Low Temperature Positron Lifetime and Doppler Broadening Measurements in B2-type FeAl Alloys
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Low Temperature Positron Lifetime and Doppler Broadening Measurements in B2-type FeAl Alloys Tomohide Haraguchi, Fuminobu Hori1, Ryuichiro Oshima1 and Mineo Kogachi2 Graduate School of Science, Osaka Prefecture University, Sakai 599-8531, Japan. 1 Research Institute for Advanced Science and Technology, Osaka Prefecture University, Sakai 599-8570, Japan. 2 College of Integrated Arts and Sciences, Osaka Prefecture University, Sakai 599-8531, Japan. ABSTRUCT Positron lifetime and Doppler broadening measurements were carried out at a low temperature, 100K, for B2-type intermetallic compounds FeAl with composition ranges from 41.2 to 50.7 at%Al systematically in order to clarify the feature of lattice defects of the alloys. Quenching temperature dependences of positron lifetimes ranging from 573 to 1173 K was examined for alloys with 41.2, 49.0 and 50.7 at%Al. Two lifetime component analyses could not be made except for a few spectra, indicating a saturation trapping of positrons at atomic vacancies. The mean positron lifetimes are in a range of 175-195 psec, which are not corresponding to vacancy clusters but to mono and/or di-vacancies. The tendency for S-parameter to increase with increase in Al content was found. This suggests that such an increase of S-parameters is attributed to change in the fraction of atoms around the vacancies, not to increase in vacancy concentration. INTRODUCTION Many intermetallic compounds have attracted a number of investigators because of their possible application capabilities as high temperature structural materials. Considerable efforts have been directed toward practical applications of ordered titanium aluminide, metal silicides, nickel aluminide and iron aluminide. A B2-type intermetallic compound FeAl is one of the practical application candidates because of its excellent features, e.g., good oxidation resistance, relative high melting point and low cost. One of difficulties for practical use is in its poor ductility at ambient temperature. As is well known, the physical and mechanical properties are strongly affected by lattice defects, such as point defects and dislocations. Recently, vacancy hardening [1, 2] and yield stress anomaly [3] were reported as the topics of FeAl system, and participation of lattice defects is suggested in these phenomena. However, accumulation of information about the lattice defects for intermetallic compounds may be deficient compared with pure metals and disordered alloys. One of the reasons is that investigation of lattice defects for intermetallic compounds is more complex than pure metals and disordered alloys because of a variety of defects in the ordered alloys, such as constitutional defects, super dislocations and anti-phase boundaries. In addition, it is well known that some B2-type alloys of NiAl, CoAl and FeAl have curious point defect structures on off-stoichiometric composition region. The constitutional defects consist of changes from anti-site atoms of transition metal atom in the transition metal rich region to vacancies on transition
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