A hierarchical microstructure due to chemical ordering in the bcc lattice: Early stages of formation in a ferritic Fe-Al-Cr-Ni-Ti alloy

C. H. Liebscher*, V. R. Radmilović, U. Dahmen, N. Q. Vo, D. C. Dunand, M. Asta, G. Ghosh

*Corresponding author for this work

Research output: Contribution to journalArticle

35 Scopus citations

Abstract

A hierarchical microstructure is obtained in an alloy with composition Fe-8.1Al-12.2Cr-1.9Mo-18.2Ni-2.0Ti (wt.%) processed by melt-spinning. The evolution of the precipitation pathways is investigated using transmission electron microscopy (TEM) techniques, atom probe tomography (APT) and first-principles thermodynamic calculations. As-solidified ribbons exhibit a random dispersion of B2-ordered precipitates (NiAl-type) in an Fe-based matrix. Subsequent aging at 700 °C yields nucleation and growth of the L21-phase (Ni2TiAl-type) within the primary B2-precipitates, leading to a microstructure exhibiting three types of hierarchy: (i) a structural hierarchy due to chemical ordering, with a chemically disordered matrix of bcc-Fe (A2), the nearest-neighbor (NN) ordered B2-precipitates (NiAl-type) and the next nearest-neighbor (NNN) ordered L21-precipitates (Ni2TiAl-type) within B2, (ii) a dimensional hierarchy with a continuous bcc-Fe matrix, coherently embedded B2-precipitates, with a size range of 60-200 nm and the coherent precipitate substructure, with L21-phase and dimensions of 15-20 nm. (iii) A spatial hierarchy where B2-precipitates are embedded in the bcc-Fe matrix and L21-precipitates nucleate and grow only within B2-precipitates. In addition, it is verified that the interface between B2 and L21 is coherent and adopts a diffuse structural profile. Monte-Carlo simulations reproduce these observations and it is found that interface energies of B2 and L21 reduce from 50 mJ/m2 at 0 K to 11 mJ/m2 at 973 K. Kinetic-Monte-Carlo simulations support the interpretation of the experimental results that the L21 nucleates within the B2 phase.

Original languageEnglish (US)
Pages (from-to)220-232
Number of pages13
JournalActa Materialia
Volume92
DOIs
StatePublished - Jun 15 2015

Keywords

  • Ferritic alloys
  • First-principles
  • Heusler alloy
  • Monte-Carlo simulations
  • Precipitation hardening
  • Transmission electron microscopy

ASJC Scopus subject areas

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Polymers and Plastics
  • Metals and Alloys

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