Effect of solute elements on the lattice parameters of magnesium

Magnesium has limited formability at room temperature due to the hexagonal close-packed (hcp) structure with an axial (c/a) ratio of 1.6240 which provides only basal slip activation. Other hcp metals with different c/a ratios such as titanium have non-basal slip at room temperature. It can be postulated that changing the c/a can change the balance of deformation mechanisms and influence formability. Solutes are known to change the lattice parameters of a metal if in solid solution and hence affect deformation behavior. The objective of this project was to determine the effects of solute additions on the c/a ratio of magnesium and to attempt and assess the resulting deformation behavior.Changes to the lattice parameters of magnesium with addition of solute elements, Ce, Li, In, and Zn, have been investigated. Binary solid solution alloys as well as multicomponent alloys were cast in permanent copper moulds and homogenized at temperatures between 375°C and 400°C. Annealed powder specimens were prepared and lattice parameters measured using a Philips PW1710 powder diffractometer with Cu-alpha radiation. The experimental X-ray diffraction (XRD) results were refined and analyzed by means of the Rietveld method while characterization of the alloys was performed with optical and scanning electron microscopy and electron probe microanalysis.The effects of solutes on lattice parameters were explained on the basis of atom size differences and the change in electron overlap of magnesium due solute additions. It was found that addition of a lower valent solute such as Li caused a contraction of the lattice parameters and a resulting axial ratio decrease. Addition of Indium, however, resulted in a contraction of the lattice parameters, but an increase in the axial ratio due to its higher valency. Conversely, neither Zn nor Ce was found to have an effect on the axial ratio.Characterization of the binary alloys revealed grain size can vary with the type and the amount of solute present in the pure Mg. Zinc was found decrease the grain size of Mg even at low solute content, while the Li and In concentrations had to be increased in order to achieve the same degree of grain refinement. Significant grain refinement was also observed with low Ce additions, mostly because of the presence of Ce-rich second phases.Preliminary deformation tests were also carried out on three different alloys in order to see if changes to the axial ratio affected formability. Rolling and compression tests revealed that alloys with a finer as-cast microstructure resulted in more consistent flow curve shapes and smoother compressed surfaces