Defect Structures of Intermetallic FeRh Alloys Induced by High-Speed Deformation
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Defect Structures of Intermetallic FeRh Alloys Induced by High-Speed Deformation Yasunori Kibata, Fuminobu Hori1, Ryuichiro Oshima1, Masao Komatsu2 and Michio Kiritani2 Graduate School of Engineering, Osaka Prefecture University Sakai 599-8531, Japan 1 Research Institute for Advanced Science and Technology, Osaka Prefecture University, Sakai 599-8570, Japan 2 Research Center for Ultra-High-Speed Plastic Deformation, Hiroshima Institute of Technology, 2-1-1 Miyake, Saekiku, Hiroshima 731-5143, Japan ABSTRACT In order to study the role of vacancy-type defects in the unique deformation-induced phase transition mechanisms of B2-L10 and B2-A1 of intermetallic compound Fe-Rh alloys, high-speed impact compression deformation has been carried out. Induced phases and substructures were examined by XRD and TEM. The relation between the phase transitions and vacancy-type defects was investigated by the positron annihilation Coincidence Doppler Broadening (CDB) method. The results showed that intermixed phases of L10 and A1 were always formed in the B2 phase, and the vacancy concentrations were rather decreased in the specimens deformed by higher strain rates, suggesting that vacancies were more consumed by the phase transition to A1 structure. INTRODUCTION FeRh alloys exhibit excellent anticorrosion and mechanical strength in a wide temperature range. [1,2] The magnetic behavior is also very interesting. A stoichiometric alloy may be used for magnetic switch devices because of its transition temperature of anti ferromagnetic-ferromagnetic phases near room temperature. FeRh has a B2 type ordered structure over a wide range of Rh below 51at.% at room temperature. Regarding the stability of B2 structure in the wide Fe-rich side of the alloy, association of two point defect species will be considered. The first is an antisite defect, and the second a structural vacancy. In addition, another marked feature of the alloy is formation of two kinds of stress-induced phases depending on the deformation rates. [3-5] One is a phase transition from B2 (α ) to L10, which is formed from the original B2 phase by comparatively mild work, and with heavier work comes out an A1 structure. It is considered that the former is a martensitic type induced by shear; the lattice deformation is explained by cooperative movement of atoms, and the coordination of atoms is kept before and after the transformation. However, the L10 is not reported as a stable phase in the phase diagram, but an A1 phase (γ ) exists as a high temperature phase. [6] The lattice change by the phase transformation is discussed elsewhere. [7] In case of B2 to A1 transition, position exchange of atoms must participate with the transformation. As one of factors of the phase transformation to take a random arrangement of Fe and Rh atoms, we suspect that vacancy type defects may be involving. Because these stress-induced transformations occur at room temperature and it is thought that the migration of atoms indispensable for transformation by atom diffusion is impossible at
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