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Nickel-base superalloys that are in service for long periods of time in fossil energy powerplants have microstructures that evolve with time. The strengthening gamma prime phase coarsens, and in some alloys, a new phase (eta) forms at grain boundaries, and its effect on creep performance is not fully understood.
Nickel-base superalloy has a strength of 20,000 psi at 2,200 degrees F, approximately double the strength of the strongest available cast nickel-base alloys. It is not subject to the formation of embrittling phases upon long time exposure at intermediate temperatures.
Nickel-based superalloys designed for use in gas turbine engines blades are difficult and expensive to properly machine without undue surface damage, due in part to their superior mechanical properties at high temperatures. Numerically controlled electrical discharge machining (EDM) has progressed to commonly be used as a machining process for initial sectioning, detailed roughing, and even some finishing operations. The objective of this study was to examine the effect of EDM on resulting surface conditions of superalloy 718 after slicing using typical rough, semi-finish, and finish condition sets with a typical wire EDM machine. Surface roughness, residual stress, and microstructures were compared for these conditions. The varied EDM conditions influenced surface roughness as intended. The residual stresses and surface roughness directly corresponding to the EDM steps were determined. While roughness decreased when progressing from rough to finish EDM conditions, tensile residual stresses unexpectedly increased. Brief assessments after EDM of two higher performance powder metallurgy disk superalloys ME209 and LSHR indicated this inverse relationship of roughness versus tensile residual stress was also present for these superalloys, where tensile residual stresses were significantly enhanced.
Nickel- (Ni-) base superalloy single crystals represent the state-of-the-art for turbine engine airfoil applications because they offer the best balance of properties under the high operating temperatures required for efficient engine operation. Current trends in alloy design take advantage of improved creep rupture strength with the addition of higher levels of refractory elements. In particular, the addition of significantly higher levels of rhenium in third-generation superalloys is key for both microstructural stability and creep rupture strength. Although refractories provide strength benefits, alloys tend to be unstable when their refractory content is high because of topologically close-packed (TCP) phases. The formation of these phases in sufficient amount is detrimental to the performance of these alloys because of their brittle nature and because they deplete the Nirich matrix of potent solid-solution strengthening elements.
Fourteen nickel-base superalloy single crystals containing a range of chromium (Cr), cobalt (Co), molybdenum (Mo), and rhenium (Re) levels, and fixed amounts of aluminum (Al) and tantalum (Ta), were examined to determine the effect of bulk composition on basic microstructural parameters, including {gamma} Prime solvus, {gamma} Prime volume fraction, topologically close-packed (TCP) phases, {gamma} and {gamma} Prime phase chemistries, and {gamma}-{gamma} Prime lattice mismatch. Regression models describing the influence of bulk alloy composition on each of the microstructural parameters were developed and compared to predictions by a commercially-available software tool that used computational thermodynamics. Co produced the largest change in {gamma} Prime solvus over the wide compositional range explored and Mo produced the biggest effect on the {gamma} lattice parameter over its range, although Re had a very potent influence on all microstructural parameters investigated. Changing the Cr, Co, Mo, and Re contents in the bulk alloy had an impact on their concentrations in the {gamma} matrix and to a smaller extent in the {gamma} Prime phase. The software tool under-predicted {gamma} Prime solvus temperatures and {gamma} Prime volume fractions, and over-predicted TCP phase volume fractions at 982 Degree-Sign C. However, the statistical regression models provided excellent estimations of the microstructural parameters and demonstrated the usefulness of such formulas. - Highlights: Black-Right-Pointing-Pointer Effects of Cr, Co, Mo, and Re on microstructure in new low density superalloys Black-Right-Pointing-Pointer Co produced a large change in {gamma} Prime solvus; Mo had a large effect on lattice mismatch. Black-Right-Pointing-Pointer Re exhibited very potent influence on all microstructural parameters was investigated. Black-Right-Pointing-Pointer {gamma} and {gamma} Prime phase chemistries both varied with temperature and alloy composition. Black-Right-Pointing-Pointer Computational thermodynamic modeling tool did not accurately predict microstructure.
Nickel-base superalloys are conventionally aged in order to develop a favorable microstructure as well as to relieve residual stresses. In this work, neutron diffraction was used to follow the evolution of residual stress during aging in Inconel 718 and Udimet 720LI for the first time by a combination of in situ and ex situ measurements. First, the quench-induced stress profiles were determined for different geometries of Inconel 718 forgings confirming that the amplitude of stress is not significantly affected by the thickness of the component. It was followed by in situ residual stress analysis using neutron diffraction during aging/annealing treatments at 650 °C, 720 °C, and 750 °C. Almost 90 pct of stress relaxation was found to occur primarily during heating to the aging temperature as a result of a combination of plasticity and early-stage creep relaxation. Creep-like stress relaxation was observed to evolve at a diminishing rate during the isothermal treatment of Inconel 718 at 720 °C and 750 °C, while no further stress relaxation occurred at 650 °C. This means that a change in hold temperature might have a more immediate impact on stress relaxation than a change in duration of heat treatment. Finally, the post-aging ex situ measurements showed that a heat treatment at 750 °C for 8 hours reduced the stresses by approximately 70 pct in Inconel 718. By comparison, when heat treating Udimet 720LI in the same way only a 20 pct stress reduction was observed, which is explained by the higher creep resistance of this alloy.
Four cast nickel-base superalloys were oxidized at 1000 and 1100 C for times up to 100 hr in static air and a Mach 1 gas stream. The oxidation resistance was judged by weight change, metal thickness loss, depletion-zone formation, and oxide formation and morphology. The alloys which formed mostly nickel aluminate (NiAl2O4) and aluminum oxide (Al2O3) (B-1900, VIA, and to a lesser extent 713C) were more oxidation resistant. Poorer oxidation resistance was associated with the appearance of chromium sesquioxide (Cr2O3) and chromite spinel (738X). Refractory metal content had little effect on oxidation resistance. Refractory metals appeared in the scale as tapiolite (NiM2O6, where M represents the refractory metal). Thermal cycling in static air appeared to supply sufficient data for the evaluation of oxidation resistance, especially for alloys which form oxides of low volatility. For alloys of higher chromium levels with high propensities toward forming a chromium-bearing scale of higher volatility, testing under conditions of high gas velocity is necessary to assess fully the behavior of the alloy.
Single crystal nickel-base superalloys used as turbine engine blades can have much higher creep and fatigue resistance than many polycrystalline superalloys due to the absence of weak grain boundaries as well as the ability to orient the blades in the primary loading direction on to the low elastic modulus [001] orientation, which significantly lowers the stresses imposed on the blades. However, grain defects can occur in these crystals, impairing these advantages. In the present study, fatigue tests of a single crystal superalloy uncovered unexpected defective grains in the test specimens, which initiated premature fatigue failures. The characteristics of these defective grains and the associated failures were evaluated and related to the observed fatigue lives.
Single crystal nickel-base superalloys used as turbine engine airfoils can have much higher creep and thermal fatigue resistance required for blades and vanes than polycrystalline superalloys due to the absence of weak grain boundaries, as well as the low elastic modulus in the [001] single crystal growth direction (Refs. 1 and 2). This has allowed an increase in the turbine rim inlet temperatures for improved performance and efficiency. However, these advantages in performance require constraints on acceptable levels of single crystal defects, such as macroscopic grain boundaries at low angles to the primary grain (i.e., low angle boundaries or LAB), higher angle boundaries, small new grains nucleated at the blade surface (a.k.a. freckles), larger new grains, and also the single crystal blades’ orientation, typically degrees off of the [001] direction and the orientation of a transverse [010] direction with respect to blade geometry. Therefore, single crystal turbine airfoils, which are frequently used with as-cast surfaces, are grain etched and inspected before and after heat treatments to ensure these aspects conform to acceptability limits.
A nickel-based superalloy that resists embrittlement by hydrogen more strongly than does nickel alloy 718 has been developed. Nickel alloy 718 is the most widely used superalloy. It has excellent strength and resistance to corrosion as well as acceptably high ductility, and is recognized as the best alloy for many high-temperature applications. However, nickel alloy 718 is susceptible to embrittlement by hydrogen and to delayed failure and reduced tensile properties in gaseous hydrogen. The greater resistance of the present nickel-based superalloy to adverse effects of hydrogen makes this alloy a superior alternative to nickel alloy 718 for applications that involve production, transfer, and storage of hydrogen, thereby potentially contributing to the commercial viability of hydrogen as a clean-burning fuel. The table shows the composition of the present improved nickel-based superalloy in comparison with that of nickel alloy 718. This composition was chosen to obtain high resistance to embrittlement by hydrogen while maintaining high strength and exceptional resistance to oxidation and corrosion. The most novel property of this alloy is that it resists embrittlement by hydrogen while retaining tensile strength greater than 175 kpsi (greater than 1.2 GPa). This alloy exhibits a tensile elongation of more than 20 percent in hydrogen at a pressure of 5 kpsi (approximately equal to 34 MPa) without loss of ductility. This amount of elongation corresponds to 50 percent more ductility than that exhibited by nickel alloy 718 under the same test conditions.
Nickel-based superalloys are in great demand for harsh-service conditions involving high temperatures and oxidative environments. Haynes 282 stands out due to its excellent high-temperature properties and easy fabricability. However, the upper operation temperature of Haynes 282 is limited due to its relatively low liquidus temperature. Equipped with high-fidelity density-functional theory calculations and high-throughput experimentation methodology, we explored new compositional spaces that exhibit higher liquidus temperature and higher strength. While maintaining the manufacturability, the newly designed alloy shows improved strength and ductility at room temperature and better oxidation resistance up to 800°C. Here, the new compositions showcase a minor change in the refractory and metalloid content can significantly impact the mechanical and oxidation performance of superalloys.
Order strengthening in nickel-based superalloys is associated with the extra stress required for dislocations to bypass the $\gamma'$ precipitates distributed in the $\gamma$ matrix. Depending on the operating conditions and microstructure, a rich variety of bypass mechanism has been identified, with various shearing and Orowan looping processes gradually 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 classical picture is that, at for a given volume fraction, the bypass mechanism changes from shearing to looping with increased particle size and within a broad coexistence size window. Another possibility, which is supported by indirect experimental evidence, is that a third “hybrid” transition mechanism may operate. Here, in this paper we use discrete dislocation dynamics (DDD) simulations to study dislocation bypass mechanisms in Ni-based superalloys. We develop a new method to compute generalized stacking fault forces in DDD simulations, based on a concept borrowed from complex analysis and known as the winding number of a closed curve about a point. We use this method to study the mechanisms of bypass of a square lattice of spherical $\gamma'$ precipitates by $a/2\langle110\rangle$ {111} edge dislocations, as a function of the precipitates volume fraction and size. We show that not only the hybrid mechanism is possible, but also that it operates as the transition mechanism between the shearing and looping regimes over a wide range of precipitates volume fraction and radii. Based on our simulation results, we propose a simple model for the strength of this mechanism. We also consider the effects of a $\gamma$/$\gamma'$ lattice misfit on the bypass mechanisms, which we approximate by an additional precipitate stress computed according to Eshelby’s inclusion theory. We show that in the shearing and hybrid looping-shearing regimes, a lattice misfit generally results in an increased bypass stress. For sufficiently high lattice misfit, the critical bypass configuration in attractive dislocation-precipitates interactions changes dramatically, and the bypass stress is controlled by the pinning of the trailing dislocation on the exit side of the precipitates, similar to what has been reported in the high-temperature creep literature.
Cast specimens of nickel-base superalloys 713C and Mar-M200 with nominal additions of 0, 0.5, and 1 wt% Si were evaluated for oxidation and corrosion resistance, tensile and stress-rupture properties, microstructure, and phase relations. Results are compared with those of an earlier study of the effects of Si in B-1900. Si had similar effects on all three superalloys. It improves oxidation resistance but the improvement in 713C and Mar-M200 was considerably less than in B-1900. Hot-corrosion resistance is also improved somewhat. Si is, however, detrimental to mechanical properties, in particular, rupture strength and tensile ductility. Si has two obvious microstructural effects. It increases the amount of gamma-prime precipitated in eutectic nodules and promotes a Mo(Ni,Si)2 Laves phase in the alloys containing Mo. These microstructural effects do not appear responsible for the degradation of mechanical properties, however.
New γ' strengthened nickel-base superalloy compositions are described that, when forged and properly heat treated, demonstrate superior resistance to fatigue crack growth accompanied by very good high temperature properties. Parts can be fabricated (e.g., using conventional cast and wrought processing) from these alloys without encountering difficulty in forging.
Dissimilar welding of γ′- and γ″-strengthened nickel-base superalloys has been investigated to identify the relationship between the microstructure of the welds and the resultant mechanical and weldability characteristics. γ′-Strengthened nickel-base Alloy 500 and γ″-strengthened nickel-base Alloy 718 were used for dissimilar welding. Gas tungsten arc welding operations were utilized for performing the autogenous dissimilar welding. Alloy 500 and Alloy 718 base metals showed various types of phases, carbides, intermetallics and eutectics in their microstructure. The results for Alloy 500 weld metal showed severe segregation of titanium to the interdendritic regions. The Alloy 718 weld metal compositional analysis confirmed the substantial role of Nb in the formation of low-melting eutectic-type morphologies which can reduce the weldability. The microstructure of dissimilar weld metal with dilution level of 65% wt.% displayed semi-developed dendritic structure. The less segregation and less formation of low-melting eutectic structures caused to less susceptibility of the dissimilar weld metal to the solidification cracking. This result was confirmed by analytic modeling achievements. Dissolution of γ″-Ni{sub 3}Nb precipitations took place in the Alloy 718 heat-affected zone leading to sharp decline of the microhardness in this region. Remelted and resolidified regions were observed in the partially-melted zone of Alloy 500 and Alloy 718. Nevertheless, no solidification and liquation cracking happened in the dissimilar welds. Finally, this was concluded that dissimilar welding of γ′- and γ″-strengthened nickel-base superalloys can successfully be performed. - Highlights: • Dissimilar welding of γ′- and γ″-strengthened nickel-base superalloys is studied. • Microstructural, mechanical and weldability aspects of the welds are assessed. • Microstructure of welds, bases and heat-affected zones is characterized in detail. • The type, morphology and distribution of the phases are thoroughly investigated. • Dissimilar welding is successfully performed without occurrence of any hot cracks.
Grain boundaries (GBs) in nickel-based superalloys have a significant influence on the creep resistance of these alloys. In this study, the trace elements of boron and zirconium were added to the newly designed LESS 1 (Ni-22Cr-20Co-2W-2Nb-1.5Al-1.5Ti-0.03C) alloy, and their effects on the GB precipitation behaviors of carbides were observed. The combined addition of boron and zirconium generates a continuous film structure containing MC carbides and M{sub 23}(C, B){sub 6}-type boro-carbides, as well as a grain boundary γ′. Based on the change in microstructure, rupture time is significantly increased. According to the observed microstructures and simulation results, it was confirmed that boron forms M{sub 23}(C, B){sub 6}-type boro-carbides and zirconium appears to stabilize MC carbides at GBs.
Specific weight changes of nickel-base superalloy B-1900 and B-1900 + 1% Si specimens were tested at 1273 K. B-1900 was losing weight at an increasing rate due to spalling of oxide scale while B-1900 + 1% Si was still gaining weight at low, nearly constant rate. Similar comparison in weight change was observed for specimens tested at 1373 K.