Order strengthening in nickel-based superalloys is associated with the extra stress required for dislocations to bypass the $γ'$ precipitates distributed in the $γ$ matrix. A rich variety of bypass mechanism has been identified, with various shearing and Orowan looping processes giving way to climb bypass as the operating conditions change from the low/intermediate temperatures and high stress regime, to the high temperature and low stress regime. When anti phase boundary (APB) shearing and Orowan looping mechanisms operate, the bypass mechanism changes from shearing to looping with increased particle size and within a broad coexistence size window. Another possibility, supported by indirect experimental evidence, is that a third "hybrid" transition mechanism may operate. In this paper we use discrete dislocation dynamics (DDD) simulations to study dislocation bypass mechanisms in Ni-based superalloys. We develop a new method to compute generalized stacking fault forces in DDD simulations. We use this method to study the mechanisms of bypass of a square lattice of spherical $γ'$ precipitates by $a/2\langle110\rangle\{111\}$ edge dislocations, as a function of the precipitates volume fraction and size. We show that the hybrid mechanism is possible and it operates as a transition mechanism between the shearing and looping regimes over a large range of precipitates volume fraction and radii. We also consider the effects of a $γ/γ'$ lattice misfit on the bypass mechanisms, which we approximate by an additional precipitate stress computed according to Eshelby's inclusion theory. We show that in the shearing and hybrid looping-shearing regimes, a lattice misfit generally results in an increased bypass stress. For sufficiently high lattice misfit, the bypass stress is controlled by the pinning of the trailing dislocation on the exit side of the precipitates.
Cobalt-based alloys with γ/γ' microstructures have the potential to become the next generation of superalloys, but alloy compositions and processing steps must be optimized to improve coarsening, creep, and rafting behavior. While these behaviors are different than in nickel-based superalloys, alloy development can be accelerated by understanding the thermodynamic factors influencing microstructure evolution. In this work, we develop a phase field model informed by first-principles density functional theory and experimental data to predict the equilibrium shapes of Co-Al-W γ' precipitates. Thr…
The tensile creep performance of a polycrystalline Co/Ni-base superalloy with a multimodal gamma prime distribution has been examined at 800C and 300MPa. The rupture life of the alloy is comparable to that of RR1000 tested under similar conditions. Microstructural examination of the alloy after testing revealed the presence of continuous gamma prime precipitates and M23C6 carbides along the grain boundaries. Intragranularly, coarsening of the secondary gamma prime precipitates occurred at the expense of the fine tertiary gamma prime. Long planar deformation bands, free of gamma prime, were als…
Titanium is commonly added to nickel superalloys but has a well-documented detrimental effect on oxidation resistance. The present work constitutes the first atomistic-scale quantitative measurements of grain boundary and bulk compositions in the oxide scale of a current generation polycrystalline nickel superalloy performed through atom probe tomography. Titanium was found to be particularly detrimental to oxide scale growth through grain boundary diffusion.
To better understand the equilibrium $γ^\prime$(L1$_2$) precipitate morphology in Co-based superalloys, a phase field modeling sensitivity analysis is conducted to examine how four phase-field parameters [initial Co concentration ($c_0$), double-well barrier height ($ω$), gradient energy density coefficient ($κ$), and lattice misfit strain ($ε_{\rm misfit}$)] influence the $γ^\prime$(L1$_2$) precipitate size and morphology. Gaussian Process Regression (GPR) models are used to fit the sample points and to generate surrogate models for both precipitate size and morphology. In an Active Learning …
A correlative approach is employed to simultaneously assess structure and chemistry of (carbide and boride) precipitates in a set of novel Co/Ni-base superalloys. Structure is derived from electron backscatter diffraction (EBSD) with pattern template matching, and chemistry obtained with energy dispersive X-ray spectroscopy (EDS). It is found that the principal carbide in these alloys is Mo and W rich with the M6C structure. An M2B boride, also exhibiting Mo and W segregation is observed at B levels above approximately 0.085 at.%. These phases are challenging to distinguish in an SEM with chem…
Gibbs-Thompson effect is the general term referring to the influence of interfaces on the course of phase transformations such as precipitation or solidification. Whilst attention is most often focused on the Gibbs-Thomson effect on nucleation, growth and coarsening, the present study considers the reverse process of precipitate dissolution in a nickel-base superalloy during in situ TEM observation. The presence of several distinct populations of gamma-prime precipitates (primary, secondary, tertiary and grain boundary) allows the differences due to particle size to be quantified and interpret…
Nickel-based superalloys play a major role in many technologically relevant high temperature applications. Understanding and predicting the evolution of the phase microstructure during high temperature creep together with the evolution of the dislocation microstructure is a challenge that up to date has not yet been fully accomplished. Our two-dimensional coupled phase-field/continuum dislocation dynamics model explains microstructural mechanisms which are important during the early stage of rafting in a single crystal system. It shows how $γ/γ'$ phases and dislocations interact giving rise to…
The excellent mechanical properties of the Ni-based superalloy IN718 mainly result from coherent $γ''$ precipitates. Due to a strongly anisotropic lattice misfit between the matrix and the precipitate phase, the particles exhibit pronounced plate-shaped morphologies. Using a phase-field model, we investigate various influencing factors that determine the equilibrium shapes of $γ"$ precipitates, minimizing the sum of the total elastic and interfacial energy. Upon increasing precipitate phase fractions, the model predicts increasingly stronger particle-particle interactions, leading to shapes wi…
To obtain comprehensive performance, heavy elements were added into superalloys for solid solution hardening. In this article, it is found by scanning transmission electron microscope observation that rather than distribute randomly heavy-atom columns prefer to align along <100> and <110> direction and form a short-range ordering with the heavy-element stripes 1-2 nm in length. Due to the strong bonding strength between the refractory elements and Ni atoms, this short-range ordering will be beneficial to the mechanical performances.
The effect of adding 2 at.% W to a model Ni-Al-Cr superalloy on the morphological evolution, spatial correlations and temporal evolution of g'(L12)-precipitates at 1073 K is studied with scanning electron microscopy and atomic force microscopy. Adding W yields a larger microhardness, earlier onset of spheroidal-to-cuboidal precipitate morphological transition, larger volume fraction (from ~20 to 30%), reduction in coarsening kinetics by one third and a larger number density (Nv) of smaller mean radii (<R>) precipitates. The kinetics of <R> and interfacial area per unit volume obey t1/3 and t-1…
In Ni-base superalloys with low Al and/or Ti content the precipitation and subsequent coarsening of gamma' particles at intermediate temperatures contribute to the degradation of the mechanical properties of the alloy. In the present paper the coarsening process is modelled and the change of the critical resolved shear stress of the alloy due to coarsening of the gamma' particles is calculated by means of statistical analysis of the depinning of a single gliding edge dislocation. It is found that the contribution of gamma' hardening to the critical resolved shear stress at 973 K reduces to mor…
In this study, the effect of rhenium (Re) addition on microstructural evolution of a new low-density Co-Ni-Al-Mo-Nb based superalloy is presented. Addition of Re significantly influences the γ' precipitate morphology, the γ/γ' lattice misfit and the γ/γ' microstructural stability during long term aging. An addition of 2 at.% Re to a Co-30Ni-10Al-5Mo-2Nb (all in at.%) alloy, aged at 900°C for 50 hours, reduces the γ/γ' lattice misfit by {\approx} 40% (from +0.32% to +0.19%, measured at room temperature) and hence alters the γ' morphology from cuboidal to round-cornered cuboidal precipitates. Th…
This work is focused on the micromechanical modelling of the low cycle fatigue of the nickel based $γ/γ'$ superalloy AM1 at high temperature. The nature of the activated slip systems in the different types of channels of the $γ$ phase is analysed, taking into account the combined effects of the applied and internal stresses. The latter are split into two contributions, misfit stresses and compatibility stresses between the elastic $γ'$ phase and the elasto-plastic $γ$ phase, which are estimated within a simplified composite approach. Internal stresses may induce slip activity and/or be relaxed…
A high-throughput correlative study of the local mechanical properties, chemical composition and crystallographic orientation has been carried out in selected areas of cast Inconel 718 specimens subjected to three different tempers. The specimens showed a strong Nb segregation at the scale of the dendrite arms, with local Nb contents that varied between 2 wt.% in the core of the dendrite arms to 8 wt.% in the interdendritic regions and 25 wt.% within the second phase particles (MC carbides, Laves phases and δ phase needles). The nanohardness was found to correlate strongly with the local Nb co…
In the present work, theoretical approaches, based on grain orientation dependent Young's modulus and Schmid factor are used to describe the influence of local grain orientation on crack initiation behaviour of the coarse grained nickel base superalloy René80. Especially for strongly anisotropic crystal structures with large grain size, such as the investigated material, the local elastic properties must be taken into account for assessment of fatigue crack initiation. With an extension of Schmid's law, the resulting shear stress amplitude, which triggers local cyclic plastic deformation, can …
The microstructural characteristics of a single crystalline nickel based superalloy (AM1) tested under high temperature fatigue at 950$^\circ$C are reported. For repeated fatigue (R$_\varepsilon$=0) through a range of cycle numbers N with imposed total strain amplitude $Δ\varepsilon^t$, two main types of behaviour are found depending on N and $Δ\varepsilon^t$. This allows a map of microstructures versus the number of cycles N and $Δ\varepsilon^t$ to be constructed. A domain called A presents anisotropic microstructures due either to a partition of the plasticity throughout the k channels, or t…
The service lives of nickel superalloys are often limited by environmental degradation. The present study compares oxidation, sulfidation and hot corrosion at 750C of three variants of a polycrystalline superalloy: a baseline alloy, a variant containing 1wt% Mn and one containing 0.5wt% Si. Mn reduced the oxidation rate without changing the scale morphology. The MnCr2O4 scale formed proved more protective against sulfidation and hot corrosion, but internal sulfides extended the damage depth. Si modified the oxide morphology to a continuous Cr2O3-Al2O3 dual layer. This provided improved protect…
We characterise the influence of the segregation behaviours of two typical alloying elements, aluminium and nickel, on the interfacial cohesive properties of copper-rich precipitates in ferritic steels, with a view towards understanding steel embrittlement. The first-principles method is used to compute the energetic and bonding properties of aluminium and nickel at the interfaces of the precipitates and corresponding fracture surfaces. Our results show the segregation of aluminium and nickel at interfaces of precipitates are both energetically favourable. We find that the interfacial cohesion…
The microstructure of a second-generation nickel base superalloy is studied using X-ray computed tomography (XCT) and scanning electron microscopy (SEM). The as-cast material contains 0.15 (+-0.001) vol% voids and these are distributed in the inter-dendritic region. The volume fraction of the voids increases to 0.21 (+-0.001) vol% after tensile deformation. Surface observations show evidence of dislocation emissions from the void surface, a mechanism possibly facilitates the expansion of the voids and contributes to the increased void volume fraction. Phenomenological parameters such as stress…
The details of phase separation of a Ni-6.5Al-9.9Mo aged at 978 K for aging times ranging from 0.125 to 1024 h are investigated by atom-probe tomography and lattice-kinetic Monte Carlo (LKMC) simulations. On the basis of the temporal evolution of the nanostructure, three experimental regimes are identified: (1) concomitant precipitate nucleation and growth (t less than 0.25 h); (2) concurrent coagulation and coalescence (t 0.25 to 16 h); and (3) quasi-stationary coarsening of gamma prime(L12)- precipitates (t 16 to 1024 h). The temporal dependencies of the mean precipitate radius and precipita…
Past models of stress-strain response under cyclic loading mainly rely on macroscopic equations which consider microstructure evolution indirectly or simply discard microstructure information. Modern materials science, on the other hand, seeks quantitive descriptions for the relations between microstrucutre and loading response. In the present work, we show a promising mesoscale phase-field framework which can describe co-evolution of phase/grain and defect microstructures, reveal microstructure mechanisms and simultaneously predict deformation properties as a natural outcome of microstrucutur…
The mechanical response of Haynes 282, a wrought gamma prime strengthened nickel based superalloy, is tailored by using both a cold deformation processing and a post straining heat treatment including a two step aging. In this work, the effect of the applied cold pre deformation by conventional equal speed rolling ESR or differential speed rolling DSR methods on the aging behavior of Haynes 282 superalloy, is examined for the first time. For this reason, the solid solution annealed alloy was subjected to 50 pct. nominal cold thickness reduction by either ESR or DSR methods, followed by a two s…
A mesoscale study of a single crystal nickel-base superalloy subjected to an industrially relevant process simulation has revealed the complex interplay between microstructural development and the micromechanical behaviour. As sample gauge volumes were smaller than the length scale of the highly cored structure of the parent material from which they were produced, their subtle composition differences gave rise to differing work hardening rates, influenced by varying secondary dendrite arm spacings, gamma-prime phase solvus temperatures and a topologically inverted gamma/gamma-prime microstruct…
Contemporary powder-based polycrystalline nickel-base superalloys inherit microstructures and properties that are heavily determined by the thermo-mechanical treatments during processing. Here, the influence of a thermal exposure alone to an alloy powder is studied to elucidate the controlling formation mechanisms of the strengthening precipitates using a combination of atom probe tomography and in-situ neutron diffraction. The initial powder comprised a single-phase supersaturated gamma only; from this, the evolution of gamma-prime volume fraction and lattice misfit was assessed. The initial …
Creep in single crystal Nickel-based superalloys has been a topic of interest since decades, and nowadays simulations are more and more able to complement experiments. In these alloys, the $γ/γ'$ phase microstructure co-evolves with the system of dislocations under load, and understanding the mutual interactions is essential for understanding the resulting creep properties. Predictive modeling thus requires multiphysics frameworks capable of modeling and simulating both the phase and defect microstructures. To do so, we formulate a coupled model of phase-field evolution and continuum dislocati…
In 2006, a novel cobalt-based superalloy was discovered [1] with mechanical properties better than some conventional nickel-based superalloys. As with conventional superalloys, its high performance arises from the precipitate-hardening effect of a coherent L1$_2$ phase, which is in two-phase equilibrium with the fcc matrix. Inspired by this unexpected discovery of an L1$_2$ ternary phase, we performed a first-principles search through 2224 ternary metallic systems for analogous precipitate-hardening phases of the form $X_{3}$[$A_{0.5}, B_{0.5}$], where $X$ = Ni, Co, or Fe, and [$A,B$] = Li, Be…
We report evidence of Re and Mo segregation (up to 2.6 at.% and 1 at.%) along with Cr and Co to the dislocations inside of γ' precipitates in a second generation Ni-based single crystal superalloy, after creep deformation at 750°C under an applied stress of 800 MPa. The observed segregation effects can be rationalized through bridging the solute partitioning behavior across the γ/γ' interface and the pipe diffusion mechanism along the core of the dislocation line. This understanding can provide new insights enabling improved alloy design.
Additive manufacturing (AM) processes produce parts with improved physical, chemical, and mechanical properties compared to conventional manufacturing processes. In AM processes, intricate part geometries are produced from multicomponent alloy powder, in a layer-by-layer fashion with multipass laser melting, solidification, and solid-state phase transformations, in a shorter manufacturing time, with minimal surface finishing, and at a reasonable cost. However, there is an increasing need for post-processing of the manufactured parts via, for example, stress relieving heat treatment and hot iso…
The stress-strain curves of most metallic alloys are often described using the relatively simple Ramberg-Osgood relationship. Whilst this description captures the overall stress-strain curve under monotonic tensile loading with reasonable overall accuracy, it often presents significant errors in the immediate post-yield region where the interplay between the elastic and plastic strains is particularly significant. This study proposes and develops a new approach to the description of the tensile stress-strain curve based on the Yield Volume Fraction (YVF) function. The YVF description provides …
The recently developed refractory multi-principle element alloy (MPEA), AlMo$_{0.5}$NbTa$_{0.5}$TiZr, shows an interesting microstructure with cuboidal precipitates of a disordered phase ($β$, bcc) coherently embedded in an ordered phase ($β'$, B2) matrix, unlike the conventional Ni-based superalloys where the ordered phase ($γ'$, L12) is the precipitate phase and the disordered phase ($γ$, fcc) is the matrix phase. It becomes critical to understand the phase transformation pathway (PTP) leading to this microstructure in order to tailor the microstructure for specific engineering applications.…
The microstructural evolution of a dissimilar metal weld (DMW) obtained by narrow-gap gas tungsten arc welding (NG-GTAW) was investigated after it was subjected to a post-weld heat treatment (PWHT). The case studied here is a joint between low-alloy steel pipes and a stainless steel steam generator using a nickel based alloy as filler material. The fusion boundary that was the focus of this work was that between the low-alloy steel (2.25Cr-1Mo) and the nickel alloy (alloy 82). The difference in matrix phase and chemical composition between the two alloys leads to a large difference in chemical…
A new computational tool has been developed to model, discover, and optimize new alloys that simultaneously satisfy up to eleven physical criteria. An artificial neural network is trained from pre-existing materials data that enables the prediction of individual material properties both as a function of composition and heat treatment routine, which allows it to optimize the material properties to search for the material with properties most likely to exceed a target criteria. We design a new polycrystalline nickel-base superalloy with the optimal combination of cost, density, gamma' phase cont…
A novel y-y'-y" dual-superlattice superalloy, with promising mechanical properties up to elevated temperatures was recently reported. The present work employs state of the art chemical and spatial characterization techniques to study the effect systematic additions of Mo, W and Fe and variations in Nb and Al contents have on the phase fraction, thermal stability, elemental partitioning and mechanical properties. Alloys were produced through arc melting followed by heat treatment. Multi-scale characterization techniques and hardness testing were employed to characterize their microstructure, th…
A systematic study of the compression creep properties of a single-crystalline Co-base superalloy (Co-9Al-7.5W-2Ta) was conducted at 950 °C, 975 °C and 1000 °C to reveal the influence of temperature and the resulting diffusion velocity of solutes like Al, W and Ta on the deformation mechanisms. Two creep rate minima are observed at all temperatures indicating that the deformation mechanisms causing these minima are quite similar. Atom-probe tomography analysis reveals elemental segregation to stacking faults, which had formed in the $γ\prime$ phase during creep. Density-functional-theory calcu…
The effects of various heat treatments on the microstructure and hardness of new Ni56Ti41Hf3 and Ni56Ti36Hf8 (atomic %) alloys were studied to evaluate the suitability of these materials for tribological applications. A solid-solution strengthening effect due to Hf atoms was observed for the solution annealed (SA) Ni56Ti36Hf8 alloy (716 HV), resulting in a comparable hardness to the Ni56Ti41Hf3 alloy containing 54 vol.% of Ni4Ti3 precipitates (707 HV). In the Ni56Ti41Hf3 alloy, the maximum hardness (752 HV), achieved after aging at 300C for 12 h, was attributed to dense, semi-coherent precipit…
Interfaces have long been known to be the key to many mechanical and electric properties. To nickel base superalloys which have perfect creep and fatigue properties and have been widely used as materials of turbine blades, interfaces determine the strengthening capacities in high temperature. By means of high resolution scanning transmission electron microscopy (HRSTEM) and 3D atom probe (3DAP) tomography, Srinivasan et al. proposed a new point that in nickel base superalloys there exist two different interfacial widths across the γ/γ' interface, one corresponding to an order-disorder transiti…
The tribological performance and underlying deformation behavior of Ni55Ti45, Ni54Ti45Hf1 and Ni56Ti36Hf8 alloys were studied using rolling contact fatigue (RCF) testing and transmission electron microscopy (TEM). TEM results of the as-machined RCF rods, prepared using focus ion beam, revealed some damage very close to the surface. TEM results after initial RCF cycling showed that additional damage was mainly confined to deformation bands that propagated several microns into the sample. These bands formed via localized dislocation slip, possibly on multiple slip systems, within the B2 matrix a…
Co-base superalloys are considered as promising high temperature materials besides the well-established Ni-base superalloys. However, Ni appears to be an indispensable alloying element also in Co-base superalloys. To address the influence of the base elements on the deformation behavior, high-temperature compressive creep experiments were performed on a single crystal alloy series that was designed to exhibit a varying Co/Ni ratio and a constant Al, W and Cr content. Creep tests were performed at 900 °C and 250 MPa and the resulting microstructures and defect configurations were characterized …
We have investigated the coarsening mechanism of intergranular Cr-rich M2B borides after creep and annealing at 850C for approximately 3000 hours in a polycrystalline nickel-based superalloy. Borides were found to be coarser after creep, with measured thicknesses in the range of 800-1100nm, compared to borides annealed in the absence of external applied load (400-600nm). The borides had a thickness of 100-200nm before exposure at 850C. Transmission electron microscopy revealed that coarsened borides have either the tetragonal I4/mcm structure, or the orthorhombic Fddd, with those two structure…
The susceptibility of nickel-based superalloys to processing-induced crack formation during laser powder-bed additive manufacturing is studied. Twelve different alloys -- some of existing (heritage) type but also other newly-designed ones -- are considered. A strong inter-dependence of alloy composition and processability is demonstrated. Stereological procedures are developed to enable the two dominant defect types found -- solidification cracks and solid-state ductility dip cracks -- to be distinguished and quantified. Differential scanning calorimetry, creep stress relaxation tests at 1000$…
Coherent misfitting precipitates in elastically stressed media such as $γ\prime$ particles in nickel-based super-alloys show various splitting patterns such as doublets, quartets, or octets due to their misfit strain energy. While it is an interesting instability phenomenon defying conventional surface thermodynamics, its mechanism is not completely clear. Through a phase-field study upon the splitting behavior and morphological evolution of coherent precipitates, we show that an interface instability driven by elastic anisotropy and a diffusion field can generate elastically induced splitting…
The Ni-base superalloy 718 is the most widely used material for turbomachinery in the aerospace industry and land-based turbines. Although the relationship between processing and the resulting properties is well known, an understanding of the specific deformation mechanisms activated across its application temperature range is required to create more mechanistically accurate property models. Direct atomic-scale imaging observations with high angle annular dark-field scanning transmission electron microscopy, complemented by phase-field modeling informed by generalized stacking fault surface ca…
Ni-based superalloys have been the subject of enormous usage in scenarios where the loading is heavy and often occurs at elevated temperatures. The strengthening mechanisms that come into play within the metallic lattice have been studied extensively as micromechanical MMC models. These continuum formulations suffer from several limitations. The underlying mechanisms at the atomistic scale have not yet been well understood. The report attempts to model the interaction of moving dislocation with cuboidal precipitates and explain the strengthening effect. The effect of particle size and inter-pa…
