title: Processing, microstructure and mechanical properties of TiₓAlᵧ/Al₂O₃ and Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC composites

authors: Cai, Z.H.

abstract: The TiₓAlᵧ/Al₂O₃ metal-ceramic composites materials [TiₓAlᵧ is α-Ti(Al,O), Ti₃Al, or TiAl] and Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC ceramic-ceramic composites with different volume fractions of ceramic reinforcement phases were synthesized using a complex combination of high energy mechanical milling, thermal treatment and powder consolidation processes. The thermal treatment process included reactions between carbon and TiO₂, and between Al and TiO₂, TiO₃ or TiOₙC₁₋ₙ, which was produced by reacting TiO₂ and carbon. Heat treating C/TiO₂ composite powder produced pure TiO₃. Pure TiO could not be obtained by only increasing the carbon to TiO₂ ratio in the C/TiO₂ composite powder. Instead, TiOₙC₁₋ₙ solid solution was formed. Reaction kinetic studies showed that higher carbon to TiO₂ ratios accelerated the reaction due to refinement of the C/TiO₂ composite structure. Reactions between Al and TiOₙC₁₋ₙ formed Al₂Ti₄C₂/Al₂O₃ ceramic-ceramic composite. When the aluminum content of the starting Al/TiOₙC₁₋ₙ composite powders was increased, Al₂Ti₄C₂ still formed, but small amounts of Ti₃Al or TiAl also formed. Producing TiC from Al and TiOₙC₁₋ₙ reaction depended on the value of n in the TiOₙC₁₋ₙ phase. When n is small enough, TiC formed; otherwise TiC did not form.

This study found that fully dense matrix phase of TiₓAlᵧ/Al₂O₃ and Al₂Ti₄C2/TiₓAlᵧ/Al₂O₃/TiC formed after sintering at 1550°C. The Al₂O₃ particles became fully sintered when the composites were sintered at 1650°C but not at 1550°C. The Al₂O₃ particles in the α-Ti(Al,O) and Ti₃Al based composites had a high coarsening rate above 1550°C, indicating that the oxygen and Al diffusion rates of α-Ti(Al,O) and Ti₃Al are very high at these temperatures. The Al₂O₃ particles in TiAl based composites also had a high coarsening rate above 1550°C, caused by TiAl being liquid at these temperatures. The Al₂O₃ particles in the Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC based composites had a much lower coarsening rate because of the Al₂Ti₄C₂ matrix the lower oxygen content and high melting point decreases oxygen and aluminum mobility.

The size and volume fraction of Al₂O₃ particles in the composites strongly affected the mechanical properties of the TiₓAlᵧ and Al₂Ti₄C₂ based composites. Hardness of α-Ti(Al,O), Ti₃Al, and Al₂Ti₄C₂ based composites was 800-1600 VHN and hardness of TiAl based composites was 150-500 VHN. Bending strength of TiₓAlᵧ/Al₂O₃ composites was 105-230 MPa and bending strength of Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC based composites was 250-460 MPa. It appeared that 10 μm might be the critical size of Al₂O₃ particles. Below 10 μm bending strength dramatically improved with deceasing the Al₂O₃ particle size in the composites. The fracture toughness of these composites was 2-4.5MPa•m¹/². Composite hardness increased and bending strength decreased with increasing Al₂O₃ particle size and volume fraction.

text: Processing, microstructure and mechanical properties of TiₓAlᵧ/Al₂O₃ and Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC composites The TiₓAlᵧ/Al₂O₃ metal-ceramic composites materials [TiₓAlᵧ is α-Ti(Al,O), Ti₃Al, or TiAl] and Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC ceramic-ceramic composites with different volume fractions of ceramic reinforcement phases were synthesized using a complex combination of high energy mechanical milling, thermal treatment and powder consolidation processes. The thermal treatment process included reactions between carbon and TiO₂, and between Al and TiO₂, TiO₃ or TiOₙC₁₋ₙ, which was produced by reacting TiO₂ and carbon. Heat treating C/TiO₂ composite powder produced pure TiO₃. Pure TiO could not be obtained by only increasing the carbon to TiO₂ ratio in the C/TiO₂ composite powder. Instead, TiOₙC₁₋ₙ solid solution was formed. Reaction kinetic studies showed that higher carbon to TiO₂ ratios accelerated the reaction due to refinement of the C/TiO₂ composite structure. Reactions between Al and TiOₙC₁₋ₙ formed Al₂Ti₄C₂/Al₂O₃ ceramic-ceramic composite. When the aluminum content of the starting Al/TiOₙC₁₋ₙ composite powders was increased, Al₂Ti₄C₂ still formed, but small amounts of Ti₃Al or TiAl also formed. Producing TiC from Al and TiOₙC₁₋ₙ reaction depended on the value of n in the TiOₙC₁₋ₙ phase. When n is small enough, TiC formed; otherwise TiC did not form.

This study found that fully dense matrix phase of TiₓAlᵧ/Al₂O₃ and Al₂Ti₄C2/TiₓAlᵧ/Al₂O₃/TiC formed after sintering at 1550°C. The Al₂O₃ particles became fully sintered when the composites were sintered at 1650°C but not at 1550°C. The Al₂O₃ particles in the α-Ti(Al,O) and Ti₃Al based composites had a high coarsening rate above 1550°C, indicating that the oxygen and Al diffusion rates of α-Ti(Al,O) and Ti₃Al are very high at these temperatures. The Al₂O₃ particles in TiAl based composites also had a high coarsening rate above 1550°C, caused by TiAl being liquid at these temperatures. The Al₂O₃ particles in the Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC based composites had a much lower coarsening rate because of the Al₂Ti₄C₂ matrix the lower oxygen content and high melting point decreases oxygen and aluminum mobility.

The size and volume fraction of Al₂O₃ particles in the composites strongly affected the mechanical properties of the TiₓAlᵧ and Al₂Ti₄C₂ based composites. Hardness of α-Ti(Al,O), Ti₃Al, and Al₂Ti₄C₂ based composites was 800-1600 VHN and hardness of TiAl based composites was 150-500 VHN. Bending strength of TiₓAlᵧ/Al₂O₃ composites was 105-230 MPa and bending strength of Al₂Ti₄C₂/TiₓAlᵧ/Al₂O₃/TiC based composites was 250-460 MPa. It appeared that 10 μm might be the critical size of Al₂O₃ particles. Below 10 μm bending strength dramatically improved with deceasing the Al₂O₃ particle size in the composites. The fracture toughness of these composites was 2-4.5MPa•m¹/². Composite hardness increased and bending strength decreased with increasing Al₂O₃ particle size and volume fraction.

year: 2002

title: What's news? Encounters with news in everyday life: A study of behaviours and attitudes

authors: Cunningham, Sally Jo

subjects: computer science

abstract: As the news landscape changes, for many users the nature of news itself is changing as well. Insights into the changing news behaviour of users can inform the design of access tools and news archives. We analysed a set of 35 autoethnographies of news encounters, created by students in New Zealand. These comprise rich descriptions of the news sources, modalities, topics of interest, and news ‘routines’ by which the students keep in touch with friends and maintain awareness of personal, local, national, and international events. We explore the implications of these insights into news behaviour for digital news systems.

text: What's news? Encounters with news in everyday life: A study of behaviours and attitudes As the news landscape changes, for many users the nature of news itself is changing as well. Insights into the changing news behaviour of users can inform the design of access tools and news archives. We analysed a set of 35 autoethnographies of news encounters, created by students in New Zealand. These comprise rich descriptions of the news sources, modalities, topics of interest, and news ‘routines’ by which the students keep in touch with friends and maintain awareness of personal, local, national, and international events. We explore the implications of these insights into news behaviour for digital news systems.

year: 2016

title: The “standard story” of anti-Māori talk in Pae Ora (Healthy Futures) Bill submissions

authors: Black, Rose

abstract: AIM: To review some common patterns of race talk in a sample of submissions made to the Pae Ora (Healthy Futures) Bill. This bill proposed a structural reform of the health system in Aotearoa New Zealand to address long-standing health inequities experienced by Māori, the Indigenous peoples, and other priority populations. METHOD: In a sample of 3,000 individual submissions made in late 2021, we found 2,536 explicit references to race. Utilising the “stan-dard story” frame of Pākehā/non-Maori race talk, five longer submissions that inferred that the Pae Ora bill was “racist” were analysed in detail. RESULTS: Many “standard story” race discourses were identified in the Pae Ora submissions. Three derived discourses included in this paper are: Pākehā as norm (monoculturalism or not seeing Pākehā as a culture), equality and the “Treaty” (equality for all to access healthcare), and one people (we are all New Zealanders). Sources such as the Waitangi Tribunal Wai 2575 Hauora report were drawn on to provide alternative discourses. CONCLUSION: Identifying Pākehā standard story discourses enables learning about language patterns systems draw on, and the development of tools and procedures to improve equity for Māori and eliminate institutional racism.

text: The “standard story” of anti-Māori talk in Pae Ora (Healthy Futures) Bill submissions AIM: To review some common patterns of race talk in a sample of submissions made to the Pae Ora (Healthy Futures) Bill. This bill proposed a structural reform of the health system in Aotearoa New Zealand to address long-standing health inequities experienced by Māori, the Indigenous peoples, and other priority populations. METHOD: In a sample of 3,000 individual submissions made in late 2021, we found 2,536 explicit references to race. Utilising the “stan-dard story” frame of Pākehā/non-Maori race talk, five longer submissions that inferred that the Pae Ora bill was “racist” were analysed in detail. RESULTS: Many “standard story” race discourses were identified in the Pae Ora submissions. Three derived discourses included in this paper are: Pākehā as norm (monoculturalism or not seeing Pākehā as a culture), equality and the “Treaty” (equality for all to access healthcare), and one people (we are all New Zealanders). Sources such as the Waitangi Tribunal Wai 2575 Hauora report were drawn on to provide alternative discourses. CONCLUSION: Identifying Pākehā standard story discourses enables learning about language patterns systems draw on, and the development of tools and procedures to improve equity for Māori and eliminate institutional racism.

year: 2023-07-21