Spinodal decomposition versus classical gamma-prime nucleation in a nickel-base superalloy powder: An in-situ neutron diffraction and atomic-scale analysis
David M Collins, Neil D'Souza, Chinnapat Panwisawas, Chrysanthi Papadaki, Geoff D West, Aleksander Kostka, Paraskevas Kontis
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 powder notably possessed elemental segregation of Cr and Co and elemental repulsion between Ni, Al and Ti with Cr; here proposed to be a precursor for subsequent gamma to gamma-prime phase transformations. Subsolvus heat treatments yielded a unimodal gamma-prime distribution, formed during heating, with evidence supporting its formation to be via spinodal decomposition. A supersolvus heat treatment led to the formation of this same gamma-prime population during heating, but dissolves as the temperature increases further. The gamma-prime then reprecipitates as a multimodal population during cooling, here forming by classical nucleation and growth. Atom probe characterisation provided intriguing precipitate characteristics, including clear differences in chemistry and microstructure, depending on whether the gamma-prime formed during heating or cooling.
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.
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…
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.
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…
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 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…
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 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…
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…
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…
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 …
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 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…
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…
Understanding the creep mechanisms and deformation response at different stresses and temperatures is crucial for design using nickel-base superalloys for high-temperature applications. In this study, the creep behaviour of a newly designed superalloy (nominated Alloy 11) at different stress and temperature was systematically investigated using SEM, STEM, EBSD and synchrotron X-ray diffraction. Material properties and mechanical response were analysed by considering such properties as lattice parameters and misfit, phase volume fraction, microstrain in reference and crept samples. Detwinning a…
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…
Imaging individual vacancies in solids and revealing their interactions with solute atoms remains one of the frontiers in microscopy and microanalysis. Here we study a creep-deformed binary Ni-2 at.% Ta alloy. Atom probe tomography reveals a random distribution of Ta. Field ion microscopy, with contrast interpretation supported by density-functional theory and time-of-flight mass spectrometry, evidences a positive correlation of tantalum with vacancies. Our results support solute-vacancy binding, which explains improvement in creep resistance of Ta-containing Ni-based superalloys and helps gui…
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…
The dynamics of homogeneous nucleation and growth of crystalline nickel from the super-cooled melt is examined during rapid quenching using molecular dynamics and a modified embedded atom method potential. The character of the critical nuclei of the crystallization transition is examined using common neighbor analysis and visualization. At nucleation the saddle point droplet consists of randomly stacked planar structures with an in plane triangular order. These results are consistent with previous theoretical results that predict that the nucleation process in some metals is non-classical due …
Combining Synchrotron X-ray Diffraction, Mechanistic Modeling, and Machine Learning for In Situ Subsurface Temperature Quantification during Laser Melting
