Diffusion Heat Treatment Research Articles

Effect of Pt on Stress Rupture Properties of Pt-Modified Nickel Aluminide Coatings at 1100 °C.

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.

Comparative environmental assessment of coated ferritic steels suited to steam turbines of coal-fired supercritical and ultra-supercritical power plants

Coated ferritic steels are the most common materials used for steam turbines construction in coal-fired power plants operating at supercritical and ultra-supercritical conditions of pressure and temperature because of their corrosion and oxidation resistance. However, the environmental implications associated to the steel-coating system have not been fully investigated to choose the best option in terms of sustainability.This work presents three types of ferritic steels as well as three coatings with its corresponding deposition technique, normally used in these environments. The designated materials are P91, P92 and VM12 ferritic steels, while the selected protective coatings are Ni50Cr applied by High-Velocity Oxi Fuel Spray (HVOF), CrN/NbN applied by High Power Impulse Magnetron Sputtering (HiPIMS) and slurry aluminide applied by painting and diffusion heat treatment. Taking these data as a starting point, comparative Life Cycle Assessments (LCA) and economic analysis are performed.This work aims to contribute to better understanding of environmental damages as well as the importance of considering the technical and environmental assessment in the steel-coating selection. The results obtained indicated that the P92 ferritic steel coated with slurry aluminide had the best performance. On the other hand, CrN/NbN coating showed the greater impacts in all the categories studied.

Electrodeposition of Dy and its grain boundary diffusion behavior of sintered NdFeB magnets

This paper presents the electrodeposition of a Dy layer onto the surface of sintered NdFeB magnets to serve as a diffusion source. Subsequently, the sintered NdFeB magnets were subjected to grain boundary diffusion treatment through electrochemical deposition at the solution temperature of 45 °C and the solution pH (hydrogenii) of 4.0. The results indicated that the coercivity of the Dy-diffused magnet increased from 1073 kA/m to 1418 kA/m compared to the original magnet after annealing at a primary diffusion heat treatment temperature of 900 °C for 6 h and subjected to the same secondary tempering heat treatment at 500 °C for 2 h, while the remanence remained relatively unaffected. SEM (Scanning Electron Microscopy) revealed that the Dy coating exhibited a compact and flat surface with a small grain size. Energy spectrum analysis confirmed a relatively uniform distribution of Dy elements in the electrodeposition coating, without any significant agglomeration. SEM-EDS (Scanning Electron Microscopy-Energy Dispersive Spectrometer) line scan demonstrated that the diffused Dy elements along the grain boundary phase primarily concentrated in the grain boundary phase itself and the epitaxial layer of the main grains. BSE (Backscattered Electron imaging) revealed the presence of a thin and continuous grain boundary phase structure between the main phases following grain boundary diffusion. XRD (X-ray diffraction) analysis demonstrated a general shift toward higher angles in the main peak of NdFeB after Dy diffusion. The remarkable increase in coercivity of the diffused Dy magnets can be attributed to the optimization of microstructure, grain boundary phase composition, and the formation of the (Dy,Nd)2Fe14B structure. The coercivity mechanism in diffusion Dy magnets was still governed by a nucleation reversal mechanism, as supported by the Kronmüller-Plot curve fitting. Tafel tests have revealed that Dy-diffused magnets have a much higher corrosion resistance with a more positive equilibrium potential, Ecorr, and a lower corrosion current density, Icorr.

Nickel Coatings on Ceramic Materials Using Different Diffusion Techniques

Diffusion bonding is a process that has proven effective for the joining of metal to ceramic, but the differences in coefficient of thermal expansion still pose challenges during and after the bonding process. This work details the exploration of traditional diffusion-bonding processes using two traditional approaches, which include bonding of a 99.9+% pure Ni foil to SiC, Si3N4, and YSZ disks using (1) a hot isostatic press (HIP), with and without added weight to promote interfacial contact, and (2) field-assisted sintering (FAST). Samples were consolidated by heating to 1200 °C and held for 6 h under vacuum before cooling to room temperature during the HIP method. For the FAST technique, bonding experiments were performed at both 800 °C and 1200 °C in a vacuum environment under 10 MPa uniaxial pressure. After the Ni was bonded to the ceramics, diffusion heat treatments were carried out in the HIP. For electroless-plated samples, the heat-treatment temperature was chosen as 825 °C to avoid melting. For electroplated samples, heat treatment occurred at 925 °C or higher. Electroplated YSZ samples were heat-treated at 1150 °C as the Ni-Si eutectic is not a concern in this system. The time at temperature varied from 6 h to 48 h depending on the material combination tested. Post-heat-treatment diffusion characteristics were analyzed using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). A main cause of poor bonding performance in the HIP samples was reduced interfacial contact, while cohesive failures in the FAST samples are likely due to the formation of brittle intermetallic Ni-Si phases. Preliminary results indicate success in bonding Ni to SiC, Si3N4, and YSZ using a diffusion-enhanced approach on electroplated specimens.

Preparation and performance of a three-dimensional porous copper-tin electrode for lithium-ion batteries

A 3D porous Cu/Sn/Cu6Sn5/Cu3Sn composite anode was successfully fabricated by electrodepositing tin on a copper foam foil and then conducting diffusion heat treatment at 200ºC for 3 h. The specific capacity of the electrode was 660.97 mAh/g at 0.2 C after 300 charge/discharge cycles. The high capacity and cycling stability of the electrode were due to the Cu-Sn phases and porous structure of the copper foam. The formation of the Cu6Sn5 and Cu3Sn phases decreased the tin content on the surface, reduced the volume expansion rate caused by lithiation/delithiation and effectively inhibited the generation of cracks and electrode pulverization. Furthermore, the number of active sites for the reaction between tin and lithium were significantly increased by the uniform distribution of tin on the surface of the copper foam and its large specific surface area.

Effect of Heat Treatment Schedules on Creep Performance of Ni-Based Superalloy Mar-M247 at 871 °C and 250 Mpa

The effects of heat treatment (H1 and H2) on the creep behavior and microstructures of Mar-M247 at 871 °C/250 MPa are studied. The results show that the as-cast microstructure is composed of eutectics, γ phase, γ’ phase, MC and M23C6 carbides, while new M6C appears in heat-treated microstructures, indicating the transformation of carbides after heat treatments. The Mar-M247 is excellent, with over 1500 h of creep life, and H1 is 48% higher than H2. The addition of post-brazing and diffusion heat treatment in H2 is detrimental to creep resistance; the two steps promoted the transformation of MC into M23C6 in advance. The increase and coarsening of M23C6 would consume more γ-phase-forming elements, weakening the solution strengthening at grain boundaries. As a result, the resistance of the grain boundary and γ/γ’ interface to dislocation motion is significantly reduced, leading to the cracks’ initiation and propagation along the grain boundaries.

Effect of Vacuum Heat Treatment on the Microstructure and Tribological Property of Ti–6Al–4V Alloy with Cu Coating

The effect of vacuum heat treatment on the interface microstructure and tribological property of Cu‐coated Ti–6Al–4V alloy is investigated herein. After the vacuum heat treatment process, a diffusion layer is formed at the interface between the Cu coating and the Ti–6Al–4V substrate. The formed intermetallic compounds at the interface between the Ti–6Al–4V substrate and Cu coating are CuTi2, CuTi, Cu4Ti3, and β‐Cu4Ti. The activation energy of intermetallic compound growth in the diffusion zone of Cu‐coated Ti–6Al–4V is 126.0 kJ mol−1, and the pre‐exponential factor is 0.1 m2 s−1. The tribological properties of the Cu‐coated Ti–6Al–4V alloy are best when subjected to diffusion treatment at 700 °C for 300 min, with weight loss reduced by 58.2% compared to the Ti–6Al–4V alloy. The wear resistance of the Ti–6Al–4V alloy can be enhanced by Cu coating and vacuum diffusion heat treatment, and the formation of the Cu–Ti intermetallic compound contributes to this improvement. These findings offer new insights for further advancements in the tribological properties of titanium alloys.

Mo-alloyed stainless steel coating with improved cavitation erosion resistance by plasma spraying a specially designed core-shell-structured powder

Plasma spray coatings are widely used to protect materials from cavitation erosion. However, the limited interlamellar bonding and porous structure of plasma spray coatings limit the potential of utilization of the coating materials for cavitation protection. In the present study, to improve the resistance of cavitation erosion for plasma spray coatings, molybdenum (Mo)-alloyed stainless steel coating with enhanced interlamellar bonding was deposited by spraying a specially designed core-shell-structured Mo-clad-304SS powder through shrouded plasma spray. The post-spray diffusion heat treatment was then also performed to further enhance the lamellar interface bonding of the coating. Owing to the high susceptibility to cohesion of the coating, cavitation erosion test was performed on the surface of resultant coatings to evaluate the quality of the interlamellar bonding within the coating. Results show that the cavitation erosion resistance of the 304SS-17Mo alloy coating with enhanced interlamellar bonding was significantly improved by 100% compared with that of conventional 304SS coating. After diffusion heat treatment, the cavitation erosion resistance of 304SS-17Mo alloy coating was further improved to as high as that of 304SS bulk due to the newly formed hard phases.

Diffusion mechanism of immiscible Fe-Mg system induced by high-density defects at the steel/Mg composite interface

Due to positive mixing heat between Fe and Mg, it is difficult to diffuse for Fe-Mg at the interface of steel/Mg laminated composites, resulting in the inability to achieve high-strength metallurgical bonding. In this paper, 20#steel/Mg laminated composites were prepared by large deformation rolling and subsequent diffusion heat treatment process. The interfacial bonding strength was improved by constructing high-density crystal defects at the interface to promote element diffusion. The mechanisms of interface morphology evolution and element diffusion were analyzed by finite element simulation and theoretical calculation. The results show after diffusion heat treatment, the bond strength of the large deformation rolled interface was increased from 14 MPa to 30 MPa. Fe-Mg transition layer with about 80 nm thickness as well as high-density vacancies, dislocations and grain boundaries were formed in the large deformation rolled interface region. During diffusion heat treatment, Mg elements diffused into grain interior and grain boundary regions of 20#steel under the effect of heat-force coupling, and the thickness of Fe-Mg transition layer increased to 150 nm, forming an Fe-based supersaturated solid solution. The interface with high-density defects constituted a non-equilibrium interface. The 20#steel internal energy in the non-equilibrium interface is able to overcome positive mixing heat of immiscible Fe-Mg system and provide the driving force for Mg elements diffusion. Promoting elemental diffusion by constructing high-density defects can be a new concept to achieve metallurgical bonding at the interface of immiscible metal laminated composites.

Characterization of Iron Aluminide Diffusion Coatings Obtained after Friction Surfacing

Iron aluminides are considered as candidate materials for high temperature applications for their excellent high temperature corrosion and oxidation resistance. In the present work, iron-aluminide coatings were developed by friction surfacing (6351 aluminum alloy deposited on a low-carbon steel substrate) followed by a diffusion heat treatment. The initial coatings were found to be geometrically homogenous and adhered well to the steel substrate. The heat treatment process was carried out at 550 °C for 48, 72 and 96 h and the resulting coatings were characterized in terms of microstructure, chemical composition, hardness distribution and phase composition. After heat treatment, the coating/substrate interface morphology was modified and presented patterns typical of Fe-Al intermetallic formation, as well as a substantial increase in hardness (>900 HV) relative to the initial as-deposited condition. With the diffusion treatment, initially Fe2Al5 was found to develop in the coatings, which was converted into FeAl2 after longer exposures.

Elemental Diffusion Behavior of Cu–Ti Alloys Prepared by Accumulative Roll Bonding‐Deformation Diffusion Process

The preparation of Cu–Ti alloys with homogenous elemental distribution is related to the engineering applications. Considering the thickness ratio and cross‐sectional geometry of components change with the increase in cumulative equivalent strain in accumulative roll bonding‐deformation diffusion (ARB‐DD) process, an elemental diffusion model is established based on different solute concentration fields. The accuracy of the model is verified by preparing Cu–Ti alloys in different states. The morphology of Ti layers evolve from a flat to a shuttle‐shaped structure due to the strong shear effect as cumulative equivalent strain increases. The diffusion heat treatment time for homogenization is predicted based on the model, and the results show that the model basically matches with the experimental results. The absolute error of prediction is within 9%, and the average error is about 4.4%. It indicates that the synergistic control of elemental diffusion behavior in ARB‐DD process can be achieved. Besides, it demonstrates that the effective diffusion activation plays a key role in determining the accuracy of the model. The difference of deformation energy storage in various cumulative equivalent strain states causes a significant change in effective diffusion activation energy, thereby producing a great influence on elemental diffusion behavior of Cu/Ti alloys.

The compositional homogenization control of Cu-Ti alloys prepared by accumulative roll bonding-deformation diffusion process

A new process of accumulative roll bonding-deformation diffusion (ARB-DD) with subsequent deformation diffusion heat treatment was proposed to solve the problem of poor composition homogeneity of alloys prepared by conventional accumulative roll bonding-diffusion heat treatment (ARB-DHT). The effects of ARB-DHT and ARB-DD on compositional homogeneity of Cu-Ti alloys were studied in comparison. And the mechanism of compositional homogeneity evolution of ARB-DDed Cu-Ti alloy was revealed. The results show that elements in Cu-Ti alloys prepared by the conventional ARB-DHT process are enriched and show a more obvious periodic distribution characteristic, and the compositional homogenization is poor. However, in ARB-DD process, the deformation process drives the Taylor lattice to reopen and form high-density nano-twin with a lamella width of 50–90 nm. Further, the high density of dislocations and stacking faults provides high diffusion rate channels for element diffusion, which effectively facilitates the diffusion and redistribution of elements in the subsequent diffusion heat treatment process and accelerates the compositional homogenization. On the contrary, in comparison with the conventional ARB-DHT process which promotes element diffusion through thermal effects, the ARB-DD process uses the synergistic effects of thermal and deformation effects to promote element diffusion, effectively enhancing the degree of compositional homogenization.

Interface heterogeneity of aluminum-steel bimetal parts manufactured via thixotropic-core compound forging

Thixotropic-core compound forging (TCF1) is a promising technology for the precision forming of aluminum-steel bimetal parts. In this paper, the hitherto unexplained mechanisms of the interface heterogeneity in TCFed aluminum-steel bimetal parts were investigated. Moreover, the effects of diffusion heat treatment on the interface heterogeneity and shear strength were studied. A thixotropic 7075 Al core and a 304L steel shell were used in TCF for the forming of a complex bimetal part. Results suggested that the interface heterogeneity in TCFed aluminum-steel bimetal parts is resulted from three aspects: (i) asynchronous material flow and the associated liquid segregation; (ii) tearing and separation of oxide films; and (iii) formation of liquid film at core surface. The local liquid segregation in distal areas of the bimetal part is helpful to improve the interface wettability and the separation of oxide films, thus accelerating the atom diffusion. In this case, reliable metallurgical bonding interface composed of Al-Fe intermetallic compounds (IMCs2) with satisfactory thickness is possible to be obtained. The diffusion heat treatment is helpful to the formation of reliable metallurgical bonding interface by improving the growth of IMCs. The optimal shear strength was tested to be 26.4 MPa after a heat treatment for 475 °C/2 h, indicating the formation of reliable metallurgical bonding. With the further increment in the heat treatment time, the interface heterogeneity will be diminished, whereas the shear strength may be deteriorated due to the overgrowth of IMCs.

Suppression of Coating Layer Crack by Diffusion Heat Treatment in Al-Si Coated HPF Steels

Suppression of Coating Layer Crack by Diffusion Heat Treatment in Al-Si Coated HPF Steels

Effect of diffusion heat treatment on microstructure and mechanical properties of a boron-containing nickel-based superalloy

Effect of diffusion heat treatment on microstructure and mechanical properties of a boron-containing nickel-based superalloy

Thermal cyclic resistance and long term inter-diffusion properties of slurry aluminide coatings modified with Si

High temperature oxidation resistant aluminide coatings fail due to their inability to maintain a protective scale. The corresponding mechanism of protection is based on of forming a thin Al2O3 layer, which depends on the availability of Al at the interface with the intermetallic coating. However, the Al content in the coating’s surface decreases with time at high temperatures due to interdiffusion with the substrate and/or frequent regeneration of the protective Al2O3 scale when it spalls. Si addition is known to enhance the life of Ni aluminide coatings under certain corrosion environments. In an effort to develop a stable, long lasting coating for industrial components exposed to metal dusting, diffusion aluminide coating has been produced by applying Al-Si slurries of various compositions on alloy 601. Aluminide coatings were prepared by applying a water base, Al slurry to which 1, 10 and 20 wt. % of Si were added, followed by a diffusion heat treatment. The microstructures of the coatings were similar, exhibiting an outer layer consisting of an Al-rich β-NiAl matrix, an almost stoichiometric β-NiAl inner layer and an interdiffusion zone between the coating and the alloy. Thermal cyclic tests in air at 1100 °C have demonstrated good adhesion and stability of all coatings. In addition, long term isothermal tests at 850 °C for up to 3 months in air showed high oxidation resistance of the coating but different degree of coating degradation depending on the composition. The coating with the lowest Si addition was the most stable.

Feasibility study for the development of Al-Si based coating on Ni-Cu alloy (UNS N04400)

Nickel-based alloys and superalloys find wide applications in marine, chemical and nuclear industries that require prolonged high-performance materials and sustenance against severe environments. Such materials are prone to corrosion due to varied kind of existing chemistries that exist within the actual operating conditions. To overcome such problems, several methods of process improvement, surface treatment and coating techniques are developed. The current research employs a different approach to develop aluminium–silicon based coatings by hot dipping technique on nickel-copper alloy (UNS N04400). An attempt has been made to deposit this coating at 750 °C, followed by diffusion heat treatment in a resistance furnace. A coating in the range of 80–100 µm thickness has been developed. Formation of intermetallic compounds are observed. The microhardness up to 140HV0.1 has been obtained for the developed coating. This paper tests the feasibility of developing aluminium–silicon based coatings on Ni-Cu alloy which can prove beneficial against various environments. The study opens a new possibility for future researches of coating.

Grain boundary modification and properties enhancement of sintered Nd-Fe-B magnets by ZnO solid diffusion

A grain boundary diffusion (GBD) process using rare earth-free compound as the diffusion source is employed to enhance coercivity of sintered Nd-Fe-B magnets. Typically, a ZnO layer was coated on the Dy-free sintered Nd-Fe-B magnets by sputtering deposition. After a solid diffusion heat treatment, the coercivity and corrosion resistance have been significantly improved. For a 4 mm-thick magnet, the diffusion of ZnO led to a maximum increase of intrinsic coercivity from 1085 to 1290 kA/m. This coercivity enhancement is much higher than those obtained in the previous investigations by GBD of non-RE compounds or alloys. Based on the microstructure analysis, the coercivity improvement is induced by the modifications of the composition and structure of grain boundary phase, which is slightly different from the conventional rare earth-based GBD. The formation of oxides at the triple junction of grains is the main reason for the enhanced corrosion resistance of diffused magnets. The present work thus demonstrated that ZnO can be employed as the GBD source for both coercivity enhancement and corrosion resistance improvement, which are important for preparing low-cost and high-performance Nd-Fe-B permanent magnets.

High temperature wear behavior of δ-Ni2Al3 and β-NiAl coatings formed on pure nickel using pack cementation process and diffusion heat treatment

Nickel aluminide coatings with three different compositions, namely δ-Ni2Al3, Al-rich β-NiAl and Ni-rich β-NiAl were produced by chemical vapor deposition on pure nickel with successive diffusion heat treatments. The influence of the coating composition on the wear resistance is systematically studied and discussed based on the stability region of the δ-Ni2Al3 to the hypo-stoichiometric β-NiAl phase using a ball-on-disk setup up to 600 °C in air with alumina as the counterpart material. Wear resistance generally increased with decreasing Al contents of the coatings tested. Nanoindentation measurements showed that Young's moduli and hardness decreased for decreasing Al contents in the NiAl system. This change is accompanied with an increase in ductility, resulting in higher resistance against cracking of the coating, which is found to be the main wear mechanism for Al-rich coatings.

Effect of the Ta addition on densification and mechanical properties of NbTi alloys prepared by SPS

In this study, we have fabricated a highly compact NbTi-xTa (x = 0, 1, 2) alloy was fabricated by spark plasma sintering (SPS) using the NbTi powder produced by the plasma rotation electrode process (PREP) and Ta powder produced by the sodium reduction method. Afterward, the alloy samples were subjected to diffusion heat treatment. The microstructure and phase compositions of the NbTi-xTa (x = 0, 1, 2) alloys were examined by using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectrometry (EDS), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Furthermore, we assessed the microhardness and compression properties of the samples. The results showed that sintering temperature and the Ta content were the key factors that affected the densification. Moreover, with the Ta content increase, the compactness of the alloy slightly decreased, while the microhardness increased. It was also indicated that the Ta addition of suppressed the grain growth and changed the NbTi solid solution from the original (110) surface to the (211) surface after the heat treatment. The first-principles calculation also showed that the Ta addition further stabilized the BCC structure of the NbTi alloy, and it was challenging to prepare HCP precipitates, such as α-Ti.