Nimonic 80A Research Articles

Effect of duplex grain size distributions on long term stress-rupture life at 600 °C/450 MPa of Nimonic 80A

Long term stress-rupture life at 600°C/450MPa of Nimonic 80A with various duplex grain size distributions has been investigated. Stress-rupture life of duplex grain size samples significantly increases from about 4512h in the bimodal condition to 6863h in the banding condition and to 10,765h in the wide-range condition. Plate-like Cr23C6 prefers to precipitate at transition region of small dynamic recrystallization (DRX) grains and large original grains in the bimodal condition, which can easily lead to grain boundary (GB) cracking. Cracking is also found at small equiaxed recrystallized GB where is enriched with strip-like Cr23C6 in sample with abnormal grains in the banding condition. Blocky Cr23C6 exhibits an orientation relationship with γ at GB in sample in the wide-range condition. Blocky TiC with average size about 7.5μm precipitates at GBs firstly and then also in the grain interior, while the fine TiC with size about 300nm is found in grain interior and the morphology changes from lageniform or oval shape to spherical one with prolonged stress-rupture life. The γ′ which maintains a coherent orientation relationship with γ is very stable. The duplex grain size distribution in the wide-range condition significantly improves stress-rupture life.

Investigation of surface integrity, material removal rate and wire wear ratio for WEDM of Nimonic 80A alloy using GRA and Taguchi method

This paper presents the investigation on surface integrity, material removal rate and wire wear ratio of Nimonic 80A using WEDM process. Taguchi's design of experiments methodology has been used for planning and designing the experiments. All of the input parameters and two factors interactions have been found to be statically significant for their effects on the response of interest. SEM was performed on the machined samples to investigate the effect and microstructure of the samples after machining. A higher pulse-on time setting leads to thicker recast layer. At lower value of pulse-on time and higher value of pulse-off time, the wire deposition on the machined surface is low.

Damage Evolution and Crack Growth in Nickel-Based Alloys during Ultrasonic Fatigue

Experimental results on the fatigue damage of quasi defect-free materials in the VHCF range are presented. For nickel-based superalloys and pure nickel the likelihood of crack initiation at favorable grain morphologies is studied. Slip band and microcrack formation at the surface was observed even in run-out samples. Hence, microcracks were evaluated regarding their propagation capabilities according to grain orientation and barrier function of grain boundaries. In the VHCF regime crack initiation can shift from surface to subsurface, consequently early crack growth has to be studied by means of optical methods and indirect detection techniques or tomographic methods. In the study presented crack initiation and crack growth was monitored through optical observation and quasi 3-D observation by means of synchrotron radiation. For an as-received and a coarse-grained condition of pure nickel Ni201 fatigue crack growth in the VHCF regime occurs at deltaK as low as 3.54 MPam1/2 for a crack growth rate da/dN = 10E-12 m/cycle. The grain size had no effect on the threshold limit but crack growth retardation at grain boundaries and crack path deflection lead to lower crack growth rates for the coarse-grained condition In the nickel-based alloy Nimonic 80A the influence of microstructure on the intercrystalline crack initiation and propagation was confirmed. Here, the combination of the misorientation angle between two adjacent grains and the orientation of their boundary with respect to the external load defines the magnitude of stress concentration at grain boundaries.

Fatigue Behavior of Precipitation Hardening Alloys in the LCF and VHCF Regime

LCF/HCF strength of precipitation hardening alloys is primarily controlled by its heat treatment condition. However, for a nickel-based superalloy and a wrought aluminium alloy it will be shown, that the VHCF behavior cannot solely be explained by the precipitation morphology. Damage accumulation is dominated by microstructure related slip localization, grain morphology and microstructural flaws. In contrast to the LCF behavior, the prediction of cyclic strength in the VHCF regime requires a detailed analysis of the competing microstructural crack initiating characteristics. Hence, new fatigue life prediction models have to be developed, which consider a statistical analysis of the failure-relevant inhomogeneities. In the case of the two materials studied, VHCF behavior is dominated by isolated and inhomogeneously distributed irreversible slip accompanied by a low dislocation density. The formation of single slip bands in favorably oriented grains in Nimonic 80A results in a decrease of the VHCF strength for the peak-aged condition. The overaged condition shows better VHCF strength due to a more homogeneous distribution of slip bands, as dislocations pile up at the overaged precipitates due to the very low strain amplitudes, while in the LCF regime the Orowan mechanism results in a weaker cyclic strength compared to the peak-aged condition. Crack initiation of the aluminium alloy EN AW-6082 on the one hand depends on the size and distribution of primary intermetallic particles of the Al-Fe-type acting as local stress raiser embedded in a strong matrix in case of the peak-aged condition. In contrast, such stress peaks are less failure-relevant due to the softer solid solution depleted matrix. Hence, the heat treatment alone does not define VHCF behavior.

Ni계 초내열합금 NIMONIC 80A의 고온변형거동

The deformation behavior of NIMONIC 80A was studied in the high temperature range of <TEX>$900{\sim}1200^{\circ}C$</TEX> and for strain rates varying between 0.02 and <TEX>$20s^{-1}$</TEX> via the hot compression test. Processing maps for hot working were constructed on the basis of the power dissipation efficiency using a dynamic material model. The results showed that the strength during hot compression increased with increasing strain rate and decreasing temperature. At low strains, the processing map of NIMONIC 80A did not reveal any instability domain regardless of the strain rate and temperature. However, at high strains, the processing map exhibited an instability domain at a low strain rate of <TEX>$0.2s^{-1}$</TEX> and within a temperature range of <TEX>$900{\sim}960^{\circ}C$</TEX>. In the instability domain, the deformed microstructure exhibited shear bands and carbide precipitation while, in the safe domain, full recrystallization occurred.

Strengthening behavior of Nb in the modified Nimonic 80A

The microstructures and mechanical properties of Nb modified Nimonic 80A were investigated. Nb increases the precipitate temperature and weight fraction of γ′ phase. The substitution of Ti by Nb and the precipitate of (Nb, Ti)C lead to the decrease of Ti composition in γ′ phase after full heat treatment. The diffusion of Nb from γ to γ′ phase makes part of the fine γ′2 particles coexist with the coarse γ′1 particles. The δ-Ni3Nb phase precipitates in γ matrix channel and exhibits an orientation relationship with the blocky (Nb, Ti)C at 845°C: (100)δ ‖ (100)MC and [010]δ ‖ [010]MC. Precipitate of (Nb, Ti)C in γ matrix channel and at grain boundaries suppresses γ′ coarsening and grain boundary sliding, respectively. The (Nb, Ti)C precipitated at grain boundary also suppresses the coalescence of cavities at high temperature. The stress-rupture life of the 1.5wt% Nb modified Nimonic 80A increases by about 25.4% at 750°C/310MPa and the creep life increases from less than 500h to more than 1300h at 750°C/240MPa. The diffusion of Nb element into γ′ phase and the precipitates of carbides significantly improve the mechanical properties of Nimonic 80A.

Microstructure evolution and stress-rupture properties of Nimonic 80A after various heat treatments

Four heat treatments have been designed to produce multi-modal size distributions of γ′ phase in γ matrix, and stress-rupture properties of Nimonic 80A have been investigated. The γ′ phase exhibits two typical coherent orientation relationships with γ matrix. The fine γ′ particles are in spherical shape when the average size is about 20nm. However, the coarse γ′ particles present a cuboidal shape with round corners when they are larger than 75nm. The Al and Ti elements diffuse from γ matrix to γ′ phase, and about 80% Al and Ti are used to form γ′ phase after various heat treatments. The increased volume fraction of the fine γ′ particles with average size of 15.8–20.3nm increases the stress-rupture life of Nimonic 80A at 750°C/310MPa. The rod-like Cr23C6 carbide at grain boundary can suppress grain boundary sliding at high temperature and benefit the stress-rupture life. The coarse γ′ particles facilitate the movement of dislocations and contribute to the ductility. The precipitate of multi-modal size distributions of γ′ phase is beneficial to the stress-rupture ductility.

Failure assessment of Nimonic 80A gas turbine blade

Nickel base superalloys are widely used in applications requiring strength at high temperature. During the operation of gas turbines, the blades and other elements of hot gas path suffer service induced degradation which may be natural or accelerated due to different causes. The failure analysis of the 3MW gas turbine first stage blades made of nickel-base alloy Nimonic 80A is presented. The blade experienced base alloy degradation due to operation at high temperature. Initially the blade was sectioned for metallographic and microhardness test. The microstructure and microhardness was compared in four different blade zones i.e. root, 30%, 60%, and 90% of the total height of the hot region of the blade. On the basis of the observed microstructures and the phases present in the alloy, the main cause of failure was found to be creep damage. Finite element method (FEM) analysis is conducted considering peak loading of centrifugal force and surface (aerodynamic) loads. Based on FEM simulation results, the life of turbine blades is predicted using the Larson–Miller method. Finally to improve the blade life two heat treatment cycles were suggested and applied. The effect of heat treatments on grain size, volume friction of γ′ primary phase, and micro hardness were investigated. A detailed analysis of the coating showed that, the loss of coatings resistant to high temperature is due to oxidation, corrosion, erosion and inters diffusion of coating-substrate, which results in diffusion of alloy elements into the coating.

Strengthening behavior of Al and Ti elements at room temperature and high temperature in modified Nimonic 80A

Nickel-based superalloy Nimonic 80A modified with various Al and Ti contents has been developed. The γ′ coherently precipitates in the γ matrix on {100}γ′/γ and {110}γ′/γ atomic planes. The volume fraction of γ′ increases significantly with the increase of Al and Ti contents. But the volume fraction of γ′ decreases after stress-rupture tests compared with that after full heat treatment, which is primarily due to the coarsening of γ′ and the increased width of γ matrix channel. The strengthening effect of Ti is more significant than that of Al at room temperature. While the strengthening effect of Al is more significant than that of Ti at 750°C/310MPa. The increased volume fraction of γ′ leads to the increased room temperature tensile strength and stress-rupture life at 750°C/310MPa. However, the η-Ni3Ti is identified after full heat treatment in the alloys when the Ti content is about 2.7wt.%, which results in decreased stress-rupture life. But the precipitate of η-Ni3Ti does no harm to the room temperature tensile properties.

Mechanical Properties of Forged Nimonic 80A Superalloy Fabricated by Vacuum Spray Casting

This study investigates the effect of heat treatment on the mechanical properties of a forged Nibased superalloy called Nimonic 80A. Nimonic 80A ingot samples were fabricated by vacuum spray casting to achieve a fine and homogenized microstructure. The ingot samples were subsequently hot-forged with the diameter of 220 mm at 1373 K. From the center to the surface of the forged Nimonic 80A, its average grain size decreased and its micro-Vickers hardness increased slightly. Solution treatment was carried out at 1353 K with 8 hours of air cooling followed by aging treatment, which was carried out in the range of 8731073 K with various times from 0.5 to 256 hours. To set the optimum aging conditions, micro-Vickers hardness tests were performed. The maximum hardness value of 388.0 Hv was obtained by aging at 973 K for 32 hours. Also, tensile tests were performed for optimum aging conditions at room temperature and 873 K. The results can be used effectively to perform reasonable heat treatment of Nimonic 80A superalloy. (Received October 10, 2011)

Evolution of microstructure and mechanical properties of Ti modified superalloy Nimonic 80A

Nickel based superalloy Nimonic 80A modified with various Ti contents has been developed. Microstructure evolutions were investigated by optical microscope, X-ray diffraction, scanning electron microscope and transmission electron microscope in relation to the room temperature tensile properties and stress-rupture properties at 750 °C/310 MPa. After full heat treatment, the precipitate changed from γ′ phase to γ′ + η phases, the volume fraction of γ′ phase, lattice misfit of γ′/γ and room temperature tensile strength increased with the increase of Ti content. With the increase of lattice misfit, the orderly {1 1 1} atomic planes in the γ′/γ interface changed to slight distorted atomic planes and edge dislocations with Burgers vector of a/3 〈1 1 1〉 were identified. The stress-rupture life first increased and then decreased with the increase of Ti content, and the decrease was primarily due to the precipitate of intermetallic η phase at grain boundaries. After stress-rupture tests, the precipitate of Cr23C6 carbide exhibited an orientation relationship with the γ′ and γ phases, the spherical morphology of γ′ phase transformed to cubic shape for the alloy with longest stress-rupture life and the morphology transformation process of γ′ phase was schematically suggested.

Studies of high temperature sliding wear of metallic dissimilar interfaces IV: Nimonic 80A versus Incoloy 800HT

Wear variations of Nimonic 80A slid against Incoloy 800HT between room temperature (RT) and 750 °C, and sliding speeds of 0.314 and 0.905 m s −1 were investigated using a ‘reciprocating-block-on-cylinder’, low debris retention configuration. These were considered alongside previous observations at 0.654 m s −1. Different wear types occurring were mapped, including high transfer ‘severe wear’ (RT and 270 °C, also 0.905 m s −1 at ≤570°C), low transfer ‘severe wear’ (0.314 m s −1 at 390 °C to 510 °C oxide abrasion assisted at 510 °C), and ‘mild wear’ (0.314 m s −1 at ≥570 °C; 0.905 m s −1 at ≥630 °C). Wear surfaces at 750 °C were cross-sectioned and profiled.

The Relaxation Behavior of High Chromium- Ni Base Superalloys

The relaxation behavior of wrought high chromium Ni-base alloys Nimonic 80A, Nimonic 101 and Nimonic 105 was studied at different temperatures. All of these alloys exhibited increase in the residual stress during the relaxation tests e.g. at 450°C for Nimonic 80A and 650°C for Nimonic 105. The observed increase in the residual stress is a manifestation of the known phenomenon of “negative creep”. The stress free aging of specimens of these alloys exhibited dimensional contraction at different temperatures ranging from 450°C to 650°C. The abnormal relaxation behavior of these alloys and the observed contraction were attributed to the precipitation and ordering of the intermetallic phase Ni2(Cr,Mo) at the test temperature. The differential thermal analysis i.e. DTA results, demonstrated endothermic peaks to correspond with the order–disorder reaction of Ni2(Cr,Mo).

Effect of precipitation condition, prestrain and temperature on the fatigue behaviour of wrought nickel-based superalloys in the VHCF range

The influence of a monotonic predeformation at room temperature on the very high cycle fatigue (VHCF) behaviour was studied in polycrystalline nickel-based superalloys (Nimonic 75 and Nimonic 80A) in three characteristic precipitation conditions (precipitation-free, peak-aged and overaged) after various degrees of predeformation. Isothermal fatigue tests were executed at temperatures up to 800 °C. In the intermediate temperature range (400–600 °C) the effect of predeformation becomes weaker with increasing temperature. This can be partially explained by the beginning of recovery. However, additional effects not related to the strain-hardened microstructure play a role, such as the formation of oxide layers and the decrease in notch sensitivity. The latter effect is important because of the surface roughening resulting from prestraining. Even for the overaged condition, 4% predeformation led to a higher VHCF fatigue resistance at 600 °C, due to the relatively stable recovered dislocation arrangement. Isothermal cyclic deformation at 800 °C revealed a pronounced decrease in cyclic lifetime for all precipitation conditions with or without a mechanical prehistory. Transmission electron microscopy was carried out in order to characterize the influence of the microstructure, the resulting dislocation slip behaviour and the relevant dislocation/particle interaction mechanisms on the VHCF behaviour. Furthermore, scanning electron microscopy observations revealed that all precipitation conditions show, independent of their predeformation condition, a transition from transgranular (room temperature to 600 °C) to intergranular (600–800 °C) microfatigue crack formation in the emerging oxide layers at the surface after VHCF tests.

Microstructural mapping in closed die forging process of superalloy Nimonic 80a valve head

Optical microscopy, XRD, TEM and FEM were used to study the microstructral map in the alloy in the upsetting and closed die forging processes. The maps of hardness, grain size, dislocation and the secondary phase γ′ in the upset and closed die forged nimonic 80a alloy valve head were investigated. It is found that the metals flowed from the asymmetric central axis outwards in the upsetting and forging process. The flowing model of metals together with the thermal field led to a heterogeneous distribution of grain size, dislocation configuration and the secondary phase γ′. It is also necessary to mention that the largest grain size, the lowest density of dislocation and secondary γ′ phase only occurred in the transitional region between the valve head and valve stem, which may weaken the strength in this region.

Strengthening mechanisms of carbon in modified nickel-based superalloy Nimonic 80A

Nickel-based superalloy Nimonic 80A with various carbon contents has been developed. Various strengthening mechanisms of carbon were analyzed by optical microscope, X-ray diffraction, scanning electron microscope equipped with energy dispersive spectroscopy and transmission electron microscope. Detailed microstructural analysis revealed that with the increase of carbon content from 0.01% to 0.10%, both the lattice parameters of γ′ and γ phases decreased, and the misfit δ between the coherent γ′ and γ phases increased slightly from 0.289% to 0.317% after full heat treatment at 1070 °C/8 h, A.C. + 700 °C/16 h, A.C. The diameter of spherical γ′ phase decreased from 22.24 nm of 0.01%C alloy to less than 15.76 nm of 0.06%C and 0.10%C alloys. Cr 23C 6 carbide precipitated at the grain boundary in the alloys with carbon content higher than 0.06%, and when carbon content increased to 0.10%, Cr 23C 6 carbide had an orientation relationship with the γ matrix: [ 00 1 ¯ ] Cr 23C 6 [ 00 1 ¯ ] γ matrix and ( 100 ) C r 23 C 6 (1 0 0) γ matrix, which can enforce the grain boundary strength. The growth of room temperature tensile strength with the increase of carbon content was primarily due to the coherent strain strengthening and the precipitate strengthening of Cr 23C 6 carbide. After stress-rupture test at 750 °C/310 MPa, the misfit δ increased from 0.182% to 0.237% with the increase of carbon content, but the values were lower than those after full heat treatment. The precipitate of blocky Cr 23C 6 in 0.10%C alloy tended to become spherical and dislocation lines were found in the γ matrix channels between γ′ phases, the combination of the dislocation strengthening and precipitate strengthening of Cr 23C 6 carbide at grain boundary was beneficial to the stress-rupture properties.

3370 Reciprocating Plunge Profile Grinding of Nickel-Based Superalloy Nimonic 80A with Ultrafine-Crystalline cBN Wheel

The purpose of this research is to clarify the grinding characteristics of the ultrafine-crystalline cBN wheel in a reciprocating plunge profile grinding of nickel-based superalloy. Reciprocating plunge V-groove grinding experiments of Nimonic 80A superalloy were conducted using the ultrafine-crystalline and mono-crystalline cBN vitrified bonded wheels. Both radial and profile wear of the ultrafine-crystalline cBN (cBN-U) wheel are less than those of the conventional mono-crystalline cBN (cBN-B) wheel, hence the steady grinding ratio of the cBN-U wheel is around 10 times higher than that of the CBN-B wheel. Grinding force in grinding with the cBN-U wheel are 10〜20 % lower than that in grinding with the cBN-B wheel. The surface roughness of groove bottom ground by the cBN-U wheel is around 10 % lower than that in grinding with the cBN-B wheel. Thus, the ultrafine-crystalline cBN wheel is suitable for the applications with a high dimensional accuracy in the profile grinding for nickel-based superalloys, because it gives less profile wear, and hence better form retention than the conventional cBN-B wheel.

On the effects of particle strengthening and temperature on the VHCF behavior at high frequency

The fatigue properties in the high-temperature range at very high number of cycles of the precipitation-hardened nickel-base alloy Nimonic 80A were investigated. With the applied global strain amplitude being well below the elastic limit, cyclic life is dominated by heterogeneously distributed and localized plastic deformation in the microstructure. The material studied was fatigued in the peak-aged and overaged condition applying high frequency testing methods. Microstructural analysis by means of scanning and transmission electron microscopy was performed to determine the dominating damage mechanism. At room temperature planar single slip and at elevated temperatures partially wavy dislocation arrangements were found, with the wavy arrangement being accompanied by dislocation climbing processes as well as Orowan loops. At room temperature the overaged Nimonic 80A showed a slightly superior fatigue behavior compared to the peak-aged condition in the VHCF range, whereas at elevated temperatures the difference in fatigue behavior at high number of cycles seemed to be of less significance. The overaged condition showed a slight increase in fatigue life at 600 °C accompanied by a tendency towards a more pronounced homogeneous dislocation movement. Isothermal cyclic deformation at 800 °C led to a pronounced decrease of cyclic life with both heat treatments exhibiting a very similar fatigue behavior. Early failure was primarily ascribed to the formation of microcracks in the emerging oxide layers.

Localized cyclic deformation and corresponding dislocation arrangements of polycrystalline Ni-base superalloys and pure Nickel in the VHCF regime

Knowledge about the fatigue behaviour of metallic materials in the range of very high cycle fatigue (VHCF, N > 10 7) is a key factor to improve the safety against failure of component parts. Even in the conventional field of application of nickel-base alloys (e.g. turbine blades) superimposed high frequency loading requires a safe life at very high number of cycles. Thus, the fatigue behaviour of structural materials in the very high cycle regime has become an important and active subject of research. In this paper, polycrystalline nickel-base alloys (Nimonic 80A and Nimonic 75) and pure Nickel (wavy slip character) were investigated in the VHCF regime by varying the precipitation conditions (peak-aged, overaged and precipitation-free), dislocation slip behaviour and test frequency. In addition, mechanisms of fatigue failure in the VHCF regime were compared to conventional damage evolution (in the LCF and HCF region) on the basis of load-controlled low frequency tests. Surprisingly the overaged condition of Nimonic 80A shows a slightly higher fatigue strength in the VHCF regime as compared to the peak-aged condition. The results obtained document that fatigue failure can still occur beyond 10 7 cycles. Transmission electron microscopy (TEM) was carried out, in order to characterize the influence of microstructure, the resulting dislocation slip behaviour and the relevant dislocation particle interaction mechanism. Studies of the development of slip markings on surface grains were performed mainly using scanning electron microscopy (SEM).

Influence of prestraining on the high-temperature fatigue behaviour of polycrystalline nickel-based superalloys in the VHCF range

The influence of a monotonic predeformation at room temperature on the very high cycle fatigue (VHCF) behaviour was studied on polycrystalline nickel-based superalloys (Nimonic 75 and Nimonic 80A) in three characteristic precipitation (precipitationfree, peak-aged and overaged) and different predeformation conditions. Isothermal fatigue tests were executed at temperatures up to 800 °C. With increasing intermediate temperature (400 °C–600 °C) the effect of predeformation becomes weaker which might partially be explained by the beginning of a recovery of the dislocation arrangement as well as attributed to effects not related to the strain hardened microstructure, such as the formation of oxide layers and the materials notch sensitivity for the testing conditions used given pronounced surface roughening due to the prestraining. Isothermal cyclic deformation at 800 °C revealed a pronounced decrease of cyclic lifetime for all precipitation conditions with or without a mechanical prehistory. Transmission electron microscopy was carried out in order to characterize the influence of the microstructure, the resulting dislocation slip behaviour and the relevant dislocation/particle interaction mechanism on the VHCF behaviour.