Study of the local structure of metastable crystalline iron-boron alloys

Abstract

The microstructure of metastable crystalline Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">100-x</inf> B <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1 \leq x \leq 9</tex> ) alloys, produced by rapidly quenching from the melt, has been studied with different techniques. Studies performed with Möss-bauer effect (ME) spectroscopy and nuclear magnetic resonance (NMR) are reported here, along with preliminary high resolution electron microscopy (HREM) and selected area electron diffraction (SAED) observations, as well as initial extended X-ray absorption fine structure (EXAFS) studies. The alloys, which appeared as bcc single phase upon X-ray diffraction (XRD) analyses, were found to be heterogeneous on a microscopic scale. Both the Mössbauer effect and nuclear magnetic resonance results showed that the alloys could not be considered as random solid solutions of boron in iron. Based on the hyperfine field values at the <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">57</sup> Fe, <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sup> B, and <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> B sites, on the relative population of probes at the different sites, and on the temperature dependence of the hyperfine fields, it was concluded that very small regions with an orthorhombic Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> B-like structure exist in the bulk. The high resolution electron microscopy bright field image observations showed regions 5-15 Å in diameter, embedded in the α-Fe matrix, and having fringe spacings different from those of bcc α-Fe. The selected area electron diffraction patterns contained weak, broad spots which were somewhat more consistent with those from orthorhombic Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> B than tetragonal Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> B. The initial extended X-ray absorption fine structure analyses were consistent with the results provided by the other techniques. A comparison is made with a previous description in which these alloys were considered as random solid solutions. Our results are discussed in terms of the short-range order (SRO) for orthorhombic and tetragonal Fe <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> B, as well as bcc α-Fe.