Structural dependence on composition of rapidly quenched Fe-B alloys

Based on the evolution of the rapidly quenched Fe_1-x{B}ₓ$ alloy density, $⁵⁷Fe$ and $¹¹B$ hyperfine fields, $¹¹B$ NMR linewidth, and magnetic moment per alloy atom, in the composition range 0.01{≤}x{≤}0.25, it is concluded that the short-range order around boron atoms in the amorphous alloys evolve from (orthorhombic) o-Fe₃B-like at x{}0.12 to (tetragonal) t-Fe₃B-like at x{}0.22--0.25. It is suggested that the microstructure of the amorphous alloys for x larger than but close to 0.12 resembles the fine dispersion of minute boride islands surrounded by iron observed in the crystalline system for x{≤}0.09, but with both boride and iron regions in amorphous states, in agreement with a model previously proposed by Dubois and Le Caer. The strong contribution of the interface boride/iron to the system free energy for alloys in the crystalline state is used to justify thermodynamically the crystalline to amorphous transition at x{}0.12 and again to crystalline at x{}0.25. Values are estimated for the free-energy formation of o-Fe₃B and of the interfaces o-Fe₃B/{α}-Fe and Fe₂B/{α}-Fe. Using a simple model for the formation of the boride particles in the rapidly quenched crystalline alloys, a relationship between particle radius and quenching rate is developed and from it a quenching rate--composition phase diagram is drawn, which reproduces qualitatively the main features of the iron-boron system.