Thermal expansion and crystal structure of cementite, Fe₃C, between 4 and 600 K determined by time-of-flight neutron powder diffraction

Abstract

The cementite phase of Fe 3 C has been studied by high-resolution neutron powder diffraction at 4.2 K and at 20 K intervals between 20 and 600 K. The crystal structure remains orthorhombic (Pnma) throughout, with the fractional coordinates of all atoms varying only slightly (the magnetic structure of the ferromagnetic phase could not be determined). The ferromagnetic phase transition, with T c 9 480 K, greatly affects the thermal expansion coef®cient of the material. The average volumetric coef®cient of thermal expansion above T c was found to be 4.1 (1) Â 10 À5 K À1; below T c it is considerably lower (< 1.8 Â 10 À5 K À1) and varies greatly with temperature. The behaviour of the volume over the full temperature range of the experiment may be modelled by a thirdorder Gru È neisen approximation to the zero-pressure equation of state, combined with a magnetostrictive correction based on mean-®eld theory.

Iron Carbide Inclusions in Lower-Mantle Diamond From Juina, Brazil

“…According to electron-diffraction data, two phases were identified among iron carbide grains, Fe 3 C and Fe 2 C. Both iron carbide phases have an orthorhombic structure, but differ in their unit-cell dimensions: Fe 3 C: a 0 = 0.509 nm, b 0 = 0.6748 nm, c 0 = 0.4523 nm (Wood et al, 2004); and Fe 2 C: a 0 = 0.4704 nm, b 0 = 0.4318 nm, c 0 = 0.2830 nm (Hirotsu & Nagakura, 1972).…”

Stability and crystal structures of iron carbides: A comparison between the semi-empirical modified embedded atom method and quantum-mechanical DFT calculations

“…The different colors represent the calculated results by means of MEAM (green/light gray), DFT-GGA (red/dark gray), DFT-LDA (blue/medium gray) and experimental values (black). 61 The filled spheres represent for the bcc family, filled squares the fcc family, and filled diamonds the hcp family (see Table II). …”

A first-principles study of cementite (Fe3C) and its alloyed counterparts: Elastic constants, elastic anisotropies, and isotropic elastic moduli

“…49 Experimentally, the EOS parameters have been determined only for Fe 3 C using diamond-and-anvil apparatus 26,5,[59][60][61] and computationally using 36,39 employing FP-LAPW and CASTEP codes. In general, our B o is higher than experimental values that could be due to the fact that experiments were conducted at room temperature or higher.…”