Ni-based Superalloy Research Articles

Weldability of new Ni-based superalloy G27: Effect of pre-weld solution annealing on the hot cracking susceptibility

The hot cracking susceptibility of a new superalloy G27 was investigated by Varestraint weldability testing. The influence of pre-weld solution annealing conditions recommended by the alloy manufacturer to obtain different grain sizes (1010 °C/1 h, 1066 °C/0.5 h, and 1110 °C/1 h) for specific applications was studied. The fusion zone solidification cracking was independent of pre-weld solution annealing, and they are associated with the γ/Laves eutectic constituents. The solution-annealed material at 1010 °C/1 h showed the lowest extent of liquation cracking, while the 1110 °C/1 h condition was the most susceptible. The increased level of liquation cracking was primarily attributed to an increased grain size of pre-weld solution-annealed material. The heat-affected zone liquation cracking was identified to be caused by constitutional liquation of NbC and supersolidus melting.

Phase formation and mechanical analysis of sintered Ni25Al25Co15Fe15Mn8Ti7Cr5 high entropy alloy

In recent years, the pursuit of cutting-edge materials has intensified, with a focus on affordability, lightweight characteristics, and exceptional performance under high-temperature conditions, to serve as alternatives to Ni-base superalloys and other conventional alloys. Potential materials suitable for high-temperature structural applications with lightweight characteristics are intermetallics such as NiAl, and TiAl, but pose numerous fabrication challenges and poor ductility behaviour at room temperature. In view of this, a novel Ni25Al25Co15Fe15 Mn8Ti7Cr5 high entropy alloy (HEA) was fabricated using spark plasma sintering (SPS). The alloy was developed at a sintering temperature of 850 °C, a heating rate of 90 °C/min, a pressure of 50 MPa, and a dwelling time of 5 min. X-ray diffraction, scanning electron microscopy, and Vickers hardness tester were used to investigate the phase formation, microstructure, and mechanical properties of the HEA, respectively. The microstructure of the sintered HEA shows a homogenous dispersion of the alloying metals. The sintered microstructures showed a mixture of simple and complex phases. The grain size analysis shows that the sintered HEA exhibited a lower grain size of 2.28 µm and a refined crystallite size of 3.159 µm. The microhardness value and relative density of the sintered HEA are 135.8 HV and 99.56 %, respectively.

Si modified aluminide coatings on a Ni-base superalloy prepared using Al-Si plasma irradiation: Growth mechanism and oxidation behavior

The growth mechanism and high temperature oxidation behavior of Si-modified aluminide coatings prepared on a Ni-base single crystal superalloy DD98M using vacuum plasma irradiation were investigated. The growth of the coatings under plasma irradiation was much faster than thermal diffusion-controlled process. The coatings comprised an aluminide outer zone, where Si existed as solid-solute, and an inter-diffusion zone where silicide particles precipitated in aluminide matrix. The crystalline structures of the aluminides were found dependent on the irradiation power. β-NiAl was formed at higher irradiation power while δ-Ni2Al3 was formed at lower irradiation power. The β-NiAl coatings had higher oxidation resistance than the δ-Ni2Al3 ones usually, with an exception of one β coating which was prepared at the highest substrate bias which underwent severe internal oxidation. The presence of numerous voids in the oxide scale of Ni2Al3 may be related to the large number of Kirkendall voids present in the as-prepared coating.

Quantitative analysis of deformation characteristics and corrosion properties of high energy laser shock peened Ni-based superalloy

This study examines the influence of high-energy laser shock peening (LSP) using 7 J and 10 J pulse energies on the sub-surface deformation characteristics of Inconel 718 superalloy. High-magnitude compressive residual stresses were induced into the samples after LSP with large residual stress depths of the order of 2 mm – the experimental observations were in good agreement with finite element analyses of the LSP process. The propagation of intense shock waves led to an increased strain hardening and dislocation densities that were experimentally quantified by synchrotron diffraction and transmission electron microscopy. Microscopic analyses revealed highly refined grain structure only at the surface without much refinement observed in the residual depth region. Alongside high degree of strain hardening, a profuse amount of adiabatic shear bands was observed in the hardened depth, indicative of simultaneous strain localisation under such high laser pulse energy. These bands occurred along common slip planes in the Ni γ-matrix and could be potential areas of instability leading to failure. The LSP-treated samples exhibited improved corrosion resistance, with higher laser pulse energy peened samples performing better.

Temperature-dependent deformation behavior of Hastelloy X Ni-based superalloy fabricated via laser powder bed fusion

Understanding the deformation behavior of additive manufacturing components across a wide spectrum of operating temperatures is crucial from an application standpoint. This study investigated the tensile deformation behavior of Hastelloy X (HX), fabricated via laser powder bed fusion (LPBF), over a temperature range of -50 °C to 900 °C through uniaxial tensile tests. The focus was on the temperature-dependent tensile properties and deformation mechanisms of HX in a fully heat-treated condition, involving initial hot isostatic pressing followed by solution treatment. The results indicate that as the temperature increases, the ultimate tensile strength decreases sharply from approximately 790 MPa at -50 °C to 610 MPa at 200 °C, then gradually declines to 530 MPa at 600 °C, and finally drops to 230 MPa at 900 °C. Meanwhile, the yield strength falls rapidly from 345 MPa at -50 °C to 230 MPa at 200 °C, then slowly reduces to 170 MPa at 600 °C, stabilizing up to 900 °C. Electron back scattered diffraction analysis revealed that with rising temperature, the predominant plastic deformation mechanism transitions from deformation twinning to slip bands and eventually to dynamic recrystallization. Remarkably, the strain hardening capacity increases with rising temperature below 600 °C, attributed to the beneficial effects of deformation twins and subsequent slip bands. Furthermore, dynamic strain aging takes place between 300 °C and 600 °C, slightly limiting the improvement of plasticity within this intermediate temperature range. Understanding these temperature-dependent deformation mechanisms in heat-treated LPBF HX would provide valuable insights into its performance over a wide range of service temperatures.

Prevention of hot cracking in Ni-based superalloy via passivation layer formation during additive manufacturing

We investigated the prevention of hot cracking in a non-weldable Ni-based superalloy by utilizing a passivation layer formed through laser-based directed energy deposition. The addition of Hf to the Ni-based superalloy facilitated the formation of a passivation layer consisting of Hf oxide on the surface of the as-deposited sample. An increase in the amount (thickness) of the passivation layers with increasing Hf content resulted in a reduction in hot cracking occurrence. Specifically, the addition of 2.5 wt% Hf led to the formation of uniformly distributed fine oxides without inducing hot cracks. This passivation layer effectively inhibited the formation of coarsened Mo oxides, which typically occur during oxidation in the liquid state and contribute to the hot cracking of Ni-based superalloy. During oxidation in the liquid state, Hf diffuses out to the surface and forms a passivation layer, thereby suppressing the formation and growth of oxides to sizes large enough to induce hot cracking.

Influence of microstructural and loading direction on the ductility and anisotropy of Ni-based superalloys

The influences of phase coarsening and loading direction on the ductility of Ni-based superalloys with phase coarsening are investigated, and the relationship between their microstructural and macroscopic mechanical properties is also explored in detail. It is found that the precipitation phase and the corresponding phase coarsening significantly influence the ductility of alloys. Furthermore, anisotropic features in ductility are observed after phase coarsening. Generally, as the volume fraction of precipitation particles or the degree of phase coarsening increases, the ductility of alloys gradually decreases; and it initially increases and then decreases gradually with the increase of loading angle, presenting an inverted ‘V’ shape. Comparatively, for the alloys with lower volume fraction of precipitation particles and higher degree of phase coarsening, the ductility shows different variation trends with the degree of phase coarsening and loading angle compared with that in materials with other microstructure features, taking the minimum value at the loading angle of 45°. The variations of ductility are closely connected to the characteristics of precipitate shape, size and distribution. The microstructure evolution and the loading direction significantly affect the initiation and movement of dislocations within the material, and result in various macroscopic ductility characteristics in the Ni-based superalloys.

Effect of Pt-Al coating on low-cycle fatigue behavior in a Ni-based single crystal superalloy at 760 °C

The effect of Pt-Al coating on low-cycle fatigue behavior of Ni-based single crystal superalloy at 760 °C was evaluated. The results show that Pt-Al coating does not change the cyclic stress response behavior of the Ni-based single crystal superalloy at 760 °C. Under the total strain amplitude from 0.6 % to 1.2 %, the cyclic stress amplitude value of Pt-Al coating samples is slightly lower than uncoated samples during cyclic deformation at the stable stage under the same applied total strain amplitude, and the fatigue life of Pt-Al coating samples is lower than uncoated ones, with an average fatigue life reduction of about 19.9 %. By the relation curve of comparing the cyclic strain amplitude with the reversals to failure, it is observed that the fatigue ductility coefficient ε′f of the Pt-Al coating sample is higher than the uncoated sample, the fatigue ductility index c and fatigue strength coefficient σ′f are lower than the uncoated sample, but the fatigue strength index b has little difference. It is found that the cyclic strength coefficient K′ and the cyclic strain hardening index n' of the Pt-Al coating sample are lower than the uncoated sample after fitting the curves of the relationship between cyclic stress amplitude and total strain amplitude. In addition, the cyclic stress amplitude of Pt-Al coating sample is slightly lower than uncoated sample under the same applied total strain amplitude. It was found by SEM that the cracks of both uncoated and Pt-Al coating samples initialized near the surface of the samples under low-cycle fatigue loading at 760 °C, and multiple crack sources began to spread into the substrate. It was found by TEM observation that at low strain amplitude (Δεt/2 = 0.6 %), the dislocations in both uncoated and Pt-Al coating samples mainly converged in γ, and a large number of dislocation debris gathered in the γ and dislocation entanglement was generated, while a small number of dislocations were observed to cut into γ'. The γ′ is seriously deformed and slightly rafted at Δεt/2 = 1.2 %, and a few dislocations are also observed in γ'.

A Monte-Carlo approach for crack initiation modeling of cast superalloys informed by crystal plasticity

Crystal plasticity simulations of materials to assess the fatigue response on the microscale are becoming increasingly popular in industrial application. However, in the case of parts made from cast Ni-base superalloys, the amount of pores in a part combined with the fine spatial discretization around the pores needed due to strong mechanical gradients quickly leads to a computationally impractical number of mesh elements. In this paper we show an alternative to explicit crystal plasticity modeling of part-scale porosity by introducing a Monte-Carlo submodel that recombines the fatigue response of single pores predicted by crystal plasticity into the fatigue response of the pore agglomerate. The model can be applied in realtime due to the use of precomputed crystal plasticity results. We demonstrate that fatigue indicator parameters predicted by the Monte-Carlo submodel agree well with those predicted by explicit crystal plasticity simulations. Lastly, we apply the proposed model to study the influence of different porosity volume percentages, pore sizes and pore morphologies on the fatigue indicator parameters of a cast MAR-M247 superalloy.

Investigation of temperature assisted corrosion failure of an aircraft turbine blade

In this study, the root cause of a recurring corrosion problem at the tip of an internally cooled high-pressure turbine blade (HPT) of a turbofan engine was investigated. A two-pronged strategy was utilized involving (i) experimental techniques to identify the blade material and corrosion products, and (ii) computational methods to analyze the fluid flow over the blade. In the experimental thrust, an array of techniques was employed to determine the chemical composition, microstructure, corrosion products and microhardness values while the airflow, pressure distribution and thermal profile over the blade were found through computational paradigms. The blade was a Ni-based superalloy containing typical gamma (γ) and gamma prime (γ′) phases, with an average microhardness value of 389 HV at room temperature. SEM analysis of the corroded area revealed oxides of iron, calcium, magnesium, aluminium, and silicon (FCMAS) most of which were not part of the base alloy. To determine the temperature profile and pressure distribution on the blade, a conjugate heat transfer analysis using Ansys CFX was conducted. A combination of experimental data from the engine ground running test station and analytical equations was used to find the required boundary conditions for the computational analysis. Pressure distribution over the entire blade was found which showed higher flow velocities and cross flows at the blade tip which potentially cause erosion at that region in the presence of impurities in the incoming air. Temperature distribution over the entire blade was also obtained, and it was found that the highest temperatures for the internally cooled blade occurred at the tip region which were in the range of ∼1060 °C–1250 °C (average ∼1114 °C). The temperature on the tip was high enough to cause the melting of the impurities that ingress the coating and cause its degradation during thermal cycling. This exposes the base alloy to FCMAS and causes corrosion. These high temperatures (1060–1250 °C) also induced phase transformation and increased the brittleness (∼22 % increase in hardness) which aggravated the presence of corrosion and resulted in crack formation and blade rejection.

Effect of Carbon on the Microstructures and Stress Rupture Properties of a Polycrystalline Ni-Based Superalloy

Effect of Carbon on the Microstructures and Stress Rupture Properties of a Polycrystalline Ni-Based Superalloy

Evaluation of critical temperatures of as-cast Co-based superalloys as a function of alloying elements and their effect on microstructure

Co-based superalloys are of great interest in the aeronautical and aerospace industry due to their superior thermal performance compared to Ni-based superalloys. This work explores the influence of elemental additions (Ni, Al, Cr, Si, and Ti) on the critical temperatures (liquidus, solidus, solvus, and eutectic) and microstructural features of several Co systems fabricated by vacuum induction melting until the final Co-30Ni-10Al-4Cr-2Si-2Ti (wt%) composition was obtained. Differential Scanning Calorimetry results indicated high thermal stability concerning the γ' phase formation. Si and Ti additions suppressed the eutectic presence in the proposed superalloy, which is desirable to avoid the incipient melting of components. Instead, the present work exhibits a low-density Co-based superalloy conformed of a continuous γ/β microstructure with γ' precipitating uniformly. These characteristics make the Co-30Ni-10Al-4Cr-2Si and Co-30Ni-10Al-4Cr-2Si-2Ti superalloys promising candidates to be subjected to high temperatures in comparison with current Co-based superalloys.

Effect of Al and Nb on microstructure and mechanical properties of novel Al-Mo-Nb-Ti-V-Zr lightweight refractory high-entropy alloys

Refractory high-entropy alloys (RHEAs) are a new family of metallic alloys which have been intensively studied in the last decade. Indeed, RHEAs likely have the potential of overcoming the high-temperature performances of conventional Ni-based superalloys. Nevertheless, up to date, their high density and poor room temperature ductility still compromise their industrial uptake. In this study, novel RHEAs compositions are investigated to provide insights on the development of strong and ductile lightweight RHEAs. Specifically, AlxMoNbyTiVZr alloys (with x = 0, 0.25, 0.5, 0.75, 1 and y = 1, 2) have been produced and characterized. These alloys have been designed to probe the possibility of reducing the density of RHEAs by increasing the Al content. At the same time, the content of Nb was also increased in the attempt to counterbalance the embrittlement effect related to Al addition.

The Effects of Microstructure on the Thermophysical Properties of the K439B Ni-Based Superalloy

The thermophysical properties of superalloys are critical for the design, fabrication, and service of hot-end components in engines. In this work, the influence of microstructure on the thermophysical parameters including heat conductivity, thermal diffusivity, heat expansion coefficient, and specific heat capacity of the K439B superalloy, which is a prospective cast superalloy for engine casings, were investigated from 100 °C to 900 °C. It has been observed that these properties increase with temperature, especially between 700 °C and 800 °C, for alloys subjected to different heat treatment processes. When compared under the same temperature, these parameters decrease with an increase in the size and volume fraction of the γ′ phase in the alloys. Meanwhile, the interfaces between the γ/γ′ phase and the boundaries between dendrites also impeded the heat treatment. It can be concluded that among the alloys with different heat treatment process, the sequence 1160 °C/4 h + 1080 °C/4 h + 845 °C/20 h exhibited the most stable heat conductivity and lowest heat expansion coefficient, making it advantageous for high-temperature service.

Deformation mechanism of Ni-based single crystal superalloy under ultrasonic surface rolling and subsequent thermal exposure

In present work, ultrasonic surface rolling (USR) is applied to DD6 Ni-based single crystal (NBSC) superalloys, and the thermal relaxation behavior of the pre-treated alloys are investigated at 650 °C. Combined with mechanical properties, the plastic deformation and recovery mechanisms of the hardened layers are revealed in detail. Results show that compressive residual stress amplitude of 1163 MPa, 75.1 % improvement of surface nano-hardness and 83.02 % reduction on surface roughness are achieved by USR. Both deformed layer thickness and slip density progressively increases with USR cycles, where initial octahedron {111}<110> slips transform to a mixture of octahedron {111}<110> and dodecahedron {111}<112> slips. Meanwhile, shear step length expansions form γ′ segment/γ terminal interface with superlattice extrinsic stacking fault (SESF) on 1¯11, which may have inferior energy threshold for γ dislocation invasions than primary ones through shortening mean free path. Also, this drives γ′/γ solute mingling to induce tortuous interfaces. While dislocation proliferation dominates γ channel hardening, the γ′ hardening derives from planar fault strengthening and net system energy augmentation. During thermal exposure, surface integrities of USRed DD6 are strongly degraded, attributable to slip trace elimination and local γ′ coarsening. Thermal exposure further compels the γ dislocation to migrate towards γ′ segment across γ′ segment/γ terminal interface. Then, they are probably consumed to motivate solutes to phase boundary to reshape the γ′ morphologies through interface rearrangements under the assistance of γ dislocation network. The orientation transformation of SESF on 1¯11 to 010 and the mixture of SESF and superlattice intrinsic stacking fault on 1¯11 are thought to be possible manipulated mechanisms for γ′ coarsening, significant impairing the work hardening of alloy due to interfacial coherency deterioration. This paper could provide guidance for USR strengthening designs of NBSC.

Discovering novel γ-γ′ Pt-Al superalloys via lattice stability in Pt3Al induced by local atomic environment distortion

The strength of Pt-Al superalloys can be effectively improved using a coherent structure composed of a disordered γ phase and an ordered γ′ phase. However, γ′-Pt3Al has a cubic phase only at high temperatures and a tetragonal phase at low temperatures, thus breaking the coherent interface and deteriorating its mechanical properties. Here, high throughput first-principles calculations and rational experiments were conducted to discover novel γ-γ′ Pt-Al superalloys. The alloying strategy stabilizes the γ′ phase through the local charge distortion (LCD) or the local lattice distortion (LLD). Alloying elements, such as, Ti, Hf, and Ta, occupy the Al site in combination with Pt, making the spherical shape of change density in Pt3Al showing obvious directionality, i.e., LCD. Therefore, the stability of the γ′ phase is proposed at a lower alloying element concentration (3.125 at.%). However, alloying elements near Pt, e.g., Ni, Co, and Ru, tend to occupy the Pt site and induce LLD to stabilize the γ′ phase at higher alloying element concentration (12.5 at.%). The shape of the change density is similar to that of Pt3Al, and no obvious LCD is found when alloying elements occupy the Pt site. LCD-induced phase stability is observed exclusively at the Al site, whereas LLD-induced phase stability is solely present at the Pt site. The Pt-12Al-6X (X = Hf, Ti, Ta, Cr, and Ni in at.%) alloys were prepared, among which Pt-12Al-6Hf possessed a stable γ′ phase, a lower lattice misfit between γ and γ′, and a higher precipitation strengthening, resulting in its hardness (442.1 HV) surpassing that of Ni-based superalloys IN718 (413.6 HV). The γ′ phase in Pt-12Al-6Hf was characterized by high-resolution transmission electron microscopy, suggesting that the Pt site of the γ′ phase was occupied by Pt, while the Al site was shared by Pt, Al, and Hf. This is consistent with the calculated results.

Tune Al/Ti to adjust FCC+L21 hetero-structured Ni-based high-entropy alloys for improved mechanical properties and wear resistance

Outstanding mechanical properties of Ni-based superalloy benefit from its coherent γ/γ’ structure via precipitation strengthening of γ matrix (FCC structure) by L12 Ni3Al-type γ’ phase. Back-stress strengthening is another effective strategy to further enhance the FCC+L12 structured Ni-based superalloy. In this work, we extend such approaches to high-entropy alloys (HEAs) by introducing different Al and Ti contents (5 at.% ∼18 at.%) into a Ni-based CrFe2Ni4 alloy to form FCC+L21 heterostructured AlxCrFe2Ni4Tiy HEAs. Detailed microstructural analysis indicates that L12 Ni3(Al,Ti)-type nanoparticles form in a (Ni,Fe,Cr)-rich FCC matrix. The volume fraction of L21 AlNi2Ti-type phase can be varied by adjusting the Al/Ti ratio and concentrations of Al and Ti. Higher Al and Ti contents promote L21 phase formation and higher Al/Ti ratio (>1) prohibits the high Ti-containing compounds such as D024η-Ni3Ti and C14 Laves Fe2Ti phases, which are hard but brittle. Corresponding Young's modulus, Poisson's ratio, hardness, and the bulk to shear modulus ratio (B/G) can be readily modified. Compressive tests demonstrate that Al1.5CrFe2Ni4Ti1.0 alloy with half FCC and half L21 phases possesses the optimal strength-ductility combination (with compressive yield strength of ∼1564 MPa and fracture strain of ∼28 %). DFT calculations were performed to elucidate relevant mechanisms. Sliding wear tests were also performed, which demonstrate superior wear resistance of the HEAs at both room and elevated temperatures, compared with a commercial Ni-based superalloy, UHT-Nickel.

Potential of carbide strengthened economical wrought nickel-based alloy for high temperature

A carbide strengthened wrought Ni-based superalloy is developed. The alloy depends on carbide dispersion strengthening. A high deformation plasticity in homogeneous alloy is shown in the 1100–1200 °C isothermal compression test and the improvement of microstructure can be achieved by recrystallization. After aging at 850–900 °C, the carbide strengthened wrought alloy appears excellent tensile strength. Carbide exists higher microstructure stability than γ′ in the same condition. The herein reported results reflect the potential of the economical wrought Ni-based superalloy to service above 800 °C.

Formation of eta phase during aging at 750–850 °C for Ni-base superalloys with different Ti/Al ratios

The formation of the η phase in two nickel-based superalloys that have similar amounts of titanium and aluminum but different Ti/Al ratios was investigated. The aging was carried out at 750 °C, 800 °C, and 850 °C for 10000 h. The microstructures were observed using SEM and TEM, and equilibria phases were calculated using a Thermo-Calc. equipped with TCNI11 database. Results showed that the number and size of η phase increased with aging temperature and Ti/Al ratios. The size of the γ′ phase was also found to be increased with aging temperature, but it was not significantly affected by the Ti/Al atomic ratio. However, the η phase was not predicted to be the thermodynamic equilibrium phase in the alloy using Thermo-Calc. with the TCNI11 database. The η phase was expected to be the equilibrium phase with Ti/Al ratios higher than 1.85 at 850 °C for Ni-27.0Cr-19.1Co-0.91Nb-0.29Mo-0.14C-xTi-yAl alloys. However, the result does not match the experimental observations or other literature. It is suggested that the effect of titanium on the formation of the η phase may be underestimated in the TCNI11 database. Therefore, it is necessary to enhance the accuracy of predicting the formation of the η phase by modifying at least one of the thermodynamic parameters, including the phase equilibrium of the ternary Ni-Ti-Al system.

Novel Fe-rich hypo-eutectic high entropy alloy developed for laser powder bed fusion in the Al-Co-Cr-Fe-Ni-Zr system

A novel heat resistant Fe-rich hypo-eutectic high entropy alloy (HEA) was manufactured using laser powder bed fusion (LPBF). The prealloyed Al1/4Co2Cr2Fe6Ni3/2Zr powder was characterized and processed using different preheating temperatures (650°C, 700°C and 800°C). The as-built microstructure showed dissimilar morphologies throughout the melt pool, with eutectic grains of ultra fine lamella spacing (λ≈25 nm) inside the core regions, and coarser dendritic and globulitic growth patterns at the interlayer boundaries (ILBs). An annealing treatment at 1000°C for 6 h yielded a homogenous composite microstructure of a Fe-rich matrix with ∼40 % of fine dispersed Zr-rich intermetallic (IM) phases of submicron size. Synchrotron high energy x-ray diffraction (HEXRD) revealed M2Zr Laves (M = Co, Fe, Ni) as the main IM for the as-built condition and the additional growth of M23Zr6 upon annealing. Further HEXRD in-situ high temperature tests examined the lattice parameter evolution and coefficient of thermal expansion (CTE) up to 1100°C. The mechanical performance of the as-built and annealed condition was evaluated through hardness and compression testing at room temperature (RT). Additional compression testing from RT to 1000°C were conducted on the annealed condition to assess the high temperature strength and give comparison to other high temperature materials like Ni-based superalloys.