Year

2023

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials, Mechatronic and Biomedical Engineering

Abstract

A bidirectional powder deposition strategy was employed to additively manufacture Ti-15Mo wt% using laser metal deposition. Phase identification, elemental analysis and microstructural characterisation were conducted in the As-Built condition and also after uniaxial tensile testing using X-ray diffraction and scanning electron microscopy along the different processing directions. In addition, electron backscattering diffraction and transmission electron microscopy were used to analyse deformation mechanisms. It was found that three distinct zones, namely the fusion, remelted and heat affected zones, evolved in all 25 deposited layers which predominantly comprised coarse columnar grains. These columnar grains were coarser up the build height due to increased distance from the substrate and slower cooling rates determined. Mo segregation was pronounced in the as-built microstructure. The fusion zone was the most solute enriched zone, followed by the remelted zone. The heat affected zone of each deposited layer featured inter-dendritic lamellas of molybdenum rich and lean inter-layers and this zone was the least solute-enriched. Deformation accommodation in β matrix was by a combination of slip, {332}〈113〉 and {112}〈111〉 twinning, α" martensite and ωD formation contrarily to the expected twinning.

The as-built alloy was subsequently subject to post-fabrication heat treatment. Microstructural characterisation was conducted in the heat-treated state and also after uniaxial tensile deformation. X-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy were employed in the heat-treated state. Electron backscatter diffraction was used in investigating the deformed microstructure. Columnar β grain refinement was achieved by fragmentation from a combined contribution from precipitated phases and deformation induced products. The three distinct microstructural zones, namely the fusion, remelted and heat affected zones, observed in each deposited layer of the as-built microstructure were retained after sub-β-solvus heat treatment but completely erased in the super-β-solvus microstructure. Accommodation of plastic deformation in β matrix was by a combination of slip and primary α" martensite which formed preferentially at grain boundaries. Elastic modulus decreased from 86.85±0.45 GPa in the as-built alloy to 72.8±0.65 GPa after heat treatment. Ultimate tensile strength of 1168± 1.12 MPa from the heat-treated sample represents only a marginal increase from that of the as-built sample of 1099± 2.3 MPa. This was accompanied by a small decrease in total elongation.

The as-built alloy was also subject to post-fabrication uniaxial thermomechanical processing at strain rates of 0.00055s-1, 0.0011s-1, 1s-1, and 4s-1 to strains of 20% and 40%. Experiments were conducted at room and elevated temperatures. Phase identification, elemental and microstructural characterisation were conducted using x-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The three distinct zones, namely the fusion, remelted and heat affected zones, identified in each deposited layer of the as-built microstructure were retained after thermomechanical processing. After processing, electron backscatter diffraction was used to analyse deformation mechanisms. Deformation accommodation in β matrix was predominantly by a combination of slip and α" martensite which formed as a primary product in parent β and at grain boundaries. However, the operation of {332}〈113〉 and {112}〈111〉 β-twinning was also determined, howbeit with a very small surface fraction. This implies a small surface fraction of secondary α" martensite forming within β-twins in the deformed microstructure. Compressive mechanical properties showed a strong dependence on strain rate as higher flow stress and compressive strength were obtained at higher strain rates.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.