Abstract
Mg-Ti alloys with up to 46wt%Ti in solid solution have been produced by PVD with in-situ mechanical working. The in-situ mechanical working performed on these alloys had little effect on the general microstructure but it did reduce porosity by closing pores and flattening surface asperities. The in-situ mechanical working effect was restricted to within 200-400nm of the flail line. All the alloys exhibited compositional inhomogeneity, columnar microstructures and typical PVD defects. The lattice parameters and the thermal stabilities of the alloys decreased with increasing Ti content, the most stable solid solution breaking up above 566 K. A Mg-2wt%Zr alloy was produced by PVD and underwent an isothermal forging treatment. The isothermal forging eliminated almost all the characteristic PVD features. The microstructure of the forged alloy consisted predominately of equiaxed grains with almost no compositional banding. The isothermal forging eliminated completely the severe cracking seen in other as deposited PVD Mg-Zr alloys. The air formed oxide film on the Mg-Ti alloys contained a mixture of MgO and TiO2 covered by Mg(OH)2 and hydromagnesite at the outermost surface. Magnesium oxide and titanium oxide were also formed at the columnar boundaries of the PVD microstructure that was not affected by in-situ mechanical working. The surface film formed on the forged Mg-Zr alloy consisted mainly of MgO with possibly some Mg(OH)2 also being present. The corrosion resistance of the PVD in-situ mechanical worked Mg-Ti alloys in 3wt% NaCI was inferior to that of pure PVD Mg. Corrosive attack was more favourable at sites of mechanical working owing to the different microstructure and higher Mg concentration at the flailed areas. This resulted in exfoliation of the alloys. The corrosion product contained a mixture of MgO, TiO2 and some Mg(OH)2 at the outermost surface. The corrosion products on the Mg-Zr alloy after total immersion corrosion tests in 3wt% NaCI were identified as MgO and Mg(OH)2. No conclusive evidence was found for the participation of Zr in the corrosion products formed on the isothermally forged Mg-2wt%Zr alloy after immersion in 3wt% NaCI. The alloying behaviour of Ti in Mg has been studied using high energy XPS and measurements of Auger parameter changes. It was found that in the presence of a core hole Ti was screened better in the alloyed state than in the pure metal (in contrast to Mg). This suggests a charge transfer from Mg to Ti. The amount of charge transfer for a given level of alloying addition was smaller in PVD Mg-Ti alloys than in the PVD Mg-V alloys studied by Diplas and co-workers. The Mg 1s plasmons were studied to identify any changes at the electronic level upon alloying. The intensity of the Mg 1s plasmons decreased as the level of Ti alloying additions increased. This is probably related to the formation of new hybrids via electron redistribution from the Mg sp band to the Ti 3d band, where the electrons are more strongly bound. The increase in the plasmon energy was due to . the increase in the valence electron density upon alloying with Ti.