High-Temperature Gigacycle Fatigue Test of Inconel 718 and Element Distribution Measurement on a Fracture Surface

Significance of the topic

Inconel 718 is a nickel-based superalloy used in high-temperature, high-stress environments such as aero-engine components, gas turbines and power-plant parts. Evaluating its fatigue behavior under gigacycle loading at elevated temperature is critical for assessing long-term reliability and safe design. Ultrasonic fatigue testing accelerates gigacycle testing by operating at 20 kHz, reducing test durations from months to hours, while high-resolution elemental analysis of fracture origins helps link microstructural defects to macroscopic failure.

Objectives and overview of the study

This work aimed to perform high-temperature gigacycle fatigue tests on Inconel 718 at 600 °C using an ultrasonic fatigue system and to identify fracture origins by elemental mapping of fracture surfaces. Key goals were to (1) demonstrate high-frequency ultrasonic testing at elevated temperature, (2) measure temperature-dependent elastic properties to calculate accurate stresses, and (3) use EPMA to characterize inclusions that initiated internal or surface fractures.

Methodology

Tests combined an ultrasonic fatigue tester with an induction heating furnace and mean-stress loading device. Important methodological elements included:

• Use of 20 kHz ultrasonic excitation to reach gigacycle counts rapidly; resonance tuning of specimen shoulder length to 20.02 kHz (determined to be 10.9 mm).
• Intermittent excitation (burst/stoppage cycles) to maintain the specimen center temperature at approximately 600 °C and keep peak temperature below ~606 °C.
• Measurement of specimen temperature distribution by thermocouples and radiation thermometer (black paint applied to measurement area) to map temperature along the specimen.
• Accounting for spatially varying elastic modulus: elastic modulus at room temperature measured by tensile testing with strain gauges, modulus at 600 °C estimated from density and specimen geometry; these values were used when calculating local stress under ultrasonic loading.
• Post-test fracture-surface observation and element mapping by electron probe microanalysis to identify inclusion chemistry and fracture origins.

Used instrumentation

Equipment specified and used in the study included:

• Ultrasonic Fatigue Testing System: USF-2000A
• Precision Universal Testing Machine: AGX-V2
• Device for mean-stress loading (custom testing jig)
• Induction heater: EASY HEAT
• Radiation thermometer: FT-H10 (KEYENCE)
• Laser Doppler Vibrometer: VibroGo
• Electron Probe Microanalyzer: EPMA-8050G

Main results and discussion

Test conditions and outcomes:

• Loading stresses: 500, 550, 570, 610, 650, 700, 750 MPa; stress ratio R = 0; maximum cycles 1 × 10^9.
• Specimens fractured between approximately 10^6 and 10^9 cycles depending on applied stress, demonstrating that ultrasonic testing at 20 kHz can produce gigacycle failures in practical timeframes (e.g., ~14 hours for 10^9 cycles at 20 kHz).
• Fracture modes varied with load: internal-origin fractures (associated with subsurface inclusions) were observed at 570, 610 and 650 MPa; surface-initiated fractures occurred at 550, 700 and 750 MPa and were associated with surface defects.

Fracture-surface analysis by EPMA:

• Fracture at 570 MPa: inclusions ~20 μm in length identified as N- and Ti-containing compounds (mapping showed Ti + N signal overlay ~1.5% of mapped area at the inclusion location).
• Fracture at 650 MPa: inclusions ~20 μm identified as Ti-, Nb- and O-containing compounds (overlay Ti + Nb + O signal ~3.3% at the inclusion site).

Discussion points:

• Temperature gradients along the specimen produced spatial variation in elastic modulus; incorporating this into stress calculations is essential for accurate S–N characterization under high-temperature ultrasonic loading.
• The presence and chemistry of internal inclusions (Ti/N-rich or Ti/Nb/O-rich) correlated with internal crack initiation at intermediate stress levels, demonstrating that micro-scale nonmetallic or intermetallic particles remain critical fatigue nucleation sites even in high-performance superalloys.
• Intermittent operation effectively limited overheating of the ultrasonic horn and maintained test temperature control while enabling high-cycle accumulation.

Benefits and practical applications of the method

Practical advantages demonstrated by the study:

• Drastic reduction of test duration for gigacycle fatigue assessment (20 kHz operation versus conventional ~100 Hz machines).
• Capability to perform accelerated fatigue tests at temperatures representative of service (600 °C) using induction heating and temperature feedback control.
• Combination of high-cycle mechanical testing with EPMA elemental mapping enables direct linkage of fracture origins to inclusion chemistry, aiding failure analysis and materials qualification.

Applications include accelerated life assessment of turbine/blade materials, design validation for aerospace components, and root-cause analysis in quality assurance of wrought/superalloy components.

Future trends and potential uses

Possible directions to extend and apply this approach include:

• Broader materials screening using high-temperature ultrasonic gigacycle testing to build S–N databases for next-generation alloys.
• Integration of in-situ monitoring (acoustic emission, high-speed thermography, or modal tracking) to detect crack nucleation and growth during ultrasonic bursts.
• Improved inclusion control and advanced casting/forging routes to reduce internal defects identified as fatigue initiators.
• Coupling experimental data with multiscale modeling to predict life under nonuniform temperature and stress fields and to translate ultrasonic test results to lower-frequency service conditions.
• Standardization efforts to harmonize ultrasonic high-temperature fatigue procedures and stress-calculation protocols that account for temperature-dependent elastic properties.

Conclusion

High-temperature gigacycle fatigue testing of Inconel 718 at 600 °C using a USF-2000A ultrasonic system successfully produced fractures in the 10^6–10^9 cycle range under various applied stresses. Accounting for temperature-dependent elastic modulus and resonance tuning of specimen geometry were essential for accurate stress application. EPMA mapping identified small internal inclusions (Ti/N- and Ti/Nb/O-rich) as origins of internal fractures at intermediate stresses, confirming the significance of micro-scale inhomogeneities for high-cycle fatigue. The combined testing and microanalysis workflow provides a rapid, effective route for fatigue life evaluation and failure investigation of high-temperature alloys.

References

1. FURUYA, Yoshiyuki, et al. Development of High-Temperature Ultrasonic Fatigue Testing System. Transactions of the Japan Society of Mechanical Engineers (Series A), Vol. 78, No. 789 (2012-5).