Ni-based High-temperature Alloy Research Articles

Simulation and experimental study on temperature field and flow field of laser cladding high-temperature Ni-based alloys

Abstract The multiphase liquid's development during the laser cladding process involves complex processes such as energy input, mass input, heat transfer, multi-phase liquid flow, and rapid solidification. Based on the coupled theory of temperature field and velocity field, a multi-field coupling model for Ni-based high-temperature alloy on the surface of 40Cr has been established. Throughout the modeling procedure, the surface tension coefficient's impact o.During the modeling process, the effect of the surface tension coefficient on the flow velocity of the molten metal in the melt pool is considered, along with the tracking of the gas/liquid free boundary through the use of the dynamic mesh technique. By comparing experimental and simulation results, it is found that the prediction error of the model ranges from -11.79% to 12.08%, demonstrating that the model has certain explanatory and predictive capabilities for laser cladding of Ni-based high-temperature alloys

Enhanced thermal performance of CeYSZ coating on Ni-based high temperature alloy with a unique Y-doped Al2O3 barrier layer via cathodic plasma electrolytic deposition

Cathodic plasma electrolytic deposition (CPED) has shown considerable potential in producing high-performance thermal barrier coatings (TBCs) for components operating in extreme thermal environments, such as gas turbines and aviation engines. However, the dendritic and porous structures greatly limited the use of CPED-coatings in some harsh environments. In this study, one Y-doped Al2O3 (Y/A) barrier layer was prefabricated before the deposition of CeO2 and Y2O3 stabilized ZrO2 (CeYSZ) TBCs using CPED. The results suggested that the prefabricated Y/A barrier layer could attenuate the plasma bombardment and promote the uniform discharge, resulting in the formation of a thinner gas film on the cathode surface, which significantly avoided the dendritic defects and improved the density of the CeYSZ coating from 6.788 g/cm³ to 6.978 g/cm³. Consequently, the thermal insulation temperature of Y/A-CeYSZ coatings increased by 83.7% than that of CeYSZ coating, and the thermal cycling performance at 1300 °C demonstrated a 52.5 % improvement. This study provided a valuable method for the development of novel CPED TBCs.

Micro mechanical property investigations of Ni-based high-temperature alloy GH4169 based on nanoindentation and CPFE simulation

In this research, the microscopic deformation behavior of GH4169 superalloy is investigated by a series of nanoindentation experiments and simulations based on crystal plasticity finite element (CPFE) model, including hardness, creep displacement and stress distribution around indentation. In this regard, the crystal plasticity constitutive model based on dislocation slip for face-centered cubic (FCC) crystal of concern is summarized and numerically implemented at first. Moreover, nanoindentation experiments are performed on specimens with various peak loads and load times to investigate the mechanical response and deformation mechanism at the micro-scale. Nanoindentation test results show that load rate, peak load, and nanoindentation size effect (ISE) all affect the microscopic deformation behavior of GH4169. The value of creep stress exponent, m, calculated from the creep experimental data is greater than 3, indicating that dislocation slip is dominant creep mechanism at room temperature.The nanoindentation simulation data based on the crystal elastic–plastic parameters determined by the trial-and-error method are in good agreement with the experimental data, demonstrating that the CPFE simulation results are credible. Therefore, this study can be used as a reference to investigate other alloy materials.

Research on the mechanism of surface damage of Ni-based high-temperature alloy GH4169 based on nano-cutting

In order to investigate how cutting parameters affect the nano-cutability of Ni-based high-temperature alloy GH4169, molecular dynamics (MD) simulations of the material have been conducted on a large-scale atomic molecular/massively parallel simulator (LAMMPS) for different cutting speeds and cutting depths. EAM potential is used between Ni, Fe and Cr, whereas Moses potential is used between the workpiece and the tool or between the tools. The results show that at the cutting depth of 1 nm, the chip shape has a tensile necking with increasing cutting speed; at the cutting depth of 3 nm, the shear line width increases with increasing cutting distance and there is an abrupt change in the evolution of both FCC and amorphous atoms at all cutting speeds. At the same cutting depth, the cutting area temperature increases with increasing cutting speed; at the same cutting speed, the cutting area temperature increases markedly with increasing cutting depth, too.The dislocation density of 1/6<112> Shockley with a cutting depth of 1 nm is higher than that of other depths at the same cutting speed,and the dislocation density of 1/6<112> Shockley is obviously positively correlated to cutting speeds at the same cutting depth.

A first-principles based description of the Hf-Ni system supported by high-temperature synchrotron experiments

Hf-Ni is an important binary system for high temperature alloys and shape memory alloys which has been investigated several times in the literature but often using samples of Hf contaminated by Zr. The thermodynamics of this system are remodeled in this work based on first-principles calculations and additional experiments using Hf with relatively low Zr contamination (0.25 wt. %). Diffusion couples in the Ni-rich portion of the Hf-Ni system heat treated at 1173, 1273 and 1373 K are used to measure phase stability and Hf solubility in the fcc phase. The solubility observed in fcc Ni from Ni/Ni50Hf50 (at.%) diffusion couples is larger than that observed in previous experiments. These results are the only source fit to during modeling of the fcc solubility to mitigate effects from Zr contamination. Data in the literature suggests that the high temperature crystal structure of the B33 NiHf phase is, in fact, the B2 structure. High temperature synchrotron measurements provide confirmation of this crystal structure. Modeling of the B2 phase was aided by first-principles calculations using special quasi-random structures (SQS). The present CALPHAD model will prove useful when designing shape memory alloys containing Hf and when modeling the Hf activity in Ni-base high temperature alloys.

Reusable chromium oxide coating with lubricating behavior from 25 to 1000 °C due to a self-assembled mesh-like surface structure

The chromium oxide coatings are deposited on Ni-based high-temperature alloy substrates through an arc ion plating system and then annealed at 1000°C in air for 2h. During annealing process, Ti and Cr atoms in the substrate diffuse to coating surface and lead to the formation of a mesh-like heave structure comprised of Cr2O3 and Cr2Ti7O17 phases. The annealed coating exhibits the excellent self-lubricating behavior with friction coefficients below 0.3 from 25 to 1000°C. Friction coefficients show no evident change even in five thermal cycles, showing a great potential as self-lubricating coating operated in a wide temperature range. While the mesh-like heave structure is damaged by friction, Ti and Cr atoms diffusion could continuously supplement the heave structure at the damaged locations at high temperature, enduing the mesh-like heave structure with the self-healing ability.

On the behaviour of minor active elements during oxidation of selected Ni-base high-temperature alloys

We have examined the distribution of active minor elements in the oxide scales developed by selected Ni-base alloys with commercial grades. Emphasis is placed upon Mn, La and Si in a chromia-forming alloy and Y in an alumina-forming alloy. Initially, La and Y have been segregated at free surfaces and then become constituents of the oxides in contact with the substrates. A continuous layer of MnCr2O4 is formed above La- and Si-modified inner chromia layer. Silicon has been homogenously distributed throughout the grain structure, however, some La is present as LaCr2O3 particles and most of the remainder has been segregated at grain boundaries. The results indicate that the collective effect of Mn, Si and La is to extend protection by chromia to temperatures in excess of 1000 °C. Yttrium in the alumina- forming alloy is found to predominantly segregate at grain boundaries of nanostructured oxide with improved mechanical strength.

Role of annealing temperatures on the evolution of microstructure and properties of Cr2O3 films

Cr2O3 films were deposited on Ni-based high-temperature alloy substrates by using a cathodic arc ion plating system and then annealed in air at different temperatures. The effects of different annealing temperatures on the microstructure, mechanical and tribological properties of the films were examined. Results showed that the as-deposited Cr2O3 films were primarily amorphous with crystallization and vaporization temperatures of 360°C and 940°C, respectively. Annealing above the crystallization temperature improved the hardness, adhesion, and wear life of the Cr2O3 films. A mesh-like heave structure comprising Cr2O3 and Cr2Ti7O17 phases formed on the film surface after annealing above the vaporization temperature. The mesh-like heave structure endued the film with excellent tribological properties in a wide temperature range from room temperature (RT) to 1000°C.

Effect of Cold Deformation on Microstructure and Mechanical Behavior of Ni-based High Temperature Alloy GH3535

The effect of extensometer induced cold-tesile deformation on microstructure and mechanical properties of Ni-based high temperature alloy GH3535 were investigated by means of OM and TEM as well as measurement of true stress-true stain curves. It was found that GH3535 alloy shows characteristics of strong work hardening; cold deformation can result in significant increase of its strength and hardness, whereas decrease of its ductility. With the increase of deformation degree grains were elongated along the deformation direction and twins became profusely lager. The work hardening kinetics of GH3535 alloy is constant with Ludwigson model, dislocation slipping and twin are the main deformation mechanism. With the increase of deformation degree the slip behavior of dislocations changes from single slip to cross slip. When the deformation degree below 30% the work hardening is mainly caused by the dislocation long-range stress field and twin, conversely, for the deformation degree above 30% work hardening is mainly caused by the dislocation short-range stress field and deformation twin.

Toughness measurement and toughening mechanisms of arc ion plating Cr2O3 films treated by annealing

Abstract Chromium sesquioxide (Cr2O3) films were deposited on Ni-based high-temperature alloy substrates by an arc ion plating technique and then annealed at different temperatures and heating rates. The influence of annealing conditions on the toughness of Cr2O3 films was calculated according to spherical indentation tests. The increase in grain size and compressive stress, variety of microstructure and surface morphology, and atom diffusion that resulted from annealing caused toughness variations. The increase in grain size closed micro-cracks along the direction of film growth. Compressive stress and a multi-crystal plane led to cracks caused by indentation that required more energy to break through the film. In the process of indentation, turning, bifurcating, and bridging of cracks on film surface was also able to dissipate energy. Atom diffusion in the process of 1000 °C annealing also played a role in grain boundary toughening. The toughness improvement of Cr2O3 film significantly improved friction life.

Microstructural study of NiCrAlY electrodeposits

Pulse electrodeposition technique was used to co-deposit Ni with NiCrAlY powder on Ni-based high temperature alloy substrate. Pure nickel anode was immersed in a standard Watt’s bath containing fine particles of NiCrAlY powder that were entrapped during electrodeposition to form a NiCrAlY electrodeposit on cathode specimen surface. Diffusion heat treatment was conducted in argon at ≈1150°C and the samples were oxidized at 1000°C in air. Scanning and transmission electron microscopy coupled with energy dispersive X-ray spectroscopy and X-ray diffraction were used to characterize the microstructure and identify the phases. Pulse electrodeposition resulted in dense and fine-grained deposit with the formation of Al2O3 oxide at the coating surface after exposure to high temperature.

Joining of C/C Composites and GH3128 Ni-based Superalloy with Ni-Ti Mixed Powder as an Interlayer

Abstract A modified SiC coating was prepared on the surface of C/C composites by a pack cementation process. Ni-Ti mixed powder was used as an interlayer to join SiC coating modified C/C composites and GH3128 Ni-based high-temperature alloy (GH3128) by a vacuum hot-pressing diffusion process. The chemical element distribution, microstructure and mechanical properties of the joint between C/C composites and GH3128 were investigated by X-ray diffractometry, scanning electron microscopy, energy-dispersive spectrometry and material universal testing machine. Results show that the SiC modified coating on the surface of C/C composites not only improves fully the wettability of C/C composites to the Ni-Ti intermediate layer, but also alleviates effectively the thermal stress in the joint due to mismatch thermal expansion. Under the same joining condition, the shear strength of the joint with SiC modified coating at room temperature is up to 22.50 MPa, whereas that of the joint without SiC coating modification is almost 0.

Microstructure and thermal cycle resistance of plasma sprayed mullite coatings made from secondary mullitized natural andalusite powder

Natural andalusite powder was calcinated at a high temperature in air to realize secondary mullitization. The resultant secondary mullitized powder was spray-dried and heat-treated to improve sprayable capability. The heat-treated spherical powder was then plasma sprayed onto Ni-based high-temperature alloy (Hastelloy C-276) to form mullite coatings. The chemical composition and phase structure of the as-sprayed and thermally cycled mullite coatings were determined by means of energy dispersive X-ray fluorescence (ED-XRF) and X-ray diffraction. The microstructure of the as-sprayed coatings was analyzed by using a scanning electron microscope; and their porosity, microhardness and bonding strength were measured. Moreover, the phase transition temperature and enthalpy of the coatings were determined by means of differential scanning calorimetry; and their thermal shock resistance was evaluated as well. Results show that the spray-dried and heat-treated powder consists of mullite and a small amount of Al2O3; while the as-sprayed mullite coatings are composed of crystalline mullite as the major phase and a small amount of amorphous glass phase. During thermal cycle test, the amorphous glass phase is partially transformed to crystalline mullite, finally leading to failure of the coatings. Whether before or after thermal cycle, the mullite coatings experience phase transition around 980°C, and the enthalpy of crystallization is determined to be −141.9×10−3J/kg and −95.48×10−3J/kg, respectively. The as-sprayed mullite coatings have a porosity of about 6.0±0.2% and possess good thermal cycle resistance, showing promising prospect in a high-temperature application.

Oxidation behaviour of selected wrought Ni-base high temperature alloys when used as flame tube material in modern blue flame oil burners

Modern blue flame burners are equipped with a flame tube that serves for recirculation of exhaust gas and for flame stabilization. The in operation material temperature, temperature changes and atmospheres at the flame tube have been measured. Exposure tests of four Ni–Cr-alloys with different concentrations of Al have been carried out under these conditions. The breakdown of the material was connected with the failure of the oxide layer and finally with the formation of internal aluminium nitrides. An approach has been developed for predicting the failure of the oxide layer and hence of the life time of the material.

Corrosion behaviour of high temperature alloys in impure helium environments

Corrosion tests with Ni-base high temperature alloys were carried out at 900 and 950°C in simulated high temperature reactor helium environments. It is shown that the carburization and decarburization behaviour is strongly affected by the Cr and Ti (Al) contents of the alloys. In carburizing environments, additions of Ti, alone or in combination with Al, significantly improve the carburization resistance. In oxidizing environment, the alloys with high Cr and Al (Ti) contents are the most resistant against decarburization. In this environment alloys with additions of Ti and Al show poor oxidation resistance. The experimental results obtained are compared with a recently developed theory describing corrosion of high temperature alloys in high temperature reactor helium environments.