| Budget Amount *help |
¥3,600,000 (Direct Cost: ¥3,600,000)
Fiscal Year 2000: ¥1,800,000 (Direct Cost: ¥1,800,000)
Fiscal Year 1999: ¥1,800,000 (Direct Cost: ¥1,800,000)
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| Research Abstract |
Some long-period superstructures based on the L1_0 structure are formed in the Al-rich TiAl region of γ-TiAl expected as high-temperature heat-resistance materials. Mechanical properties of the Al-rich TiAl alloys are controlled depending on periodic-structure, morphology and distribution of these superstructures. In this study, processes of ordering and precipitation were clarified in the (Al, Ga)-rich (Al, Ga) Ti region using rapidly solidified ribbons containing the superstructures with a low degree of order.
Firstly, it was found by the detailed TEM observation that stability of these superstructures formed in the Al-rich TiAl region depends strongly on annealing temperature and Al concentration. Profit annealing changes the structure and periodicity of stable superstuctures. The stable superstructure was, for example, changed from Al_5Ti_3 to r-Al_2Ti through h-Al_2Ti. The transformation process was understood on the basis of crystal symmetry and lattice misfit of superstructures.
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he next year, an investigated compositional area was expanded from the Ti-Al binary to the Ti-Al-Ga ternary region in order to clarify the ordering process and transitional superstructures. In the Ti-Ga binary system, the Ga_3Ti_2-type superstructure with 5-fold periodicity with respect to the L1_0 matrix exists, thus the phase stability was discussed among three superstructures with different periodic structures from 3 to 5 fold-periodicity. In the (Al, Ga)-rich (Al, Ga) Ti region, continuous solid-solution area of the L1_0 matrix phase exists and some superstructures with different periodicities appeared depending on annealing temperature and composition in this region. The stable phase changes from the 2 : 1 to 3 : 2 structure through the 5 : 3 structure with a decrease in Al concentration or annealing temperature. This transitional process was explained by an continuous change in {310) concentration waves.
The above results can resolve phase stability and transitional process of the superstructures closely related to the mechanical properties of γ-TiAl alloys. Less
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