High-temperature Low-cycle Fatigue Behavior Research Articles

Study on High-Temperature Low-Cycle Fatigue Behavior of the FGH96 Superalloy Based on Internal Stress Division

In order to deeply explore the high-temperature cyclic characteristics of the FGH96 superalloy under different strain amplitudes, the high-temperature low-cycle fatigue behavior of the FGH96 superalloy was analyzed from the perspective of internal stress evolution. Four sets of strain amplitude (0.5%, 0.6%, 0.8%, and 1.2%) controlled high-temperature low-cycle fatigue tests were carried out on the FGH96 superalloy at 550 °C, and the internal stress was divided into back stress and effective stress through the cyclic stress-strain curves. The results show that the cyclic softening/hardening characteristics of the FGH96 superalloy under different strain amplitudes are closely related to the evolution of internal stress. The strain amplitude has a significant effect on the back stress of the FGH96 superalloy but has little effect on effective stress. At low strain amplitudes (0.5% and 0.6%), the back stress evolution rate of the FGH96 superalloy is lower than effective stress, and the material mainly exhibits cyclic softening. At high strain amplitudes (0.8% and 1.2%), the back stress evolution rate of the FGH96 superalloy is significantly higher than effective stress, and the material exhibits cyclic hardening. The combined effect of back stress and effective stress is the main reason for the different low-cycle fatigue behaviors of the FGH96 superalloy under different strain amplitudes.

A universal constitutive model considering strain range dependence effect and transient behaviour for both cyclic softening and hardening steels

High-temperature low-cycle fatigue (HTLCF) behaviour of 2.25CrMoV and 316L were investigated by employing internal stress. Results reveal that 2.25CrMoV and 316L show cyclic softening and hardening–softening behaviour, respectively. Moreover, they exhibit strain range dependence effect (SRDE) and transient Bauschinger effect (TBE). The SRDE of 316L was observed throughout the entire fatigue procedure, whereas that of 2.25CrMoV was only observed in the initial softening stage. It is noteworthy that the SRDE is controlled by both back and effective stresses in the case of 316L, and it is primarily dominated by the back stress for 2.25CrMoV. Based on the internal stress analysis, a universal constitutive model was developed. Compared with existing models, a cycle-dependent kinematic hardening factor φ was coupled into kinematic hardening variables to improve the description of the hysteresis loops. Besides, a unified modelling method for SRDE was developed based on a kinematic-isotropic combined hardening rule extended with a strain memory surface. Furthermore, a novel two-dimensional proportionality factor δ related to the plastic modulus of cyclic and monotonic responses was incorporated into the short- and medium-range kinematic hardening variables to account for TBE. Finally, the validity of the proposed model was discussed from the hysteresis loop, peak stress evolution, and peak internal stress evolution. The good agreement between experimental data and simulated results suggests that the proposed model is robust in predicting cyclic softening and hardening–softening behaviours.

High Temperature Fatigue Properties of Boron-doped Superalloy B1914

The B1914 superalloy is engaged in the power and aerospace industry as a material for critical hot section of gas turbines. Typical findings in these critical components with a high number of operations are damages by low cycle fatigue where high stresses comparable to the material yield strength are developed due to mechanical loading or thermal gradients causing large plastic deformation. The present contribution refers to the high-temperature low cycle fatigue behaviour of polycrystalline nickel-based superalloy B1914. Cylindrical specimens were fatigued under strain control with constant total strain amplitude in symmetrical cycling at 800°C and 900°C in air. The microstructure is based on a γ matrix (face-centered cubic lattice) with coherent L12 γ´ precipitates and γ/γ´ eutectic. The higher content of grain boundary stabilizing boron allows for lower carbon contents and thus reduces the carbide content that can facilitate fatigue crack initiation. Cyclic hardening/softening curves and fatigue life diagrams were determined. An increase in testing temperature accelerated fatigue crack initiation at grain boundaries associated with carbides. The SEM and TEM analysis of fatigued material further the discussion of fatigue behaviour of the B1914 superalloy.

Study of High-Temperature Low-Cycle Fatigue Behavior and a Unified Viscoplastic-Damage Model for Heat-Resistant Cast Iron

Study of High-Temperature Low-Cycle Fatigue Behavior and a Unified Viscoplastic-Damage Model for Heat-Resistant Cast Iron

Comportamiento a fatiga de baja frecuencia del compuesto Rene®80 recubierto de aluminuro de platino cerca y por encima del DBTT

La superaleación Rene®80 a base de Ni se utiliza para fabricar álabes de turbinas de gas en motores a reacción. La vida útil de algunas palas de turbina de motores a reacción está limitada por la fatiga de baja frecuencia y esta propiedad se ha visto fuertemente afectada por los revestimientos. La temperatura de transición de dúctil a frágil (DBTT) es el factor más importante que afecta las propiedades mecánicas de las aleaciones recubiertas. En este estudio, se evaluó el comportamiento de fatiga a alta temperatura y baja frecuencia del compuesto Rene®80 sin recubrimiento y recubierto con aluminuro de platino (Pt-Al) a temperaturas de 871 °C (cerca del DBTT) y 982 °C (por encima del DBTT). Los resultados de las pruebas de fatiga, en condiciones de deformación controlada a 871 °C para R = 0 y una tasa de deformación de 2×10-3 s-1, en un intervalo de deformación total de 0,8%, mostraron una disminución de la resistencia a la fatiga de las muestras recubiertas de aproximadamente un 14%, en comparación con las no recubiertas. Sin embargo, el aumento de la temperatura de prueba de 871 °C a 982 °C, condujo a un aumento en el comportamiento a fatiga de baja frecuencia del compuesto Rene®80 recubierto en aproximadamente un 10% en comparación con las muestras sin recubrir.

High-Temperature Low-Cycle Fatigue Behaviour of MAR-M247 Coated with Newly Developed Thermal and Environmental Barrier Coating

This study investigates the strain-controlled low-cycle fatigue (LCF) behaviour of an untreated and surface-treated MAR-M247 superalloy in a symmetrical push-pull cycle with a constant strain rate at 900°C in laboratory air. A newly developed experimental thermal and environmental barrier coating (TEBC) system, consisting of a 170 μm thick CoNiCrAlY bond coat (BC) and a bilayer ceramic top coat (TC), with an interlayer and an upper layer, was deposited using air plasma spray techniques. The ceramic interlayer with an average thickness of 77 μm was formed from agglomerated and sintered yttria-stabilized zirconia. An experimental mixture of mullite (Al6Si2O13) and hexacelsian (BaAl2Si2O8) at a ratio of 70/30 vol.% was sprayed as the upper layer. The average thickness of the TC was 244 μm. The specimen sections were investigated using a TESCAN Lyra3 XMU scanning electron microscope (SEM) to characterise the microstructure of both the TEBC and the substrate material. The fatigue damage mechanisms in the TEBC-coated superalloy were studied. The fatigue life curves in the representation of the total strain amplitude versus the number of cycles to failure of the TEBC-coated and uncoated superalloy were assessed. TEBC was found to have a slight, positive effect on the fatigue life of MAR-M247.

High-Temperature Low-Cycle Fatigue Behavior of HS80H Ferritic–Martensitic Steel Under Dynamic Strain Aging

In this work, low-cycle fatigue tests were performed on HS80H ferritic–martensitic steel with the strain amplitudes ranging from 0.5 to 2.0% at room temperature and 350 °C. The cyclic stress response at 350 °C was found to be different from that at room temperature due to the effect of dynamic strain aging and showed a significant secondary hardening when the strain values were 0.5 and 0.7%. Furthermore, the dynamic strain aging effect also resulted in an abnormal increase in fatigue life when the strain was 0.7%, which was due to the change in elastic strain. Additionally, the elastic strain and fatigue life were bilinear relations in the double logarithmic coordinates. Finally, the transmission electron microscope observations showed that the dynamic strain aging led to the change in substructure, while the grain was refined.

Low cycle fatigue behavior and microstructure evolution of a novel 9Cr–3W–3Co tempered martensitic steel at 650 °C

A series of uniaxial strain-controlled fatigue tests were performed at 650 °C to explore the high-temperature low cycle fatigue (LCF) behavior and corresponding microstructure evolution of a novel 9Cr–3W–3Co martensitic G115 steel. The fatigue test results showed that the steel exhibited a cyclic softening behavior and the degree of softening S increased with the total strain amplitude, which was related to the decrease of dislocation density and subgrain development. Further, statistical analysis of the subgrain size after LCF tests at different strain amplitudes showed that the development of subgrain or cell depended on both the plastic strain level and cycles, among which the effect of plastic strain level is dominant. The variation of degree of softening ΔS was used to make comparison between G115 steel and P92 steels to evaluate the effects of the strain amplitudes on the respective S. The G115 steel experienced the minimum relative increment compared with P92 steel. This resulted from the strengthening by fresh and fine Cu-rich precipitates. The number density of Cu-rich particles increased with the total strain amplitude and that weakened the effect of strain amplitude on degree of softening. SEM was employed to investigate the fracture morphology; representative images of the G115 steel after the LCF tests showing three regions: fatigue crack initiation sites, stable crack growth region, and rupture region. Larger the strain amplitude was, more the crack initiation sites formed, which corroborates the significant decrease in fatigue life with increase in the total strain amplitude. In addition, a modified plastic strain energy approach based non-Masing behavior of G115 steel is proposed to predict the fatigue life.

High-temperature low-cycle fatigue behavior and microstructural evolution of an improved austenitic ODS steel

Abstract

Effect of tensile dwell on high-temperature low-cycle fatigue and fracture behaviour of cast superalloy MAR-M247

Hot isostatically-pressed, nickel-based superalloy MAR-M247 was tested under fatigue and creep-fatigue conditions. Continuous cycling and cycling with 10min tensile dwells in each cycle were applied in symmetrical strain control cycle with constant total strain amplitude at 900°C in air. Cyclic hardening/softening curves, cyclic stress-strain curves and fatigue life curves were obtained. The introduction of tensile dwells resulted in lower fatigue life in total strain representation. Surface relief, specimen sections and fracture surface observations revealed crack initiation sites, crack density and crack paths, and help to clarify the effect of dwells on fatigue behaviour.

High-temperature low-cycle fatigue behavior and microstructural evolution of an ODS steel based on conventional T91

The aim of this work is to investigate the low-cycle fatigue (LCF) behavior of a ferritic-martensitic (F/M) oxide dispersion strengthened (ODS) steel based on commercial T91 (mod. 9Cr-1Mo steel) and to correlate it to the microstructural evolution after cyclic straining at different temperatures. Therefore, LCF tests were performed at temperature between 500 and 650°C with strain amplitudes ranging from ±0.4 to ±0.9%. Cyclic stress response and cyclic strain-life resistance were analyzed in detail and compared with the matrix material. The F/M ODS steel showed complex continuous cyclic softening but to a lesser extent than the matrix material and a slight initial cyclic hardening at 650°C. Due to their high strength at elevated temperatures, strain-life resistance was superior at total strain amplitudes below approx. 0.6%. Minor microstructural changes were observed after testing at 500 and 600°C. With increasing temperature, the initially tangled dislocation structure was partially replaced by straight and parallel dislocations showing double-slip cross over. At 650°C, the F/M subgrain structure changed into more equiaxed grains. However, the formation of three-dimensional (3D) dislocation structures, e.g. dislocation cells or slip band formation, was not observed.

High-temperature low-cycle fatigue behavior of novel austenitic ODS steels

The focus of this study is to investigate the low-cycle fatigue (LCF) behavior of two newly developed austenitic 17Cr13Ni–W ODS steels. The LCF tests were performed at 650°C with strain amplitudes ranging from 0.4 to 0.9%. The cyclic stress response and the cyclic strain-life resistance were analyzed in detail and compared with two non-ODS austenitic stainless steels (304 and 316L(N)) and a ferritic-martensitic ODS steel based on T91. The austenitic ODS steels showed stabilized cyclic response after a slight initial cyclic hardening. Due to their high strength at elevated temperatures, the strain-life resistance became superior with decreasing total strain amplitude compared to the other materials considered.

Deformation and damage mechanisms of a bimodal 12Cr-ODS steel under high-temperature cyclic loading

The present study reports on the high-temperature low-cycle fatigue (LCF) behavior of a ferritic bimodal 12Cr oxide dispersion strengthened steel. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The measured cyclic stress response showed four distinct stages which include instant initial cyclic softening followed by gradual cyclic hardening thereafter continuous linear cyclic softening and finally crack initiation and growth stage. The rate and amount of these stages depend on both strain amplitude and testing temperature. The cyclic stress-strain and strain-life relationships were obtained through the test results, and related LCF parameters were calculated. Microstructural investigations, using electron microscopy, were carried out to shed light on the deformation mechanisms. At 550°C, no appreciable microstructural evolution occurred, which is consistent with the manifestation of Masing behavior. A propensity towards cross-slip and hence the formation of three-dimensional dislocation structures increased with increase in loading amplitude and testing temperature, which assisted in accumulating higher inelastic strain and led to slightly higher cyclic hardening at these conditions. Furthermore, W-enriched Laves phase particles precipitated at grain boundaries. The evolution of cell structures with strain amplitude at 650°C resulted in a small deviation from Masing behavior. The damage studies revealed multiple surface initiated transgranular cracks under higher strain amplitudes and a single surface initiated transgranular crack under lower strain amplitudes. The different stages of the crack path, whose durations depend on the testing conditions, revealed distinct fatigue fracture features.

High-Temperature Low-Cycle Fatigue Behavior of MarBN at 600 °C

This paper presents the high-temperature low-cycle fatigue (HTLCF) behavior of a precipitate strengthened 9Cr martensitic steel, MarBN, designed to provide enhanced creep strength and precipitate stability at high temperature. The strain-controlled test program addresses the cyclic effects of strain-rate and strain-range at 600 °C, as well as tensile stress-relaxation response. A recently developed unified cyclic viscoplastic material model is implemented to characterize the complex cyclic and relaxation plasticity response, including cyclic softening and kinematic hardening effects. The measured response is compared to that of P91 steel, a current power plant material, and shows enhanced cyclic strength relative to P91.

High-temperature low-cycle fatigue behaviour of HIP treated and untreated superalloy MAR-M247

High-temperature low-cycle fatigue behaviour of HIP treated and untreated superalloy MAR-M247

High-temperature low cycle fatigue behavior of a gray cast iron

The strain controlled low cycle fatigue properties of the studied gray cast iron for engine cylinder blocks were investigated. At the same total strain amplitude, the low cycle fatigue life of the studied material at 523K was higher than that at 423K. The fatigue behavior of the studied material was characterized as cyclic softening at any given total strain amplitude (0.12%–0.24%), which was attributed to fatigue crack initiation and propagation. Moreover, this material exhibited asymmetric hysteresis loops due to the presence of the graphite lamellas. Transmission electron microscopy analysis suggested that cyclic softening was also caused by the interactions of dislocations at 423K, such as cell structure in ferrite, whereas cyclic softening was related to subgrain boundaries and dislocation climbing at 523K. Micro-analysis of specimen fracture appearance was conducted in order to obtain the fracture characteristics and crack paths for different strain amplitudes. It showed that the higher the temperature, the rougher the crack face of the examined gray cast iron at the same total strain amplitude. Additionally, the microcracks were readily blunted during growth inside the pearlite matrix at 423K, whereas the microcracks could easily pass through pearlite matrix along with deflection at 523K. The results of fatigue experiments consistently showed that fatigue damage for the studied material at 423K was lower than that at 523K under any given total strain amplitude.

Low cycle fatigue properties of CLAM steel at 823 K

China Low Activation Martensitic (CLAM) steel is considered to be the main candidate material for the first wall components of future fusion reactors in China. In this paper, the low cycle fatigue (LCF) behavior of CLAM steel is studied under fully reversed tension–compression loading at 823K in air. Total strain amplitude was controlled from 0.14% to 1.8% with a constant strain rate of 2.4×10−3s−1. The corresponding plastic strain amplitude ranged from 0.023% to 1.613%. The CLAM steel displayed continuous softening to failure at 823K. The relationship between strain, stress and fatigue life was obtained using the parameters obtained from fatigue tests. The LCF properties of CLAM steel at 823K followed Coffin–Manson relationship. Furthermore, irregular serration was observed on the stress–strain hysteresis loops of CLAM steel tested with the total strain amplitude of 0.45–1.8%, which was attributed to the dynamic strain aging (DSA) effect. During continuous cyclic deformation, the microstructure and precipitate distribution of CLAM steel changed gradually. Many tempered martensitic laths were decomposed into subgrains, and the size and number of M23C6 carbide and MX carbonitride precipitates decreased with the increase of total strain amplitude. The response cyclic stress promoted the recovery of martensitic lath, while the thermal activation mainly played an important role on the growth of precipitates in CLAM steel at 823K. In order to have a better understanding of high-temperature LCF behavior, the potential mechanisms controlling stress–strain response, DSA phenomenon and microstructure changes have also been evaluated.

High-temperature low-cycle fatigue behavior of K40S cobalt-base superalloy

An investigation was made on the strain-controlled low-cycle fatigue (LCF) of K40S cobalt-base superalloy at 900 °C in ambient atmosphere. The results show that K40S alloy possesses high LCF resistance in comparison with X-40 alloy. Under the testing conditions in this study, K40S alloy exhibits a cyclic stress response of initial hardening followed by softening. The cyclic stress response behavior has been attributed to dislocation-dislocation interactions and dislocation-precipitate interactions. The high response stress can lead to a large stress concentration at locations where inelastic strains of high amplitude accumulate, which account for the decreasing fatigue life with increasing strain rate. The well-distributed carbide particles are the “secondary” crack initiation sites. The secondary crack initiation relaxes the stress concentration at the crack tip, reducing the driving force of crack propagation. High-temperature LCF failure of K40S alloy results from the interaction of the mechanical fatigue and environmental oxidation.

Dependence of the high-temperature low-cycle fatigue behaviour of the monocrystalline nickel-base superalloys CMSX-4 and CMSX-6 on the γ/γ′-morphology

Strain-controlled high-temperature (950 and 1050°C) low-cycle fatigue (LCF) experiments were conducted on monocrystalline specimens of the γ′-hardened nickel-base superalloys CMSX-4 and CMSX-6. The goal was to investigate the effect of different γ/γ′-morphologies on the LCF behaviour. For this purpose, the LCF tests were performed on specimens with three different initial γ/γ′-morphologies, namely cuboidal γ′-particles, γ/γ′-rafts perpendicular and γ/γ′-rafts parallel to the stress axis, respectively. The raft structures had been introduced by a small high-temperature creep pre-deformation (plastic strain ≤0.4%) in tension or compression. The LCF tests showed that the fatigue lives were reduced for specimens with γ/γ′-rafts perpendicular to the stress axis and extended for specimens with γ/γ′-rafts parallel to the stress axis. Specimens with cuboidal γ/γ′-particles exhibited intermediate fatigue lives. Fractographic investigations revealed the reasons for this behaviour. The fatigue cracks usually followed along the γ-channels or the γ/γ′-interfaces and avoided cutting the γ′-phase. Thus, fatigue cracking was facilitated when the γ/γ′-rafts were perpendicular to the stress axis and obstructed, when the γ/γ′-rafts lay parallel to the stress axis.

The influence of texture and grain structure on the high temperature low-cycle fatigue behaviour of the ODS nickel-based superalloy PM 1000

The high temperature low-cycle fatigue (HTLCF) behaviour of the recently developed oxide dispersion strengthened (ODS) alloy PM 1000 is investigated at 1123 K using total strain controlled symmetrical wave shapes ( R = −1). In order to reveal the influence of grain structure and texture (a) a 〈100〉-fibre structured bar material with a grain aspect ratio (GAR) of 10 along the longitudinal direction and (b) a pancake structured sheet material with a GAR of 4 and 〈100〉011-cube on edge texture have been studied. The grain structure determines the fracture path, but cannot be correlated with the cyclic lifetimes. Instead, lifetime can be predicted on the basis of the different Young's moduli of the grain structures.