MOSSBAUER-EFFECT INVESTIGATION OF THE PENTAGONAL APPROXIMANT PHASE INTHE FE-NB SYSTEM

The formation of the pentagonal Frank-Kasper phase (mu phase) in the F e50+xNb50-x system near x=0 has been investigated. The effects of comp osition and heat treatment on the phase diagram of this system have be en studied. Only the as-cast x=O alloy was found to consist of a pure mu phase. X-ray diffraction patterns indicate the existence of a phase with a high degree of structural order but considerable chemical diso rder. A Mossbauer spectrum of the mu-phase material exhibits a room-te mperature quadrupole split doublet with Delta=0.209 mm/s and an isomer shift delta=-0.289 mm/s relative to alpha-Fe. A shell model analysis of the spectrum indicates a structure with a high degree of order. Res ults are discussed in terms of measurements on related icosahedral and decagonal phase materials.