title: Consolidation of Al-TiO₂ and Ti₃Al-Ai₂O₃ composite powders

authors: Han, Ching Zen

abstract: Pressureless sintering, hot pressing and sinter-HIP of Ti₃Al-Al₂O₃ and Al-TiO₂ composite powders have been carried out under inert atmospheres. The Al-TiO₂ composite powders were produced by discus milling Al/TiO₂  powder mixtures followed by ball milling. The Ti₃Al-Al₂O₃ composite powders were produced by heating Al/TiO₂ powder mixtures. Densification, sintering behaviour, microstructural development and mechanical properties of the composites were investigated for the variables of pressure and temperature at different powder particle sizes. It was shown that higher bulk density and lower porosity were obtained by hot pressing compared to pressureless sintering. A temperature of 1600°C is required to achieve >95% theoretical density of the composites when fabricated by pressureless sintering Ti₃Al-Al₂O₃ composite powder. However, fully densified composites could be fabricated by hot pressing l.2μm Ti₃Al-Al₂O₃ composite powder at 1474°C under vacuum for a one hour holding period. The growth of Al₂O₃ particle size and the development of microcracks at the Ti₃Al-Al₂O₃ interfaces were evident with the increasing temperature above 1500°C for the composites produced by pressureless sintering and hot pressing. 

Near fully densified Ti₃Al-TiO-Al₂O₃ and Ti₃Al-Al₂O₃ composites (96% of theoretical density) with ≤4μm Al₂O₃ particle size in the composites could be achieved by pressureless sintering Al-TiO₂ composite powder at 1480°C in argon for 4-5 hours of holding period. Fully dense Ti₃Al-Al₂O₃ composite microstructures could be achieved by hot pressing four hours discus milled composite powder subjected to 24 hours of ball milling giving a starting median particle diameter of 7μm, at 1387°C under vacuum. The XRD traces show that the Ti₃Al phase in the Ti₃Al-Al₂O₃ composites is maintained above 50% XRD peak intensity up to 1500°C. The composite microstructure demonstrates that both the Al₂O₃ and Ti₃Al phases are continuous and exhibit an interpenetrating network. 

Near fully densified Ti₃Al-TiO-Al₂O₃ composites (≤98% of theoretical density) were fabricated by sinter-HIP of Al-TiO₂ composite powders with a starting median particle diameter of 7.7μm. The coarsening of the Al₂O₃ particles was suppressed after hot isostatic pressing at 1350°C in argon for two hours at 200MPa for the Ti₃Al-TiO-Al₂O₃ composites prepared by pressureless sintering Al-TiO₂ composite powder at 1480°C. 

Vickers hardness of the composites increased with increasing densification and increased Ah03 ceramic phase content. Vickers hardness of ≤17.5GPa was achieved for the fully densified Ti₃Al-TiO-Al₂O₃ composite whereas Vickers hardness of ≤15GPa was achieved for the fully densified Ti₃Al-Al₂O₃ composites with Ah03 particle size of ≤4μm. In the sinter-HIP experiments, the Vickers hardness increased by up to 2GPa due to the decreased total porosity after hot isostatic pressing of the pre-sintered Ti₃Al-TiO-Al₂O₃ composites with 96% theoretical density. Biaxial strength of ≤350MPa and ≤278MPa were achieved for Ti₃Al-TiO-Al₂O₃ and Ti₃Al-Al₂O₃  composites, respectively. In this study, fracture toughness as high as 7.5±0.8MPa.m½ was obtained when the phase structure exhibited fine (:'.S4μm) and homogeneous microstructure with 50% XRD peak intensity of Ti₃Al ductile phase in
the matrix.

text: Consolidation of Al-TiO₂ and Ti₃Al-Ai₂O₃ composite powders Pressureless sintering, hot pressing and sinter-HIP of Ti₃Al-Al₂O₃ and Al-TiO₂ composite powders have been carried out under inert atmospheres. The Al-TiO₂ composite powders were produced by discus milling Al/TiO₂  powder mixtures followed by ball milling. The Ti₃Al-Al₂O₃ composite powders were produced by heating Al/TiO₂ powder mixtures. Densification, sintering behaviour, microstructural development and mechanical properties of the composites were investigated for the variables of pressure and temperature at different powder particle sizes. It was shown that higher bulk density and lower porosity were obtained by hot pressing compared to pressureless sintering. A temperature of 1600°C is required to achieve >95% theoretical density of the composites when fabricated by pressureless sintering Ti₃Al-Al₂O₃ composite powder. However, fully densified composites could be fabricated by hot pressing l.2μm Ti₃Al-Al₂O₃ composite powder at 1474°C under vacuum for a one hour holding period. The growth of Al₂O₃ particle size and the development of microcracks at the Ti₃Al-Al₂O₃ interfaces were evident with the increasing temperature above 1500°C for the composites produced by pressureless sintering and hot pressing. 

Near fully densified Ti₃Al-TiO-Al₂O₃ and Ti₃Al-Al₂O₃ composites (96% of theoretical density) with ≤4μm Al₂O₃ particle size in the composites could be achieved by pressureless sintering Al-TiO₂ composite powder at 1480°C in argon for 4-5 hours of holding period. Fully dense Ti₃Al-Al₂O₃ composite microstructures could be achieved by hot pressing four hours discus milled composite powder subjected to 24 hours of ball milling giving a starting median particle diameter of 7μm, at 1387°C under vacuum. The XRD traces show that the Ti₃Al phase in the Ti₃Al-Al₂O₃ composites is maintained above 50% XRD peak intensity up to 1500°C. The composite microstructure demonstrates that both the Al₂O₃ and Ti₃Al phases are continuous and exhibit an interpenetrating network. 

Near fully densified Ti₃Al-TiO-Al₂O₃ composites (≤98% of theoretical density) were fabricated by sinter-HIP of Al-TiO₂ composite powders with a starting median particle diameter of 7.7μm. The coarsening of the Al₂O₃ particles was suppressed after hot isostatic pressing at 1350°C in argon for two hours at 200MPa for the Ti₃Al-TiO-Al₂O₃ composites prepared by pressureless sintering Al-TiO₂ composite powder at 1480°C. 

Vickers hardness of the composites increased with increasing densification and increased Ah03 ceramic phase content. Vickers hardness of ≤17.5GPa was achieved for the fully densified Ti₃Al-TiO-Al₂O₃ composite whereas Vickers hardness of ≤15GPa was achieved for the fully densified Ti₃Al-Al₂O₃ composites with Ah03 particle size of ≤4μm. In the sinter-HIP experiments, the Vickers hardness increased by up to 2GPa due to the decreased total porosity after hot isostatic pressing of the pre-sintered Ti₃Al-TiO-Al₂O₃ composites with 96% theoretical density. Biaxial strength of ≤350MPa and ≤278MPa were achieved for Ti₃Al-TiO-Al₂O₃ and Ti₃Al-Al₂O₃  composites, respectively. In this study, fracture toughness as high as 7.5±0.8MPa.m½ was obtained when the phase structure exhibited fine (:'.S4μm) and homogeneous microstructure with 50% XRD peak intensity of Ti₃Al ductile phase in
the matrix.

year: 2005

title: Online estimation of discrete densities using classifier chains

authors: Geilke, Michael

subjects: computer science

abstract: We propose an approach to estimate a discrete joint density online, that is, the algorithm is only provided the current example, its current estimate, and a limited amount of memory. To design an online estimator for discrete densities, we use classifier chains to model dependencies among features. Each classifier in the chain estimates the probability of one particular feature. Because a single chain may not provide a reliable estimate, we also consider ensembles of classifier chains. Our experiments on synthetic data show that the approach is feasible and the estimated densities approach the true, known distribution with increasing amounts of data.

text: Online estimation of discrete densities using classifier chains We propose an approach to estimate a discrete joint density online, that is, the algorithm is only provided the current example, its current estimate, and a limited amount of memory. To design an online estimator for discrete densities, we use classifier chains to model dependencies among features. Each classifier in the chain estimates the probability of one particular feature. Because a single chain may not provide a reliable estimate, we also consider ensembles of classifier chains. Our experiments on synthetic data show that the approach is feasible and the estimated densities approach the true, known distribution with increasing amounts of data.

year: 2012

title: Young Maori males experiences in a secondary school context as sporting, academic and cultural identities.

authors: Porteous, Erin

subjects: Male Maori

abstract: This research focused on the journey of male Māori through secondary school and the effect on their identities in terms of sport, culture and academia. The two key areas of focus were how the culture of the school, including cultural stereotypes, impacts on male Māori learner self-concept, and whether the effect of creating an all Māori boys form class has resulted in an increased sense of belonging and engagement. 
The research was conducted as a case study underpinned by an interpretive methodology, using a focus group interview. The group was made up of five boys, chosen from a newly formed all boys’ Year 11 Māori form class. The boys were selected in conjunction with their form teacher and the schools Kaitiaki, in order to reflect a range of sporting, academic and cultural levels. 
The research findings found that although the boys view of their experiences in school seemed positive overall, they described common incidences and reactions to racial discrimination. The exposure to embedded racial stereotypes and behaviours caused them to create their own solutions, often through threatening violence or aggressive reactions. It appears that when young male Māori are more closely connected to Māori culture, they are more likely they are to create these responses. 
Teachers remain integral to the engagement and belonging of a young male Māori in secondary school. In support of the findings from Kia Eke Panuku (2013), this study found that male Māori require positive relationships with their teachers to engage in the subject and school in a general sense. Relationships based on ako and manaakitanga are key to these students viewing their experiences in school as successful.
Another finding showed the importance of sport and physical activity to young male Māori, though success in this area has been a limiting factor in contributing to overall success in secondary school. The placing of male Māori as a ‘physical being’ (Hokowhitu, 2003a; Hokowhitu, 2003b; Hokowhitu, 2004) creates spaces where Māori can be successful, such as practical based subjects like physical education. This success is perceived as acceptable yet often isolates them away from success in other areas (Erueti & Palmer, 2014), feeding into the stereotype itself.
The success of the all boys’ Māori form class was scrutinised by this study, and found the intended outcomes had not necessarily been met. The overall impact was that the boys had been grouped with a Māori form teacher and the intentions of additional support had been overlooked due to time pressures and deficit thinking in terms of capabilities of the class members.  For the Māori boys in this study, the challenges of overcome historical cultural stereotypes added to the challenge of succeeding at secondary school. The expectation that they walk successfully between the realms of Māori and pākehā to achieve in the current education system coupled with the feeling they needed to act ‘pākehā’ in order to achieve is particularly disturbing. The inequity for them in denying part of their identity seems a hefty price to pay for educational success.
This study highlights the need for schools, communities and individual teachers to recognise these racial and systemic inequities and work tirelessly to develop strategies to eliminate them, in order to create to take a giant step towards equity for all New Zealanders within education.

text: Young Maori males experiences in a secondary school context as sporting, academic and cultural identities. This research focused on the journey of male Māori through secondary school and the effect on their identities in terms of sport, culture and academia. The two key areas of focus were how the culture of the school, including cultural stereotypes, impacts on male Māori learner self-concept, and whether the effect of creating an all Māori boys form class has resulted in an increased sense of belonging and engagement. 
The research was conducted as a case study underpinned by an interpretive methodology, using a focus group interview. The group was made up of five boys, chosen from a newly formed all boys’ Year 11 Māori form class. The boys were selected in conjunction with their form teacher and the schools Kaitiaki, in order to reflect a range of sporting, academic and cultural levels. 
The research findings found that although the boys view of their experiences in school seemed positive overall, they described common incidences and reactions to racial discrimination. The exposure to embedded racial stereotypes and behaviours caused them to create their own solutions, often through threatening violence or aggressive reactions. It appears that when young male Māori are more closely connected to Māori culture, they are more likely they are to create these responses. 
Teachers remain integral to the engagement and belonging of a young male Māori in secondary school. In support of the findings from Kia Eke Panuku (2013), this study found that male Māori require positive relationships with their teachers to engage in the subject and school in a general sense. Relationships based on ako and manaakitanga are key to these students viewing their experiences in school as successful.
Another finding showed the importance of sport and physical activity to young male Māori, though success in this area has been a limiting factor in contributing to overall success in secondary school. The placing of male Māori as a ‘physical being’ (Hokowhitu, 2003a; Hokowhitu, 2003b; Hokowhitu, 2004) creates spaces where Māori can be successful, such as practical based subjects like physical education. This success is perceived as acceptable yet often isolates them away from success in other areas (Erueti & Palmer, 2014), feeding into the stereotype itself.
The success of the all boys’ Māori form class was scrutinised by this study, and found the intended outcomes had not necessarily been met. The overall impact was that the boys had been grouped with a Māori form teacher and the intentions of additional support had been overlooked due to time pressures and deficit thinking in terms of capabilities of the class members.  For the Māori boys in this study, the challenges of overcome historical cultural stereotypes added to the challenge of succeeding at secondary school. The expectation that they walk successfully between the realms of Māori and pākehā to achieve in the current education system coupled with the feeling they needed to act ‘pākehā’ in order to achieve is particularly disturbing. The inequity for them in denying part of their identity seems a hefty price to pay for educational success.
This study highlights the need for schools, communities and individual teachers to recognise these racial and systemic inequities and work tirelessly to develop strategies to eliminate them, in order to create to take a giant step towards equity for all New Zealanders within education.

year: 2016