Structures, interfaces and thermodynamic stability of nanocrystalline phases in rapidly solidified Fe-based amorphous nanocomposite ribbon, powder and coating

Abstract

Fe-based amorphous nanocomposite with nominal composition Fe 58.82 Cr 11.12 Mo 1.52 Si 4.16 B 15.12 P 8.88 C 0.39 (at%) has been processed through melt-spinning, melt-spinning followed by ball milling and thermal spray deposition of coating onto a mild steel substrate. X-ray diffraction and electron microscopy characterization of the processed alloy reveals that the melt-spun ribbon is mostly amorphous with uniform distribution of very fine crystals of α-Fe and oP30 (Fe 2 B 7 ) phase. In the melt-spun followed by ball milled powder mainly cI58 (Fe 18 Cr 6 Mo 5 ), tP30 (Fe 62 Cr 34 Mo 4 ) and mI32 (Fe 5 C 2 ) phases are obtained. However, the tP30 (Fe 62 Cr 34 Mo 4 ) phase while ball milling undergoes distortive changes locally to produce oC68, tP60 and tP58 phases with similar composition. The interface between the two structures is considerably strained. In the coating, cF118 (Fe 18 Cr 6 Mo 5 ) and oI92 (Fe 62 Cr 34 Mo 4 ) are observed in the amorphous matrix. Essentially the cF118 and oI92 can be seen as a structural derivative of cI58 and tP30 phases respectively. Local composition fluctuation and availability of a suitable nucleation condition leads to the nucleation of crystalline phases in the amorphous matrix in the melt-spun ribbon. In the melt-spun followed by ball milled powder further distortive changes are observed locally, which leads to the formation of polymorphically related phases to tP30 phase. Even though all the phases are crystallographically quite complicated, they are all derived from either the tP30 (Fe 62 Cr 34 Mo 4 ) phase or cI58 (Fe 18 Cr 6 Mo 5 ) phases. Thermodynamic calculations through Miedema’s model indicates that the nominal composition of the alloy lies at the boundary of glass forming composition. The crystal compositions, although different from the nominal composition of the alloy, lies in the crystal forming composition range. Local composition fluctuation, favorable thermodynamic conditions for nucleation and growth leads to the crystal nucleation. Although all the phases are crystallographically different, they have similar polyhedral order in them. The polyhedral structures cannot be correlated with Frank-Kasper or Bernal deltahedral phases. This is a significant difference between metal-metalloid glasses with its metal-metal counterpart. • Hagg phases and Fe-Cr-Mo intermetalic phases evolve in a Fe-based BMG forming alloy after processing through different solidification routes. • Different polymorphs of Fe-Cr-Mo intermetallics evolve after processing through different solidification routes. • Contrary to the metal-metal glasses, polyhedral structures of the intermetallics do not directly corroborate with Frank-Kasper polyhedra or Bernal deltahedra. • Formation of structurally different polymorphs in the same alloy indicates that the energy difference between two polymorphs is very little. • Miedema calculations show that the alloy composition lies in the boundary region of the glass forming composition range and composition fluctuation is responsible for nucleation of nanocrystalline phases.