Effect of aging on coarsening- and creep resistance of a Ti-modified Fe–Ni–Al–Cr–Mo ferritic steel with L21/B2 composite precipitates

Sung Il Baik, Michael J.S. Rawlings, David C. Dunand*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

The FBB8-ferritic superalloy (Fe- 10Ni- 6.5Al- 10Cr- 3.4Mo- 0.25Zr- 0.005B, wt.%) modified with 2 wt.% Ti additions, was aged between 700 and 860 °C, leading to coarsening of its coherent L21-Ni2TiAl/B2–NiAl composite precipitates. The temporal evolution of these submicron precipitates upon aging, studied via scanning electron microscopy (SEM), is consistent with the diffusion-limited Lifshitz-Slyozov-Wagner (LSW) and Philippe-Voorhees (PV) coarsening models with an activation energy Q = 253 ± 72 kJ/mol and interfacial energies for the precipitates of 83, 78 and 75 (±26) mJ/m2 for 700, 780 and 860 °C, respectively. Creep tests performed at 700 °C reveal threshold stresses ranging from σth = 94 MPa (after over-aging at 860 °C, with significant coarsening) to σth = 187 MPa (after peak-aging at 700 °C). Creep resistance is reduced with increasing aging temperature and/or creep temperatures (from 700 to 860 °C) and the following mechanisms are identified: (i) composite L21/B2 precipitate coarsening, (ii) partial loss of coherency of precipitates by misfit dislocations at the L21/B2 precipitate/matrix interface, (iii) absence of nano-sized secondary precipitates. Nevertheless, the alloy exhibits a high creep resistance when aged and creep-tested at 780 °C, with a threshold stress σth = 67 MPa, which is similar to that exhibited at 700 °C, σth = 69 MPa, by the same alloy without 2% Ti addition (FBB8), which contains weaker, single-phase B2–NiAl precipitates.

Original languageEnglish (US)
Article number138987
JournalMaterials Science and Engineering A
Volume776
DOIs
StatePublished - Mar 3 2020

Keywords

  • Coarsening
  • Composite (multi-phase) precipitates
  • Creep
  • Creep/coarsening activation energies
  • Ferritic based superalloy
  • Interfacial energy
  • L2-B2 phases
  • Precipitate strengthening

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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