In this thesis aluminium alloys containing small additions of both tin (~ 0.1 wt %) and gallium (~ 0.05 wt %) are shown to dissolve anodically at high rates in sodium chloride media at room temperatures; current densities > 0.2 A cm -2 can be obtained at potentialsclose to the open circuit potential, ~ -1.5 V vs SCE. Alloys that do not contain both tin and gallium were shown not to dissolve at such a negative potential. The tin exists in the alloys as a second phase, typically as ~ 1 µm inclusions (precipitates) distributed throughout the aluminium structure, and anodic dissolution occurs to form rounded pits around the tin inclusions. The pits were different in structure from the crystallographic pits commonly observed with Al and other alloys. The change in pit structure and the negative shift in dissolution potential indicate that the AlMgSnGa alloys dissolve by a different mechanism. Although the distribution of the gallium in the alloy could not be established, it is also shown to be critical in the formation of these pits as well as maintaining their activity. The stability of the alloys to open circuit corrosion and the overpotential for high rate dissolution, both critical to battery performance, is shown to depend on factors in addition to elemental composition; both heat treatment and mechanical working influence the performance of the alloy. The correlation between alloy performance and their microstructure has been investigated.iii Imaging of the surface with a resolution of 10 -20 µm was used for the direct observation of the anodic dissolution of aluminium alloys containing Sn and Ga. The resolution allows confirmation that hydrogen evolution occurs from the Sn inclusions.

The microstructure and fatigue performance of three sub-solvus heat treated nickel based disc superalloys for turbine disc applications are reported. The alloy variants studied are RR1000, N18 and Udimet 720 Low Interstitial (U720Li), with the latter tested both in a standard and large grain variant (LG). Their microstructures are examined in terms of grain and gamma prime size (γ'). Fatigue crack growth (FCG) rates for all materials at 650ºC show that RR1000 provides the best performance, followed by U720Li-LG, N18 and U720Li. Some of the variations in FCG rate between the alloys are due to reduction in grain boundary oxidation processes with increased grain size, but more subtle interplays between grain boundary character, alloy composition and slip character are also important.

The effects of temperature, dwell and environment on the fatigue crack growth behaviour of two nickel base turbine disc superalloys RR1000 and Udimet 720Li are presented in the present paper. Fatigue tests were carried out at room temperature in air using a 20 Hz sinusoidal loading waveform and at elevated temperatures of 650 and 725°C in both air and vacuum environments using a trapezoidal loading waveform with dwell times at a maximum load of 1 or 20 s. The fatigue crack propagation resistance of both materials is rationalised in terms of the influence of microstructure as well as the effects of alloy chemistry.

A number of model Ni based superalloy bimetallic discs have been manufactured, each having a cast and wrought U720Li hub bonded to a cast IN738LC ring by means of hot isostatic pressing augmented by isothermal forging. The microstructures of both the as hipped and HIP plus forged model discs showed a complex precipitation reaction at the interface between the two alloys. Different types of precipitate were identified at the bond line, which had formed as a consequence of interdiffusion and recrystallisation at the interface during hipping. Residues from a pre-HIP acid cleaning process were also detected. Preliminary forging trials conducted on the model discs found poor control of the deformation and shape of the interface due to temperature and die lubrication issues and resulted in strong recrystallisation of the IN738LC and internal cracking. Subsequent full scale disc manufacture was able to overcome these problems, although further work is continuing in the microstructural and mechanical characterisation of these parts.

Crack nucleation/initiation at high temperatures has been studied in CMSX4 in both air and vacuum environments, to elucidate the effect of oxidation on the notch fatigue initiation process. In air, crack nucleation/initiation occurred at subsurface interdendritic pores in all cases. The subsurface crack grows initially under vacuum conditions, before breaking out to the top surface. Lifetime is then dependent on initiating pore size and distance from the notch root surface. In vacuum conditions, crack nucleation/initiation has been observed more consistently from surface or close-to-surface pores, indicating that surface oxidation is in-filling/'healing' surface pores or providing significant local stress transfer to shift initiation to subsurface pores.