatibility with ScSZ. The results of electrical conductivity test and the first principle computation indicate that the La doping concentration and A-site deficiency will simultaneously influence the amount of Ti4+ to Ti3+ and the oxygen vacancy concentration in (LxS1-x)1-yT in reducing atmosphere, thus affecting its conductivity.

Prediction Model of Aluminized Coating Thicknesses Based on Monte Carlo Simulation by X-ray Fluorescence

An aluminized coating can improve the high-temperature oxidation resistance of turbine blades, but the inter-diffusion of elements renders the coating’s thickness difficult to achieve in non-destructive testing. As a typical method for coating thickness inspection, X-ray fluorescence mainly includes the fundamental parameter method and the empirical coefficient method. The fundamental parameter method has low accuracy for such complex coatings, while it is difficult to provide sufficient reference samples for the empirical coefficient method. To achieve accurate non-destructive testing of aluminized coating thickness, we analyzed the coating system of aluminized blades, simulated the spectra of reference samples using the open-source software XMI-MSIM, established the mapping between elemental spectral intensity and coating thickness based on partial least squares and back-propagation neural networks, and validated the model with actual samples. The experimental results show that the model’s prediction error based on the back-propagation neural network is 4.45% for the Al-rich layer and 16.89% for the Al-poor layer. Therefore, the model is more suitable for predicting aluminized coating thickness. Furthermore, the Monte Carlo simulation method can provide a new way of thinking for materials that have difficulty in fabricating reference samples.

Microstructure evolution and interdiffusion of PtAl coated a third generation single crystal superalloy during thermal exposure

Microstructure evolution and elemental interdiffusion of PtAl coated third generation single crystal superalloy at 1000 °C and 1100 °C have been investigated. Interdiffusion coefficients and elemental diffusion fluxes are determined by HitDIC software based on the numerical inverse method. The interdiffusion zone (IDZ) with σ and R topologically close-packed (TCP) phases and secondary reaction zone (SRZ) with σ and P phases are formed after exposure at 1000 °C and 1100 °C. Interdiffusion flux of Al is the largest, followed by Pt, Ni, Co and Ta. However, most of the inward diffused Al is absorbed by IDZ, and the enrichment of Ta in IDZ/SRZ primarily accounts for the formation of SRZ. The increasing outward diffusion of refractory elements, such as Re, W and Ta mainly leads to larger thickness of SRZ at 1100 °C. Moreover increasing interdiffusion of Al and Ni further promotes the growth of SRZ and transformation from β-NiAl to γ′-Ni3Al phases. The content of reserved Al in the β layer is the same after thermal exposure at both temperatures, due to the effect of Pt.

Effects of annealing temperature on interface microstructure and element diffusion of ultra-thin Cu/Al composite sheets

In this research, the ultra-thin Cu/Al metallic composite sheets of 0.06 ∼ 0.09 mm were successfully prepared by the self-designed four-high laboratory micro mill. The cold rolling reduction of single pass was up to 70%. The effect of different annealing temperatures on microstructure and element diffusion of the Cu/Al composite sheet interface was investigated at the microscale. The results show that with the increase of annealing temperature, the average grain size of Cu and Al increases gradually, the mutual diffusion of Cu/Al elements increases. Compared with the macroscale, the grain refinement of the ultra-thin Cu/Al composite sheets can promote the interdiffusion of Cu and Al elements. When the annealing temperature was 350 ∼ 450 °C, a relatively large number of micropores and microcracks appeared at the interface of Cu/Al composite sheet. When the annealing temperature was 350 °C, a diffusion layer was formed at the interface of the ultra-thin Cu/Al composite sheet, and the interface bonding method was metallurgical bonded. When the annealing temperature was 500 °C, the mutual diffusion degree of elements was the largest at the interface.

Interface Analysis of MOCVD Grown GeTe/Sb2Te3 and Ge-Rich Ge-Sb-Te/Sb2Te3 Core-Shell Nanowires.

Controlling material thickness and element interdiffusion at the interface is crucial for many applications of core-shell nanowires. Herein, we report the thickness-controlled and conformal growth of a Sb2Te3 shell over GeTe and Ge-rich Ge-Sb-Te core nanowires synthesized via metal-organic chemical vapor deposition (MOCVD), catalyzed by the Vapor–Liquid–Solid (VLS) mechanism. The thickness of the Sb2Te3 shell could be adjusted by controlling the growth time without altering the nanowire morphology. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were employed to examine the surface morphology and the structure of the nanowires. The study aims to investigate the interdiffusion, intactness, as well as the oxidation state of the core-shell nanowires. Angle-resolved X-ray photoelectron spectroscopy (XPS) was applied to investigate the surface chemistry of the nanowires. No elemental interdiffusion between the GeTe, Ge-rich Ge-Sb-Te cores, and Sb2Te3 shell of the nanowires was revealed. Chemical bonding between the core and the shell was observed.

Microstructure and mechanical property of titanium steel by vacuum roll-cladding with interlayer

The titanium clad steel plates were prepared by vacuum roll-cladding (VRC) in three ways: without interlayer, with Fe interlayer and Ni interlayer. With the increase in temperature, the TiC layer becomes thicker and the metal compounds increase at the interface during VRC without interlayer. The bonding strength is 266 MPa at 900°C. The Fe interlayer effectively inhibits the diffusion of C element and reduces the amount of TiC. The fracture is brittle and partial ductile, and the interface bonding strength improved. The Ni interlayer can prevent the interdiffusion of Ti and Fe elements, but brittle TiC and Ti-Ni compounds are formed at the interface. The bonding strength is low, and the fracture is brittle, which occurs at the Ti-Ni interface.