Development of Ni–Fe–Al-based alloys precipitating cubic γ′ for fabrication of nanoporous membranes

During this study, new Ni-base alloys precipitating cubic γ′ were developed which shall be used for the production of polycrystalline nanoporous membranes. The polycrystalline nanoporous membranes are produced through a combination of cold rolling and heat treatment in order to get directional coarsening of the γ′-phase which is selectively dissolved afterwards. Conventional Ni-based superalloys have a γ/γ′ -microstructure with cubic γ′-precipitates and show the needed etching behaviour but their high strength and limited ductility at room temperature do not allow to produce the polycrystalline nanoporous membranes by means of the before mentioned method. Thus, the new alloys with simpler composition were developed which have a γ/γ′ microstructure. The alloy Ni–13Fe–8Al–4Ti (composition in atomic percent) which was produced by Schmitz (Cullivier, ISBN 978-3-86955-523-2, 16) served as basis and showed the promising characteristics. To obtain cubic γ′-precipitates, the misfit was estimated to values of at least |0.2| % by a method presented by Mishima (Acta Metall 33:1161–1169, 23). Further, the phase compositions as well as phase volume fractions of γ-matrix and γ′-phase were calculated by means of Thermocalc^® simulations (database: TTNi7). The etching behaviour of the new alloys was adjusted by adding chromium and molybdenum which passivate the γ-matrix so that the γ′-precipitates dissolve during the leaching process. The well-aligned cubic γ′-precipitates were obtained by partially replacing titanium by niobium. Furthermore, the hardness could be significantly lowered compared to conventional superalloys by reduction of alloying elements. Hence, the promising alloys were found to get directional coarsening of the γ′-precipitates in a combined process of cold rolling and heat treatment.     

Effects of aging treatments on the Microstructure of the Ni-base superalloy IN-738

The effects of aging treatments on the microstructure of the Ni-base superalloys IN-738 HC and IN-738 LC were investigated. Changes in (i) size, morphology, and distribution of the γ′-phase, (ii) the amount of M₂₃C₆-carbide, and (iii) the occurrence of additional phases and phase instabilities were quantitatively determined. The results indicate the possibility to deduce an unknown thermal history from the observed microstructure.     

Anomalous deformation behavior and twin formation of Ni-base superalloys at the intermediate temperatures

The tensile behaviors of polycrystalline Ni-base superalloys have been studied in the temperature range of 25-980 °C. Anomalous increase of yield strength was observed in precipitation hardened superalloys at intermediate temperature. The alloy with high γ′ volume fraction showed a remarkable increase of yield strength at intermediate temperature. A peak of yield strength was observed in the alloy with low γ′ volume fraction at intermediate temperature while solid solution strengthened alloys did not have such peak. Abrupt decrease of ductility in the intermediate temperature regime was observed not only in the γ′ strengthened superalloys but also in the solid solution strengthened superalloy. This result implies that γ′ precipitation is not a substantial cause for the occurrence of the ductility minimum in the superalloys. It was found that twinning was an important deformation mechanism of the superalloys at intermediate temperature where ductility was abnormally low. Deformation twins formed easily in the superalloys whose reduction of stacking fault energy was high regardless of strengthening mechanisms because alloys with low stacking fault energy was prone to extend stacking faults.     

Investigating variability of fatigue indicator parameters of two-phase nickel-based superalloy microstructures

Variability of fatigue properties of Nickel-based superalloys induced by microstructure feature uncertainties is investigated. The microstructure at one material point is described by its grain size and orientation features, as well as the volume fraction of the γ′ phase. Principal component analysis (PCA) is introduced to reduce the dimensionality of the microstructure feature space. PCA and kernel PCA (KPCA) techniques are presented and compared. Reduced representations of input features are mapped to uniform or standard Gaussian distributions through polynomial chaos expansion (PCE) so that the sampling of new microstructure realizations becomes feasible. A crystal plasticity constitutive model is adopted to evaluate fatigue properties of two-phase superalloy microstructures under cyclic loading. The fatigue properties are measured by strain-based fatigue indicator parameters (FIP). Adaptive sparse grid collocation (ASGC) and Monte Carlo (MC) methods are used to establish the relation between microstructure feature uncertainties and the variability of macroscopic properties. Convergence with increasing dimensionality of the reduced surrogate stochastic space is studied. Distributions of FIPs and the convex hulls describing the envelope of these parameters in the presence of microstructure uncertainties are shown.     

Load history effects in ductile cast iron for wind turbine components

Fracture-mechanics experiments were carried out on samples of ductile cast iron to investigate the fracture behaviour under cyclic and random loading. Under cyclic loading, the crack growth rate was described well by the ESACRACK model. Fatigue crack growth behaviour depends on the graphite particle size. Increasing particle size leads to higher threshold-values ΔK_th, lower da/dN values and higher transition to static fracture K_fc. The investigation of load history effects with low–high and high–low transitions shows that crack growth acceleration is independent of the transition type. The computation of the a–N curves based on different load history models yields non-conservative results.     

Directional coarsening of γ′ phase in single crystal nickel based superalloys during tensile creep

Abstract

The γ′ precipitate rafting kinetics and morphological evolution for two model single crystal superalloys have been studied. The microstructure of the alloys after different stages of tensile creep at 1040°C and under a range of stresses are examined using TEM and SEM. The chemical compositions of both γ and γ′ phases are analysed by energy dispersive spectrometry. Results show that a meshlike γ′ raft structure is formed along the direction normal to the stress axis during primary creep. The applied stress causes a decrease in the coherent strain energy at the γ′/γ interfaces in the (001) crystal plane. The released energy is the driving force for the diffusion of elements, leading to the formation of the γ′ rafts. A longer time is required for the formation of γ′rafts in alloy 2 owing to its higher content of the refractory element W which obstructs the migration of the other elements in the diffusion field of the γ′ rafts.     

Finite element analysis and experimental study of microstructure evolution of nitrogen alloyed ultralow carbon stainless steel during hot deformation

Abstract

Hot compression experiments of a nitrogen alloyed ultralow carbon stainless steel were performed in the temperature range of 1223–1423 K, at strain rates of 0.001–1 s^?1, and with deformation amounts of 30–70% on a Gleeble-3500 thermal-simulator. Based on the results from thermo-physical simulation experiments and metallographic analyses, a physically-based constitutive model and a dynamic recrystallisation (DRX) model of the studied steel were derived, and the developed models were further embedded into a finite element method (FEM) software. The microstructure evolution of the studied steel under various hot deformation conditions was simulated by FEM, and the effects of deformation amount, strain rate and temperature on the microstructure evolution were clarified. The results obtained from the finite element analysis were verified by the experiments. The finding confirms that the thermal-mechanical FEM coupled with the developed constitutive model and DRX model can be used to accurately predict the microstructure evolution of the studied steel during hot deformation.     

Microstructure-sensitive HCF and VHCF simulations

This paper provides some background and historical review of how microstructure-sensitive finite element simulations can play a role in understanding the effects of stress amplitude, R-ratio, and microstructure on fatigue crack formation and early growth at notches, including pores and non-metallic inclusions for Ti alloys and Ni-base superalloys. The simulations employ fatigue indicator parameters (FIPs) computed over finite volumes that relate to processes of fatigue crack formation and early growth at the scale of individual grains. It is argued that both coarse scale (uncracked, mesoscale) and fine scale FIPs (computed in the vicinity of cracks in single grains or crystals) serve as a driving force for crystallographic fatigue crack growth, and correlate directly with the cyclic crack tip displacement (CTD). Furthermore, variability in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) responses is computationally assessed using multiple statistical volume elements and the distribution of FIPs of extreme value character. The concepts of marked correlation functions and weighted probability density functions are reviewed as a means to quantify the role of multiple microstructure attributes that couple to enhance the extreme value FIPs in the HCF regime. An algorithm for estimation of the cumulative probability distribution of cycles for crack formation and growth from notches in HCF and VHCF is also described.     

Comparison of thermodynamic database models and APT data for strength modeling in high Nb content γ–γ′ Ni-base superalloys

In response to the increasing need for higher operating temperatures in advanced gas turbine engines, new alloying concepts are required to develop novel nickel-base superalloys with enhanced temperature capabilities. Recent studies have shown that polycrystalline Ni-base superalloys containing elevated levels of Nb additions exhibit superior properties at elevated temperatures when compared to existing commercial Ni-base superalloys. In order to design, develop and fully exploit this innovative class of superalloys, an understanding of how alloying elements partition to each phase is essential. Using atom probe tomography (APT), compositions of the constituent phases were measured in four high Nb content γ–γ′ Ni-base superalloys and the results were compared to thermodynamic database models from Thermo-Calc. Results were also used in predicting the solid solution strength behavior of the four alloys. The differences in phase composition predictions from thermodynamic models resulted in dissimilarities between the generated strength behavior curves and those from the experimental work.     

Review of Microstructure–Mechanical Property Relationships in Cast and Wrought Ni-Based Superalloys with Boron,Carbon, and Zirconium Microalloying Additions

Cast and wrought Ni-based superalloys are materials of choice for harsh high-temperature environments of aircraft engines and gas turbines. Their compositional complexity requires sophisticated thermo-mechanical processing. A typical microstructure consists of a polycrystalline γ-matrix, strengthening Ni₃(Al,Ti) γ′ precipitates, carbides (MC, M₆C, and M₂₃C₆), borides (M₂B, M₃B₂, and M₅B₃), and other inclusions. Microalloying additions of B, C, and Zr commonly improve high-temperature strength and creep resistance, although excessive additions are detrimental. Grain boundary (GB) segregation may improve cohesion and displace embrittling impurities. Finely dispersed carbides and borides are desired to control grain size via GB pinning. However, excessive decoration of GBs may lead to failure during processing and in-service. Hence, a systematic review on the roles of B, C, and Zr in cast and wrought Ni-based superalloys is required. The current state of knowledge on GB segregation and precipitation is reviewed. Experimental and modeling results are compared across various processing steps. The impact of GB precipitation on mechanical properties is most well researched. Co-precipitation in proximity to GBs interacting with local microstructure evolution and mechanical properties remains less explored. Addressing these gaps in knowledge allows a more complete understanding of processing–microstructure–properties relationships in advanced cast and wrought Ni-based superalloys.     

HIGH TEMPERATURE CYCLIC DEFORMATION OF A DIRECTIONALLY SOLIDIFIED Ni-BASE SUPERALLOY

Abstract— The high temperature low cycle fatigue behaviour of a directionally solidified Ni-base superalloy hardened by about 65% volume fraction of γ'-precipitates was investigated in order to determine the fatigue life parameters for longitudinal (L) and longitudinal transverse (LT) grain orientations. The fatigue resistance was compared with that of two oxide dispersion strengthened (ODS) Ni-base superalloys with a similar elongated grain structure.
The fatigue life of the alloy can be adequately predicted by Basquin and Coffin-Manson empirical relationships and the fatigue ductility parameters in these relationships show a similar trend with the tensile ductility properties.
The studied alloy exhibits a fairly stable cyclic stress response, with only a slight stress softening. Fatigue crack initiation occurs mainly at shrinkage pores on the surface or sub-surface of the specimens. The crack growth direction is predominantly perpendicular to the applied load. The fracture mode in the LT-direction is transgranular and fatigue life is shorter by a factor of about six compared to the L-direction. The fatigue life of the alloy is longer than that of the ODS Ni-base superalloys with which it is compared.     

γ′ precipitation in some model nickel based superalloys

Abstract

An investigation is reported of the precipitation of γ′ from supersaturated γ in model Ni based superalloys containing (at.-%) 75%Ni, 15 or 12·5%Al, 10 or 7·5%Mo, 2·5%Ta or Won aging in the range 1073–1373 K for periods up to 100 h. For short aging periods the γ/γ′ mismatch was positive, but on prolonged aging the y lattice parameter increased and that of γ′ decreased giving negative mismatch; this change is attributed substantially to the partitioning of Mo to γ.

MST/1047     

Microstructure and load sensitive fatigue crack nucleation in Ti-6242 using accelerated crystal plasticity FEM simulations

This paper investigates microstructure and load sensitive fatigue behavior of Ti-6242 using cyclic crystal plasticity finite element (CPFE) simulations of statistically equivalent image-based microstructures. A wavelet transformation induced multi-time scaling (WATMUS) method [1], [2] is used to perform accelerated cyclic CPFE simulations till crack nucleation, otherwise infeasible using conventional time integration schemes. A physically motivated crack nucleation model in terms of crystal plasticity variables [3] is extended in this work to predict nucleation. The crack nucleation model is based on dislocation pile-up and stress concentration at grain boundaries, caused by inhomogeneous plastic deformation in the polycrystalline microstructure. The model is calibrated and validated with experiments. The dependence of yield strength on the underlying grain orientations and sizes is developed through the introduction of an effective microstructural parameter Plastic Flow Index or PFI. To determine the effects of the microstructure on crack nucleation, a local microstructural variable is defined in terms of the surface area fraction of soft grains surrounding each hard grain or SAFSSG. Simulations with different cyclic load patterns suggest that fatigue crack nucleation in Ti-6242 strongly depends on the dwell cycle hold time at maximum stress.     

A model of composite laminated Reddy beam based on a modified couple-stress theory

In this paper, a new anisotropic constitutive relation based on a modified couple-stress theory is defined for composite laminated Reddy beam. The theory contains only one material length-scale parameter in each ply of the composite laminated beam. The example of a cross-ply simply supported beam subjected to transverse load q₀sin(πx/L) is presented. Numerical results show that the proposed beam model can capture the scale effect of the microstructure. The proposed model can be reduced to several models of the modified couple-stress theory by adopting the assumptions in Timoshenko beam, Bernoulli–Euler beam and material isotropy. It can be seen that the deflections and stresses obtained by the proposed beam model are smaller than those based on Timoshenko and Bernoulli–Euler beam assumptions.     

Influences of Re on low-cycle fatigue behaviors of single crystal superalloys at intermediate temperature

Low-cycle fatigue behaviors of Ni-base single crystal superalloys containing different Re contents have been investigated at 760 °C. During heat treatment, Re retards γ′ phases coarsening and equalizes the distribution of γ′ phases. As Re content increases, fatigue life increases and slip bands distribute more inhomogeneously. Moreover, adding Re not only reduces stacking fault energy of the matrix, but also promotes the element segregation to increase the lattice misfit. However, the larger lattice misfit does not lead to the formation of dislocation networks, but which activates dislocation movement and promotes dislocations cross-slip and climbing movement under high temperature and applied stress. On the other hand, with the addition of Re, cyclic deformation behaviors change from cyclic hardening to cyclic stability, mainly depending on a transformation of deformation mechanisms from slip bands cutting through γ and γ′ phases to stacking faults shearing.     

Micromechanical methodology for fatigue in cardiovascular stents

A finite element based micromechanical methodology for cyclic plasticity and fatigue crack initiation in cardiovascular stents is presented. The methodology is based on the combined use of a (global) three-dimensional continuum stent-artery model, a local micromechanical stent model, the development of a combined kinematic–isotropic hardening crystal plasticity constitutive formulation, and the application of microstructure sensitive crack initiation parameters. The methodology is applied to 316L stainless steel stents with random polycrystalline microstructures, based on scanning electron microscopy images of the grain morphology, under realistic elastic–plastic loading histories, including crimp, deployment and in vivo systolic–diastolic cyclic pressurisation. Identification of the micromechanical cyclic plasticity and failure constants is achieved via application of an objective function and a unit cell representative volume element for 316L stainless steel. Cyclic stent deformations are compared with the J₂-predicted response and conventional fatigue life prediction techniques. It is shown that micromechanical fatigue analysis of stents is necessary due to the significant predicted effects of material inhomogeneity on micro-plasticity and micro-crack initiation.     

Observation of the Dissolution of Topologically Close Packed Phases by an Additional Heat Treatment in Third Generation Nickel‐Based Single Crystal Superalloy Turbine Blades

Topologically close packed (TCP) phases degrade the superior creep and rupture properties of Ni‐based single crystal superalloys. Initially, small TCP phases are formed in the dendrite core during the early stages of the manufacturing process, such as solution heat treatment, and surrounded by a gamma prime (γ′) phase. Then, TCP phases continue to develop during full heat treatment. However, an additional heat treatment induces diffusion of refractory metals from the TCP phases into the γ′ phase and consequently, the TCP phases clearly disappear. After dissolution, the regions where the TCP phases existed are altered to a normal microstructure composed of γ channels and a normal cubic γ′ phase. Based on the observation result, the mechanism of the dissolution of TCP phases is discussed.

     

Evolution of microstructure of nickel base superalloy at high temperatures

Abstract

The evolution of primary and secondary γ′ precipitates in the high γ′ volume fraction Rene 80 Ni based superalloy has been examined during aging at elevated temperatures for periods up to 1750 h. While the increase in average dimension of particles followed the cube rate Lifshitz, Slypzof and Wagner (LSW) law, r³_t – r³₀=Kt, there were significant discrepancies between the experimental and theoretical particle size distributions (PSDs) and inconsistency with the kinetic constants associated with the two populations of particles. These differences are attributed to the influence of elastic coherency strains which have not been considered in conventional capillarity driven coarsening models. During thermal exposure at 871°C, coalescence of primary cuboidal γ′ was predominant in early stages of aging, while the microstructure was relatively stable at longer aging times. The stability of the microstructure at longer aging times is attributed to the formation of the network of closely spaced dislocations at the γ/γ′ interface which would cause the loss of internal misfit stresses associated with the growth. Secondary spheroidal γ′ particles were initially coarsened and their volume fraction gradually decreased until they completely dissolved after 500 h at 871°C or 1 h at 982°C.     

Nanoporous Superalloy Membranes: A Review

The ability to produce metallic membrane materials with porosity on the nanoscale from Ni‐based superalloys, hitherto used exclusively for high temperature applications, has been discovered 15 years ago. The basic principle is to first convert the initial γ/γ′ microstructure, containing isolated γ′‐precipitates, into a bi‐continuous network where both phases are in themselves continuous and interpenetrate each other. Then, one of the two phases is selectively removed, so that a rigid structure consisting of the remaining phase with pores on the location of the removed phase results. This article reviews the progress made so far. In that time period, a number of ways to fabricate these unique materials have emerged, utilizing 1) single crystals and polycrystals as precursor materials as well as 2) coarsening of coherent and incoherent γ′‐precipitates to realize bi‐continuity of the microstructure. Consequently, a family of superalloy membranes has emerged with specific microstructures, properties, advantages, and limitations. It is the intention of this article to give an overview on these various manufacturing routes, as well as on resulting microstructures and properties. Finally, possible fields of applications are outlined. It is demonstrated that the particular manufacturing process from a solid to the porous material leads to certain advantages, such as the ability to structure the material in porous and solid areas as required by the application.