title: Effect of thermomechanical processing on the mechanical properties of cost-effective powder metallurgy titanium-based materials

authors: Romero Villarreal, Carlos

subjects: Titanium alloys

abstract: Despite their excellent balance of properties, titanium alloys are still a niche material whose range of applications is limited by the high cost of their manufacturing. With recent developments, powder metallurgy (PM) approaches, in particular the blended elemental (BE) route, are able to reduce considerably the cost of manufacturing while keeping a reasonable performance, mostly in terms of tensile strength and ductility. The cost reduction of the BE approach derives from the simplification of the process by removing a large amount of re-melting and ingot breakdown operations, regardless of the use of the approach as a net-shape technology. However, in terms of fatigue behaviour, the performance of these alloys is generally worse than conventional processes unless additional steps are applied. This thesis aims to study the mechanical behaviour, with emphasis on the fatigue behaviour, of two cost-effective titanium alloys in order to find the key processing, microstructural and texture parameters that control their performance, with the goal of achieving similar properties to conventional wrought alloys with a reduction of the cost. These alloys, a conventional Ti-6Al-4V composition and a non-conventional low-cost Ti-5Fe composition, were processed following the blended elemental approach and thermomechanical processing, via hot extrusion, was subsequently applied. A variety of microstructures and textures were achieved under different extrusion conditions, which were further modified by applying additional heat treatments.
The tensile behaviour of both Ti-6Al-4V and Ti-5Fe alloys is strongly dependent on the processing conditions. Fully lamellar microstructures (from processing in the β phase) showed similar strength levels but considerably lower ductility compared to alloys with bimodal microstructures and stronger texture (from processing in the α+β phase). In the case of Ti-5Fe, moreover, there was a Fe partitioning effect that, when the processing temperature is low enough, leads to a sharp increase in both the strength and the ability to deform past necking took place. Heat treatments in Ti-6Al-4V alloys were useful in order to increase the strength as well as to keep the ductility similar or even increase it to higher values. The tensile behaviour of Ti-5Fe alloys was more tuneable, achieving higher differences in strength and ductility with the as-extruded conditions, even though this happened by trading-off. Overall, Ti-6Al-4V alloys outperformed Ti-5Fe alloys as their strength was typically higher (associated to their higher amount of alloying elements) as well as their ductility, due to their higher ability to deform past necking, which was very limited for Ti-5Fe alloys in most cases.
The fatigue behaviour of the alloys was found to as dependent on the chemistry as on the microstructure and the texture. While the Ti-6Al-4V alloy is not affected by the residual porosity left after thermomechanical processing, the Ti-5Fe alloy shows pores in the region where fatigue damage begins. In the Ti-6Al-4V alloy, it was found that texturing had a much stronger effect on the fatigue strength of the alloy than microstructural refinement. Therefore, the PM Ti-6Al-4V alloy processed in the α+β phase had a higher fatigue strength than alloys processed in the β phase, regardless if the latter has fully lamellar or acicular microstructure. In the case of the Ti-5Fe alloy, two conditions processed in the α+β phase were studied, and it was found that the Fe concentration in transformed β (which is higher when there is a larger amount of primary α) had a critical effect on the fatigue behaviour, overriding the effect of texture or microstructural refinement. This leads to the Ti-5Fe alloy processed at a lower temperature in the α+β phase showing a much lower fatigue strength compared with the alloy processed just below the β transus, despite the fact that its texture and microstructure were, in theory, optimal for fatigue behaviour.
It is concluded that, in order to process cost-effective Ti alloys with fatigue performance comparable to wrought alloys with conventional compositions (with fatigue strength in the range of 650 MPa), thermomechanical processing in the α+β phase must be applied to make use of the strong texturing of the alloy. However, in the case of alloys containing β-eutectoid elements like Fe, the range of temperatures is limited as the partitioning of the alloying element results in severe reduction of the fatigue strength.

text: Effect of thermomechanical processing on the mechanical properties of cost-effective powder metallurgy titanium-based materials Despite their excellent balance of properties, titanium alloys are still a niche material whose range of applications is limited by the high cost of their manufacturing. With recent developments, powder metallurgy (PM) approaches, in particular the blended elemental (BE) route, are able to reduce considerably the cost of manufacturing while keeping a reasonable performance, mostly in terms of tensile strength and ductility. The cost reduction of the BE approach derives from the simplification of the process by removing a large amount of re-melting and ingot breakdown operations, regardless of the use of the approach as a net-shape technology. However, in terms of fatigue behaviour, the performance of these alloys is generally worse than conventional processes unless additional steps are applied. This thesis aims to study the mechanical behaviour, with emphasis on the fatigue behaviour, of two cost-effective titanium alloys in order to find the key processing, microstructural and texture parameters that control their performance, with the goal of achieving similar properties to conventional wrought alloys with a reduction of the cost. These alloys, a conventional Ti-6Al-4V composition and a non-conventional low-cost Ti-5Fe composition, were processed following the blended elemental approach and thermomechanical processing, via hot extrusion, was subsequently applied. A variety of microstructures and textures were achieved under different extrusion conditions, which were further modified by applying additional heat treatments.
The tensile behaviour of both Ti-6Al-4V and Ti-5Fe alloys is strongly dependent on the processing conditions. Fully lamellar microstructures (from processing in the β phase) showed similar strength levels but considerably lower ductility compared to alloys with bimodal microstructures and stronger texture (from processing in the α+β phase). In the case of Ti-5Fe, moreover, there was a Fe partitioning effect that, when the processing temperature is low enough, leads to a sharp increase in both the strength and the ability to deform past necking took place. Heat treatments in Ti-6Al-4V alloys were useful in order to increase the strength as well as to keep the ductility similar or even increase it to higher values. The tensile behaviour of Ti-5Fe alloys was more tuneable, achieving higher differences in strength and ductility with the as-extruded conditions, even though this happened by trading-off. Overall, Ti-6Al-4V alloys outperformed Ti-5Fe alloys as their strength was typically higher (associated to their higher amount of alloying elements) as well as their ductility, due to their higher ability to deform past necking, which was very limited for Ti-5Fe alloys in most cases.
The fatigue behaviour of the alloys was found to as dependent on the chemistry as on the microstructure and the texture. While the Ti-6Al-4V alloy is not affected by the residual porosity left after thermomechanical processing, the Ti-5Fe alloy shows pores in the region where fatigue damage begins. In the Ti-6Al-4V alloy, it was found that texturing had a much stronger effect on the fatigue strength of the alloy than microstructural refinement. Therefore, the PM Ti-6Al-4V alloy processed in the α+β phase had a higher fatigue strength than alloys processed in the β phase, regardless if the latter has fully lamellar or acicular microstructure. In the case of the Ti-5Fe alloy, two conditions processed in the α+β phase were studied, and it was found that the Fe concentration in transformed β (which is higher when there is a larger amount of primary α) had a critical effect on the fatigue behaviour, overriding the effect of texture or microstructural refinement. This leads to the Ti-5Fe alloy processed at a lower temperature in the α+β phase showing a much lower fatigue strength compared with the alloy processed just below the β transus, despite the fact that its texture and microstructure were, in theory, optimal for fatigue behaviour.
It is concluded that, in order to process cost-effective Ti alloys with fatigue performance comparable to wrought alloys with conventional compositions (with fatigue strength in the range of 650 MPa), thermomechanical processing in the α+β phase must be applied to make use of the strong texturing of the alloy. However, in the case of alloys containing β-eutectoid elements like Fe, the range of temperatures is limited as the partitioning of the alloying element results in severe reduction of the fatigue strength.

year: 2020

title: A molecular-based assessment tool for characterising New Zealand freshwater zooplankton communities

authors: Watson, Nathan Thomas Noble

abstract: The ability to adequately assess ecosystem health is essential for informed resource management. Freshwater zooplankton respond rapidly to environmental changes in pest fish populations and nutrient loads and can therefore be used to monitor ecosystem health and provide a surrogate for lake biodiversity. The Zooplankton Molecular-Based Assessment (ZooMBA) described here is a technique for assessing zooplankton communities using short fragments of DNA sequences and a recently developed, online database of reference sequences ("DNA barcodes"). Users can collect their own zooplankton samples using standard collection techniques and either pre-process samples or send samples directly to appropriate laboratory facilities for molecular analyses. Resulting data can then be used to provide accurate species inventories, or cumulatively, can be used to compute indices of lake trophic status (e.g. rotifer Trophic Level Index).

text: A molecular-based assessment tool for characterising New Zealand freshwater zooplankton communities The ability to adequately assess ecosystem health is essential for informed resource management. Freshwater zooplankton respond rapidly to environmental changes in pest fish populations and nutrient loads and can therefore be used to monitor ecosystem health and provide a surrogate for lake biodiversity. The Zooplankton Molecular-Based Assessment (ZooMBA) described here is a technique for assessing zooplankton communities using short fragments of DNA sequences and a recently developed, online database of reference sequences ("DNA barcodes"). Users can collect their own zooplankton samples using standard collection techniques and either pre-process samples or send samples directly to appropriate laboratory facilities for molecular analyses. Resulting data can then be used to provide accurate species inventories, or cumulatively, can be used to compute indices of lake trophic status (e.g. rotifer Trophic Level Index).

year: 2014

title: Exploring the experiences of young Māori mothers

authors: Shepherd, Cindy-Joe Theresa Aroha

subjects: Young Māori mothers

abstract: This thesis explores the lived experiences of three young Māori mothers (aged 16 
years old) in Aotearoa, New Zealand, focusing on their strengths, challenges, and resilience. 
By employing a Kaupapa Māori methodological framework, the research aims to reframe 
narratives about young Māori mothers towards a strengths-based perspective, advocating for 
protection and nurturing Māori whakapapa. Using qualitative methods, the study explores the 
pūrākau (stories) of three young Māori mothers attending a teen parent unit in Waikato. 
The study highlights how, by living within both individualistic and collective realms, 
these young mothers actively challenge stigma, assert their worth, and develop unique 
underlying strengths. The findings, structured around emergent themes from their collective 
experiences, reveal a dichotomy: these young mothers must navigate the individualistic 
pressures and high expectations of motherhood while also drawing strength from collective 
responsibilities upheld within their whānau and supportive community services. Despite 
facing significant societal stigma and systemic barriers, the mothers demonstrate immense 
strength. Their resilience is grounded in whānau connections, proactive self-care, and 
engagement with taiao, wairua, and tino rangatiratanga—processes that also support 
intergenerational healing. 
This research underscores the importance of culturally responsive support systems 
that honour Indigenous knowledge and collective well-being. It offers insights for reorienting 
policy and practice to empower young Māori mothers, recognising their tino rangatiratanga 
and transformative agency by embracing their multifaceted realities.

text: Exploring the experiences of young Māori mothers This thesis explores the lived experiences of three young Māori mothers (aged 16 
years old) in Aotearoa, New Zealand, focusing on their strengths, challenges, and resilience. 
By employing a Kaupapa Māori methodological framework, the research aims to reframe 
narratives about young Māori mothers towards a strengths-based perspective, advocating for 
protection and nurturing Māori whakapapa. Using qualitative methods, the study explores the 
pūrākau (stories) of three young Māori mothers attending a teen parent unit in Waikato. 
The study highlights how, by living within both individualistic and collective realms, 
these young mothers actively challenge stigma, assert their worth, and develop unique 
underlying strengths. The findings, structured around emergent themes from their collective 
experiences, reveal a dichotomy: these young mothers must navigate the individualistic 
pressures and high expectations of motherhood while also drawing strength from collective 
responsibilities upheld within their whānau and supportive community services. Despite 
facing significant societal stigma and systemic barriers, the mothers demonstrate immense 
strength. Their resilience is grounded in whānau connections, proactive self-care, and 
engagement with taiao, wairua, and tino rangatiratanga—processes that also support 
intergenerational healing. 
This research underscores the importance of culturally responsive support systems 
that honour Indigenous knowledge and collective well-being. It offers insights for reorienting 
policy and practice to empower young Māori mothers, recognising their tino rangatiratanga 
and transformative agency by embracing their multifaceted realities.

year: 2024-06-23