4D quantifications of intermetallics in solidifying aluminium alloys

Song, Zihan (2023). 4D quantifications of intermetallics in solidifying aluminium alloys. University of Birmingham. Ph.D.

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Abstract

High-speed synchrotron X-ray tomography was used to investigate the growth dynamics and mechanisms of faceted intermetallic compounds (IMCs) in aluminium alloys under different solidification conditions (temperature gradients, cooling rates and external magnetic field). Different IMCs in four aluminium alloys are studied including Al13Fe4 in Al-5wt%Fe alloys, Al2Cu in Al-45wt%Cu alloys, Al3Ni in Al-10wt%Ni alloy and β-IMCs in W319 (Al-Si-Cu) based alloys.

4D quantifications (3D plus time) were achieved in determining the nucleation rates, nucleation densities, volume fractions and growth velocities of Al\(_{13}\)Fe\(_4\) IMCs with various faceted morphologies. In both Al-45wt%Cu and Al-10wt%Ni, two stages of the formation processes were identified including the growth of the basic unit and the growth of the faceted dendrite. The transition of various morphologies of the basic unit was observed and the relationship between these morphologies was determined. The growth mechanism of the faceted dendrite was proposed to be self-repeated layer-by-layer stacking of the basic units (such as L-shaped in Al\(_2\)Cu or V-shaped in Al\(_3\)Ni).

In addition, the work studied the effect of magnetic fields on solidification in Al-45wt%Cu and W319 alloys. A transverse magnetic field of 0.5T was used to control the solidification processes while the sample was rotating. Highly refined Al\(_2\)Cu intermetallic compounds were obtained including the in Al-45wt%Cu alloys which were much finer than those without the imposition of the magnetic field. Under the magnetic field, fine α-Al dendrite with smaller primary dendrite arm spacing in W319 alloys was obtained. The macro segregation zone was almost eliminated, while the solid/liquid interface was altered from tilted to flat. Gradient volume distribution of the secondary β-IMCs was observed.

This work first reveals the growth mechanisms of different faceted IMCs with various morphologies. It also demonstrates that rotating the sample under a transversal magnetic field is a simple yet effective method of controlling the morphologies and volume distribution of crystals via altering the temperature and composition profiles in the melt.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):