Strengthened Nickel Base Superalloy Research Articles

Introduction on Research and Application of Nickel Base Superalloy GH4169

In this paper, the research and application of nickel base superalloy GH4169 in recent years was introduced. GH4169 alloy is a precipitation strengthened nickel base superalloy, which has been widely used in aeroengine, petroleum, nuclear industry and other fields. The phase composition, three common heat treatments as well as the application prospect were elaborated. At last, the service temperature range and application area of GH4169 alloy was believed to be further expanded with the adjusted alloy composition and the corresponding hot working process.

On the strengthening and embrittlement mechanisms of an additively manufactured Nickel-base superalloy

The γ′ phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ′ precipitated with various size and fraction depending on heat treatment process. For room-temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built sample while dislocations bypassing the precipitates via Orowan looping in the γ′ strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ′ strengthened samples have higher work hardening rate than as-built sample but encounter premature failure. Experimental evidence shows that the embrittlement mechanism in the γ′ strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strength and ductility of Nickel-base superalloy is discussed in detail.

On the Strengthening and Embrittlement Mechanisms of an Additively Manufactured Nickel-Base Superalloy

The γ' phase strengthened Nickel-base superalloy is one of the most significant dual-phase alloy systems for high-temperature engineering applications. The tensile properties of laser powder-bed-fused IN738LC superalloy in the as-built state have been shown to have both good strength and ductility compared with its post-thermal treated state. A microstructural hierarchy composed of weak texture, sub-micron cellular structures and dislocation cellular walls was promoted in the as-built sample. After post-thermal treatment, the secondary phase γ' precipitated with various size and fraction depending on heat treatment process. For room temperature tensile tests, the dominated deformation mechanism is planar slip of dislocations in the as-built samples while dislocations bypassing the precipitates via Orowan looping in the γ' strengthened samples. The extraordinary strengthening effect due to the dislocation substructure in the as-built sample provides an addition of 372 MPa in yield strength. The results of our calculation are in agreement with experimental yield strength for all the three different conditions investigated. Strikingly, the γ' strengthened samples have higher work hardening rate than as-built sample but encountered premature failure. Experimental evidence shows that the embrittlement mechanism in the γ' strengthened samples is caused by the high dislocation hardening of the grain interior region, which reduces the ability to accommodate further plastic strain and leads to premature intergranular cracking. On the basis of these results, the strengthening micromechanism and double-edge effect of strengthening and ductility of Nickel-base superalloy is discussed in detail.

Dynamic recrystallization phenomena during laser-assisted friction stir processing of a precipitation hardened nickel base superalloy

Laser-assisted friction stir processing was used to modify the microstructure of an as-cast gamma prime strengthened nickel base superalloy. The grain structure of top region of the stir zone, where full dissolution of gamma prime precipitates occurred during heating, is controlled by discontinuous dynamic recrystallization mechanism and the pinning effect of refined carbides and cooling gamma prime precipitates. In the bottom of the stir zone, the partial dissolved gamma prime precipitates play important role in grain structure development by their pinning effect in retarding grain growth during cooling as well as by a continuous recrystallization mechanism.

Fatigue induced deformation and thermodynamics evolution in a nano particle strengthened nickel base superalloy

AbstractIn‐situ neutron‐diffraction and temperature measurements were simultaneously applied to investigate low‐cycle‐fatigue behaviour of a nano‐precipitate strengthened nickel‐based superalloy. Two transitions in the temperature‐evolution are observed subjected to cyclic loading. Two models are compared with the measured temperature evolution. One is based on bulk stress, and the other is based on lattice‐strain evolution. The calculated thermoelastic responses in both models qualitatively agree with the measured bulk‐temperature evolution for the first transition. The in‐situ neutron‐diffraction results reveal that the first transition is associated with the cyclic hardening/softening dislocation‐structural transformation. However, the second transition, which is observed at larger number of fatigue cycles during the steady cycles, does not correlate with the dislocation evolution. A phenomenological model is applied to describe the second temperature‐transition stages. The energy dissipation evolutions in the second fatigue stage indicate the initiation and the growth activities of fatigue microcrack. The data reported here may be useful for cohesive zone model.

Superalloys: an introduction with thermal analysis

Nickel based superalloys are commonly used materials in the aero industry and more specifically in the hot section of aero engines. These nickel and nickel iron based superalloys are precipitation strengthened alloys with a face centered cubic gamma matrix. Alloy 718, Allvac 718Plus and Waspaloy have been of great interest in the present study. Alloy 718 is a precipitation strengthened nickel-iron based alloy having gamma double prime phase (Ni 3 Nb) as a main strengthening phase up to 650 °C. Waspaloy, another precipitation strengthened nickel base superalloy, has a very good strength at temperatures up to ~750 °C whereas Allvac 718Plus is a newly developed nickel based precipitation strengthened superalloy which retains good mechanical properties at up to ~700 °C. These three alloys were investigated in terms of how their respective solidification process reveals upon cooling.Latent heat of soloidification has been estimated for all three alloys. Differential thermal analyses (DTA) have been used to approach the task. It was seen that Waspaloy has the smallest solidification range whereas Allvac 718Plus has the largest solidification interval in comparison.

A dislocation density based material model to simulate the anisotropic creep behavior of single-phase and two-phase single crystals

The primary and secondary creep behavior of single crystals is observed by a material model using evolution equations for dislocation densities on individual slip systems. An interaction matrix defines the mutual influence of dislocation densities on different glide systems. Face-centered cubic (fcc), body-centered cubic (bcc) and hexagonal closed packed (hcp) lattice structures have been investigated. The material model is implemented in a finite element method to analyze the orientation dependent creep behavior of two-phase single crystals. Three finite element models are introduced to simulate creep of a γ′ strengthened nickel base superalloy in 〈1 0 0〉, 〈1 1 0〉 and 〈1 1 1〉 directions. This approach allows to examine the influence of crystal slip and cuboidal microstructure on the deformation process.

Mechanism of dislocation shearing of gamma in fine precipitate strengthened superalloy

The dislocation structure of a fine gamma precipitate strengthened nickel base superalloy during steady state creep has been studied. Extensive shearing of gamma particles by partial dislocations, creating stacking faults in both gamma and gamma phases, have been observed in specimens under stresses ranging from 120 to 250 MPa. Detailed analysis of the fault nature and dislocation dissociation revealed that a unit matrix dislocation of a2110 dissociated into two partials of a3112 and a6112. Extrinsic stacking faults were created in both gamma and gamma phases, presumably being created by the movement of the leading a3112 partial, while the movement of the a6112 partial eliminates the stacking fault in the matrix, but leaves a partial dislocation loop around the faulted gamma precipitates.

High temperature oxidation and sulfate induced corrosion of an oxide dispersion strengthened Ni base superalloy

The oxidation and sulfate induced corrosion behavior of an oxide dispersion strengthened nickel base superalloy, MA 6000, has been studied at 900°C and 1000°C. The experimental program involved thermogravimetry. X-ray diffraction and scanning electron microscopy. The kinetics of high temperature oxidation in air increased with increasing temperature. The experimental activation energy for oxidation in air indicated that a chromia-rich scale formed the protective oxide layer. In the case of sulfate induced corrosion, however, increasing temperature decreased the scale growth rate. This result has been explained by considering the relative stability of nickel sulfide at 900° and 1000°C. Metallographic studies of the scales are also presented.

Prediction of rupture lifetime in thin sections of a nickel base superalloy

The prediction of rupture lifetime of a component can be based on the data generated from round, flat and tubular specimens. An investigation on the influence of specimen geometry on the creep behavior of Inconel alloy X-750 showed that the tubular specimen exhibited better creep performance when rupture lifetime data were compared on the basis of section size. However, the time to rupture data of all three specimen geometries merged together when compared on the basis of the volume to surface area ratio (V/S) indicating that there is a definite relationship between V/S and the rupture lifetime. This analysis is now extended in another gamma prime strengthened nickel base superalloy. The chemical composition of the alloy in wt%, is as follows: 0.07C-0.66Si-0.07Cu-0.79Fe-19.1Cr-2.35Ti-1.52Al and remainder nickel. Analysis of the rupture lifetime data of a gamma prime strengthened nickel base superalloy shows that time-to-rupture is controlled by the volume to surface area ratio. Furthermore, the normalization of the volume to surface area ratio with the grain size shows that the time-to-rupture data of two grain sizes, 55 and 250 [mu]m, can be represented by a single curve.

Directional recrystallisation in Inconel MA 6000 nickel base oxide dispersion strengthened superalloy

The directional recrystallisation process which leads to the growth of highly anisotropic ‘new’ grains in an oxide dispersion strengthened nickel base superalloy is investigated to elucidate the mechanism of recrystallisation and the process variables important in controlling microstructural evolution. It is found that after extrusion and hot deformation, the alloy consists of an ultra fine equiaxed grain microstructure characteristic of a material which has undergone dynamic primary recrystallisation. Thus, the directional recrystallisation which follows when the hot rolled sample is annealed in a moving temperature gradient (zone annealed) is really a secondary recrystallisation phenomenon. By comparing grain boundary mobility with the rate of zone annealing, it was theoretically predicted and experimentally observed that directional recrystallisation should give way to equiaxial recrystallisation when the interface velocity is less than the speed of zone annealing. The variables involved in the anisothermal zone annealing process appear to rationalise when the effects of temperature and time are consolidated into a single parameter, the kinetic strength of the heat treatment. Directional recrystallisation is found to occur only when a critical value of the kinetic strength is exceeded, irrespective of the peak temperature or traverse speed during zone annealing.MST/1268

酸化物分散強化型ニッケル基超合金の開発

酸化物分散強化型ニッケル基超合金の開発

An Oxide Dispersion Strengthened Nickel-base Superalloy with Excellent High Temperature Strength

Oxide dispersion strengthened nickel-base superalloys with compositional variants were prepared by mechanical alloying, and their secondary recrystallization characteristics and creep rupture strength were examined. Their matrix compositions were determined by modifying cast superalloys containing a high volume fraction of γ' phase. One of the presently investigated alloys, Alloy 98, was shown to form directionally recrystallized grain structures by zone anneal and surpass monocrystalline superalloys in the creep rupture stress range below about 320MPa. Higher tantalum concentration in Alloy 98 than in other oxide dispersion strengthened alloys is considered to improve creep rupture strength. Aluminum addition, higher than about 6wt%, seems to degrade recrystallization properties and consequently decrease creep strength. M23C6 carbide precipitates besides γ' phase and oxide dispersoids were observed in Alloy 98 by means of transmission electron microscopy. Also observed was a formation of coalescent oxides and oxide denuded areas around them, and the higher creep strength will be expected by improved process to minimize these dispersion defects.

Low-Pressure Plasma-Spray Coatings for Hot-Corrosion Resistance

The hot corrosion of low-pressure plasma-sprayed coating, GT-29 (Co-29Cr-6Al-1Y) on a γ′-strengthened nickel base superalloy, IN-738, was characterized at both 871 °C and 983 °C in a simulated gas turbine environment. The test results show that at 871 °C the dense, defect-free, low-pressure, plasma-sprayed GT-29 coatings provide good corrosion protection up to 8000 hr. The mechanism of protection was the formation of a dense, adherent Al2O3 scale underneath a mixed oxide scale; scale was maintained by a uniformly dispersed Al rich β(CoAl) reservoir. At 982 °C, the coating corrosion protection exceeded 5000 hours and also utilized a protective Al2O3 scale. Due to the higher test temperature, interdiffusion of O, S, Al, Co, and Cr was higher and internal sulfidation/oxidation of the coating occurred; however, the rates were much lower than uncoated IN-738. Field tests were run (13051 and 19842 hr) to evaluate the low-pressure, plasma-sprayed GT-29 coatings’ corrosion resistance on turbine buckets. Metallographic inspection of the field-tested coatings verified the simulated burner-rig test results and demonstrated that the low-pressure plasma process provides an attractive alternative to other coating processes for producing hot-corrosion resistant coatings.

Fine Microstructure of a Mechanically Alloyed Oxide Dispersion Strengthened Nickel-Base Superalloy

The previous paper briefly described the fine microstructure of a mechanically alloyed oxide dispersion strengthened nickel-base solid solution. This note examines the fine microstructure of another mechanically alloyed system. This alloy differs from the one described previously in that it is more generously endowed with coherent precipitate γ forming elements A1 and Ti and it contains a higher volume fraction of the finely dispersed Y2O3 oxide. An interesting question to answer in the comparative study of the creep and stress rupture of these two ODS systems is the role of the precipitate γ' in the mechanisms of creep and stress rupture in alloys already containing oxide dispersoids.The nominal chemical composition of this alloy is Ni - 20%Cr - 2.5%Ti - 1.5% A1 - 1.3%Y203 by weight. The system receives a three stage heat treatment-- the first designed to produce a coarse grain structure similar to the solid solution alloy but with a smaller grain aspect ratio of about ten.

Ni基固溶強化型超耐熱合金の合金設計

Ni基固溶強化型超耐熱合金の合金設計