Microstructural characterisation and modelling of dilute magnesium-tin-aluminium alloys

Douglas, Gareth (2018). Microstructural characterisation and modelling of dilute magnesium-tin-aluminium alloys. University of Birmingham. Ph.D.

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Abstract

The ageing process of two magnesium alloys with compositions of Mg-1.75Sn-1.93Al and Mg-1.29Sn-2.85Al(at%) have been investigated. Three ageing times, 40, 72 and 160 hrs, were selected to correspond to the early growth, peak hardness and coarsened stage of the alloys and the hardness measured. Subsequently, the precipitates in the alloys have been classified by 4 morphologies, basal plates, 〈112 ̅0〉 laths, pyramidal laths and prismatic rods. and identified as Mg2Sn before the number density and size of the precipitates was measured. This has been achieved through a combination of X-ray Diffraction, Scanning Electron Microscopy and Transmission Electron Microscopy. Moreover, an Orientation Relationship of (0001)α//(111)β,[112 ̅0]α//[110]β was found to be common to three of the four morphologies with the pyramidal laths not conforming to a well-defined orientation relationship. This is then linked through observation to the lack of a common habit plane or growth direction for these pyramidal precipitates.
The obtained data has then been used to model the nucleation and growth of the precipitates using a Kampmann-Wagner Numerical framework, where key parameters such as the diffusion rate of Sn and the interfacial energy of the precipitates were fitted. This leads into a simulated strength against the ageing time which has been compared to the initial hardness measurements made. This has good agreement with not only the overall number density and precipitate sizes but also the size distribution of the precipitates
The effect of the two microstructures on the differing mechanical properties has been discussed, highlighting the importance of number density over the different precipitate morphologies observed, and future experiments outlined.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

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