Anisotropic microstrain broadening in cementite, Fe₃C, caused by thermal microstress: comparison between prediction and results from diffraction-line profile analysis

Abstract

X-ray powder diffraction data of an Fe 3 C powder consisting of polycrystalline particles shows pronouncedly anisotropic microstrain broadening of the Bragg reflections. The extent and anisotropy of the broadening can quantitatively be attributed to thermal microstresses induced by anisotropic thermal shrinkage from the preparation temperature of 873 K to the ambient analysis temperature, in conjunction with the elastic anisotropy of Fe 3 C. research papers J. Appl. Cryst. (2012). 45, 944-949 A. Leineweber Anisotropic microstrain broadening in cementite 945 research papers J. Appl. Cryst. (2012). 45, 944-949 A. Leineweber Anisotropic microstrain broadening in cementite 949

C-vacancy concentration in cementite, Fe3C1−, in equilibrium with α-Fe[C] and γ-Fe[C]

“…Note that there are no hints at a polycrystalline character of the cementite particles. Within such polycrystals, thermal microstresses might build up, which would affect the (apparent) lattice parameters to some extent [27].…”

Thermal expansion anisotropy as source for microstrain broadening of polycrystalline cementite, Fe3C

“…Nevertheless, elastically accommodated misfits between different grains, so-called second-order stresses, are also known to cause microstrain (e.g. MacEwen et al, 1990;Manley et al, 2002;Brown et al, 2009;Leineweber, 2012;Koker et al, 2014) and can, in principle, also exist in dislocation-free polycrystals, for which line broadening due to dislocations can be neglected.…”

“…a sample where cohesion between the differently oriented crystallites is maintained via grain boundaries. The model system is cementite, Fe 3 C, powder consisting of polycrystalline particles of about 5 mm in diameter composed of differently oriented crystallites of about 1 mm in size (Leineweber, 2012). In these particles microstress-induced microstrain builds up as a result of thermal expansion (shrinkage) after preparation at 873 K. An earlier study on that material (Leineweber, 2012) was based on in-house X-ray diffraction (XRD) data taken at ambient temperature.…”

“…The model system is cementite, Fe 3 C, powder consisting of polycrystalline particles of about 5 mm in diameter composed of differently oriented crystallites of about 1 mm in size (Leineweber, 2012). In these particles microstress-induced microstrain builds up as a result of thermal expansion (shrinkage) after preparation at 873 K. An earlier study on that material (Leineweber, 2012) was based on in-house X-ray diffraction (XRD) data taken at ambient temperature. The present work features powder synchrotron XRD and powder neutron diffraction (ND) data on cementite covering a temperature range of 10-973 K. The data reveal simultaneously the strongly anisotropic thermal expansion of the orthorhombic cementite and the strongly temperaturedependent microstrain broadening.…”

A transition of ω-Fe3C → ω′-Fe3C → θ′-Fe3C in Fe-C martensite

“…Among all the carbides, the most well-known one is θ-type Fe 3 C cementite, which possesses orthorhombic crystal structure (space group Pnma) with its lattice parameter being a θ = 4.524 Å, b θ = 5.088 Å and c θ = 6.741 Å 1,2 . Although the θ-Fe 3 C cementite has been studied extensively due to its importance and popularity in carbon steels [3][4][5][6][7][8][9][10][11][12][13][14][15][16] , its formation mechanism remains unclear. This is particularly true for the θ-Fe 3 C formation during martensitic transformation.…”