E-Thesis 146 views 108 downloads
The effects of microstructure and microtexture generated during solidification on deformation micromechanism in IN713C nickel-based superalloy / Gang Liu
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DOI (Published version): 10.23889/Suthesis.53058
Nickel-based superalloy IN713C produced by investment casting method are used for turbine blade of turbocharger in modern vehicles. IN713C alloy possesses good strength, fatigue, creep and high temperature oxidation resistance that make the alloy suitable to be used in harsh service environment such...
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Nickel-based superalloy IN713C produced by investment casting method are used for turbine blade of turbocharger in modern vehicles. IN713C alloy possesses good strength, fatigue, creep and high temperature oxidation resistance that make the alloy suitable to be used in harsh service environment such as in the heating part of turbocharger. However, this material suffers from microstructure and microtexture heterogeneity produced during solidification. This microstructure heterogeneity across the component will inevitably give rise to local stress and strain accumulation which may facilitate crack initiation and affect crack propagation. Fatigue, both LCF (Low Cycle Fatigue) and HCF (High Cycle fatigue) are the common failure modes of turbine blade component in turbocharger. It is of industrial and academic interests to identify and classify the features of fracture surface of each failure mode. The necessity of optimisation of fatigue property for the newly developed turbocharger component parts is becoming critical and a fundamental research for understanding fatigue deformation micromechanism and the influence of microstructure (dendrite structure, carbides / oxidised carbides, grain size, etc.) and microtexture (individual crystallographic orientation, cluster of grains, etc.) is required. In the current investigation, LCF and HCF fatigue tests are conducted on real turbine blades as wel as on bars produced via investment casting. Various microstructure characterisation tools were used to identify the deformation micromechanics during LCF and HCF fatigue conditions. The results showed that in real turbine blades where there are much less casting defects than in the testing bar, the fatigue crack initiated from blade surface and crack propagation process was mainly dominated by oxidation-assistant process with oxidised carbides during LCF test. During the late stage of crack propagation, the interdendrite area was found to deform differently from the surrounding area to accommodate accumulated strain heterogeneity. Whilst for HCF, facet was initiated from slip planes with the highest Schmid factor and assisted by small porosity in most cases. As for the fatigue tests conducted on test bars produced via investment casting, the dendrite structure played a vital role in crack propagation mechanism. Based on the observations throughout this study, a concept of ‘crack propagation unit (CPU)’ was proposed. From this proposed micro deformation mechanism, a new perspective of Hall-Patch effect of small grain size in casting alloys (containing dendrite structure) was further elucidated duirng both LCF and HCF. Finally, solidification trials were performed to study the exact correlation between solidification cooling rate and microstructure evolution including grain size and structure, gamma prime, carbides and other phases.
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Superalloy, deformation micromechanism, microstructure, microtexture