Fatigue microcrack distributions and the reliability of a nickel base superalloy

Low cycle fatigue failure in nickel base superalloys is strongly influenced by the initiation, growth and interaction of a high density of distributed microcracks. The latter is of immense significance late in the fatigue life as several microcracks can link together to form a rapidly growing critical crack. This study details the room temperature growth behavior of microcracks in populations on the surface of a Ni base superalloy selected for use in advanced turbine engines. Gathered with the needs of the reliability engineer in mind, it is shown that the growth patterns of these microcracks must be considered in order to avoid making nonconservative fatigue lifetime predictions.     

High temperature fatigue of a polycrystalline nickel base superalloy

Abstract

A high temperature fatigue crack growth study on an experimental disc alloy of approximate composition Ni-14.75Cr-14-19Co-4.75Mo-3Al-3.75Ti-1.75Ta-0.7Hf-0.06Zr-0.02C-0.0175B wt- has been undertaken. Comparison of constant load fatigue crack growth tests conducted in air and a vacuum at 725C and in air at room temperature indicate that an oxidising environment has a major influence on crack growth rates over a wide range of applied stress intensity range. In particular it contributes to enhanced embrittlement of grain boundary regions, promoting an early transition to intergranular failure along with a concomitant increase in growth rate. Constant stress intensity factor range tests at high R ratio showed that a decrease in frequency at 725C in both air and a vacuum caused an increase in the crack growth rate per cycle due to time dependent crack growth. This was most significant in air at 725C rather than in vacuo, although in both instances low frequency tests were accompanied by wholly intergranular crack growth. At lower R ratios the influence of time dependent processes is less pronounced, especially in a vacuum. For the purposes of prediction a straightforward linear summation model using sustained load crack growth results combined with high frequency growth rates was found to model reasonably the influence of frequency on crack growth rates for the air tests at 725C. This can be rationalised by the observation that in air intergranular failure proceeds by linking of prior cracked or embrittled grains some distance ahead of the crack tip. At lower values of stress intensity range a slight under prediction of growth rates was evident, but improvements could be made through the use of triangular waveform data. For the vacuum tests, the linear summation model was consistently found to over predict growth rates due to the transitory nature of the sustained load crack growth rates under repeated loading and was not generally deemed suitable. Under these conditions damage occurs close to the crack tip and direct interaction between time dependent and time independent mechanisms will occur. This is not taken into account by a summation approach and more accurate modelling of damage formation in the varying strain fields ahead of the crack tip is required to predict this.     

Novel powder packing theory with bimodal particle size distribution-application in superalloy

Powder packing behavior plays an important role in determining sintering ability of powder and the resultant performance of materials. In this study, a novel powder packing theory with bimodal particle size distribution is proposed by considering the loosening effect, wall effect and wedging effect. This theory is applied in PM nickel base superalloy by using mixture of coarse particles and fine particles. Microstructures of alloy sintered by vacuum hot pressing (HP) are observed by optical microscope (OM) and electron backscatter diffraction (EBSD). The prediction result by this theory is in good agreement with the experimental results. The enhanced sintering ability of powder containing appropriate fractions of coarse particle and fine particle is ascribed to the filling of fine particles to the voids between coarse particles, which enhanced the density of sample after sintering. Tensile behavior and the fracture morphology of alloys with various particle distributions are analyzed in details, suggesting the higher reliability of the present theory.     

Creep behaviour of single crystal nickel base superalloy

Abstract

The creep activation energy and structure constant at the different creep stages have been calculated, and the microstructures have been observed by SEM and TEM. The results showed that the internal stress σo decreased with an increase in temperature. Over the stress and temperature range, there are different activation energies, time exponents, and structure constants at different creep stages. The change in microstructure has an influence on creep resistance in this superalloy (Ni-6.0AI-7.0Ta-8.5Mo, wt-%). It is shown that the dislocation climb is the major deformation mechanism during tensile creep stages I and II, but during the tertiary stage, the creep resistance decreased as a result of dislocations shearing into the γ′ rafts. Creep fracture occurs mainly by the cavities and microcracks produced at the γ′/γ phase interface due to the interaction of multislips.     

High temperature deformation of nickel base superalloy Udimet 520

Abstract

The high temperature deformation behaviour of nickel base superalloy Udimet 520 was characterised using hot compression isothermal tests. Hot compression tests were conducted between 900 and 1150°C with strain rates of 0.001, 0.01, 0.1 and 1 s^-1. Testing at ≤ 950°C led to sample fracture for all the applied strain rates. The flow behaviour at 1000, 1050 and 1075°C indicated the occurrence of dynamic recovery. For specimens tested at 1100, 1125 and 1150°C, recrystallisation is the softening mechanism. The strain rate sensitivity factor m was estimated for various thermomechanical histories. The activation energy for the hot deformation was determined to be 780 kJ mol¹. The Zener–Hollomon parameter was also determined and its variation with grain size was studied with deformation conditions. The microstructures of all samples were examined by both optical and scanning electron microscopy. The presence and variations in the morphology and size distribution of deformed and recrystallised grains were determined and related to the deformation conditions.     

Low temperature effects on the fracture behaviour of a nickel base superalloy

The mechanical properties of a solution treated and double aged nickel - 18% iron - 18% chromium alloy (Inconel 718) were studied to assess its utility at temperatures in the ambient-to-cryogenic range. Uniaxial tensile property measurements using unnotched specimens at decreasing temperatures between 295 and 4 K show that yield and ultimate strengths increase by 20% and 29%, respectively, while ductility remains virtually constant. Fracture mechanics tests using 2.54 cm thick compact specimens revealed that the fatigue crack growth resistance of this alloy improves slightly at extreme cryogenic temperatures, and its plane strain fracture toughness, K_lc, increases from 96.3 MPa m^{$\text{1}\text{2}$} at 295 K to 112.3 MPa m^{$\text{1}\text{2}$} at 4 K. These results are compared with similar data for Inconel 750 alloys     

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.     

Notch fatigue crack initiation studies in a high strength nickel base superalloy

It is with great pleasure that I offer this paper for the Special Issue honoring Professor Takeo Yokobori on the occasion of his 70th birthday. I regard Professor Yokobori as a personal friend, having known him for over twenty five years professionally and socially. His impact on the discipline of fracture mechanics has been most significant, not only by his own technical contributions but, perhaps, more so because he put into motion the means whereby those interested in the field of fracture mechanics could meet together and to share their work with others. I refer, of course, to the formation of the International Conference on Fracture at Sendai, Japan in 1965 and to the present international journal, Engineering Fracture Mechanics. The following paper is therefore dedicated to Professor Yokobori as my contribution in honoring his distinguished career.     

Physical basis for model with various inelastic mechanisms for nickel base superalloy

Abstract

Two mechanical behaviour models for N – 18 alloy are proposed. The material is a powder metallurgy nickel base superalloy hardened by 60% volume of the ordered γ′ phase. The behaviour of alloy N – 18 is modelled by classical constitutive equations involving plasticity and creep. The experimental data used include stress relaxation and creep tests. An updated version of the first model is proposed and compared to the experimental data set. A new model is also presented with equations based on physical concepts. Material parameter identification is performed for each model, and experimental results are in good agreement with theoretical simulations.     

Superplasticity in a magnesium alloy prepared with bimodal grain size distributions developed by dynamic recrystallisation

In the present work the influence of bimodal grain size distributions on superplastic behavior, of a magnesium alloy, was investigated. Samples with different volume fraction of fine grains have been prepared, and their strain rate-stress relation during superplasticity has been measured. Additionally, the predictions of two deformation models, based on the isostrain and the isostress conditions, were compared with the experimental data. The isostrain model allows the major experimental observations to be readily explained and predicted.     

Numerical investigation of fracture behavior of nanostructured Cu with bimodal grain size distribution

Nanostructured metals with bimodal grain size distribution, composed of coarse grain (CG) and nanograin (NG) regions, have proved to have high strength and good ductility. Here, numerical investigation, based on the mechanism-based strain gradient plasticity theory and the Johnson–Cook failure model, focuses on effects of (1) distribution characteristics of the CG regions and (2) the constitutive relation of the NG with different grain sizes on fracture behavior in a center-cracked tension specimen of bimodal nanostructured Cu. High strain rate simulations show that both of them directly influence load response and energy history, and importantly, they are closely related to the fracture pattern. This study shows that both CG region bridging and crack deflection toughen the bimodal nanostructured Cu significantly, while debonding enhances the overall ductility moderately. Simulations also show that with volume fraction of the CG regions increasing, both structural strength and ductility of the bimodal nanostructured Cu specimen can be improved.     

Identification of the effective grain size responsible for the ductile to brittle transition temperature for steel with an ultrafine grain size ferrite/cementite microstructure with a bimodal ferrite grain size distribution

This research investigated the mechanism responsible for the ductile to brittle transition temperature for the newly developed steels with a bimodal, ultrafine grain size, ferrite/cementite microstructure (UGF/C), which are produced by caliber warm rolling followed by annealing. The microstructure of the steel was characterised. Charpy impact tests were carried out in the temperature range from 373 K to 4.2 K and the fracture surfaces were analysed. The effective grain size responsible for the ductile-to-brittle transition temperature corresponded to the grain size of the large grain size regions. The mechanism of this phenomenon was attributed to the characteristics of the grain boundaries, as high angle grain boundaries are more effective in impeding cleavage crack propagation. The grain size of the large grain size regions was important in determining the DBTT because these grain boundaries were high angle grain boundaries, whereas the small gain size regions were dominated by the low angle grain boundaries.     

The influence of base metal grain size on isothermal solidification during transient liquid-phase brazing of nickel

The influence of base metal grain size on isothermal solidification during transient liquid-phase brazing with Ni-11 wt% P filler metal has been investigated. Single-crystal, coarse-grained and fine-grained nickel base metal were brazed at 1150C for various holding times. The eutectic width decreased linearly with the square-root of the brazing time in single-crystal, coarse-grained and fine-grained nickel base metals. The completion time for isothermal solidification decreased in the order single-crystal, coarse-grained and fine-grained nickel base metal. The difference in isothermal solidification rates produced when brazing the different base metals is explained qualitatively by the influence of base metal grain boundaries on the apparent mean diffusion coefficient of phosphorus in solid nickel.     

Microstructural characterisation of deformation behaviour of nickel base superalloy IN625

Abstract

Simple compression and microscopy techniques were employed to characterise the microstructural origin of the deformation behaviour of nickel base superalloy IN625 during large strain testing. The alloy exhibited a four-stage strain hardening response similar to that previously reported for low stacking fault energy face centred cubic alloys. At strains lower than about ?0·06 (stage A), a falling regime of the hardening rate was observed. This stage was followed by a second stage (stage B) of slow increasing hardening rate, which was found to be coincident with the formation of Lomer–Cottrell locks. The second falling regime of strain hardening (stage C) was seen in the strain range of ?0·25 to ?0·65. The occurrence of this stage was attributed to the increasing ease of dislocation cross-slip with increasing strain and consequently to the decreasing Lomer–Cottrell lock efficiency in hindering dislocation movement. Beyond a strain of ?0·65, a final slightly constant hardening regime (stage D) was developed. The initiation of this stage was concurrent with the onset of deformation twinning in the microstructure.     

ODS ferritic steel engineered with bimodal grain size for high strength and ductility

An attractive way to enhance the ductility of ODS ferritic steels is to develop an alloy with a bimodal grain size distribution, in which the micron-sized coarse grains provide high ductility. The nanometer-sized fine grains enhance the tensile strength. The microstructures were obtained by blending the gas-atomized powders and mechanical alloyed powders, followed by hot forging and annealing. The homogeneously distributed nanometer-sized oxide nanoparticles can also be detected. Mechanical properties tests revealed a great improvement in ductility in comparison with other ODS ferritic steels, and high strength over the whole range of test temperatures, owing to the fine grains and oxide nanoparticles. The combination of high ductility and high strength makes this ODS ferritic steel much promising in high-temperature application.     

Thermomechanical relaxation of residual stress in shot peened nickel base superalloy

Abstract

The thermal and thermomechanical behaviour of the relaxation of the residual stresses of a shot peened Astroloy superalloy under tensile cyclic loads has been evaluated by X-ray diffraction and investigated. The stress relaxation under purely thermal conditions (550 and 650°C) and thermomechanical conditions (pulsating tensile loading at 650°C) as afunction of the exposure time is presented. The purely thermal relaxation is interpreted by annihilation and reorganisation of the crystalline defects induced by shot peening, whereas the mechanical relaxation is linked to cyclic plasticity of materials. In consequence, the thermomechanical relaxation is essentially due to the complex mechanism of the concurrent thermal and mechanical effects. A model is used to predict the residual stresses induced by the specified shot peening conditions and their relaxation under the specified thermal/thermomechanical conditions.

MST/1963     

Characterisation of hot isostatically pressed nickel base superalloy Inconel* 718

Abstract

Inert gas atomised Inconel 718 superalloy powder was characterised for various important properties and subsequently consolidated by hot isostatic pressing (hipping) at 1200° C and 120 MPa for 3 h. The density of the as compacted material was nearly the same as its theoretical density. Optical microscopy of as hipped material showed a fine grained structure with no porosity but having annealing twins and prior particle boundaries (PPBs). Electron probe microanalysis (EPMA) studies revealed that the PPBs were decorated with Al, Ti oxides, and MC type carbides enriched with Nb and Ti. In addition to these phases, the presence of very fine γ"-Ni₃Nb and γ'-Ni₃(Al,Ti) precipitates in the matrix were revealed by TEM analysis, which indicates that the compacted material was partially aged during the slow cooling stage of hipping. Tensile tests conducted on the as hipped material showed that the ultimate tensile strength (UTS) and ductility values were comparable to those obtained in the (solution treated and two step aged) wrought alloy 718, although its yield strength was marginally lower at room temperature.     

Ductility of nanocrystalline materials with a bimodal grain structure

A simple model describing the tensile deformation behavior of a nanostructured material with a combined nano- and microcrystalline grain structure has been developed. The nanocrystalline matrix ensures a high mechanical strength, while the microcrystalline grains provide an increase in the ductility of such materials. The proposed model of tensile straining of the material with bimodal grain structure predicts that the regime of uniform straining until neck formation must depend on the dimensions and volume fractions of the nano- and microcrystalline grains. It is shown that a growth in the ultimate uniform strain is related to an increase in the ratio of the yield stress to the coefficient of strain hardening and in the parameter of plastic strain distribution between the nano- and microcrystalline components.     

Properties,processability and weldability of a novel affordable creep resistant nickel base superalloy

Abstract

A recently designed heat resistant alloy for operation ～750°C in power plant, named FT750_DC, of composition Ni–20Cr–5Fe–3·5W–2·3Al–2·1Ti–0·07C–0·4Si–0·005B (wt-%), is fully characterised. Creep rupture and damage is investigated, as well as Charpy impact toughness. The alloy processability is also assessed: castability (comparison between measured microsegregation and Scheil simulation), forgeability (hot tensile ductility, strain hardening and recrystallisation behaviour), γ′ aging heat treatment (gamma-prime precipitation kinetics) and weldability (GTAW/TIG). It is demonstrated that this material, made affordable by avoiding the use of expensive alloying elements, with a creep rupture life in excess of 100 000 h at 750°C under 100 MPa, is easily castable, forgeable and weldable. It can therefore be favourably compared to concurrent alloys for fossil fuel or nuclear power plant applications (like the coal fired ultra supercritical steam power plant, or the nuclear high temperature gas cooled reactor) currently under assessment, such as alloys 230, 617, 625, 740, HX, etc.     

Oxidation assisted fatigue crack growth under complex non-isothermal loading conditions in a nickel base superalloy

This paper deals with the prediction of fatigue crack growth at high temperatures in the N18 nickel base superalloy, which is employed by Snecma for turbine disc applications. This material and other nickel base superalloys were widely studied in the past under isothermal conditions and constant amplitude fatigue. Dwell time effects are observed which are attributed, in this material, to grain boundary oxidation. The main objective of this research is to use this knowledge to model the fatigue crack growth rate in the N18 nickel base superalloy when complex “missions” are encountered. This implies variable amplitude and non-isothermal loading conditions (450–650 °C). For this purpose, an incremental fatigue crack growth model which was originally developed for isothermal variable amplitude loading conditions was extended so as to be applicable to non-isothermal loading conditions. In addition, the incremental form of the fatigue crack growth law in this model is very useful to account for the coupling effect between fatigue and time-dependent phenomena such as creep or oxidation. In the present case, the effect of the environment was modelled as a competition between two phenomena: a detrimental effect of grain boundary oxidation ahead of the crack tip and a beneficial effect of the growth of a passivation layer of oxides on the freshly created crack surfaces. The model was used to simulate fatigue crack growth under complex cycles at high temperature and the comparisons with experimental results are satisfactory.