title: Turning waste into useful materials: fabrication of titanium alloy matrix composite from the Ti-6Al-4V swarf

authors: Zhai, Yutao

subjects: Ti-6Al-4V

abstract: The rapid development of various high-precision technologies in the aerospace, military and civil fields has put forward higher and higher requirements for the performance of materials. Traditional titanium alloys have become inadequate in the face of the comprehensive requirements of the application environment, so composite materials have increasingly become the focus of material researchers. titanium boride (TiB) is the most attractive reinforcement because of the balance and stability of its thermal, physical, and mechanical properties. Boron is essentially insoluble in Ti and its alloys, so it does not cause lattice embrittlement. In this thesis, a Ti-6Al-4V matrix composite reinforced by in-situ synthesized TiB whiskers were successfully fabricated by hot pressing of a mixture of Ti-6Al-4V alloy swarf and boron powder.

Hot pressing temperature significantly affects the densification and properties of the fabricated titanium alloy matrix composites. With the increase of hot processing temperature from 1050ºC to 1250ºC, the porosity of the fabricated composites was decreased from 12.8% to 2.7%. When the hot-pressing temperature was reached to 1250ºC, the Ti-6Al-4V-3TiB composite could be successfully consolidated with a well-bonded prior swarf boundaries (no obvious voids were appeared at the prior swarf boundary) and less TiBx phases (most are TiB), leading to a reasonable ultimate tensile strength (1121MPa) and yield stress (1028MPa) were achieved.

The amount of boron addition (1vol%, 3vol%, 5vol%, 7vol%) influences the phase constitutions, microstructure, and mechanical properties of the fabricated swarf Ti-6Al-4V matrix composites hot-pressed at 1250°C. The UTS and YS of the fabricated composite was first increased when the boron addition was increased from 1vol% to 3vol%, and then decreased with the boron addition was further increased to 7vol%. However, the ductility of the fabricated composites was continually decreased with increasing the boron addition amount, and this is attributed to the increase amount of agglomerated TiB reinforcement phases. The Ti-6Al-4V-3TiB (adding 3vol% of boron) had the optimal mechanical properties among all of the fabricated composites, having a UTS of 1108MPa-1127MPa, YS of 1025MPa-1034MPa, and a strain to fracture of 3.45% - 3.98%.

Comparing to the hot-pressed Ti-6Al-4V-3TiB composite, The post-processed (extrusion at 950°C) composite had finer grain size and smaller lamellar spacing, which contributed to slightly increase the composite’s ductility to 4.18% - 5.19%, but the strengthen effect of TiB reinforcement effect was reduced due to the formation of voids between the reinforcing phase and the matrix phase introduced by severe plastic deformation.

Various heat treatments were carried out for the hot-pressed and hot extruded Ti-6Al-4V-3TiB composites to tailor the composite’s microstructure for achieving desired strength-ductility balance. After STA-955 (955℃ 1 hr water quenching + 550℃ 6hrs air cooling) and STA-855 (855℃ 1 hr water quenching + 550℃ 6hrs air cooling), the strength of hot-pressed and hot extruded composites were improved, due to the acicular secondary α phase and nano-size secondary α phase, respectively. The hot extruded composite had the best mechanical properties after STA-955, with a UTS of 1259MPa - 1263MPa, YS of 1165MPa - 1170MPa, and a strain to fracture of 3.15% - 3.28%.

text: Turning waste into useful materials: fabrication of titanium alloy matrix composite from the Ti-6Al-4V swarf The rapid development of various high-precision technologies in the aerospace, military and civil fields has put forward higher and higher requirements for the performance of materials. Traditional titanium alloys have become inadequate in the face of the comprehensive requirements of the application environment, so composite materials have increasingly become the focus of material researchers. titanium boride (TiB) is the most attractive reinforcement because of the balance and stability of its thermal, physical, and mechanical properties. Boron is essentially insoluble in Ti and its alloys, so it does not cause lattice embrittlement. In this thesis, a Ti-6Al-4V matrix composite reinforced by in-situ synthesized TiB whiskers were successfully fabricated by hot pressing of a mixture of Ti-6Al-4V alloy swarf and boron powder.

Hot pressing temperature significantly affects the densification and properties of the fabricated titanium alloy matrix composites. With the increase of hot processing temperature from 1050ºC to 1250ºC, the porosity of the fabricated composites was decreased from 12.8% to 2.7%. When the hot-pressing temperature was reached to 1250ºC, the Ti-6Al-4V-3TiB composite could be successfully consolidated with a well-bonded prior swarf boundaries (no obvious voids were appeared at the prior swarf boundary) and less TiBx phases (most are TiB), leading to a reasonable ultimate tensile strength (1121MPa) and yield stress (1028MPa) were achieved.

The amount of boron addition (1vol%, 3vol%, 5vol%, 7vol%) influences the phase constitutions, microstructure, and mechanical properties of the fabricated swarf Ti-6Al-4V matrix composites hot-pressed at 1250°C. The UTS and YS of the fabricated composite was first increased when the boron addition was increased from 1vol% to 3vol%, and then decreased with the boron addition was further increased to 7vol%. However, the ductility of the fabricated composites was continually decreased with increasing the boron addition amount, and this is attributed to the increase amount of agglomerated TiB reinforcement phases. The Ti-6Al-4V-3TiB (adding 3vol% of boron) had the optimal mechanical properties among all of the fabricated composites, having a UTS of 1108MPa-1127MPa, YS of 1025MPa-1034MPa, and a strain to fracture of 3.45% - 3.98%.

Comparing to the hot-pressed Ti-6Al-4V-3TiB composite, The post-processed (extrusion at 950°C) composite had finer grain size and smaller lamellar spacing, which contributed to slightly increase the composite’s ductility to 4.18% - 5.19%, but the strengthen effect of TiB reinforcement effect was reduced due to the formation of voids between the reinforcing phase and the matrix phase introduced by severe plastic deformation.

Various heat treatments were carried out for the hot-pressed and hot extruded Ti-6Al-4V-3TiB composites to tailor the composite’s microstructure for achieving desired strength-ductility balance. After STA-955 (955℃ 1 hr water quenching + 550℃ 6hrs air cooling) and STA-855 (855℃ 1 hr water quenching + 550℃ 6hrs air cooling), the strength of hot-pressed and hot extruded composites were improved, due to the acicular secondary α phase and nano-size secondary α phase, respectively. The hot extruded composite had the best mechanical properties after STA-955, with a UTS of 1259MPa - 1263MPa, YS of 1165MPa - 1170MPa, and a strain to fracture of 3.15% - 3.28%.

year: 2020

title: The interaction of buoyant river plumes with vegetation and consequences for sediment transport and deposition in coastal regions

authors: Vundavilli, Venkata Satya Sai Ram Hemanth

subjects: Buoyant river plumes

abstract: Mangrove forests are one of the most prominent vegetated coastal habits in tropical and subtropical areas. These wetland ecosystems typically inhabit estuaries and tidally influenced river banks. The presence of trees helps to protect coastal regions through the dissipation of tidal currents and wave energy, forming an essential barrier against coastal erosion. River plumes are the primary mechanism of sediment delivery to these vegetated coastlines. As the buoyant freshwater merges with saline ocean waters, horizontal advection of the freshwater establishes the shape of the river plume and influences the distribution of sediments along the coast. In this thesis, I investigate the principal dynamics associated with the interaction of buoyant river plumes with vegetation. In particular, we investigate the interplay between mangrove vegetation, hydrodynamics, and sediment dynamics, specifically: (1) the principal drivers of sediment deposition within a mangrove-lined river delta, (2) the influence of river forcing and winds on the sediment transport, and (3) sediment dynamics of two adjacent and interacting coastal river plumes.  

Understanding the hydro-morphodynamics of a river plume in a mangrove environment:
We developed a 3-D idealized Delft3D morphological model to explore the effects of vegetation on the river plumes and associated sediment transport patterns. Using an idealized model based on the Firth of Thames (FoT) mangrove forest located on the North Island of Aotearoa-New Zealand, we observed that, while sediment deposition occurred in the forest and tidal flats region of the model domain, the fringe region (between the vegetation and mudflat) experienced erosion. Compared to the eastern side, sediment deposition in the western side of the model domain was more prominent owing to the influence of Coriolis (Southern hemisphere). By examining the momentum balances on the surface and bottom layers in different regions of the river plume, we found that the principal balance between the bottom shear stress (enhanced by the presence of vegetation) and baroclinic pressure gradient largely controlled the sediment deposition in the riverine sections of the domain. Additionally, we found that vertical advection and diffusion during flood tide enhanced erosion in the fringe region of the mangrove forest. The advection of suspended sediment into the forest was controlled by factors including longer duration of high water slack at the forest fringe region, pressure gradients, and inertial acceleration. In the near-field region (close to the river mouth) of the river plume, during ebb tide, the barotropic and baroclinic pressure gradients coupled with Coriolis accelerations deliver sediments into the forest and mudflat regions. In the mid-field regions of the river plume, the magnitude of changes in the bed elevation was smaller due to larger Coriolis acceleration which arrested the spreading of the river plume. Furthermore, the far-field region (away from the river mouth) of the river plume experienced erosion due to more substantial tidal influence. Conversely, in the shallow forested regions, reduced movement of sediment in the offshore direction leads to an overall flood dominance.

Linking sediment transport within river plumes to varying river flows and winds:
Subsequent modeling experiments were carried out to investigate how the forcing factors of riverine discharge and wind velocities influence sediment transport within an idealized moderate-sized freshwater river plume. We assessed total sediment transport along with the relative contributions of riverine and bed-sourced sediment into a mangrove forest. Instantaneous and tidally averaged sediment fluxes were evaluated to investigate the critical linkages between the riverine flows, tidal influence, and winds in the presence of vegetation. In the near-field region of the river plume, riverine and bed-sourced sediment were dominant and found to be directed into the forest, indicative of an accretionary environment. In the mid- and far-field regions of the river plume, bed-sourced sediment was found to be the dominant contributor to total sediment transport and was directed out of the forest, indicative of sediment erosion. While the mass loads (directed into the mangrove forest) increased with an increase in riverine discharge, mass loads were relatively similar for large flow events. The river sediment delivered into the mangrove-forested regions was found to push into and through the forest front for large river discharges. The presence of 5 m/s wind velocities could alter the sediment deposition patterns in the mid- and far-field regions of the river plume. Despite the significant river momentum, strong winds (10 m/s) were also able to alter the sediment transport in the near-field region of the river plume. Moreover, results showed that in the far-field region of the river plume, strong winds combined with the effects of riverine discharge, Coriolis, and bed-shear stress affected the overall mass movement of sediment. In the case of easterly winds, a combination of tidal effects and wind stress controlled the sediment deposition into the forest. In the case of westerly winds, as the plume was pushed away from the forest, tidal effects controlled the total sediment transport. 


Understanding the effects of coalescing buoyant river plumes on sediment transport:
We investigated how the interaction between plumes from two mangrove-lined rivers in close proximity affected coastal sediment transport patterns. In particular, I examined the links between the river plume coalescence and the trapping of sediment by mangroves, using an idealized 3-dimensional numerical. Sediment fluxes and relative contributions of riverine- and bed-sourced sediments were quantified along the western, central (located between the rivers), and eastern mangrove forests. The two river plumes coalesced into a single river plume that flowed along the edge of the central mangrove forest. As the river plumes were deflected to the left by Coriolis (Southern hemisphere), the total sediment fluxes were largest for the central and the western forest, with only modest sediment deposition along the eastern mangrove forest. Also, modeled transport fluxes along the central mangrove forest were the largest close to the rivers, consistent with the satellite imagery of the FoT field site. Flow variations through either river resulted in changes to the sediment transport patterns into the mangrove forests. In particular, an increase in flow discharge through the western river resulted in reduced contributions of the eastern river towards the total sediment transport fluxes. Conversely, an increase in flow discharge through the eastern river inhibited the contribution of western river-associated sediment towards the total sediment transport flux through the central, eastern, and western mangrove forests. Furthermore, this study revealed that 5 m/s wind velocities altered the expansion, coalescence, and overall sediment transport fluxes of the river plumes. The total sediment fluxes through the central and western mangrove forests were greatest for easterly winds. However, the total transport into the eastern mangrove forest was largest for the southerly winds, as the bulge of the eastern river plume was pushed into mangrove forests.

These modeling experiments help elucidate the interaction of river plumes with mangrove vegetation. The work presented in this thesis highlights the complex non-linear interactions of hydro-morphodynamics, sediment transport, the effects of river flows and winds within mangrove environments, and the fate of riverine sediments in the bay of the Firth of Thames. Finally, the results presented here provide insights into the sediment trapping capabilities of mangroves under variable conditions and thus may help to predict their ability to function as a coastal defense mechanism under future sea-level rise and sediment supply scenarios.

text: The interaction of buoyant river plumes with vegetation and consequences for sediment transport and deposition in coastal regions Mangrove forests are one of the most prominent vegetated coastal habits in tropical and subtropical areas. These wetland ecosystems typically inhabit estuaries and tidally influenced river banks. The presence of trees helps to protect coastal regions through the dissipation of tidal currents and wave energy, forming an essential barrier against coastal erosion. River plumes are the primary mechanism of sediment delivery to these vegetated coastlines. As the buoyant freshwater merges with saline ocean waters, horizontal advection of the freshwater establishes the shape of the river plume and influences the distribution of sediments along the coast. In this thesis, I investigate the principal dynamics associated with the interaction of buoyant river plumes with vegetation. In particular, we investigate the interplay between mangrove vegetation, hydrodynamics, and sediment dynamics, specifically: (1) the principal drivers of sediment deposition within a mangrove-lined river delta, (2) the influence of river forcing and winds on the sediment transport, and (3) sediment dynamics of two adjacent and interacting coastal river plumes.  

Understanding the hydro-morphodynamics of a river plume in a mangrove environment:
We developed a 3-D idealized Delft3D morphological model to explore the effects of vegetation on the river plumes and associated sediment transport patterns. Using an idealized model based on the Firth of Thames (FoT) mangrove forest located on the North Island of Aotearoa-New Zealand, we observed that, while sediment deposition occurred in the forest and tidal flats region of the model domain, the fringe region (between the vegetation and mudflat) experienced erosion. Compared to the eastern side, sediment deposition in the western side of the model domain was more prominent owing to the influence of Coriolis (Southern hemisphere). By examining the momentum balances on the surface and bottom layers in different regions of the river plume, we found that the principal balance between the bottom shear stress (enhanced by the presence of vegetation) and baroclinic pressure gradient largely controlled the sediment deposition in the riverine sections of the domain. Additionally, we found that vertical advection and diffusion during flood tide enhanced erosion in the fringe region of the mangrove forest. The advection of suspended sediment into the forest was controlled by factors including longer duration of high water slack at the forest fringe region, pressure gradients, and inertial acceleration. In the near-field region (close to the river mouth) of the river plume, during ebb tide, the barotropic and baroclinic pressure gradients coupled with Coriolis accelerations deliver sediments into the forest and mudflat regions. In the mid-field regions of the river plume, the magnitude of changes in the bed elevation was smaller due to larger Coriolis acceleration which arrested the spreading of the river plume. Furthermore, the far-field region (away from the river mouth) of the river plume experienced erosion due to more substantial tidal influence. Conversely, in the shallow forested regions, reduced movement of sediment in the offshore direction leads to an overall flood dominance.

Linking sediment transport within river plumes to varying river flows and winds:
Subsequent modeling experiments were carried out to investigate how the forcing factors of riverine discharge and wind velocities influence sediment transport within an idealized moderate-sized freshwater river plume. We assessed total sediment transport along with the relative contributions of riverine and bed-sourced sediment into a mangrove forest. Instantaneous and tidally averaged sediment fluxes were evaluated to investigate the critical linkages between the riverine flows, tidal influence, and winds in the presence of vegetation. In the near-field region of the river plume, riverine and bed-sourced sediment were dominant and found to be directed into the forest, indicative of an accretionary environment. In the mid- and far-field regions of the river plume, bed-sourced sediment was found to be the dominant contributor to total sediment transport and was directed out of the forest, indicative of sediment erosion. While the mass loads (directed into the mangrove forest) increased with an increase in riverine discharge, mass loads were relatively similar for large flow events. The river sediment delivered into the mangrove-forested regions was found to push into and through the forest front for large river discharges. The presence of 5 m/s wind velocities could alter the sediment deposition patterns in the mid- and far-field regions of the river plume. Despite the significant river momentum, strong winds (10 m/s) were also able to alter the sediment transport in the near-field region of the river plume. Moreover, results showed that in the far-field region of the river plume, strong winds combined with the effects of riverine discharge, Coriolis, and bed-shear stress affected the overall mass movement of sediment. In the case of easterly winds, a combination of tidal effects and wind stress controlled the sediment deposition into the forest. In the case of westerly winds, as the plume was pushed away from the forest, tidal effects controlled the total sediment transport. 


Understanding the effects of coalescing buoyant river plumes on sediment transport:
We investigated how the interaction between plumes from two mangrove-lined rivers in close proximity affected coastal sediment transport patterns. In particular, I examined the links between the river plume coalescence and the trapping of sediment by mangroves, using an idealized 3-dimensional numerical. Sediment fluxes and relative contributions of riverine- and bed-sourced sediments were quantified along the western, central (located between the rivers), and eastern mangrove forests. The two river plumes coalesced into a single river plume that flowed along the edge of the central mangrove forest. As the river plumes were deflected to the left by Coriolis (Southern hemisphere), the total sediment fluxes were largest for the central and the western forest, with only modest sediment deposition along the eastern mangrove forest. Also, modeled transport fluxes along the central mangrove forest were the largest close to the rivers, consistent with the satellite imagery of the FoT field site. Flow variations through either river resulted in changes to the sediment transport patterns into the mangrove forests. In particular, an increase in flow discharge through the western river resulted in reduced contributions of the eastern river towards the total sediment transport fluxes. Conversely, an increase in flow discharge through the eastern river inhibited the contribution of western river-associated sediment towards the total sediment transport flux through the central, eastern, and western mangrove forests. Furthermore, this study revealed that 5 m/s wind velocities altered the expansion, coalescence, and overall sediment transport fluxes of the river plumes. The total sediment fluxes through the central and western mangrove forests were greatest for easterly winds. However, the total transport into the eastern mangrove forest was largest for the southerly winds, as the bulge of the eastern river plume was pushed into mangrove forests.

These modeling experiments help elucidate the interaction of river plumes with mangrove vegetation. The work presented in this thesis highlights the complex non-linear interactions of hydro-morphodynamics, sediment transport, the effects of river flows and winds within mangrove environments, and the fate of riverine sediments in the bay of the Firth of Thames. Finally, the results presented here provide insights into the sediment trapping capabilities of mangroves under variable conditions and thus may help to predict their ability to function as a coastal defense mechanism under future sea-level rise and sediment supply scenarios.

year: 2023

title: Re-creating legal space for the first law of Aotearoa-New Zealand

authors: Joseph, Robert

subjects: law

abstract: Some of the historic and contemporary statutory and case law examples of a hybrid Maori-British common law legal system in Aotearoa-New Zealand are discussed. The importance and role performed by the First Law, the tikanga Maori customary law is highlighted.

text: Re-creating legal space for the first law of Aotearoa-New Zealand Some of the historic and contemporary statutory and case law examples of a hybrid Maori-British common law legal system in Aotearoa-New Zealand are discussed. The importance and role performed by the First Law, the tikanga Maori customary law is highlighted.

year: 2009