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: Program Development and Specification Refinement in the Schema Calculus

authors: Henson, Martin C.

subjects: computer science

abstract: We introduce a framework for program development and specification refinement in the schema calculus of Z. We provide illustrative examples outlining the major design decisions based on an interpretation of operation schemas as sets of programs.

text: Program Development and Specification Refinement in the Schema Calculus We introduce a framework for program development and specification refinement in the schema calculus of Z. We provide illustrative examples outlining the major design decisions based on an interpretation of operation schemas as sets of programs.

year: 2000

title: Moehewa: Death, lifestyle & sexuality in the Maori World

authors: Nikora, Linda Waimarie

abstract: Customary death ritual and traditional practice have continued for the Māori (indigenous) people of Aotearoa/New Zealand, despite intensive missionary incursion and the colonial process. This paper critically considers what occurs when the deceased is different, in a most significant way. What happens when you die – and you are Māori and any one, or a combination, of the following: a queen, takatāpui,[1] butch, like that, gay, she-male, lesbian, transsexual, a dyke, intersex, tomboy, kamp, drag, homosexual, or just queer? Who remembers you and how? Same sex relationships today are still discouraged or denied, although traditional chant and Māori visual narratives record such liaisons and erotic experience as joyously normal (Te Awekotuku, 2005). And yet some people choose to remain in the closet. In this paper, we present three case studies, and invite the reader to reflect with us on mourning rituals or tangi – Māori death rites, in a same sex relationship, or for a gay, lesbian or transsexual family member. In this way, we open up a space for dialogue about such matters in our intimate and kin communities.

text: Moehewa: Death, lifestyle & sexuality in the Maori World Customary death ritual and traditional practice have continued for the Māori (indigenous) people of Aotearoa/New Zealand, despite intensive missionary incursion and the colonial process. This paper critically considers what occurs when the deceased is different, in a most significant way. What happens when you die – and you are Māori and any one, or a combination, of the following: a queen, takatāpui,[1] butch, like that, gay, she-male, lesbian, transsexual, a dyke, intersex, tomboy, kamp, drag, homosexual, or just queer? Who remembers you and how? Same sex relationships today are still discouraged or denied, although traditional chant and Māori visual narratives record such liaisons and erotic experience as joyously normal (Te Awekotuku, 2005). And yet some people choose to remain in the closet. In this paper, we present three case studies, and invite the reader to reflect with us on mourning rituals or tangi – Māori death rites, in a same sex relationship, or for a gay, lesbian or transsexual family member. In this way, we open up a space for dialogue about such matters in our intimate and kin communities.

year: 2016