Turbine Blade Alloy Research Articles

Age-hardening behavior of Inconel X-750 superalloy

In order to develop highly efficient aircraft engines, the use of alloys for turbine blades that combine high creep resistance and lower density brings substantial benefits. That may be achieved by the introduction of a reinforcing phase, such as a discontinuous reinforcement. So far, most researched particle-reinforced Ni-based superalloys made use of sub-micrometer particles, such as oxides and borides, often produced by in-situ reactions. These are proposed solely as dislocation immobilizers, in a role comparable to that carried out by γ’ precipitates, while metal matrix composites (MMCs) containing micro-scale particles offer a load-transfer potential. To combine both approaches, a recently designed polycrystalline Ni-based γ’-strengthened superalloy reinforced with TiC particles was produced by low energy mixing of powders, followed by pressure sintering. The composite, along with a non-reinforced reference, was submitted to compression creep tests at 973 K (700 °C), with stresses varying from 280 to 500 MPa. Selected specimens had their microstructure evaluated with techniques such as EBSD and XRD. When simulating a working turbine, the material density influences directly the centripetal forces acting on a section of a turbine blade, and consequently the creep rates. In the proposed composite, not only an increase in the threshold stress by a load-transfer component was obtained, but also the density reduction affected remarkably the creep rates. Moreover, the presented MMC not only presented a superior creep resistance in relation to the non-reinforced alloy, but also exhibited higher stability of primary and secondary γ’ volumes after 500 h in creep tests.

Improved Performance Rhenium Containing Single Crystal Alloy Turbine Blades Utilizing PPM Levels of the Highly Reactive Elements Lanthanum and Yttrium

Turbine inlet temperatures have now approached 1650°C (3000°F) at maximum power for the latest large commercial turbofan engines, resulting in high fuel efficiency and thrust levels approaching or exceeding 445 kN (100,000 lbs.). High reliability and durability must be intrinsically designed into these turbine engines to meet operating economic targets and ETOPS certification requirements. This level of performance has been brought about by a combination of advances in air cooling for turbine blades and vanes, computerized design technology for stresses and airflow, and the development and application of rhenium (Re) containing, high γ’ volume fraction nickel-base single crystal superalloys, with advanced coatings, including prime-reliant ceramic thermal barrier coatings (TBCs). Re additions to cast airfoil superalloys not only improve creep and thermomechanical fatigue strength but also environmental properties, including coating performance. Re slows down diffusion in these alloys at high operating temperatures [1]. At high gas temperatures, several issues are critical to turbine engine performance retention, blade life, and integrity. These are tip oxidation in particular for shroudless blades, internal oxidation for lightly cooled turbine blades, and TBC adherence to both the airfoil and tip seal liner. It is now known that sulfur (S) at levels, <10 ppm but >0.2 ppm in these alloys reduces the adherence of α alumina protective scales on these materials or their coatings by weakening the Van der Waal’s bond between the scale and the alloy substrate. A team approach has been used to develop an improvement to CMSX-41 alloy which contains 3 percent Re, by reducing S and phosphorus (P) levels in the alloy to <2 ppm, combined with residual additions of lanthanum (La) + yttrium (Y) in the range 10-30 ppm. Results from cyclic, burner rig dynamic oxidation testing at 1093°C (2000°F) show thirteen times the number of cycles to initial alumina scale spallation for CMSX-4 [La + Y] compared to standard CMSX-4. A key factor for application acceptance is of course manufacturing cost. The development of improved low reactivity prime coats for the blade shell molds along with a viable, tight dimensional control yttrium oxide core body are discussed. The target is to attain grain yields of single crystal CMSX-4 (ULS) (La + Y) turbine blades and casting cleanliness approaching standard CMSX-4. The low residual levels of La + Y along with a sophisticated homogenisation/solutioning heat treatment procedure result in full solutioning with essentially no residual γ/γ’ eutectic phase, Ni (La, Y) low melting point eutectics, and associated incipient melting pores. Thus, full CMSX-4 mechanical properties are attained. The La assists with ppm chemistry control of the Y throughout the single crystal turbine blade castings through the formation of a continuous lanthanum oxide film between the molten and solidifying alloy and the ceramic core and prime coat of the shell mold. Y and La tie up the <2 ppm but >0.2 ppm residual S in the alloy as very stable Y and La sulfides and oxysulfides, thus preventing diffusion of the S atoms to the alumina scale layer under high temperature, cyclic oxidising conditions. La also forms a stable phosphide. CMSX-4 (ULS) (La + Y) HP shroudless turbine blades will commence engine testing in May 1998.

GTD111 Alloy Material Study

Very few property data on this common turbine blade alloy have been published. As longer hours of service are accumulated, maintenance considerations such as developing optimum component life strategies and repair processes become important. The lack of specific material data hampers the effort of users and repair facilities to achieve optimum service from this alloy. This study measured some of the basic mechanical and metallurgical characteristics of this polycrystalline nickel base superalloy. Tensile and short-term creep rupture properties as well as microstructural and fracture characteristics are presented. Both the as-heat-treated and thermally exposed characteristics at two different temperatures are examined.

MC Carbides in the Hf Containing Ni Based Superalloy MarM002

Unexposed MarM002 alloy turbine blades and blades exposed at 700-1000oC for up to 250 days were studied by TEM, SEM, EDX and XRD. The shapes, distribution and compositions of the MC carbides do not vary significantly during exposure at 700 and 800oC. All MC carbides contain significant amounts of Hf, Ta, Ti and W, and they are present with a very broad range of compositions. The latter is caused by a miscibility gap in the MC carbide phase which causes the formation of MC carbides to shift from Hf lean in the initial stages of solidification to Hf rich in the later stages. In the unexposed samples the Hf lean (Ti,Ta)C carbide is the most numerous one. The shape of the complex MC carbide diffraction peaks are explained semi-quantitatively on the basis of the compositional range of the MC carbides.

The fatigue and fracture of turbine blade alloys in high-pressure hydrogen : DeLuca, D.P. and Cowles, B.A. JOM (1996) 48 (11), 61–64

The roles of microstructure and environment in influencing mechanisms of damage during high-temperature, low cycle fatigue were investigated in a lithium-containing aluminium alloy in the peak-aged condition, and tested over a range of plastic strains. Reduction in fatigue life and resistance were observed at an elevated test temperature. Localized oxidation and embrittlement at grain boundaries are promoted by the applied stress, and play an important role in accelerating crack initiation and subsequent crack propagation. Evidence is presented to rule out contributions from microstructural changes through the coarsening of precipitate particles and the formation of new precipitate phases, and damage due to creep. The apparent acceleration of crack initiation and enhanced crack propagation are ascribed to a complex interaction of four mechanisms: intrinsic microstructural features, the embrittling effect of the environment, the slip mode arising from a combination of mechanical and microstructural contributions, and applied stress. The abnormal plastic strain-fatigue life by behaviour is attributed to differences in the distribution of deformation as a function of plastic strain amplitude.

Development of alloyed and dispersion-strengthened MCrAlY coatings

An on-going research program has investigated the effects of the separate and combined additions of Ta, Pt and an oxide dispersion to basic MCrAlY overlay coatings. The goals were improved oxidation and hot corrosion resistance, greater diffusional stability, and coating thermal expansion reduction in order to be closer to nickel base turbine blade alloys. It was found in cyclic burner rig testing at 1050°C that each individual addition improved resistance of the baseline compositions. Oxide and Pt additions reduced thermal expansion. Creep resistance was greatly increased by oxide additions, which also strongly increased yield strength. Yield strength was modestly increased by Pt at low temperature and significantly increased by Ta at low to moderate temperatures.

The fatigue and fracture of turbine blade alloys in high-pressure hydrogen

In June 1995, the NASA space shuttle Discovery lifted off for the first time with an improved high-pressure oxidizer turbopump fitted to one of its three main engines. In 1997, an advanced high-pressure fuel turbopump will enter service as well. Both systems will make extensive use of nickel-based superalloy microstructures specially tailored for improved resistance to hydrogen embrittlement. This article reviews a study on cast single-crystal and equiaxed nickel superalloys and the subsequent approach used to improve their hydrogen resistance.

Development of design principles for a creep-limited alloy for turbine blades

Recent advancements in turbine-blade materials engineering are reviewed in light of general superalloy research and the author’s work on a new powder metallurgy IN-792 creep-limited alloy for application in blades of gas-turbine engines. The developed set of principles presented in this paper incorporates all the factors that must be taken into consideration in selecting and designing an alloy for turbine blades.

The root area parameter model for evaluation of effects of various artifical small defects and mutual interaction of small defects on fatigue limit: Toriyama, T. and Murakami, Y. J. Soc. Mater. Sci. Jpn (Oct 1993) 42(481), 1160–1166 (in Japanese)

There have been few studies to date describing fatigue-crack propagation thresholds under mixed-mode loading conditions in the presence of cracks, that are small as compared to the characteristic microstructural dimensions. To address this need, the variation in mixed-mode, high-cycle fatigue-crack growth thresholds with crack size is reported for a Ti-6Al-4V turbine blade alloy with a fine-grained bimodal microstructure. Specifically, threshold behavior is examined for large through-thickness cracks >4 mm in length), short through-thickness cracks (~200 μm in length), and microstructurally-small surface cracks (10–50 μm in diameter) under combined mode I and mode II loading at load ratios (ratio of minimum to maximum load) ranging from 0.1 to 0.8. For mode-mixities ranging from pure mode I to predominantly mode II, large crack, mode I ΔKI,TH thresholds were found to decrease substantially with increasing phase angle. However, by characterizing in terms of the range in strain energy release rate, ΔGTH, incorporating both mode I and mode II contributions, it was observed that the pure mode I threshold could be regarded as a ‘worst case’ under mixed-mode loading in this alloy. By estimating the effective crack-driving force actually experienced at the crack tip, the observed increase in the mixed-mode ΔGTH threshold with mode-mixity was attributed to an increasing influence of crack-tip shielding due to crack closure and crack-surface interference. Equivalent thresholds for through-thickness short cracks, where the crack wake and hence the effect of such shielding is minimized, were consequently far less sensitive to mode-mixity and corresponded in magnitude to the shielding-corrected large-crack thresholds. This effect was accentuated for the measured thresholds of microstructurally-small surface cracks; such small-crack, mixed-mode ΔGTH thresholds not only displayed a minimal influence of mode-mixity but were up to two orders of magnitude smaller than those for corresponding large cracks at the same load ratio and mode-mixity conditions.

A contribution to the problem of the endurance limit of nitrided service parts: Kern, T.-U. and Spies, H.-J. Proc. Conf. Surface Engineering, Bremen, Germany, 1993, pp. 187–189

Mixed-mode, high-cycle fatigue-crack growth thresholds are reported for through-thickness cracks (large compared to microstructural dimensions) in a Ti–6Al–4V turbine blade alloy with a bimodal microstructure. Specifically, the effect of combined mode I and mode II loading, over a range of phase angles β=tan−1(ΔKII/ΔKI) from 0° to 82° (ΔKII/ΔKI∼0–7), is examined for load ratios (ratio of minimum to maximum loads) ranging from R=0.1 to 0.8 at a cyclic loading frequency of 1000 Hz in ambient temperature air. Although the general trend for the mode I stress-intensity range at the threshold, ΔKI,TH, is to decrease with increasing mode mixity, ΔKII/ΔKI, and load ratio, R, if the crack-driving force is alternatively characterized in terms of the strain-energy release rate, ΔG, incorporating contributions from both the applied tensile and shear loading, the threshold fatigue-crack growth resistance increases significantly with the applied ratio of ΔKII/ΔKI. The pure mode I threshold, in terms of ΔGTH, is observed to be a lower bound (worst case) with respect to mixed-mode (I+II) behavior. These results are compared with mixed-mode fatigue thresholds for short cracks, where the precrack wake has been machined to within ∼200 μm of the precrack tip. For such short cracks, wherein the magnitude of crack-tip shielding which can develop is greatly reduced, the measured mixed-mode fatigue-crack growth thresholds are observed to be markedly lower. Moreover, the dependence of the mixed-mode fatigue-crack growth resistance on the applied phase angle is significantly reduced. Comparison of the large- and short-crack data suggests that the increase in the large-crack fatigue threshold, ΔGTH, with an increasing mode mixity (ΔKII/ΔKI) is largely due to shielding from shear-induced crack-surface contact, which reduces the local crack-driving force actually experienced at the crack tip. Quantification of such shielding is described in Part II of this paper.

Fatigue Crack Initiation In 403 Stainless Steel In Simulated Steam Cycle Environments: Hydroxide And Silicate Solutions

AbstractThe fatigue crack initiation behaviour of a 12%Cr turbine blade alloy (403 stainless steel) was studied at a single alternating stress level of 448 MN m−2 in deaerated solutions of NaOH at pH 10,12, and 14 and in 1M Na4SiO4 solution (pH 10) at 100°C. Fatigue life in the hydroxide solution increased with increasing pH and was also good in the silicate solution. Long cracks of similar appearance were found to be initiated on the surface of the steel in all the solutions except NaO H at pH 14, in which short incipient microcracks of a transgranular nature initiated on the specimen surface, regardless of the selective attack that occurred at grain and phase boundaries. It was found that increasing the pH of the hydroxide solution and the presence of the silicate anion both cause the initiated cracks to propagate into the bulk of the material in a transgranular mode, thereby resulting in a longer fatigue life.

Test methods for accelerated development, design and life assessment of high-temperature materials

A new conceptual framework involving an integrated approach to materials development, component design and life prediction for high-temperature applications is outlined. Termed ‘Design for Performance’, this provides an alternative to the traditional approach which requires the creep to rupture testing of many specimens over long periods. Accelerated development of new alloys for higher-temperature turbine blades and discs, and assessment of remaining life of operating components, is now possible. The tests described are a refined stress relaxation test — from which extensive stress versus strain-rate data may be generated over a range of temperatures — and a constant displacement rate crack growth test. These tests provide a separation of the creep strength and fracture resistance criteria required in design. Extensive studies on the cast alloy IN738 have shown that the creep strength is quite insensitive to prior exposure and microstructure, but fracture resistance is very sensitive to thermal history. Relative to the standard heat treatment, fracture resistance can be seriously impaired or substantially improved. It is also shown that calibration with traditional design methods for creep strength is possible. Once minimum performance standards have been established for the original design, these may be used as a basis for life management. For example, decisions regarding component replacement, repair or refurbishment may be made as one or more of these minimum performance standards are approached during service. Thus, the methodology provides a consistent basis for life management for both new and serviced components. It is argued that not only is this approach much faster, more efficient and less costly, but that the framework on which it is based is technically superior to the traditional approach.

Contemporary heat-resistant and cast alloys for gas turbine blades

Contemporary heat-resistant and cast alloys for gas turbine blades

Case histories on surgical implants and their causes: Zitter, H. Werkst. Korros. Sept. 1991 42, (9), 455–466 (in German)

Mixed-mode, high-cycle fatigue-crack growth thresholds are reported for through-thickness cracks (large compared to microstructural dimensions) in a Ti–6Al–4V turbine blade alloy with a bimodal microstructure. Specifically, the effect of combined mode I and mode II loading, over a range of phase angles β=tan−1(ΔKII/ΔKI) from 0° to 82° (ΔKII/ΔKI∼0–7), is examined for load ratios (ratio of minimum to maximum loads) ranging from R=0.1 to 0.8 at a cyclic loading frequency of 1000 Hz in ambient temperature air. Although the general trend for the mode I stress-intensity range at the threshold, ΔKI,TH, is to decrease with increasing mode mixity, ΔKII/ΔKI, and load ratio, R, if the crack-driving force is alternatively characterized in terms of the strain-energy release rate, ΔG, incorporating contributions from both the applied tensile and shear loading, the threshold fatigue-crack growth resistance increases significantly with the applied ratio of ΔKII/ΔKI. The pure mode I threshold, in terms of ΔGTH, is observed to be a lower bound (worst case) with respect to mixed-mode (I+II) behavior. These results are compared with mixed-mode fatigue thresholds for short cracks, where the precrack wake has been machined to within ∼200 μm of the precrack tip. For such short cracks, wherein the magnitude of crack-tip shielding which can develop is greatly reduced, the measured mixed-mode fatigue-crack growth thresholds are observed to be markedly lower. Moreover, the dependence of the mixed-mode fatigue-crack growth resistance on the applied phase angle is significantly reduced. Comparison of the large- and short-crack data suggests that the increase in the large-crack fatigue threshold, ΔGTH, with an increasing mode mixity (ΔKII/ΔKI) is largely due to shielding from shear-induced crack-surface contact, which reduces the local crack-driving force actually experienced at the crack tip. Quantification of such shielding is described in Part II of this paper.

Statistical design of fatigue crack growth test programmes: Salivar, G.C. and Hoeppner, D.W. ASTM J. Test. Eval. Nov. 1988 16, (6), 508–515

Mixed-mode, high-cycle fatigue-crack growth thresholds are reported for through-thickness cracks (large compared to microstructural dimensions) in a Ti–6Al–4V turbine blade alloy with a bimodal microstructure. Specifically, the effect of combined mode I and mode II loading, over a range of phase angles β=tan−1(ΔKII/ΔKI) from 0° to 82° (ΔKII/ΔKI∼0–7), is examined for load ratios (ratio of minimum to maximum loads) ranging from R=0.1 to 0.8 at a cyclic loading frequency of 1000 Hz in ambient temperature air. Although the general trend for the mode I stress-intensity range at the threshold, ΔKI,TH, is to decrease with increasing mode mixity, ΔKII/ΔKI, and load ratio, R, if the crack-driving force is alternatively characterized in terms of the strain-energy release rate, ΔG, incorporating contributions from both the applied tensile and shear loading, the threshold fatigue-crack growth resistance increases significantly with the applied ratio of ΔKII/ΔKI. The pure mode I threshold, in terms of ΔGTH, is observed to be a lower bound (worst case) with respect to mixed-mode (I+II) behavior. These results are compared with mixed-mode fatigue thresholds for short cracks, where the precrack wake has been machined to within ∼200 μm of the precrack tip. For such short cracks, wherein the magnitude of crack-tip shielding which can develop is greatly reduced, the measured mixed-mode fatigue-crack growth thresholds are observed to be markedly lower. Moreover, the dependence of the mixed-mode fatigue-crack growth resistance on the applied phase angle is significantly reduced. Comparison of the large- and short-crack data suggests that the increase in the large-crack fatigue threshold, ΔGTH, with an increasing mode mixity (ΔKII/ΔKI) is largely due to shielding from shear-induced crack-surface contact, which reduces the local crack-driving force actually experienced at the crack tip. Quantification of such shielding is described in Part II of this paper.

Cyclic crack growth and closure effects in low alloy ferritic steels during creep-fatigue at 550 °C : Skelton, R.P. High Temp. Technol. Aug. 1989 7, (3), 115–128

There have been few studies to date describing fatigue-crack propagation thresholds under mixed-mode loading conditions in the presence of cracks, that are small as compared to the characteristic microstructural dimensions. To address this need, the variation in mixed-mode, high-cycle fatigue-crack growth thresholds with crack size is reported for a Ti-6Al-4V turbine blade alloy with a fine-grained bimodal microstructure. Specifically, threshold behavior is examined for large through-thickness cracks >4 mm in length), short through-thickness cracks (~200 μm in length), and microstructurally-small surface cracks (10–50 μm in diameter) under combined mode I and mode II loading at load ratios (ratio of minimum to maximum load) ranging from 0.1 to 0.8. For mode-mixities ranging from pure mode I to predominantly mode II, large crack, mode I ΔKI,TH thresholds were found to decrease substantially with increasing phase angle. However, by characterizing in terms of the range in strain energy release rate, ΔGTH, incorporating both mode I and mode II contributions, it was observed that the pure mode I threshold could be regarded as a ‘worst case’ under mixed-mode loading in this alloy. By estimating the effective crack-driving force actually experienced at the crack tip, the observed increase in the mixed-mode ΔGTH threshold with mode-mixity was attributed to an increasing influence of crack-tip shielding due to crack closure and crack-surface interference. Equivalent thresholds for through-thickness short cracks, where the crack wake and hence the effect of such shielding is minimized, were consequently far less sensitive to mode-mixity and corresponded in magnitude to the shielding-corrected large-crack thresholds. This effect was accentuated for the measured thresholds of microstructurally-small surface cracks; such small-crack, mixed-mode ΔGTH thresholds not only displayed a minimal influence of mode-mixity but were up to two orders of magnitude smaller than those for corresponding large cracks at the same load ratio and mode-mixity conditions.

Thermal fatigue crack propagation and failure life evaluation on CrMoV and SUS304 steels : Nitta, A., Ogata, T. and Kuwabara, K.J. Soc. Mater. Sci., Jpn. Apr. 1989 38, (427), 423–429 (in Japanese)

The role of crack-tip shielding in influencing mixed-mode (mode I+II) fatigue-crack growth thresholds for large, through-thickness cracks in a Ti–6Al–4V turbine blade alloy is examined under high-cycle fatigue loading conditions, i.e., at a loading frequency of 1000 Hz in ambient temperature air for load ratios (Kmin/Kmax) of R=0.1–0.8. Techniques are developed to quantify crack-tip shielding with respect to both the mode I and mode II applied loading, enabling an estimation of the shielding-corrected, crack-driving forces actually experienced at the crack tip (ΔKI,TH,eff and ΔKII,TH,eff or ΔGTH,eff). In Part I, it was shown that when the crack-driving force is characterized in terms of the range in strain-energy release rate, ΔG, which incorporates contributions from both the applied tensile and shear loading, the mixed-mode (I+II) fatigue-crack growth resistance increases monotonically with the ratio ΔKII/ΔKI. When the fatigue-crack growth thresholds are expressed in terms of the near-tip (shielding-corrected) crack-driving force, this increase in crack-growth resistance with increasing mode mixity is markedly reduced. Moreover, for all mode mixities investigated, the near-tip mixed-mode fatigue threshold is lower than the applied (global) value, with the effect being particularly pronounced under shear-dominant loading conditions. These observations illustrate the prominent role of crack-tip shielding for the mixed-mode loading of fatigue cracks with crack-wake dimensions large compared with microstructural size scales; specifically, they indicate that the elevation of the ΔGTH fatigue-crack growth threshold with increasing applied mode mixity is largely due to a shear-induced enhancement of crack-tip shielding.

Study on mechanism of fatigue crack propagation by variable loading [formula omitted] decreasing test method : Masuda, H. and Matsuoka, S.J. Soc. Mater. Sci., Jpn. Apr. 1989 38, (427), 410–415 (in Japanese)

The role of crack-tip shielding in influencing mixed-mode (mode I+II) fatigue-crack growth thresholds for large, through-thickness cracks in a Ti–6Al–4V turbine blade alloy is examined under high-cycle fatigue loading conditions, i.e., at a loading frequency of 1000 Hz in ambient temperature air for load ratios (Kmin/Kmax) of R=0.1–0.8. Techniques are developed to quantify crack-tip shielding with respect to both the mode I and mode II applied loading, enabling an estimation of the shielding-corrected, crack-driving forces actually experienced at the crack tip (ΔKI,TH,eff and ΔKII,TH,eff or ΔGTH,eff). In Part I, it was shown that when the crack-driving force is characterized in terms of the range in strain-energy release rate, ΔG, which incorporates contributions from both the applied tensile and shear loading, the mixed-mode (I+II) fatigue-crack growth resistance increases monotonically with the ratio ΔKII/ΔKI. When the fatigue-crack growth thresholds are expressed in terms of the near-tip (shielding-corrected) crack-driving force, this increase in crack-growth resistance with increasing mode mixity is markedly reduced. Moreover, for all mode mixities investigated, the near-tip mixed-mode fatigue threshold is lower than the applied (global) value, with the effect being particularly pronounced under shear-dominant loading conditions. These observations illustrate the prominent role of crack-tip shielding for the mixed-mode loading of fatigue cracks with crack-wake dimensions large compared with microstructural size scales; specifically, they indicate that the elevation of the ΔGTH fatigue-crack growth threshold with increasing applied mode mixity is largely due to a shear-induced enhancement of crack-tip shielding.

Stress-strain histories in coatings on single crystal specimens of a turbine blade alloy

The stress-strain response of an electron beam, physical vapor deposited (EB-PVD) NiCoCrAlY overlay coating applied to a single crystal nickel base superalloy (CMSX-3) is studied by subjecting stepped-disk fatigue specimens to thermal cycles with different rates of heating (from 520 to 1090†C in 4.5, 6, and 20 s) and cooling (to 520°C in 7 and 15 s). Rate-dependent material anisotropy is modelled using a creep (or flow) potential for a cubic crystal and the associated flow rule. An anisotropic creep finite element analysis of the substrate reveals that continued cycling does not affect the strain ranges, which play a role in coating degradation. However, continued cycling does produce an increase of the peak tensile stresses (from 13 MPa for the first cycle to about 250 MPa at the 6000th cycle for 4.5 s heating history). Most of this transition occurs before the 500th cycle. As a consequence of these tensile stresses, crack propagation rates into the substrate are expected to be more severe than the predictions from a thermo-elastic analysis. However, the conventional thermo-elastic analysis is very accurate at estimating the strain ranges. For the coating, an isotropic creep finite element analysis shows that the mechanical strain range (Δεe + Δεc) and tensile peak stresses responsible for crack initiation and crack growth, with 0.52% strain range in the substrate, are 0.83% and 460 MPa for the 4.5 s heating cycle, due in large part to the thermal expansion mismatch between the coating and substrate.

Ｎｉ基耐熱合金の高温腐食疲労強度の推定

Previously, the authors have proposed several methods to estimate corrosion fatigue strength at room temperature.In the present study, a method to deduce the corrosion fatigue strength from the fatigue strength of corroded specimens, was proposed and applied to estimate the corrosion fatigue strength of nickel-base heat resisting alloy for gas turbine blades in a high temperature sulfidation environment.The constants in the equation for estimation were obtained from the high temperature fatigue strength of the specimens corroded by the burner-rig test, and the estimated strength was in good agreement with the experimental data. It is made clear that this method can be applied to the estimation of corrosion fatigue strength at high temperatures.