Ordering in α-iron-silicon single crystals investigated by neutron scattering

The state of order of primary iron-rich iron-silicon single crystals has been investigated by neutron scattering. The atomic fraction cSi of silicon was between 0.076 and 0.103; the temperatures TA from which the specimens were quenched ranged from 723 K to 873 K. The observed (200) and (111) superlattice reflections are much sharper than those usually produced by short-range order. The experimental results are interpreted as follows. All specimens investigated have the DO3 superlattice. The long-range order parameter is always very close to the highest value possible at given cSi. The antiphase domains are very small. In most cases their diameter WD is below 5 nm. Only if cSi approaches the critical concentration, c*Si(TA), does WD become large. c*Si(TA) describes the alpha - alpha 2 phase boundary in the current Fe-Si phase diagram. The present results, however, preclude the interpretation of the function c*Si(TA) as a phase boundary; c*Si(TA) only has bearing on WD.