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Yao, M.J.,Dey, P.,Seol, J.B.,Choi, P.,Herbig, M.,Marceau, R.K.W.,Hickel, T.,Neugebauer, J.,Raabe, D. Elsevier Science 2016 Acta materialia Vol.106 No.-
<P>We report on the investigation of the off-stoichiometry and site-occupancy of kappa-carbide precipitates within an austenitic (gamma), Fe-29.8Mn-7.7Al-1.3C (wt.%) alloy using a combination of atom probe tomography and density functional theory. The chemical composition of the kappa-carbides as measured by atom probe tomography indicates depletion of both interstitial C and substitutional Al, in comparison to the ideal stoichiometric L'12 bulk perovskite. In this work we demonstrate that both these effects are coupled. The off-stoichiometric concentration of Al can, to a certain extent, be explained by strain caused by the kappa/gamma mismatch, which facilitates occupation of Al sites in kappa-carbide by Mn atoms (Mn-Al(gamma) anti-site defects). The large anti-site concentrations observed by our experiments, however, can only be stabilized if there are C vacancies in the vicinity of the anti-site. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>
Strengthening and strain hardening mechanisms in a precipitation-hardened high-Mn lightweight steel
Yao, M.J.,Welsch, E.,Ponge, D.,Haghighat, S.M.H.,Sandlö,bes, S.,Choi, P.,Herbig, M.,Bleskov, I.,Hickel, T.,Lipinska-Chwalek, M.,Shanthraj, P.,Scheu, C.,Zaefferer, S.,Gault, B.,Raabe, D. Elsevier 2017 ACTA MATERIALIA Vol.140 No.-
<P>We report on the strengthening and strain hardening mechanisms in an aged high-Mn lightweight steel (Fe-30.4Mn-8Al-1.2C, wt.%) studied by electron channeling contrast imaging (ECCI), transmission electron microscopy (TEM), atom probe tomography (APT) and correlative TEM/APT. Upon isothermal annealing at 600 degrees C, nano-sized kappa-carbides form,, as characterized by TEM arid APT. The resultant alloy exhibits high strength and excellent ductility accompanied by a high constant strain hardening rate.& para;& para;In comparison to the as-quenched kappa-free state, the precipitation of kappa-carbides leads to a significant increase in yield strength (similar to 480 MPa) without sacrificing much tensile elongation. To study the strengthening and strain hardening behavior of the precipitation-hardened material, deformation microstructures were analyzed at different strain levels. TEM and correlative TEM/APT results show that the kappa-carbides are primarily sheared by lattice dislocations, gliding on the typical face-centered-cubic (fcc) slip system {111 }<110>, leading to particle dissolution and solute segregation. Ordering strengthening is the predominant strengthening mechanism. As the deformation substructure is characterized by planar slip bands, we quantitatively studied the evolution of the slip band spacing during straining to under stand the strain hardening behavior. A good agreement between the calculated flow stresses and the experimental data suggests that dynamic slip band refinement is the main strain hardening mechanism. The influence of kappa-carbides on mechanical properties is discussed by comparing the results with that of the same alloy in the as-quenched, kappa-free state. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved</P>
Strain hardening by dynamic slip band refinement in a high-Mn lightweight steel
Welsch, E.,Ponge, D.,Hafez Haghighat, S.M.,Sandlö,bes, S.,Choi, P.,Herbig, M.,Zaefferer, S.,Raabe, D. Elsevier 2016 Acta materialia Vol.116 No.-
<P>The strain hardening mechanism of a high-Mn lightweight steel (Fe-30.4Mn-8A1-1.2C (wt%)) is investigated by electron channeling contrast imaging (ECCI) and transmission electron microscopy (TEM). The alloy is characterized by a constant high strain hardening rate accompanied by high strength and high ductility (ultimate tensile strength: 900 MPa, elongation to fracture: 68%). Deformation microstructures at different strain levels are studied in order to reveal and quantify the governing structural parameters at micro- and nanometer scales. As the material deforms mainly by planar dislocation slip causing the formation of slip bands, we quantitatively study the evolution of the slip band spacing during straining. The flow stress is calculated from the slip band spacing on the basis of the passing stress. The good agreement between the calculated values and the tensile test data shows dynamic slip band refinement as the main strain hardening mechanism, enabling the excellent mechanical properties. This novel strain hardening mechanism is based on the passing stress acting between co-planar slip bands in contrast to earlier attempts to explain the strain hardening in high-Mn lightweight steels that are based on grain subdivision by microbands. We discuss in detail the formation of the finely distributed slip bands and the gradual reduction of the spacing between them, leading to constantly high strain hardening. TEM investigations of the precipitation state in the as-quenched state show finely dispersed atomically ordered clusters (size < 2 nm). The influence of these zones on planar slip is discussed. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>