The effect of thermal exposure on the microstructure and mechanical properties of nickel-base superalloys

The three nickel-base superalloys B-1900, TRW-NASA VIA, and René 80 were studied utilizing metallographic and residue analysis techniques in conjuction with mechanical property tests to determine the effect of thermal exposure on the microstructure and mechanical properties. Exposure times of 10, 100, 1000, and 5000 h at temperatures from 1400 to 2000°F (760 to 1093°C) were evaluated. Four minor phases-MC, M₆C, M₂₃C₆, and M₃B₂-plus gamma-prime were observed in the gamma matrix of these alloys. Significant variations in the mechanical properties were observed to occur with thermal exposure. Microstructural evaluation indicated that these variations in properties were due primarily to gamma-prime agglomeration or ripening. Perturbations noted in a number of the mechanical property vs exposure temperature curves in the 1500 to 1900°F (816 to 1038°C) temperature range appeared to be due to the precipitation and growth of M₆C and/or M₂₃C₆ carbides.     

The influence of cobalt,tantalum, and tungsten on the microstructure of single crystal nickel-base superalloys

The influence of composition on the microstructure of single crystal nickel-base superalloys was investigated. Co was replaced by Ni, and Ta was replaced by either Ni or W, according to a matrix of compositions based on MAR-M247. Substitution of Ni for Co caused an increase inγ′ solvus temperature, an increase inγ-γ′ lattice mismatch, and the precipitation of W-rich phases in the alloys with high refractory metal levels. Substitution of Ni for Ta caused large decreases inγ′ solvus temperature,γ′ volume fraction, andγ-γ′ lattice mismatch, whereas substitution of W for Ta resulted in smaller decreases in these features. For the alloys withγ′ particles that remained coherent, substitution of Ni for Co caused an increase inγ′ coarsening rate. The two alloys with the largest magnitude of lattice mismatch possessedγ′ particles which lost coherency during unstressed aging and exhibited anomalously low coarsening rates. Creep exposure at 1000 °C resulted in the formation ofγ′ lamellae oriented perpendicular to the applied stress axis in all alloys.     

Soldification segregation in ruthenium-containing nickel-base superalloys

Ruthenium-containing multicomponent Ni-base superalloys with large variations in refractory alloying elements (Re, Ru, Ta, and W) have been investigated with respect to solidification, segregation characteristics, and the tendency to develop grain defects during directional solidification. Phase transformation temperatures and the effects of alloy composition on the liquidus temperature were determined by differential thermal analysis (DTA). The liquidus temperatures for most Ru-containing superalloys are generally higher than those of current commercial single-crystal superalloys. The partitioning behavior of individual constituents under the influence of alloy chemistry was characterized using a quantitative segregation mapping technique combined with a Scheil-type analysis. Whereas ruthenium partitioned preferentially to the dendrite cores during soldification, segregation of Ru is much less pronounced than Re and W. A higher degree of rhenium segregation was observed in Ru-containing superalloys. For the fixed processing conditions and moderate levels of Ru+Re, single-crystal solidification occurred without freckle formation or convection-induced breakdown of the solidification front. However, with high levels of Ru (9.6 ∼ 14.1 wt pct) and Re (7.2 wt pct), grain defects or the complete breakdown of single-crystal solidification was observed. Results from segregation and DTA analyses were used to estimate the corresponding Rayleigh numbers present during solidification of the experimental alloys. The Rayleigh criterion is effective for predicting the conditions under which the grain defect formation occurs during directional solidification of Ru-containing superalloys.     

Study of microporosity formation in nickel-base superalloys

The susceptibility of nickel-base superalloys to microporosity formation is studied experimentally and theoretically. Analysis of a model introduced herein leads to formulation of a microporosity index, ΔP *, which is expressed in terms of solidification parameters and alloy system properties. This index can be determined from information obtained by quantitative differential thermal analysis. The effect of composition on the formation of microporosity is evaluated. Thus, aluminum, titanium, and cobalt are found to increase, and chromium to decrease microporosity. The effect of carbon is beneficial or detrimental depending on the aluminum content in the alloy.     

Effects of silicon on the oxidation,hot-corrosion,and mechanical behavior of two cast nickel-base superalloys

Cast specimens of nickel-base superalloys 713C and Mar-M200 with nominal additions of 0, 0.5, and 1 wt pct Si were evaluated for oxidation and corrosion resistance, tensile and stress-rupture properties, microstructure, and phase relations. Results are com-pared 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 in-creases the amount of γ precipitated in eutectie 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.     

Estimation of gamma phase composition in nickel-base superalloys

The gamma-gamma prime region of the Ni?Al?Cr?Ti system at 750°C has been analyzed through the use of phase rule principles and analytic geometry to allow estimation of gamma phase composition when the composition of a two-phase quaternary alloy is known. By application of currently used phase computation methods with the above calculation, the gamma phase composition of several nickel-based superalloys was estimated and found (except for tungsten) to be in good agreement with compositions for these alloys as reported in the literature.     

The effects of tantalum on the microstructure of two polycrystalline nickel-base superalloys: B-1900 + Hf and MAR-M247

Changes in the γ/γ'/carbide microstructure as a function of Ta content were studied in conventionally cast B-1900 + Hf and both conventionally cast and directionally solidified MAR-M247.* The effects of tantalum on the microstructure were found to be similar in both nickel-base superalloys. In particular, the γ' and carbide volume fractions increased approximately linearly with tantalum additions in both alloys. The γ' phase compositions did not change as tantalum additions were made with the exception of an increase in the tantalum level. Bulk tantalum additions increased the tantalum, chromium, and cobalt levels of the γ phase in both alloy series. The increase in the concentrations of the latter two elements was attributed to a decrease in the γ phase fraction with increasing bulk tantalum level and nearly constant γ' /γ partitioning ratios. It was demonstrated that the large increase in the γ ' volume fraction was a result of tantalum not affecting the partitioning ratios of the other alloying elements. The addition of tantalum led to a partial replacement of the hafnium in the MC carbides, although the degree of replacement was reduced by the solutionizing and aging heat treat-ment. In addition, chromium-rich M₂₃C₆ carbides formed as a result of MC carbide decomposition during heat treatment.     

Advances in nickel-based cast superalloys

Nickel-based superalloys have served as the most competitive high temperature structural materials under highly stressed and aggressive operating conditions in a variety of applications for more than 60 years. The most demanding among all the applications has been the gas turbine aerofoil castings of modern aero-engines. These turbine parts operate in extremely aggressive environment of high velocity hot combustion gas-air mixture carrying highly corrosive ingredients at high pressure. Gas turbine aerofoil materials should therefore possess adequate resistance to creep, fatigue and aggressive environment. Materials design for such application therefore has been extremely challenging, particularly since the engine designers always aim at higher turbine entry temperature (TET) in order to achieve greater engine thrust and better fuel efficiency. In spite of enormous efforts made in the recent past towards developing ceramics and their composites, Ni-based superalloys continue to be most reliable blade and vane materials offering always the highest TET. This has been possible through better alloy design, improved blade cooling schemes, protective coatings and directional solidification (DS) of either columnar grains or single crystals (SC) along the most favorable 〈001〉 texture. During the last six decades, TET has gone up by about 500K. This article covers recent advances in cast Ni-based superalloys, including our own efforts in this direction. Extensive research at DMRL has led to the development of new generation Ni-based superalloys, designated as DMD-4 and DMS-4 for DS and SC processing, respectively. Simultaneously, expertise has been developed to cast DS and SC components for aero-engines. Technology has also been established for pilot scale production of these components.     

Effect of nitrogen content on the microstructure and mechanical properties of a cast nickel-base superalloy

The effects of nitrogen content on the microstructure and the mechanical properties of a cast nickel-base superalloy (CNS) have been investigated experimentally. Experimental results demonstrated that the grain structure of CNS samples was refined by increasing the nitrogen content, but the area percentage of microporosity has been augmented with increased nitrogen content. Increasing the nitrogen content resulted in the morphology evolution of carbide from an acicular or ‘Chinese hieroglyphs’ type to blocky one, while negligible change of the morphology of γ′ precipitates was observed. Finally, it was found that the tensile strength has no obvious variation as the nitrogen content increases from 5 to 26?ppm, but it reduces sharply when the nitrogen content is raised to 34?ppm. The elongation decreases gradually with increasing nitrogen content.     

Recrystallization and texture development in dispersion-strengthened nickel-base superalloys

The recrystallization heat treating cycle is shown to be a critical factor in determining the final texture of an extruded and hot-rolled yttria-dispersed superalloy. It appears that recrystallization in such systems is not a dynamic process,i.e., no recrystallization occurs during hot-rolling operations. Further, pole figures obtained from fully recrystallized specimens demonstrate that the temperature from which the recrystallization treatment is started determines the final crystallographic orientation of the system. (110){001} and (110){110} texture will be developed only if the cycle is started within a specific temperature range; initial temperatures either too low or too high promote other orientations. CLAUDIA J. BURTON, formerly Graduate Student, Columbia University     

Creep-rupture in powder metallurgical nickel-base superalloys at intermediate temperatures

To gain insight into the factors which control the creep-rupture properties of powder metallurgical nickel-base superalloys at intermediate temperatures (650 to 775°C), a comparative study was conducted on the alloys AF115, modified MAR-M432 (B6) and modified IN100 (MERL76). Creep-rupture properties in these alloys were characterized in terms of the stress and temperature dependence of the secondary creep rate, ε^S, andrupture time,t _R . Within the limited stress ranges used, the stress dependence of both ε^S andt _R at 704°C can be represented by power laws ε^S andC ⁿ andt _R =Mσ ^-p ; whereC,M, n, andp are constants. The stress exponentsn andp are approximately equal for both AF115 and B6 with values of 16 and 7, respectively. In the case of MERL76,n andp are different, with values of 15 and 5, respectively. The apparent activation energies,Q, are 700, 370 and 520 KJ mol^-1 for AF115, B6 and MERL76, respectively. For these alloys, long creep-rupture lives are associated with large values ofn andQ. The sig-nificant differences inn andQ values between AF115 and B6 were related to creep re-covery processes for which the lattice misfit between the gamma and the gamma prime was identified to be an important parameter. However, the unequaln andp values in MERL76 compared with those in AF115 and B6, were traced to differences in fracture mode. Failures in AF115 and B6 were initiated at carbide particles at grain boundaries. In contrast, fracture in MERL76 was initiated at grain boundary triple junctions. The rupture lives of AF115 and B6 can be modeled reasonably well by the growth of cavities during secondary creep and propagation of a surface-nucleated crack during the tertiary creep.     

The influence of molybdenum on the γ/’phase in experimental nickel-base superalloys

The influence of 1, 3, and 5 at. pct Mo on the γ’precipitate has been studied in experimental wrought nickel-base superalloys containing about 14 at. pct Cr and 6-1/2, 9, or 12 at. pct Al, or 2 at. pct Al plus 4 at. pct Ti. Concentrations of all other elements were quite low to limit the observed effects to those of molybdenum alone. Molybdenum markedly increases the γ’ solvus temperature, as determined by the sensitive and relatively simple technique of differential thermal analysis; correspondingly, the weight fraction of γ’ increases with molybdenum additions for a given aging treatment. Molybdenum dissolves extensively in the γ’of the titanium-free alloys, but it dissolves to a considerably smaller extent in the γ’of the titanium-bearing alloys. Molybdenum substitutes for chromium in y’, but does not alter the aluminum or titanium contents of this phase. Lattice parameters of both the matrix and the γ’are increased markedly by molybdenum, in proportion to the molybdenum contents of these phases. The resulting effects on lattice-parameter mismatch correlate rather well with observed γ’morphology, which tends to change from spheroidal to cuboidal in titanium-free alloys, and from cuboidal to spheroidal in 2 at. pct Al-4 at. pct Ti alloys, as molybdenum is added to these alloys. J. W. Freeman was formerly Professor of Metallurgical Engineering, University of Michigan, Ann Arbor, Mich. (Deceased, November, 1970). This paper is based upon a thesis submitted by W. T. Loomis in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Horace H. Rackham School of Graduate Studies, The University of Michigan.     

The creep-damage constitutive and life predictive model for nickel-base single-crystal superalloys

A two-state-variable creep-damage constitutive and life predictive model that has been built is discussed in this article. The cavitation-controlled damage mechanism and microstructural deg-radation, i.e., material damage mechanism, are considered. The latter is derived mainly from the rafting and derafting of the precipitate γ′. The model has been verified by the creep ex-periments of nickel-base single-crystal DD3 at 760 °C and 850 °C. The steady creep and tertiary creep can be predicted satisfactorily. The active slip systems are confirmed as octahedral 〈112〉 {111} based on the lattice rotation. The parameter C reflecting material damage mechanism depends on the crystallographic orientation and can be assigned to the value C _〈001〉 ^α along 〈001〉 crystallographic orientation and C _〈011〉 ^α along 〈011〉 orientation partially. The life in different crystallographic orientations can be predicted satisfactorily.     

Elevated temperature mechanical properties and residual tensile properties of two cast superalloys and several nickel-base oxide dispersion strengthened alloys

The elevated temperature tensile, stress-rupture and creep properties and residual tensile properties after creep straining have been determined for two cast superalloys and several wrought Ni-16Cr-4Al-yttria oxide dispersion strengthened (ODS) alloys. The creep behavior of the ODS alloys is similar to that of previously studied ODS nickel alloys. In general, the longitudinal direction is stronger than the long transverse direction, and creep is at least partially due to a diffusional creep mechanism as dispersoid-free zones were observed after creep-rupture testing. The tensile properties of the nickel-base superalloy B-1900 and cobalt-base superalloy MAR-M509 are not degraded by prior elevated temperature creep straining (at least up to 1 pct) between 1144 and 1366 K. On the other hand, the room temperature tensile properties of ODS nickel-base alloys can be reduced by prior creep strains of 0.5 pct or less between 1144 and 1477 K, with the long transverse direction being more susceptible to degradation than the longitudinal direction.     

Effect of grain-boundary characteristics on castability of nickel-base superalloys

Hot tearing susceptibility of two Ni-base superalloys, IN792 and CM247, during directional solidification was studied. Effects of grain-boundary (GB) misorientation on castability were examined by casting single-crystal (SC) and bicrystal (BC) specimens. Crack-free specimens were obtained in castings with GB misorientation angles of less than ∼12 deg. Severe cracking occurred if the GB angle was greater than 25 deg. Secondary dendrite arm length right at the GB was found to be larger than in the GB vicinity and to increase with GB misorientation. The amount of eutectic melt and foreign element segregation is also larger at the GB. The greater susceptibility of the GB to hot tearing is almost certainly caused by reduced strength compared to the grain interior. The reduced strength is either due to reduced secondary arm bridging because of geometrical constraints or due to the existence of a thin liquid film as a result of stronger segregation.     

A critical strain model for the creep fracture of nickel-base superalloys

Fracture toughness samples of NIMONIC 115 were creep tested at 704°C in Mode I (tension) and Mode III (torsion) loading. In Mode III loading the rupture lives were two orders of magnitude shorter than in Mode I. The effects of loading mode are shown to agree with predictions based on a critical strain fracture model. Earlier test results with a number of different superalloys also are consistent with a strain controlled fracture model. Improved resistance to crack growth during creep at intermediate temperatures can be achieved by increasing Young’s modulus, yield strength, grain size and the critical strain value.     

Evolution of aluminide coating microstructure on nickel-base cast superalloy CM-247 in a single-step high-activity aluminizing process

This study deals with the aluminizing of a directionally cast Ni-base superalloy, namely CM-247, by a single-step process using a high-activity pack. It is observed that significant incorporation of Al into the substrate surface during aluminizing continues over a period of about 1 hour and is not restricted merely to the first few minutes, as reported in the literature. Based on the microstructural details of the coatings formed at various stages of aluminizing, it is concluded that the coating growth in the above process takes place primarily by inward Al diffusion initially, followed by an intermediate stage when the growth involves both inward Al and outward Ni diffusion. In the final stages, the outward diffusion of Ni dominates the coating formation process. The above mechanism of coating formation is different from the one that prevails in the conventional two-step high-activity coating process in that the reaction front for the formation of NiAl remains spatially stationary despite the outward diffusion of nickel during the intermediate stage. It is also shown in the present study that the content of the Al source in the pack affects the coating structure significantly. It is further demonstrated that the microstructure of the aluminide coatings depends not only on the amount of Al incorporated in the sample during aluminizing but also on the time over which the uptake of this Al takes place.