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Made available in DSpace on 2014-10-09T12:36:18Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:59:18Z (GMT). No. of bitstreams: 1 03981.pdf: 3648582 bytes, checksum: 289e3f8da88b110ea883c9749d9e35bc (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

Made available in DSpace on 2014-10-09T12:29:28Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T13:59:40Z (GMT). No. of bitstreams: 1 12907.pdf: 1852096 bytes, checksum: 4b1d5ce099ad8f5b1da0845dac78c66e (MD5) / Dissertacao (Mestrado) / IEA/D / Universidade Federal do Rio de Janeiro - UFRJ

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Process irregularities in remelting furnace operation have an impact on the ingot solidification structure which depends on the local thermal conditions and the alloy concerned. In this work, a laboratory investigation into the structures resulting from interruptions in solid growth is presented in order to understand the cause and effect relationship between the solidification interruption and banding. The results demonstrate the range of structure, segregation and precipitation changes which are experienced by a range of alloys including Inconel 718, Nimonic 80A, Waspaloy andM50. In the case of alloys which form primary precipitates, the interruption period is shown to give rise to structure changes. Segregations are found due to the interruption. We also comment on the differences observed between industrial examples of banding and the laboratory samples which may be due to the absence of liquid movement in the latter technique. It is concluded that the interruption in solidification condition during a directionally solidification itself can produce banding through its effects on the morphology of precipitates. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

High manganese steels offer an outstanding combination of high strength and ductility owing to their high sustained strain hardening rates. The strain-induced deformation products in these steels (mechanical twins and ε-martensite) increase the work hardening rates by acting as barriers for dislocation motion. A significant determinant of the deformation products in these steels is the value of stacking fault energy (SFE), which is in turn strongly dependent on the alloy manganese and carbon contents. The main objective of present work is to investigate the microstructural evolution and mechanical behaviour of both homogenous and compositionally graded high-Mn steels, where C compositional gradients were introduced into the latter. The starting materials in this work were cold rolled Fe-22Mn-0.6C and Fe-30Mn-0.6C steels. For both starting alloys, decarburization and annealing heat-treatments were used to obtain four homogenous alloys with carbon contents of approximately 0, 0.2, 0.4 and 0.6 wt. % with similar grain sizes. Decarburization heat treatments were also applied to obtain three carbon graded Fe-22Mn-C alloys (G1, G2, G3) and one carbon graded Fe-30Mn-C alloy. Microstructural observations determined the deformation products to be mechanical ε-martensite for the 22Mn-0C and 22Mn-0.2C alloys and mechanical twins for the 22Mn-0.6C, 30Mn-0.2C, 30Mn-0.4C and 30Mn-0.6C alloys. For the 22Mn-0.4C and 30Mn-0C alloys, both mechanical twins and ε-martensite were observed during deformation. For all the carbon graded Fe-22Mn-C alloys, the dominant deformation products changed from mechanical ε-martensite at the near-surface layer to a mixture of mechanical twins and ε-martensite or mechanical twins only at the inner cross-section layers. In the case of carbon graded Fe-30Mn-C alloy, the deformation products altered from a combination of mechanical ε-martensite and twins at the near-surface layer to mechanical twins at the inner cross-section layers. For all the homogenous alloys, the ultimate tensile strength and uniform elongation increased with increasing alloy carbon content. The work hardening behaviour of these steels was successfully modelled using a modified Kocks-Mecking model, in which the work hardening was the sum of the dislocation glide contribution and the phase transition contribution – mechanical twinning and/or mechanical ε-martensite formation – as dictated by the formation kinetics of both deformation products. For both alloy systems, the mechanical properties of the carbon graded alloys were not as good as the monolithic 22Mn-0.6C and 30Mn-0.6C alloys due to their lower sustained high work hardening rates. Both the mechanical ε-martensite and twin formation were found to follow a sigmoidal kinetic with strain. In the case of twin formation homogenous alloys, the saturated volume fraction of twins was directly proportional to the alloy SFE. For the ε-martensite formation homogenous alloys, the ε-martensite volume fraction at fracture was found to be strongly dependent on alloy SFE, where it declined sigmoidally with increasing alloy SFE. It was also found that the ε-martensite volume fraction at fracture – approximately 0.7 – was independent of SFE for SFE  6 mJ/m2. This indicated that the critical damage mechanism was determined by the kinetics of the ε-martensite formation, which was in turn dictated by the alloy SFE. Finally, it was found that the stress for the onset of mechanical twinning – and consequent increase in the work hardening rate – for the higher SFE, twinning dominated alloys was linearly proportional to the alloy SFE. / Thesis / Doctor of Philosophy (PhD)