Dwaine L. Klarstrom

Enhanced fatigue resistance of a nickel-based hastelloy induced by a surface nanocrystallization and hardening process

Improvements in the fatigue resistance of a nickel-based alloy have been achieved via a surface nanocrystallization and hardening (SNH) process. The enhanced fatigue resistance is related to the surface nanocrystallization, work hardening, and compressive residual stresses induced by the SNH process.

Temperature evolution during low-cycle fatigue of ULTIMET® alloy: experiment and modeling

Abstract The temperature variations of a cobalt-based ULTIMET alloy subjected to low-cycle fatigue were characterized by a high-speed, high-resolution infrared thermography. The change of temperature during fatigue, which was due to the thermal-elastic–plastic effect, was utilized to reveal the accumulation of fatigue damage. A constitutive model was developed for predicting the thermal and mechanical responses of ULTIMET alloy subjected to cyclic deformation. The model was constructed in light of internal state variables, which were developed to characterize the inelastic strain of the material during cyclic loading. The predicted stress–strain and temperature responses were found to be in good agreement with the experimental results.

Cyclic deformation behavior of HAYNES® HR-120® superalloy under low-cycle fatigue loading

Abstract The cyclic deformation behavior of HAYNES ® HR-120 ® superalloy at different temperatures ranging from 24 to 982 °C was investigated by performing fully reversed total strain-controlled low-cycle fatigue tests under the total strain ranges of 0.4–2.3%. It was noted that in most cases, increasing the temperature from 24 to 982 °C significantly decreased the fatigue lives. The alloy exhibited the cyclic hardening, softening, or stable cyclic stress response, which was dependent on the temperature and total strain range. Dynamic-strain aging was found to occur at both temperatures of 761 and 871 °C. The precipitation of secondary-phase particles was also observed above 761°C. The change in the microstructure due to cyclic deformation was evaluated through scanning electron microscopy and transmission electron microscopy. In addition, an advanced infrared thermography system was employed to monitor the temperature evolution during fatigue at 24 °C. It was noted that during low-cycle fatigue, the steady-state temperature of the specimens increased from 2 to 120 °C above room temperature, depending on the strain range and fatigue life. Thus, the measured temperature can be used to predict fatigue life. A model based on energy conservation and one-dimensional heat conduction was used to predict the temperature evolution resulting from low-cycle fatigue.

High-frequency metal fatigue: the high-cycle fatigue behavior of ULTIMET ® alloy

Abstract ULTIMET® alloy is a relatively new commercial Co–26Cr–9Ni (wt.%) alloy, which exhibits good resistance to both wear and corrosion. A state-of-the-art high-frequency, 1000-Hz, material test system was used to study the high-cycle fatigue behavior of ULTIMET alloy up to 109 cycles. Fatigue experiments were conducted at high (1000 Hz) and conventional (20 Hz) frequencies in air at room temperature. The effects of the test frequency, the temperature increase during fatigue, and the change of crack initiation sites from the surface to subsurface on fatigue life are discussed. Although the fatigue life was comparable at test frequencies of 1000 and 20 Hz, the equilibrium temperature at 1000 Hz was considerably higher than that at 20 Hz. The fractographic study showed different morphologies of fracture surfaces at various frequencies. The high-cycle fatigue behavior of ULTIMET alloy at both high- and low-frequencies exhibited a typical two-stage fatigue-crack-growth process, i.e., (a) stage I fatigue-crack initiation in which the cracks formed on those planes most closely aligned with the maximum shear–stress direction in the grains of the fatigue specimen; and (b) stage II fatigue-crack growth in which the maximum principal tensile stress controlled crack propagation in the region of the crack tip.

Fatigue behavior in nickel-based superalloys: A literature review

In this literature review, the present understanding regarding the effects of microstructure, loading conditions, and environments on the fatigue behavior of nickel-based superalloys is reviewed.

Development of a New Nickel-Base Superalloy

A wrought nickel-base superalloy based on the Ni-Cr-W system was developed for high-temperature applications. The new alloy is solid solution and carbide strengthened. It is essentially free of cobalt, and it resists the formation of detrimental intermetallic compounds after prolonged exposure to elevated temperatures. Various mechanical, oxidation and physical properties of the new alloy were measured, and the microstructural features were characterized. These were compared with those of other solid solution-strengthened superalloys. Also, the fabricability of the alloy was evaluated. A number of advantages of the new alloy are defined.

Nondestructive evaluation of fatigue damage in ULTIMET® superalloy

Acoustic emission (AE) and positron spectroscopy were used to study fatigue damage in ULTIMET alloy. The linear-location method of AE was employed for identifying the positions of crack initiation for a cylindrical specimen subjected to fatigue. Positron lifetime spectroscopy, as a sensitive nondestructive technique, was utilized to reveal the fatigue damage. The results obtained by the AE system were generally in good agreement with those of the average positron lifetimes by positron spectroscopy. The detected crack initiation was further investigated by scanning electron microscopy, which was found to be consistent with the AE and positron spectroscopy results. The crack-initiation stage of ULTIMET alloy subjected to high-cycle fatigue was characterized.

Influence of subsolvus thermomechanical processing on the low-cycle fatigue properties of haynes 230 alloy

Strain-controlled low-cycle fatigue tests have been conducted in air at elevated temperature to determine the influence of subsolvus thermomechanical processing on the low-cycle fatigue (LCF) behavior of HAYNES 230 alloy. A series of tests at various strain ranges was conducted on material experimentally processed at 1121 °C, which is below the M23C6 carbide solvus temperature, and on material fully solution annealed at 1232 °C. A comparative strain-life analysis was performed on the LCF results, and the cyclic hardening/softening characteristics were examined. At 760 °C and 871 °C, the fatigue life of the experimental 230/1121 material was improved relative to the standard 230/1232 material up to a factor of 3. The fatigue life advantage of the experimental material was related primarily to a lower plastic (inelastic) strain amplitude response for a given imposed total strain range. It appears the increase in monotonic flow stress exhibited by the finer grain size experimental material has been translated into an increase in cyclic flow stress at the 760 °C and 871 °C test temperatures. Both materials exhibited entirely transgranular fatigue crack initiation and propagation modes at these temperatures. The LCF performance of the experimental material in tests performed at 982 °C was improved relative to the standard material up to a factor as high as 2. The life advantage of the 230/1121 material occurred despite having a larger plastic strain amplitude than the standard 230/1232 material for a given total strain range. Though not fully understood at present, it is suspected that this behavior is related to the deleterious influence of grain boundaries in the fatigue crack initiations of the standard processed material relative to the experimental material, and ultimately to differences in carbide morphology as a result of thermomechanical processing.

Low-cycle fatigue behavior of HAYNES® HR-120® alloy

Abstract Low-cycle fatigue behavior of the HAYNES HR-120 alloy in the temperature range from 24°C to 982°C and total strain range from 0.4% to 2.3% was investigated under an axial total strain control mode in laboratory air. It was noted that increasing temperature generally led to a substantial decrease in the fatigue life of the alloy. It was found that the alloy could exhibit cyclic strain hardening or softening, which was closely related to the imposed total strain range and testing temperature. The microstructural evolution due to low-cycle fatigue deformation was characterized using optical, scanning-electron, and transmission-electron microscopy. It was observed that the precipitation of second-phase particles would occur at or above 761°C. The presence of these precipitates could be taken into account the greater the cyclic-hardening behavior and the reduction of fatigue life, with increasing temperature. In addition, strain fatigue parameters were determined at different temperatures, based on Coffin–Manson and Holloman equations.

Phenomenological aspects of the high-cycle fatigue of ULTIMET® alloy

Abstract ULTIMET® alloy is a commercial Co–26Cr–9Ni wt.% superalloy, which possesses good resistance to both wear and corrosion. The microstructure of ULTIMET® alloy in the as-received condition exhibited a single face-centered-cubic phase with relatively fine, uniform grains, and annealing twins. Stress-controlled fatigue tests were performed at room temperature with different R ratios, in air and vacuum. The experimental method, uniform design, was employed to plan fatigue tests in order to study systematically the effects of the testing variables. A statistical model was formulated to estimate the effects of maximum stresses, R ratios, and environmental conditions on the S–N curves. The statistical analysis showed that these three factors had significant effects on fatigue life, but there was no interaction effect within the ranges of parameters investigated. Interestingly, there were plateau regions in the S–N curves of this alloy regardless of the environment. The plateaus were around a maximum stress level of 600 MPa, which was approximately equal to the material yield strength of 586 MPa. Fractographic studies showed that fatigue cracks were generally initiated either on the specimen surface or subsurface, and the crack-initiation sites were cleavage-like in nature, typical of stage I crack initiation. Fatigue-fracture surfaces had a crystallographic appearance. The stress-induced phase transformation of ULTIMET® alloy during fatigue was characterized by X-ray diffraction. The plateaus of S–N curves were associated with the stress-induced phase transformation and the change of the crack-initiation site from the surface to subsurface.