Ni Superalloy Research Articles

The formation and characterization of large twin related domains

The enhanced properties of grain boundary engineered metals are a result of their unique microstructures, which contain large clusters of twinned grains, called twin related domains. These large twin related domains in grain boundary engineered Ni were found to form through recrystallization. Orientation mapping showed that sparse nucleation and multiple twinning resulted in twin related domains containing hundreds of grains connected together in complex morphologies. A correlation was found between the size of the twin related domains and the overall twin boundary fraction. The same correlation was also observed in Cu and a Ni superalloy, showing that this is a general observation for grain boundary engineered microstructures. This finding can be understood through the topology of the twin related domains and an accompanying scaling relation is provided. The crystal orientations contained within each twin related domain were observed to depend on both the spatial correlation of twinning variants and the degree of branching in the twin boundary network. The results suggest a natural way of quantifying grain boundary engineered microstructures and provide a step toward making a closer connection between processing, microstructure, and performance.

Numerical and physical simulation of rapid microstructural evolution of gas atomised Ni superalloy powders

The rapid microstructural evolution of gas atomised Ni superalloy powder compacts over timescales of a few seconds was studied using a Gleeble 3500 thermomechanical simulator, finite element based numerical model and electron microscopy. The study found that the microstructural changes were governed by the characteristic temperatures of the alloy. At a temperature below the γ' solvus, the powders maintained dendritic structures. Above the γ' solvus temperature but in the solid-state, rapid grain spheroidisation and coarsening occurred, although the fine-scale microstructures were largely retained. Once the incipient melting temperature of the alloy was exceeded, microstructural change was rapid, and when the temperature was increased into the solid+liquid state, the powder compact partially melted and then re-solidified with no trace of the original structures, despite the fast timescales. The study reveals the relationship between short, severe thermal excursions and microstructural evolution in powder processed components, and gives guidance on the upper limit of temperature and time for powder-based processes if desirable fine-scale features of powders are to be preserved.

Characterization and Comparison of Inconel 625 Processed by Selective Laser Melting and Laser Metal Deposition

Inconel 625 (IN625) is a Ni superalloy ideal for the aeronautic and aerospace industries because of its superior mechanical and corrosion resistance properties. However, the production of components with complex geometries by traditional processes is very expensive, so additive manufacturing (AM) technologies can be applied to reduce the costs. In this work, IN625 alloy is processed by two different AM processes: selective laser melting (SLM) and laser metal deposition (LMD). First, the different powders are characterized, and then a parameter optimization is performed to obtain the highest possible density (over 99.8%) and hardness, showing very fine microstructures. The LMD process allows the highest build‐up rate, but results in less dimensional accuracy and a lower hardness than the SLM process.

Corrosion evaluation of alloys and MCrAlX coatings in molten carbonates for thermal solar applications

Stainless steels (SS) 310, 321, 347, Incoloy 800H (In800H), alumina-forming austenitic (AFA-OC6), Ni superalloy Inconel 625 (IN625), and MCrAlX (M: Ni, and/or Co; X: Y, Hf, Si, and/or Ta) coatings were corroded in molten carbonates in N2 and bone-dry CO2 atmospheres. Electrochemical tests in molten eutectics K2CO3–Na2CO3 and Na2CO3–K2CO3–Li2CO3 at temperatures higher than 600°C were evaluated using an open-circuit potential followed by a potentiodynamic polarization sweep to determine the corrosion rates. Because the best-performing alloys at 750°C were In800H followed by SS310, these two alloys were selected as the substrate material for the MCrAlX coatings. The coatings were able to mitigate corrosion in molten carbonates environments. The corrosion of substrates SS310 and In800H was reduced from ~2500µm/year to 34µm/year when coated with high-velocity oxyfuel (HVOF) NiCoCrAlHfSiY and pre-oxidized (air, 900°C, 24h, 0.5°C/min) before molten carbonate exposure at 700°C in bone-dry CO2 atmosphere. Metallographic characterization of the corroded surfaces showed that the formation of a uniform alumina scale during the pre-oxidation seems to protect the alloy from the molten carbonate attack.

Rough-cut fast numerical investigation of temperature fields in selective laser sintering/melting

This paper investigates selective laser sintering/melting (SLS/SLM) processes by a thermal model solved with a finite difference scheme. The density equation is semi-empirical based on the Arrhenius equation and experimental results taken from literature. Energy distribution function in the laser beam is appropriately considered. The main input parameters studied were as follows: laser speed, preheating temperature and laser power. The output parameters were maximum temperature and consolidation area. SLS was tried on polycarbonate powder and SLM on a Ni superalloy. Variations of temperature and density with time at the centre of the beam and at another two points at increasing distances from it were found to be qualitatively similar irrespective of the laser speed and power. Maximum temperature reached is not affected significantly by preheating temperature, by contrast to the consolidated depth and consolidation area. The latter are also affected by laser power. Increase in laser speed leads, as expected, to a decrease in consolidation depth and area as well as a decrease in maximum temperature. Continuous and pulsed lasers exhibited large differences; e.g. for the same speed and power, a pulsed laser results in much lower temperature gradients during cooling. The model can be used to obtain a preliminary picture of the response of any material to SLS/SLM processing provided that the appropriate input data is available, which is a crucial factor not to be underestimated.

Determining the oxidation behavior of metal powders during heating through thermogravimetric and evolved gas analysis using a coupled thermogravimetry-gas chromatography-mass spectrometry technique

The oxidation behavior of a fine Ni superalloy (NiSA) powder, Inconel 625, within an alumina support powder during heating has been determined using combined thermogravimetry (TG) and evolved gas analysis (EGA). A gas chromatographic-mass spectrometer (GCMS) was used for EGA. The GCMS was able to semi-quantitatively measure the rate at which air was removed from the furnace due to a He purge, where no pre-evacuation was used. It also was able to measure the effectiveness of a three cycle evacuation and He purge in reducing the oxygen content in the furnace and identify an important alumina off-gassing process that exposed the NiSA powder to H2O and CO2. The parallel use of the TG signal was able to quantitatively determine the relationship between NiSA oxidation and relative concentrations of the oxidizing gases created by the differing furnace pre-conditions and the alumina powder off-gassing process. It was concluded that the developed TG-GCMS method can be widely applied to quantitatively study oxygen pick-up in metal powders under industrially relevant processing conditions.

The characteristic of deformability of Fe–Ni superalloy during high-temperature deformation

The characteristic of deformability of Fe–Ni superalloy during high-temperature deformation

Design of functionally graded multilayer thermal barrier coatings for gas turbine application

Progress in design of thermal barrier coatings (TBCs) is based upon application of new materials and deposition techniques. In this work traditional NiCrAl bond coat YSZ top coat were deposited on Ni superalloy substrate by inexpensive dry detonation spraying, while finishing layers of oxide nanocomposites (LaAlO3–La2Zr2O7, LaAlO3–LaCuAl11O19) were deposited on YSZ by slip casting. Complex oxides were prepared via Pechini route. Thin (~ 20 μm) finishing layers were deposited by slip casting of suspensions of oxides mixture in isopropanol with addition of polyvinylbutyral. Genesis of the texture, composition and real/defect structure of bulk nanocomposite materials and deposited layers after annealing in air up to 1300 °C as well as after series of thermal shocks by heating up to 1200 °C by H2–O2 burner were studied by combination of diffraction (high resolution SEM and TEM with EDX, XRD on synchrotron radiation) and spectroscopic (UV–Vis, EXAFS, laser-excited Dy3 + luminescence spectra) methods. Thermal conductivity of nanocomposites and TBCs was determined by using NETZSCH LFA 457 MicroFlash. Nanocomposites were shown to retain porosity as well as nanosizes of disordered domains of oxide phases even after sintering at high temperatures. A good adhesion and compatibility of all layers in TBCs were demonstrated, while in general disordering of the oxides structure in deposited layers was higher than that in bulk materials due to the effect of depositing procedure and interaction between layers. This provides a low thermal conductivity of nanocomposites and functionally graded TBC. After 90 thermal shocks neither layers spallation nor cracks were revealed, 8YSZ and finishing layers retaining porosity, nanocrystallinity and disordering required for a low thermal conductivity and cracks trapping.

On the mechanistic basis of fatigue crack nucleation in Ni superalloy containing inclusions using high resolution electron backscatter diffraction

A series of interrupted three-point bend low-cycle fatigue tests were carried out on a powder metallurgy FHG96 nickel superalloy sample containing non-metallic inclusions. High resolution electron backscatter diffraction (HR-EBSD) was used to characterise the distribution and evolution of geometrically necessary dislocation (GND) density, residual stress and total dislocation density near a non-metallic inclusion. A systematic study of room temperature cyclic deformation is presented in which slip localisation, cyclic hardening, ratcheting and stabilisation occur, through to crack formation and microstructurally-sensitive propagation. Particular focus is brought to bear at the inclusion–matrix interface. Complex inhomogeneous deformation structures were directly observed from the first few loading cycles, and these structures were found not to vary significantly with increasing number of cycles. A clear link was observed between crack nucleation site and microstructurally-sensitive growth path and the spatially-resolved sites of extreme values of residual stress and GND density.

Assessment of surface integrity of Ni superalloy after electrical-discharge, laser and mechanical micro-drilling processes

Currently, electrical discharge machining (EDM) is the established manufacturing process for making holes through Ni superalloy turbine blades to facilitate blade cooling. Surface integrity of these sub-millimetre holes is of paramount importance since it directly influences the fatigue life of the component. In this paper, EDM is compared against laser and mechanical micro-hole drilling of Inconel 718 alloy. The motivation was to assess alternative manufacturing methods for raising the quality threshold of micro-drilled holes. Mechanical and metallurgical characterisation of surface and subsurface regions of the holes produced by the three processes was undertaken using backscatter electron (BSE) microscopy, electron backscatter diffraction (EBSD) and nano-indentation techniques to assess surface hardness, grain misorientation, plastic deformation and the heat-affected zone. The results suggest that mechanical micro-drilling offers improved mechanical, metallurgical and geometrical properties compared to both laser and EDM processes.

Picosecond laser shock peening of base Nimonic 263 material and laser welded Nimonic 263 alloy

Laser welding is a welding process with a high energy density and numerous advantages over conventional welding methods (narrow heat affected zone, lower heat input, less distortion, and the like.) Mechanical treatment laser is an innovative technique for improving the properties of fatigue resistance, abrasion and corrosion resistance. In this paper, mechanically processed by laser two groups of samples; base materials and laser welded joints. Sheets made of Ni superalloy 2mm thick -millisecond Nd: YAG laser, while the mechanical treatment made picosecond Nd: YAG laser. Welds are tensile tested, and microstructure was observed by optical and scanning electron microscopy. The microhardness was measured by Vickers method with a load of 10 N.

Characterization of Alumina Scales Grown on a 2nd Generation Single Crystal Ni Superalloy During Isothermal Oxidation at 1050, 1100 and 1150 °C

This paper concerns microstructural characterization of alumina scales formed on a 2nd generation single crystal Ni superalloy during isothermal oxidation in dry oxygen at 1050, 1100 and 1150 °C for 100 h. Samples for high resolution characterization of Al2O3 scales were prepared using a focused ion beam (FIB) method. High resolution TEM and S/TEM techniques were used for a detailed characterization and a direct comparison of the phase composition and chemistry of the oxide scales formed during high temperature oxidation. The growth of transient θ-Al2O3 and its transformation to α-alumina is addressed for each oxidation temperature along with the differences in the diffusion of reactive elements, such as Hf, Zr and Y, through grain boundaries of the α phase. The θ to α transformation front was proven to move from the metal-scale to the scale-gas interface. The results presented in this paper indicate that after 100 h of oxidation at 1050 and 1100 °C there are still some θ-alumina grains remaining and even in the regions where the transformation to α was finished the surface retained the whisker-like morphology.

Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading

Electron backscattered diffraction (EBSD) has been employed to study a polycrystalline nickel superalloy containing a complex non-metallic agglomerate under thermal loading. Heterogeneous distributions of elastic strains are observed near the inclusion due to its complex geometry and these have been quantified. Lattice rotations were also related to geometrically necessary dislocation (GND) density (∼1014m-2), indicating the development of localized plasticity arising from the mismatch in thermal expansivity between the Ni polycrystal and the inclusion. A crystal plasticity finite-element (CPFE) model which explicitly represents the full detail of the complex microstructure was developed to interpret the experimental measurements, and good quantitative and qualitative agreement has been obtained. However, a limitation of the EBSD technique when investigating polycrystal systems is that full-field, transgranular strain measurement remains difficult due to the necessity to reference a lattice spacing within a grain for strain calculation. An inverse reference shifting methodology has been developed using CPFE modeling to overcome this problem, thereby allowing like-for-like and grain-by-grain strain comparisons to be made. The method, in conjunction with high-resolution EBSD, shows promise for the determination of full-field strains and rotations in polycrystalline materials, and provides key information for fatigue nucleation in these material systems.

A hybrid arc spray forming technique for the manufacture of nickel superalloy IN617

Spray forming produces cast microstructures with comparatively low macro‐ and micro‐structural chemical segregation and is thus well‐suited for the manufacture of complex chemistry, multi‐component alloys that otherwise show strong elemental segregation. Although spray formed Ni superalloys have shown properties equivalent or superior to their conventionally cast/wrought counterparts, they have not been adopted commercially because of the difficulties in ensuring a high process yield and the complexity and associated cost of large‐scale Ni superalloy melting. In this paper, we describe a hybrid arc spray forming (HASF) process in which costly, large‐scale alloy melting as pre‐cursor to spray forming is avoided by the use of a consumable wire feedstock. To achieve thermal conditions of melt spray forming – essential to produce a refined, polygonal grain structure – a customised secondary atomisation system has been developed. Fe‐0.8 wt%C and Ni superalloy IN617 microstructures and preliminary mechanical properties suggested that hybrid arc spray forming may offer an attractive combination of convenience, low cost and mechanical performance.

Evolution of deformation and annealing textures in Incoloy 800H/HT via different rolling paths and strains

In this report, we characterize the deformation and annealing textures of Incoloy 800H/HT, following different rolling conditions that produced different textures in this material. Incoloy 800H/HT is an austenitic Fe–Ni super alloy and is considered to be a candidate material for Gen IV nuclear reactors. Fossil fuel plants have used this alloy for decades; however, as grain structure and texture parameters can strongly affect its physical and mechanical properties in-service, engineers should consider some structural modifications before using this alloy in nuclear reactors.In this study, we used Thermo-Mechanical Processing (TMP) to alter the texture of Incoloy 800H/HT. We applied various thickness reductions (10%, 30%, 50%, 70%, and 90%) to this alloy using two different rolling paths, followed by annealing. Our detailed study of the deformation and annealing texture evolution shows that upon different rolling paths, the final deformation texture and the annealing texture were different. Brass texture was the dominant component for the uni-directional rolled (UDR) samples, while a combination of brass (B) and ND-rotated brass (BT) were the dominant components in the cross-rolled (CR) samples. Annealing textures of UDR samples were mainly Goss, copper, S, recrystallized brass ({236}〈385〉) and minor copper twin {552}〈115〉 components. At lower deformations (<50%), the annealed CR samples showed tilted cube, S, recrystallized brass ({236}〈385〉) and minor Goss twin (113)〈33¯2〉. However, at higher rolling reductions, the B+BT deformation texture was retained for the CR samples.

Mechanical properties and microstructure of large IN713LC nickel superalloy castings

When approving large castings for use, mechanical values are measured by room temperature tensile test on test bodies made from the centres of the castings. These test bodies often exhibit lower values than required, in particular ultimate tensile strength and elongation. The presence of fragile niobium carbides in the structure of the alloy was identified as the main cause of this situation, which originated as a result of the slow speed cooling. Because of large grain size compared to gauge length diameter, crystal orientation of individual grains also plays an important role. Interdendritic carbides are oriented parallel with dendrites thus parallel with directions in Ni superalloy crystal lattice. Grain oriented with axis parallel to tensile direction has platelet carbides oriented perpendicular to the tensile axis. In that position carbides crack easily at low grain deformation. Longitudinal metallographic cuts of selected tested specimen gauge lengths were prepared and grain orientation of individual grains was investigated by EBSD, Specimens, where grain oriented with axis parallel to tensile direction were found, performed poor elongation values 3%, compared to 7% where no such oriented grains were found.

The Effect of Long-Term Annealing on Microstructure of Aluminide Coatings Deposited on MAR M200 Superalloy by CVD Method

The paper presents the influence of long-term heat treatment on aluminide coating. The MAR M-200 nickel superalloy was use as base material. The aluminide coating was deposited in low-activity chemical vapour deposition process using Ion Bond BPX Pro 325s device with following parameters: temperature 1040°C, time 12 hours, pressure 150 mbar. The samples (14 mm diameter) were annealed at 1020°C in vacuum for 12, 16 and 20 hours respectively. The XRD phase analysis and SEM microstructural observation with EDS analysis were conducted. The phase composition analysis showed that after deposition the NiAl phase is a main component of the coating. After the 20-hour-long annealing presence of Ni3Al phase in the coating was observed. Changes in structure of coatings and Al content during heat treatment were detected as well. The maximum duration of heat treatment performed for Ni superalloy with aluminide coating without significant changes in coating phase composition and microstructure is 16 hours.

The Influence of Thermal Treatment upon Nanostructure and Composition of YZrO Based Ceramics Obtained by Atmospheric Plasma Spraying

This paper presents a concept of thermal barrier coating used to prevent exfoliation and increase the maximum working temperature of a turbine engine. The coating consist of a ZrO2/20%Y2O3 ceramic layer and a NiCrAlY adherent layer, deposited by atmospheric plasma spraying (APS), on a Ni super alloy sample usually used for the manufacturing of turbine blades. The sample was subjected to thermal treatment and after that analyzed. SEM images of the samples were taken, to analyze their behaviour to thermal treatment. . Reference sample (without thermal treatment) and samples subjected to 5, 10 and 15 hours heat treatment were investigated using X-ray diffraction in 25-130o 2θ interval.

(Cr1−x Al x )N as a candidate for corrosion protection in high temperature segments of CCS plants

Three (Cr1−xAl x )N (x = 0.33, 0.5, 0.66) coatings were deposited on Ni-super alloy IN718 using reactive magnetron sputtering. The oxidation behavior of all coatings at 900 °C up to 500 h in air was studied. Furthermore, the corrosion behavior of the (Cr0.33,Al0.66)N coating at 900 °C (corrosion type I) by spraying a uniform salt scale of Na2SO4 on the sample surface (1 mg/cm2) was investigated. It was found that the coated samples indicate significantly higher oxidation and corrosion resistance compared to uncoated Ni super alloy. This is mainly due to the formation of protective Cr2O3 and Al2O3 layers on the coating surface. With the increase of Al content, the coatings exhibited improved oxidation resistance. The formation of thin and adherent Al2O3 scale on the surface of Al rich coating is the reason for its better oxidation behavior. The detailed structures of the oxide scales and the interdiffusion between coating and substrate were studied using energy dispersive X-ray spectroscopy-analysis.

A comparison on thermomechanical properties of plasma-sprayed conventional and nanostructured YSZ TBC coatings in thermal cycling

Conventional and nanostructured YSZ coatings were deposited on IN-738 Ni super alloy by atmospheric plasma spray technique. The oxidation resistance of the coating was measured at 1100°C using an atmospheric electrical furnace. According to the experimental results, nanostructured coatings showed a better oxidation resistance than conventional one. The improved oxidation resistance of the nanocoating could be explained by the change of structure to a dense and more packed structure in this coating. The thermomechanical properties of the coating were tested using thermal cycles, thermal expansion and elastic modulus tests during which nanostructured YSZ coating again showed a better performance by structural stability. The results indicate that the thermal stress within the nanostructured YSZ is lower than of the conventional YSZ.