Masters Degrees (Mechanical Engineering)
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Browsing Masters Degrees (Mechanical Engineering) by Author "Aghion, E. E."
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Item High temperature fatigue crack growth behaviour of TIMETAL 21S in an oxidizing environment.(1995) Ferreira, Jacques Henri.; Aghion, E. E.The high temperature fatigue crack growth behaviour of the newly developed, metastable titanium-based alloy, TIMETAL 21S, was investigated in an inert and an oxidizing environment. The investigation adopted a two pronged approached, namely, to initially establish the pure microstructural behaviour under oxidizing and inert environments at various elevated temperatures, and consequently, to establish the environmental effects on the fatigue crack growth behaviour in the various environments at high temperature. The effect of the oxidizing environment on the metastable alloy and on the mechanical and chemical events occurring at the fatigue crack were studied by using optical and scanning electron microscopy, including ED X analysis, x-ray diffraction, and Auger Electron Spectroscopy (AES) . For the microstructural investigation, the TIMETAL 21S samples were exposed for 5 hours to a pure argon and argon + 20% O2 environment at 300°C to 750°C in increments of 50°C. The results showed that in the oxidizing environment a more homogeneous nucleation of the alpha phase had occurred at higher temperatures and that the oxide Ti02, in addition to the alpha case, had predominantly formed on the exposed surfaces. AES analysis showed that dissolution of the oxygen into the alloy occurred even at low temperatures. An LEFM approach was used to investigate fatigue crack growth rate (FCGR) of C(T) specimens at 375°C, 450°C, 550°C and 620°C in the argon and argon + 20% oxygen environment. The crack growth rates were monitored using load-line compliance and the beachmarking method - a method by which beach marks were impressed on the fracture surface to track the progressing crack. The results showed that the crack growth rates were lower in the oxidizing environment and was influenced by a synergistic effect of the temperature, stress intensity at the crack tip and the environment. In addition to the phenomena of crack tip shielding (a process whereby the effective crack tip driving force experienced at the crack tip was locally reduced), other mechanisms such as slip character modification and secondary cracking ahead of the crack tip, leading to crack tip blunting and branching, had to be incorporated to fully explain the crack growth behaviour. The tests conducted in the inert environment effectively excluded the effect of oxygen on the crack growth behaviour and substantiated that various mechanisms ultimately determined the FCGR in TIMETAL 21S at elevated temperatures.Item The influence of sulphidizing attack on the mechanism of failure of coated superalloy under cyclic loading conditions.(1998) Govender, Gonasagren.; Aghion, E. E.A systematic study of the effect of sulphidizing atmosphere on the High Temperature Low Cycle Fatigue (HTLCF) properties of coated and uncoated unidirectionally solidified MARM002 nickel base superalloy was performed at 870°C. The coating systems investigated were, aluminide coating, three types of platinum modified aluminide coatings, and platinum coating. The creep-plasticity mode of the strain range partitioning method was used for creep-fatigue loading. A constant loading regime (Strain range 6.6 x 10-3 ) was used to test the samples in argon, air and Ar + 5%S02 and a lower strain range of3.8 x 10-3 was used to investigate the creep-fatigue properties in Ar + 5%S02 only. The results were analysed using scanning electron microscopy including spot analyses (SEM-EDS), Auger electron spectroscopy (AES) and X-ray diffraction (XRD) techniques. The synergistic effect of sulphidizing environment and the creep fatigue loading (Strain range - 0.66%) resulted in accelerated failure in all the materials systems tested, except for the TYPE I platinum aluminide coated sample. This coating displayed a "self-healing" mechanism which enhanced its fatigue life under sulphidizing conditions. In general, the coatings had an adverse effect on the fatigue properties of the material systems. This was due to the poor mechanical properties of the coating. The mechanical properties of the coating was influenced by the coating microstructure and the chemical composition. The modification of the NiAI zone with platinum in the platinum aluminide coatings improved the fatigue properties of the coating by altering the crack propagation mechanism in the NiAl zone. The higher the platinum content in this region the more brittle it became. The platinum modified aluminide coating showed an improvement in the corrosion fatigue properties in the S02 containing environment at the higher strain range when compared with the uncoated, aluminide coated and platinum coated samples. However, at the lower strain range all the coating systems performed worse than the uncoated alloy. This was mainly due to the brittle failure of the coating. The platinum modified aluminides performed the worst due to the presence of brittle platinum aluminide phases. The interdiffusion and interaction of platinum with the substrate alloying elements, resulted in this coating being ineffective for corrosion protection. The resultant coating layer produced poor corrosion-fatigue properties. Although the coating systems did show evidence of resistance to sulphidation and oxidation there were relatively ineffective under the combination of sulphidizing environment and fatigue loading due to their poor mechanical properties. The mechanism of sulphidation was consistent for all the material systems tested with oxidation proceeding first and sulphidation proceeding at the corrosion scale/substrate interface. The crack propagation in the coating and substrate was controlled by the sulphidation attack at the crack tip and failure of the oxide scales formed in the cracks.