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Deformation of dual phase titanium alloys: experiment and modelling
thesisposted on 14.02.2017, 02:19 by Zhong, Jikang
The deformation behaviour of dual phase titanium alloys has been investigated using experimental and modelling methods. The motivation for conducting this investigation was to improve understanding of the deformation behaviour of dual phase titanium alloys in order to contribute to the long term goal of reducing manufacturing cost and improving production efficiency. Dual phase titanium alloys have been studied under two deformation conditions: high speed machining and uniaxial compression, which represent respectively high rate and low rate deformation. The microstructure and texture of the serrated chips obtained during high speed cutting of the dual phase Ti-6Al-4V alloy have been studied. The cutting speed, depth of cut and the orientation of the sample were found to have a significant influence on the deformation of serrated chips. Adiabatic shear bands were found in the serrated chips due to non-uniform deformation of Ti-6Al-4V alloy and deformation heating. In addition, there is reasonable agreement between the predicted texture using the visco-plastic self-consistent (VPSC) model and the measured texture using the electron backscatter diffraction (EBSD) technique indicating that the deformation in an adiabatic shear band is due to shear. The deformation behaviour of dual phase Ti-Mn alloys has been characterised with uniaxial compression at different temperatures and strain rates. The flow stress plateau phenomenon was observed in all the Ti-Mn alloys in the low to medium temperature range (27~500°C), which is attributed to the occurrence of dynamic strain aging (DSA). The flow softening behaviour was observed in dual phase Ti-Mn alloys in the high to sub-transus temperature range (600~800°C), which is attributed to the change in volume fraction of β phase during deformation due to the rise in temperature caused by deformation heating. The yield point phenomenon was observed when the Ti-Mn alloys are above their β transus temperatures. This phenomenon has been attributed to the multiplication of dislocations at the beginning of plastic deformation and dynamic recovery in the following plastic deformation. The strain rate sensitivity of Ti-Mn alloys was low in the low to medium temperature range and thus the effect of strain rate on deformation is not obvious. However, it was high in the high to sub-transus temperature range and thus the effect of strain rate on the deformation is significant. The 0.2% proof stress was found to increase with the volume fraction of β phase. The strain rate sensitivity was found to decrease with the volume fraction of β phase in the low to medium temperature range because the strain rate sensitivity of the α phase is higher than the β phase. However, the strain rate sensitivity was found to increase with the volume fraction of β phase at high temperatures because the strain rate sensitivity of the β phase becomes larger than the α phase. A considerable temperature rise was observed during compression of the Ti-Mn alloys and this indicates that the effect of deformation heating on deformation is not negligible. A composite model has been constructed to predict the plastic stress-strain curves of the dual phase Ti-Mn alloys using the in-situ behaviour of the component phases. The partition of stress and strain between the component phases has been accounted for by using three partitioning assumptions: iso-strain, iso-stress and iso-work. There was good agreement between the predicted curves and the experimental curves. It was found that the in-situ behaviour of the α phase is different in different dual phase Ti-Mn alloys and it exhibits a linear relationship with the volume fraction of β phase when the dominant matrix is the same. The modelling results in the low to medium temperature range indicate that most of the plastic deformation occurs in the localised shear band, which is consistent with the experimental observation that the dual phase Ti-Mn alloys were more susceptible to the formation of localised shear bands. In addition, it was found that a change in the dominant matrix in dual phase Ti-Mn alloys significantly influences the in-situ behaviour of the α phase. The modelling results in the high to sub-transus temperature range indicate that the flow softening behaviour observed in dual phase Ti-Mn alloys is related to the change in volume fraction of the β phase and the diffusion of Mn during the deformation.