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Title: Magneto-structural transformations via a solid-state nudged elastic band method: Application to iron under pressure

We extend the solid-state nudged elastic band method to handle a non-conserved order parameter, in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volume collapse during the pressure-induced transformation in iron—from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface (yet all the atomic and cell degrees of freedom are continuous); the calculated pressure jump of 25 GPa is related to the observed 25 GPa spread in measured coexistence pressures arising from martensitic and coherency stresses in samples. Our results agree with experiments, but necessarily differ from those arising from drag and restricted parametrization methods having improperly constrained or uncontrolled degrees of freedom.
Authors:
 [1] ;  [1] ;  [2]
  1. The Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011-3020 (United States)
  2. (United States)
Publication Date:
OSTI Identifier:
22493520
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 143; Journal Issue: 6; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 36 MATERIALS SCIENCE; AUSTENITE; BCC LATTICES; CUSPED GEOMETRIES; DEGREES OF FREEDOM; ENTHALPY; EQUILIBRIUM; HCP LATTICES; IRON; MAGNETIZATION; MARTENSITE; MARTENSITIC STEELS; ORDER PARAMETERS; PHASE TRANSFORMATIONS; POTENTIAL ENERGY; PRESSURE DEPENDENCE; PRESSURE RANGE GIGA PA; SOLIDS; STRESSES; SURFACES