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The objectives of this work are to characterize the flow behaviour of the Ti-6Cr-5Mo-5V-4Al (Ti6554) alloy at high strain rates and elevated temperatures using the Johnson-Cook (JC) model and a modified Zerilli-Armstrong (ZA) model, and to make a comparative study on the predictability of these two models. The stress-strain data from Split Hopkinson Pressure Bar (SHPB) tests over a wide range of temperatures (293-1173K) and strain rates (10 3 -10 4 s -1 ) were employed to fit parameters for the JC and the modified ZA models. It is observed that both the JC and the modified ZA models have good capacities of describing the flow behaviour of the Ti6554 alloy at high strain rates and elevated temperatures in terms of the average absolute error. The modified ZA model is able to capture the strain-hardening behaviour of the Ti6554 alloy better as it incorporates the coupling effects of strain and temperature. However, dynamic recovery or dynamic recrystallization that may happen at elevated temperatures should be taken into consideration when selecting data set for parameters fitting for the modified ZA model. Also the modified ZA model requires more stress-strain data for the parameters fitting than the JC model.

Application prospects in automotive and aerospace industry have led to extensive studies on AA6xxx alloys in recent years. Varying amounts of Mg, Si and Cu and heat treatments are used to achieve the desired mechanical properties in these alloys. In this investigation, a series of tests have been designed and carried out on model AlMgSi(Cu) alloys to investigate the effects of Cu, Mg and Si composition and heat treatments on the corrosion properties for a range of Cu content levels (0.03, 0.15 and 0.80 wt%, respectively).The results indicate that the localized corrosion susceptibility of the model alloys primarily increased with Cu content. The Si and Mg content and ratio do not appear to have a significant effect on the local corrosion behaviour. Heat treatment can improve the corrosion resistance. However, this effect is small compared to that of the Cu content for the range of model alloys investigated. Intergranular corrosion will occur by micro-galvanic coupling between the cathodic AlMgSi(Cu) (Q) phase precipitates and the aluminum matrix adjacent to the particles. The increasing susceptibility to intergranular corrosion with Cu content can be attributed to an increased formation of Q phase particles.

The stress-strain behaviour and microstructural evolution of the Ti-6Cr-5Mo-5V-4Al (Ti6554) alloy was systematically investigated using Split Hopkinson Pressure Bar (SHPB) tests over a wide range of strain rates from 1000 s -1 to 10000 s -1 and initial temperatures from 293K to 1173K. Dislocation slip is the main deformation mechanism for plastic flow of the Ti6554 alloy at high strain rates. The flow stress increases with increasing strain rate and decreasing temperature. Also the flow stress is more sensitive to temperature than to strain rate. For high strain rate deformations, the strain hardening rate is found to be negative at 293K and increases with increasing temperatures. Flow softening observed at 293K is potentially caused by adiabatic heating. The increment in the strain hardening rate with increasing temperatures may be the result of interactions between thermally activated solute Cr atoms and mobile dislocations. When the temperature is raised to 873K, a novel Į precipitate 2 morphology consisting of globular Į aligned in strings was observed in specimens deformed at strain rates of 4000 and 10000 s -1 . It has hardening effects on the ȕ matrix and is purported to nucleate on dislocations introduced by the high strain rate deformation. Adiabatic shear bands were observed in specimens deformed at higher temperatures (873K). The microstructure inside the shear bands is harder than that outside of the shear bands in the Ti6554 alloy.

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