Defect Properties and Magnetism in B2-type CoFe Alloys
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Defect Properties and Magnetism in B2-type CoFe Alloys Keiichi Harada, Hiroki Ishibashi and Mineo Kogachi Department of Materials Science, College of Integrated Arts and Sciences, Osaka Prefecture University, Sakai, 599-8531, Japan ABSTRACT Point defect properties have been studied for B2-type Co1-cFec alloys ( 0.4 ≤ c ≤ 0.6 ) by X-ray diffraction and density measurements. The lattice constant increases with increase in composition c and indicates a distinct change in the slope beyond c = 0.5 . Further, it shows a decrease with increase in quenching temperature. The obtained vacancy concentration is very low for any composition and temperature concerned. Thus, it can be concluded that defect type in B2 CoFe is an antisite atom. The change in the lattice constant with quenching temperature can be related to the long-range order (LRO): it is well approximated by a linear form with respect to square of the LRO parameter. CoFe alloys in both B2 and higher temperature A2 phase regions show ferromagnetism. Observed results are discussed taking a correlation between the defect structure and the magnetic behavior into account. INTRODUCTION Intermetallic compounds are formed around the stoichiometric compositions (e.g., AB and A3B) in many binary alloys, A-B. Because of their excellent physical and mechanical properties, they have attracted many researchers from viewpoints of understanding of the mechanism as well as practical fabrication as functional and structural material. Point defects are intrinsically generated in these compounds through deviation of the composition from the stoichiometry and elevating the temperature from absolute zero. They are well recognized to affect strongly the physical and mechanical properties and thus it is essential to clarify the point defect structure and behavior. We have studied such problems by density, X-ray and neutron diffraction and positron annihilation measurements for the intermetallic compounds NiAl, CoAl, FeAl and AuCd with B2-type ordered structure [e.g., 1-6]. In these B2 alloys, point defects are composed of antisite atom and vacancy, which are divided into the antisite A atom and the antisite B atom for the former and the A-vacancy and the B-vacancy for the latter [1]. Through the work mentioned above, it was shown that their point defect behavior at finite temperature is well described as a hybrid state in which four kinds of defects coexist [7]. The peculiar mechanical property found in B2 FeAl, well known as the vacancy hardening [8], was successfully interpreted from detailed experiments on vacancy concentration [4]. Further, the dynamic behavior of vacancies during ageing was recently studied for AuCd alloys in both B2 and martensite phases in order to clarify the role of point defects on the martensite ageing effect [6]. In the present report, the defect properties in B2 CoFe are studied. The Co-Fe system forms a B2-phase over a wide range of composition and at low-temperature. The B2-phase alloys show ferromagnetism and transform to a disordered A2 (bcc) s
