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.     

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.     

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     

Crack initiation and propagation during high-temperature fatigue of oxide dispersion-strengthened superalloys

The mechanisms of crack initiation and propagation have been investigated in two oxide dispersion-strengthened (ODS) Ni-base superalloys under conditions of symmetric low-cycle fatigue (LCF) and creep-fatigue. The behavior of both ODS alloys is compared with that of conventional alloys of otherwise similar composition. While the improvement in fatigue resistance previously reported for ODS metals and alloys is confirmed by the present study for temperatures below about0.6T _m, the potential advantage of dispersion strengthening is not being exploited by the current generation of ODS superalloys at higher temperatures; crack initiation is found to occur prematurely due to the presence of recrystallization defects in the form of fine grains. The mechanism of crack initiation at fine grains is creep-type cavitation on boundaries transverse to the applied stress. Experimental results indicating the influence of temperature, loading frequency, and waveshape on the crack initiation rate are presented and discussed in detail. A qualitative correlation between waveshape and creep-fatigue life is suggested based on the macroscopic inelastic strain rate which is determined by the waveform and limits in turn the rate at which cavity growth can be accommodated. Formerly with Max-Planck-Institut für Metallforschung     

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.     

Features and effect factors of creep of single-crystal nickel-base superalloys

The creep behavior of two single-crystal nickel-base superalloys with [001] orientation has been studied by measuring the creep curves, internal friction stress of dislocation motion, transmission electron microscopy (TEM) observation and energy-dispersive X-ray analysis (EDAX) composition analysis. The results show that over the stress and temperature range, there are different creep activation energies, time exponents, and effective stress exponents in two alloys at different creep stages. The size and volume fraction of the γ′ phase in the tantalum-free alloy is obviously decreased with the elevated temperature. This results in the decrease of the internal friction stress during steady-state creep. Higher levels of tungsten in the alloy result in a smaller strain value and lower directional-coarsening rate during primary creep. During steady-state creep, the primary reason for the better creep resistance of the other alloy is that it contains more Al and also Ta, which maintains a high volume fraction of γ′ phase. The dislocation climb over the γ′ rafts is the major deformation mechanisms during steady-state tensile creep. The fact that the strain rate is decreased with the increase of the size and volume fraction of the γ′ rafts may be described by a modified constitutive equation that is based on a model of the rate of dislocation motion.     

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.     

Internal hydrogen effects on tensile properties of iron- and nickel-base superalloys

Two nickel-base alloys [superalloys INCONEL 718 (IN718) and INCONEL 625 (IN625)] and one iron-base superalloy (A286) were chosen to study the effects of internal H charging on their room-temperature slow strain rate mechanical behavior. Uniform internal H contents ranged from 0 to 50 wt ppm H (0 to 3000 at. ppm H), and a strain rate of 8.5 X 10^-7 m/s was used with notched strip specimens. The three alloys showed varying losses in strength and ductility, and the strongest alloy, IN718, showed a decrease of 67 pet in ductility for a dissolved H content of 40 wt ppm. Superalloy A286 showed a corresponding 50 pet decrease in ductility, and IN625 showed a 29 pet loss in ductility. Fractographic evidence and the marked decrease in strength of the alloys lead the authors to conclude that the enhanced localized plasticity mechanism for H embrittlement is possibly operative in these face-centered cubic (fcc) alloys.     

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.     

Study of solidification features of nickel-base superalloys in relation with composition

The influence of the six major alloying elements: carbon, chromium, cobalt, molybdenum, titanium, and aluminum on the solidification sequence of nickel-base superalloys was investigated. The microstructure was found to depend greatly on aluminum and titanium contents. During solidification the liquid is enriched in titanium and molybdenum, whereas the dendrite cores are richer in cobalt. Aluminum and chromium segregate in the liquid or in the dendrite center, depending on alloy nominal composition. Chemical analysis of the carbides showed that their composition changes during solidification, thus affecting the composition of the residual liquid. The composition of carbides is strongly influenced by titanium and molybdenum nominal content in the alloy. Statistical analysis of the transformation temperatures obtained by DTA showed that titanium and aluminum influence the entire solidification sequence.     

Fatigue and creep-fatigue deformation of several nickel-base superalloys at 650 °c

Specimens of seven nickel-base superalloys for gas turbine disk application that had been failed in fatigue and creep-fatigue at 650 °C were examined by transmission electron microscopy to observe the effects of composition and microstructure on the deformation characteristics of the alloys. The alloys were Waspaloy, HIP Astroloy, H+F Astroloy, H+F René 95, IN 100, MERL 76, and NASA IIB-7. The amount of bulk deformation observed in all the alloys was low. At inelastic strain amplitudes less than about 10^-3 only favorably oriented grains exhibited yielding, and the majority of those had 〈110〉 near the tensile axis. Deformation occurred on octahedral systems for all the alloys except MERL 76, which also exhibited primary cube slip. The difference in slip behavior between MERL 76 and its parent composition, IN 100, was attributed to the addition of Nb. Deformation occurred in well-defined slip bands in the alloys that contained only fine aging γ′, 0.01 to 0.06 μm in size. Alloys which also contained a population of larger aging γ′ particles, 0.1 to 0.3 μm, exhibited more homogeneous deformation. Deformation in the creep-fatigue cycle, which employed a 15 minute dwell at the maximum tensile strain of the cycle, was not greatly different from fatigue deformation except that a few extended faults were formed.     

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.     

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.     

感应燃烧红外吸收法测定镍基高温合金中痕量硫

应用红外碳硫测定仪 ,对镍基高温合金中痕量硫的测定方法进行了研究。考察了影响低硫测定的主要因素 ,采用医用氧气、W -Sn助熔剂、坩埚预处理等途径降低并稳定了空白。测定范围 :硫的质量分数 0 .0 0 0 1%～ 0 .0 0 0 5 % ,RSD <3. 0 %。     

Superplasticity and internal friction of nickel-base superalloy

Temperature spectrums of internal friction, in other words, specific points have been investigated and discussed in the wide temperature range from room temperature to melting point for nickel-base superalloy in order to measure a grain boundary peak which is associated with superplasticity. Large boundary peaks of Pα and Pβ have been observed at the temperature range from 1173 to 1423 K. The activation energy of former Pα peak has been calculated to be 428–434 kJ·mol⁻¹ at the temperatures of 1213–1283 K, and this peak seems due to the grain boundary sliding which can be accomodated by the motion of lattice or local dislocations. The activation energy of later Pβ peak has been calculated to be 341–379 kJ·mol⁻¹ at the temperatures of 1333–1373 K, and this peak seems due to the grain boundary sliding which can be accommodated by the diffusive flux of nickel or the other elements. Furthermore, it has been examined whether non-destructive tests would be possible by using internal friction experiments instead of tension tests for superplasticity.