To predict and better understand the creep-fatigue behaviour of austenitic cast iron D5S under tension and compression dwell at 800 degrees C, a physics-based crystal plasticity model that describes the complex rate-and temperature-dependent deformation of the material as a function of the dislocation density is implemented. In addition to the tension and compression dwell direction, the effect of three different dwell times (30, 180 and 600 s) on the creep-fatigue properties is investigated. The dislocation density-based crystal plasticity simulations are compared to experimental tests from a prior work. While relaxation tests and low-cycle fatigue (LCF) tests without dwell assist in systematically identifying the material parameters, creep-fatigue (CF) data is used to validate the predictions. The virtual testing is performed on a large-scale representation of the actual test specimen with a polycrystalline structure. To analyse the fatigue damage mechanism, small-scale predictions are also conducted using a micromechanical unit cell approach. Here, a single graphite nodule frequently found in the material is embedded into the austenitic matrix. In the present work, a close agreement is achieved between the predicted CF behaviour and the experimental results. Consistent with the experimental findings, the simulation results show that the addition of compression dwell leads to an uplift of the overall tensile stress level, which significantly reduces the fatigue life of the material. The unit cell studies demonstrate that during this uplift, a strong localisation of stresses and strains arises at the graphite/matrix interface, triggering the nucleation and growth of cavities and/or debonding.

The influence of ferritic nitrocarburizing on the high-temperature corrosion-fatigue properties of the Si-Mo-Al cast iron SiMo1000 is investigated in the present study. It was found that nitrocarburizing effectively increases the surface hardness, but dramatically decreases the fatigue and oxidation resistance of SiMo1000. The fatigue resistance is reduced due to two types of microcracks formed after nitrocarburizing. The oxidation resistance is dramatically diminished due to the formation of microcracks, and the depletion of aluminum in the matrix from nitride precipitation during the exposure at 800 degrees C. The corrosion-fatigue synergy is found to cause severe decarburization (i.e. graphite depletion) in nitrocarburized SiMo1000.

Creep-fatigue tests with either tension or compression dwell and reference low-cycle fatigue test without dwell were conducted on the austenitic cast iron D5S in atmospheric air at 800oC to investigate the influence of dwell time on lifetime and the corresponding failure mechanisms. The addition of tension or compression dwell reduces the lifetime, by up to 50% and 80% in the tested range, respectively. Compared with tension dwell, compression dwell is found to be more detrimental and could further reduce the lifetime by up to 60%. Tension and compression dwell are seen to cause a thicker and thinner gauge length, respectively. Tension dwell causes the formation of creep pinholes at the graphite/matrix interface and subgrain boundaries, leading to the formation of internal microcracks. Compression dwell results in forming large cavities at the graphite/matrix interface. The intermetallics in an untested sample are found to contain G phase, Ni31Si12, ?-Fe, and M7C3. The intermetallics in an LCF-tested sample are found to contain G phase, Ni31Si12, M7C3, and ? phase. Cracks inside intermetallics are found to form at the interfaces between Ni31Si12/? phase, M7C3/? phase, and Ni31Si12/M7C3.

To increase the understanding of the corrosion-fatigue mechanisms in two cast iron-alloys used in exhaust manifolds, low-cycle fatigue tests at 800 degrees C in argon and synthetic diesel exhaust as well as isothermal oxidation tests in the exhaust atmosphere are conducted. The corrosion impacts on the fatigue life of the materials are quantitatively evaluated from comparing the G-N curves, and examined from characterization through SEM, EDX, EBSD and EPMA. The materials show very different behaviors to the synergistic effect of corrosion and fatigue. Different theories have been suggested based on the findings.

The influence of graphite morphology on the corrosion-fatigue mechanism in the cast compacted graphite iron SiMo1000 is investigated. Two batches of SiMo1000 with the same chemical composition but different nodularity are tested using low-cycle-fatigue tests in argon and a synthetic exhaust atmosphere at 800 degrees C. Decreased graphite nodularity is found to significantly reduce the corrosion resistance of SiMo1000, causing severe decarburization on the sample surface. Besides, the fatigue life is also dramatically reduced with decreased graphite nodularity. The synergy of decarburization and fatigue is found to reduce fatigue life in one of the batches.

In the present study, low-cycle fatigue (LCF) tests and oxidation tests in controlled atmospheres are carried out at 800ºC on two ductile cast irons SiMo51 and SiMo1000. The LCF tests are conducted in argon and synthetic exhaust gas, whereas oxidation tests are carried out in the latter atmosphere. S-N curves and weight-gain curves are presented. The crack growth mechanisms and oxidation mechanisms are investigated, as well as the synergistic effects. A surprising finding of increased fatigue resistance in the oxidizing atmosphere is partly explained.

Low-cycle fatigue tests and creep-fatigue tests with either tension or compression dwell were conducted on the austenitic cast iron Ni-resist D5S in atmospheric air at 800^oC to investigate the influence of dwell time on lifetime and the corresponding failure mechanisms. The addition of tension or compression dwell reduces the lifetime, by up to 50% and 80% in the tested range, respectively. Distinct creep-oxidation-fatigue interactions are observed from the microstructural characterization of the creep-fatigue tested specimens. The intermetallics in the untested sample contain G phase, Ni₃₁Si₁₂, α-Fe, and M₇C₃. χ phase was found after LCF tests.