Oxidation-assisted Crack Growth Research Articles

Early Spalling Analysis of Large Particles in High-Cr Steel during Thermal Fatigue: Relevant Mechanisms.

The aim of this study was to investigate the surface deterioration of high-Cr roll steel caused by the spalling of larger particles during thermal fatigue. The mechanisms of surface deterioration due to spalling of larger particles are discussed. Using a laboratory thermal fatigue test that replicates hot rolling conditions, samples were tested cyclically (up to 4500 times) at maximum cycle temperatures of 500, 600 and 700 °C, followed by water cooling. Specimens with surface deterioration were selected for analysis, revealing important influencing parameters, i.e., the combination of test temperatures, chemical composition, thermal stress and microstructural properties, leading to oxidation-assisted crack growth in different directions and consequent surface deterioration due to early spalling of larger particles. Here, we describe the mechanisms of crack propagation, especially in the lateral direction, and their relation to the subsequent spalling of larger particles, which depend on the influence of the local chemical composition on the microstructural constituents, as well as their distribution and properties. The results obtained in this study can be used in the development of roll steel microstructures with improved resistance to the identified mechanisms of surface degradation.

Oxidation-Assisted Crack Growth in Single-Crystal Superalloys during Fatigue with Compressive Holds

The mechanism of oxidation-assisted growth of surface cracks during fatigue with compressive holds has been studied experimentally and via a model that describes the role of oxide and substrate properties. The creep-based finite element model has been employed to examine the role of material parameters in the damage evolution in a Ni-base single-crystal superalloy Rene N5. Low-cycle fatigue experiments with compressive holds were conducted at 1255 K and 1366 K (982 °C and 1093 °C). Interrupted and failed specimens were characterized for crack depth and spacing, oxide thickness, and microstructural evolution. Comparison of experimental to modeled hysteresis loops indicates that transient creep drives the macroscopic stress–strain response. Crack penetration rates are strongly influenced by growth stresses in the oxide, structural evolution in the substrate, and the development of \(\gamma ^{\prime }\) denuded zones. Implications for design of alloys resistant to this mode of degradation are discussed.

Oxygen diffusion and crack growth for a nickel-based superalloy under fatigue-oxidation conditions

Advanced microscopy characterisation and numerical modelling have been carried out to investigate oxygen diffusion and crack growth in a nickel-based superalloy under fatigue-oxidation conditions. Penetration of oxygen into the material and the associated internal oxidation, which leads to material embrittlement and failure, have been found from Focused Ion Beam (FIB) examinations. Applied fatigue loading tends to enhance the extent of internal oxidation for temperatures at 750°C and above. Using a submodelling technique, finite element analyses of oxygen penetration at grain level have been carried out to quantify the fatigue-oxidation damage and calibrate the diffusion parameters based on the measurements of maximum depth of internal oxidation. The grain microstructure was considered explicitly in the finite element model, where the grain boundary was taken as the primary path for oxygen diffusion. A sequentially coupled mechanical-diffusion analysis was adopted to account for the effects of deformation on diffusion during fatigue loading, for which the material constitutive behaviour was described by a crystal plasticity model at grain level. Prediction of oxidation-assisted crack growth has also been carried out at elevated temperature from the finite element analyses of oxygen diffusion near a fatigue crack tip. A failure curve for crack growth has been constructed based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The predictions from the fatigue-oxidation failure curve compared well with the experimental results for triangular and dwell loading waveforms, with significant improvement achieved over those predicted from the viscoplastic model alone.

Fatigue crack growth behavior of a newly developed Ni–Co-base superalloy TMW-2 at elevated temperatures

Fatigue crack growth (FCG) rates of a new superalloy TMW-2 in air was studied by a fracture mechanics test method. Compact tension specimens were tested under load control with a triangular wave form to investigate the effects of temperature (400, 650, and 725°C) and load ratio (0.05 and 0.5) on FCG rates. The results showed that the FCG rates increased significantly with increasing the temperature. Compared with the creep effects, the results showed that the degradation of mechanical properties and the oxidation assisted crack growth may dominate the FCG rates of TMW-2 at elevated temperatures. Load ratio had a little (at 400°C) or moderate (at 650 and 725°C) influence on the FCG rates, which increased as the load ratio increased. The load ratio effects were successfully accounted for by applying the Walker model. The fractographic observations showed that the fracture mode was transgranular at 400°C and was mixed transgranular and intergranular at 650 and 725°C.

Fatigue crack growth characteristics of a new Ni–Co-base superalloy TMW-4M3: effects of temperature and load ratio

The fatigue crack growth (FCG) behavior of a newly developed Ni–Co-base superalloy TMW-4M3 has been investigated in this study. Experiment was carried out in laboratory air with compact-tension specimen. The effects of temperature (400, 650, and 725 °C) and load ratio (0.05 and 0.5) on crack growth rate were studied in the Paris regime. The results revealed that the crack growth rate increased with increasing temperature under a given load ratio. The influence of load ratio on crack growth rate was pronounced, especially at higher temperature and low stress intensity factor range. Fractographic observations showed that fracture mode was transgranular at 400 °C, mixed transgranular and intergranular at 650 °C, and predominantly intergranular at 725 °C. The possible explanations for the crack growth behavior were discussed based on the degradation of mechanical properties and the oxidation assisted crack growth, as well as the crack closure effect. A comparison of FCG rate was also made between TMW-4M3 and the commercial superalloy U720Li.

Prediction of crack growth in a nickel-based superalloy under fatigue-oxidation conditions

Prediction of oxidation-assisted crack growth has been carried out for a nickel-based superalloy at elevated temperature based on finite element analyses of oxygen diffusion, coupled with viscoplastic deformation, near a fatigue crack tip. The material constitutive behaviour, implemented in the finite element code ABAQUS via a user-defined material subroutine (UMAT), was described by a unified viscoplastic model with non-linear kinematic and isotropic hardening rules. Diffusion of oxygen was assumed to be controlled by two parameters, the oxygen diffusivity and deformation-assisted oxygen mobility. Low frequencies and superimposed hold periods at peak loads significantly enhanced oxygen concentration near the crack tip. Evaluations of near-tip deformation and oxygen concentration were performed, which led to the construction of a failure envelop for crack growth based on the consideration of both oxygen concentration and accumulated inelastic strain near the crack tip. The failure envelop was then utilised to predict crack growth rates in a compact tension (CT) specimen under fatigue-oxidation conditions for selected loading ranges, frequencies and dwell periods. The predictions from the fatigue-oxidation failure envelop compared well with the experimental results for triangular and dwell loading waveforms, with marked improvements achieved over those predicted from the viscoplastic model alone. The fatigue-oxidation predictions also agree well with the experimental results for slow-fast loading waveforms, but not for fast-slow waveforms where the effect of oxidation is much reduced.

OXIDATION-ASSISTED FATIGUE CRACK GROWTH BEHAVIOR IN ALLOY 718-PART II. APPLICATIONS

In previous work by the authors, a quantitative model based on a two-stage oxidation mechanism was developed to describe the oxidation-assisted crack growth behavior in alloy 718. This model is used here to predict the crack growth rate in this alloy at 650°C for two different loading conditions: one is a continuous cycle with a hold time duration of 300 s imposed at minimum load, the other is a continuous cycle without a hold time duration. The results obtained from applying the model to these loading profiles were then compared with those obtained experimentally. Good agreement was observed between the two data sets. Details of the model calculations are discussed, and suggestions to further extend the model capabilities are made in this paper.

Oxidation-assisted crack growth during high cycle fatigue of a 1%CrMoV steel at 550°C

Fatigue tests have been performed on cast 1%CrMoV steel at 550°C under vacuum. Crack growth rates were obtained in the region where linear elastic fracture mechanics could be applied and were found to be substantially slower under vacuum than rates obtained in air. At low stress intensity amplitudes ( ΔK = 6 MN m −3 2 ), rates were a maximum of 100 times slower than in air and similar to growth rates at room temperature in air. Frequency effects on growth rate at 550°C under vacuum were much less marked than in air. Although always transgranular, there was an increased tendency for crack branching in air at 550°C as cycle frequency was increased. It is suggested that this effect was caused by a change in the dominant oxidation mechanism from diffusion-controlled growth to oxide fracturing.